Analysis of Low-Frequency Stability in Grid-Tied DFIGs by Nonminimum Phase Zero Identification

For power converter-interfaced wind-generation systems, low-frequency instability issues arising from the interactions among converter outer power flow control (PFC) loops have got increased attention recently. This paper explores the possible origin of such instability in grid-tied doubly fed induction generator systems. To comprehensively analyze this multiloop interacted system, a reduced transfer matrix model is first constructed allowing for the PFC dynamics. Then the individual channel analysis and design approach is employed to decompose such closed-loop multivariable system into components such as local subsystem and loop interactions. Subsequently, frequency-response behavior analyses of these components are individually conducted at various operating points to study their relative contribution to overall stability. Results indicate that local active power control loop will induce a right-half-plane zero (RHPZ) for certain high-power operating conditions, and the resulting inverse response leads system unstable. Meanwhile, the critical operating point at which the RHPZ arises is analytically derived, and demonstrated to be close to the actual dynamic stability boundary.