Sub-synchronous controller interaction (SSCI) of series-compensated DFIG wind farms

One of the problems faced by the integration of large renewable energy sources - such as wind turbines- is that often the load centres are very far from the production centres. This forces the use of series capacitor compensation in order to increase the power transfer and the transient stability limits. The presence of series compensation introduces resonance into the system and this resonance could interact with the control system of the connected DFIG wind farm, thus causing the system (under certain conditions) to experience unstable resonant oscillations at the subsynchronous frequency range. This thesis studies the sub-synchronous controller interaction (SSCI) of DFIG-based wind-farms with series-compensated transmission lines and suggests a STATCOM solution to damp the resulting oscillations. This is done by studying the IEEE First Benchmark Model (FBM) for computer simulation of sub-synchronous resonance as test system. Analysis is conducted in both time domain and frequency domain to understand the parameters that affect the system’s stability and create the unstable sub-synchronous oscillations. The software employed is PSCAD/EMTDC. The frequency-domain analysis includes the use of the net positive damping criteria to assess the stability of the system For the mitigation of the SSCI oscillations, a STATCOM installed at the DFIG wind farm bus is employed. It is assumed that this STATCOM is there for the purpose of reactive power compensation and grid code compliance of the wind farm. For SSCI oscillation damping, a controller is designed and tested. The damping controller is based on detecting the presence of sub-synchronous oscillation and then determining its frequency on-line. Next this frequency is used to extract the unstable component from the active power signal. This extracted signal is then multiplied by an empirical gain and afterwards used to modulate the voltage reference of the STATCOM voltage controller. This method was found to damp both oscillations on voltage and power signals