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INTRODUCTION
Due to the increasingly prominent energy problem and environmental pollution, electric vehicles (EVs) are gradually replacing traditional automobiles equipped with internal combustion engines. Meanwhile, the continuous development of outstanding performance batteries and high-efficiency motors also has spurred dramatic interest in EVs, which are regarded as representatives of new energy vehicles. In addition, with the emergence and development of the concept of smart grid, the reliable, economic, efficient and clean performance of smart grid and its user-friendly interaction will give EVs brighter prospects and a new round of improvements. Therefore, V2G technology, the product of interaction between the smart grid and EVs, will become more and more attractive. According to the V2G concept, EVs are not only traditional power consumers, but also mobile distributed generators (DGs) returning the extra power to the grid when they are idle, which can improve energy management of the grid. Considering the large scale applications of EVs and the advantages of V2G system, such as fast response and no additional cost, EVs are the promising devices for reactive power local compensation.
The increasingly serious energy shortage and environmental pollution issues have drawn the attention of relevant authorities to the importance of energy conservation and emission reduction. “V2G” is used generically for both V2G and G2V energy flows: energy stored in a battery returns to the grid to support it at peaks of consumer energy demands, and the energy is transferred from the grid to vehicles via batteries during off-peak periods. Therefore, the V2G concept improves the performance of the power grid in terms of efficiency, stability and reliability. In the near future, V2G will become an indispensable part of the electric vehicle industry.
CONCEPT OF INTEGRATION OF EV WITH GRID:
BEVs incorporated with bidirectional power flow capable converter when plugged in, the power can flowboth ways:when electric power stored in electric vehicles flows to power grid, it is called vehicle-to-grid, the opposite flow of electric power, which means charging batteries in EVs, is referred to as grid-to- vehicle.
In general, by serving in two modes: G2V and V2G, BEVs can provide benefits to the power system operation. The G2V mode can be used to charge BEVs atreduced cost when the power system load is reduced and generation capacity is abundant,such as during night time. The V2G mode may be used whendemand is high or supply isaccidentally lost. Most of the time vehicles sit idle parked at homes, streets, parking lots, or garages; henceBEVs battery capacity can be fully utilized during such times. BEVscould serve as decentralized energy storage in a smart grid and can act as either a load or a generator as needed. PEVs/BEVs may be an attractive integral part of a smart grid,when aggregated in sizeable numbers and capable to operate in the V2G mode.With the help of advanced control and communication methods, BEVs can work as a generation resource connected to the power grid via bidirectional power transfer through energy exchange stations. This could increase the flexibility of the electrical distribution system operation since the additional storage will allow for more widespread use of solar and wind generation, which are currently underutilized due to lack of storage. By participating in the ancillary services market, BEVs based V2G will improve the grid’s stability and reliability.
Economic Incentives for Participants:
When the vehicles of participants are not in use, they can serve as storages connected to the power grid which provide power during peaks. In return, for the use of the vehicles the firms which provide power would pay the participants for their service based on the amount of power supplied from the vehicle. Companies simulating V2G predict that the participants can earn up to $300 per month (cost of recharging is taken into account).
Renewable Energy Integration:
One of the major problems associated with harnessing renewable energy resources is its storage. Energy storage has long been a subject of research within the electric community as it would take advantage of low cost periods of generation. Added storage would become a resource for the voltage regulation needs discussed previously. The remote and challenging locations also act as barriers to the procurement of energy. Solar energy peaks around 1 pm, whereas wind energy peaks overnight around 4.30 am. The standard model for the peak electricity demand predicts highest demand between 4 and 8 p.m. The peak, is thus shifted from the optimal hours of the solar/wind generation capacities leading to major problems in integrating renewable energy resources. Wind curtailment also remains a big issue in harnessing wind energy. Sometimes the energy generation overloads the grid and some turbines are then shut down or temporarily impaired. The most important role for V2G may ultimately be in emerging power markets to support renewable energy.
About this Project:
This project works on power converter topology with two-stage architecture. The voltage source PWM converter is used as the front-stage, and the post-stage is composed of the symmetrical half-bridge LLC resonant converter.
LLC RESONANT TOPOLOGY:
LLC resonant topology lowers switching losses, boosts efficiency.
With the current global energy crisis, the focus is on efficiency and electronic products are facing the daunting challenge of delivering high performance, while consuming less power. It will be difficult to meet these efficiency specifications with conventional hard switched converters.
But one such topology is the LLC resonant converter. The LLC resonant topology allows for zero voltage switching of the main switches thereby dramatically lowering switching losses and boosting efficiency. Efficiencies of 93 to 96% can be achieved with LLC resonant converters. Resonant converters have been around for a long time. However, only now are they seeing increased acceptance as controllers become more available and there is an absolute need to improve efficiency. Perhaps the best way to understand the LLC resonant converter is to first study the conventional series resonant converter
Benefits of Resonant Converters:
Operating a converter in resonant mode, at the point where the impedance between the input and output of the circuit is at its minimum, provides improved efficiency.
The LLC converter can operate at resonance, at nominal input voltage, and is able to operate at no load. In addition, it can be designed to operate over a wide input voltage. Both zero voltage and zero current switching are achievable over the entire operating range.
The SHB LLC resonant converter could realize ZVS/ZCS operational mode in the whole load range. It reduces the switching loss effectively and slows down the transient over-voltage and over-current of the switches. This topology can solve the problem which is accepted that it is difficult for the lagging arm to achieve soft switching using the traditional ZVS bridge phase-shift PWM converter or the bridge ZVS PWM converter. Compared with the single resonant capacitor topology, using split resonant capacitor, the ripple and root mean square (RMS) of the input current through resonant capacitors are both smaller. The split resonant capacitor receives only half RMS current of the single resonant capacitor and the capacitance of the split resonant capacitor is also only half of the single resonant capacitor.
The DC characteristic of LLC resonant converter could be divided into ZVS region and ZCS region. For this converter, there are two resonant frequencies. One is determined by the resonant components Lr and Cr. The other one is determined by Lm, Cr and load condition.
CONCLUSION:
This project focuses on the rational utilization of large-scale electric vehicle energy storage medium, explores the design ways of reducing electrical stress of LLC resonant circuit components, presents a novel bidirectional power converter for electric vehicles in V2G Systems with two-stage architecture and its control strategy. The single-phase voltage source PWM converter (VSC) is used as the front-stage, and the post-stage is composed of the symmetrical half-bridge (SHB) LLC resonant DC/DC converter. The SHB LLC resonant DC/DC converter has a symmetrical structure and split capacity. It has the advantages such as isolated, high efficiency, high power density, small size, wide output voltage range, good dynamic performance and low cost.