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Combined ground-fault relay protection in 6–10 kV power grids

In the present work, engineering decisions for combined ground-fault relay protection in 6–10 kV power grids are addressed. For objects in electrical grids having a voltage of 6–10 kV, simple relay protection was used: overcurrent protection and current cutoff. The most common algorithms for single phase-to-earth fault relay protection were analysed and compared. The modelling allowed a scheme for automatic transfer switch to be designed. Operation algorithms of relay protection and automatics were established, considering the properties of an electrical grids section and the characteristics of possible operating conditions. In 6–10 kV electrical grids, an increase in the damage rate of cable lines with outdated thinned insulation was revealed. It was determined that long-term grid operation under a single phase-to-earth fault should be avoided since double earth faults may occur due to an increase in the voltage of healthy phases relative to earth by √3 times. The calculations performed to evaluate selectivity and sensitivity of the current protection of selective zero-sequence showed limited application (due to the insufficient sensitivity) of such protection from single phase-to-earth fault in some cases. The advantages of circuit designs, which use electromagnetic and inductive relays as protection fault detectors, and advanced compact elements in the form of microcircuits with electronic interconnection as logical elements, were investigated. It is established that the proposed solutions can be used by energy providers to increase the reliability and fail-safety of 6–10 kV distribution networks with the minimum capital expenditures to relaying systems. The presented analysis of systems and devices for phase-to-earth fault relay protection showed that power grids having isolated neutrals are the most problematic in terms of maintenance. Simplified circuit designs for combined protection circuits can be used for facilitating the maintenance of relaying devices and automatics due to their openness.