Harmonic instability in phase controlled rectifiers

The phenomenon of harmonic instability in phase-controlled power converters has been known for a long time. The subject was discussed by Ainsworth in the context of high voltage DC converters back in the 1950s. Harmonic instability is the abnormal operation of a power converter system due to the harmonic voltage distortion of the power source caused by the harmonic currents of the converter system itself. This effect is often found in power converters such as rectifiers but is often not recognized for what it is. The consequences can range from mild overheating and increased acoustic noise, severe instability and system shutdown, to transformer failure. Phase-controlled rectifiers are the most common type in industrial use today. They include thyristor process rectifiers, large motor drives and saturable core reactor controlled rectifiers. This paper discusses the elements of the feedback loop that causes harmonic instability, and lists symptoms by which it can be recognized. A more detailed look is taken at the zero crossing type of triggering control, as this is the most common type in use today, in both analog and digital implementations. Steps to avoid or to fix the system problems are discussed.