Small-Signal Stability Analysis of Type-4 Wind in Series-Compensated Networks

Type-4 wind is claimed to be immune from subsynchronous resonances (SSRs) that have been experienced by Type-3 wind with radial connection to series compensated lines. In this paper, we examine this claim through simplified analytical model building, analysis based on linearized models, and validation against electromagnetic transient (EMT) testbeds with full details. Two analytical models of Type-4 wind farm with radial connection to a series compensated line are built in $dq$-frames. The main difference of the two models is in grid-side converter (GSC)'s control mode, with one model assuming real power control and the other assuming dc-link voltage control. Relying on the analytical models, an efficient approach is demonstrated to obtain frequency-domain impedance models. Small-signal analysis is carried out using eigenvalue analysis and frequency-domain impedance model-based analysis. Potential stability risk is demonstrated, which is due to interaction of a mode associated to voltage source converter (VSC) in weak grid (termed as weak grid mode) and a mode associated to network LC resonance. The weak grid mode is influenced by grid strength and VSC control parameters, including phase-locked-loop (PLL) parameters. The small-signal analysis results are validated against two EMT testbeds with full details in MATLAB/SimPowerSystems and PSCAD/EMTDC, respectively.