I - PRÉSENTATION GÉNÉRALE



SOURCE IMPEDANCES

OF THE CANADIAN DISTRIBUTION SYSTEMS

( Residential and Industrial )

Amadou Oury BA** , Roger BERGERON*/ ** & André LAPERRIÈRE***

Institut de Recherche d’Hydro-Québec

1800 Bd. Lionel Boulet J3X 1S1

Varennes (PQ), CANADA

(*) - Chairman of CSA WG 311.4 working group; (**) - Researcher IREQ / Hydro-Quebec; (***) - Researcher LTEE / Hydro-Quebec

ABSTRACT - Up to nowadays, one uses in Canada approximate values for the residential ( or industrial ) impedance while in Europe, the IEC 725 standard has already determined an accurate value for the european distribution system. This paper presents the assessment of the low-voltage power supply impedances of canadian distribution systems which are expected not to be exceeded in 95% of situations: theses values take into account all the residential ( or industrial ) distribution system components, from the utility source to the residential ( or industrial ) cable connected to the appliance ( or apparatus ) terminals. Measurements performed at 70 sites were used to validate the impedance values proposed by this report.

I - INTRODUCTION

Since the 80-thies, all three-phase 4-wires, residential distribution systems of european countries have been normalized by IEC as a 230/400V distribution system. In fact, the IEC-725 standard, which is at the beginning of this work, has adopted a unique value of a reference impedance for this normalized system. One knows the huge advantage of the unique impedance value choice, such as the existence of a one test procedure for all the european manufacturers and the easy fixation of european standards relative to the compatibility level and the emission limits,…,etc.

Because of the legendary difference between the european and north american types of distribution systems, the adopted values by the IEC-725, despite certain efforts of the IEC experts, are not applicable to the north american type of distribution system. For this reason, some canadian and american organisms are working to the determination of values that will be used as reference impedances for the north american distribution systems.

The purpose of this paper is to evaluate the impedance values of the canadian residential and industrial distribution systems. These calculated impedance values are then compared with those determined by experimental measurements.

II - GENERAL PRESENTATION

As the european distribution system, the north american distribution system begins from primary lines (medium voltage lines or MV-lines ) that originate at substations supplied from high voltage system ( HV-system ). Each of these MV feeders, constituting a three-phase system (4-wires), leaves the substation and supplies an ensemble of single phase transformers to which are connected many secondary circuits ( low voltage circuits or LV-circuits ). The nominal values of distribution system voltages fall generally between 4.16 and 34.5kV, with the particularly common range being around 25kV for the canadian one; whereas the distribution transformers and LV circuits are designed in order to maintain the voltage received by customers within standard tolerances over the full range of loading conditions. In contrary to the unique single phase 230V supply, the north american system provides the dual 120/240Varrangement ( see figures 1,4 ). For residential distribution system, probably the most commonly used distribution transformers are single-phase units rated in the range of 100kVA-25kV. The industrial distribution system is similar to the residential one, with the particularity of being composed of groups of 3 single-phase transformers the secondary windings of which provide 347V(L-N) and 600V(L-L) voltages ( see figures 1 and 4 ). Compared to the european type, the north american distribution system has in general longer MV-feeders and shorter LV-circuits, with the consequences that follow from the point of view of impedance values. Moreover the single-phase transformers feeding from 3 to 7 consumers, are larger than the european three-phase transformers, but these later have higher kVA.

Therefore, for the canadian distribution system, the equivalent impedance seen from the appliances (or apparatus ) electrical outlets ( or receptacles ) is, as shows the scheme of figure 1, composed of the sum of the following various impedances:

- the MV-line ( primary feeder ) impedance,[pic];

- the distribution transformer impedance ,[pic];

- the LV-line ( secondary line ) impedance, [pic];

- the service drop phase conductor impedance[pic]

( or the neutral [pic]);

- the inside house cable drop phase conductor [pic]

( or the neutral [pic]).

Figure 1 shows the scheme of an distribution system, from the HV/MV-distribution substation up to the inside residential ( or industrial ) cable drop.

Figure 2 represents the equivalent scheme of the distribution system presented by figure 1.

[pic]

Figure 1: Scheme of the residential distribution system

[pic]

Figure 2 : Equivalent scheme of the distribution system

III - BASIC THEORITICAL EQUATIONS

To evaluate the equivalent impedance of a distribution system such as the one presented by the figure1, one determines first of all the impedances, expressed in Ohm (or in p.u.), of each equivalent circuit components. Then, one computes the distribution system equivalent impedance. According to the unit in which the impedance is expressed, we distinguish 2 methods of impedance computation: the ohmic method and the p.u.method. In this work, if we choose the ohmic method then, the basic equations to be used are the following.

The short-circuit utility impedance, the resistance of which is negligible, can be found using the equation (1) [6,7].

[pic] (1)

[pic] - Secondary (LV) nominal voltage (L-L);

[pic] - Utility short-circuit kVA ( 3-ph. ).

To compute the primary feeder circuit reactance and resistance, we must refer them to the secondary voltage level using (2) and (3) [8].

[pic] (2)

[pic] (3)

[pic]- Primary (MV) circuit nominal voltage (L-L);

[pic]- Secondary (LV) circuit nominal voltage (L-L) .

As to the distribution transformer, its reactance and resistance are respectively determined according to equations (4) and (5) [7].

[pic] (4)

[pic] (5)

For the resistances and reactances of all other components which are at the secondary voltage level, one can directly use their values expressed in Ohm. For example, we will use for:

- the secondary line ( LV-line ): [pic]and [pic];

- the service drop conductor: [pic] and [pic].

If one considers the impedance per unit length ([pic]) of a conductor with ([pic]) length, its total impedance is found using equation (6).

[pic] (6)

As shows the scheme of figure 2, the equivalent impedances analytical expressions of the phase ([pic]) or the neutral ([pic]) conductors feeding a residential (or industrial ) load are respectively given by relations (7) and (8).

[pic] (7)

[pic] (8)

This way of equivalent impedance determination of the distribution system is similar to the short circuit analysis of the faulted power system, when the fault is on the appliance ( apparatus ) terminals.

IV - IMPEDANCE OF RESIDENTIAL SYSTEM

The main electrical parameters of the residential distribution system are presented in the Annex I. For a 50MVA utility short circuit power and a 50kVA 14.4kV-120/240V transformer, we determine the impedances of the distribution system components.

1 - The source impedance

The source impedance, which is the short circuit impedance, is determined according to the equation (1), taking into account the 50 MVA utility short circuit power ( see Annex I ).

2 - The distribution transformer impedance

The chosen distribution transformer is the following : 50

kVA, 14.4/25kV-120/240V. The value of its equivalent impedance is found using equations (4) and (5) (see Annex I ).

3 - The primary Line impedance

The primary feeder is a three-phase four-wire (or three-wire) common-neutral line. The usual operation voltage of the primary system coming from the distribution substation bus is equal to 25 kV(L-L). According to the reference [1], the conductor 477 MCM ACSR ( 242 MM2 ) is used for the overhead line, while the cable 750 MCM Al ( 380 MM2 ) is used for the underground line. In the Annex I, the primary line conductor characteristics are presented. Using relations (2) and (3), one can find the MV-line impedance for the 120/240V voltages ( see Annex I ).

4 - Secondary system Line impedance

As the case of the service drop conductor, the secondary system conductor is specified by the Hydro Quebec’s standards, according to the kVA of the used distribution transformer. For this reason, when one chooses a given conductor, one obtains at the same time its impedance per unit length and so the total line impedance with the use of relation (6). For the secondary system, the following conductor characteristics are chosen [1].

- mean length of secondary conductors : 100 m ;

- phase conductor: 2-350MCM AL,

with [pic];

- neutral conductor : 2-350 MCM AL,

with [pic].

So, the total value of the secondary line impedance ( for the phase ) is equal to: [pic].

As to the total impedance of the secondary line (neutral),

it is equal to : [pic].

5 - Service drop conductor impedance

Residential load is fed from the secondary distribution system via the service drop conductor which connects the secondary system to the residential electrical panelboard. The distribution system design standards fixe the characteristics of the secondary system conductor, according to the distribution transformer; as well as the service drop conductor according to the customer service entrance. For the service drop conductor, the following characteristics are chosen [1].

- mean length of the service drop : 200 m ;

- the phase conductor is a 350 MCM AL

(or 3/0AWG AL) with [pic];

- the neutral conductor is a 350 MCM AL

(or 3/0AWG AL) with [pic].

It comes that the values of the phase and the neutral (or return ) impedance of the service drop conductor are equal to: [pic]

6 - Impedance of the Inside house feeder

The impedance at the service entrance was defined in the above section 5. This is valid for assessing the effect of the current at the service entrance. However, before to be supplied, the customer use a no 14 gage cable up to the apparatus into consideration which add a more impedance. Since several equipment can be connected on the same outlet, this impedance value may be important for assessing the compatibility level and the emission limits.

The characteristics of the inside house cable are the following :

- mean length of the inside house cable: 70 m ;

- the phase conductor is a No. 4 Cu ,

with [pic];

- the neutral conductor is a No.4 Cu.

It comes that the values of the impedances (phase, neutral) are equal to: [pic].

7 - Impedance of Residential breaks.

The feeder cable can be cut in 24 sections before to supply the appliance ( or apparatus ). These breaks are needed to introduce up to 24 power outlets according to the radial distribution scheme. The voltage drop shall not exceed 5% from the supply side of the consumer’s service[3]. Since the break is quasi-resistive then, one can find the total value of the 48 outlets ( 24 breaks one way and 24 return way ). As a matter of fact, the equivalent resistance of the 24 outlets is the following:

[pic]

8 - Equivalent Impedance of the Residential distribution system

The impedances of the distribution system components are presented in Annex I ( see table I.1 ). In this table, are presented the equivalent impedance values ( for the phase and neutral/return conductors) of the 120/240V residential distribution system; these impedance values are seen from the electrical outlets.

In order to find values for comparison, measurements [5] have been performed at 70 houses in Quebec by the Laboratoire des Technologies Électrochimiques et des Électrotechnologiques d’Hydro-Québec ( or LTEE ). We present, in table 1, some measurement results from residential distribution systems, the distribution transformer of which is an 50 kVA. Theses performed impedance measurements took into account 2 parameters: the voltage drop conductor length between the transformer and the house, as well as the transformer capacity.

Tableau 1: Experimental impedance values of the

canadian residential distribution system

| Experimental impedance values |

|of the canadian Residential system |

| | |

|Length |Impedance |

|( [pic]) |([pic] ) |

| 31 | 42 |

| 38 | 51 |

| 46 | 57 |

| 62 | 76.5 |

| 77 | 88 |

| 92 | 148 |

| 154 | 193 |

| 169 | 195 |

The table 2 presents the impedances of the residential distribution system used by european utilities [4].

Tableau 2: European distribution system impedances

| |

|Voltage 230/400V , 50 Hz |

| | |

|Phase |Neutral |

| | | | |

|R([pic]) |X([pic]) |R([pic]) |X([pic]) |

| | | | |

|0.24 |0.15 |0.16 |0.1 |

IV - INDUSTRIAL SYSTEM IMPEDANCE

The main electrical parameters of the industrial distribution system are presented in Annex II. For a 50 MVA utility short circuit power and a 1000 kVA 14.4/25kV-347/600V distribution transformer, we have determined the impedances of the industrial distribution system components. We have used the same procedure as that for the determination of the residential distribution system impedances .

The impedances of these various components are calculated and listed in Annex II ( table II.1). The last row contains the equivalent impedance values ( for the phase and neutral conductors ) of the 347/600V industrial distribution system.

Tableau 3: Theoretical impedance values of the canadian

residential and industrial distribution systems

| |

|Source Impedances ( seen from electrical panel ) |

| distribution | Phase | Neutral |

|system | |( or return phase ) |

| Residential | | |

|120V(L-N) |0.0997 + j 0.0356 |0.0974 + j 0.027 |

| Residential | | |

|208V(L-L) |0.0997 + j 0.0356 |0.0997 + j 0.0356 |

| Residential | | |

|240V(L-L) |0.1035 + j 0.0536 |0.0974 + j 0.027 |

| Industrial | | |

|347V(L-N) |0.285 + j 0.0675 |0.2974 + j 0.04 |

| Industrial | | |

|600V(L-L) |0.285 + j 0.0674 |0.285 + j 0.0674 |

[pic]

Figure 3: European distribution system

[pic]

Figure 4: North american distribution system

VI - PRÉSENTATION OF RESULTS

After the computation performed by using formula of equations ( 1 to 8 ), we have obtained the residential and industrial impedance values, seen from the electrical supply outlets (see Annex I and II). However, we will only present impedances seen from electrical panel, that is the equivalent impedance of the system beginning from the substation up to service drop. Table 3, being the resume of tables of Annex I and II, gives the different impedance values of the distribution systems seen from the electrical panelboard.

In the case of the 120V residential distribution system, the impedance value, seen from the electrical panel, is equal to [pic] for the phase conductor and

equal to [pic] for the neutral conductor. The equivalent impedance values of the 240V residential distribution system are equal to [pic]and [pic], respectively for the phase and neutral conductors ( see table 3 ).

As to the 347V(L-N) industrial distribution system, we have found impedance values of [pic] and [pic], respectively for the phase and the neutral conductors; while we have obtained for the 600V(L-L) distribution system an equivalent impedance equal to [pic]( see table 3 ).

One notes that the computed impedance values of the 120/240V residential system are in the same order as the experimental impedance values measured by the LTEE (see Table 1). Indeed, for a service drop conductor length (between the transformer and the house ) equal to 92m, the measured impedance value is [pic]; this value is close to the theoretical impedance of the residential distribution system.

In the opposite, the reference impedances of the 120/240V canadian distribution systems are lower than the 230/400Veuropean systems one ( see table 2 ). For example, the [pic] impedance value of the european distribution system has to be compared with the [pic] canadian residential distribution system: we note that reactance of the canadian residential distribution system is 4 times lower than the reactance of the european distribution system. This difference can be explained essentially , as we already mentioned, by the fact that the european distribution system is longer than the canadian one.

V - CONCLUSION

The purpose of this work is to evaluate the acceptable values which can be used as standard for the equivalent impedances of the residential and industrial distribution systems in Canada. The so called ohmic method, well known in the analysis of the faulted systems, has been used for the calculation. Table 4 represents the reference impedances accepted for the canadian distribution systems.

We have found for example that, in the case of the 120/240V residential distribution system, the global phase to neutral impedance is equal to[pic]. As to the 347/400V industrial distribution system, the global phase to neutral impedance is equal to [pic].

These theoretical impedance values of residential distribution system are similar to the measurement results performed by the LTEE. However, one notes that the computed impedance of the canadian residential lines is smaller then the european distribution system : it is probably due to the fact that the european distribution system is 3 to 4 times longer than the north american one.

Tableau 4: Reference impedances of the canadian

distribution systems

| |

|Impedances of Residential and Industrial systems |

|( seen from electrical panelboard ) |

| 120V (L-N) | phase conductor … | 0.0997 + j0.0356 |

|Residential |neutral conductor … |0.0974 + j0.027 |

|distribution | | |

|System |Impedance |0.19 + j 0.062 |

| |phase to neutral | |

| 208V (L-N) | phase conductor … | 0.0997 + j0.0356 |

|Residential |phase conductor … |0.0997 + j0.0356 |

|distribution | | |

|System |Impedance |0.20 + j 0.071 |

| |phase to phase | |

| 240V (L-L) | phase conductor … | 0.1035 + j0.0536 |

|Residential |return conductor … |0.0974 + j0.027 |

|distribution | | |

|System |Impedance |0.20 + j 0.080 |

| |phase to return | |

| 347V (L-N) | phase conductor … | 0.285 + j0.0675 |

|Industrial |neutral conductor … |0.2974 + j0.04 |

|distribution | | |

|System |Impedance |0.58 + j 0.107 |

| |phase to neutral | |

| 600V (L-L) | phase conductor … | 0.285 + j0.0674 |

|Industrial |phase conductor … |0.285 + j0.0674 |

|distribution | | |

|System |Impedance |0.57 + j0.135 |

| |phase to phase | |

VI - REFERENCES

[1] Norme de distribution. Construction du réseau aérien (B.41.11), Hydro-Québec, Mars 1994.

[2] Norme de distribution. Construction du réseau souterrain (B.41.21), Hydro-Québec, Mars 1994.

[3] Canadian Electrical Code, Part I, CSA Standard C22.1- 1982, Jan. 1982.

[4] Considérations sur les impédances de références à utiliser pour la détermination des caractéristiques de perturbation des appareils électrodomestiques et les équipements analogues, Norme CEI-725, 1981.

[5] J-P. Martel & A.Laperrière, Mesure et validation du domomètre en situation réelle, LTEE 94-069 HQ, Shawinigan Août 1994.

[6] Engineering dependable protection for an distribution system; Part I : a simple approach to short-circuit calculations, A.I.A, File No.31d6., 1968.

[7] Gönen T, Electric power distribution system engineering, Mc Graw-Hill Book Company, New York 1986.

[8] Gönen T., Electric power transmission system engineering, John Wiley & sons, New York 1986.

[9] J. Carr, North American and european distribution systems compared, Power Technology International, Spring 1996.

ANNEX I

I.1 - Electrical parameters of the residential system

Source : 50 MVA

Primary ( MV )feeder [1] :

- length : 10 km

- phase conductor : 477 MCM, ACSR

([pic] ) .

Transformer: 50 kVA , 14.4/25kV-120/240V

( R= 0.52%; X=2.15% )

or ( R = 0.78 % ; X=2.58 % ) pour 120V [6,7].

Secondary (LV) circuit [1]:

- length : 100 m

- phase conductor: 2-350 MCM Al

([pic]);

- neutral conductor : 2-350 MCM Al

Service drop conductor [1] :

- length : 200 m

- phase conductor :3/0 AWG

([pic] );

- neutral conductor :3/0 AWG Al

Inside house conductor [7] :

- length : 70 m

- phase conductor: no. 14 Cu

([pic] );

- neutral conductor : no. 14 Cu

Electrical breaks :

[pic]

I.2 - Equivalent Impedance of the Residential distribution system

Table I.1: Equivalent Impedance of the canadian Residential distribution system

| |

|Source: SCC = 50 000kVA; XFO : SXFO = 50kVA, 14.4/25kV - 120/240V ; 60 Hz |

| | | |

| |Voltage 120 V |Voltage 240 V |

| | Phase | Neutral | Phase | return Phase |

| | R ([pic]) | X ([pic]) | R ([pic]) | X ([pic]) | R ([pic]) | X ([pic]) | R ([pic]) | X ([pic]) |

| Utility | | 0.0009 | | | | 0.0012 | | |

|Source | | | | | | | | |

| Primary | 0.0001 | 0.0003 | | | 0.0001 | 0.0004 | | |

|System | | | | | | | | |

|Distribut. | 0.0022 | 0.0074 | | | 0.006 | 0.025 | | |

|XFO | | | | | | | | |

| Second. | 0.01045 | 0.0052 | 0.01045 | 0.0052 | 0.01045 | 0.0052 | 0.01045 | 0.0052 |

|System | | | | | | | | |

| Service | 0.0869 | 0.0218 | 0.0869 | 0.0218 | 0.0869 | 0.0218 | 0.0869 | 0.0218 |

|Drop | | | | | | | | |

|Residen. | 0.0997 | 0.0356 | 0.0974 | 0.027 | 0.1035 | 0.0536 | 0.0974 | 0.027 |

|panel | | | | | | | | |

| Z ([pic]) | | | | |

|Res. panel |0.0997 + j0.0356 |0.0974 + j0.027 |0.1035 + j0.0536 |0.0974 + j0.027 |

|Int. house | 0.1125 | 0.0076 | 0.1125 | 0.0076 | 0.1125 | 0.0076 | 0.1125 | 0.0076 |

|Cable | | | | | | | | |

| Break | 0.04 | | 0.04 | | 0.04 | | 0.04 | |

|Contact | | | | | | | | |

| Total | 0.2522 | 0.0432 | 0.2449 | 0.0346 | 0.256 | 0.0612 | 0.2449 | 0.0346 |

|Resident. | | | | | | | | |

|Z ([pic]) | | | | |

|Resident. |0.252 + j0.043 |0.245 + j0.0346 |0.256 + j0.0612 |0.245 + j0.0346 |

ANNEXE II

II .1 - Electrical parameters of the Industrial system

Source : 50 MVA

Primary ( MV ) feeder [1]

- length: 10 km

- phase conductor : 477 MCM ACSR

([pic] ).

Transformer: 1000 kVA, 14.4/25kV-347/600V

([pic]; [pic])

Secondary (LV) circuit [1]:

- length :150 m

- phase conductor : 3-750 MCM Al

([pic]);

- neutral conductor : 350 MCM Al

([pic])

Service drop conductor [1] :

- length : 100 m

- phase conductor: 3-no8 AWG

([pic]) ;

- neutral conductor : 3-no8 AWG

([pic])

Inside house conductor [7] :

- length : 120 m

- phase ( and neutral ) conductor : 3/0 Al

([pic]) ;

Electrical breaks :

- for industrial (motor ) [pic]

II.2 - Equivalent Impedance of the Industrial distribution system

Table II.2: Equivalent Impedance of the canadian Industrial distribution system

| |

|Source: SCC = 50 000kVA; XFO : SXFO = 1000kVA, 14.4/25kV - 347/600V ; 60 Hz |

| | | |

| |Voltage 347 V |Voltage 600V |

| | Phase | Neutral | Phase |

| | | | | | | |

| Utility | | 0.0072 | | | | 0.0072 |

|Source | | | | | | |

| Primary | 0.0007 | 0.0023 | | | 0.0007 | 0.0022 |

|System | | | | | | |

|Distribut. | 0.0032 | 0.0184 | | | 0.0032 | 0.0184 |

|XFO | | | | | | |

| Second. | 0.0149 | 0.0148 | 0.0313 | 0.0156 | 0.0149 | 0.0148 |

|System | | | | | | |

| Service | 0.2661 | 0.0248 | 0.2661 | 0.0248 | 0.2661 | 0.0248 |

|Drop | | | | | | |

|Industrial | 0.2849 | 0.0675 | 0.2974 | 0.0404 | 0.2849 | 0.0674 |

|panel | | | | | | |

| Z ([pic]) | | | |

|Ind. panel |0.2849 + j0.0675 |0.2974 + j0.0404 |0.2849 + j 0.0674 |

|Int. house | 0.0522 | 0.0131 | 0.0522 | 0.0131 | 0.0522 | 0.0131 |

|Cable | | | | | | |

| Break | 0.024 | | | | 0.024 | |

|Contact | | | | | | |

| Total | 0.3611 | 0.0806 | 0.3496 | 0.0535 | 0.3611 | 0.0805 |

|Industrial | | | | | | |

|Z ([pic]) | | | |

|Industrial |0.3611 + j0.0806 |0.3496 + j0.0535 |0.3611 + j 0.0805 |

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