Generally, during operation of a wind turbine, wind impacts the rotor blades and the blades transform wind energy into a mechanical rotational torque that drives a low-speed shaft. The low-speed shaft drives a gearbox that subsequently steps up the low rotational speed of the low-speed shaft to drive a high-speed shaft at an increased rotational speed, wherein the high-speed shaft rotatably drives a generator rotor. In many conventional wind turbine configurations, the generator is electrically coupled to a bi-directional power converter that includes a rotor-side converter (RSC) joined to a line-side converter (LSC) via a regulated DC link. Each of the RSC and the LSC typically includes a bank of pulse width modulated switching devices, for example insulated gate bipolar transistors (IGBT modules). The LSC converts the DC power on the DC link into AC output power that is combined with the power from the generator stator to provide multi-phase power having a frequency maintained substantially at the frequency of the electrical grid bus (e.g. 50 HZ or 60 HZ).
The above system is generally referred to as a doubly-fed induction generator (DFIG) system, whose operating principles include that the rotor windings are connected to the grid via slip rings and the power converter controls rotor current and voltage. Control of rotor voltage and current enables the generator to remain synchronized with the grid frequency while the wind turbine speed varies (e.g., rotor frequency can differ from the grid frequency). Also, the primary source of reactive power from the DFIG system is from the RSC via the generator (generator stator-side reactive power) and the LSC (generator line-side reactive power). Use of the power converter, in particular the RSC, to control the rotor current/voltage makes it is possible to adjust the reactive power (and real power) fed to the grid from the RSC independently of the rotational speed of the generator. In addition, the generator is able to import or export reactive power, which allows the system to support the grid during extreme voltage fluctuations on the grid.
Typically, the amount of reactive power to be supplied by a wind farm to the grid during steady-state and transient conditions is established by a code requirement dictated by the grid operator, wherein a wind farm controller determines the reactive power demand made on each wind turbine within the wind farm. A local controller at each wind turbine receives and allocates the reactive power demand between the generator sources (e.g., between generator-side reactive power and line-side reactive power).
As the generator speed approaches synchronous speed, the rotor fundamental frequency approaches DC where the thermal cycling of the IGBTs is greatest, resulting in a peak temperature of the rotor side IGBT at or near the synchronous speed. This results in a reduction of the total output current capability of the RSC, and thus a reduction in the reactive power capability of the RSC. Typically, the switching frequency on the rotor side of a DFIG power convertor is maintained at an elevated frequency (e.g., about 2000 or 3000 Hz) for all rotor speeds. While this elevated switching frequency is desirable for most operating speeds, at or near synchronous generator rotor speeds, it generates peak temperatures and thermal cycling stresses in the IGBTs and limits the reactive power capability of the DFIG system.
In addition, operation of a DFIG generator at or near synchronous speed is even more complicated because current harmonics are feed through the generator from the rotor side to the stator side and then directly to the transmission utility grid. These harmonics must be controlled to levels dictated by utility grid harmonic requirements.
U.S. Pat. No. 8,853,876 describes a system and method for operating a DFIG power generation system in a wind turbine. A control command is generated to control a switching frequency of the switching elements in the power converter to an adjusted switching frequency that is substantially equal to a fundamental frequency of the load when the generator is at or near synchronous speed. By reducing the switching frequency with reductions in the generator speed, power losses in the switching elements may be reduced. With such a reduction in power loss, the temperature rise in the switching elements may also be reduced, which may provide an extra margin in the output current capability of the power convertor and may also increase the component life of the switching elements. In addition, by closely matching the switching frequency with the fundamental frequency of the grid, a reduction in the amount of harmonics fed through to the line side of the converter may also be obtained, thereby decreasing the harmonic distortion to the grid.
U.S. Pat. No. 9,625,921 describes a method for temperature regulation of IGBTs in a power converter to reduce thermal stresses and extend the life of the devices. When the IGBTs are not within a predetermined temperature range, the switching frequencies of the devices are modified to bring the IGBTS within the temperature range.
A system and method that operate a power converter in a power generation system, such as a wind turbine DFIG system, in a manner to enhance the reactive power generation capability of the system in real time while maintaining harmonic distortions within limits would be desirable in the industry.