Precise measurement of power system frequency and phasor using SDFT and comparison with DFT based approach

摘要

Power system condition is reflected by frequency and phasor more accurately. State of power system is shown by phasor while real time balance between generation and load is reflected by frequency. Frequency undergoes variation from the standard value when there is a difference between generation of electrical power and load. Some power system protection and control applications, e.g., frequency relay for load shedding, load-frequency controller requires precise and fast estimation of the frequency. So frequency and phasor regarded as indices for the operating power system in practice. To gain efficient and safe operation of power system caused due to any significant variation from nominal frequency forces protective devices to take corrective actions. As phasor is important parameter in the frequency monitoring and control of power systems and it is not constant at off-nominal frequencies, its measurement at every instant in power system network with greater accuracy and fast speed is necessary. Discrete Fourier Transforms (DFT) is one of the most important tools in Digital Signal Processing (DSP). It can calculate signal's frequency spectrum, it can find system's frequency response and it allows systems to be analyzed in the frequency domain. Conventional DFT method produces errors in calculating phasor and frequency if frequency varies from standard value. Smart Discrete Fourier Transforms (SDFT) has all merits of DFT such as recursive computation, ease of implementation and real time suitability. SDFT also takes care of deviations in frequency, harmonics and noise. This paper presents a SDFT based algorithm for measuring frequency, amplitude and phase angle of an ac signal and its comparison with conventional DFT method. Different signals in the presence of noise and harmonics are tested with the presented algorithm.