Terms and conditions of single phase to earth fault current full compensation in 610 kV cable networks with neutral point grounded via arc suppression coil

Abstract

To improve the reliability of power supply of consumers, a significant part of 6–10 kV cable networks (in Russia about 20 %) operate with resonant neutral grounding via arc suppression coil (ASC) (single-phase earth fault capacitive current compensation). This neutral grounding mode provides suppression of arcing earth faults accompanied by dangerous overvoltage and reduction of current at the point of fault by compensating the capacitive component of the elementary frequency. Field experience of operation of 6–10 kV compensated cable networks has shown that the reason of the decrease of the efficiency of the neutral grounding mode is an increase of the residual current at the point of fault. It happens due to the active component of the elementary frequency and higher harmonic components that are not compensated via ASC up to the values when long time arc duration and transition of the earth fault to the short circuit become possible. The existing solutions of the problem of full compensation of the current of a single-phase earth fault are not sufficiently developed and substantiated. Thus, the solution of the problem to increase the efficiency of grounding the neutral of the network via ASC can be achieved by developing and implementing methods and devices of the full compensation of the earth fault current, including the active component of the elementary frequency and higher harmonic components. To achieve the terms and conditions of full compensation of the current components of a single-phase earth fault, the classical theory of electrical circuits has been used. To verify the validity of the results obtained, simulation modeling in the Matlab software package has been carried out. For an electrical network with neutral grounding via ASC, the authors have obtained a general mathematical description of the terms and conditions of full current compensation at the point of earth fault, including higher harmonic components and transient components. The authors have defined the requirements for accuracy to determine the moment of a single-phase earth fault and the voltage value of the damaged phase when insulation fault occurs. Possible options of algorithms to compensate higher harmonics in the current of stable earth fault are proposed. The obtained general mathematical description of the terms and conditions of full current compensation at the point of a single-phase earth fault in 6–10 kV compensated cable networks allows us to substantiate existing methods and develop new ones to compensate both the total current at the point of fault and its components.