Aspects of conversion of the Nelson River bipole 2 HVDC converters from 12-pulse to 6-pulse operation

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Kolt, Robert A.
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In the early months of 1979, Manitoba Hydro was confronted with a serious problem involving the converter transformers for the bipole 2 operation of the Nelson River HVDC transmission system. The transformer problem left the negative pole of bipole 2, pole 3, without a converter transformer at each converter station. Since the transformer outage was forseen to be a lengthy one, the system had a reduced transmission capability firstly due to the direct loss of the negative pole and secondly due to the necessity to operate the positive pole continuously in a monopolar mode which affects adversely the lifetime of the earth electrode. Studies were therefore undertaken by Manitoba Hydro to investigate the feasibility of making the equipment, designed for 12-pulse, perform in a 6-pulse mode. The results of the studies proved favourable and with the approval of the HVDC Working Group, who are the dc equipment suppliers, the negative pole of the bipole 2 was operated in a 6-pulse configuration successfully for a year. The cost of the studies and modifications to the equipment were minimal and both Manitoba Hydro and the Working Group benefited greatly from this engineering exercise.