Research On The Security Evaluation And Control Of Power System With Wind Farms And Electric Vehicles

Energy and environmental issue is currently the main focus of the world. In China,in order to achieve the goal of energy-saving and emission-reduction, the powerindustry has taken specific initiatives to develop all kinds of renewable energyresources at the generation side to de-carbonize the power generation. At the sametime, promote new types of electric drives to replace the traditional petrochemicalbased loads at the demand side. These measures lead to the fast development of largescale wind farms and electric vehicles. Uncertainty is the shared characteristic of windgeneration and electric vehicle. The power output of wind farm depends on the windspeed with intermittent and fluctuation nature; the charging power of electric vehiclehas closely relationship with the traffic patterns and is able to get access to the gridanytime and anywhere. With the increasing penetrations of wind farms and electricvehicles, the resulting great deal of uncertainties will significantly impact theoperation of power system at many aspects, such as voltage stability, frequencystability, etc. This dissertation studies the new security evaluation and controlstrategies for the power system with high penetration levels of wind farms and electricvehicles. The major work is categorized as follows:1. Improve the traditional solution method to determine the boundaries of voltagestability region, in order to make it suitable for on-line security monitoring andcontrol of power system with intimittent renewable energy generations (such as windfarms). The main tasks of this part include the following two aspects:1) The traditional method of using one hyper-plane to approximate the boundary ofsecurity region has the deficiencies both at calculation speed and accuracy, which isunable to satisfy the requirements of on-line monitor and control. In this part, a newperturbation approach is introduced to obtain the local boundaries of voltage stabilityregion in the cut-set space (CVSR) of power system. Firstly, the power flow tracingalgorithm is used to determine the generator-load pair which is most sensitive to thepower flow on each interface line. The power flow on each interface line is perturbedaccurately towards at both power increasing and decreasing directions byimplementing control on the generator-load pair. Then, using the operation pointsafter perturbation to determine the critical points related to voltage stability. Furture,obtain the local approximation boundaries of CVSR at the above two perturbationdirections. Finally, the accurate CVSR local boundaries are obtained by translation and weight. Verification results from several systems show that the proposed methodcan reduce the fitting error of CVSR effectively.2) Based on the above perturbation method, a new approach to determing the localboundaries of voltage stability region in power injection space (IVSR) with windfarms is presented. It can be used for voltage stability analysis and on-line securityevaluation with considering the output undertainty of wind farms. Firstly, power flowtracing and double layer dispatch model are used to determine the generators that areclosely relat+ed to the wind farms, in order to balance the power fluctuations causedby wind speed variation. Then, modal analysis is used to obtain the key generators toachieve an effective dimensionality reduction for IVSR. Finally, the forecasting outputpower (or wind speed) of wind farms is divided into several subintervals. For eachinterval, the corresponding local IVSR boundaries are calculated by the perturbationbased method. The parallel process is used to accelerate the computation speed. Thepresented approach is validated by several power systems. It is revealed that theapproach can give the local IVSR boundaries at different wind speeds and be used forpower system voltage stability analysis and on-line security evaluation with largescale wind farms, which has a good engineering application prospect.2. A preliminary research is carried out to model the electric vehicles and study thecorresponding impacts on the power system. The purpose is to seek revelant analysistheories and control strategies for power system in order to provide technical supportfor the future integration of electric vehicles. The main work is divieded into thefollowing two parts:1) Combined with the Origin-Destination analysis from the intelligenttransportation theory, a Spatial Temporal Model (STM) is developed to evaluate thesecurity of power supply with high penetration levels of electric vehicles. STM candetermine the impacts on power system caused by electric vehicles’ charging powerwith spatial temporal characteristic. STM is able to guide the planning and upgrade ofthe power grid.2) Under the environment of Vehicle-to-Grid (V2G), four architectures of usingelectric vehicles to support the operation of power system, Aggregator, Virtual PowerPlant (VPP), MicroGrid and Energy Hub are compared and analyzed. Then, usingelectric vehicles to contribute to the primary frequency response is investigated indepth. On this basis, a voltage stability control stragety is presented when emergencyhappens, which uses the complementary characteristics between wind farms and electric vehicles to improve the voltage stability of system by exerting controls.3. The interface power flow control strategy of power system with intermittentpower injections is studied in-depth in order to develop new power flow controltechnologies for operators to control the operation of power system effectively. Themain works are divded into the following two parts:1) A new interface power control method based on DC power flow and sensitivityanalysis is presented. Firstly, DC power flow is used to calculate the matrix ofgeneration shift distribution factor (GSDF). Further, according to the needs of powercontrol, using the information of GSDF matrix, the control scheme is obtainedthrough nonlinear optimization. It is revealed that, the method not only can control thetotal power of the interface accurately, but also can take the directional variation ofpower flow on each interface line into considersation, with a good applicationprospect.2) The above linearized DC power flow can not consider the error caused by thevoltage problem on the power transmission of power system. As a result, an interfacepower flow control method based on surface approximation is proposed. This methodcan take fully account of the nonlinear relationship between power flow and thevariation of power generation as well as the impacts from reactive power. The controlprecision is further improved.