Effects of DC ground electrode on converter transformers

The effects of transferred DC voltage to a converter station ground from the DC ground electrode were studied with a time-domain analysis model of a converter substation and interconnected system. The model explicitly represents all grounds of the system (AC and DC) and the nonlinear magnetization characteristics of the converter transformer. The transferred DC voltage from DC ground electrode to station ground has been shown to cause the flow of direct current in the windings of the converter transformer. The effects of these currents have been studied with a parametric analysis. The major parameters considered were the separation distance between station ground and DC ground electrode, and the source voltage. The study suggests that for separation distances larger than 10000 feet, the transferred DC voltage to the station ground is less than 1% of the DC ground potential rise. For smaller separation distances, a substantial percentage of the DC ground potential rise is transferred to the station ground, which causes the flow of direct current in the winding of the converter transformers