Study on Commutation Failure and its Prevention Strategy in ±1100kV UHVDC

Commutation failure is normal fault of line-commutated converters at the inverter stations. With the development and wide application of UHVDC transmission technology, the DC voltage and transmission capacity have been continuously improved, and the interaction between AC and DC system has been aggravating, so the effect of commutation failure on the AC/DC system has become increasingly prominent. With the increase of DC transmission distance, the coupling effect between bipolar DC lines becomes more and more serious, when one pole has a DC line fault, a commutation failure of the other pole may be caused on. The electromagnetic induction characteristics between bipolar transmission lines are analyzed, and the measure of improved extinction angle controller combining with DC voltage mutation is proposed in this paper. According to the characteristics of inducting DC current and DC voltage of non-fault pole, the extinction angle controller is improved in this measure based on the characteristics of bipolar line coupling effect. At the same time, by discriminating the fault pole with the DC voltage mutation index and compensating the extinction angle, the commutation failure of non-fault pole can be avoided. The effectiveness and adaptability of the proposed measure are verified by the electromagnetic transient model, which has been applied in the Changji-Guquan ±UHVDC project.