The loss of the converter valve is one of the key indexes to evaluate the VSC HVDC transmission system.In this paper

in order to improve the availability of VSC HVDC converter valve

the three harmonic wave is injectedto the modular wave. By this way

the redundancy degree of the converter valve can be improved. The formula of thelosses of converter valve is obtained under this way. The impact of the three harmonic superposition rate on the lossesof converter valve is discussed in detail. Example results show that the losses of converter valve are reduced with theincrease of three harmonic superposition rate. But the redundancy degree and losses is not enough good at this time.So we get the optimal allocation between redundancy degree and losses by genetic algorithm. The results show thatwhen the three harmonic superposition rate reaches 24.3% when the losses and redundancy get performance bal-ance

then the total loss is 1.485% and redundancy degree is 12.7%.