Modeling Of Charging Loads Of Electric Vehicles And Analysis Of Impacts On Distribution Systems

Electric vehicles(EVs)which are fueled by electricity bring about zero or very low emissions.Promotion of EVs is regarded as one of the solutions for alleviating problems caused by energy crisis,environment pollution and climate warming based on relevant researches.EVs are low-carbon travel as well as consumption part of intermittent renewable energy.Especially,the advent of plug-in EVs(PEVs)can realize the vehicle-to-grid(V2G)mode or grid-to-vehicle(G2V)mode,shift the peak load to improve the load rate as charging load,and provide frequency regulation and spinning reserve for a power system concerned by feeding electric energy back to the power system.However,large-scale connection of EVs could bring about impacts on power grids due to charging/discharging uncertainties in time and space dimensions without coordination.Based on recent researches of large test data EVs charging load profile is consistent with distribution system(DS)daily load curve under condition of uncontrolled.With the increase of penetration level of the EVs,EV charging could result in a new peak in the load profile and further increase of the peak-valley difference,which,in turn,will cause a series of problems in the distribution network,such as higher probability risk of equipment operation due to increased harmonic,line and transformer overload,and so on.The problems above directly impact on system reliability.Overload further cause increased voltage drop,and more power loss of DS.The growing generation capacities demands increase generation cost,impacting on system economic operating.In this context,systematic and intensive study on impacts optimization of large electric vehicles charging on distribution system operation and charging modeling have been done in this dissertation for the accurate simulation of charging load and its characteristics as well as the impacts on DS from EVs charging and the Associated Strategies.Meanwhile,the huge potentials can be explored and a lot of auxiliary services,such as theoretical basis of EVs promotion,the enhancement of power supply reliability and increase of operation economics can be achieved.Specifically,the contents of this dissertation are as follows:1.The key points and state-of-the-art of researches on charging load description,charging impact simulation and coordinated charging strategy are summarized.Based on charging behavior mechanism,the influence factors and their relationships of charging load are investigated.It is suggested that accurate simulation of the influence factors about charging load including initial charging states and starting charging time points be the direction of charging load modeling improvement.The impacts on the system are summarized by system reliability,power quality and operating economy when a large quantity electric vehicles access to the system for charging.Simulation results and regulation methods are analyzed.The influence factors regulation of EVs charging load should be more considered,and the coordination of Spatial and temporal distribution of charging load is worth to study deeply.This is the theoretical basis for the further research about electric vehicles coordinated charging in the time and space dimensions.2.The stochastic features and interrelation of influence factors of charging load are accurately simulated by Probability method.The distribution of daily average trips of EVs is attained based on the fitting results of traveling statistics.The time interval of adjacent charging is expressed by time coefficient.The distributions of initial states of charging(SOC)of batteries at different times is attained by the expression relationship between SOC and daily average trips of EVs.The distribution of starting charging time points of EVs are verified based on the fitting results of starting charging statistics at different times,considering specific constraints about scenarios.Considering cumulative effect of continuous charging in the time dimension,a probability model for describing EV charging loads is established and then solved by Monte Carlo simulations with demonstration of actual data of electric taxis in Shenzhen.3.Considering the difference of charging demand and charging method in different charging regions,a spatial and temporal model for describing EV charging loads was established and then solved by Monte Carlo simulations.Then,the bus voltage deviations and total branch loss increments are employed as evaluation indexes for assessing the impacts of EV charging loads on distribution system concerned.Considering different spatial and temporal distribution and different permeability of EVs,taking a modified version of the IEEE 30-node distribution system for example,the simulation is implemented.The calculation indicates that the bus voltage deviation and branch loss are increasing with the increase of automobile permeability.There is no obvious improvement that charging loads are uniformly distributed among the charging stations when planning.Charging station planning should consider the characteristics of the site in different areas,in order to decrease branch loss.4.Regulation mechanism of influence factors of charging load aimed for load shifting from charging effect on DS has been specially proposed.Charging time intervals and distribution of the starting charging time points are considered as regulative variables.The maximum difference between the peak and valley loads and the load rate are regarded as the evaluation indexes of regulation.Through the different division of charging time intervals and different distribution of the starting charging time,three methods are presented accordingly for load shifting from charging effects on distribution system within station.Theoretical basis and regulation targets are provided for EVs agents.The results of Evaluation indexes of different methods show that: compared with the impacts of uncoordinated charging load on system,it is very significant to regulate charging time intervals according to the period of peak and valley,and the regulation of distribution of starting charging time points is also very necessary,which is related to regulation effect.5.The optimal option of electrical connecting nodes of EV Charging Stations in DS has been investigated,in view of the optimal locating of EV charging loads in the space dimension.On the basis of daily charging load profile,the percentages of bus voltage deviations and branch loss increments are described.Then a comprehensive evaluation index is employed for assessing the impacts of EV charging stations on power flow in the distribution system concerned,and the electrical connecting points of EV charging stations in the distribution system are then optimally determined.Finally,a modified version of the IEEE 33-node distribution system is served for demonstrating the proposed method.The various scenarios indicate that the impacts on power flow aggravate along with the increase of charging load,at the same load level the impact is different when electrical connecting nodes of charging station modify.