Dynamic Models for Wind Turbines

Author: National Renewable Energy Laboratory^[1]

As wind turbine technology matures and wind power penetration levels increase, interconnecting a large-scale wind power plant (WPP) into the bulk power system has become a more important issue. Large-scale WPPs can have a significant impact on the grid, and the topic has been a matter of interest in the United States since the late 1970s and early 1980s. This was a period when wind turbine technology was starting to become viable, and concerns about the effects of large-scale WPPs on the grid began to be voiced. The intermittent and variable nature of wind, the reliance of most wind power plants on induction generators, and the fact that wind generation tends to displace conventional generation, negatively affect system stability.

Right now, there is a need for wind turbine dynamic models, with potential users being power system planners and operators, researchers, consultants, wind plant developers. Reliability entities also need validated, non-proprietary models to meet reliability standards such as those set by the North American Electric Reliability Corporation (NERC). The purpose of these models is to observe the impact of wind turbine generators (WTGs) on the power system during dynamic events such as loss of load, loss of generation, loss of line, loss of wind, short circuits and voltage ride-through. Interconnection studies require steady-state and dynamic transient models of a WPP along with its collector system. Failure to perform proper interconnection studies could lead to non-optimal designs and operations of the WPP. Numerical power system simulation tools developed specifically for power systems and dynamic modeling, such as PSCAD/EMTDC, SIMPOW, PSLF, or PSSE may be used for these interconnection studies. General purpose modeling software such as MATLAB/Simulink may also be used. The dynamic models of wind plants for power system studies are not usually built-in in these software tools, and have to be developed independently. Model development is an involved process, as is model validation. Models developed for system stability studies also need to be able to reproduce events on a timescale ranging from milliseconds to tens of seconds. Existing models are proprietary and manufacturer-specific, and are bound by the manufacturer’s non-disclosure agreements. They are usually positive-sequence models, and hence, cannot model unbalanced faults. In addition, they are usually not detailed; they often model the generator alone, and do not model aerodynamics and mechanics of the wind turbine and generator. Most models are also not validated using real data. The need for robust generic wind turbine and wind power plant models has been the motivation behind the research described here.

Background

Proprietary and manufacturer-specific models of wind turbines are typically favored for use in wind power interconnection studies. While they are detailed and accurate, their usages are limited to the terms of nondisclosure agreement, thus stifling model sharing. The primary objective of the work described herein was to develop universal manufacturer-independent wind turbine and wind power plant models that can be shared, used, and improved without any restrictions by project developers, manufacturers, and engineers. The emphasis is on development and validation of standardized “textbook models,” similar to those for other power system apparatus. In addition to the primary objective, the secondary objective was to use these models to perform many other studies such as on inertial response of wind turbines during a unit trip on the grid, and to model controls which allow wind turbines to provide inertial support under such conditions. The salient features of these models are:

• They are generic and manufacturer-independent models;
• Selected models have been validated with real data;
• They are detailed analytical models intended for power system stability studies;
• They are three-phase, time-domain models implemented in PSCAD/EMTDC but portable to other modeling software, and can model balanced and unbalanced faults, frequency excursions and other dynamic events;
• They can successfully represent the diversity of wind turbine technologies currently in use;
• They can model fast and slow phenomena: electromagnetic transients (1ms) to system-wide controls (50s);
• They are scalable (from single turbine to large wind power plant);
• They are comprehensive:
  • They can model wind behavior (wind ramps/gusts etc.);
  • They include basic wind turbine aerodynamic characteristics;
  • They include basic wind turbine mechanical characteristics;
  • They include generator and power electronic converters (if present);
  • They include controls for mechanical and electrical systems;
  • They include collector system (interface to grid) of wind power plant.

Some of the above features, while desirable, also have associated tradeoffs. Generic models will always be approximate, and can be relied on for good estimates rather than precision. They do however have the advantage that they do not need large data-sets for validation. Also, three-phase time-domain models are computationally intensive and require more time and computing power than frequency-domain models. However they do provide greater detail in short time scales. Allowing scalability of models from single wind turbines to large wind power plants has some drawbacks; namely, that the wind power plant’s collector system, i.e., the dispersed electrical equipment necessary for collecting the wind power plant’s output power and feeding it into the grid needs to be reduced to a single-line representation. One of the complicating factors in this work was the diversity of wind power technologies in use. This was overcome by classification of wind turbines into four basic types based on the WECC classifications, and modeling each of these types separately.

Direct-Connected Induction (Asynchronous) Generators

Sometimes referred to as fixed-speed wind turbine generators, called so because they operate with less than 1% variation in rotor speed, employ squirrel-cage induction machines directly connected to the power grid. Fixed-speed wind turbines are low-cost, robust, reliable, simple to maintain, and proven in the field. A large number of fixed-speed wind turbines have been installed over the past decade and a half, and more continue to be installed. While variable-speed wind turbines form the bulk of new installed capacity, a niche for fixed-speed wind turbines still exists. Therefore, it can be expected that fixed-speed wind turbines will continue to play a role in the power systems of the future. From a modeling standpoint, a fixed-speed wind turbine consists of the following components:

• Turbine rotor and blade assembly (prime mover);
• Shaft and gearbox unit (drivetrain and speed changer);
• Induction generator;
• Control system.

The interaction between each of the components listed above determines how much kinetic energy is extracted from the wind. Modeling of the electrical subsystems is fairly straightforward, as power system modeling software usually includes a built-in induction machine model. However, modeling of the aerodynamics and mechanical drive-train is more challenging. These components are modeled based on the differential and algebraic equations that describe their operation.

Wound-Rotor Induction Generator with External Resistance Control

While fixed-speed (type 1) wind turbines are simple and robust, they have a significant disadvantage: they cannot optimally extract power from the wind. It would be preferable to have the generator continue to output rated power at high wind speeds. To achieve this, variable-speed wind turbines are employed. While largely relying on the same concepts as fixed-speed wind turbines at lower-than-rated wind speeds, they typically incorporate blade pitch and output power controls to optimize power extraction at higher-than-rated wind speeds. The Type-2 turbines use rotor resistance control to achieve output power control. The successful implementation of a type 2 wind turbine generator model requires a consideration of the following concepts:

• Rotor resistance control, its basis in machine theory and the induction machine equivalent circuit,
• Methods of achieving optimal power output based on rotor resistance control
• Implementation of the control methods using a modified version of the fixed-speed wind turbine model

Doubly-fed Asynchronous Generator – DFAG

A rotating machine is said to be a generator when it is converting mechanical input power to electrical output power. When induction machines are operated at speeds greater than their synchronous speeds, they act as generators. DFAGs operate on the same principles as conventional wound-rotor induction generators with additional external power electronic circuits on the rotor and stator windings to optimize the wind turbine operation. These circuits help extract and regulate mechanical power from the available wind resource better than would be possible with simpler squirrel-cage induction generators. From a modeling standpoint, a full converter permanent magnet alternator (PMA) wind turbine consists of the following mechanical and electrical subsystems:

• Aerodynamic model for rotor;
• Mechanical two-mass model for drivetrain;
• Reference power calculation block;
• Pitch controller;
• Induction generator model;
• Rotor-side converter;
• Grid-side inverter;
• Unit transformer and grid representation.

The interaction between each of the components listed above determines the wind turbine model’s steady-state and dynamic response. Modeling of the aerodynamics and mechanical drivetrain is based on the differential and algebraic equations that describe their operation.

Variable Speed Turbine with Full-Rated Power Converter

This technology has a number of significant advantages. It effectively decouples the generator from the grid, improving fault response. It allows the turbine to operate over a wide speed range, leading to improved power extraction from the wind. The converter interfacing the turbine to the grid has to handle the entire output of the generator (unlike in a DFAG turbine where the converter handles only 30% to 40% of the generator output) and hence is more costly and lossy, but also provides more headroom to supply reactive power to the grid. The permanent magnet alternator itself has no rotor windings, reducing excitation losses and reducing the size of the generating unit with respect to competing technologies. Absence of rotor slip rings reduces maintenance requirements. This combination of factors is driving the increasing penetration of full converter wind turbines, especially for offshore wind power plants. From a modeling standpoint, a full converter permanent magnet alternator wind turbine consists of the following mechanical and electrical subsystems:

• Aerodynamic model for rotor;
• Mechanical two-mass model for drive-train;
• Reference power calculation block;
• Pitch controller;
• Permanent magnet alternator (PMA) model;

References