Identification of Sub-Synchronous Interaction in MMC Systems using Frequency Scanning

The interaction between the Modular Multi-level Converter (MMC) and the ac system can result in oscillation, even instability in the sub-synchronous frequency range. Rather than using an analytical model of the MMC which is complicated and often ignores details of commercial proprietary controllers, this paper proposes the application of frequency scanning of the MMC modelled on a real-time digital simulator (RTDS). The frequency response of the MMC looking from the Point of Common Coupling (PCC) bus can be extracted from the scan in either the dq0 domain or the modified PNO domain, which would be discussed in detail. The system behavior can be studied using the Generalized Nyquist stability Criterion (GNC) or equivalently, the Bode plot. The approach can predict the Critical Short Circuit Ratio (CSCR) of the ac system resulting in marginal stability, as well as the oscillation frequency. The result is then validated by real-time simulation on RTDS.