Control Choices to Allow the Parallel Operation of Grid-Forming Type-III Wind Turbines

Grid codes are now requesting grid forming capability to renewable energy sources. The provision of this service is being thoroughly investigated for type-IV wind turbines (full electronic interface) with the virtual synchronous machine (VSM) control technique. However, this issue has been barely addressed for type-III wind turbines. In this turbine topology, it is of special interest to keep the current controller of the rotor-side converter (RSC) unaltered since many functions that are required for the safe operation of the machine and to comply with grid codes depend on it. In this article, two control alternatives to implement a VSM without modifying the structure of the inner current controller of type-III wind turbines are proposed. The first one is based on a current controller plus a novel flux-based virtual inductance, while the second one has a more traditional structure based on a current controller, a voltage controller, and a virtual impedance. Results show that a vector voltage controller is not an essential requirement to provide grid-forming capability. In addition, the operation of a wind farm consisting of six parallel-connected type-III wind turbines is studied analytically for both alternatives. The parallel operation of the wind turbines is verified by using real-time simulations performed in OPAL-RT. Finally, the control alternatives were experimentally validated in a real doubly fed induction generator rated at 28 kVA.