INVESTIGATION OF GIC RELATED EFFECTS ON POWER TRANSFORMERS USING MODERN DIAGNOSTIC METHODS

Geomagnetically Induced Currents (GIC) are supposed to be a threat to power transformers worldwide. Recent surveys show correlation between solar storm activities, the cause of GIC, and transformers failing within a relatively short time period. The scope of this contribution is to create and investigate a laboratory setup emulating GIC effects by injecting DC currents into a three phase power transformer. Transformer under test is a 30 kVA oil immersed transformer energized by a 400 V variable ratio transformer. The load consists of a symmetric, star connected three phase resistance. A DC source situated between the secondary neutral terminal and the neutral point of the load injects a current emulating GIC. The effects of the DC offset on core hysteresis curve, partial discharge (PD) appearance and the transformers vibration behaviour are presented. The investigation shows half wave saturation of the core due to the DC offset contrary to nominal operating conditions. Hence a shift of the hysteresis loop can be monitored. Measured PD levels increase, which is potentially the reason of transformer failures after a GIC event. The measured vibration patterns of core and tank show a DC dependency. This change in the vibration pattern can be used as an easy to apply GIC indicator.