Study On The Theory Of Protection Failure Boundary For The Positive Sequence Voltage As Polarization Of Impedance Relay

Distance protection has been widely applied in power system protection. However, due to the presence of transition resistance, it is restricted in the application of distance protection.The model of protection failure boundary is proposed in this paper, which is based on the fault distance and transition resistance size, and it is used to evaluate the ability of impedance relay to tolerance transition resistance. It is also used to explore real reason for transition resistance which causes the impedance relay non-operation or misoperation.Firstly, the normal operation situation of power system voltage, current and all sequence impedances is analyzed. And the situation of single phase to ground fault through transition resistance, the situation of phase to phase fault through transition resistance and the situation of three-phase to ground through transition resistance are analyzed. The formulas for measured impedance at the fault conditions are derived in the paper. Through some equivalent mathematical models, it relational expression between transition resistance and fault distance is clear and the equations for solving the transition resistance is gained. The operating characteristics of the impedance relay in which positive sequence voltage is used as polarization are analyzed explicitly in this paper. This kind of relay has many advantage, for example, its protection area is stable and it has strong ability of relay to tolerance transition resistance. Then protection failure boundary resistance is defined, which is making the relay correct operation as a maximum tolerated transition resistance. According to the model of protection failure boundary, the protection failure boundary resistance and corresponding fault distance can be calculated, calculation results are used to draw the curve of protection failure boundary resistance. The concept of protection failure coefficient is introduced to assess the ability of relay to tolerate transition resistance and resist risks. Modeling simulation is calculated by the PSCAD, which will present the measured impedance trajectories, the curve of protection failure boundary resistance and protection failure coefficient at the fault conditions. Then the factors are analyzed and compared, which influence on ability of relay to tolerate transition resistance. The model of protection failure boundary for the positive sequence voltage as polarization of impedance relay is also established when the polarizing voltage is dephasing. The simulation experiment results prove that the polarizing voltage dephasing can really improve the ability of the relay to tolerate transition resistance.