Modeling and Analysis of Harmonic Resonance Involving Renewable Energy Sources

 The resonance between power-factor-correction capacitors and the line reactance is a common cause for harmonic problems in traditional power systems. Grid-connected inverters integrating renewable energy sources into the grid exhibit capacitive output impedance at harmonic frequencies and may also resonate with the grid impedance. In addition to possible amplification of current and voltage harmonics, such resonances may lead to inverter control instability and other dynamic problems. This paper presents an overview of inverter modeling and system analysis methods for the characterization and mitigation of harmonic resonances involving renewable energy sources. The concept of sequence impedances is generalized to nonlinear circuits and used to model three-phase inverters at harmonic frequencies. A new harmonic resonance analysis method based on the Nyquist stability criterion is presented. Possible ways to shape the inverter output impedance to avoid resonances with the grid are highlighted, and the opportunity for adaptive inverter control based on real-time grid impedance measurement is discussed.