The increasing penetration of variable-speed wind turbines in the electricity grid will result in a reduction of the number of connected conventional power plants. This will require changes in the way the grid frequency is controlled. In this letter, a method is proposed to let variable-speed wind turbines emulate inertia and support primary frequency control. The required power is obtained from the kinetic energy stored in the rotating mass of the turbine blades.@en

This paper presents the results of a simulation of system operation in the Netherlands in the presence of future large-scale wind energy production. The study is aimed at identifying bottlenecks in system planning and operation due to wind integration, in particular base-load and ramp rate problems. These may constraint the amount of wind that can be accommodated given a projected production park of dispatchable units and yearly load profile by 2012. Wind data from 2004-2005, interpolated to existing locations for onshore and planned locations for offshore wind parks, were used to create a realistic yearly wind energy output profile. The unit commitment and economic dispatch formulation includes ramp rate constraints for generation schedules and reserve activation as well as minimum up- and down times. Of particular interest in this study are the combined heat & power (CHP) units, which impose additional constraints coupling their heat and energy production. Since no insight was available into the aggregated predictability of wind generation, both a 0-MW prediction, where conventional units are scheduled to meet the total load, and a perfect prediction have been investigated. No forms of electrical or heat storage were considered. The results show no ramp rate problems in the Dutch system by 2012, however base-load problems may arise at high wind penetration levels, only to be prevented by wasting available wind resources.@en

Abstract Distributed Generation is increasing in nowadays power systems. Small scale systems such as photovoltaic, biomass or small cogeneration plants are connected to the distribution level, while large wind farms will be connected to the transmission level. Both trends lead to a replacement of large synchronous generators as the dominating generation technology. Up to now, transient stability of transmission systems has been analysed to a satisfactory degree of accuracy with a simplified representation of the distribution systems. In future, distributed generation will more and more influence the behaviour of the system. Stiff, inverter-based local generation technologies may improve the system stability; however, increasing electrical distances between large synchronous generators in operation will impede the system stability. These (and other) diverging effects have to be studied in detail. This overview paper summarises the latest findings and reveals future research questions. It is concluded that the accuracy and validity of the currently applied dynamic models for transient stability analysis of power systems with high penetration of DG should be further investigated.@en

Stochastic generation, i.e., electrical power production by an uncontrolled primary energy source, is expected to play an important role in future power systems. A new power system structure is created due to the large-scale implementation of this small-scale, distributed, non-dispatchable generation; the `horizontally-operated¿ system. Modeling methodologies that can deal with the operational uncertainty introduced by these power units should be used for analyzing the impact of this generation to the system. In this contribution, the principles for this modeling are presented, based on the decoupling of the single stochastic generator behavior (marginal distribution-stochastic unit capacity) from the concurrent behavior of the stochastic generators (stochastic dependence structure-stochastic system dispatch). Subsequently, the stochastic bounds methodology is applied to model the extreme power contribution of the stochastic generation to the system, based on two new sampling concepts (comonotonicity¿countermonotonicity). The application of this methodology to the power system leads to the definition of clusters of positively correlated stochastic generators and the combination of different clusters based on the sampling concepts. The stochastic decomposition and clustering concepts presented in this contribution provide the basis for the application of new uncertainty analysis techniques for the modeling of stochastic generation in power systems. Keywords: Stochastic power generation; Distributed generation; Steady-state analysis; Uncertainty analysis; Monte-Carlo simulation; Risk management@en

Energy management systems and smart measuring devices at residential level have received significant research attention in the past years. Most of the technological developments designed for energy and/or cost savings employ real-time consumption monitoring, supervision and control. One of the possibilities that these kinds of systems provide to consumers is that they can give an overview of the energy consumption, which enables consumers to take advantage of the available information and take steps towards saving energy. The designs of such systems are based mostly on historical data and forecasting techniques of a household or a neighbourhood. This paper focuses on the analysis of electricity and heat profiles of a group of houses consisting of different housing types and resident categories. The effects of load flexibility are analysed, as well as the energy consumption profiles of electrical appliances; possible shifting of consumption periods are considered and discussed.@en

In a liberalized electricity market, the use of phase shift ing transformers or other power ¿ow controlling devices allows the transmission system operator to utilize the avail able grid infrastructure in a more optimal way. However, each phase shifter adds a degrees of freedom to the control problem, making optimization more dif¿cult. In this paper, the coordination problem is solved by using Particle Swarm Optimization (PSO). The goal is to give an overview of how PSO is used to solve this particular problem, and to demon strate a new area of application for the method.@en

This paper presents a data-driven approach for estimating the degree of variability and predictability associated with large-scale wind energy production for a planned integration in a given geographical area, with an application to The Netherlands. A new method is presented for generating realistic time series of aggregated wind power realizations and forecasts. To this end, simultaneous wind speed time series - both actual and predicted - at planned wind farm locations are needed, but not always available. A 1-year data set of 10-min averaged wind speeds measured at several weather stations is used. The measurements are first transformed from sensor height to hub height, then spatially interpolated using multivariate normal theory, and finally averaged over the market resolution time interval. Day-ahead wind speed forecast time series are created from the atmospheric model HiRLAM (High Resolution Limited Area Model). Actual and forecasted wind speeds are passed through multi-turbine power curves and summed up to create time series of actual and forecasted wind power. Two insights are derived from the developed data set: the degree of long-term variability and the degree of predictability when Dutch wind energy production is aggregated at the national or at the market participant level. For a 7.8 GW installed wind power scenario, at the system level, the imbalance energy requirements due to wind variations across 15-min intervals are ±14% of the total installed capacity, while the imbalance due to forecast errors vary between 53% for down- and 56% for up-regulation. When aggregating at the market participant level, the balancing energy requirements are 2-3% higher. Copyright © 2008 John Wiley & Sons, Ltd.@en

Nowadays the number of dispersed generators (DG) is growing rapidly. This change greatly influence the power system dynamics. A distribution network, where DG are connected to the grid, cannot be considered as passive any more. So in future it will not be possible to use simple equivalents of distribution networks for power system dynamic modeling as it was done before. In dynamic studies the whole power system cannot be represented in detailed manner because of huge system dimension. Therefore special techniques have to be applied for aggregation and order-reduction of distribution networks with DG. In this paper brief review of existing techniques has been given, and after that dynamic reduction using Hankel norm approximation has been performed for a part of a distribution network consisting of DG of different types.@en

This paper deals with the inclusion of VSC-HVdc transmission schemes into stability-type simulations by hybrid methods. These methods allow selected parts of the network to be simulated in detail by including electro-magnetic behaviour of devices and network elements whereas the remainder of the network is simulated in a simplified fashion with emphasis on electro-mechanical interactions. The paper discusses the interface methods needed for coupling both simulations, and presents a simple but robust interface, particularly designed for VSCs. Several aspects that determine the performance of the hybrid simulation are discussed and tested by simulations on an example network. The hybrid simulations have been implemented in Matlab and verified against a full EMT-type simulation in PSS®NETOMAC.@en

Abstract In this study, a commercially available unit commitment and economic dispatch (UC-ED) tool is extended for the simulation of wind power integration in an international environment. An existing generation unit database for the Netherlands is extended to include conventional generation portfolios of neighbouring areas to the Netherlands. Furthermore, wind power in Germany is modelled such that the spatial correlation between wind speeds at different locations in the Netherlands and Germany is maintained. These additions allow the assessment of the benefits of international exchange for wind power integration and a comparison with other integration solutions. The UC-ED tool is applied for annual simulations of a power system with generation portfolios foreseen for the year 2014. Four variants for international exchange possibilities are investigated for different wind power penetrations. The opportunities of the following integration solutions are assessed: use of conventional generation in isolated systems, use of international markets, flexible combined heat and power (CHP), pumped hydro energy storage, compressed air energy storage and interconnection to a hydro-based system. The solutions are placed in an order of potential with respect to technical, economical and environmental aspects. The results show that the advantages of international exchange for wind power integration are large and provide an alternative for the development of energy storage facilities.@en

Wind power plants show different behavior than conventional (synchronous) generators. As the traditional power systems mainly consisted of centralized generation by synchronous machines feeding passive loads, it was well-understood how the system reacted in normal operation as well as during disturbances. As wind power plants are foreseen to increase in size and the amount of installed wind power will grow, the relative contribution of equipment not exhibiting this common behavior increases. At the same time power electronics offer opportunities for additional features to stabilize the power system. Transmission system operators impose requirements on the (dynamic) capabilities of connected new generation resources (including wind power plants) which are specified in grid codes. In this paper, the importance of such requirements is explained by looking at the needs of the power system and by showing simulation results for a test network. The paper facilitates a detailed understanding of the underlying phenomena related to grid code requirements with a focus on low-voltage ride-through and voltage support by reactive current boosting.@en

Abstract On the one hand, several developments make the operation of the distribution grids more and more complex. On the other hand developments in energy storage and the availability of loads which are not time critical bring up possibilities to provide more flexibility in the grid and to use the available system more efficiently. In this paper, these and other advantages of as well distributed energy storage as load management of not time critical loads are discussed. An approach is described to analyse the available capacity in the grids, to investigate how this capacity can be used by applying storage or load management and to compare the benefits of storage and load management. This approach is illustrated using a real medium voltage network and measured data. It shows the part of the capacity in the distribution grids which is now unused, but can be made available by applying storage or load management of non-critical loads. The size of the storage and the non-critical loads which are needed to use this capacity are determined. The characteristics of future loads, the need to support integration of distributed generation and the desired level of reliability of supply are factors that determine the size of the storage and how storage or load management can be applied best.@en

This paper presents an assessment of the kinetic energy reserve that could be made available by aggregating a distributed group of wind farms. The size of reserve available for a single turbine and the range of wind speeds where it can be assumed available is computed and compared with the kinetic energy delivered by synchronous generators during typical transients. The size and availability of an aggregated reserve is then computed using wind speed data from 39 locations on and off the shores of the Netherlands. The dependence of this reserve on the area and density of sites being aggregated is presented. Finally, the aggregated reserve is compared on a statistical basis with the energy required for 87 frequency dip incidents in the European continental grid. It is concluded that although improved communications and control would be necessary, the technical potential could meet or exceed the need for inertial response forty percent of the time.@en

This paper presents the concept of an 'empty network' and shows how the power balance can be controlled in such a system. In this study, an 'empty network' is defined as a transmission system in which no rotating mass is present. All generators are connected to distributed systems and 'hidden' behind power electronic interfaces. One generator creates a neat 50 Hz voltage that serves as a frequency reference for the other generators. Consequently, a power imbalance cannot be detected in the classical way, as an altered system frequency. Therefore, a novel control system to maintain the power balance is needed. In this paper, voltage deviations are used to detect power imbalances, and remedies to eliminate the negative consequences of using the voltage deviations to detect the power imbalances are proposed and discussed. Keywords: empty network; power balance control; distributed generation; distribution systems; power electronic interfaces; voltage deviations; power imbalances.@en

Keywords wind power ¿ market ¿ power system balancing ¿ program responsible parties Abstract Wind power is becoming a large-scale electricity generation technology in a number of European countries, including the Netherlands. Owing to the variability and unpredictability of wind power production, large-scale wind power can be foreseen to have large consequences for balancing generation and demand in power systems. As an essential aspect of the Dutch market design, participants are encouraged to act according to their energy programs, as submitted day-ahead to the system operator. This program responsibility shifts the burden of balancing wind power away from the system operator to the market. However, the system operator remains the responsible party for balancing any generation/load imbalances that may still be arising in real time. In this article, features that are unique for the Dutch market design are presented and their implications on the system integration of wind power are investigated. It is shown that the Dutch market design penalizes the intermittent nature of wind power. A discussion of opportunities and threats of balancing wind power by use of market forces is provided. Last, an outline is given of future work. Copyright © 2006 John Wiley &Sons, Ltd. -------------------------------------------------------------------------------- Received: 19 July 2005; Revised: 5 October 2005; Accepted: 23 January 2006@en

The ongoing liberalization process in Europe together with the growing penetration of renewable energy sources (RES), e.g. wind power, require an internationally oriented transmission planning approach that considers the increased uncertainties in terms of trade, location of generation and output of intermittent RES. This paper identifies and ranks bottlenecks, which is the first step of the transmission expansion planning process of interconnected transmission grids. A round-the-year approach is adopted by combining market simulations with static security analysis. Many combinations of load and generation (including renewables) are created and analyzed, using unit dispatch based on cost optimization. For each combination the branch loadings are determined for normal and contingency situations. A statistical risk-based approach for ranking the most severe bottlenecks is developed. The method is illustrated on a modified New England test system where wind power was added at several buses. A regional analysis and a detailed per-area analysis are presented.@en

This paper presents the concept of an "empty network" and shows how the power balance can be maintained in such a system. In this study, an "empty network" is defined as a power system in which no rotating mass is present; all generators are grid-connected via power electronic interfaces, so that the possible inertia of the generators is 'hidden' from the network. One generator creates a neat 50 Hz voltage that serves as a frequency reference for the other generators. Conventional power systems are mainly supplied by large centralized synchronous generators. These generators are connected directly to the grid so that there is a coupling between the generator rotor speed, and thus the system frequency, and the power balance in the system. Therefore, an unbalance manifests itself by an altered system frequency. In the empty network, the frequency is fixed, as it is created by a power electronic device, and an unbalance cannot be detected in the classical way. In this paper, an empty network is simulated on an RTDS (real time digital simulator). Voltage deviations are used to detect the power unbalances. Simple test systems that consist of 1,2 and 3 buses are applied, in which all generators are modeled as current sources, except for the frequency reference, which is a voltage source. A load jump is simulated to cause a power unbalance in the system. The study shows that by using voltage deviations as control signals, the power balance can be maintained in an empty network.@en

Abstract A production cost simulation model for the NorthWestern part of the UCTE system is developed in this paper. Information from various sources was gathered and combined to produce an integrated representative model of the areas under study. Specifically, the technical and economical limits of wind power integration within the power system are highlighted, by means of observing and analyzing various unit commitment schedules under different scenarios. The main question refers to which level of wind penetration the power system can operate in a safe and reliable fashion, while the incoming wind power is completely integrated and correlations between system variables in adjacent areas are taken into account. By answering this question, the system planner will have the flexibility to arrange in the future the power system operation accordingly so as to decrease the operation costs to a minimum and increase the efficiency of the total system to maximum. An investigation of this problem statement can only be executed with global models, which take into account both technical and economic constraints and further optimize the operation of the power system.@en

This paper proposes a new approach - set up within the REALISEGRID research project - for speeding up approval procedures for building transmission infrastructure. The experiences of Transmission System Operators from Netherlands, Italy, France, Austria and Germany are studied in order to generalize and pinpoint the main weaknesses and strengths of current approval procedures. Recommendations concerning how to reduce the time needed for approval procedures are made. The problem of consensus improvement is also analysed. Suggestions for better structuring of the decision process in transmission planning, with high involvement of stakeholders, are made. A systemic approach for speeding up authorization procedures is proposed by combining the country survey and the consensus improvement findings. Actions are defined on different time horizons.@en

The development of transnational offshore grids will play a major part in the grid integration of large-scale offshore wind power. Up to now, such an infrastructure has never been built and, when finished, it will probably consist of a combination of different transmission technologies. Therefore, the effects of these grids on the power system must be studied thoroughly The North Sea transnational grid research project intends to study technical and economic aspects regarding the development of an electrical network in the North Sea. It aims to provide a comprehensive technical reference for policy makers in the Netherlands. This paper describes the objectives of the North Sea transnational grid research project and the approaches to be used. Moreover, an overview of other ongoing or recently finished research projects regarding transnational offshore grids in Europe will be provided. These are compared to the North Sea transnational grid project by assessing where the technicaleconomic focus lies, the time horizons and methods used.@en