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Bi-directional converter – A Nanogrid

1Manjula. J, 2T.B.Dayananda

Department of Electrical & Electronics, Visvesvaraya Technological University, 1IV Sem, M.Tech, ,

2Associate Professor, Department of Electrical and Electronics,

Dr. Ambedkar Institute of Technology, Bangalore-560056

Email: 1 team1projects@

Abstract— Due to increase in population, there is increase in power demand in our day to day life, so there is a need for development of an alternative solution which reduces the dependency on the grid. Since fossil fuels are exhaustible, we can consider renewable energy as an alternative source of energy. Whenever there is an outage or black-out we need to have alternate source as dc-nanogrid which then supplies energy to the residential applications and also it can be used to inject back the energy to the grid itself.

In this paper, a nanogrid which consists of a bi-directional converter used for power conversion is analysed.

I. INTRODUCTION

In today’s world “global warming” is posing a serious threat to mankind. As we all know fossil fuels are subjected to exhaustion after a certain period of time. Down the line in another 50 years we will not be left with these fossil fuels. Hence nature is alarming us to take a call to preserve them and also at the same time to make use of other alternative sources of energy such as wind energy, biomass, solar etc. By this way greenhouse emissions can be reduced to a greater extent.

Renewable energy sources such as solar, wind, biomass, hydropower, and geothermal can provide sustainable energy services, based on the use of routinely available, indigenous resources.[1] Due to cost fluctuations of oil & gas, the demand for renewables-based energy systems is increasing. Fossil fuels are non-renewable resources of energy. The oil, natural gas and coal we use today are gone forever. However, biomass fuels are renewable because the growth of new plants and trees replenishes the supply.

The term “nanogrid” (fig 1) may be defined as a small-scale generators and loads upto 20KW in size, and loads are located within short radius of the sources. The available renewable energy sources can also be directly interfaced to the nanogrid at higher reliability. In rectification mode, a buck converter steps down the level of voltage and is stored in the battery whereas in regenerative mode, a boost converter comes into scenario to supply back the power to the grid. The charging and discharging action of the battery action is taken care of by a bi-directional converter. The centralized power generation model is more economical and also more reliable source of energy production.

An electric power system (EPS) is the greatest and most complex machine ever built. It is a network of electrical components which consists of wires, cables, transformers, towers, circuit breakers all bolted together in some fashion. It is used to supply, transmit and use electric power, which can be divided into the categories of power generation, power transmission, power distribution, and power consumption. The EPS is a mechanical system with only modest use of sensors, minimal electronic communication and almost no electric control. The traditional model of large base-load AC centralized electrical power generation and its distribution via long transmission lines causes huge losses of energy and costs required to operate such systems.

Many steps have been taken to mitigate the potential blackouts, particularly to give way for new technologies which makes EPS more reliable and also can sustain high economy which is based on power sensitive equipment.

Battery-based hybrid and electrical vehicles and solid-state based lighting are transforming the transportation and lighting industries, both of which are powered by electric current. A DC nanogrid is the key enabler of the “zero energy module” with a very minimum wastage in transmission and conversion. So, it can be used to treat 100% energy needs of a building.

II. DG SYSTEMS

Distributed Generation (DG) is a type of electrical generator or static inverter producing alternating current that (a) has the capability of parallel operation with the utility distribution system, or (b) is designed to operate separately from the utility system and can feed a load that can also be fed by the utility electrical system. This is sometimes referred to simply as “generator”.

Distributed Generation is a back-up electric power generating unit that is used in many industrial facilities, hospitals, campuses, commercial buildings & departmental stores. These back-up units are primarily used by customers to provide emergency power when grid-connected power is unavailable and they are installed within the consumer premises where the electric demand is needed. The transportation cost is reduced since it is installed very close to the demand centre; hence there are no associated transmission losses.

Distributed generators include induction and synchronous electrical generators as well as any type of electrical inverter capable of producing A/C power. An Emergency or Standby Generation System is designed so as to never electrically interconnect or operate in parallel with the utility system. An Interconnected Generation System is any generator or generation system that can parallel (or has the potential to be paralleled via design or normal operator control), either momentarily or on a continuous basis, with the utility system.

Distributed Generation can be easily interfaced with non-generating technologies such as power storage devices like batteries and flywheels in addition to generators, while DG is limited to small scale (less than 20 MW) electrical generation located close to point of use. Unlike central power plant generation, DG often utilizes the waste heat from the generation process as an additional form of energy for space or process heating, dehumidification, or for cooling through absorption refrigeration.

Distributed generation, which can be referred as small-scale electricity generation, is a fairly new concept in the economics literature about electricity markets, but the idea behind it is not new at all. In the early days of electricity generation, distributed generation was the rule, not the exception. The first power plants only supplied electricity to customers in the close neighbourhood of the generation plant. The very first grids were DC based, and therefore, the supply voltage was limited, as was the distance that could be used between generator and consumer. Balancing demand and supply was partially done using local storage, i.e. batteries, which could be directly coupled to the DC grid. Along with small-scale generation, local storage is also returning to the scene.

When compared with AC transmission, DC transmission is far more efficient with improved controllability and reduced cost over long distances. These days, HVDC technology is used because of fast power flow control and also for reactive power compensation. Also, DC is more stable when compared to AC and also can be easily integrated with energy storage devices.[7]

A typical DG conversion system is made of two main energy converting stages. The first stage is the prime fuel converting block wherein prime fuel is converted into mechanical energy and in the second stage mechanical energy is converted into electrical power using an induction generator or synchronous alternator. Fuel cells and PV converter are very good examples of DG systems.

The uses of DG are:

➢ Future demand can be handled with ease without any investment in expanding the existing system.

➢ This reduces right of way costs.

➢ Reduces fossil fuel consumption.

➢ Reduces peak supply burdens of the utility grid.

➢ Improves power quality.

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Fig 1. Nanogrid structure of home

(source:)

III LITERATURE SURVEY

In recent years, due to the growing concern over energy shortage and environmental pollution, the concepts of distributed generation (DG) systems, smart grid systems, and dc-based hybrid power systems have become progressively more popular, especially with the decreasing costs of various clean renewable energy sources like wind, bio-mass, solar, and fuel-cell systems. These DG systems would be connected to the utility grid under normal operating conditions; but additionally, these systems have the capability to sustain a local system by sourcing power directly from the renewable energy sources and energy storage devices if necessary.[5] The frequency-response based design procedure for the proposed control system is presented in detail for all the converter operating modes.

Autonomous load shedding can be implemented in a nanogrid that uses DC bus signalling for source scheduling by shedding loads when the dc bus voltage decreases to a level that signals an overload condition. This paper explains the control requirements for the system interface converters that is required to permit load shedding and explains a procedure for implementing a prioritized load shedding scheme in a practical system.[7]

In this paper DC bus signaling acts as a means of generator scheduling and power sharing in a nanogrid under steady-state conditions. [20] DC bus signaling is a novel control strategy that is a hybrid of the voltage level signaling and voltage droop schemes. Discrete voltage levels on the nanogrid bus indicate the state of the system and determine the behaviour of each source.

In dc-microgrid applications, a power distribution system needs a bi-directional inverter to control the power flow between dc bus and ac grid, and to regulate the dc bus to a certain range of voltages, in which dc load may change abruptly. This will result in high dc-bus voltage variation. In this paper, we take into account this variation and propose an on-line regulation mechanism according to the inductor current levels to balance power flow and enhance the dynamic performance. [21] Additionally, for power compensation and islanding protection, the bi-directional inverter can shift its current commands according to the specified power factor at ac grid side.

A dc nanogrid - hypothetical future sustainable home electronic power distribution system with dc-based distribution and the presence of renewable energy sources is dealt in this paper.. Description of the nanogrid and the testbed for its system-level operation analysis is presented in this paper together with initial experimental results and conclusions.[32]

All the sub-systems like micro, mini & nano grids are interconnected with the help of energy control centres which acts as energy routers. Smart metering, on-line communication, data collection & evaluation, sensing faults, regulation and bi-directional power flow operation are the pre-requisites of an energy control centre.

A Smart grid is the use of sensors, communications, computational ability and control in some form to enhance the overall functionality of the EPS. This system becomes smart by sensing, communicating, applying intelligence, exercising control and through feedback, continually adjusting. Optimization is achieved this implementation which also ensures high reliability by mitigating environmental impact, manage assets and also the cost of installation & maintenance.

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Fig 2. Smart Grid

(souce:)

As in fig 1, a 380v dc bus is connected to many renewable energy sources, storage devices & loads via power converters. Nowadays, 380v is used as a standard voltage in dc data centres and 48v is used for consumer devices and is accessible by 48v batteries. This system is referred as electronic-based dc nanogrid. This is a zero net energy consumption module. In this voltage is regulated by connecting battery directly to the dc bus.

The operating voltage of DC bus is chosen between 360V to 400V easily so that power sharing can be carried out with voltage regulation. [7]The static V-I graph of ECC is shown in fig 3. ECC takes energy from the grid in the first quadrant hence output current Ig is positive whereas in the second quadrant it is sourced back to the grid. The converter can reprogram its V-I characteristic in the second quadrant to limit the current value which corresponds to demanded power on the request of operator. A droop control scheme is used to detect changes in the system and to adjust the operating points accordingly.

IV PROPOSED SYSTEM

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Fig 3. Single phase-full bridge converter

This topology is very widely used to transfer the energy between ac grid to dc storage system. This system is connected to a buck converter so that the voltage level is stepped down before storing it to the battery. Here the circuit operates in two-modes: one in rectification mode and the other in re-generative mode. In rectification mode, the ac- is converted into dc and is used to charge a dc-link capacitor, which is stepped down and stored in battery. Like-wise during re-generation mode, the d from the battery is stepped up to much higher level and is capacitor is charged and is inverted with the help of bi-directional converter so that dc is converted to ac. With the help of filter present before the grid, unwanted ripple is removed. A controller is developed to control the firing angle of the IGBT’s used in the circuit.

An additional requirement of the inverter is to have the capability of preventing the DG from islanding. Islanding is a condition which occurs when a generator or an inverter and a portion of the grid system separate from the remainder of large distribution system and continues to operate in an energized state This poses a potential threat to other equipments which is in the vicinity and hence it should be taken care of well in advance that is during planning itself.[15-17]

The high-frequency switching of the current and voltage in power electronics can generate fast di/dt and dv/dt noises referred as EMI noises. This electromagnetic interference can harm the neighbor component and also deteriorates the system operation. By proper grounding techniques and by the use of EMI filter the EMI can be reduced to a greater extent.

V CONCLUSION

In the field of power electronics, this bidirectional converter gives fast dynamic response and also faults are regulated easily which can be practically implemented in our day to day life. This can be used to replace existing electromechanical devices with digital control system. A nanogrid can be used as an alternate source of energy at home. It reduces the burden on utility grid and when the battery is full can also be used to share the load along with the grid. This is also very economical and is an efficient way to manage power. Renewable energy sources can be directly connected to the grid which further reduces the cost.

VI FUTURE WORK

Since large number of power converters are injected to the grids, it causes instability in the grid. Therefore proper care should be taken to synchronize it. The ability of the converter can be increased with soft-switching and interleaving converter technologies. All the EMI aspects need to be taken care of at the design level itself which reduces the total cost of nanogrid. Adaptive voltage control can be used to optimize the system’s performance. DC stability study, ground-fault & short-circuit protections need to be considered for the converter implementations.

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