Research Of Fast Distance Relay For Transmission Lines

Distance protection is widely used in HV, EHV and UHV system. Improving the performance of distance protection is of great significance to the security and stability of power system. Because of the transient characteristic of capacitive voltage transformers(CVT) and the complicated fault feature caused by large scale wind farms connected to the power system, the realization of a fast and reliable action of distance protection is facing a series of new challenges.The equal transfer process-based distance protection algorithm can overcome the transient characteristics of CVT, and its excellent performance in fault locating puts forward higher requirements on the speed of fault phase selection. Based on the equal transfer process-based distance protection, six kinds of transmission line differential equations are established to describe different types of single-phase fault and phase to phase fault. The correlations between the fault types and the least-squares fitting error of transmission line differential equations are analyzed. The least-squares fitting error of the differential equation which is consistent with the real fault type is smaller, while others are bigger. A fault phase identification method by comparing the relative size of fitting error when solving differential equations under different fault types is proposed. The simulation results show that the proposed method can quickly identify any types and phases of faults with a typical time of 5~6ms and can be unaffected by the fault location and fault resistance. Meanwhile, the method could also work in the weak-infeed conditions and has sufficient reliability.The complex fault feature of wind power system leads a serious impact on the action performance of traditional distance protection element based on power frequency algorithm. To develop a fast distance protection applied to wind farm outgoing lines is of great urgency. The equivalent model of a large scale DFIG-based wind farm connected to power system with the capacity of low-voltage ride-through(LVRT) is established on PSCAD/EMTDC platform. When a fault occurs at the wind farm outgoing transmission line, the fault voltage and fault current contain a large number of harmonic, and the activation of crowbar protection leads a fundamental frequency inconsistency between the fault current and the bus voltage at wind farm side. The fault current fundamental frequency depends on the DFIG rotor speed before failure. Based on the theory of equal transfer process, a fast distance protection algorithm applied to the DFIG-based wind farm outgoing transmission line is proposed, which consists of three major steps: re-structuring of the voltage at the fault point, low-pass filtering and solving the transmission lines differential equations. A variety of simulation tests show that the proposed equal transfer process-based distance protection can overcome the complicated transient characteristics of DFIG fault voltage and current and trip correctly and quickly no matter what kind of fault occurs in the protection scope, which performs significantly better than the traditional Fourier algorithm.The conventional phase-to-phase distance relay using positive sequence polarizing voltage malfunctions when a three-phase short-circuit occurs at the DFIG-based wind farm outgoing transmission line, thus it is necessary to develop new directional element. The performance of conventional directional element based on power frequency algorithm is analyzed. When a three-phase short-circuit occurs at the DFIG-based wind farm outgoing transmission line, the traditional direction element can trip correctly if the fault current through the distance relay is provided by the power system, otherwise, if the fault current through the distance relay is provided by DFIG, the traditional direction component will not be able to get accurate and stable judgments about the fault direction. A new direction relay algorithm for wind farm outgoing transmission line is proposed. The positive direction criterion is employed when the relay is at the power system side, while the opposite direction criterion is adopted when the relay is at the wind farm side. The directional feature is confirmed by judging for a period of time after fault occurs. The simulation results show that the new direction relay at both sides of the outgoing transmission lines can both provide accurate fault directions by 30 ms no matter what position the three-phase fault occurs on the outgoing transmission line.In addition, a comprehensive summary about the whole work is given and the next step of research needed to be further accomplished is pointed out.