A Theory for Resonance-Generated Harmonics of Grid-Connected Converters

Conventional power system harmonic theory treats power converters as harmonic sources. Possible amplification by resonances in the grid impedance is also considered. However, this harmonic emission-amplification theory requires a pre-existing source of harmonics and cannot explain most new harmonic problems encountered in recent years. This paper presents a theory for resonance-generated harmonics involving grid-connected converters to support system analysis based on sequence impedance. In this mechanism, continuous harmonic resonance is identified as the result of system operation in a critically stable mode achieved through a self-adjusting process when the operation is initially unstable. A key factor that creates this self-adjustment mechanism is the change in converter impedance characteristics when the initially unstable operation causes harmonics to grow. The self-adjusting process can be described using impedance models and is distinctively different from limited cycles and similar sustained oscillation behavior studied in nonlinear system theory. The mechanism is tested extensively in an experimental setup and is supported by analytical models and quantitative analysis that can also predict the amplitude of harmonics generated by such system resonance.