New method of searching for the out-of-step separation interface based on reactive power

When a multi-machine system losses synchronism, it may be separated into two or more electrical islands along with the separation interfaces, which are generally composed of several tie-lines. The characteristics of the separation interface tie-lines are as follows. First, the active power on these lines crosses zero and oscillates periodically. Second, there exits one point that is called the out-of-step center on the separation interface, whose voltage fluctuation amplitude is significantly larger than that of other points. The farther away from the out-of-step center, the smaller the fluctuation amplitude is. An obvious voltage gradient shows on the separation interface during the process of out-of-step. Third, the reactive power flows into the interior from the both sides of the separation interface. The integral value of reactive power in one period of out-of-step can reflect the characteristics of separation interface. Based on the above characteristics, a novel theoretical method of capturing the separation interface is presented in this paper.