A New Modified Down-sampling DFT based Phasor Estimation Techinque for Power System Protective Relays

Decaying DC offset poses a significant challenge in power systems, especially during fault occurrences. Short circuits generate this Decaying DC offset (DDC) in fault current waveforms. The presence of DDC offset in protection and control systems can lead to system failures or incorrect operations. Phasor estimation techniques based on Discrete Fourier Transform (DFT) encounter difficulties in accurately and promptly extracting fundamental characteristics from fault signals due to decaying DC components (DDCs). This complicates the reliable and swift extraction of essential information by protective relays during fault situations. This paper proposes a modification down-sampling algorithm based on DFT, which is derived from the Multiple Delayed Signal Cancellation (MDSC) to estimate the fundamental phasor current signal in a faulted high power transmission line containing one or more Decaying DC (DDC) components such as a primary DDC or secondary DDC caused by the current transformer. Given the improvement in the accuracy of phasor measurement and the elimination of multiple DDCs, a full cycle and an even sample set combination of DFTs is proposed. The DDC error can be obtained by subtracting the full cycle DFT from the even sample set DFT. This method is immune to multiple DDCs. Numerical and simulation tests are implemented to validate the proposed technique, and its performance is compared with four related phasor estimation methods. In the presence of DDCs with the hindrance of noise and harmonics, the proposed method performs well, and the test results show that it can attain fast and accurate phasor estimation. This method is even suitable for eliminating the secondary DDC caused by the current transformer.