Critical Short Circuit Ratio Calculation for MIMO Systems Using Disk Margins

The frequency-based model of power systems including power converters is typically multiple-input multiple-output (MIMO). As the complexity of control loops increases and the number of converter units grows, the coupling terms of the model will strengthen, making it increasingly challenging to reduce it to single-input single-output (SISO) models. In such cases, using classical gain and phase margins to calculate the stability boundary of the MIMO system model may lead to inaccurate results. Disk margins, as a multiloop analysis method, can offer a more accurate and robust measure of stability margin for MIMO systems. The open loop-based disk margins can also be converted into grid impedance margins, establishing a connection between margins and the critical short-circuit ratio, which is a crucial metric for evaluating grid strength. This relationship allows for calculating the range of short-circuit ratios inside which the grid-converter system can operate stably.