Christian Feltes

Reactive Power Capability of Wind Turbines Based on Doubly Fed Induction Generators

With the increasing penetration of wind turbines (WTs) grid utilities require extended reactive power supply capability not only during voltage dips but also in steady-state operation. WTs with doubly fed induction generators (DFIG) are able to control active and reactive power independently. The reactive power capability is subject to several limitations resulting from the voltage, current, and speed, which change with the operating point. This paper discusses the steady-state reactive power loading capability of DFIG-based WTs by taking into account the most important physical phenomena restricting the reactive power supply of DFIG-based WT systems. The active-reactive power diagram is systematically derived by considering the typical power-speed relationship and converter loading limits. The authors discuss also some special operating modes limiting the reactive power capability together with aspects of modeling and control that give rise to these limitations.

Enhanced Fault Ride-Through Method for Wind Farms Connected to the Grid Through VSC-Based HVDC Transmission

This paper describes a new control approach for secure fault-ride through of wind farms connected to the grid through a voltage source converter-based high voltage DC transmission. On fault occurrence in the high voltage grid, the proposed control initiates a controlled voltage drop in the wind farm grid to achieve a fast power reduction. In this way overvoltages in the DC transmission link can be avoided. It uses controlled demagnetization to achieve a fast voltage reduction without producing the typical generator short circuit currents and the related electrical and mechanical stress to the wind turbines and the converter. The method is compared to other recent FRT methods for HVDC systems and its superior performance is demonstrated by simulation results.

Variable Frequency Operation of DFIG based Wind Farms connected to the Grid through VSC-HVDC Link

High voltage DC transmission using voltage source converters is a quite new technology for the interconnection of offshore wind farms. It facilitates variable frequency operation in the wind farm grid, which can be used for improving the performance of the wind turbines. In this paper a new approach for the coordinated control of voltage source converter based HVDC and wind turbines equipped with doubly-fed induction generators is introduced. Variable frequency can be used to reduce the machine slip. Thus a lower rated converter is sufficient for the same speed range. Alternatively, the slip-limiting operation can be used for extending the speed range while keeping the converter rating at the same level. The basics of variable frequency operation of doubly-fed induction generators are discussed, and the performance is evaluated by a simulation model in MATLAB/Simulink.

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.

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.

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.

Response of DFG-based wind farms operating on weak grids to voltage sags

The integration of wind power into the electrical power systems is continuously increasing all over the world. In many power systems the wind resources are located in remote areas far away from the load centres. Then the short circuit ratio (SCR) at the point of common coupling (PCC) could be small and needs special consideration in wind turbine control and operation. In this paper the authors investigate the effect of the SCR on the behaviour of wind farm following sudden voltage drop due to grid short circuits. For this a 90 MW wind farm has been simulated taking into account realistic DFG and corresponding controller implementations. The objective of this paper is to provide an insight in to the capability of DFG-based wind farm to participate in voltage support during grid faults especially in weak power systems.