STATCOM application for mitigation of subsynchronous resonance in wind farms connected to series-compensated transmission lines

Abstract

Large-scale integration of wind farms in transmission networks has led to several challenges; one of which is the need for increased transmission capacity to transport bulk amounts of wind power. Series compensation is an established means of enhancing the power transfer capability of existing transmission lines and is being increasingly considered for integrating large wind power plants. However, seriescompensated transmission lines may cause subsynchronous resonance (SSR) in turbine-generators, which can lead to electrical instability at subsynchronous frequencies and potential turbine-generator shaft failures. This chapter presents a thorough mathematical analysis of SSR in a double-cage induction generator (IG) based wind farm connected to a series-compensated transmission line and its mitigation. The prediction of SSR is done for a wide range of wind farm sizes and series compensation levels through eigenvalue studies and equivalent circuit analysis. Two types of STATCOM controllers are proposed and tested with an aggregated wind farm model. In the proposed controller-I the DC voltage remains uncontrolled and only the angular difference between the STATCOM terminal and the Point of Common Coupling (PCC) bus voltage is controlled. In controller-II, the DC link capacitor voltage is controlled by the phase angle difference between the STATCOM terminal and PCC bus voltage. Both the modulation index and angle are controlled in this controller. Studies are conducted for three different induction generators used in commercial wind farms. Fault studies are also conducted at different locations.