Study on Asynchronous Self-excitation and Its Protection Measures in Generator

摘要

Series compensation transmission technology will cause SSR, mounting block filters in the power plant is a solution to avoid SSR. In China, A power plant adopted block filters has been found asynchronous self-excitation phenomenon in synchronous generators. Self-excitation in synchronous generator can be divided into synchronous self-excitation and asynchronous self-excitation, when the generator is in a state of excessive capacitive loads, the current and voltage will grow, it is an abnormal operation mode, and it has threatened the safety operation of the generator and power system. This paper analyzes the cause of asynchronous self-excitation and introduces the study of asynchronous self-excitation, also this paper proposes protection technology and its applications. As the modeling shows: asynchronous self-excitation is a kind of resonance, it is only related with electrical parameters of the system. In A power plant which installed block filters, asynchronous resonance self-excitation of sub synchronous frequency range occurred because of the matching of the power system parameters and the block filter parameters. As the impedance of block filters is nonlinear, it performs strongly capacitive characteristics around the resonant frequency, matching with the power system parameters, a series resonant circuit is formed and asynchronous self-excitation is inspired. So, to avoid asynchronous self-excitation, the easier way is to improve the damping of generator and the power system, and adjust quality factor of the resonant circuit to inhibit asynchronous self-excitation. In order to prevent the system from the asynchronous self-excitation effectively, this paper proposes a protection measure, if the asynchronous selfexcitation happens, the device will disconnect the generator from system to protect the unit from harm. According to analysis of this paper: asynchronous self-excitation is a current resonance. After studying the asynchronous current trend, this paper puts forward the following three protection criteria: criterion of divergence protection judges the asynchronous current trend and then exports the control signals; criterion of inverse-time over current protection starts to cumulate if asynchronous current is oscillating or diverging, when integrated value exceeds the set value, it acts; criterion of definite time over current protection works as backup protection, once asynchronous current value is greater than set value, it trips. Based on the above ideas, this paper designs an asynchronous selfexcitation protection device.At present, the study results - asynchronous self-excitation protection device has been applied to practice for monitoring and protecting the system from asynchronous selfexcitation. Since the device was put into operation, the device has not only monitored multiple asynchronous self-excitation phenomenons, but also recorded valuable experimental data for the system. In the meantime, it also proves this study is reliable and practical.