On-line Monitoring Of Mid-long Term Voltage Stability Based On The Analytical Identification Method Of Thevenin Equivalent Parameter

In recent years,with the rapid growth of the power load,large-scale access of new energy and implementation of electricity market,the operating point of power system is approaching the stability limit,the severity of the voltage stability is gradually highlighted.Reasonable on-line monitoring index of voltage stability can provide operators with correct guidance and help to prevent voltage stability accidents in power systems.This paper focuses on the relationship between analytical impedance ratio and the characteristics of power system dynamic components,the analytical identification algorithm of Thevenin equivalent(TE)considering the startup of the generator's over excitation limiter(OXL),and mid-long term voltage stability monitoring based on the analytical power margin index.The analytical impedance ratio index is applied to the analysis of mid-long term voltage stability.The impacts of the characteristics of power system dynamic components on analytical impedance ratio index is analyzed by simulation.The simulation results show that the tap adjustment of OLTC and the typical ZIP load do not affect the effect of applying analytical impedance ratio index to the voltage stability monitoring.But the startup of the generator's OXL causes the analytical impedance ratio index of some sub-weak nodes not to reach 1 near the maximum transmission power point.And the smaller of the excitation limit,the smaller the analytical impedance ratio index near the maximum transmission power point.An analytical Thevenin equivalent parameter identification algorithm considering the startup of the generator's OXL is proposed.The nodes whose OXL starts up are processed as PQ or PI_f nodes to participate in the calculation of the sensitivity parameters,and the improved analytical impedance ratio index is obtained.For the mid-long term voltage instability caused by the continuous load growth,the improved analytical impedance modules of any load node can be matched before the maximum transmission power point or the maximum point.For the voltage instability caused by the big disturbance,the improved analytical impedance ratio index can reach 1 in advance,which plays a leading warming role to the impending voltage instability,and also provides sufficient time for operation dispatchers to take measures to maintain the voltage stability.The performance of the analytical power margin index for monitoring mid-long term voltage stability is analyzed in depth.The analytical power margin index under PV mode can keep good linearity with the increase of the load.Although the continuity and linearity of the analytical power margin index under PQ mode are deteriorated due to the influence of the generator's OXL startup,the analytical power margin index can truly reflect the overrun of the generator excitation,and the reflected network power transmission capacity is closer to the reality of the system.