Earth fault protection for compensated electric networks based on frequency filters

Syvokobylenko, V. F.; Lysenko, V. A.

Purpose. Development of selective microprocessing earth fault protection in compensated electrical networks of 6 - 35 kV. Use of the Goertzel algorithm for the separation of components of frequencies higher than fundamental frequency from the zero sequence voltage and currents. Methodology. The methods of mathematical modeling and analysis of transients in compensated electric networks of 6 - 35 kV, Fourier transform and Goertzel algorithm, construction of earth-fault protection algorithms, and experimental research are used. Findings. The results of mathematical modeling show that in compensated electric networks of 6 - 35 kV at phase-to-ground fault the reactive power direction in a in a damaged feeder1can be similar to the one in an undamaged feeder; therefore, conventional protections cannot perform selective operation. This action can be provided by protections based on separation of components of fixed frequencies of 200 - 300 Hz from the zero sequence voltage and currents. Their reactive power in an undamaged connection is always directed off tyres, because is hardly compensated by the reactor regardless of degree of its cavity tuning. When applying band-pass frequency filters, the required protection responsivity is not always provided at phase-to-ground fault through resistances exceeding 10 - 15 Ohm. Moreover, failure of steady operation of filters is possible due to location of their poles on a circular curve of a unit radius, while use of blocks of numerical differentiation of current and voltage can result in running failure at alternate arcing ground. The application of the Goertzel algorithm for high-frequency components detection is proposed. It is implemented through an infinite impulse-response filter of the second order with two real coefficients in back coupling and one complex coefficient in circuit of direct relation. The results of simulation of the behavior of protection in the system of computer algebra Mathcad confirmed the reduction of calculation costs and stable work, regardless of the presence of aperiodic components at different initial phases of the voltage at the time of fault. As part of the protection, a trigger unit is provided that allows the operation if the voltage of the zero sequence exceeds the setpoint, which is 12 - 15 % of the nominal value. In order to ensure reliable operation of the protection, permanent and arcing faults provide activation of protection impulses in the event of a fault. The positive results of protection algorithm were achieved on the mathematical model of the network as well as when testing a microprocessor prototype of a protective system in a laboratory environment. Originality. For the first time for ground-fault protection in 6 - 35 kV compensated networks, it is proposed to separate components of frequencies higher than fundamental frequency from the zero sequence voltage and currents using frequency filters created on the basis of the Goertzel algorithm are proposed; the algorithm is an improved modification of the Fourier transform; this increased the sensitivity, speed and stability of work protection. Practical value. The mathematical model of the compensated electric network allows analyzing the behavior of protection at permanent and arcing ground faults. Selective protection from phase faults on the ground has been developed, which allows increasing the reliability of the power supply systems with a voltage of 6 - 35 kV.


	Наукова періодика України		Naukovyi visnyk Natsionalnoho Hirnychoho Universytetu