W.L. Kling

Aggregated modelling of wind parks in power system dynamics simulations

Increasing numbers of wind turbines are connected to electrical power systems, in order to reduce the adverse environmental impact of conventional electrical power generation. A tendency can be observed to erect these turbines in wind parks, connected to the high voltage transmission grid. These parks effect the dynamic behaviour of power systems, because in wind turbines generator types that are different from the conventional synchronous generator are used. To investigate the impact of a wind park on the dynamics of the power system to which it is connected, an adequate model is required. In order to avoid the necessity of developing a detailed model of a wind park with tens or hundreds of wind turbines and their interconnections and to calculate the wind speed signal for each individual turbine, aggregated wind park models are needed. In the paper, aggregated models for wind parks equipped with either constant or variable speed wind turbines are presented. It is shown that results obtained with an aggregated model and with a detailed model show a high degree of correspondence, both for normal operation and for disturbances.

Phase shifting transformers: principles and applications

The purpose of this paper is to give a short overview of existing technologies regarding phase shifting transformers (PSTs). A classification is made based on the symmetrical or asymmetrical and on the direct or indirect character of the PST. As a case-study, the PSTs in Meeden, The Netherlands are studied more profoundly. Furthermore, a model is developed on a real-time digital simulator (RTDS) in order to demonstrate the capabilities of the PST

Dynamic Stability of Power Systems with Power Electronic Interfaced DG

In this paper, the transient stability of power systems with a high penetration level of power electronic interfaced (converter connected) distributed generation is explored by means of computer simulations. Small 2 and 3 bus test systems are used. The converter is modeled as a three-phase full-bridge IGBT voltage source converter (VSC). The control setting is such that during the actual disturbance the converter connected DG stays connected to the system but with a current limiter. The simulations are performed by using MATLAB SimPowerSystems

Impacts of distributed generation penetration levels on power systems transient stability

Concerns on environmental and economical issues drive the increasing developments that support small scale generators to be connected close to distribution networks, i.e. distributed generation (DG). When connected in small amounts, the impact of DG on the power system transient stability will be negligible, however, when the penetration of DG increases, its impact is no longer restricted to the distribution network but starts to influence the whole system, including the transmission system transient stability. In this paper, the transmission system transient stability is investigated when a fault is applied in all possible branches (regarding the N-1 security analysis). In this studie the penetration level of DG implementation is raised in two ways: (1) a load increase is covered by DG implementation (with a constant centralized generation) or increased CG output, and (2) a reduction of centralized generation is covered by DG (with a constant load).

Technical benefits of distributed storage and load management in distribution grids

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.

Investigating impacts of distributed generation on transmission system stability

Driven by the increasing environmental concerns and the increasing amount of new generation technologies, it is expected that many new generation technologies, including renewable generation, will be connected to the electrical power system. A lot of these new technologies will be connected at the distribution level. When the penetration of this distributed/dispersed generation (DG) is low, the impacts of the DC on the transmission system transient stability may be neglected. However, when the penetration of DG strongly increases, its impact is no longer restricted to the distribution network but begins to influence the whole system. In this paper the impact of DG on the transmission system transient stability is investigated by considering different types of DG technologies and various penetration levels. It is observed that both the types of DG technologies and the penetration levels of DG in the power system have a strong influence on the transmission system transient stability.

Statistical Wind Speed Interpolation for Simulating Aggregated Wind Energy Production under System Studies

In this paper we present a statistical interpolation method to generate a time series of system-aggregated wind power production values that can be used as an input to system operations planning tools such as unit commitment (UC) and economic dispatch (ED). We use historical wind speed data measured at several locations, in order to estimate average wind patterns and express the covariance between locations as a function of their distance. Then, for a new set of locations where wind parks are planned, we create wind time series for the study period such that the spatial correlation between the sites is taken into account. Depending on the system under study, this may be of specific importance due to the concentration of areas with favorable wind conditions, resulting in strong correlations between wind park outputs. These cross-correlations are essential when evaluating system adequacy and security in planning mode, in the presence of large-scale wind power. The resulting aggregated wind power time series are finally fed into the UC-ED module to help evaluate the amount of total system reserve required to maintain an adequate level of reliability. The method is applied to a simplified version of the Dutch power system

Analytical Approach to Grid Operation With Phase Shifting Transformers

Analytical expressions are derived to gain insight in the operating principles of phase shifting transformers (PSTs) in a highly meshed grid. To this extent, the dc load flow algorithm is adapted to account for such devices. This leads to a linear expression for the relation between PST settings and the active power flow in the lines. Based on these equations, the total transfer capacity (TTC) can be described mathematically, which allows for optimization. Furthermore, the linear least squares method is used to distribute a cross-border transport evenly over the interconnectors.

Dynamic models for transient stability analysis of transmission and distribution systems with distributed generation: An overview

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.

System integration of large-scale wind power in the Netherlands

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