Introduction to stochastic programming

This paper provides an overview of grid code technical requirements regarding the connection of large wind farms to the electric power systems. The grid codes examined are generally compiled by transmission system operators (TSOs) of countries or regions with high wind penetration and therefore incorporate the accumulated experience after several years of system operation at significant wind penetration levels. The paper focuses on the most important technical requirements for wind farms, included in most grid codes, such as active and reactive power regulation, voltage and frequency operating limits and wind farm behaviour during grid disturbances. The paper also includes a review of modern wind turbine technologies, regarding their capability of satisfying the requirements set by the codes, demonstrating that recent developments in wind turbine technology provide wind farms with stability and regulation capabilities directly comparable to those of conventional generating plants.

A review of grid code technical requirements for wind farms

(1990). ENERGY FUNCTION ANALYSIS FOR POWER SYSTEM STABILITY. Electric Machines & Power Systems: Vol. 18, No. 2, pp. 209-210.

Energy function analysis for power system stability

With rapid development of wind power technologies and significant growth of wind power capacity installed worldwide, various wind turbine concepts have been developed. The wind energy conversion system is demanded to be more cost-competitive, so that comparisons of different wind generator systems are necessary. An overview of different wind generator systems and their comparisons are presented. First, the contemporary wind turbines are classified with respect to both their control features and drive train types, and their strengths and weaknesses are described. The promising permanent magnet generator types are also investigated. Then, the quantitative comparison and market penetration of different wind generator systems are presented. Finally, the developing trends of wind generator systems and appropriate comparison criteria are discussed. It is shown that variable speed concepts with power electronics will continue to dominate and be very promising technologies for large wind farms. The future success of different wind turbine concepts may strongly depend on their ability of complying with both market expectations and the requirements of grid utility companies.

Overview of different wind generator systems and their comparisons

This paper presents a new method based on the cost-benefit analysis for optimal sizing of an energy storage system in a microgrid (MG). The unit commitment problem with spinning reserve for MG is considered in this method. Time series and feed-forward neural network techniques are used for forecasting the wind speed and solar radiations respectively and the forecasting errors are also considered in this paper. Two mathematical models have been built for both the islanded and grid-connected modes of MGs. The main problem is formulated as a mixed linear integer problem (MLIP), which is solved in AMPL (A Modeling Language for Mathematical Programming). The effectiveness of the approach is validated by case studies where the optimal system energy storage ratings for the islanded and grid-connected MGs are determined. Quantitative results show that the optimal size of BESS exists and differs for both the grid-connected and islanded MGs in this paper.

Sizing of Energy Storage for Microgrids

This paper presents a stochastic cost model and a solution technique for optimal scheduling of the generators in a wind integrated power system considering the demand and wind generation uncertainties. The proposed robust unit commitment solution methodology will help the power system operators in optimal day-ahead planning even with indeterminate information about the wind generation. A particle swarm optimization based scenario generation and reduction algorithm is used for modeling the uncertainties. The stochastic unit commitment problem is solved using a new parameter free self adaptive particle swarm optimization algorithm. The numerical results indicate the low risk involved in day-ahead power system planning when the stochastic model is used instead of the deterministic model.

A Stochastic Model for the Optimal Operation of a Wind-Thermal Power System

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.

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

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

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

In Germany 18 GW wind power will have been installed by the end of 2005. Until 2020, this figure reaches the 50 GW mark. Based on the results of recent studies and on the experience with existing wind projects modification of the existing grid code for connection and operation of wind farms in the high voltage grid is necessary. The paper discusses main issues of the suggested requirements by highlighting major changes and extensions. The topics considered are fault ride-through, grid voltage maintenance respective voltage control, system monitoring and protection as well as retrofitting of old units. The new requirements are defined taking into account some new developments in wind turbine technologies which should be utilized in the future to meet grid requirement. Monitoring and system protection is defined under the aspect of sustainability of the measures introduced.

Advanced grid requirements for the integration of wind turbines into the German transmission system

This paper discusses issues of German grid codes relating to wind turbines. With the high utilization of wind power a simultaneous loss of several thousand MW wind generation became a realistic scenario in the German power system. Therefore, the main requirements concern the fault ride through capability of wind turbines. Accordingly, disconnection of wind turbines and wind farms above 15 % nominal voltage at the grid connection nodes is not allowed. Besides, following network faults wind turbines have to supply a definite reactive current depending on the instantaneous voltage. Furthermore, they must return quickly to normal operation. The frequency range wind turbines have to tolerate is about 47.5-51.5 Hz. According to the wishes of German transmission grid operators, large wind farms have to be treated in the future like conventional power plants.

Istvan Erlich

Papers

A Stochastic Model for the Optimal Operation of a Wind-Thermal Power System

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

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

Advanced grid requirements for the integration of wind turbines into the German transmission system

Grid code requirements concerning connection and operation of wind turbines in Germany