Gusev

The aim of this research is to enhance reliability of power grids with single-phase shielded cables by preventing transition from a single-phase fault to a line-to-line short circuit. Arc-flash processes during a single-phase fault and sheath failure were analyzed in this study. Maximum duration of single-phase faults and its dependence on the method of cable installation were determined. The results of the study are important for estimating failure effects of single-phase faults on cables, for reducing maintenance and restoration costs of cable network. A mathematical simulation of electromagnetic and heat transient processes in a single-phase fault point was carried out using ELCUT software. The accuracy of the model was proved by a number of laboratory experiments with arcing short circuits. It was established that a single-phase fault transforms into a line-to-line short circuit for several seconds in case of a triangle configuration of three single-phase shielded cables with no gap between them. In order to prevent this process it was recommended to apply single-phase shielded cables of 6 – 35 kV to grids with low-resistance neutral grounding and with fast single phase-to-earth faults.

One way to reduce fault currents in a high-voltage power network is application of current limiting coils. However, mounting of current limiting coils influences transient processes in case of short circuits. Rates of transient recovery voltage development at the contacts of a switch in case of commutation in circuits with current limiting coils exceed permissible values. In order to break a circuit successfully, it is required that the rate of recovery voltage in a switch circuit should not exceed the limiting normalized values permissible for the specified switch type. This paper considers the influence of current limiting coils’ properties, such as inductivity and natural resonance frequency, on transient recovery voltages.

 Currently, operation experience of vacuum and gas-insulated circuit breakers substituting oil ones in the  chain "generator - transformer" is not enough. Therefore, in each case, installation of switches of a new generation requires an analysis of the switching processes. For conducting a research, mathematical models of the block scheme for low power hydro-plants and switches of various types among EMTP-RV were created. The authors estimated the levels of overvoltage occurring in switching by power-producing switches for various combinations of parameters of the main equipment unit; they analyzed the influence of parameters and places of installation of overvoltage protection. The main provisions of the research can be used by design organizations for reconstruction of low-power hydro-plants; in preparation of instructions for use of overvoltage protection when overvoltage is caused by commutation of vacuum switches in circuits of power-producing stations.