Commutation Failure Studies Of The First ±660kV Transmission Project

HVDC transmission system has many technical and economic advantages in large-capacity long-distance transmission and regional network connection. Up to now, more than ten EHVDC/UHVDC projects, such as±800kV XiangShang DC line project and±660kV Yindong-Jiaodong DC line project, have been built up and put into operation. China has become a country with the world's largest capacity, fastest growing, highest voltage level in HVDC transmission. But it also brings many new technical problems at the same time. Commutation failure is one of the most common faults in HVDC projects. It may cause undesirable consequences such as DC voltage reduction, temporary interruption of transmitted power, over-voltage of weak AC systems, and DC blocks due to repeated commutation failures. Yindong-Jiaodong HVDC line is the first±660kV HVDC project in the world and it has many features different from previous DC projects. Take its converter transformer short-circuit reactance for example, it is larger than that of existing±500kV HVDC projects, which makes commutation failure more easily to happen. Therefore, it is important to study commutation failures of Yindong-Jiaodong HVDC line.This thesis presents a comprehensive study of Yindong-Jiaodong HVDC line commutation failures from several aspects, including simulation criteria, electromagnetic transient simulation, electromechanical transient simulation, preventive strategies, and so on. The main work and achievements are as follows:Firstly, commutation failure simulation tools and commutation failure simulation criteria are compared and discussed. Theoretical analysis shows that, the commutation failure threshold voltage is definite to a fixed critical extinction angleγct, when the HVDC operating condition is fixed. A specific threshold voltage can be used to determine whether a commutation failure will occur under specific HVDC operating mode.Secondly, the electromagnetic transient simulation program PSCAD/EMTDC and electromechanical transient simulation program PSS/E are jointly used to analyze the influence of Shandong Power Grid AC faults to commutation failures of Yindong-Jiaodong HVDC line. At first, the electromagnetic transient simulation platform of Yindong-Jiaodong HVDC line is built and AC fault simulations are performed. In this way, the commutation failure critical voltage under symmetrical faults is 0.887p.u., when Yindong-Jiaodong HVDC line is in bipolar operating mode. Then the threshold voltage is used in the CDC6T model of electromechanical transient simulation program to perform Shandong Power Grid AC fault simulations. Simulation results show that most 500kV line faults of Shandong Power Grid cause commutation failures, besides the part west from the line of Yuncheng 500 Station, Liaocheng Plant 500 Station and Dezhou 500 Station; Parts of 220kV line faults will cause commutation failures, which includes 220kV lines in Qingdao,220kV lines near Laiyang 500 Station in Yantai and 500 stations in Weifang, and a few of 220kV lines in Rizhao and Zibo area. Commutation failure area caused by 220kV faults significantly shows a trend of distributing along the 500kV lines. After that, by further building the detailed electromagnetic transient platform of Shandong 500kV grid, independent electromagnetic transient simulation of 500kV line faults are performed. It shows that the commutation failure probability of electromagnetic transient simulation is less than 100%in critical commutation failure areas, where the postfault voltage is near the threshold voltage. Besides these areas, the conclusions of these two methods are consistent about the result that whether a commutation failure will occur after AC faults. Meanwhile, the influence of Shandong Power Grid AC unsymmetrical faults to commutation failures of Yindong-Jiaodong HVDC line is discussed.Finally, commutation failure prevention measures and their effect are discussed. Through increasing the capacitor shunts of converter station, improving the speed of fault clearing and sending out trigger pulse in advance, parts of Yindong-Jiaodong HVDC line commutation failures caused by AC faults can be avoided. It should be considered that coordinate with other measures to improve the preventive effect, and take appropriate control measures to avoid continuous commutation failres.In this thesis, a combined simulation method using the electromagnetic transient program and the electromechanical transient program is presented. The electromagnetic transient program is used to determine the commutation failure threshold voltage, and this value is applied in the electromechanical transient program to study commutation failures of large scale AC-DC hybrid systems. This method, which reserves the simulation size and speed advantage of the electromechanical transient program and the accuracy advantage of the electromagnetic transient program, has strong engineering practicality and is beneficial to better analyze commutation failures of Yindong-Jiaodong HVDC line.