Research On A Novel UHV/EHV Transmission Line Protection Based On Longitudinal Tapped Impedance

Full current differential protection is one of the main protection of UHV/EHV transmission lines. With the raise of voltage grade and the increase of transmission distance, the distributed capacitive current increases, which influences the sensitivity and selectivity of the longitudinal differential protection severely. In order to overcome the adverse influence of capacitance current, many compensation methods are proposed, but these methods all need the line capacitance parameter and it is hard to compensate for the capacitive current accurately in the transient period. Full current differential protection is also influenced by the load current and fault resistance, in order to overcome the adverse influence, one method is to apply different protection operation settings, high settings are applied in the transient period and low setting are applied after the transient period; another method is to apply the zero sequence current differential protection or fault current differential protection as the supplement of full current differential protection, thus constituting a new protection scheme. Both the two methods have their own problem, the first method can’t solve the problem fundamentally; for the second method, the zero sequence current differential protection doesn’t have the phase selection ability and it may refuse to operate when ground fault with high resistance occurs on open-phase running line, and also that the fault current differential protection can only be used in the first and second cycles.Aiming at the problems existed in EHV/UHV line protection, a novel protection principle based on longitudinal tapped impedance is proposed in this paper, theoretical analysis shows that when no fault or external fault occurs, the computed impedance is approximately equal to the capacitive impedance of the line, when internal ground fault occurs, the computed impedance of faulty phase is approximately equal to the parallel value of the line capacitive impedance and fault resistance and when internal phase to phase fault occurs, the computed impedance of faulty phase is affected by fault resistance and system impedance. This new protection principle based on longitudinal tapped impedance doesn’t need to compensate for the capacitive current of the transmission lines, is not affected by load current and is not affected by the system impedance. This new protection principle has a good selectivity and natural phase selective ability, when fault with high resistance occurs, it has high sensibility.This paper simulates the longitudinal tapped impedance protection operation behaviors for different kind of fault occurred in or out of line, the simulation results shows that for external fault, the simulation result steady value has a small difference with the theoretical value, a detailed analysis and solving method is presented in this paper. The simulation results also shows that the longitudinal tapped impedance fluctuates in the transient period for the external fault with small fault resistance. Based on the characteristics mentioned above, a new protection scheme which attaches the longitudinal tapped impedance protection to the full current differential protection, the full current differential protection is mainly used to reflect the severe fault occurred on line and the longitudinal tapped impedance protection is mainly used to reflect the mild fault. In order to avoid false operation of longitudinal tapped impedance protection in the transient period, it can be brought into service 20ms after fault and a direction element should be added on it.The new protection scheme overcomes the problem of zero sequence current differential protection, it can operate quickly because it doesn’t need to operate after a time delay to avoid the asynchronous closing of breakers. This protection scheme has the natural phase selectivity, and it can operate sensitively when ground fault with high resistance occurs on an open-phase running mode line. The new protection scheme also overcomes the problem of only applying for one or two periods exist in fault component current differential protection. This new protection scheme shows a good prospect of application.