Research On The Control Method Of Bi-directional Charging For Electric Vehicles And Its Coordination With Generating System In V2G Scenario

Electric vehicles(EVs)will play a vital role in the future's transportation systems since this technology is promising for environment,energy security,and improved fuel economy.One issue is whether nowadays' power gird is able to sustain the growing demand due to more EVs and by what method people should use to charge these vehicles.Although the increased EVs seems to pose a large liability to the power grid,it can actually become an asset to the grid if being operated properly.The grid can benefit greatly from having reserves that can store or release energy at the appropriate times.When a large number of EVs are aggregated,the huge total battery capacity is sufficient to stabilize the fluctuations in the power grid.EVs serve either as loads or as a distributed power source is known as the vehicle-to-grid(V2G)concept.This paper has focused on the research and application of the V2G technology,which has drawn increasing attention from both industry and academia,from the following aspects:1.Bi-directional power converter control technology for EV charging&discharging;2.V2G control strategy for distributed grid-connected EVs;3.V2G control strategy for EV charging station;4.Power quality control for island or grid-connected EVs with local loads;5.Electric Vehicle Charging and Discharging Coordination with wind/thermal power generating units.Firstly,Enabling EVs to fulfill V2G will require a bi-directional interface between the grid and EVs.This bi-directional charger must have capability to charge EVs' batteries and also have the ability to return the energy back to the grid.This paper have proposed a virtual synchronous machine(VSM)-based control strategy to control the AC/DC converter between EVs and the power gird.Based on the proposed metiond,a EV can be integrated into the power grid and behave in the same way as large synchronous generator&motor do,and is able to provide the grid damping and inertia,which would help the operation of the grid be more smoother and reduce the impact of EV charging on the power gird.This characteristic is vital when the penetration level of EVs is large.By using the proposed control method,the grid-connected EV is able to realize bi-directional real power and reactive power control and provide high quality frequency and voltage regulation services due to the inner frequency-and voltage-droop mechanisms of the VSM,which has laid a solid foundation for the realization of V2G.Then,According to the different ways of EVs integrated into the power grid,in order to make EVs "smart" enough to participate in V2G autonomously,this paper have proposed two kinds of V2G control strategy for distributed grid-connected EVs and EVs that are integrated in a charging station respectively.As for distributed grid-connected EVs,two T-S fuzzy controllers are designed to decide the reference charging and discharging power of EVs as well as the operational mode of the VSM controller,which are depending on the EV's battery state,users' offset and the present Grid condition.Hence the distributed grid-connected EVS are able to participate in frequency regulation and reactive power compensation for the grid while meet EV users' drive needs;As for EV charging station,a T-S fuzzy controller is designed to generate the reference charging power of the charging station by considering grid frequency and the energy demand of EVs inside the station.A self-adjusted drooping coefficient method is designed to adapt the change of EVs' stored energy in a CS unit while the drooping mechanism of the VSM controller is employed,and a power distribution strategy is proposed to ensure the charging/discharging power of each EV is subject to its' maximum limit.Due to the inner frequency-and voltage-drooping mechanisms of the VSM,a charging staion is able to self-adjust its' input&ouput real and reactive power according to the grid frequency and voltage,thus the charging station is able to behave in the same way as a virtual peak and frequency power plant.After that,considering EVs have the potential of serving as the backup power supply in vehicle-to-house(V2H)or vehicle-to-building(V2B)scenario,in order to ensure the power quality of EVs provided for the grid and the load while EVs are with local loads,an improved VSM control strategy,which is cascaded by an original VSM controller proposed by chapter 2 and a H? repetitive controller has been proposed to control both the real&reactive power and the output voltage of the AC/DC converter between EVs and the grid.Genetic algorithm is employed to optimize the steady-state performance of the designed controller.The stability of the close-loop system is verified by check the H ? norm from input to output.Simulation results have demonstrate that the proposed controller is able to keep the output voltage of the AC/DC converter stable and clear while in island or grid-connected mode with different kinds of local loads.Simulation results have also shown that by using the proposed method,the current THD that EVs inject to the grid is reduced as well.At last,this paper proposed a optimal strategy for coordinated operation of electric vehicles(EVs)charging and discharging with wind-thermal system.By aggregating a large number of EVs,the huge total battery capacity is sufficient to stabilize the disturbance of the transmission grid.Hence,a dynamic environmental dispatch model which coordinates a cluster of charging and discharging controllable EV units with wind farms and thermal plants is proposed.A multi-objective particle swarm optimization(MOPSO)algorithm and a fuzzy decision maker are put forward for the simultaneous optimization of grid operating cost,C02 emissions,and wind curtailment.Simulations are done in a 30 node system containing traditional thermal plants,carbon capture and storage(CCS)thermal plants,wind farms,and EV aggregations.The results are presented to prove the effectiveness of the proposed strategy.