The Mechanism And Control Methods Of The Faults Occurred At The Marginal Zone Of The Interconnected Grid Damaging The Inter-Region Stability

Along with the operation of the1000kV Jindongnan-Nanyang-Jingmen Ultra High Voltage (UHV) AC Demonstration Project, the North China grid and the Central China grid became one synchronized interconncected system through the UHV line. It was found in a number of simulations that short-circuit faults occurred in Jianshan substation and its neighbourhood in Sichuan grid, which was the marginal zone of the interconnected grid, would result in significant power fluctuation on the UHV line and even system splitting. However, the faults occurred in Hubei grid which located near the UHV line only have much less effects on system stability. Further, the simulation results also indicated that the values of short-circuit current of the buses near the UHV line were larger than that of the buses in the marginal zone. This phenomenon is divergent from the traditional view. With the problem, the transmission capacity of the UHV line could not be brought into full play, and there was risk which threats the security and stability of the interconnected grid as well.This paper studied the phenomenon from four aspects, which were analyzing the influencing factors, bringing in the branch potential energy method, studying based on a3-generator equivalent system and proposing control methods. The main work was as follows:1. The details of the phenomenon was stated. Then a number of simulations were made in different modes of operation to analyze various factors which had effects on it, and a preliminary pattern of the factors affecting system stability was concluded.2. The branch potential energy method was introduced to analyze the problem. This paper expounded the reason of faults in marginal zone resulting in system splitting by studying the mechanism from the aspects of identifying the vulnerable cut set and analyzing the spreading progress of the transient energy respectively. The results indicate that there are two main factors. The first factor is the transient energy collection on the UHV interconnected line caused by the faults in the marginal zone. The second factor is that the UHV line is the critical cut set of the interconnected system. 3. After analyzing the long-chain structure of the interconnected grid, this paper built a3-generator equivalent system. Based on this system, the effects of system characteristics changing were quantified with critical clearing time index. The analysis indicates that the post-disturbance generator group angle swing in the regional grid can affect the amplitude of equivalent potential and then lead to the change in the transient stability of interconnected systems. When the marginal grid is under the power transmission mode, the faults occurred in it make the effects on system stability more serious, but the faults occurred in the central zone only have less effects.4. This paper proposed a series of control methods to restrain the effects of the faults in the marginal grid based on the ideas of restraining the impact of the faults and enhance the ability to resist faults impact of the vulnerable links. Then simulations based on the actual interconnected grid were made, which certified the validity of those control methods.