Analysis of fault-induced inrush current of converter transformer in LCC HVDC system considering DC control and protection

Abstract

Converter transformers are vital devices in line-commutated-converter (LCC) high-voltage direct-current (HVDC) systems. This paper introduces a new inrush current induced by a valve-side asymmetric ground fault on a converter transformer. It is defined as a “fault-induced inrush current (FIIC).” Taking a valve-side single-phase-to-ground (VSSPG) fault on a rectifier-side converter transformer as an example, the FIIC induced by this fault and its characteristics are first analyzed. Subsequently, the differences of the FIICs caused by the VSSPG faults occurring at the transformers in wye-grounded-wye (Y0/Y) and wye-grounded-delta (Y0/Δ) connections are discussed. After that, the effect of HVDC control and protection (C&P) on the characteristics of FIICs is considered. Finally, the impact of the FIIC on the transformer differential protection is analyzed. It is concluded that the transformer differential protection has a risk of being inappropriately blocked because of the FIIC. A suggestion for the HVDC C&P to apply a shorter protection action delay is proposed to reduce the risk. The theoretical analysis is verified through simulations performed in PSCAD/EMTDC. This study will be valuable for the research of inrush currents of converter transformers.