Study On Impacts Of MOV Nonlinear Characteristic On Protection And Countermeasures

UHV AC project has been applied in China gradually, however, supported by a weak AC grid, the transmission capability is restricted considering transient stability. In order to improve the transmission power limit, make full use of the transmission capability of each UHV AC line and do well to future development, the series compensation is usually fixed in the UHV AC long transmission line. Although lots of researches have been done on the conventional protection for the line that installed with series compensation, the series compensation is usually treated as a liner capacitor. In a practical project, once the series compensation transmission line operates in a fault state, there will be over voltage on the capacitor, unless the related actions were taken, the capacitor of the compensation will be discharged, which will increase the cost of the project. In order to ensure the safety of capacitors of series compensation, the MOV is usually fixed in parallel to the capacitors, which will limit over voltage.MOV is a nonlinear resistance, such non-liner characteristic brings new challenges to the rely protection. So research on the impacts that MOV have on relay protection and the related solutions will be practical and meaningful.The influence of series compensation considering the nonlinear characteristics of MOV on fault characteristics of transmission lines and its corresponding strategies are investigated. The research is based on UHV project. The feasibility of replacement of series compensation with the equivalent linear model when line faults happens is discussed through the research of technical parameters, three working modes and linearization of series compensation. The reasons of the appearance of reverse voltage and reverse current when different types of faults occur at different positions especially the single-phase ground fault are analyzed. The generated condition of Low frequency component and the factors that affect its amplitude and frequency is discussed. Then, the influence of series compensation on distance protection is investigated. The influence of asymmetric breakdown of MOV on zero and negative sequence direction component is discussed according to the fault feature of variables and the self-control protection of series compensation. The directional loss problem occurs when PT is installed on the bus side, forward fault happens and PT is installed on the line side and can be solved by mho relay with memory voltage. The protection scope shortening problem can be solved by the value of "positive breakdown voltage of MOV" and the real-time automatic adjusting of the measured current of the protection. For the wrong operation of zero and negative sequence directional protection caused by the asymmetric breakdown of MOV, it can be solved by setting the threshold of positive sequence current. Based all mentioned theories, a method that the lines are divided into different regions to realize protection, the method of partition protection is proposed based on the fault feature of different regions.