Improved LVRT for grid connected DFIG using enhanced field oriented control technique with super capacitor as external energy storage system

During faults, severe inrush current of magnitude 2-5 times reaches DFIG stator and rotor terminals damaging its windings. Many control schemes are developed to limit to these inrush currents to 2 times but face issues like over speeding of generator, dc voltage fluctuations etc. To overcome the issues and limit the current within 2 times for faults, enhanced field oriented control technique (EFOC) was implemented in the Rotor Side Control (RSC) of DFIG converter. This technique can control oscillations in torque, speed and flux components of DFIG during and after faults. New equations and generator converter control schemes are proposed. This converter topology uses a super capacitor energy storage system (SCESS) in parallel to a normal capacitor for additional reactive power support to further to improve performance of DFIG during the faults. The SCESS helps in maintaining nearly constant voltage profile across the dc link capacitor. In EFOC technique, the reference value of rotor flux changes its value of super-synchronous slip speed to a small value of zero during the fault with the injecting rotor current at the rotor slip frequency during normal operation. In this process dc-offset component of flux is controlled for decomposition during faults. The system performance with symmetrical and asymmetrical fault is analyzed using simulation studies.