This paper summarizes the work of the Ad Hoc Task Force on Wind Generation Model Validation. The paper describes the concept of model validation, how this applies to wind turbine generation systems, and then gives clear examples of the most recent efforts to achieve model validation for wind turbine power plants. The document ends with a summary of the learning from the work presented and the conclusions which can be derived. Recommendations are made on the path forward for wind turbine generator modeling and model validation, primarily focused on generic models (i.e., standardized and publicly available) for stability analysis in power system studies.

Model Validation for Wind Turbine Generator Models

The doubly-fed induction generator (DFIG) is a 'special' variable speed induction machine and is widely used as modern large wind turbine generators. It is a standard, wound rotor induction machine with its stator windings directly connected to the grid and its rotor windings connected to the grid through an AC/DC/AC PWM converter. The AC/DC/AC converter normally consists of a machine-side converter and a grid-side converter, both of which are controlled by decoupled d- q control approaches. In order to gain proper control of a DFIG and appropriate DFIG integration with the grid, it is important to understand the power control characteristics of the two AC/DC converters. This paper focuses on the analysis of traditional decoupled d-q vector control approaches for control of DFIG wind turbines. A typical d-q control concept that has been used widely in DFIG converter control is reviewed in the paper. Then, detailed study is performed to investigate the power control characteristics of DFIG converter using decoupled d-q control approaches. Deficiencies of conventional d-q control mechanisms are discovered and analyzed both analytically and through computer simulation. An extensive simulation study is performed to examine the power control characteristics of DFIG PWM converter under different d-q control conditions.

Analysis of Decoupled d-q Vector Control in DFIG Back-to-Back PWM Converter

In recent years, computer models of wind turbines for power system stability studies have been developed and supplied to power system operators worldwide. The development of such models is of particular significance for stability studies on systems experiencing very large penetrations of wind energy such as the electricity system in the Republic of Ireland. There is a large variation in the complexity of models developed and no widespread agreement on the level of modelling detail required. The use of multiple wind turbine models for large-scale stability studies is also proving difficult. In addition, the majority of models have not been validated. This paper will examine model development and use from the perspective of the Irish grid operator. The validation of developed models will be discussed, and a generic modelling approach will be proposed for large-scale power system stability investigations.

Wind Turbine Modelling for Power System Stability Analysis—A System Operator Perspective

A method for controlling dynamic power factor or the reactive power of a wind farm is provided. The wind farm comprises a number of wind turbines connected to a utility grid driven with a requested power factor or a requested reactive power. The wind turbine output voltage is controlled to a specific voltage set point. In the method, the wind farm power factor is measured and compared with the power factor requested for the utility grid, or the wind farm reactive power is measured and compared with the reactive power requested for the utility grid, respectively; the ratio of the wind farm voltage to the utility grid voltage is adjusted, and the output voltage of the individual wind turbines is regulated to correspond to the specific voltage set point; the steps are repeated until the power factor of the wind farm electricity corresponds to the requested reactive power.

Wind farm and method for controlling the same

Mathematical modeling of synchronous and induction generators for wind turbines using state-space representations is presented. Emphasis is given to those models suitable for control schemes of variable-speed wind turbines and their application for different power system studies. The state-space representations provide a convenient way to assess different configurations of fixed and variable-speed wind turbines based on synchronous and induction generators. The modeling approach here presented allows both transient and small-signal stability analysis. As a case study, the performance of fixed and variable-speed wind turbines under faults and voltage sags is assessed. The state-space models are used to investigate the capability of different wind turbine technologies to satisfy Grid Code requirements. An eigenanalysis is included to show the flexibility of the models.

State-Space Modeling of Wind Turbine Generators for Power System Studies

Wind energy conversion systems comprise mechanical and electrical equipment and their controls. Modeling these systems for power system stability simulation studies requires careful analysis of the equipment and controls to determine the characteristics that are important in the timeframe and bandwidth of such studies. Just as important, the characteristics must be reviewed to put aside factors that can be important for wind turbine/wind farm design but do not play a decisive role for the wind turbine/wind farm response from a system standpoint or whose characteristics are not relevant to the frequency range typical for power system stability performance. With this in mind, models of wind turbine units and wind farms are being developed for a commercial power system load flow and stability simulation package. All significant air-dynamical, mechanical, and electrical factors have been reviewed and are modeled appropriately. Load flow models allow aggregation of wind farms comprising tens to hundreds of wind turbine units and easy addition of an equivalent of the wind farm to the model of the existing system. Dynamic models represent different types of constant speed and variable speed technologies currently employed by manufacturers, with their controls.

J.W. Feltes

Papers

Modeling wind farms for power system stability studies