Valve saturable reactor iron losses of ultrahigh-voltage direct current converter

Thyristor-based line-commutated converters (LCCs) are widely used to achieve AC/DC and DC/AC power conversions in UHVDCs (ultrahigh-voltage direct currents) transmission. Valve saturable reactors are used to protect power thyristors. To improve the efficiency of bulk electricity transmission over long distance, the operating voltage and current of UHVDCs and therefore the thermal stresses applied to the valve saturable reactors have steadily increased over the last decades. Valve saturable reactor iron losses were studied by accounting for the reverse recovery characteristics of the valve thyristors and the non-linear characteristics of the saturable reactors. The impact of UHVDC current on valve saturable reactor iron losses was quantified and verified. The valve saturable reactor iron losses of an LCC are generated not only at the turn-on and turn-off periods of the valve, but also at the off-state of the valve. The amount of the losses are affected by the AC busbar stray capacitances of the converter, the valve firing and extinction voltages, the rates of change of valve commutation current, and the reverse recovery current of the valve thyristors. The conclusion obtained will help to understand better the forming mechanism of valve saturable reactor iron losses and optimise the valve saturable reactor design.