Research On Unbalance Factors And Its Compensation Strategy Of 220kV Cable Transmission System

With the development of the economy,people's quality of life has gradually improved,power quality has become critical in our daily life.Phase balance is one of the most important indicators to measure power quality.The imbalance of power transmission lines and loads affects the safe operation of the power grid.As a result,solving the above two problems is of great significance for improving the power quality of the grid.In this dissertation,the basic theory of three-phase unbalance in transmission lines is studied firstly.The self-impendance and the mutual impedance are calculated according to the Carson line model and the "wire-earth" loop theory.The cable metal sheath is equivalent to a wire in the calculation of sequence impendance,and t he method of matrix reduction is used to eliminate the influence of the metal sheath on the electrical parameters of the cable line,in the meanwhile,the calculation method of the electromagnetic unbalance about the single and multiple transmission line s is introduced.Secondly,the combination of theoretical analysis and ATP-EMTP simulation is used to analyze the influence of different factors such as the distance between phases,cross-bonding mode of metal sheath,metal sheath grounding resistance and arrangement of phases on electrical parameters and electromagnetic unbalance of cable lines.According to this analysis,it is concluded that the method of increasing the phase distance,the equidistant cross-bonding of the core wire,and adjusting the phase sequence arrangement can reduce the electromagnetic imbalance of the line to the greatest extent.Then,the compensation method of load imbalance is studied and the static synchronous compensator based on modular multilevel converter is utilized.The topology and operating principle of MMC-STATCOM are analyzed,in the meanwhile,the methods of sub-module capacitor voltage balancing,circulating current suppressing and power controling are designed under grid balanced conditions.The effectiveness of proposed control method s is verified by simulation using PSCAD/EMTDC.Finally,in order to solve the problem of reactive power compensation under three-phase unbalanced conditions of the power grid,the STATCOM voltage compensation principle at PCC,the MMC mathematical model under unbalanced condition and the unbalanced circulating current caused by unbalanced compensation are analyzed.The multi-layer control strategy which consists of the positive-,negative-and zero-sequence control strategy,the sub-module capacitor voltage balancing strategy,and the unbalanced circulating current suppressing strategy is proposed.Simulation results in PSCAD/EMTDC platform demonstrate that the multi-layer control strategy can compensate unbalance loads,balance the sub-module capacitor voltage,suppress the circulating current and eliminate second-order harmonic in the dc voltage.