From the perspective of optimal LCC(Life cycle cost)，this paper studied the method to develop a reasonable maintenance strategy of power equipment. The deterioration theory in the field of mechanical engineering was led in， based on that we analyzed the failure rates of each repair period and eatablished the model of the optimal problem， then，through a case study， we got the optimal maintenance strategy by using GA, finally, the optimization result and the influence factors are analyzed on a briefly way.

Commutation failure (CF) is one of the common faults in HVDC system, a new issue arising in multi-infeed HVDC systems is whether all the inverters will suffer from commutation failures concurrently, and the interaction between AC and DC systems is much more complicated. According to the mechanism of commutation failure, influencing factors and judgment basis, this paper proposed a commutation failure detection method based on the critical multi-infeed interaction factor (CMIIF). A three-infeed HVDC system was modeled in PSACD/EMTDC, The simulation results of the test system and an actual system proved the accuracy and validity of the proposed method.