Improvement of the System for Suppressing Ferroresonant Processes in 10 kV Electrical Grids

Abstract

Ferroresonant processes (FRP) in 10 kV electrical grids with isolated neutral are considered. These processes occur between the network’s capacitance to ground and the nonlinear inductance of the grounded primary windings of voltage transformers (VT) at the moment of eliminating a single-phase earth fault in the network. In most cases, FRPs in electrical networks are self-extinguishing within several cycles of the power frequency. However, under certain ratios of the network’s capacitive earth-fault current and the voltage transformer parameters, the FRP can become stable (long-lasting) and cause thermal damage to the primary windings of VTs and other electrical equipment.

The duration of such FRPs is limited by the operation time of the devices designed to prevent (suppress) them, or, in their absence, by the time until VT damage occurs or the network configuration changes. To suppress stable FRPs and protect VTs from damage, protection devices are used, their action is directed on the connection of a damping resisting to the open-delta winding of the VT. This principle has been implemented, in particular, in PZF-type protection devices, which have been in operation in 10–35 kV electrical grids for more than 25 years. Over the years of PZF operation, cases of ineffective suppression of FRPs in networks have been recorded. Such events occurred due to changes in the network configuration and, accordingly, the number of operating VTs equipped with PZF devices.

FRPs were studied in 10 kV radial electrical grids with different numbers of operating VTs, and the ranges of capacitive earth-fault currents at which FRPs are stable were determined. It has been established that each number of VTs in 10 kV networks corresponds to its own range of capacitive earth-fault currents. As the number of VTs in the grid increases, the ranges of capacitive earth-fault currents at which stable FRPs occur also expand.

Method for suppressing stable FRPs in 10 kV electrical grids has been proposed, and its effectiveness has been investigated in grids with the isolated neutral. The essence of the method is based on the short-term connection of an active resistor to the neutral of a power transformer. The proposed method for suppressing stable FRPs in 10 kV isolated-neutral electrical networks, as the studies have shown, is effective regardless of the number and type of VTs in the network, as well as regardless of whether FRP suppression devices are installed or absent. Based on the results of the study, the optimal resistance value was determined, the connection of which to the neutral of the power transformer leads to the suppression of stable FRPs. Principle for the technical implementation of the developed “power transformer neutral–resistor” method has been developed.