Study On Power System Delay Margin

Modern power systems are featured with extra high voltage level, long-distance transmission lines, large capacity generators, spaciously inter-connections and huge inter-area exchanges. Any instability accidents may cause enormous losses and catastrophic results. So studies on the power system security and stability have been paid much more attention nowadays. Recently, PMU/WAMS (Phasor Measurement Unit/ Wide Area Measurement System) system based on the so-called GPS technique has developed rapidly. It can monitor the large scope power system dynamics precisely and is very suitable to the power system coordination control. However, there is obvious time delay in the measuring data of WAMS, ranging from tens to several hundreds of milliseconds. And past studies revealed that the time delay had soundly impact on power system stability analysis and controller design. So it should be properly considered. In this thesis, we mainly focus on determining the system time delay margin, which means the maximum delay time that system can sustain without losing the small signal stability. The main work of this thesis is as following:Firstly, necessary theoretical background, such as small signal stability for the normal dynamic system and time-delay system, is briefly reviewed. The characteristic equation of the time-delay system is deduced. Since the characteristic equation of the time-delay system is transcendental, it is very difficult to directly solve and perform the stability analysis.Secondly, an effective method to determine system delay margin is introduced. The transcendental characteristic equation of the time-delay system is first transferred into a polynomial equation based on Rekasius transformation. Rekasius transfor -mation is identical when there is a system eigenvalue on the imaginary axis. Further, Routh stability criterion is used to determine the critical eigenvalue, the critical oscillation frequency and the system delay margin.Finally, a single-machine-infinite-bus system and a 3-generator-9-bus system are employed to do some verification studies. It is found that when the generation output and exciter gain turn large, system delay margin decreases. While, the interaction between generator damping and system delay margin is very complicated.