F. Shewarega

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.

Impact of large wind power generation on frequency stability

This paper explores the available control options for enabling wind power generation plants to participate on the maintenance of system frequency following a major power imbalance. Taking the currently employed control structures for wind generators as the baseline case, possible expansions and additional features have been discussed. The options include voltage or alternatively frequency dependent active power control. The responses of these control schemes vis-a-vis their frequency supporting capability in a power system contingency situation have been simulated and with one another compared. It was found out that at the conceptual level there are indeed a range of options which would place wind generating plants in a position to support system frequency in an emergency situation

Consideration of wind farm wake effect in power system dynamic simulation

This paper deals with the modelling of the wake effect in wind farms for use in dynamic power system simulation software. First, basic relationships describing the leeside wind speed as a function of the incoming wind speed under ideal conditions are put together. Through aerodynamic efficiency considerations the ideal power-coefficient of the turbine is then related to the actual power coefficient provided by the turbine manufacturer. Finally, mathematical expressions are formulated for the wake wind speed in relation to location within the park and the wind direction. The model thus obtained is implemented on a power system simulation package as an additional feature. Together with the full range of capabilities of the package for steady state as well as dynamic analysis, the model enables an in-depth study of the wake effect and its impact on the farm in terms of output power and its interactions with the network. Some results illustrating the impact of the wake effect on steady state system operation have been presented.

Impact of large offshore wind farms on power system transient stability

This paper deals with the impact of large scale wind power integration on the transient stability performance of power systems. Typical offshore wind farm topologies including the medium and high voltage submarine cables, the generator, the multi-stage transformers, the controllers were simulated using real-world values. Voltage and power at the point of common coupling and the swing curves of conventional synchronous generators following a major grid fault were computed and the critical fault clearing times were determined and compared with one another for different levels of wind integration. It was found out that using the currently established controller structure and parameter settings, the transient stability performance of the system deteriorates with increasing wind integration. The study has also revealed that there are a range of options to improve the performance of the system even to the extent of improving on the performance of the system beyond the baseline scenario where there is no wind power generation at all.

Modeling of Wind Turbines Equipped with Doubly-Fed Induction Machines for Power System Stability Studies

This paper deals with the modeling of the doubly-fed induction generator (DFIG) for stability studies. Using the space-phasor representation and the underlying quasi stationary model, suitable control algorithms for the simulation of the DFIG operating on an interconnected system is developed. The schemes include the pitch-angle/speed control and the decoupled control of the real and reactive power outputs. The model is then implemented on a large test network. Simulations have been carried out to study the control behavior of wind turbines to wind speed changes and the response to three phase grid faults. The results demonstrate the suitability of the models and the overall control architecture for stability studies and give insight into the scope of possible targeted control interventions. The short-circuit test in particular highlights the relationship between the control philosophy adopted and the possibility of the DFIG actively participating in supporting the network voltage

Dynamic interaction of large offshore wind farms with the electric power system

The paper explores the influence of large offshore wind farms on the performance of the system to which they are connected. To meet the emerging requirements of the power system with regard to voltage and frequency, controllers with extended features have been used that enable the control of the terminal voltage and the participation of large wind farms on system frequency control. The models of the wind turbine and the electrical machines together with the proposed control structure are integrated into a power system simulation environment Then, the impact of planned offshore wind farms on the transient stability performance of parallel operating conventional power plants and the bus voltage profile of the network during fault for alternative wind generator types are investigated. Additionally, the response of the wind farm to a major load change in a large multi-machine network is simulated and the results discussed.

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.

Evaluating the Mean-Variance Mapping Optimization on the IEEE-CEC 2014 test suite

This paper provides a survey on the performance of the hybrid variant of the Mean-Variance Mapping Optimization (MVMO-SH) when applied for solving the IEEE-CEC 2014 competition test suite on Single Objective RealParameter Numerical Optimization. MVMO-SH adopts a swarm intelligence scheme, where each particle is characterized by its own solution archive and mapping function. Besides, multi-parent crossover is incorporated into the offspring creation stage in order to force the particles with worst fitness to explore other sub-regions of the search space. In addition, MVMO-SH can be customized to perform with an embedded local search strategy. Experimental results demonstrate the search ability of MVMO-SH for effectively tackling a variety of problems with different dimensions and mathematical properties.

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.