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The cascaded multilevel convertor has the following advantages: simple structure, less harmonic, and less switching loss. It also can easily achieve high voltage and large power capacity. However, due to the structure of cascaded, the number of the device increases with the increment of capacity, meanwhile the reliability of the system reduces. Based on the cascaded multilevel systems possible failures and the characteristic of carrier phase shift modulation, this paper proposes a fault diagnosis method suitable for the cascaded multilevel system. Firstly, the paper introduces the basic principle of the proposed approach, and then it deduces the major implementation steps, finally the correctness and reliability of the method are validated by the simulation.

The voltage stability can be maintained by the series compensation equipment while the voltage sag of distribution system side occurs. The series compensation equipment of cascade H-bridge type has many advantages such as lower harmonic injection into the power system, lower switching losses, easily to be extended and so on. However, due to the complexity of structure and control of the multilevel converter increase the possibility of device failure. In order to improve the reliability of series compensation equipment, this paper analyzes the possible fault type of series compensation devices and combined with the characteristics of different types of faults, proposing a fault diagnosis method for the series compensation equipment of cascade H-bridge type. This method only needs to detect the voltage and current of output side in series compensation equipment, the fault can be successfully dignosed by a certain equivalent and computation .This method is not affected by voltage and current changes, and easy to implement. This device can rapidly diagnosis and remove faults device in continuous operation, then by changing the modulation of inverter makes the series compensation device work normally. The simulation examples proves the validity of this method.