Autonomous Fail-Normal Switch for Hybrid Transformers

Large power transformers (LPTs) are critical components affecting grid resiliency in high-intensity, low probability events. Hybrid transformers (HTs), consisting of standard transformers integrated with series-connected fractionally-rated converters, have been proposed to improve the resiliency and functionality of LPTs. However, when integrating with relatively less reliable power electronics, it is critical not to impact the reliability of otherwise highly reliable passive transformers. Simultaneously, the power electronics must be protected from system transients such as high fault currents (1020 kA in 13 kV applications). This paper proposes a Fail-Normal Switch (FNS), which will be an integral part of the transformer and bypasses the power electronics to retain the basic transformer functionality in the case of system or converter faults. The FNS consists of a combination of anti-parallel SCRs and a normally closed mechanical switch. The FNS is a fully autonomous unit embedded with local intelligence, making it unaffected by the converter controller failures. In addition to the capability of handling high fault currents with a sub-cycle response, various failure modes under practical scenarios, such as control power failure, communication failure, SCR gate drive failure, and the corresponding failure handling methods using a multi-layered protection scheme, are presented. An FNS prototype to be used with a 24 kV, 5 MVA HT is built and tested experimentally for reliable operation under various failure modes.