Friedrich Koch

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.

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.

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.

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.

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 the 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 offshore wind farm connected through a long submarine cable to the high voltage grid.

The Effect of Large Offshore and Onshore Wind Farms on the Frequency of a National Power Supply System; Simulation Modelled on Germany

The paper describes the simulated effect of large windfarms on the frequency of the interconnected grid system within which they are operating. The study is modelled on Germany. The features of the well-tested and widely used power system simulation package Power System Dynamics (PSD) were expanded so that windfarms with all commonly used wind turbine generators and their control structures can be simulated. In particular, the windfarm interaction with the power grid can be studied. Additionally, the effects of topology and wind conditions at the location of the wind turbines on the output power have been incorporated into the simulation. The result reveals that the stochastic nature of wind power at significant capacity may cause noticeable frequency fluctuation on the whole grid. Without countermeasures, the fluctuating power may use a disproportionately large component of the primary control power reserve of the grid system. The paper describes options for the integration of large offshore and onshore windfarms in the task of primary frequency control.