A Simple Approach to Determine Power System Delay Margin

Since there exist significant time delays in the information of phasor measurement unit (PMU) and wide area measurement system (WAMS), it is an important issue to properly evaluate the influence of time delay in power system stability study and controller design. In this paper, we mainly focus on the determination of the maximum time delay that system can sustain without losing small signal stability. Here we denote the maximum time delay as delay margin. Based on Rekasius substitution and Routh stability criterion, a simple approach to determine power system delay margin is introduced in this paper. Rekasius substitution is first used to transfer the system transcendental characteristic equation into a polynomial equation. Then Routh stability criterion is applied to determine the critical eigenvalue, the corresponding oscillation frequency and the system delay margin. Finally, a single-machine-infinite- bus system is employed to do some verification studies. The correctness and effectiveness of the proposed approach are proved. Further, impacts of the exciter gain, generator mechanical output and generator damping to the system delay margin are deeply discussed. It is found that when the value of generation output and exciter gain increase, system delay margin will decrease. While relationship of generator damping and the delay margin is somewhat complex. Smaller generator damping does not mean smaller delay margin. Since time delays widely exist in the WAMS measurement, more studies should be done to properly consider their influence in future.