Effects of DC Ground Electrode on Converter Transformers

DC transmission systems are normally equipped with a DC ground electrode for emergency or normal operation with or without ground return. In all cases, the DC ground stabilizes the DC voltage. During operation with ground return, a DC ground potential rise occurs at the DC ground electrode and DC voltage is transferred to the station ground. The transferred DC voltage causes the flow of DC electric current through the converter transformers. The effects of this phenomenon are investigated in this paper. Specifically, a study of DC transfer voltage to the station ground and its effects on the converter transformers is presented. The primary tool for this investigation is a time domain simulation program which comprises a set of generic power system models. Any number of power system elements can be interconnected in appropriate ways to form specific converter station configurations to simulate monopolar, homopolar, or bipolar operation. The converter is modeled with a linear system which has variable topology depending on the state of the valves. A number of innovations have been introduced in the model of the converter. The developed algorithm minimizes computations at each time step as well as computer storage requirements. The model also provides an accurate representation of turn-off and turn-on times. The transformer model takes into account the nonlinearity in the magnetizing inductance. The ground electrodes are modeled with equivalent circuits. The parameters of the equivalent circuits are computed based on the grounding system geometry and soil data.