Research On Synchronized Phasor Measurement Algorithms In Power System

With the scale expanding of the interconnected power grid and the increasing of the installed capacity, the security and stability monitoring and control of power system is more and more important. Wide Area Measurement System (WAMS) based on synchronized phasor measurement technology can monitor and analyze the running state of power system. WAMS has important significance for reinforcing the reliability of power system. With the continuous development of the wide area measurement technology, the wide area measurement technology has been widely used in power system and improved the capability of safe and stable power system operation. The phasor data of power system is a significant parameter which can describe the dynamic running state of power system. Phasor Measurement Unit (PMU) is the basic unit of WAMS. Since PMU can measure the phasor data of power system, PMU provides an important tool for the real-time monitoring of power system. Since excellent synchronized phasor measurement algorithms improve the measurement accuracy and calculation speed of PMU, research on synchronized phasor measurement algorithms has important significance for improving the performance of WAMS.The common synchronized phasor measurement methods are zero-crossing detection method and discrete fourier transform (DFT) algorithm. The principle of DFT algorithm is comparatively simple. The realization of DFT algorithm is relatively easy. And DFT algorithm has comparatively good inhibition for harmonics. As a result, DFT algorithm has been widely used in the synchronized phasor measurement in power system. But when power system frequency changes and deviates from rated frequency, and the sampling frequency and the power system frequency are no longer synchronous, the spectrum leakage of DFT algorithm causes synchronized phasor measurement errors.On the basis of the variable window DFT algorithm based on fixed frequency sampling, this paper uses a method based on three sampling points to measure power system frequency. Then the DFT window is varied according to the frequency variation to compute the phasor. The calculation quantity of this algorithm is comparatively small. And this algorithm has certain advantages in calculation speed. Comparing with the traditional DFT algorithm, this algorithm reduces the error caused by spectrum leakage when power system frequency deviates from rated frequency, and the synchronized phasor measurement accuracy improves.On the basis of the analysis of the error of the traditional DFT algorithm caused by nonsynchronous sampling, this paper proposes a modified DFT algorithm based on fixed frequency sampling to further improve the synchronized phasor measurement accuracy. This algorithm makes use of two consecutive synchronized phasor values calculated by the traditional DFT algorithm and accurate frequency measurement to realize the modification of the synchronized phasor measurement results calculated by the traditional DFT algorithm. The calculation quantity of this algorithm is relatively large, but the measurement accuracy is further improved.Since the calculation frequency of PMU can be much lower than the sampling frequency, an adaptive DFT algorithm based on fixed high frequency sampling is proposed at the end of this paper. This algorithm calculates synchronized phasor with a frequency which is much lower than the sampling frequency, and chooses the data sampled with a fixed frequency by PMU according to the equal angle sampling principle before the DFT calculation. This algorithm relatively accurately calculates the synchronized phasor and solves the problem about the data resynchronization of the traditional adaptive sampling algorithm.