Mitigation of Half-Cycle Saturation of Adjacent Transformers during HVDC Monopolar Operation - Part I: Mitigation Principle and Device Design

When HVDC transmission systems operate in the ground return mode, transformers in adjacent ac systems frequently suffer half-cycle saturation. In this two-part paper, an effective mitigation method using power electronic switches is proposed that does not compromise the ability of the protection system to detect fault currents. In Part I, a study is conducted to find the physical performance of the half-cycle saturation and give a technical solution to the inrush-like half-cycle saturated currents. Power electronic switches, opening the neutral connection to ground, are utilized to mitigate the saturation without compromising the reference ground of the transformer. The determination of the switch parameters, switching frequency, and duty cycle is performed. Details of the mitigation transient are studied by comparing simulations and laboratory experiments to validate the efficacy of the proposed power electronic switch for the mitigation of half-cycle saturated currents. In Part II, the influence of the proposed switch on the detection of ground faults is investigated. Proper parameter and operation strategies are selected to effectively detect the ground faults and ensure the fast response of protective relays. The techniques proposed are applicable to mitigate geomagnetically induced current.