Investigation Of On-line Voltage Stability Monitoring And Load Shedding Control Based On Multi-port Equivalence

With the expanding scale of the grid, sustainable load growth and the gradualimplementation of electricity market, power system operates closer and closer to thestability limit. the international system has occurred several collapse due to voltageinstability, causing enormous economic losses and social impacts, hence, seekingreasonable monitoring and controlling method for voltage stability have becomevery important.This paper firstly discusses the mid-long term voltage stability, analyzing thedynamic response of the components which play imporant roles in the mid-longterm voltage stability. In the analysis of mid-long term voltage stability, the systemgenerally have a slow process of losing stability before voltage collapse, startup ofthe generator’s over excitation limiter OXL, tap adjustment of the on-load tapchanger OLTC and recovery characteristics of the load power etc. all play importantroles in the system.10bus system and Nordic32system are tested through PSAT toanalysis the effect of dynamic characteristics of the system and device for mid-longterm voltage instability, when a large disturbance such as disconnection or shortcircuit occurs in the overload transmission line.Generators are normally considered as PV nodes or ideal voltage sources in thestudy of voltage stability index. This assumption is not applicable to the entireprocess of voltage instability, in the analysis of mid-long term voltage stability,reactive power shortage may cause OXL starts, automatic voltage regulator AVR canno longer maintain the terminal voltage constant. In this paper, the voltage stabilityindex of the conventional multi-port equivalent circuit has been improved, applyinggenerator model which can well describe the action of OXL to the multi-portequivalent circuit, then the dynamic conversion from AVR to OXL can be wellexpressed. The idex can identify the maximum load power point as the warningpoint of collapse during the mid-long term instability process. Simulation resultsshow that the improved index performs more excellent than the traditional one, andit can play a good warning for the impending mid-long term voltage collapse.Based on the improved multi-port equiva lent network, a load shedding schemeis proposed to prevent voltage instability, the deduced load shedding sensitivity notonly can interpret the dynamic change of generators effectively, but also determinethe locations for load shedding correctly. Combined with the derived voltagestability index which can identify the maximum load power point, the load sheddingscheme in this paper can not only warn mid-long term voltage collapse effective ly,but also shed load fast and accuratly. Simulation results show that compared to the traditional under voltage load shedding, the method of this paper can reduce theamount of load shedding, and improve system voltage recovery rate.