Authors

Modeling of soft connection of nonlinear load to high-voltage network by switch with phase-by-phase control of main contacts

The article uses developed dynamic mathematical models to explore the problems of processes that arise during the uncontrolled connection of a reactive load to high-voltage three-phase networks with a solidly grounded neutral. When switching transformer or reactor electrical equipment to the network at an unfavorable moment, shock inrush currents can occur that are tens of times higher than the rated current. These currents contain aperiodic components that magnetize the steel cores of the devices. Then it is necessary to adjust the settings of the relay protection against current surges, which leads to a decrease in its sensitivity and performance when triggered in real short circuit modes. An effective technical solution to reduce shock currents is the use of a controlled phase-by-phase drive of the main contacts of switches. Simulation of circuit dynamics was carried out in the MatLab system and the MultiSim software to assess favorable switching moments. An analytical expression is derived for neutralizing the aperiodic component of the flux linkage of magnetic cores. Under this condition, non-sinusoidal surges of magnetizing currents do not exceed the specified values controlled by the protection. The difficulty of shock-free connection in practice of power transformers in idle mode, containing secondary windings in star and delta circuits, is noted. Then in the secondary windings, with the initial setting of the phase-by-phase switching of the main contacts of the switch, the symmetry of the phase flux linkages is broken. The results of the simulation confirmed a possible solution to the soft switching problem in this case. It consists of changing the design of the transformer by introducing high-voltage switches into the delta phases of the corresponding secondary winding, which must be open during the start-up of the transformer and then closed at a predictable moment. A block diagram of the operation algorithm of the information part for soft phase-by-phase switching of the main contacts of a high-voltage circuit breaker has been generated. The developed package of dynamic mathematical models allows, based on processing data on instantaneous values of network phase voltages, to form a shock-free phase-by-phase connection of a reactive load with the absence of aperiodic current components. Read more...