Power System Reliability Assessment And Weak Bus Identification Based On Voltage Stability

Parts of this thesis were surported by Research Project Supported byShanxi Scholarship Council of China (2014-030)“Static voltage stabilityanalysis method based on flow calculation” and Shanxi Province Science andTechnology Activities Project for the Selected Scholar Back from Oversea Study“Effect ofAC/DC hybrid micro grid control mode on system voltage stability”.Power system instability and voltage collapse in particular may lead to amassive and protracted loss of load. This affects both reliability and economicaleffectiveness of the power system operation. Reactive power is closely relatedwith the system voltage stability and voltage collapse. It is important for thesystem stable operation. The effect of stability on reliability is a complexproblem of power system operation. Nevertheless, due to the complexity ofstability-reliability interactions, the important practical problem has not receivedadequate attention. It includes the effect of reactive power shortage on reliability,weak bus identification considering the system fault states and the probabilitydistribution of system stability based on sharp fluctuation of load.The focus of this paper is to investigate bus reliability from aspects ofsystem reactive and real power resources and network redundancy based on busvoltage stability. The effect of load random fluctuations on system reliabilitybased on bus voltage stability is also researched. The main objective is toprovide some reliability indices which can represent system operationalreliability level and the margin to system instability.This paper proposes a new technique to evaluate reliability indices whichtake into account both real and reactive power shortage. Reactive powershortages associated voltage violations due to system failures are considered. Athree-stage load-shedding technique for post contingencies is implemented todetermine the contributions of real and reactive power on the system reliability and to find an optimal way to release network violation. A reactive powerinjection technique is proposed to determine possible reactive power shortageand location. It is inproved that this method could provide the system reliabilityinformation from active power balance, reactive power balance and voltagestability points of view.This paper proposes a technique to examine network reliability from busvoltage stability point of view. Network reliability under various operatingconditions and states are evaluated and presented by probabilities of systemvoltage being in normal, under-voltage, critical, and collapse zones. Two busvoltage indices, the MV (Minimum Voltage, MV) and BVC (Bus Voltage Change,BVC) are proposed to identify the weakest bus for each contingency. Twoexpected bus voltage indices, the EBVD (Expected Bus Voltage Drop, EBVD)and EBV (Expected Bus Voltage, EBV) are proposed to evaluate the marginfrom the actual voltage level to the collapse point of a bus for a contingency.Those indices can be directly observed by system operator though monitoringsystem voltage level. It is inproved that the indices are more intuitive,economical and effective in practical applications.Considering the load uncertainty and the change in the network topology,the power system collapse due to the sharp fluctuation of load is studied. Theprobabilistic load flow based on Latin hypercube sampling is proposed to obtainnumerous states of system collapse under different load conditions. Aquantitative evaluation method for voltage stability assessment based on risk isproposed. Several risk indicators are proposed from voltage, node type andnetwork load bearing capacity point of view. The weak areas of the system areidentified by these risk indicators. It is inproved that this method is effective,and feasible. It is consistent with the MV, BVC, EBVC and EBV indices.