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IEEE P1818 / D1 Guide for the Design of Low Voltage Auxiliary Systems for Electric Power SubstationsOverviewScopeThis guide will consider the components of both the AC and DC systems and provide guidelines and recommendations for designing the appropriate systems for the substation under consideration. This guide includes the low voltage auxiliary systems from the source(s) to the distribution point(s). Reliability requirements and load characteristics are discussed and distribution methods are recommended.PurposeThe low voltage AC and DC auxiliary systems comprise very important parts of the substation equipment. The design of the AC and DC auxiliary systems facilitates the safe and reliable operation of the substation. This guide considers various factors that affect the design of the AC and DC auxiliary systems such as reliability, load requirements, system configurations, personal safety and protection of auxiliary systems equipment.Normative ReferencesDefinitions4.0Design of Substation AC SystemsThe objective of this section is to provide the required information for the substation engineer to design safe and economical AC station auxiliary system as applicable for each substation. The station power for most substations can be represented by the block diagram shown in figure 4-1 below. Detailed information is given in the section given next to each part.Figure 4-1: Block Diagram for Typical Substation AC Station PowerThe above figure represents an ultimate station power configuration that can be applied to any substation depending on substation size reliability and load requirements. One source is designated as the normal feed, and a second or third source is used as a backup. A loss of the normal or preferred source will result in transferring the load to the backup source. In substations with multiple sources, the sources are normally connected to a transferring scheme. One or more AC panels are used to serve the substation load as required.As a first step to the design process, the design engineer must review the design criterion for the station power; the number of sources, source type: three phase or single phase, required transformer rating, connection and other factors that may affect the final configuration of the station power for the applicable substation. The design criterion is discussed in section 4.1 below.4.1 Design CriteriaIn general, the design criteria of the AC auxiliary system are determined by the demand load of the connected KVA of the substation loads, as well as the voltage ratings and number of phases of the substation equipment to be supplied. Auxiliary transformers and other station power components should consider substation expansion and/or anticipated growth rate. Timing of any proposed expansion may dictate initial installation or deferral of station power components. Some loads may be identified as critical, which requires AC service to be maintained at all times. Depending upon such critical loads, the substation may require two or three AC station service sources with the ability to transfer loads between sources.Due to the importance of the station power to the operation and reliability of the substation, the following factors must be considered in order to determine the required station power configurations:4.1.1 System Stability System stability considerations are important for the reliability requirements of the station power. If the loss of a certain substation will result in system disturbance not only within the owner utility system but might also have a cascading effect on neighboring utilities which may result in a blackout condition in the area.4.1.2 Customer Service and Loss of RevenueSome substations serve very critical loads such as hospitals, manufacturing complexes, governmental offices, schools and other important loads, or serve large blocks of load where the substation reliability requirements are high. Some substations are connected to power plants where the loss of the substation equipment may result in tripping the plant which results in loss of revenue for the utility. More than one station power source may be warranted for these types of substations. Other less critical substations may have limited effect on the customers’ service and one source for the station power may be justified.4.1.3 Equipment ProtectionSubstation equipment protection considerations must be given to all substations regardless of the size, however high and extra high voltage substations contain high cost equipment such as transformers where the cooling system is considered very important to the operation of this equipment and therefore a backup source is generally required. Protective relays or other electronic control equipment located in high temperature areas may require a continuous cooling system and therefore the second source is generally required.4.1.4 Design ConsiderationsReview & Comment –The designer may consider the following list when designing an AC system for Substation:Location of AC equipment indoor/outdoorNumber of AC PanelsEssential loads connectionsNon-essential loads connectionsConductor type and sizeVoltage drop calculationsNEC requirements4.1.5 Selection of auxiliary system voltageReview & Comment –Several secondary voltage levels are available for ac auxiliary systems. When determining the voltage level needed, the designer may use a standard voltage level determined by the designer’s power system or use a variation to maintain the voltage levels of the equipment being supplied. Either way, the designer needs to remember the factors in section 4.2: Connected Ultimate Load.4.2 Load RequirementsAC auxiliary power systems are vital to the successful operation of a substation. The AC auxiliary system supplies auxiliary AC power to all AC loads located within the substation. Because of the critical nature of much of these loads, it is necessary to calculate all of the required AC loads within the substation to adequately design the AC auxiliary power system.The governing design requirements of the substation AC auxiliary system should be safe, reliable operation and allowance for future load growth. Some applications may require additional uninterruptable power supplies.In both transmission and distribution substations, the substation AC auxiliary systems are typically used to supply loads such as, but are not limited to:Essential LoadNon Essential LoadMaintenance LoadConstruction Load4.2.1 Essential LoadThese loads are related to equipment operation and are necessary to the proper function of the substation.a) Transformer cooling, oil pumps, and load tap changers b) Substation battery charging systemsc) Circuit breaker air compressors and charging motors d) Power circuit breaker control circuitse) Power equipment heating circuits f) Communications equipmentg) Relaying, supervisory, alarm, and control equipment h) AC/DC converter – uninterruptable power suppliesi) AC powered motor operated disconnect switches4.2.2 Non-Essential LoadProvide description –a) Outdoor lighting, security systems and receptaclesb) Control building or switchgear building lighting, HVAC, and receptacles4.2.3 Maintenance LoadProvide description –4.2.4 Construction LoadProvide description –4.3 One-Line DiagramThe designer may consider the following list when designing one-line diagram.Transformer connection requirements when three-phase transformer is usedTransformer KVA and voltage ratingPossible available sourcesDetermine preferred sourcesNumber of AC Panels requiredMethod of connecting the AC panels to the transformer4.3.1 Distribution SubstationA distribution substation AC station service auxiliary system may be as simple as a step down transformer with secondary voltage to an AC panelboard to distribute service loads. Depending upon the size and criticality of the distribution substation, an alternate source and manual or automatic transfer switch may be incorporated.Provide Samples -Convert to Block Diagram –4.3.2 Transmission SubstationThe AC station service auxiliary system normally requires a primary and a backup station service source. Typically, the primary source is fed within the substation from a substation bus or transformer tertiary. The backup source may also come from a substation bus or transformer tertiary, or from an external source outside the substation or by a backup generator. The design should include the most reliable and efficient means for two auxiliary system sources. The service loads should be fed through an automatic transfer switch to provide redundancy and reliability to serve the substation AC loads.Many AC auxiliary systems in transmission substation will include AC panels in the substation yard to serve outdoor yard equipment and lighting circuits. Depending upon the voltage, yard equipment may be served from AC panels within the control building. Large three phase equipment loads may require yard panels rated 480 VAC, which would require voltage step down for other substation loads. Circuits within the control building should be served from AC panels within the building.Provide Samples -16Convert to Block Diagram –4.4 Station power Source4.4.1 Single-Phase or Three-PhaseAC auxiliary systems are critical in a substation. AC power is used to provide load requirements for all essential and non-essential loads within the substation. The designer should perform a load analysis on all the AC loads in the substation in order to decide whether a single-phase or three-phase source is needed. Economic analysis and determination of essential and non-essential loads should be considered before making a final decision.4.4.1.1 Single-Phase AC SourceSingle-phase transformers are typically used in smaller substations where the load demand does not warrant a three phase source. If the substation has a potential expansion or serves critical loads, a backup station power circuit may be considered. These two circuits shall be from independent sources. A tertiary winding of the power transformer may also be used for an AC source if available. Given the significance of the substation, a backup generator shall also be considered for providing an auxiliary source during an outage. The load analysis shall determine the size of the single-phase transformers. All critical or essential loads shall be served at any time. The single-phase source is used when all the substation loads can operate at 120 or 240 volts, and the KVA demanded by the loads is not large enough to require high ampacity conductors and breakers. It is very important for the designer to specify the rated AC voltage on the equipment specification.4.4.1.2 Three-Phase AC SourceSubstations with three-phase voltage equipment and high station power load requirements should consider a three-phase AC source. Large bulk substations with high MVA autotransformers are common examples of substations with a three-phase AC source. The fans on high MVA autotransformers and some oil pumps can be rated at 480 Volts. The 240/120 volt Delta and 240/120 volt Wye connections are common three-phase AC station power connections since they can also supply 120 volts when it is a four wire connection. Standard three-phase AC sources can be obtained with a three-phase transformer, a three-phase banking of single-phase transformers, or a backup generator for auxiliary source. With this type of AC source, there are several types of different connections in order to fit the load requirements. The standard IEEE C57.105 provides a guide for application of transformer connections in three-phase distribution systems. Some utilities utilize the three-phase AC source as a standard AC power supply to the substations. After the load analysis, the designer shall determine the rating of three-phase voltage, the type of connection and weather a three- phase transformer or transformer bank is needed.4.4.2 Station Power Available SourcesAvailable sources are represented in figure 4-1. The source deemed the most reliable source is typically designated and used as primary or normal source. The second source is designated as a backup source and used only when the primary source has been lost. A third source, if used, will be used as a second back up and will be connected only if both the primary and second sources have been lost.The designer must determine the power source to serve the substation AC load as described in section 4.2. In most cases, the designer may want to consider a primary and a secondary source based on the design criteria. Four sources that are commonly used as substation AC auxiliary power sources include:1. Power Transformer Tertiary2. Substation Bus3. Distribution Line4. Standby GeneratorsEach source has advantages and disadvantages. Substation location, substation equipment, and bus configurations may dictate which source is preferred. In substations where AC auxiliary power reliability is critical, multiple sources of AC auxiliary power may be utilized with a transfer scheme to switch between a normal AC auxiliary power source and an emergency AC auxiliary power source.4.4.2.1 Power Transformer TertiaryWhen used in a substation, the tertiary of a three winding or autotransformer can provide a good source for station power applications. When the primary and secondary windings are connected “wye”, a third or tertiary winding connected in delta must be used for stabilizing purposes. A tertiary winding presents a low impedance path to zero sequence currents and harmonics, thereby reducing the zero sequence impedance presented to the outside world, while avoiding the problem of tank heating. The tertiary winding typically has a volt-ampere rating between 20-30% of the volt-ampere rating of the primary winding, and typically has a medium voltage rating up to 40kV. If there are plans to use the transformer tertiary for a sub-transmission or distribution source and/or for station power purposes, the tertiary winding is brought out to external bushings for user connection. Otherwise, the tertiary winding is buried inside the transformer. See figure 4-2 for transformer connection.The volt-ampere rating of the tertiary winding typically exceeds the maximum volt- ampere requirement of a substation’s AC auxiliary power load and is, therefore, an extremely adequate AC auxiliary power source.Consideration should be given to the available fault current at the tertiary bus for phase-to-phase and three-phase faults. In case of the current magnitude exceeding the interrupting rating of the protective equipment such as the fuse or the circuit breakers, several options can be employed to reduce the fault current. These options include installing current limiting fuses, resistors, reactors or increasing the transformer tertiary impedance.Another consideration should be given to the detection of the ground fault on the tertiary bus. The tertiary buses on three phase power transformers are generally short and may not require any ground fault protection. However, when single phase power transformers are used to construct a three phase bank, bus runs that connect and form the tertiary bus become much longer and are more likely to be subject to a phase to ground fault. A single phase to ground fault on the tertiary will not generate fault current in the tertiary delta and will not trip the distribution transformer high side protection, nor will the fault trip a typical power transformer bank differential scheme. Some utilities choose to install a ground detection network on the tertiary delta to detect this single line to ground fault, signal an alarm, but not trip the power transformer bank. A common method for constructing this ground detection network is to install three small delta connected transformers with a grounding resistor across the open delta. There will be zero voltage dropped across the grounding resistor under normal operating conditions. There will be a voltage drop across the grounding resistor due to circulating current in the broken delta during a phase to ground fault and that indicates that a phase to ground fault on the delta tertiary has occurred.Figure 4-2: Tertiary Delta Bus with Grounding Resistor (Normal System Conditions)Figure 4-3: Tertiary Delta Bus with Grounding Resistor (A Phase Grounded)One disadvantage to using a power transformer tertiary as an AC auxiliary power source is that when the power transformer is removed from service due to either maintenance or failure, the AC auxiliary power source is also removed. This possibility generally leads utilities to install an emergency back-up AC auxiliary service for such cases. This emergency service is generally provided from either a second power transformer located within the substation, an unaffected high voltage bus, an offsite distribution feeder, or an emergency generator.Another disadvantage to using a power transformer tertiary as an AC auxiliary power source is that special Load Tap Changer (LTC) designs are required to stabilize the tertiary winding voltage (IEEE C57.12.10 Appendix A.1.2). Adding these LTC’s to the transformer’s tertiary winding will add additional cost to the power transformer.4.4.2.2 Substation BusAnother available source for station power is to use a high voltage bus as a source. This possible source is normally used when other sources are not available due to its relatively high cost. A Power Voltage Transformer (PVT), sometimes known as a Station Service Voltage Transformer (SSVT), is the device that may be used to transform the bus voltage to the AC auxiliary voltage. These devices are available for voltages between 34.5 to 230 kV. One or more SSVT’s might be required as required by the station power load. See figures 4-4, 4-5, 4-6, 4-7, 4-8 & 4-9 for possible connections.The SSVT is normally located within the line or bus relays zones of protection. A fault on the SSVT will be cleared by the protective relay faster than any high side fuse. Also the size of required fuse may not be available for certain voltage level. The protection engineer must be consulted for the final when determining the required SSVT protection. Low side over current protection of the secondary conductors used for auxiliary station service are typically applied as close to the secondary terminals as possible. Surge protection is typically needed on the high side connection of the SSVT. If arresters protecting other equipment in the station are close enough to protect the SSVT, a dedicated arrester for the PVT may not be required. Guidance on surge protection and separation effects can be found in IEEE C62.22, Guide for the Application of Metal- Oxide Surge Arresters for Alternating-Current Systems. Depending on the bus configuration and the importance of the switching station, multiple PVT’s may be required to provide adequate auxiliary AC service. The figures below show some bus arrangements and possible SSVT placements.For lower voltage substation busses up to 34.5 kV, standard distribution transformers can be used as the source, whether single phase or banked for three phase service.(There were figures here, but they did not copy or move well when I shuffled the document - not sure they are necessary)Where two station service feeds are provided within the switching station, either both by bus, or one by bus and the other by some other source, the feeds typically are switched on the secondary side with either a manual or automatic transfer switch. A typical arrangement is shown in Figure 4-10.4.4.2.3 Distribution Line / FeederDistribution feeders may be used as a normal or emergency AC auxiliary power source. Utilizing the distribution feeder as the normal source can be beneficial if that feeder is not directly supplied by the utility substation. In that case, the AC auxiliary power source would still be energized in the event of a de-energized utility substation. The disadvantage of using a distributor feeder is that the utility substation is depending upon the reliability of that feeder to supply AC auxiliary power. There is the risk of losing AC auxiliary power from the distributor feeder even though the utility substation is still energized.Some utilities use a distributor feeder as a back-up to a power transformer or bus normal AC auxiliary supply. There are engineering aspects to consider when using an outside distribution feeder as an emergency back-up source. When using the distribution feeder as a back-up source, consideration has to be taken as how to switch between the two supplies. This is done through a manual or automatic transfer switch. Consideration also must be given as to whether or not to switch the neutral connections when switching the live lines. If the distribution feeder has already been transformed to an AC auxiliary voltage (i.e. 120/240 volt), the neutral has been single point grounded, most likely, at the transformer secondary connection point. Conversely, the normal AC auxiliary source is typically single point grounded to the substation ground grid. In that case, it is desirable to switch the neutral from the normal source to the neutral of the emergency source. If both neutrals are single point grounded onto the same ground, then it is not necessary to switch the neutrals of the AC auxiliary supplies.4.4.2.4 Standby GeneratorsGenerators may also be used as an AC auxiliary power source. In substations, generators are typically used as an emergency power source instead of a permanent power source. This is due to the disadvantages of using generators as a permanent AC auxiliary source. Choosing to use generators as a permanent AC auxiliary source will require additional design considerations. Fire protection systems will need to be designed to protect the substation equipment from a generator file. Fuel storage systems will need to be installed to house the fuel needed to run the generators. The generators may also be housed in a separate building structure, which requires the installation of a ventilation system.Generators used as an emergency AC auxiliary power source have more merit than as a permanent source. As an emergency source, there is not the same need for fire protection installation, fuel storage system, or building ventilation (if the generators are located outdoors in the switchyard).4.5 Conductor SelectionThis section covers both the primary and secondary conductors. The primary conductor can be either a bare conductor or insulated cable depending in the location of the transformer. When the transformer is located near the source, a bare conductor either AA or copper can be used. When the transformer is located away from the source, Insulated cable Copper or All Aluminum can be used. The cable is normally installed inside a conduit and buried below grade.Insulated cable is normally used for the secondary conductor. The following must be considered when selecting either the primary or the secondary conductor. There are three main requirements for the conductor used for the auxiliary power system. IEEE 525 provides information on the selection and application of cables and conductor for AC auxiliary power systems.4.5.1 Cable Insulation Voltage RatingThe cable insulation voltage rating is selected based on the phase to phase operating voltage and the expected clearing fault clearing time. In general, for a clearing time equal to 1 minute, 100% insulation is selected, for fault clearing time greater than one minute but less than one hour, 133% insulation is selected. In either case, the protective relays must clear the fault before the insulation can be damaged as calculated in the sections below. The primary cable standard voltages are 5, 8,15,25,28 and 35kV. The secondary cable insulating is normally given in two ratings; 600V or 1000V.4.5.2 Cable Insulation TypeThe insulation type must be also selected to meet the location condition such as dry, wet or both. Table 4-2 shows some of the insulation types that have used in substation application. Refer NEC for more information.Table 4-2 Selected Cable Insulation TypeTHW‐2Thermoplastic Insulation (usually PVC), Heat Resistant (90°C rating),suitable for Wet locationsTHWN‐2Same as THW except Nylon jacket over reduced insulation thickness.Also rated THHN.THHNThermoplastic Insulation (usually PVC), High Heat Resistant (90°Crating), dry locations only, Nylon jacket. Also rated THWN.XHHW‐2Cross-linked Polyethylene Insulation (X) High Heat Resistant (90°Crating) for Wet and dry locations.RHHRubber Insulation (we actually use cross-linked polyethylene becauseit qualifies for rubber), High Heat Resistant (90°C rating) for dry locations only.RHW‐2Rubber Insulation (again, cross-linked polyethylene is used by most manufacturers), Heat Resistant (90°C), Suitable for Wet locations.USE‐2Underground Service Entrance. Most utilize cross-linked polyethyleneinsulation rated for 90°C in direct burial applications. Product is usually triple rated RHH—RHW—2-USE-2.4.5.3 Conductor Size CalculationsThe following factors must be considered when selecting the conductor size:Required AmpacityTemperature CorrectionAllowable Connector TemperatureVoltage Drop limitations4.5.3.1 Required AmpacityThe conductor size must be selected with a proper size to carry the maximum station power load. This is important to prevent conductor overheating and insulation damage and therefore fire. As a first step the ampacity requirements of the conductor must be determined. Station power load consists of a continuous load which is expected to continue for 3 hours or more. The non-continuous load is expected continue for short time only. The following substation load can be classified as continuous load:Control Building Air condition loadTransformer cooling loadsLighting loadBattery Charger loadLoad that is non-continuous can be identified as follows:Breaker motor loadTest equipment loadMotor operator load.Branch conductors should be sized to meet equipment load requirements. An overload factor of 1.25 should be applied. Equipment rating data will provide the load requirements. Some loads are of short duration, such as breaker spring or compressor motors. Others are continuous duration, such as heaters and transformer fans. As loads are aggregated at panels the conductors supplying the panel can be sized using a load factor based on the anticipated coincidence the of branch loads.Once the continuous and non-continuous load is determined, the primary and secondary conductors required size can be determined as discussed below:4.5.3.1.1 Primary ConductorAccording the NEC 215.15 (B1) the rating of the conductor shall be selected to equal to at least the KVA rating of the transformer. Since the station power primary current is normally low due to transformer low kVA rating and the high primary voltage, the conductor size is normally selected to exceed the NEC requirements. Therefore no overheating will occur and the voltage drop is acceptable. When an insulated cable is used, considerations must be given to the conductor strength in order to prevent breakage and damage during pulling.4.5.3.1.2 Secondary ConductorFor secondary conductors rated 600V or less, the NEC requires the conductor be rated equal to the non-continuous load plus 125% of the continuous load as defined in section 4.3.4.1 above. Using the NEC appropriate ampacity tables, a preliminary conductor size can be selected.4.5.3.2 Temperature CorrectionsIf the ambient temperature at the substation location is different than the ambient temperature used for calculating the NEC ampacity tables, the selected conductor ampacity should be corrected for the new ambient temperature. The following equation can be used to calculate the ampacity rating of the conductor based on the new ambient temperature: 4.1Where:I' = ampacity corrected for ambient temperatureI = ampacity shown in the tablesTc = temperature rating of conductor (°C) Ta' = new ambient temperature (°C)Ta = ambient temperature used in the table (°C)4.5.3.3 Correction for number of conductorsWhen more than three current carrying conductors either single or multi conductor cables are used for station power, the conductor rating should be de-rated according to NEC table 310.15(B)(3)(a) is shown below as table 4-2Table 4-2 (NEC Table 310.15(B)(3)(1) Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or CableNumber ofConductors1Percent of Values inTable 310.15(B)(16) through Table 310.15(B)(19) as Adjusted for Ambient Temperature if Necessary4-6.807-9.7010-20.5021-30.4531-40.4041 and Above.354.5.3.4 Voltage Drop VerificationsThe voltage for both the primary and secondary conductors must be maintained at low values. For station power applications, the voltage drop is normally low for the primary conductors and is not an issue. For the secondary conductors, depending on the distance between the transformer and AC panel must be maintained. Voltage drop limits may be given either as a target percent drop (typically 3% for a branch circuit or 5% including the feeder) or as the equipment voltage limitations. In substations with long cable lengths, the voltage drop considerations may require a larger conductor size than the ampacity requirements.4.6 Station Power Transformer4.6.1 Single-PhaseSingle-phase distribution transformers are manufactured with one or two primary bushings. The single-primary-bushing transformers can be used only on grounded wye systems if they are properly connected. An example, single-phase transformer connected to a three-phase 2,400-volt L-L to obtain l20-volt single-phase is shown. The connections are the same for the following voltage levels: 4,800 volt L-L, 7,200 volt L-L, 13,200 volt L-L, and 34,400 volt LA single-phase transformer connected to a three-phase 2,400-volt L-L system is shown below to obtain 120/240-volt single-phase, three-wire service. Normally the wire connected to the center low-voltage bushing will be connected to ground. Grounding the wire to the center bushing limits the secondary voltage above ground to 120 volts, even though the wires connected to the outside secondary bushings have 240 volts between them.The single-phase distribution transformer connected to 4,160Y/2,400 volts is shown below to obtain 120/240-volt single- phase secondary service. Other standard three-phase system voltages are 12,470Y/7,620V, 13,200Y/7,620V and 13,800Y/7,970 V.4.6.2 Three-Phase ConnectionSingle-phase transformers can be connected to obtain three-phase secondary voltages. The four common connections are shown below.4.6.2.1 Open Delta ConnectionsThe open delta bank is often the most economical choice for serving small three phase loads, particularly when commonly available distribution transformers can be used. The cost of the additional KVA capacity in the two transformers will normally be much less than the cost of an additional transformer, fuse, and installation labor.Open delta banks will carry 57.7% of the equivalent three phase capacity.Example:3- 25 KVA transformers 3 phase capacity = 75 KVA.2- 25 KVA transformers 3 phase capacity = 57.7% of 75 KVA or 43.275 KVA4.4.2.2 Common Three Phase Connections?Delta-Delta?Wye-Wye?Delta-Wye or Wye-Delta4.6.2.3 Personnel SafetyAll engineering, construction, and maintenance shall adhere to specific codes and standards as well as the Owner’s operating policy to ensure personnel safety.4.6.2.4 FerroresonanceFerroresonance is a condition that creates a high voltage between the transformer primary winding and ground. The high voltage can be as much as five times of the primary voltage. In such cases the transformer, cables insulation, or other equipment can be damaged. When present, the transformer will make sounds that are not of its normal hum.Ferroresonance occurs under the following conditionsThree phaseUngrounded primary and transformer groundedLong primary cable, producing a high capacitanceNo load on the bank or lightly loaded4.6.3 Station Power Transformer Types and RatingsIn this section the following transformer requirements will be considered:Quantity of station power transformers requiredStation power transformer ratingsTransformer impedanceTransformer connections4.6.3.1 Quantity of station power transformers requiredStation load dictate the number of transformers that must be used for station service transformer. For substations with single-phase and light to medium loads, single-phase transformer is used. For substations with three-phase high loads, a three transformer is normally selected. Other loads such as maintenance equipment may dictate the number of transformers to be used for station power.4.6.3.2 Station Power Transformer RatingsThe capacity of a transformer is determined by the amount of current it can carry continuously at rated voltage without exceeding the design temperature. Transformer rating is given in kilovolt-amperes (kVA) since the capacity is limited by the load current which is proportional to the kVA regardless of the power factor. The standard kVA ratings are given in Table 4-1 below.Table 4-1: Standard Ratings of Distribution Transformer kVAOverhead TypePad Mounted TypeSinglePhaseThreePhaseSingle PhaseThreePhase5152575103037.5112.515455015025757522537.5112.5100300501501675007522575010030010001675001500250200033325005004.6.3.3 Transformer Impedance4.6.3.4 Transformer ConnectionsDepending on number of transformer selected for station power applications two types of connections are employed:4.6.3.4 .1 Single Phase Transformer ApplicationSingle-phase distribution transformers are manufactured with one or two primary bushings. The single-primary-bushing transformers can be used only on grounded wye systems. For this connection the H1 bushing is connected to one of the available phases while the other bushing is connected to ground as shown in figure 4-11 below.Figure 4-11: Single Phase to Ground ConnectionWhen a delta system is available, a phase to phase voltage is applied between the two bushings H1 and H2 as shown in figure 4-12 below.Figure 4-12: Single Phase Transformer with Phase to Phase ConnectionsThe primary voltage can be any of the following; 2400,4800,13,200 and 34500 Volts. The Secondary voltage can be any of the following 120/240 and 480/120 Volts.4.6.3.4.2 Three Phase Transformer ConnectionsThree-phase transformer connection can be achieved by using two or three single-phase transformers and connected as required. The user can also specify three-phase transformer connected as specified by the user. When three-phase transformer is required a pad mounted three-phase transformer is normally used for the station power applications. A pad mounted three-phase transformer is applicable to below grade connection from both the primary and the secondary’s sides.When selecting a three-phase transformer the following must be considered before selecting the transformer connection:The required secondary voltageSafetyFerro resonance condition4.6.3.4.3 Secondary VoltageThe following secondary Transformer connection must be selected in order to obtain the secondary voltage that will be required for the application.Three-Phase Secondary Connections–DeltaThree-phase transformers or banks with delta secondaries will have simple nameplate designations such as 240 or 480. If one winding has a mid-tap, say for lighting, then the nameplate will say 240/120 or 480/240, similar to a single-phase transformer with a center tap. Delta secondaries can be grounded at the mid-tap or any corner.Three-Phase Secondary Connections–WyePopular voltages for wye secondaries are 208Y/120, 480Y/277, and 600Y/347.4.7 Transfer Switch4.7.1 GeneralThe need for an auxiliary power system transfer switch is related to the criticality of the substation. If only one station service power source is available, a transfer switch may not be required. If there are no critical AC system requirements, the DC battery system may be sufficient to operate the critical DC systems until the AC station service power is restored.Most substations are provided with two sources of station service AC power. The two sources of station service power are generally designated as the primary source and the alternate (or backup or secondary) source. Both sources should be of equal reliability.To simplify the operation of the transfer between sources, a “break before make” operation is suggested. This will ensure that sources that are out of phase with one another do not operate in parallel. In the case of manual operation of the transfer switch, it may be desirable to disable or lock out one source while the other source is being used. In both cases, sufficient training should be provided to operators to ensure that sources are not paralleled.Since the auxiliary power sources can be supplied at different voltages than the utilization voltage in the substation, the transfer switch can be applied at either the primary or secondary voltage. The higher voltage application results in lower current rated equipment. 13.8kV, 12.47kV, 4.16kV, 480V and 240/120V are common auxiliary power voltages and the transfer switch can be applied at any of these voltages. The auxiliary power source can be either three-phase or single-phase depending on the station service requirements. Transfer switches typically can be purchased with two, three or four poles. A four pole switch has the ability to switch the neutral and is necessary on a system that has separately derived system.Smaller rated transfer switches can be wall mounted. Floor mounted switches are common. Transfer switches can be purchased for indoor or outdoor mounting.The transfer switch may be as simple as two input sources with switching devices and one output to the load. The transfer system may be as elaborate as a unit switchgear consisting of two input switching devices, two transformers, two main circuit breakers, one tie circuit breaker and multiple branch circuit breakers.Figure 4.9-1 Simple Transfer switchFigure 4.9-2 Complex Transfer SystemAnother consideration when designing the transfer system is the reliability of the transfer switch. It may be prudent to make provisions to bypass the switch in the event of the switch’s failure, maintenance or replacement. This may be accomplished by having a third source routed to the substation AC load center that is left normally open and locked out until it is needed. It may be more cost effective to route another set of conductors from either or both of the primary and alternate source to the substation AC load center. Similar to the transfer operation, training and procedure should be provided to the operator so that it will be unlikely to parallel sources for a bypass option.4.7.2 Manual Transfer SwitchFor less critical substations a manual transfer switch will provide the capability of transferring from the primary to the alternate source. The manual transfer switch will be a much simpler and lower cost switch than an automatic transfer switch. However, the use of the manual transfer switch will require station alarms to alert operations personnel of the loss of the primary source and dispatching personnel to the substation to operate the manual transfer switch. Proper design of the DC battery system is required to provide continuous operation of critical systems (system protection functions, control and breaker tripping) while personnel responds and manually operates the transfer switch.If the substation has only one source of AC power, a manual transfer switch may still be desirable as a connection point for a temporary AC alternate source, such as a portable generator.The manual transfer switch consists of two (2) manually operated switching devices (usually circuit breakers) capable of interrupting the load current of the transfer switch. The two switching devices are typically mechanically interlocked to prevent both AC sources from being connected in parallel. Fault current interruption capability is not required in the transfer switch, but could be included or provided separately. Indication of source status (hot or dead) is not typically provided. Some point of alarm is necessary to detect the loss of the primary AC source.4.7.3 Automatic Transfer SwitchCritical substations or substations with critical AC loads will require a transfer switch that will automatically transfer from the primary source to the alternate source when the primary source is lost.Transfer should occur under the following conditions:1) There should be a time delay on loss of the primary source. This is to prevent transfer for momentary problems with the primary source.2) The alternate source is available. This is to prevent transfer to a dead source.Automatic return to the primary source should occur only after the primary source has been restored for a specified time period to prevent return to an unstable source.The automatic transfer switch consists of two (2) electrically operated switching devices (usually circuit breakers) capable of interrupting the load current of the transfer switch. The two switching devices can be electrically and/or mechanically interlocked to prevent both AC sources from being connected in parallel. Fault current interruption capability is not required in the transfer switch, but could be provided or added separately. Detection and indication of source status (hot or dead) is required. Time delays and control sequencing is necessary to prevent transferring to a dead alternate source or to prevent nuisance transferring to unstable sources. Indicating lights and relays are usually provided. Alarm indication of transfer should be provided. Close and latch capability must also be consider in equipment rating.4.7.4 Alternate MethodsIf both sources are designated as primary sources, the AC load can be divided between the two sources with the transfer switch system consisting of the two normally closed primary circuit breakers and a normally open transfer circuit breaker.4.8 AC Panel Bus rating4.9 AC PanelsReview and Comment -4.9.1 Present and Future Load AccommodationThe number of branch circuits is dependent on the ultimate station build out. The station arrangement drawing should provide the number of each type of equipment (transformers, circuit breakers, etc,). The voltage and current requirements of each piece of equipment will determine the branch circuit breaker rating requirements. For future equipment installations, the requirements should be a worst case estimation. Allowance for future “unknowns” or spare branch circuit breakers should be provided.4.9.2 Load Classification (or Segregation)It may be desirable to separate critical loads to different AC panels.Non-critical loads can be connected to different panels from the critical loads.4.9.3 Number of Panels requiredThe number of panels will depend on several items;1) Voltage and phase requirements - Generally, the higher the voltage of the auxiliary power system, the fewer circuit breakers that can fit in the panel. Two and three pole circuit breakers will require more space within the panel2) Load classification - With critical AC loads, additional panels may be required to allow for segregating the loads3) Equipment location in the station - If the substation is large, additional panels installed to serve specific groupings of equipment may be desirable. This will reduce the length of the branch circuit runs.4) Number of branch circuits required - Provision for future additions and unplanned additions should be made. If extensive future development is foreseen, it may be better to plan on providing some of the future development requirements with the addition of future panels.4.9.4 Panel RatingVoltageMain BreakerBranch BreakersNeutral BusGround Bus4.10 Circuit ProtectionThe protection of AC auxiliary power systems is vital to the successful operation of the substation. AC auxiliary power system circuit protection is necessary to ensure the proper protective device coordination operation of the system. This ensures proper clearing of faults in the AC auxiliary power system.4.10.1 Available Short Circuit CurrentThe available short circuit current in AC auxiliary power systems is largely determined by the AC auxiliary power system station service transformer. The VA rating, transformer impedance, and voltage rating are the key components in calculating the available short circuit current.For single phase AC auxiliary systems, the maximum available short circuit current available at the transformer secondary bushings is calculated by:Where,Isc = Maximum available short circuit currentVA = Volt Ampere rating of the station service transformerV = Voltage rating of the AC auxiliary power systemZxfmr = Per Unit Impedance of the station service transformerFor three phase AC auxiliary systems, the maximum available short circuit current available at the transformer secondary bushings is calculated by:AC auxiliary power system cabling adds extra impedance into the system so that the maximum available short circuit current seen at the station transformer secondary bushings is not necessarily the maximum short circuit current that is seen at AC auxiliary distribution panelboards or end use equipment. The short circuit current at any point downstream from the station service transformer can be calculated by adding the impedance of the conductor to the impedance of the transformer on a per unit basis.Where,Zpu = impedance per unitZ = actual impedanceZbase = impedance reference baseVAbase = volt ampere reference base (typically the VA rating of the station service transformer)The per unit impedance added to an auxiliary ac circuit can be calculated byWhere,? = Ohms to Neutral per 1000 feet (Values found in NEC Table 9) L = total conductor lengthOnce the per unit impedance of the circuit length is known, the maximum available short circuit current available at the end of that length can be calculated by4.10.2 Fault calculationsAt the sourceAt the transformer low sideAt the AC panel4.10.3 Transformer Protection4.10.4 Panel Protection4.10.5 Feeder Protection4.10.6 Selection of Circuit BreakersAppropriate circuit breaker selection is important for the protection and fault clearing coordination of the AC auxiliary power system. There are three important ratings to consider when properly selecting circuit breakers for AC auxiliary power system protection. These are the AC voltage rating, maximum AC current interrupting rating and the AC trip rating.The AC voltage rating of the circuit breaker should be equal to the operating voltage of the AC auxiliary power system. Typical AC circuit breaker voltage ratings are 120, 120/240, 208Y/120, 240, 277, 347, 480Y/277, 480, 600Y/347, and 600 volts.The maximum AC current interrupting rating is the maximum AC short circuit current that an AC circuit breaker can successfully interrupt. The power circuit breakers selected for use in an AC auxiliary power system must have a maximum AC current interrupting rating equal to or higher than the actual maximum AC current that the circuit breaker will see during service to effectively operate. AC auxiliary power system circuit breakers have typical maximum AC current interrupting ratings of 7.5 kA, 10 kA, 14 kA, 18 kA, 20 kA, 22 kA, 25 kA, 35 kA, 42 kA, 50 kA, 65 kA, 85 kA, 100 kA, 125 kA, 150 kA, and 200 kA.The AC trip rating is the maximum AC continuous current that an AC circuit breaker will allow to flow through it. When this rating is exceeded, the circuit breaker will operate. The maximum AC continuous current required to supply an AC load should be considered when selecting the AC trip rating of the circuit breaker. Typical AC continuous current trip ratings range from 10 amps to 6000 amps.4.10.7 Selection of Circuit FusesAppropriate fuse selection is important for the protection and fault clearing coordination of the AC auxiliary power system. The important ratings to consider when properly selecting fuses for AC auxiliary power system protection are voltage rating and current rating.The AC voltage rating of the fuse should be equal to the operating voltage of the AC auxiliary power system. Typical AC fuse voltage ratings are 125, 250, 300, 480, and 600 volts.The AC current rating of a fuse is the maximum AC continuous current that an AC fuse will allow to flow through it. When this rating is exceeded, the fuse will blow, thus opening the circuit. The maximum AC continuous current required to supply an AC load should be considered when selecting the AC fuse rating. Typical AC continuous current fuse ratings range from 1 amp to 6000 amps.4.11 Equipment SpecificationsDocuments for specifying equipment include the necessary information for manufactures or suppliers to prepare and submit a firm proposal to furnish the requested equipment. The equipment specification is usually comprised of both commercial and technical requirements.The commercial requirements are typically a set of terms and conditions that address how, when and to whom the proposals are to be returned. Other information included may be legal considerations such as taxes or liabilities. Commercial requirements will not be discussed in further detail in this section.The technical requirements include the description of the necessary performance requirements for the equipment. The information in the description should include, as needed, the operational criteria of the equipment related to its design, construction, testing, and shipment.Subjects that need to be addressed when specifying aux power equipment include voltage/current levels, service conditions, code requirements/restrictions, delivery dates, delivery/transportation to site, and temporary storage of equipment.Designers should be aware that the standard equipment that is offered by suppliers may not meet the robust requirements needed for some substations. For instance, the size and layout of the substation may warrant that larger cables be used between equipment. These larger cable sizes will require larger cable bending space and termination sizes and hence bigger enclosure sizes.Numerous standards have been written to specify requirements of equipment to be used in AC auxiliary power systems. These standards cover transformers, surge arresters, transfer switches, panelboards, medium and low voltage fuses, medium and low voltage circuit breakers, etc.Some of these standards are:IEEE C57.12.00 - General Requirements for Liquid-Immersed Distribution, Power, and Regulating TransformersIEEE C62.22 - Guide for the Application of Metal-Oxide Arresters for Alternating-Current SystemsUL 1008 - Transfer Switch EquipmentNEMA PB-2 – Dead front Distribution SwitchboardsUL 248 - Low-Voltage Fuses - Parts 1 through 16: General RequirementsUL 489 (NEMA AB 1) - Molded Case Circuit Breakers, Molded Case Switches and Circuit Breaker EnclosuresUL 891 - Dead-Front SwitchboardsUL 991 - Safety Tests for Safety-Related Controls Employing Solid-State Devices4.11.1 NEMA standard for indoor/outdoor operationNEMA (National Electrical Manufacturers Association) creates ratings for equipment based on expected performance. NEMA does not require independent testing to ensure that the manufacturer is compliant to the standard. Compliance to the standard is up to the manufacturer.Standard NEMA 250-2008 describes types of enclosures for electrical equipment up to 1000 volts maximum. NEMA publishes descriptions of their enclosure types for both non- hazardous and hazardous locations. They also define which enclosure types may be used for indoor/outdoor use and which enclosure types may be used for indoor use only.The design engineer should choose the type of enclosure specific to environmental, atmospheric and site conditions. For example, a NEMA Type 1 enclosure provides a minimum degree of protection for indoor use in a non-hazardous location while a NEMA Type 3R enclosure provides a minimum degree of protection for outdoor use in a non- hazardous location. The degree of protection offered by these types of enclosures may be sufficient for a particular substation environment.4.12 Operation and Maintenance Considerations4.12.1 Isolation Switch Requirements4.12.2 Equipment Accessibility4.12.3 Standby Backup AC SystemThe purpose of the standby AC system would be to provide continued AC power to essential systems for a set period of time after all sources to the auxiliary power system are unavailable. The essential systems may be defined as the DC power systems that provide the power required for relaying, control, telemetry, and communications and any AC power needed for breaker operation.Factors that may determine the need for a standby backup AC system are the criticality of the substation, the battery life for the essential systems, and the reliability of the AC sources for the auxiliary system. If there is a possibility that an event can occur where the minimum time period to provide DC power will be exceeded, a standby backup AC system may be considered.The standby backup AC system should be a stand-alone unit that provides power without the support of the overall electric power system. A manual start for the system will be desirable considering that telemetry and communications functions may be disabled. Isolation of the sources to the aux power system is necessary before connecting the standby backup AC system to prevent inadvertent paralleling.The standby backup AC system used in substation normally consists of a diesel generator. The diesel generator is normally used in the substation for one of the following reasons:Used as back up to the primary source, when only one source is available and the substation requires two redundant AC sources.Used as the third source when two sources are available and the substation requires three AC sources.Under emergency condition when all the normal AC sources are not available, the stand by generator is used to restore the system.When the generator is used as a backup to one or more of normal AC system, the station load can be transferred to the generator automatically by the use of an automatic transfer switch or by manual transfer as required.4.12.3.1 Standby Generator for System CollapseIn anticipation of a possibility power system collapse, the station power sources will also be lost. In this case a stand by generator is installed at several predetermined substation will be used to re-energize the lost power system. The purpose of this generator is to provide the AC power to a pre-determined substation loads which is defined as essential loads. The essential loads are summarized below:Breaker Load - The breaker load consists of the charging motor inrush current normally added for the first breaker and the continuous motor current added for any additional breakers.Battery charger - One or two battery chargers as applicable must be included as part of the Generator load.RTU/Relay Load - Any RTU, Relays and control equipment load.Lighting load - Limited lightning both for the control building and the yard should be included in case the emergency condition occurs during night.Air Condition/ Heating Load - Limited air condition /heating load should be considered in case generator is required to run for several days during high or low temperature conditions.Stand by Voltage Rating - The generator voltage is selected based on the load voltage.5.0 Design of Substation DC System5.1 Design CriteriaPrior to the start of the DC System design the designer should consider several factors that are crucial to successful implementation. Typically in substation applications, the primary purpose of DC auxiliary systems is to provide a reliable power source for the power system protection. DC systems provide power to operate protective relays, monitoring equipment, and control circuits that operate power circuit breakers or other fault isolating equipment. The DC systems are designed to provide power for these protection systems during outages and when the power systems are intact. Several key factors are listed below. (Do we show a block diagram here? Given all the options of quantities and connections, what would we show?)5.1.1 ReliabilityThe reliability requirements of the power system are typically defined by the system protection design. For example the design requirements for transmission equipment is likely different than the requirements for distribution equipment. These designs determine the robustness requirements for the systems. System reliability standards should be reviewed to determine if back-up equipment or automatic switching is required in the event of one piece of equipment failing.5.1.2 RedundancyThe redundancy requirements of the power system can be viewed as components of the power system protection. Typical components include: AC current and voltage sources to relays, protective relaying, DC power supply, DC circuit protection, auxiliary relays (lockout relays), breaker trip coils, and control circuitry. This philosophy is illustrated in typical transmission protection systems compared to distribution protection systems. For example, transmission systems typically are designed with redundancy throughout the components listed above while distribution systems may only be designed with single components.The redundancy design can also be illustrated by examining what would happen if the battery (or DC source) failed during a fault condition on the power system. In a typical transmission system, the over-reaching distance elements of the line relaying on the remote ends would trip the breakers at the remote ends to isolate a fault located within the reach of the distance element (the reach to the remote terminal may be an overreaching zone that is time delayed). This provides some redundancy, however it would not provide complete redundancy for the line protection as only one end would trip for a fault beyond overreach of the line length. Thus, the batteries and relaying on the remote ends provide some redundancy for the battery (or DC source). Compared to the distribution system, unless there are ties to another substation, there typically is no redundancy for failure of the battery in the substation. The example above illustrates that it is important to design the DC system for the protection requirements. If the local substation has only a few transmission lines, the tripping of remote ends to isolate the fault during loss of DC source at the local substation may or may not result in a system stability condition. It is more likely to be a stability concern for large transmission substations. Thus, design of the DC system should coordinate with system protection and system stability requirements. In regard for the distribution system design, consideration for system back-up for failed equipment such as mobile substations or field ties to an alternate source provide a more economical or acceptable solution to system redundancy requirements. There may be regulatory requirements requiring redundancy.5.1.3 EnvironmentThe environment that a DC System is exposed to can impact the reliability of battery performance, the capacity of the battery, and the life of the battery. Key environmental components include: temperature variations, vibration, altitude, cleanliness, and ventilation. Some applications may be susceptible to seismic considerations.5.1.4 Design considerationsThe DC system design should be based on capacity and performance. It is of great importance that all applicable criteria are reviewed to insure that the most reliable, cost effective equipment has been selected for the life of the installation. Factors to consider:Load on the DC system when the maximum output of the battery charger is exceeded.Demand on the battery when the output of the charger is interrupted.Duration of the battery standby duration (e.g. 2, 4, 8, 12 hours), when auxiliary AC power is lost.Battery Life - What is the projected minimum life of the installation? Are battery life cycle costs to be factored in cost of operation?Cost/Reliability - What was the cost and quality of the battery initially selected? Does operational history align with published life/costs?Operating Temperatures - Will the battery be subjected to temperature extremes? When Ac is lost what is the expected minimum or maximum temperature the building the battery is in can be expected to reach and how long to reach it? Should the extreme temperature be averaged in selecting battery size?Maintenance Intervals - The overall reliability of the battery depends on proper maintenance.Location - Will the battery be located where required maintenance can be completed? Is the battery properly ventilated? Will any associated equipment be susceptible to damage from corrosive lead acid fumes?Vibration/Shock - Will the battery be located near rotating equipment? Lead-acid batteries easily shed their active materials from the surface of the plates, affecting battery life.Weight/Size - Physical size and weight can play a significant role in determining the type of battery to be selected. Is there enough room for the battery and rack in the proposed location? Can the location of the battery accept the floor loading? Can the battery cells be replaced with all adjacent equipment installed or are lifting measures required (E.g. a multi-cell jar can easily weigh over 50 kg)? Is adequate space allocated to get either a permanent or portable lifting device installed?Design Process - Does the design process account for verification of the DC system loads for all additions or changes?Changed state loads. - does the design need to account for loads that may change state. Examples are breaker spring charging motors that run on DC on loss of AC or SCADA computer monitor that is fed from an inverter source that fails to DC on loss of its normal AC service.Is emergency lighting required? If so can an alternate source be provided? Does the DC system have alternatives in the substation emergency power system?The design considerations need to accommodate both the owner’s requirements and those of any regulatory agency, Authority Having Jurisdiction (AHJ), or quasi-regulatory agency. Other considerations may include those of any insurer or transmission operator (e.g. black start plans). For example a black start or system restoration plan may require more than one attempt to close in a transmission path and re-establish a secure source of the station AC service. During these attempts beaker spring motors may have to charge on the station battery which may be overlooked in an existing load case and may need to be accounted for in a new design. 5.2 Typical Equipment Served by the DC SystemThe DC system in a substation serves many critical and non-critical functions and equipment. Some typical equipment served may include:Circuit breakersCircuit switchersMotor operatorsProtective relay systemsSCADAFire protection/detectionEmergency lightingSecurity systemsPumpsRadio or other communication systems.While most of the equipment is required to be operational at all times, some may be defined as non-critical and may be segregated to reduce loads in the event where the battery of the DC system is required to carry substation loads without the battery charger available. Consideration should be given to limit the amount of non-critical loads connected to the battery to provide reliability to the system protection and to limit the size of the battery. The equipment may require DC voltages at different values such as 125 VDC for circuit breaker controls and 12 VDC or 24 VDC for a radio communication system. The designer will have to determine the best method to supply the various voltages. It is not generally recommended to tap a larger voltage battery for lower voltages (i.e. 24 volt tap on a 125 VDC battery). If alternate voltages are required to be supplied from a single battery, DC-DC converters are typically utilized for smaller non-critical loads at a lower voltage or a second DC system dedicated to the communications equipment could be installed. It is not recommended to install many DC-DC converters to provide different voltages. Vendors should be consulted to determine if alternate power supplies can be used. 5.3 One-Line DiagramTo start the design process it is recommended the designer create a one-line diagram showing the battery (or batteries), charger (or chargers), DC panels and all connected loads. Consideration should also be given for future load growth. A review of the overall substation one-line may aid in determining future possible additions. 5.3.1 Number of Battery SystemsThe designer should evaluate the criticality of the substation facilities and owner’s preference or regulatory requirements. Protective relay systems are normally designed with two independent systems. The systems are inclusive from the DC feeds to independent trip coils in the circuit breakers. The designer should review whether separate battery systems and panels are required, a single battery system with independent DC panels or one battery system and panel. Independent systems may provide better opportunities for maintenance or replacement in the event of equipment failure or the need to upgrade in the future.The number of battery systems may depend on the voltage level of the equipment. For example, if a communication system requires 48 VDC and the substation equipment is 125 VDC, the designer needs to consider whether the communication equipment would be supplied by its own battery as noted in clause 5.2 and charger or be supplied by a DC-DC converter. The decision will have to consider reliability and control building space among other issues. The number of battery systems has a direct impact on the size of the control house as battery systems typically occupy wall space that can dictate building size. Depending on local requirements, additional preventative measures may be required with multiple battery systems.5.3.2 Load transferThe designer needs to account for any DC load transfer requirements. Load transfer could be automatic or manual and serves to back up one DC system in the event of a charger failure from another system or similar event. The need to transfer and the details of a transfer scheme can be dictated by owner’s preference or design criteria, criticality of the substation, or other similar reasons. All equipment that could serve additional load upon transfer must be sized appropriately for that additional load.There may be regulatory or quasi regulatory requirements that require the ability to transfer the DC load to ensure reliability of the protection systems for the electric transmission system. In North America, NERC and the regional transmission organizations have established requirements for DC system reliability. 5.4 DC Batteries5.4.1 Battery typesBattery types and their characteristics are discussed extensively in other IEEE guides. However a brief discussion of them will be held so the designer can make themselves aware of the issues. Common types of batteries used in substation applications include: Valve Regulated Lead Acid (VRLA), Vented Lead Acid (VLA), Nickel Cadmium (N-C). This may change with time due to continued development of new DC technologies. The most common battery types used in substation applications are flooded Lead-Acid batteries. Common flooded lead acid batteries are available with the following variations: Electrolite Lead Calcium Lead SeleniumLead-Antimony Plate DesignFlat PlateTubular PlatePlante’ PlateThe selection of which type of battery to use should be based on reliability and economic criteria. The designer through the use of the referenced IEEE guides, manufacturer’s specifications, and owner’s preference should familiarize themselves with the impact of each type of battery on the design of the overall DC system. Considerations for selecting different types of batteries should include, battery load requirements, environmental conditions exposed to the battery (temperature ranges, moisture), battery life, design, duty cycle, capacity and planned maintenance cycle.In substation applications, the battery is not exposed to many deep cycles so the ability to accommodate many cycles may not be as important compared to other factors such as battery life and maintenance. Typically the battery charger will support substation loads with the battery available to supply energy for short duration activities such as breaker trips and closes where the battery charger response time cannot support the transient.5.4.2 Criterion for Battery RatingThe referenced IEEE guides (IEEE- 485 Recommended Practice for Sizing lead-Acid Batteries for Stationary Applications) list the requirements a designer needs to consider for obtaining the battery rating. However to aid the designer some considerations will be repeated here. In addition, this guide places emphasis on substation specific application considerations. 5.4.2.1 Continuous LoadsFirst using the one-line or equivalent document the designer should review all the continuous loads such as protective relays, SCADA systems, emergency lighting, indicating lights, communication equipment (power line carrier, radio, telecom, microwave, fiber optic), security systems, fire protection, etc. Continuous loads can be obtained for new substations by reviewing vendor literature or calculations from previous designs. For upgrades at existing facilities, the data may need to be obtained by field testing as vendor data may not be readily available. When reviewing the literature the continuous loads should be evaluated at the final battery voltage (End of Discharge or Minimum Cell Voltage) selected (i.e. 105 volts). For example, if a device has a load of 125 watts one may be tempted to have the load at 1 Amp for a 125 VDC system. However, at final battery voltage of 105 volts the load would be 1.19 Amps.Care should be taken to tabulate all known loads. The designer should also review the design for “phantom” loads that may be added by personnel other than the substation designer. For example the control building may be designed by another person who includes a fire protection system to meet local codes and also add DC emergency lighting. Future additions should be considered in the battery design for at least half the battery design life to prevent battery from being replaced uneconomically. Substation designers should consider limiting loads connected to substation batteries used primarily for protection purposes to provide a more reliable source to the protective system. If emergency lighting is required with no other available source, timers could be installed to reduce the time the lights are connected to the battery. This will keep the continuous loads and the battery size to a reasonable limit. 5.4.2.2 Momentary LoadsMomentary Loads are those such as breaker open or close that occur at various times through the duty cycle (see IEEE-485). Many substation momentary loads such as breaker operations, lockout relays and communication system operations operate in times frames of several cycles and careful analysis using IEEE and the battery manufacturer may be required. For example an EHV system may detect a fault in ? cycle initiate communications for 1 cycle, operate protective devices in ? cycle and open the circuit breaker(s) in 2 cycles. The whole operation is over in less than 5 cycles from detection. Typical sizing per IEEE 485 looks at loads of 1 minute as the shortest period. After all momentary loads and initial battery size selected, it may be advisable to work with the battery vendor to ensure the selected battery can respond to the expected loads and duration of the load. From IEEE 485 – re-draw this or something similar?If a discrete load sequence can be determined, the peak one minute load can be determined more accurately than if the loads are just all summed. For example, if a substation bus trips on differential via a lockout relay (LOR) that trips three breakers with logic that opens a motor operated disconnect after the breakers open, the peak current would be either the LOR current, the sum of the three breaker trip coil currents or the MOD locked rotor current. The single max current (breaker trips or locked rotor of MOD) would be used as the peak one minute load. This reduces the likely hood of an overly conservative battery size. It requires careful examination of the trip sequence to understand the peak momentary loads. Newer computer analysis programs are available to assist in the analysis. As described in IEEE 485, all load cases should be carefully analyzed to ensure the proper case is identified. A traditional load case that may have been used over an eight hour period for example may not be applicable in a situation where the substation may be required to cycle multiple loads or an extended period in order to restore the system after a blackout. When sizing momentary loads for motor operated disconnects, the locked rotor value should be used for the DC load of the motor operator to accommodate for conditions that prevent the operation of the disconnect from opening or closing under typical force This includes iced switches or switches that have not been operated for a while the blade may be stuck in the switch jaw due to corrosion or other obstruction.Another important issue when determining the worse case momentary load is whether to consider a breaker-failure situation where a breaker fail relay can operate a group of devices around a failed breaker to isolate the fault. When utilized, breaker failure relaying is a form of the secondary power system protection that requires a second contingency to operate. If breaker failure protection is used, a second contingency to operate the breaker fail may provide the worst case tripping scenario and this contingency should be used to properly size the battery. In many cases, the breaker-fail operation may put a larger load on the battery and both loads may occur within a minute time frame, because the breaker fail would occur in a matter of cycles. This would exceed the more normal typical worse case. If the original trip included a motor operated device, it would still be operating when breaker fail occurred and thus should be included in both conditions prior and after the breaker fail operation to determine the worst case scenario.As mentioned above, restoration from “black-start” or system restoration scenario may need to be considered. During “black-start” or system restoration several trip and close cycles may be required to restore the transmission system after a collapse. It would not be uncommon for two or three attempts to be made to get the system to hold. As part of the “black start” or system restoration all the station breakers would be opened prior to closing in a selected transmission path. 5.4.2.3 Battery voltage and number of cellsThe normal DC operating voltages at most utilities are: 54 volts, for 48 volt nominal systems and 130 volts, for 125 volt nominal systems. The float voltages (voltage in the nominal charged condition) for an individual cell will vary from approximately 2.17 volts per cell to 2.25 volts per cell depending on the type of battery and number of cells. In most cases, these batteries are equalize charged (continuation of the regular charge at a higher voltage to bring the battery back to a fully recharged condition) at approximately 2.33 volts per cell.The number of cells connected in series is based on the required minimum and maximum voltages of the battery load. Typical lead calcium and lead selenium battery individual nominal cell voltage is approximately 2.25 volts per cell. These batteries require 23 or 24 cells for the 48 volt system and 58 or 60 cells for a 125 volt system.The maximum acceptable cell voltage is approximately 2.33 volts per cell. At this point excessive battery gassing (evolution of hydrogen and oxygen) occurs and the maximum voltage limit of the connected equipment is approached. The minimum voltage for these battery cells is typically 1.75 volts per cell, which is normally considered fully discharged. As the voltage falls to this level the ability of connected equipment to operate may become questionable. Typically, breaker trip coils will operate at half their rated voltage but other DC operated equipment may not function properly at or around 1.75 volts per cell. Make sure to check connected equipment ratings if there are any questions. For a 58 cell 125 volt system a minimum voltage of 1.81 volts per cell is used to maintain minimum terminal voltage of 105 VDC.The voltage of the battery is calculated by using the following formula:The number of cells and the end voltage of a battery system can be calculated using the following formulas:5.4.2.4 Duty Cycle The duty cycle of a battery is defined in IEEE485 as the loads a battery is expected to supply during specified time periods. The duration of the duty cycle and the specific loads on the battery during that time period determines the size of a battery based on IEEE485 battery sizing. An important consideration for determining the length of the duty cycle is the response time required to replace a failed battery charger. For example, a realistic sequence of events that would follow a battery charger failure may include the following:Charger fails and initiates an alarm to SCADADispatcher notices alarmDispatcher attempts to contact substation personnelSubstation personnel drives to substationSubstation personnel investigates alarmSubstation personnel determines that charger has failed and notifies dispatcher that a technician is needed to repair the chargerDispatcher attempts to contact technicianTechnician drives to substationTechnician attempts to repair chargerTechnician determines that the charger cannot be repaired and the substation supervisor is notifiedSubstation supervisor locates spare chargerSubstation supervisor attempts to contact additional substation personnelAdditional substation personnel report to service center to pick up vehicle and chargerAdditional substation personnel drives to substationCharger is replacedIt is not difficult to imagine this process taking longer than the 8 hour duration typically used in substations. Under certain circumstances (particularly during major storms where multiple outages are being worked at the same time) the acknowledgement of the initial alarm is likely delayed due to other priorities thus increasing the battery duty cycle duration. The availability of personnel to respond to an alarm may also increase the duration especially during weekends or holidays. The battery may function properly supporting continuous load during an extended time to replace the charger but may not fulfill its design basis if called upon. Remote devices may be needed to clear a fault having a greater impact.Another important impact is loss of AC to the control house (add reference to applicable AC sections). Similar to loss of the charger the battery will be called upon to support all station loads. However many control houses may not have been designed to limit temperature minimums or maximums without the heating or cooling systems available. The designer should review the building capability to ensure battery can meet its duty cycle. 5.5 Battery ChargersBattery chargers are discussed in detail in other IEEE guides. The battery charger is normally used to provide the continuous loads of the station and as a means to either maintain charge on the battery, recharge after an event, or to provide an equalize charge to bring the battery back into specification when specific voltages are outside manufacturer’s tolerances.There are four types of battery chargers commonly available as described in IEEE 1375. They are:Ferroresonant & controlled ferroresonantPhase Control SCRMagnetic Amplifier ChargersHigh frequency switch mode power supply (SMPS)Battery charger type depends largely on owner’s preference or design criteria.5.5.1 Battery Charger SizingBattery charger sizing is based on the amount of continuous load plus constant times the ratio of discharge divided by recharge time as seen below:A=L+1.1(CH) A = Charger capacity in AmperesL = Continuous Load in AmperesC = Discharge in Ampere-hoursH = Recharge time in hoursFor C the designer should use the actual discharge in Amp-hours if known from the sizing calculation (either manual or via computer program). If the total removed Amp-hours is not known from calculation, a very conservative method is to use the 8 hour Amphour rating of the battery. For H, the designer should consider the owner’s preference or design criteria. Typical times of 8-24 hours are used. While a faster recharge time may restore a fully discharged battery faster this may cause other problems. A faster recharge may lead to plate damage of the battery due to overheating or the charger being oversized for day to day operations. The designer needs to review the probability of a worst case event happening during recharge and use that to help determine battery size. For large charger sizes, the designer may consider installing two chargers operating in parallel. Since under normal operating conditions, the full capacity of the charger is not needed, it can allow for routine maintenance or even a single charger failure to occur without an effect on battery performance. The constant of 1.1 allows for some variance in loads from the original design and margin for change in performance.Sizing – The following formula is used to determine the required DC output of the battery charger. I= calculated battery charger output, DC ampsA= amp-hours to be replacedt= time in which the battery should be rechargede= charger efficiency factorIc= continuous DC load currentd = design margin factork= Altitude adjustment factorat 3,300 feet1.1 at 5,000 feet1.7 at 10,000 feet5.5.2 Battery Charger ConnectionsThe designer should review the owner’s preference or design criteria regarding the method of connecting the battery charger to the DC system. All connection methods have benefits and drawbacks. The charger can be connected at various points in the system including:Directly to the battery terminalsSource side of battery disconnect switch, if one existsLoad side of battery disconnect switch, if one existsDC panel main lugsDC panel branch circuitDC bus terminal block (Insert sketches of these connections – are there others?)If the charger is connected directly to the battery or on the source side of the disconnect switch, it could be considered a reliable method of charging the battery, since there are minimal points of failure in between the charger and battery. However, since the charger also serves to supply power to continuous loads under normal operation, a fault on the battery or removal of the battery for replacement (by opening the battery disconnect switch or disconnecting the main battery leads/cables) may disconnect the charger from the loads.If the charger is connected on the load side of the battery disconnect switch or at the DC panel, it will maintain connection to the continuous loads even in the event of a battery failure or replacement. However, if the charger gets disconnected from the battery due to an event at the DC panel, the battery loses its means to re-charge.5.5.3 Charger Circuit ProtectionAlthough the charger may be equipped with integral AC and DC circuit breakers or fuses, the designer may consider external protection as well. The AC feed breaker from the main AC source should be protected in accordance applicable local codes. The DC output may need to be connected with another overcurrent device to coordinate with the overall DC system. This overcurrent device could be either a fuse or circuit breaker depending in owner preference, local codes, or coordination needs. Both the AC and DC external protection should be used to protect the external circuit and cabling. The current limiting characteristics of the selected charger should be reviewed in accordance with IEEE 1375.5.6 DC PanelsThe DC panels are used to distribute power to various loads in a substation and can come in many varieties. Panels can come with overcurrent protection on the main feed or main lug only (where the main DC feed connects directly to the DC bus). Branch circuits can be protected by circuit breakers, fuses, fuses with knife blade isolation, or combinations of these such as a circuit breaker in the positive leg and knife switch isolation in the negative. The designer should review applicable local codes and owner’s preference as to what type should be used.5.6.1 Critical and Non-critical LoadsThe designer should review if there is separation required by local codes, owner’s preference or design criteria. This could be based on whether there is a need to separate loads as critical or non-critical. Critical loads are those that would be required to have DC power under unusual system conditions such as loss of power to the site, black start path, loss of the charger, etc. For example, it may be determined that during a black start condition, it may be beneficial to only have close DC and available and only DC for one trip circuit/protective relay system. Panels would then be segregated with a main DC breaker being tripped by a contact from some system input such as SCADA or the charger loss of AC. While non-critical loads would play a vital role under normal conditions, during severe events they may be overlooked until the system is stable.5.6.2 Circuit SizeThe designer should size the DC panel to accommodate the required number of circuits needed for existing load as well as planned load growth. Branch circuits should be sized in accordance with the NEC, local codes or owner’s design criteria as applicable. Branch circuits should coordinate with any downstream devices such as fuses or circuit breakers within the downstream device such as a transformer or circuit breaker. The circuit side should match with installed cable to prevent a miscoordination between sizes. Circuit size should also account for any voltage drop. Voltage drop includes the effects of current through all interconnecting cable to and from the remote device. This is different from AC sizing where the impedance only one way needs to be considered. The cable should be sized so the device can operate at minimum battery voltage (i.e 105 VDC on a 125 VDC battery) so that the minimum device voltage (90 VDC typical minimum pick-up) is available at the remote device. It may be prudent to build some conservatism in the design calculation to allow for variations in field conditions due to cable lengths, device tolerances, etc.5.7 Load Transfer MethodsIf it is determined that load transfer is required, the designer must ensure that the additional load is accounted for in calculations that size the battery, charger, cables, etc. that are part of the DC system that will accommodate the added load. The specific details and method of transfer must also be determined:5.7.1 Manual TransferManual transfer of DC load can be accomplished by methods including:Disconnect switch(es)Temporary cablesIn order to ensure that manual load transfer is accomplished in a safe manner, proper switching procedures, electrical isolation, physical locks and other methods can be utilized. The equipment (cable, switch, lugs, etc.) that actually transfers the load from one system to the other must be sized for the expected load to be transferred as well as future load growth. Some sort of picture?5.7.2 Automatic TransferAutomatic transfer could be accomplished via transfer switches similar to those used on AC systems. Some sort of picture?5.8 Design considerations5.8.1 Battery MonitoringThe battery and DC system has many options for monitoring. The battery charger itself may be equipped with monitoring functions such as loss of DC, low DC, battery grounds, and loss of charger AC. Some microprocessor based chargers have programmable flexibility to provide many other forms of battery monitoring such as battery temperature, impedance, and an on-line partial battery capacity test. Many microprocessor based relays have the option to monitor the DC source voltage to the relay and can provide additional alarm capability. An auxiliary relay may be used to monitor for systems where automatic monitoring may not be available. Through the use of communication links, continuous loads may be monitored from the charger directly to a SCADA RTU or other similar device. A DC shunt may be used to measure battery current directly and connect to a monitoring device. Reference IEEE 1491 Guide for Selection and Use of Battery Monitoring Equipment in Stationary Applications.5.8.2 Battery Installation5.8.2.1 Battery location5.8.2.1.1 Fire ConsiderationsWhile the battery is not normally a direct fire hazard, several conditions may present hazards. If the battery main terminals become shorted between the main terminals and there is no protection (fuse or circuit breaker) as allowed by IEEE-1375 for overcurrent, the short circuited battery would become a fire hazard. Thermal runaway conditions also present fire hazards. Another common hazard is the generation of hydrogen gas produced by lead-acid batteries (not applicable to VRLA or Ni-Cd batteries) that occurs during charging, especially when an equalizing charge is applied. Removal of any potential hydrogen build-up should be considered by the designer. This build-up may be removed through normally building exhaust or leakage, direct exhaust of the battery area or by inclusion of fresh air into the building ventilation system. IEEE-484 notes several other recommendations. The designer should be aware of any restrictions imposed by the AHJ in regards to battery ventilation. IEEE 979 provides guidance for fire protection in substation applications. IEEE 1375 provides some additional guidance as well on physical protection of batteries. Local codes or the owner’s preference should be reviewed as to whether the battery should be housed in its own room or enclosure. The battery charger also does not present any direct fire hazard. However they generate heat as part of the AC-DC conversion and care should be taken to restrict flammable material from being located above the vent openings.Working clearance meeting the requirements of the NESC Table 125-1 (or local codes) should be used to provide safe access to the equipment for workers and in the event of an emergency.5.8.2.1.2 Safety ConsiderationsAs discussed in section 5.8.2.1.1, working space meeting the requirements of NESC Table 125-1 or local codes should be maintained. In retrofit designs in older station, the designer should check clearances that may have been inadvertently compromised over the life of the substation or in replacing equipment that was installed prior to code applicability. Consideration should also be given to a method for removing battery cells in the future. Space for a lifting device or permanent device may be needed. Typical substation battery cells weigh 20-70 kg (44-154 pounds). Lifting cells of that weight can be very difficult for maintenance from upper steps or tiers of a battery rack. An eyewash station (or equivalent device) that conforms to local codes should be installed to support workers in the event of acid contact. Provisions should be made depending on owner’s requirement for storing the specific gravity tester and an acid resistant cloak. For building safety, acid resistant paint should be used in the battery area to prevent damage to walls and floor. Consideration should be given to using a spill containment system around the battery to absorb acid in the event of a catastrophic cell failure.5.8.2.1.3 Reliability ConsiderationsThe designer should review owner’s preference or local codes for separation of multiple battery systems. Physical separation or barriers may be required for multiple systems to ensure that in case of a catastrophic event (e.g. fire or short circuit) on one DC system, it does not readily propagate to the other DC system(s). This can include physical separation by air gap or installation of a barrier (a wall or locating batteries in separate rooms). As the battery system is crucial in allowing most substation equipment to successfully operate, care should be given to provide as much as reasonably possible protection to the battery system.Reliability is also dependent on battery area temperature. Battery area temperature should be monitored and kept constant, as discussed in clauses 5.8.2.1.5 and 5.8.2.3). Owner’s operating practice for response to building high or low temperature should be reviewed to determine effect on battery performance and reliability. Low or high temperatures outside the design of the battery load profile can affect reliability.Reliability of the DC system is also affected by placement of DC panels. Separation of batteries and chargers could be ineffective if the DC panels are side by side, as a single panel fire could take out both systems. Cable routing should also be reviewed. It is common to separate the cables of separate DC systems in order to prevent a single event from taking more than one system out of service.5.8.2.1.4 Battery Room Door RequirementsIf the battery is placed in its own room due to owner’s preference or local codes, the battery room door should have a fire rating equal to or exceeding fire rating of the walls. The battery room door should also incorporate all necessary signage to inform workers of potential hazards of the area, such as acid-containing, explosive mixtures, etc. as required by AHJ. Interior signage should ensure that personnel can identify and find the door. Depending on room design and local codes, the battery room door may also need to incorporate a blast louver to relive pressure in the event of a hydrogen build-up and explosion. The battery room door should have a panic par on the inside and open outward into the control room or outside to allow safe egress of personnel in the event of an emergency. Requirements for securing the door such as locks should be reviewed by the designer.5.8.2.1.5 Battery Area TemperatureAs discussed in IEEE-484 and IEEE-485 battery temperature plays a key role in battery performance. Battery specifications are generally published at 25°C (77°F) and temperatures that vary from this can affect performance. During the battery sizing calculation the designer should consider the minimum and maximum temperature that the battery area could reach. For example, in a cold weather climate in winter, the battery area could easily reach 13°C (55°F) during a loss of AC to the substation, depending on building insulation levels during the needed response time. Conversely, in a warm weather climate in summer, the same loss of AC could drive the battery area to over 40°C (104°F). Normal operating practices should also be reviewed to determine baseline conditions as part of the battery calculation. If the owner keeps the battery area at 18.3°C (65°F) during winter months as an energy conservation method, battery performance will be below published data and needs to be accounted for in the design calculation. Batteries that are installed outdoors or in small enclosures (that are not temperature controlled like a building) may be subject to large variations in temperature.5.8.2.2 Acid Spill ContainmentThe designer should review applicable local codes regarding acid containment. It is typical practice to install a spill containment system that contains the acid to an area immediately adjacent to the battery and neutralizes it for safe handling and disposal. Use of acid resistant paint on the floors and walls of the battery area is also recommended to minimize any damage to the building in the event of a spill. If permanent spill containment is not installed, the designer should review local codes or owners preference to determine if on-site temporary acid adsorbent material or temporary containment is required.The designer should review the footprint required for a containment system to ensure that adequate worker access is maintained. The designer should review any tripping hazard that may be presented by installation of a mechanical containment system.(Need to review NFPA-1 more, I think containment is now required. . I reviewed NFPA -1 the 2006 version and containment is required for greater than 208 liters (55 gal) which for most station batteries would qualify We may want the group to see whether they have determined if control buildings and battery areas are subject to NFPA -1 in the US). 5.8.2.3 Battery RacksWhen selecting a battery rack, there are several things that should be considered including temperature differences, weight of the battery, available space and maintenance requirements. Battery racks generally come in three types – step, tier, or stepped tier. A step rack is designed so the battery levels are “stepped" from one another (usually offset by the depth of a cell). A tiered rack has the levels of batteries on top of each other. A stepped tier is a combination of the two.For substation applications, steps and tiers are usually limited to two levels. Step racks generally have a larger footprint than an equivalent tiered rack and cells can be easier to access. Tiered racks tend to save floor space due to their smaller footprint. Other considerations with larger batteries include height and weight. The European din size requirements of some batteries requires the increase in battery size to increase the height rather than the width or length of the battery jar. This can create height issues for larger battery sizes. Weight can also be an issue especially for taller racks. The height variations between upper and lower levels of a battery rack are a concern. Height variations will cause differences in cell temperatures within the same battery system. Since temperature can have such a drastic effect on battery characteristics, interconnecting cells at different temperatures can lead to an early failure of the battery system. As a general rule, temperature gradients in excess of 3°C should be avoided.Battery racks should have an acid resistant coating applied to the structural frame to preserve its integrity. It may also be advantageous to have a liner on the support rails of polyethylene or similar material to further protect the rails from damage and provide electrical isolation. In areas of seismic concern, the battery rack should be specified as to its correct seismic zone so the correct bracing can be applied. A seismic rack has the same basic design as a non-seismic rack with additional bracing applied to hold the rack and cells in place. Cross bracing is used on the rack and an additional set of bracing is applied around the tops of the jars to prevent the jars from shifting in the seismic event.5.8.3 Circuit Considerations5.8.3.1 Grounded and Ungrounded SystemsSubstation batteries used for operation and control of interrupting devices and protection system, SCADA etc. are typically ungrounded. Communication systems such as those used by telecom companies are typically a positively grounded 24 or 48 VDC systems. The designer should be aware of the difference and not mix the two. Direct contact input to opposite systems should be avoided and use of interposing relays or devices should be used. Addition of unintentional grounds should be reviewed during the design and installation process.5.8.3.2 Isolation of Main DC CablesAs discussed further in section 5.8.3.3.2, the battery is the source of fault current for the DC system. The cables between the main battery terminals and the first overcurrent protection (breaker or fuse) are unprotected (unless using a mid-point fuse). Thus designs should place the main battery overcurrent protection as close to the main terminals as possible to reduce this exposure. A short to any portion of the battery main terminals can produce extreme heat and fire hazard for a shorted battery. Any damage to the cables from the battery can subject a worker to the full short circuit capability of the battery. The designer should review the owner’s preference to separate the positive and negative cables of the battery to reduce the possibility of a direct short circuit being applied to the battery. When separating the cables they should be placed in a non-magnetic conduit to prevent induced fields from causing other potential hazards. If the cables are not run in separate conduits, the designer should try and minimize the cable length to the DC load center to prevent risk of damage. With multiple battery systems, care should be taken to avoid routing main DC cables near one another to preserve independence and reliability.IEEE 1375, Guide for the Protection of Stationary Battery Systems, gives additional guidance on the methods of protecting the main DC feed to the load device from the battery. They include:Battery fuse (in both positive and negative leads for ungrounded systems)Battery circuit breaker (including both positive and negative leads for ungrounded systems)Battery disconnect switch (fused or non-fused) that allows the battery to be disconnected from the load circuits (note that for this application the battery charger must be connected between the switch and the load)Mid-span battery fuse which protects for external faults and limits output short circuit current to half of the entire battery ratingCable onlyIEEE 1375 gives a description of the advantages and disadvantages of each method.Do we include figures from IEEE 1375? They are helpful I think. Joe to provide re-drawn figures? 5.8.3.3 Circuit Protection and Coordination5.8.3.3.1 Coordination of Overcurrent ProtectionThe designer needs to review the coordination between all devices in the DC circuit in accordance with the NEC, local codes or owner’s design criteria. Overcurrent protection devices should be sized such that an upstream device does not trip for a downstream operation. For example, if a DC panel circuit feeds both a relay panel fuse and a circuit breaker trip coil, it would not be prudent to have the panel breaker or fuse operate if the relay panel fuse operates due to a protective relay power supply or circuit failure.5.8.3.3.2 Short Circuit LevelsSince the battery is the primary current source in case of short circuit, the battery data sheet or manufacturer should be consulted to determine available fault current. The interrupting devices in downstream circuits should be reviewed for their DC ratings. Many devices may appear to have sufficient interrupting capability but do not have the appropriate Asymmetrical Interruption Current (AIC). Without proper AIC a breaker may not interrupt the current. It may weld close or open without the ability to dissipate the energy. These conditions could result in damage to equipment, injury to personnel and/or other un-intended operations. Similar conditions apply to fuses used for interrupting faults.The designer should consider protection of the main DC feed by use of circuit breakers of fuses. Section 5.8.3.2 and IEEE 1375 give more guidance on protection of the battery main feed.5.8.3.3.3 Fuse and Circuit BreakersThe designer should consider local codes as well as owner's preference or design criteria when selecting circuit breakers or fuses. Fuses may have a lower initial installed cost but may require additional spare material to be stored on site to allow for replacement in the event of an operation. Fuses may also require a fuse monitor to be installed to detect and provide indication that they have operated. Circuit breakers may have a higher initial installed cost but they provide indication they have operated and usually do not require replacement after they have operated.5.8.4 Equipment rating5.8.4.1 Indoor and Outdoor Equipment RatingsThe DC equipment should be selected to be of the proper rating for their intended location. Outdoor rated equipment may be installed within indoor substation locations, but indoor rated equipment should not be installed outdoors. It may be advantageous to have some DC panels placed closer to the loads they support such as circuit breakers in a large transmission substation. In this application outdoor rated equipment may be required (do we reference the NEMA standards?)5.8.4.2 Equipment RatingAs discussed previously, the DC equipment should be rated for interruption of fault current. If a main breaker is used, it should be able to interrupt the maximum short circuit current available from the battery for the life of the battery. The designer should review interrupting capability during a battery replacement. Continuous current rating should match or exceed the current drawn by existing loads and allow for future growth. Voltage rating should match or exceed the maximum battery voltage (i.e. 250 VDC for a 125 VDC battery).5.9 Maintenance Provisions5.9.1 Isolation SwitchesThe designer should review local codes and owner’s preference or design criteria regarding the need to provide isolation switches for the battery and charger. Main isolation switches can allow a temporary battery to be installed during maintenance, upgrades or replacement. Since a substation may be in-service for more than the 20 year design life of a battery, it is reasonable to assume that the battery will be changed at least once during the life of the substation. Since it is usually not feasible to shut down an entire substation during a battery change out, providing an isolation switch where a temporary battery can be connected can be advantageous during upgrades or emergencies such as battery failure. Similar logic can be applied to chargers, though in case of a charger failure or replacement, it is usually easier to connect a charger temporarily than a battery.5.9.2 Equipment AccessibilityAs discussed previously, access per NESC table 125-1 or other local codes should be maintained. Table 125-1 provides minimum clearances, but owner’s preference and design criteria should be reviewed as well. Battery cells/jars can be weight enough that ordinary workers may not be able to lift it without mechanical assistance. Access room may need to be maintained for mechanical lifting devices to install or remove battery cells/jars. Safe working clearances between the battery and other equipment of 30 inches side to side and 36 inches in front or behind should be maintained. Overhead lifting devices may be needed from building supports to remove the cells as well.Battery charger weights may be such that provisions may need to be made for access for lifting devices to replace it as well. The charger may weigh over 100 kg (220 lbs.). The battery chargers also may generate a significant amount of heat and care should be taken to ensure that access may be made to service the equipment without risk of thermal damage.5.9.3 Back-up SuppliesThe designer should review owner’s preference for any back-ups. Based upon the importance of the substation, there may be a need for back-up equipment (either charger or battery bank). As discussed previously, if provisions are made during design then back-up supplies can easily be connected. If back-up supplies are required, the design needs to account for the time frame required to facilitate timely or permanent connection of any back-up supplies, including the location of back-up or temporary connections. Also, the designer needs to review if automatic actions are required to place any back-up supplies in service.DC Open items:Do we add figures from IEEE 1375Do we add a Clause to put it all together (Battery charger, panels) with some figures. We can steal from 1375 for some of it or re-create our own.Add Clause on various methods to connect charger to the battery along with appropriate figures. I have seen charger directly on batter, at the battery disconnect, at a junction box with a terminal block, thru a DC panel, etc.4. Resolution of non-answer from NESC ommitee on IEEE 1375 allowing unprotected cables. Talk to bruce Dietzman and Gary Engmann about it. It may be that the importance of the DC system may override section 161.c of NESC. Utilities that use this method may be in conflict with adoption of NESC by their local states or other municipalities.AC Section ReferencesDistribution Transformer Handbook, First Edition Transformer Connections, General Electric October 1951 ................
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