Type-4 Wind Turbine Generators
Type-4 wind turbine generators are fully decoupled from the power grid through AC-DC-AC conversion by the back-to-back converters and this makes the turbine models operate at a wide range of speed conditions. Type-4 wind turbine models can be built with permanent magnet synchronous generators (PMSG) or induction machines (IM) or synchronous machines (SM) with wound rotor. The type-4 wind turbine models are shown in Fig. 1.
Fig. 1. Type-4 wind turbine generator model.
The frequency of the generator is depending on the speed of the wind turbine. The converters connected on the generator side convert the AC power with variable frequency (output of wind turbine generators) to DC power. The converters connected on the grid side will convert the DC power to AC power at the grid frequency. The characteristic of the connected power grid is independent of the type-4 wind turbine generators since they are fully decoupled from each other via back-to-back converters. Similar to the type-3 wind turbine model, the real and reactive power are controlled by the back-to-back converters to control the WT speed, mechanical loads, and terminal voltage.
References
Walling, R. A., Ekrem Gursoy, and Bruce English. "Current contributions from Type 3 and Type 4 wind turbine generators during faults." In PES T&D 2012, pp. 1-6. IEEE, 2012.
Muljadi, Eduard, et al. "Understanding dynamic model validation of a wind turbine generator and a wind power plant." 2016 IEEE Energy Conversion Congress and Exposition (ECCE). IEEE, 2016.Â