Analysis Of Cross-voltage Fault In Mixed-voltage Quadruple-circuit Transmission Lines And Its Influence On Relay Protection

Due to the special construction method of mixed-voltage multi-circuit transmission lines on the same tower,cross-voltage short-circuit faults are easy to occur which will result in great damages to the system stability,if the relay protection cannot operate accurately.This paper focuses on the cross-voltage fault in mixed-voltage quadruple-circuit lines which includes the short-circuit calculation and analysis of the adaptability of relay protection and its countermeasures.First of all,this paper puts forward a new method of realizing the short-circuit calculation of cross-voltage short-circuit faults by regarding the cross-voltage fault as a double-fault based on the theory of multiple fault analysis.Considering the difference of electrical and magnetic connection as well as the construction method,the way proposed is not only adaptable for systems with weak-electrical and strong-magnetic connection or strong-electrical and weak-magnetic connection,but also systems with transmission lines totally or partly on the same tower.This method avoids complex transformation of decoupling which simplifies the process of calculation.Then,taking the phase selection based on superimposed phase-to-phase currents for example,the adaptability of phase selector when confronting cross-voltage faults is analyzed and a new method of phase selection is proposed.In this paper,superimposed phase-to-phase currents of different faults are deduced based on the new short-circuit calculation method,and its influencing factors are analyzed.Further,the action of the phase selection components under different influencing factors is analyzed in detail according to the example given.The analysis shows that when the cross-voltage fault occurs,the phase selector based on superimposed phase-to-phase currents may regard the two-phase fault as a single-phase fault by mistake.In order to make up for the above shortcomings,an improved phase selection method is proposed.By adding an auxiliary criterion consisting of the ratio of the superimposed phase currents,it is possible to avoid the occurrence of mis-selection effectively when confronting cross-voltage faults.Besides,based on the proposed cross-voltage fault analysis method,the specific expression of impedance measured at the location of protective relay is derived,and the properties of each additional impedance are analyzed in detail in order to clarify the adaptability of the distance protection.In this paper,the cross-voltage non-ground fault is taken as an example.It can be deduced that the measured impedance of the grounding distance protection has three parts of additional impedance due to the zero-sequence coupling between the transmission lines and the voltage at fault point when non-ground fault occurs,which may result in the mis-operation of distance protection.Simulation results in PSCAD verified the correctness of the cross-voltage short-circuit calculation method and analysis of relative protections.