Impact of DC bias on differential protection of converter transformers

Abstract

Converter transformers in high-voltage direct current (HVDC) systems are often subjected to DC bias induced by earth return currents (ERCs) from HVDC systems or by geomagnetically induced currents (GICs). DC bias may adversely affect the performance of converter transformers, but its impact on the protection of converter transformers remains uncertain. This paper thoroughly investigates the response of converter transformer differential protection under the effect of DC bias. The network was simulated in PSCAD/EMTDC, and the modeled components included the converter transformers, the current transformers (CTs), and the DC bias signatures. The fault currents generated by the simulator were downloaded into a numerical relay and its operating behavior was observed. The results indicated the pre-existing harmonics, induced by DC bias, do not cause harmonic blocking to incorrectly inhibit the relay for in-zone faults; and any spurious trip, caused by DC bias, can be prevented by appropriate harmonic blocking and the use of a higher slope 2 in the differential current versus bias current characteristic. It was demonstrated the low-frequency characteristic of GIC did not pose any new threats to differential protection. This indicates the low-frequency characteristic of GIC is negligible when investigating the impact of GIC.