Transformer neutral blocking device to prevent the flow of geomagnetically induced currents in power systems

Geomagnetically-induced currents (GIC), which cause detrimental effects to electrical equipment and power system operation, enter power systems through grounded neutral leads of wye-connected transformers. The Electric Power Research Institute (EPRI) has sponsored research into strategies which mitigate the effects of geomagnetic disturbances. One such strategy involves the use of a Neutral Blocking/Bypass Device (NB/BD) which can be inserted in power transformer neutral-ground connections to block the flow of GIC while maintaining a low-impedance AC grounding path. The device consists of distribution capacitors which provide the blocking function for the quasi-dc geomagnetically-induced currents. The capacitors are rapidly bypassed by a combination of a sparkgap and circuit breaker to solidly ground the neutral during power system disturbances. The sparkgap provides the fast operating response speed, while the circuit breaker provides a secure metallic bypass of the capacitor. In both simulation, and in rigorous field test, the bypass operation will occur rapidly (less than 1 msec) and will both prevent ferroresonant over-voltages and preserve proper protective device operation.