The doubly-fed induction generator (DFIG) system is a popular system in which the power electronic interface controls the rotor currents to achieve the variable speed necessary for maximum energy capture in variable winds. Because the power electronics only process the rotor power, typically less than 25% of the overall output power, the DFIG offers the advantages of speed control with reduced cost and power losses. This PLECS demo model demonstrates a grid-connected wind turbine system using all of PLECS' physical modeling domains. The system model includes a mechanical model of the blades, hub, and shaft, a back-to-back converter including thermal loss calculations, a magnetic model of the three-phase transformer, and the transmission line and grid.
The electrical power circuit consists of a DFIG, whose stator is directly connected to the grid via a transformer, while the rotor winding is connected via slip rings to a back-to-back converter. The grid-side of the converter is connected to the tertiary winding of the transformer, which feeds the generated power into the 10 kV medium voltage network through a 20 km-long cable. The transmission line is modeled with a distributed parameter line component.
The machine’s rotor, and the gearbox, hub and blades of the propeller together make up the mechanical part of the wind turbine. They are coupled elastically with each other, which introduces resonant oscillations into the system.
The value of the wind torque applied on the turbine blades comes from a look-up table, where the value varies against the wind and shaft rotation speeds (transformed to the high-speed side of the gearbox).
Magnetic Transformer Circuit
The three-winding transformer is built up with primitive components from the PLECS Magnetic component library. Compared to a conventional model using a purely electrical equivalent circuit, the layout of the core structure is more intuitive to understand and it is possible to model complex non-linear effects like saturation and hysteresis in the three-leg core.
The back-to-back converter comprises separate machine-side and grid-side portions, which are connected with each other via a DC-link capacitor.
The machine-side converter regulates the torque of the DFIG and thus the rotational speed with a double loop structure, where the outer speed loop generates the reference signal for the inner current loop. The current control is carried out in rotational framework (d-q) with stator flux orientation. In addition, the machine-side converter also regulates the reactive power injection of the DFIG.
The grid-side converter transfers the active power from the machine-side converter into the grid through an LCL filter, and maintains the DC-link voltage at 950 VDC. The methods of active damping, feedforward as well as integrator anti-windup are adopted for the PI controllers, and the converters operate using space vector PWM (SVPWM) modulation.
The converter model can be simulated in either an "Averaged model" or a "Switched model with thermal" configuration. With the averaged converter model there are no harmonic components present in the switching frequency and the simplification offers improved simulation speed. In the switched model the conduction losses and switching losses of the IGBTs, as well as the effect of the cooling system, can be investigated.
As the simulation starts up the DFIG operates at synchronous speed. At t = 3 s, the speed reference signal jumps from 157 rad/s to 175 rad/s (-10% slip rate) to track the peak power output of 2 MW. Then, shortly after the system enters the new balanced state, a voltage sag occurs on the 10 kV stiff network at t = 12 s. This fault is modeled as the borderline condition corresponding to the worse case scenario as defined by the Germany Grid Code of 2007. As a result of the fault, a transient with high frequency oscillation in both the electrical and mechanical systems can be observed. After the grid voltage recovers, the system returns to its steady state.
Further documentation in PDF format can be downloaded here. Demo models for this application example are provided inside both PLECS Blockset and PLECS Standalone.