Voltage fluctuations on a major load throw off

Impact of a major load throw off with SVC: The Power Quality measurements were carried-out at 33kv primary bus, across a 150mva, 33kv/1120v, Electrical Arc Furnace Transformer, supplying to a 180 Ton EAF Furnace, connecting through a set of 2nos. 2000A, 33kv, in parallel, SF6 Circuit Breakers. The EAF transformer was protected against momentary voltage fluctuations, both voltage sags and voltage swells, by one Siemens Germany supplied, 190mvar Static VAR Compensation (SVC) System, comprising of a 190mvar Thyristor Controlled Reactor (TCR), and a combination of 180mvar Net Capacitive harmonic filters distributed among 50mvar in 2nd, 30mvar in 3rd, 30mvar in 4th, and 70mvar in 5th harmonic filters.

The load throw off incident was triggered, due to a regulation malfunction in the AC EAF furnace control system, that regulate electrode positioning into the molten metal bath. The 3phase, 1120v electrodes of the EAF furnace comprise of 3nos large electrodes positioned in a delta configuration. It started slipping, deeper into the molten charge of 180Ton iron bath, raising furnace current. Within 20seconds from the start of regulation malfunction, the over current relay tripped 2x2000A, 33kv SF6 circuit breakers, switching off the Electrical Arc Furnace Transformer.