A combined dq-MIMO and sequence-SISO impedance-based stability assessment scheme for IBR interfaced system

The electromagnetic transient (EMT)-type impedance scanning methods conducted in positive sequence domain and dq-frame (p-scan and dq-scan, respectively) have been widely used for identifying inverter-based resource (IBR) instabilities. They obtain the sequence domain single-input single-output (sequence-SISO) and dq-frame multi-input multi-output (dq-MIMO) impedance models of an IBR, respectively. The dq-MIMO impedance-based stability assessment (IBSA) is more accurate compared to the sequence-SISO IBSA as it considers the mirror frequency effect (MFE). However, unlike the sequence-SISO, the dq-MIMO IBSA cannot differentiate the resonance frequency from its mirror frequency, because the dynamics in dq-frame incorporate both the positive and negative sequence impedances of IBR. This paper proposes a new IBSA scheme to eliminate this shortcoming. First, a dq-MIMO IBSA is carried out to obtain the accurate dq-frame resonance frequency and stability margin. Then a sequence-SISO IBSA is conducted to determine the phase domain resonance frequency. The sequence-SISO impedance models are transformed from the dq-MIMO model. The proposed scheme has lower computational costs than the existing dq-MIMO IBSA methods. The effectiveness of the proposed scheme is validated on a test system incorporating full-size converter-based wind park (FSC-based WP). This paper also demonstrates the possibility of weak grid instability occurrence in supersynchronous frequency range.