Research On Reactive Power Reserve Optimization Considering Voltage Stability

Along with the development of China's large power grid interconnection,voltage instability has become one of the main threats to the safity and reliability of power system's operation.Voltage stability of power system mainly rely on the reactive power supporting at different locations in the system.And voltage collapse usually occurs in the regions where there are heavy loads.Secure voltage profiles cannot be guaranteed when some regios are short of reactive power reserve.Most of the accidents related to voltage stability in power system are due to the lack of reasonable reactive power management.To deal with various disturbances that are dangerous to voltage stability,reasonable arrangement of reactive power reserve of reactive power sources can improve the system voltage stability margin effectively and ensure the safity and stability of system operation.This study proposed a method for reactive reserve assessement and optimization to improve both long-and short-term voltage stabilities simultaneously,and the optimization of reactive power reserve is further studied to coordinate the long-and short-term voltage stability.The main research results of this study are summarized as follows:1)Two reactive power reserve definitions are proposed in this study to evaluate the levels of system reactive power reserve,namely long-term voltage stability-related reactive power reserve(LVRPR)and short-term voltage stability-related reactive power reserve(SVRPR).For LVRPR,the generator participation factor is used to describe the relative contributions of generator reactive support to system voltage stability.For SVRPR,a definition is first proposed and a voltage support coefficient based on the trajectory sensitivity is proposed to evaluate the contribution of reactive power reserve.2)A reactive power reserve optimization method to improve long-term voltage stability margin is proposed.There exist some deficiencies in reactive power reserve evaluation based on generator participation factor.To exert more generator's potential reactive support,the optimization model is decomposed to two sub-problems: generator master problem and capacitor sub-problem.Generators and capacitors are taken into account separately when solving the upper and lower problem respectively.3)A SVRPR optimization model to improve short-term voltage stability is proposed.The contingencies with short-term voltage security problem are included in the model.To evaluate the contribution of various var sources to the short-term voltage stability,the first-order trace sensitivity based on the time domain simulation results is adopted.The objective of the model is to maximize the SVRPR,satisfying all the restrictions under the steady-state operation condition.The power system is ensured to have enough capability to suffer the Contingency set by the enhancement of the SVRPR.Thus,the method proposed can be used to prevent the happening of short-term voltage collapse during system's contigencies.4)A reactive power reserve optimization model to improve short-term voltage stability is built based on the second-order trace sensitivity method.The var sources second-order trace sensitivities with respect to the weak buses in transient process are taken into account to evaluate SVRPR's contribution to short-term voltage stability more presizely.Hence,optimization based on the second-order trace sensitivity can improve short-term voltage stability more effectively.5)A bi-objective optimization model that improves both the LVRPR and SVRPR is proposed to incorporate both long-and short-term voltage stabilities.The study focus on imoroving short-term voltage stability by optimization of SVRPR,ensuring enough long-term voltage stability margin.To solve the multi-objective model,the Pareto method is introduced.And,the NBI technique is used to produce the uniform Pareto optimal solutions so as to provide comprehensive and precise decision support information to the system operator.Case study shows that the LVRPR optimization and SVRPR optimization can effectively improve static voltage stability and transient voltage stability respectively.However,it will not do well to improve the other kind voltage stability if only one of the reactive power reserve optimization taken into account.By means of the bi-objective reactive power reserve optimization which coordinate the long-and short-term voltage stability,uniformly distributed Pareto front solutions set can be obtained to find a more reasonable optimization result according to the trade-off between LVRPR and SVRPR.Thus,comprehensive improvement of the long-and short-term voltage stability can be achieved.