A Study Of Negtive And Zero Sequences Current Compensated And Controlled In Transmission Lines Operated In Two Phases

This thesis studies main problems in achieving ultra high voltage power transmission lines operating on non-full phase long-running mode. A new approach using FACTS controllers to eliminate negative sequence currents and restrain zero sequence currents is proposed for the first time, which realizes ultra high voltage power transmission lines operating on long time-non full phase mode, which is one of effective means to guarantee constant power supply and satisfy requirements of system stability, which will make full use of existing power system resources, power transmitting and distributing system. With the help of successful fault location and on-line inspection technologies, many new ways can be applied to power system disaster defence and disaster elimination, and it is meaningful for practical applications and academic research. Especially for power transmission lines installed with FACTS appliances, it benefits much from changing control strategies and extending functions of eliminating negative sequence currents and restraining zero sequence currents. As for not only system with single-circuit power transmission lines or interconnected lines, but also power transmission lines which are in the weakly interconnected systems, economic and technological value is more noticeable. By using this technology, some parallel power transmission lines are even not constructed or be builded late. So, its economic value is obvious.This thesis presents a scheme, which uses FACTS controllers to achieve compensation and control of negative sequence and zero sequence currents under condition that power transmission lines operate on long time-non full phase mode. And the scheme is as follow.1) Under circumstances that one of UPFC, ASVG and APF are installed on both sides of power transmission lines, if permanent grounding fault of single phase occurs on power transmission lines, the installed UPFC, ASVG and APF can be used to achieve compensation and control of negative sequence and zero sequence currents for two-phase running, and it can guarantee constant power supply and system stability. Moreover, cascade compensation of UPFC can be used to improve and adjust transmission power for two phases running, too.2) Under circumstances that UPFC is not installed on both sides of power transmission lines, but ASVG and APF are installed, it can be acquired the samecompensation and control of negative sequence and zero sequence currents for two-phase running.3) Under circumstances that UPFC with cascade compensation functions is installed on one side of power transmission lines, cascade compensation functions can be used to restrain zero sequence currents of abnormal lines operating on long time-non full phase mode. In order to prevent or eliminate interference resulted from zero sequence currents to communication signals nearby; the detailed scheme is described as follows.a) If we add a voltage source to normal phase in series mode, ensure voltage fall between two normal phases is equal in amplitude and opposite in direction, and make running mode from two normal phases to signal phase, zero sequence currents can be restrained accordingly.b) If we add a voltage source to normal phase in series mode, which is proportional to zero sequence currents in amplitude and opposite in direction, zero sequence currents in abnormal lines can be restrained accordingly.Simulation studies are performed for schemes mentioned above. And the simulation results verify the schemes presented in this thesis are valuable for long time-non full phase mode.Because UPFC and ASVG usually control voltage, reactive power and phase displacement under normal condition, its main circuits are always three-phase inverters with three-leg. It can only provide passages for positive sequence and negative sequence currents. In order to accomplish the targets, main circuit topologies of UPFC and ASVG should be changed into three-phase four-wire system based on three-phase inverters with three-leg using grounding branch or three-phase inverters with fou...