UHV Knowledge | Characteristics and role of converter station equipment

The converter station is a system for mutual energy conversion between DC and AC in DC transmission projects. In addition to the same equipment as AC substations such as AC fields, DC converter stations have the following unique equipment: converter, converter transformer, AC-DC filter and reactive power compensation equipment, and flat wave reactor.  The main function of the converter is to carry out AC-DC conversion, from the initial mercury arc valve to the development of electronically and optically controlled thyristor valve, the converter unit capacity is constantly increasing.

The converter transformer is the key equipment for AC-DC conversion in DC converter stations. Its network side is connected to the AC field and the valve side is connected to the converter, so its valve side winding needs to be subjected to AC and DC compound stress. As the converter transformer operation is closely related to the non-linearity caused by the commutation of the converter, it has different characteristics from ordinary power transformers in terms of leakage resistance, insulation, harmonics, DC bias magnetism, on-load regulation and testing.

The AC-DC filter provides an incoming path for the characteristic harmonics generated during converter operation. A large number of harmonics are generated during converter operation, consuming 40% to 60% of the reactive power of the converter capacity. The AC filter also provides reactive power while filtering. When the reactive power provided by the AC filter is not sufficient, special reactive power compensation equipment is also required.

The flat wave reactor prevents lightning and steep waves on the DC side from entering the valve hall, thus saving the converter valve from the stress of these overvoltages; it smooths out the ripple in the DC current. In addition, in the event of a DC short circuit, flat wave reactors reduce the probability of commutation failure by limiting rapid changes in current.

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