Background: In distribution 6–10 kV networks with an insulated neutral for earth fault protection, zero sequence current directional protection devices are commonly used. According to the operation data, the main disadvantage of such kind of protection is the possibility of their functioning failures in transient conditions with the most dangerous for network intermittent arc earth faults. Therefore, the selection and justification of the implementation principles of zero sequence current directional protection which can provide high dynamic stability of functioning is a relevant objective. Nowadays, new developments of electrical power systems relay protection devices, including earth fault protection of medium voltage distribution electrical networks, are implemented only on a microprocessor base. Therefore, the effectiveness analysis of the implementation principles of zero sequence current directional protection is relevant for modern digital protection devices.
Materials and Methods: When analyzing the dynamic stability of the functioning of zero sequence directional current protection, in view of the complexity of transients during intermittent arc earth faults in medium voltage electrical networks with an isolated neutral; simulation was used in Matlab with SimPowerSystem and Simulink expansion packs. In the context of this study, only transient currents and voltages were considered as the main factor influencing dynamic stability of the functioning of zero sequence current directional protection; the influence of other factors, for example, the errors of the primary zero sequence current and voltage transducers, the scheme of formation of compared quantities, and others, was not taken into account in simulation models.
Results: In terms of the studies, the causes of possible functioning failures of digital current earth fault directional protection in dynamic operation modes are defined. It is shown that the usage of orthogonal components of fundamental frequency of zero sequence voltage and current in current directional protection devices eliminates the failure of their operation with any kind of arc earth faults.
Conclusions: To ensure high dynamic stability of operation under the influence of transients during arc intermittent earth faults, current directional protection, which operates with this type of damage should be performed on the basis of monitoring the phase relationships of the fundamental frequency components of 50 Hz of zero sequence voltage and current, but not their full values.