Voltage Stability Constrained Optimal Power Flow Based On An Improved Load Impedance Modulus Margin Index

With the development of the domestic economy,people's increasingly need for electrical power raised.However,electrical power system is developing toward the large-scale power grid,high voltage class and long-distance transmission on account of the restraint of the environment and construction cost,which result in the reactively weakness network structure and insufficient reserve generating capacity.Besides,the electrical power system usually operates under the heavy loading condition because of the increasing desire of the electrical power company to the economic benefits in electrical market.Those above reasons lead the electrical power system operation condition closer to its extreme power transmission capacity and the larger possibility of the voltage instability.Therefore,it is very necessary to take into account the voltage stability constraint problem in the conventional optimal power flow(OPF).Nowadays,the major difference of voltage stability constrained optimal power flow(VSC-OPF)analysis is the utilization of various voltage stability assessment(VSA)indicators.Besides,the constant power load model is usually used in the establishment of the VSC-OPF.Nevertheless,the load model with static voltage characteristic,which affecting the VSA,rarely taken into consideration.In this paper,a novel computing method of load impedance modulus margin(LIMM)index with load static voltage characteristic is proposed,and then the novel voltage stability index is incorporated into the conventional OPF.Consequently,a new VSC-OPF approach is modeled.The main research contents and academic achievements are presented as follow.1)In this paper,the load impedance modulus margin(LIMM)index is the first time to be incorporated in to the classical optimal power flow to establish the VSC-OPF(LIMM-OPF).Besides,LIMM index has been improved so as to be more flexible to the VSC-OPF approach.In order to verify the availability of the proposed VSC-OPF,two VSC-OPF approaches based on L-index(L-OPF)and the minimum singular value(MSV)of power flow Jacobian matrix(MSV-OPF)respectively are compared with the LIMM-OPF.And then,the simulation results obtained in the standard test systems are analyzed and the performances of the LIMM-OPF approach are discussed.The proposed VSC-OPF can simultaneously improve voltage stability and minimise active power losses and the reactive power output of the generators under stressed condition of the power system.Besides,it also extends the application scale of the LIMM index.2)The PV curves are obtained by use of the continuous power flow in the single-machine infinite bus system based on four static load models(constant current load model,constant impedance load model,constant power load model and ZIP load model)to analyze the influences of different static load models on the voltage stability.On basis of the above research,a new calculating method of LIMM index with load static voltage characteristic of ZIP load model is proposed,and then it is utilized to build the VSC-OPF.In order to analyze performances of the proposed VSC-OPF approach,OPF is used as the contrast.The simulation results obtained by IEEE 30 nodes standard test system shows the validity of the proposed VSC-OPF.Besides,the influences of the different static load model on the proposed VSC-OPF are discussed based on the simulation results in IEEE 30 nodes system.Finally,in order to analyze the influence of the ZIP parameter errors on LIMM-OPF,the results after LIMM-OPF optimized are compared in the different errors of the ZIP parameters.The simulation results show that LIMM-OPF based on the proposed calculating method of the LIMM index has strong robustness.