Formation of the Signal Amplitude in Digital Relay Protection Devices when the Frequency Deviates from the Nominal One

In digital relay protection systems, the controlled parameters of signals are often their amplitude values. They are usually determined from samples of the orthogonal components of the signals, which are formed by nonecursive digital Fourier filters. At a normal frequency, the amplitude values are determined without additional error. In modes with frequency deviation from the nominal frequency, fluctuations in the received amplitude values occur in the range from minimum to maximum levels. Due to the use of Fourier filters for the formation of orthogonal components, the amplitude determination time is at the level of the power frequency period. The previously developed method of forming the signal amplitude ensures that it does not oscillate at a frequency other than the nominal one. It is based on the use of dynamic cosine and sine of the sampling angle, which are calculated from the instantaneous values of the orthogonal components. When the frequency deviates from the nominal frequency due to the resulting oscillations in the samples of these components, the frequency range of reliable obtaining of dynamic cosine and sine is limited, which causes obstacles to determining the amplitude with an acceptable error. Fast determination of the signal amplitude is achieved through the use of a nonlinear correction factor in the procedure for its formation. However, it is cumbersome to obtain it, taking into account the possible nature of the change in the controlled signal. In this paper, the signal amplitude is determined as the half-sum of the amplitudes of the sine and cosine orthogonal components. At the same time, frequency deviation from the nominal value does not cause significant changes in the magnitude of the controlled amplitude. The use of a transition characteristic for the amplitude of the signal, consisting of sections close to linear, made it possible to simplify the method for determining the mode of signal variation. The conducted computational experiments have confirmed that the developed method for determining the signal amplitude is quite simply implemented at the program level and prevents its oscillations in the frequency range of 45-55 Hz. The proposed technique ensures that the amplitude value of the signal is obtained in less than half of the power frequency period.

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