Application of UPFC to mitigate SSR in series-compensated wind farms

Flexible AC transmission systems have shown effective functionalities in promoting the system operation security and service reliability. Facing with series-compensated lines, subsynchronous resonance (SSR) may jeopardise the stability and mechanical facilities. The control strategy and simulation model of a unified power flow controller (UPFC) based on the dq decoupling control are deduced and constructed. Considering the impacts of grid-connected wind farms, SSR characteristics of the series-compensated system with UPFC are studied by the complex torque coefficient method and time-domain simulation. The results verify the capability of UPFC in attenuating SSR in wind farm integrations by utilising subsynchronous damping controllers coordinated with UPFC's main controllers. It also suggests that the shunt transformer−converter has a slight effect on SSR, whereas the series transformer−converter affects the system resonance characteristics significantly. Simulations are carried out on a typical power system model integrated with wind turbines.