Methodology for evaluating the impact of GIC and GIC capability of power transformer designs

When power Transformers are subjected to high levels of GIC, high levels of a short - duration magnetizing current pulse; one per cycle, results. These translate into high levels of VAR swings and significant current harmonics; which are injected into the power system to which the transformer is connected to. If not planned for, these VAR swings could cause a dip in the system voltage. Also, the current harmonics could cause differential relays to operate and capacitive components in power systems, such as SVC (s), to trip. This could cause the stability of the grid to be compromised. Also, the resulting high magnitudes of harmonic rich magnetizing current causes corresponding leakage flux that causes temperatures of the windings & structural parts to rise. The magnitude of all above effects and their impact on the transformer, and the power system, depends on the magnitude and duration of the GIC pulses as well as the design of the transformer. This paper provides a methodology to evaluate the impact of GIC on power transformers; including the calculation of the resulting magnetizing current pulses, and associated VAR swings and harmonics, as a function of the magnitude of GIC. Part of this evaluation is determining the resulting increase in core losses, core noise, and load loss. The paper also presents a methodology used by the authors to evaluate the GIC Capability of transformer designs for a wide range of magnitudes and durations of GIC pulses. The paper presents examples of the data provided upon performing these studies on large power transformer designs. The paper also explains how utility engineers / operators could use this data to ensure proper operation of the power system and power equipment during a GMD event.