Equivalent Circuits for a Hybrid DC/AC System

When an inverter-based resource is subject to grid unbalance conditions, the current in the system has a negative 60-Hz component and a positive 180-Hz component along with the fundamental component. The negative-sequence circuit should have both the -60 Hz component and +180 Hz component presented and coupled. This paper explains the interaction between the negative-sequence and third harmonic component and provides a detailed derivation of the negative-sequence circuit. We also provide a generalized equivalent circuit for an inverter-based resource. For validation, we rely on electromagnetic transient (EMT) simulation, to design the experiment, produce time-domain data, and carry out fast Fourier transform (FFT) to perform harmonic analysis and compare those with the analysis results.