Jens Fortmann

Modeling of Wind Turbines Based on Doubly-Fed Induction Generators for Power System Stability Studies

This paper deals with modeling of the doubly-fed induction generator (DFIG) and the corresponding converter for stability studies. To enable efficient computation, a reduced-order DFIG model is developed that restricts the calculation to the fundamental frequency component. However, the model enhancement introduced in this paper allows the consideration of the alternating components of the rotor current as well, which is necessary for triggering the crowbar operation. Suitable models are presented for the rotor and grid side converters as well as the dc-link, taking into account all four possible operating modes. The proposed model for speed and pitch angle control can be used when wind and rotor speed variations are significant. Simulation results are presented for model verification purposes and also for demonstrating the dynamic behavior of a large offshore wind farm connected through a long undersea cable to the high voltage grid.

Model Validation for Wind Turbine Generator Models

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.

High voltage ride-through of DFIG-based wind turbines

With the rapid increase of large offshore wind farms in Europe, a new problem associated with the response of wind turbines to temporary overvoltages has arisen. This problem has not been a focus of discussion up to now. The majority of wind turbines use voltage source converters with a DC-link. When the grid voltage exceeds a certain limit the current flow through the line-side converter may reverse, resulting in a rapidly increasing DC voltage. To handle such situations, special countermeasures are required. This paper identifies and outlines the problem and recommends possible measures to ride through the overvoltage safely. Additionally, active voltage control structures to limit the overvoltages are proposed.

Effect of wind turbine output current during faults on grid voltage and the transient stability of wind parks

This paper deals with the effectiveness of fault-induced current injection into the system by wind turbines. First, a brief review of the grid code requirement on wind turbines to support voltage profile during fault is reviewed. Based on first principles, analytical expressions quantifying the effectiveness and the limits of the voltage support effort are formulated. This has then been extended to include the limits imposed on output current by the transient stability requirements of the wind park, in which stability constrained current limits through the link wind park — the point of the interconnection and the boundary conditions for stable operation have been derived. Using sample computations, the effect of the violation of this stability limit during fault on the wind park has been analyzed. Finally, a control scheme has been proposed, which on the basis of the voltage dip experienced by the network, reduces the real part of the wind park output current with the objective of enhancing the transient stability margin. Again the effectiveness of the proposed scheme has been demonstrated using sample computations.

Offshore Wind Power Generation Technologies

This paper provides an overview of the current state of the technology of offshore wind-based power generation and the technological challenges with emphasis on the electrical parts. First, a brief review of the core control functions, their correlation with operational behavior, and the grid-supporting capability of the machine during normal operation as well as during contingency situations are provided. This is followed by the discussion of basic considerations in wind farm collector design, including topology, grounding options, and outlay of the offshore substation. Then, issues related to offshore turbine foundation and typical dimensions of the offshore substation platform are discussed. The platform is designed to accommodate the main and grounding transformers, the switch gear, and other assorted accessories. Next, options for the transmission link from the offshore plant to the grid onshore are reviewed. Finally, a discussion of issues related to grid integration together with currently applicable special grid code requirements concludes the paper.

New Generic Model of DFG-Based Wind Turbines for RMS-Type Simulation

New requirements for the validation of simulation models based on measurements in many grid codes show that existing generic approaches for generator and converter models of doubly fed generator systems (DFG) may not be accurate enough. The authors show that by applying a detailed analysis of the generator equations and the converter control design, a reduction of the model complexity is possible while maintaining a high level of accuracy. The generator model presented in this paper allows an improved representation of the stationary and dynamic response of wind turbines equipped with DFG systems especially during grid faults and during voltage recovery. The model is designed to represent modern DFG systems independently of vendor specific hardware and software. The results of simulations are compared to measurements of a voltage dip involving wind turbines. The generator model has been proposed as extension to the WECC/IEEE generator models and has been accepted as reference for IEC TC88 working group 27 (standard IEC 61400-27-1) on modeling and model validation of wind turbines.

Code Shift: Grid Specifications and Dynamic Wind Turbine Models

Grid codes (GCs) and dynamic wind turbine (WT) models are key tools to allow increasing renewable energy penetration without challenging security of supply. In this article, the state of the art and the further development of both tools are discussed, focusing on the European and North American experiences.

Determination of dynamic wind farm equivalents using heuristic optimization

As a result of the increasing share of wind power the dynamic behavior of power systems will change considerably. To carry out stability studies in the future wind turbine and wind farm dynamic models will be indispensable. Generic models seem to provide the required simplicity and accuracy. But the parameters cannot be derived directly from the mathematical models of the generator and converter system, numerical identification methods are needed. In this paper the authors introduce a new heuristic optimization method called Mean Variance Mapping Optimization (MVMO) which provides excellent performance in terms of the accuracy of the generic model parameters and convergence behavior. The fitness evaluation is performed using time domain simulation in each iteration step. The procedure and the level of accuracy that can be reached are demonstrated using an 18 machine, 90 MW test wind farm consisting of DFIG based wind turbines.

Dynamic performance evaluation of DFIG-based wind turbines regarding new German grid code requirements

Current grid codes require the fault ride-through capability of modern wind turbines. During grid faults the reactive current control of the wind turbines should be used to support the grid voltage. With modern protection devices the fault durations are normally in a range of some hundred milliseconds or less. However this time window may be decisive for the stability of the conventional generators connected to the grid and consequently for the whole system. In this regard the dynamic response of the voltage support by the generation units is a very important issue. New German grid codes address this subject by specifying timing rules for the system response to grid faults. While wind turbines equipped with full-size converters can fulfil these rules with moderate effort due to their fast converter control, DFIG-based wind turbines are facing a new big challenge, which requires a dedicated control. This paper shows an extended control approach that deals with a highly dynamic response to grid faults. Simulation results prove the good performance of this control and validate it based on the new requirements.

Negative sequence control of DFG based wind turbines

The paper deals with the control of negative sequence voltages and currents in wind turbine systems caused by grid fault or unsymmetrical system operation. The ensuing stator and rotor currents lead to additional thermal stress. Moreover, the interaction between the different sequence components of the current and voltage in the stator as well as rotor cause oscillating torque leading to mechanical strain on the drive-train. A control approach for limiting or eliminating the negative sequence current and the resulting alternating torque is discussed. This is followed by the description and derivation of the rotor side converter (RSC) for the positive as well as negative sequence current controllers. The procedure is repeated for the grid side converter (GSC), and the limitations imposed on the controllers by practical operational considerations are explained. On the basis of simulation examples using representative wind turbine system data, the effectiveness of the proposed control methods has been demonstrated.