Systems and Control
Induction machines are the energy conversion devices of choice in commercial wind turbine design. In addition to their robustness and reliability, they provide a “softer” coupling between the grid and the mechanical system of the turbine. Wind turbine manufacturers have also moved beyond the basic induction generator systems with technologies for improving control and overall efficiencies. These technologies have a definite impact on the electrical and dynamic performance of wind turbines, even to the extent of masking or overriding the dynamic characteristics that would normally be associated with rotating machinery. Almost all of the wind turbines deployed in large wind generation facilities in the U.S. over the past decades can be generally described by one of the following configurations:
- Direct-Connected Induction (Asynchronous) Generator (Type I)
- Wound-Rotor Induction Generator with External Resistance Control (Type II)
- Doubly-fed Asynchronous Generator – DFAG (Type III)
- Variable Speed Turbine with Full-Rated Power Converter (Type IV)
Mechanically, the turbine must be protected from rotational speeds above some value that could lead to catastrophic failure. Mechanical brakes are provided for stopping the turbine in emergency conditions, but are not used in normal operations. Controlling the power (and hence, torque) extracted from the moving air stream is the primary means for protecting the turbine from over-speed under all but emergency shutdown conditions. In fairly steady conditions, the power extracted from the air stream by the turbine blades can be characterized by:
- ρ = air density (nominally 1.22 kg/m3)
- R = radius of area swept by the turbine blades
- υ = speed of moving air stream
- Cp = “coefficient of performance” for the composite airfoil (rotating blades)
Wind Turbines Technology Trends
The value of variable speed technology for large wind turbines has been proven in the marketplace over the past decade, and will be the predominate technology going forward. Variable speed operation has benefits in terms of managing mechanical loads on the turbine blades, drive train, and structure. The grid-side benefits are also significant, and include dynamic reactive power control, increased dynamic control over electric power generation, and opportunities for further enhancement of grid-integration features of the turbine.
The term generic model refers to non-proprietary dynamic models that can be used to represent wind turbine generators (WTGs) with similar physical and control topology, regardless of the manufacturer. In principle, generic WTG models should exhibit the following characteristics:
a) allow for an easy exchange of model data between interested parties,
b) facilitate comparisons of system dynamic performance between different simulation programs,
c) allow for the implementation of WTG models in different simulation programs, and
d) provide a mechanism by which manufacturers can tune the model parameters to best represent their equipment, without having to reveal proprietary information.
Vendor Specific Models
Vendor Specific Models (VSMs) are designed according to the turbine vendor’s specification. This implies that the internal mechanism and control settings can be quite different even for the same type of wind turbine. Furthermore, for confidentiality reasons, critical components and parameters for many VSMs are hard-coded in the models and are not disclosed to users. Often, a VSM is regarded as “blackbox” with essentially unknown characteristics.
- Working Group Joint Report – WECC Working Group on Dynamic Performance of Wind Power Generation & IEEE Working Group on Dynamic Performance of Wind Power Generation of the IEEE PES Power Stability Controls Subcommittee of the IEEE PES Power System Dynamic Performance Committee, “Description and Technical Specifications for Generic WTG Models – A Status Report.”