Stability Criterion for Inertial and Primary Frequency Droop Control in MTDC Grids with Implications on Ratio-based Frequency Support

Multiple strategies have been proposed to exchange frequency support amongst asynchronous AC areas through Multiterminal DC (MTDC) grid. Majority of these strategies rely on basic droop control mechanisms involving either inertial or primary frequency droop, or both. This paper addresses the requirements on those droop coefficients, which ensure small-signal stability. To that end, a stability criterion is hypothesized based on certain observations. Using a reduced Nth-order model, the hypothesis is analytically proved for the cases when either inertial droop or primary frequency droop control is active. Moreover, implication of this hypothesis on a recently proposed ratio-based frequency support method is analyzed to derive new stability constraints over and above the existing performance requirements. To support the stability hypothesis, numerically constructed stability boundaries in the droop coefficient space are studied. The stability boundary obtained from small-signal analysis of the full-order nonlinear model is shown to approximately match the boundary obtained from the Nth-order model, which further strengthens the hypothesis. Finally, various time-domain simulation studies are performed on full-order model to validate the stability hypothesis.