Stephan Engelhardt

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.

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.

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.

Improving the Reactive Power Capability of the DFIG-Based Wind Turbine During Operation Around the Synchronous Speed

The doubly fed induction generator (DFIG) equipped with self-commutated insulated gate bipolar transistor (IGBT) voltage source converter (VSC) is one of the most popular topologies used in wind power systems. It has the ability to control active and reactive power independently. The reactive power capability is subject to several limitations which change with the operating point. Around synchronous operating point, a special attention is needed since the limitation of maximum junction temperature of the IGBTs cause a reduction on maximum permissible output current at the rotor side. This paper investigates the thermal behavior of the converter using semiconductor losses and thermal model based on the IGBT manufacturer datasheet. Different pulse-width modulation (PWM) types, including continuous and discontinuous types are applied and the results of reactive power capability are compared. Simulation results show that appropriate selection of PWM type is necessary at around synchronous speed to increase the maximum permissible rotor current as well as reactive power capability.

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.

Comparison of the grid support capability of DFIG-based wind farms and conventional power plants with synchronous generators

Current grid codes stipulate the same or similar behaviour during grid faults for wind turbines as for conventional power plants based on synchronous generators. But the technology and control of both devices is completely different. Since modern wind turbines use IGBT-based frequency converters, they provide a very fast control with the disadvantage of tight thermal limits for the IGBTs. The excitation control of synchronous generators is rather slow and has nearly no effect on the transient process during faults. Only the synchronous generator itself provides good grid support during severe faults by virtue of its large overload capability. This paper compares both generation techniques through conceptual discussion and based on simulation results. The results show that there are significant differences in terms of their behaviour during grid faults. The results also point to the need for a revision of the current grid codes with respect to more dedicated requirements.

Capability and limitations of DFIG based wind turbines concerning negative sequence control

Modern wind turbine systems based on doubly-fed induction generator (DFIG) technology offer negative sequence control capability during grid faults or unsymmetrical system operation. This paper deals especially with the effects of current and voltage limitation of a typically rated converter system design based on positive sequence control requirements. At first a short summary of the most common alternative negative sequence control objectives is given. Then the corresponding rotor side converter (RSC) control algorithms are described. This is followed by typical characteristics of current capabilities as well as voltage constraints. Finally detailed calculations regarding control possibilities and limitations in the positive and negative sequence are shown.

Wind turbine modeling, LVRT field test and certification

The need to validate wind turbine models for the use in grid integration studies is addressed in an increasing number of grid codes. In several countries (Spain, Australia, UK, Germany) requirements for simulation models are now required for the grid access of wind turbines. In Germany, triggered by the new renewable energy law (EEG), a guideline for validating electrical simulation models of wind turbines has been designed. The model validation approach chosen will be described. The aim of this validation approach is to quantify the error between measurement and simulation. This is necessary in order to give a reliable figure for the model uncertainty for the use of the model in studies. Results of FRT-measurements with balanced and unbalanced faults will be compared to the results of an RMS DFIG model. It can be shown that the validation approach can be applied with success both to balanced and unbalanced faults.