Vladislav Akhmatov

An aggregate model of a grid-connected, large-scale, offshore wind farm for power stability investigations—importance of windmill mechanical system

Abstract An aggregate model of a large-scale offshore wind farm, comprising 72 wind turbines of 2xa0MW rating each, is set up. Representation of the shaft systems of the wind turbines shall be taken into account when a simplified aggregate model of the wind farm is used in voltage stability investigations. Because the shaft system gives a soft coupling between the rotating wind turbine and the induction generator, the large-scale wind farm cannot always be reduced to one-machine equivalent and use of multi-machine equivalents will be necessary for reaching accuracy of the investigation results. This will be in cases with irregular wind distribution over the wind farm area. The torsion mode of the shaft systems of large wind turbines is commonly in the range of 1–2xa0Hz and close to typical values of the electric power grid eigenfrequencies why there is a risk of oscillation between the wind turbines and the entire network. All these phenomena are different compared to previous experiences with modelling of conventional power plants with synchronous generators and stiff shaft systems.

Advanced simulation of windmills in the electric power supply

An advanced model of a grid-connected windmill is set up where the windmill is a complex electro-mechanical system. The windmill model is implemented as a standardised component in the dynamic simulation tool, PSS/E, which makes it possible to investigate dynamic behaviour of grid-connected windmills as a part of realistic electrical grid models. This means an arbitrary number of wind farms or single windmills within an arbitrary network configuration. The windmill model may be applied to the study of electric power system stability and of power quality as well. It is found that a grid-connected windmill operates as a low-pass filter, whereby the two following observations are made: (1) interaction between the electrical grid and the mechanical systems of grid-connected windmills is given by a low frequency oscillation as the result of disturbances in the electric grid; (2) flicker, which is commonly explained by the dynamic wind variation, may also be caused by mechanical eigenswings in the windmill mechanical construction.

Modelling and Transient Stability of Large Wind Farms

The paper is dealing with modelling and short-term voltage stability considerations of large wind farms. A physical model of a large offshore wind farm consisting of a large number of windmills is implemented in the dynamic simulation tool PSS/E. Each windmill in the wind farm is represented by a physical model of grid-connected windmills. The windmill generators are conventional induction generators and the wind farm is ac-connected to the power system. Improvements of short-term voltage stability in case of failure events in the external power system are treated with use of conventional generator technology. This subject is treated as a parameter study with respect to the windmill electrical and mechanical parameters and with use of control strategies within the conventional generator technology. Stability improvements on the wind farm side of the connection point lead to significant reduction of dynamic reactive compensation demands. In case of blade angle control applied at failure events, dynamic reactive compensation is not necessary for maintaining the voltage stability.

Contribution to a dynamic wind turbine model validation from a wind farm islanding experiment

Abstract Measurements from an islanding experiment on the Rejsby Hede wind farm, Denmark, are used for the validation of the dynamic model of grid-connected, stall-controlled wind turbines equipped with induction generators. The simulated results are found to be in good agreement with the measurements and possible discrepancies are explained. The work with the wind turbine model validation relates to the dynamic stability investigations on incorporation of large amount of wind power in the Danish power grid, where the dynamic wind turbine model is applied.

Simulation Model of the Transmission Grid for a Large Offshore Windfarm, Used in Education and Research at the Technical University of Denmark

A small test software-model for the transmission system containing a number of central power plants, consumption centres, local wind turbines and a large offshore windfarm has been implemented in the simulation tool Powerfactory (DigSilent). This model demonstrates the interaction between electricity-producing wind turbines and a realistic transmission system, for instance short-term voltage stability and ride-through capability of the wind turbines. The model has been developed by the Danish Transmission System Operator Energinet.dk for use by the Centre for Electric Technology (CET) at Technical University of Denmark. It has been applied in the education of electric power engineers and in research projects. An example of its use is presented.

Integrating dispersed generation into the Danish power system - present situation and future prospects

Since the early 80s a huge amount of dispersed generation (DG) has been implemented into the Danish power systems. Today the Danish system has a share of 18.5% electricity consumption produced by wind turbines and 26.5% produced by combined heat and power units (CHP), of which the biggest part is installed in the western part of Denmark. The paper shows the technical measures as well as utilization of market mechanisms applied by the Danish system operator, Energinet.dk, to handle the challenging situation of safe and reliable system operation. Future prospects with respect to the internationally growing wind power capacity and respective need for a market for ancillary services are presented

Full-load Converter Connected Asynchronous Generators for MW Class Wind Turbines

Wind turbines equipped with full-load converter-connected asynchronous generators are a known concept. These have rating up to hundreds of kW and are a feasible concept for MW class wind turbines and may have advantages when compared to conventional wind turbines with directly connected generators.* The concept requires the use of full-scale frequency converters, but the mechanical gearbox is smaller than in conventional wind turbines of the same rating. Application of smaller gearbox may reduce the no-load losses in the wind turbines, which is why such wind turbines with converter connected generators may start operation at a smaller wind speed. Wind turbines equipped with such converted connected asynchronous generators are pitch-controlled and variable-speed. This allows better performance and control. The converter control may be applied to support the grid voltage at short-circuit faults and to improve the fault-ride-through capability of the wind turbines, which makes the concepts relevant for large wind farms. The Danish transmission system operator Energinet-DK has implemented the general model of wind turbines equipped with converter connected asynchronous generators with the simulation tool Powerfactory (DlgSilent). The article presents Energinet-DKs experience of modeling this feasible wind turbine concept.

Static Synchronous Compensator (Statcom) for Dynamic Reactive-Compensation of Wind Turbines

This paper reviews the common control characteristics of a Statcom and presents the control strategies which may be applied for compensation of large wind farms. The Statcoms control and protection are modelled with the simulation tool Powerfactory (DigSilent). The analysis evaluates (i) a realistic test grid containing local wind turbines in the distribution network, and (ii) a large offshore windfarm connected to the transmission system.

Excessive Over-Voltage in Long Cables of Large Offshore Windfarms

Large offshore windfarms are connected to onshore transmission grids through long high-voltage cables. When a large offshore windfarm goes into isolated operation with its high-voltage cable, for example by an action of the on-shore main-circuit breaker, there is a risk of significant over-voltage in the cable and the windfarm transformer. This paper analyses such a situation for both an existing and a planned large offshore windfarm in Denmark.

Planning under Uncertainty - Securing Reliable Electricity Supply in Liberalized Energy Markets

The Danish power system is characterized by a high share of dispersed generation (DG) and wind power plants. Today about 24 % of the electricity consumption is produced by wind turbines and about 19 % by combined heat and power units (CHP). The paper shows, how the Danish transmission system operator (TSO), Energinet.dk, handles the tasks of providing secure system operation today and in future, using technical and market mechanisms as well.