Enhanced Control and Operation of Doubly-Fed Induction Generator Based Wind Farm during Network Unbalance

This proposal intends to study the system control and the operation of wind farms based on Doubly-Fed Induction Generators (DFIG), connected to either a transmission system or embedded within a distribution system, when the network voltage is unbalanced. The unbalanced voltage can either be a large transient unbalance caused by asymmetric AC faults or a small steady-state unbalance. The project will first develop a mathematical model which includes a DFIG and two back-to-back connected converters under unbalanced supply. The impact of unbalanced network voltage on the operation of the DFIG and the associated converters will be examined. This project will further explore strategies that can be employed by the DFIG under unbalanced network voltage for providing enhanced operation such as improving the system operation and recovery during and after asymmetric fault, maintaining a balanced stator current, minimising DFIG electric torque pulsation or making a contribution to the rebalancing of the network voltage. Methods for controlling DFIG systems during network voltage unbalance, by providing precise control of the stator/rotor current and if required, the asymmetric active and reactive power outputs, will then be investigated. Experimental tests will be carried out to validate the proposed control strategy.