A Flexible Over-Frequency Generator Tripping Method Based on Power Flow Tracing

Abstract

With the rapid development of DC transmission in recent years in China, frequency stability has become a key issue for the normal operation of the entire asynchronous interconnected electric system. When the external DC transmission line experiences a fault, the frequency of the electric grid at sending end will inevitably rise, which may further cause frequency stability problem. Therefore, in order to prevent system frequency instability, over-frequency generator tripping devices are widely installed in major power plants. This paper proposes an over-frequency generator tripping scheme based on the power flow tracing technology. Some significant strategies including predetermination of maximum generator tripping capacity in each round, calculation of the power flow contribution level for each generator, sorting and grouping of generators to be tripped, optimized proportion of generator tripping, and the inspection of over tripping phenomenon are discussed elaborately. Finally, the validity and effectiveness of the proposed strategy is verified by conducting case studies. Introduction After the asynchronous interconnection through DC transmission, frequency stability will become one of the main factors threatening the safe and stable operation of the power grid [1]. It is known that the power system frequency is closely related to the active power, and the frequency change is affected by the generator output adjustment, the load power fluctuation and so on. When the power supply provides more active power than the load demand, the system power is excessive, and the frequency will increase. On the other hand, when the power supply is less than the load demand, the system power is insufficient and the frequency will decrease. Due to the constant change of load power, and coupled with the uncertainty of the current large-scale access to new energy power generation devices such as wind power and photovoltaic energy, the power of the entire power system is in a real-time dynamic balance. In normal operation condition, although the system frequency fluctuates, it does not change much and remains stable at around 50 Hz. Frequency stability is the premise of stable operation of power system. After large-scale DC transmission projects have been put into operation one after another, the current power grid presents a trend of asynchronous interconnection. At this time, if the DC line fails, it will cause a large power surplus and a series of frequency stability problems. As a measure of stability, the over-frequency generator tripping device monitors the system frequency above the allowable frequency, and then cuts off some generators after a short delay, so that the entire system frequency can be restored to the power frequency 50Hz. Traditional over-frequency generator tripping measures are limited to a specific historical background, and bear the following shortcomings [2,3,4]: 1) Based on subjective experience and lack of theoretical guidance. 2) Poor flexibility and not suitable for different modes of operation. 3) Easy to cause overcut or undercut of generators. 2nd International Conference on Electrical and Electronic Engineering (EEE 2019) Copyright © 2019, the Authors. Published by Atlantis Press. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/). Advances in Engineering Research, volume 185