Мultifrequency Protecting Method Against Earth-Faults of Phase in the Compensated Electric Networks

Abstract

Introduction. A significant proportion of earth faults in 6-35 kV networks is a transient and short-lived process, which is followed by an electric arc. Problem. In such cases, earth-fault protection that responds to steady-state current and voltage is not able to operate properly. Also, the use of the Petersen coil to compensate for the capacitive earth fault current complicates the protection function because it significantly reduces the single phase earth fault current in steady state. Purpose. To develop selective single-phase earth faults protection algorithm using harmonic components that occur in zero-sequence currents and voltage in the transient process. Method. A mathematical model of the power supply system is applied to study the frequency components of currents and voltage of zero sequence in compensated electrical networks with phase-to-earth faults, and a mathematical model is used to test the operation of the developed protection algorithm. The results showed that, the reactive power for harmonic components of the frequency greater than 100 Hz, which are separated from the current and voltage of zero sequence in compensated electrical networks on the damaged feeder, is positive regardless of the degree of compensation of the capacitive current. That may be the basis of the principle of directional protection. Originality. Phase-to-earth fault selective protection algorithm has been developed. In that algorithm, first derivatives of currents and voltages of zero sequence are found, to reduce the influence of aperiodic components. And then, by using of the Fourier transform, a number of harmonic orthogonal components are extracted from them. Reactive power is calculated for each of frequency component and their total sum is found. If that sum excess of threshold, the relay will make a decision. The reliability of the developed protection algorithm is confirmed by the results of mathematical modeling and verification of the test sample at the laboratory stand and by means of field signals that were recorded by digital loggers at the substations. References 10, table 1, figures 6.

Improving the Efficiency of Fault Protection Systems of Electrical Grids Based on Zero Sequence Voltages and Currents Wavelet Transforms

“…In particular, in [5] it is proposed to use digital filters with infinite pulsed response to extract the information parameter from currents and voltages of zero sequence. In [6], it is proposed to use the Fourier transform to obtain the phase and amplitude characteristics of the voltage and current of zero sequence. In [7], the properties of Fourier transform and wavelet transform are compared on the example of electric arc detection.…”

“…The neutral of the grid of the first section is grounded through a reactor with resonant inductance of 0.161 H, and the phase capacitances to ground for feeders Ф1-Ф3 connected to this section are 1, 8 and 12 μF, respectively, interphase capacitances are 2 μF, steady-state earth fault current is 72.3 А. The solution of differential equations in the mathematical model is performed by the implicit method with a calculation step h = 0.667 ms The frequency [5] and multifrequency [6] protections against SEFs proposed in recent years are based on the use of higher harmonics in currents and voltages that occur in the first moment of the fault and therefore depend on the instantaneous value of the phase voltage on the damaged phase. The degree of distortion of sinusoidal voltages and currents depends on this voltage, which can be seen, for example, from Fig.…”

“…In 6-35 kV electrical grids operating with neutral isolated or grounded through a Petersen coil, the transients in arc phase-to-ground faults are transient, and therefore there are questions about the correctness of the application of protection algorithms based on the use of frequency [5] and multifrequency [6] components in currents 3i 0 and voltages 3u 0 of zero sequence obtained by means of Fourier spectral transforms. For such processes it is more rational to use wavelet transforms (WTs), which allow to decompose the input, time-varying signal x(t) into frequency-time components in the form of coefficients CW(a,b,t) which depend on the selected mother function g(a,b,t) as well as on the coefficients of frequency scale a and time shift b [8].…”

“…From the obtained expression we select one of the components of the mother function g(t), for example, imaginary (sine) gS(n). Then the expressions for determining the k-th values of the coefficients of wavelet transforms will look like: Note that the frequency scale coefficient a is used to set the wavelet duration in the region of the desired frequency range f n , the value of which we take equal to 250 Hz, which corresponds to the preferred harmonics at arc faults [6]. As can be seen from (4), the wavelet transform is a convolution of the values of the input signal (current, voltage and their derivatives) with the same number of values of the mother wavelet function.…”

“…5,c) at zero initial voltage. Compared to multifrequency protection based on Fourier transform [6], the developed protection has sensitivity almost an order of magnitude higher.…”