Research On Transient Current Character Of UHV Transmission Line And The Affect On Protection

Usually, the UHV transmission line is long and the phase-to-ground capacity and phase-to-phase capacity are big. Compared to the 500kV and 750kV system, the UHV system has the features of larger harmonic components and distributed capacitance current. All of these will have adverse influence on protection.At first, the UHV transmission line model is built based on lumped parameter. After the approximate expression of transient current is deduced, the transient current's character is analyzed qualitatively.Secondly, two 1150kV transmission line models have been chosen. One is the line without shunt reactor and the other is the line with two shunt reactors. Simulations that include line faults, the breaker closing to no-load line and permanent faults are done with PSCAD/EMTDC. The following conclusion is found by analyzing the transient current using Discrete Fourier Transform.(1) Shunt reactor has almost no affect on harmonic wave in transient current, namely the transient current's character of two line models is nearly same.(2) There is no obvious harmonic wave in faulty-phase under single-phase-to-ground fault. Under phase-to-phase fault, two-phases-to-ground fault and three-phases fault, the harmonic wave obeys the same rule: the harmonic wave appears in transient current that is far away from fault location. That means to the protection at the beginning of the line, when the faults are near the end of the line, there will be obvious harmonic wave. And farther the fault location is, more close to fundament frequency the harmonic wave is.(3) When the breaker closes to no-load line and the opposite breaker is open, the harmonic wave in transient current appears between second and third harmonics, and the biggest amplitude exceeds half of the fundament current wave's amplitude.(4) When the breaker closes to permanent faults, the character of transient current in faulty-phase is similar to that of line faults, and the feature of transient current in non-faulty phase is similar to situation in closing to no-load line.Based on the simulation for transient current, the phase segregated current differential protection with capacity current compensation is analyzed with UHV transmission line model. And the result is that this protection can't satisfy sensitivity and reliability at the same time.