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The use of three-level neutral-point-clamped (3L-NPC) voltage source inverters for drive applications in the megawatt range is becoming a standard solution. In combination with the same converter circuit in rectifier operation as active front end (AFE), four-quadrant operation is also available. An existing plant, which has been in operation with eight converters for several years, has shown some failures, which leads to considerations of failure mechanisms and failure rates. All failures were on the AFE. A first failure of a power semiconductor leads to subsequent breakdowns of others. Some failures are analyzed and failure conditions are described. Some failure reasons can be excluded and a probable reason is pointed out. Finally, a recommended solution is described and its result after its implementation is discussed.