An improved phase difference correction method considering rate of  change of frequency of the power systems

LIN Shenli， CHEN Longdao

(College of Electrical Engineering, Zhejiang University, Hangzhou 310027, China)

Abstract: Aiming at the problem that the phase difference correction method had a large error and sometimes might fail when the fundamental frequencies of the power signals varied with time, an improved phase difference correction method considering the rate of change of frequency(ROCOF) was presented. The fact that the spectral leakage from asynchronous sampling was the main source of the measurement error for phase difference correction method in power harmonic analysis was pointed out. When the fundamental frequencies of the power signals varied with time which led to a remarkable frequency offset, the accuracy of the phase difference correction method was low because of the asynchronous sampling. The modified normalized frequency correction value based on the ROCOF was deduced. During online measurement, the rate of change of frequency was estimated by calculating the difference quotient of fundamental frequency of last measurement and this measurement. After that, the result of this measurement was modified with the ROCOF. The variablefrequency power signals were simulated using the method considering ROCOF and the original method respectively. The results indicate that the accuracy of the phase difference correction method considering ROCOF is higher than the original method. The accuracy of amplitudes is improved by one order of magnitude and the time length of the window is 50% of that prescribed by IEC standard. This method satisfies the requirements of accuracy and realtime performance in the online harmonic measurements of the variablefrequency power systems.

Key words: harmonic measurement; frequency offset; rate of change of frequency(ROCOF); the normalized frequency correction value; phase difference correction method