Saturation Mechanism of Voltage Transformer during Single-Phase-Ground Fault Recovery and Novel Solution Based on Active Thyristor

Saturation of electromagnetic voltage transformers (VTs) is observed after the single-phase-ground (SPG) fault recovery in ungrounded power distribution system, threatening the safety of VT. To address this problem, the saturation mechanism of VT during SPG fault recovery is revealed and a novel solution is proposed. Contributions of this article are as follows. First, by deducing the zero-state response and zero-input response of the transient voltage applied on VT during SPG fault recovery, it is found the transient voltage includes an ac voltage component and a dc bias voltage (DCBV) component. Under the effect of DCBV, the iron core of VT is forced into saturation, causing excessive overcurrent and potential ferroresonance. Second, an active solution is proposed by configuring a centralized protection device on the common bus, which is constructed by a low-capacity three-phase three-limb reactor in series with thyristor module. The basic working principle is to eliminate the DCBV actively. The benefit of the proposed solution is one protection device on the common bus can prevent the saturation of all VTs caused by DCBV, without causing measurement errors and excessive operating losses during normal state. All these findings are validated by theoretical analysis, simulation and experiments.