Research On Power Harmonic Measurement In The Frequency Dynamic Process Based On FFT

With the widespread application of nonlinear equipment,especially in recent years,a large number of new energy grid project put into use,the power system harmonic pollution is becoming increasingly serious.Accurate and real-time measurements of power harmonic parameters are the prerequisite and important technical means to control harmonic pollution.The harmonic measurement methods based on Fast Fourier Transform(FFT)have been widely used in on-line monitoring of power harmonic due to their advantages such as fast calculation speed and easy engineering realization.However,the active power outputs of new energy power generations such as wind power and photovoltaic power are uncertain,which may cause the generation and load imbalance,which will lead to the offset of the frequency of the power system,and even the wide frequency fluctuation and frequency collapse.In the frequency dynamic process,there are some problems such as spectral leakage from asynchronous sampling,which will lead to remarkable errors when using the power harmonic measurement methods based on FFT.And worse,the measurements may fail.The improved methods under asynchronous sampling are divided into two kinds according to the sampling method:the constant sampling and the adaptive sampling.The sampling rate of a constant sampling method is constant,and the effect of the asynchronous sampling is reduced by algorithms such as interpolation in the time domain or the frequency domain.An adaptive sampling method adaptively adjusts the sampling rate by tracking the frequency of the nodal point in real time so that the actual sampling sequence is close to the ideal synchronous sampling sequence.The adaptive sampling methods include hardware quasi-synchronization and software quasi-synchronization.The harmonic measurement methods based on constant sampling and adaptive sampling are improved respectively considering the frequency dynamic process.For the constant sampling methods,the rate of change of frequency in the dynamic process of frequency is considered.Taking the phase difference correction method as an example,the modified normalized frequency correction value based on the rate of change of frequency is deduced,which helps to improve the accuracy of the power harmonic parameters of continuous measurement in the frequency dynamic process.For the adaptive sampling methods,the real-time fundamental frequency of the nodal point measured is predicted according to the fundamental frequency of the previous measurement and the rate of change of frequency calculated in the previous measurement during the continuous measurement.The sampling rate is adjusted according to the predicted fundamental frequency in time to make the sampling rate match the real-time frequency of the nodal point,which helps to reduce the spectral leakage caused by the frequency dynamic process.The power signals with different frequency variation models are simulated and analyzed.The total system error is evaluated by considering the accuracy of the transformers.The results show that the proposed methods have higher accuracy and real-time performance than the original methods,and are more suitable and practical for harmonic measurements in frequency dynamic process.