  This paper proposes a non-invasive fault location method for modular
multilevel converters (MMC) considering light load conditions. The prior-art
fault location methods of the MMC are often developed and verified under full
load conditions. However, it is revealed that the faulty arm current will be
suppressed to be unipolar when the open-circuit fault happens on the submodule
switch under light load. This leads to the capacitor voltage of the healthy and
faulty submodules rising or falling with the same variations, increasing the
difficulty of fault location. The proposed approach of injecting the
second-order circulating current will rebuild the bipolar arm current of the
MMC and enlarge the capacitor voltage deviations between the healthy and faulty
SMs. As a result, the fault location time is significantly shortened. The
simulations are carried out to validate the effectiveness of the proposed
approach, showing that the fault location time is reduced to 1/6 compared with
the condition without second-order circulating current injection.
