Analysis On Calculation Of The Cross-Voltage Short-Circuit Fault In Mixed-Voltage Transmission Lines System And Influence On The Relay Protection

With the development of economic, land resource is becoming scarce in China. Many new build mixed-voltage transmission lines in city were reconstructed based on dual-circuit lines. Transmission lines take on the important task of transmitting electrical energy, which is the lifeline of the power system. The relay protections of the transmission lines must operate correct once fault occurs.The phrase mixed-voltage transmission lines was defined as transmission lines with several different voltage levels, the phrase cross-voltage fault was defined as cross-line fault in which the lines leveled in different voltages separately.Coupling between the mixed-voltage transmission lines are complex, and parameters are no longer symmetrical anymore, which make the traditional fault analysis method cannot be used, the relay protection in the systems are impact seriously. In the meantime, cross-voltage fault is prone to occur due to topology of the lines, which will cause sever influence on the power system security. Hence, analysis on the cross-voltage fault and influence of the relay protection is important. The main work done in this paper is list as follow:(1) Multi-composite sequence network method was proposed to calculate fault current in mixed-voltage transmission lines system with cross-voltage fault in strong-magnetic and weak-electric condition. Higher order matrix operation was avoid, all kinds of cross-voltage fault can be calculated simply and quickly.(2) With superposition principle and multi-composite sequence network method, the cross-voltage fault in mixed-voltage transmission lines with strong-electric connection was decomposed into two parts, one is the strong-magnetic and weak-electric system, the other is the additional equivalent current source part. Combination of the two methods can fix all types of mixed-voltage transmission lines systems. Results show that:the two methods solve the cross-voltage fault perfectly, and the fault characters is no longer the same as the normal single line fault.(3) The problem of over-voltage in fault point of the lower-voltage system during the cross-voltage was proposed, the computational expressions of the over-voltage in unearthed cross-voltage fault were given. Relationship between the over-voltage and the phase-angle difference/impedance ratio were analyzed. Results shows that:the over-voltage was exist, and would turn to the maximum value when phase-angel was 0°and the impedance ratio was the minimum value.(4) Influence on the main principles of relay protection when cross-voltage fault occur in mixed-voltage transmission lines were analyzed. Relationship between criteria of the phase-selector/distance protection/zero-sequence protection/ differential current protection and the two electromotive force phase-angle difference/impedances of the systems were analyzed.All the results were tested in a 500 kV/220 kV four-circuit transmission lines build in PSCAD.