Novel Control of DFIG with ESD to Improve LVRT Capability and to Perform Voltage Support during Grid Faults

A novel control method for a doubly-fed induction generator (DFIG) equipped with an energy storage Device (ESD) to enhance its transient voltage support capacity during grid faults. Specifically, the ESD connected to the DC-link capacitor of the DFIG is controlled to regulate the transient DC-link voltage. This is actually equivalent to absorb the power imbalance between the captured wind power and the power delivered to the grid by the DFIG during the fault so that the DC-link voltage and the rotor current dynamics can be greatly improved. Moreover, the rotor speed and the pitch angle are also controlled to initiatively react to the low voltage transient, which includes increasing the rotor speed to store the unbalanced energy as kinetic energy and augmenting the pitch angle to decrease the captured wind power. Thus, due to less unbalanced power flowing into the rotor, the DFIG dynamics can be further ameliorated and the required ESD capacity can be obviously saved. As the grid side converter (GSC) can be free from managing the transient DC-link voltage, it is manipulated to devote its whole capacity to inject reactive power to the grid for voltage support during the fault. By simulating a single-machine-infinite-bus power system, comparisons with other control methods are conducted to prove that the novel proposed control method can well ensure the DFIG LVRT capability but save the ESD capacity, and also significantly improve the terminal voltage dynamics during the grid fault.