Design Of A Phase-Selective Closing System For Closing Oscillations In Rural Power Grid

With the continuous development of China’s rural power grid,the problems that arise in the operation of the grid are becoming more complex.Among them,in rural power grids,the long length of distribution lines,large load variations and limited power supply capacity can lead to strong grid oscillations during closing operations,posing a considerable challenge to the safety and stability of the grid.To solve this problem,the voltage initial phase angle of the circuit,power factor angle and frequency were determined to be important parameters affecting the closing effect through short-circuit experiments of switching appliances.The FFT and Hilbert transforms were applied to the signals respectively to calculate the closing parameters of the signals.By comparing the accuracy of the calculated parameters between the two methods,the Hilbert transform,which has a higher calculation accuracy,was selected to process the signals.By comparing the accuracy of the two methods,the Hilbert transform is chosen to process the signal with a higher accuracy.The shortcomings of the Hilbert transform in calculating the closing parameters with endpoint effects are proposed.A modular hardware design is carried out for the phase selection and closing system,the algorithm for calculating the closing parameters is written into the PLC and a touch screen display interface is designed.The main elements are as follows:(1)The FFT transform and Hilbert transform are used to find the closing parameters for the signal respectively,and the accuracy of the two algorithms for calculating the closing parameters is compared when the signal frequency fluctuates in the range of 49.8-50.2Hz.When the signal has +7% amplitude fluctuation and 25 d B noise disturbance,the calculation accuracy of the FFT transform and Hilbert transform for the disturbance is analysed.(2)To address the problem of endpoint effect in the Hilbert transform process,which leads to signal spectrum leakage,the solution of adding windows to the signal first is proposed.After adding the rectangular window,Blackman window,Hanning window and Hamming window to the signal,the spectral leakage of the signal is analysed.The Hanning window has the best effect in suppressing the spectral leakage,so the Hanning window is chosen to add a window to the signal.The calculated results of the Hilbert transform of the closing parameters were analysed before and after the windowing of the signal when the signal frequency fluctuated in the range of 49.8-50.2 Hz.(3)A modular hardware design is used for the phase selection and closing system.The S7-1200 CPU 1211 C AC/DC/RLY PLC was selected as the control core for the phase-selecting and closing device to process the sampled data.A sampling module circuit was designed with a CHB-25 NP current sensor and a CHV-25 P voltage sensor as the core.The data sampled by the sensors is transformed by a conditioning circuit into a signal amplitude that can be processed by the PLC.Drive module to SCR as the core,design drive circuit in the PLC output closing command,the drive module can quickly and accurately control the circuit of SCR closing.Design touch screen interface,through the touch screen can issue a closing command and access to signal frequency and power factor angle,and can view the real-time voltage signal and current signal waveform before and after the closing.(4)The analysis of the test effect of the phase selection and closing parameters shows that the Hilbert transform algorithm with Hanning window has high accuracy of frequency and initial phase measurement when the frequency fluctuates;the simulated load experiment shows that the designed phase selection and closing system has good overall execution effect of phase selection.The phase selection and closing system has high phase selection accuracy.