Analysis of Dynamic Behaviour of Electric Power Systems With

ØRSTED•DTU, Electric Power Engineering

1

Description of an Industrial Ph.D. project

“Analysis of dynamic behaviour of electric power systems with large amount of wind power” Project description

Participants:

Danish Academy of Technical Sciences, NESA A/S, Technical University of Denmark.

Ph.D.-student:

Vladislav Akhmatov, M.Sc., [email protected]

Supervisors:

Arne Hejde Nielsen (DTU), Hans Knudsen (Danish Energy Authority), Jørgen Nygaard Nielsen (NESA), Niels K Poulsen (DTU), J Kaas Pedersen (DTU).

Period:

November 1999 – April 2003 (deadline for the Ph.D. Thesis).

Presentation of the Thesis (The date of publication) June 2003.

Abstract: Four basic wind turbine concepts are treated with respect to modelling in dynamic simulation tools, maintaining of transient voltage stability and uninterrupted operation when the transmission power network is subjected to a three-phased short-circuit fault. The work deals with two issues: the modelling part with focus on complexity of the simulating models of the four wind turbine concepts and the simulating part with investigation of transient voltage stability. The Thesis will also contain a less number of special cases with relation to grid-connection and control of wind turbines and the model validation examples. In the modelling part, the four basic wind turbine concepts are described. All the wind turbine models are implemented in the simulation tool PSS/E (from PTI) as user-written models. (1) Fixed-speed wind turbines equipped with induction generators. The largest manufacturers of this concept are NEG-Micon, Bonus Energy, Nordex. The induction generators are represented by the fifthorder transient model instead of the common third-order model. It is important for accuracy of the results with relation to over-speeding and voltage profile at the grid faults. The shaft system is given by the two-mass model, e.g. with representation of the heavy turbine rotor and the tinny generator rotor connected via the soft shaft. The turbine rotor is given by its aerodynamic model, where the aerodynamic models of different complexities are compared. The blade-angle control is represented by its generic control model. By simple examples, it is shown that the electrical and mechanical parameters of the wind turbine and its induction generator influence on transient voltage stability. By a suitable choice of the parameters, demands of dynamic reactive compensation can be reduced. The controllability of the blade-angle control can be also applied for preventing of fatal over-speeding and so improving of transient voltage stability. (2) Partially variable -speed wind turbines with induction generators and variable -rotor-resistance (VRR) feature. The representation is similar to the given above, but with a generic representation of the VRR. By simple examples, it is demonstrated that the VRR and the pitch-control can contribute to improving

Prepared by Vladislav Akhmatov, date:05/02/2003


2


of transient voltage stability. Demands of dynamic reactive compensation can be reduced. The Danish manufacturer Vestas Wind Systems offers Opti-Slip® wind turbines. (3) Variable-speed wind turbines with doubly-fed induction generators (DFIG). The wind turbines of this concept can be ordered from severe large manufacturers such as Vestas Wind Systems, GE Wind, Gamesa Eolica, NEG-Micon, Nordex. The model is with a transient model of DFIG, a detailed representation of the back-to-back converter and its generic control system, the shaft system with representation of torsional oscillations, and the aerodynamic model of the pitch-controlled wind turbine. The converter blocking sequences are discussed. The IGBTs of the converter are protected against overcurrent and thermal over-loads, which is why the converter blocking will occur when the voltage drop at the grid fault initiates the current transients in the generator and in the converter. Blocking at the grid faults is a common behaviour of these converters. Simulations will be meaningless, when neglecting the converter blocking at the grid faults. An interrupted operation feature with blocking and re-starting of the back-to-back converter is suggested. It is also shown that the grid-side converter can be used for reactive power control at grid faults. A detailed and accurate representation of the back-to-back converter, its (generic) control, its protection and sequences is essential in voltage stability investigations. (4) Variable -speed wind turbines with direct-driven, multi-pole, synchronous generators excited by permanent magnets (PMG) and full-load converters . The largest manufacturer is Lagerwey the Windmaster. The model contains a model of PMG, a model of a full-load back-to-back converter and its generic control and a model of the mechanical system. Again, concerns about the converter blocking and risk of the wind turbine tripping at the grid faults will be present in this concept. The full-load converter can be tripped by over-current (protection of IGBTs against over-current and thermal overloads) and by over-voltage in the dc-link. An uninterrupted operation feature with blocking and restarting of the full-load converter is suggested. It is shown that the grid-side converter can contribute to reactive power control at grid faults (operating as a Statcom). The suggested feature can be also applied for wind turbines with electrically excited synchronous generators. A detailed and accurate representation of the full-load converter, its (generic) control, its protection and sequences is essential in voltage stability investigations. The manufacturer Enercon offers the wind turbines with multi-pole synchronous electrically excited generators and full-load converters. The converter sequences discussed in the Thesis can be also useful for general understanding of the concept with direct-driven, multi-pole synchronous generators, independently from what kind excitation is applied, and the action of full-load converters at the grid faults. In the simulating part the simulating models of the wind turbines are used in a number of simulating cases. Here the power system model is with representation of a number of power plants, consumption centres, a large number of small wind turbine sites, many combined heat-power (CHP) units, and one large offshore wind farm. The wind turbines and the CHP units are equipped with their relay models, which can trip these decentralised units at abnormal operation in the power system. The large offshore wind farm is assigned to the Danish specifications for connection of large wind farms to transmission power networks. The power supply from wind turbines and CHP units corresponds to 50% of power consumption in the grid.



3


Investigations are carried out for a number of cases. With respect to the wind technology those are where the large wind farm is with: (1) (2) (3)

Fixed-speed, active-stall controlled wind turbines equipped with induction generators, Variable -speed, pitch-controlled wind turbines with DFIG, Variable -speed wind turbines with PMG and full-load converters.

With respect to the access to the strong power grid those are where the large wind farm is: (1) (2)

Grid-connected to a weak network, Grid-connected to a relatively strong network.

All the cases are discussed and explained. Practical application of the results: (1)

(2)

The wind turbine models developed and implemented in the simulation tool PSS/E during this project have been applied in investigations of transient voltage stability. The work is with relation to incorporation of large amount of wind power in eastern Denmark and abroad. A feature for stabilisation of large wind farms at grid faults with use of the blade-angle control is suggested in this Ph.D. project. The feature will be applied at the Rødsand offshore wind farm in Denmark, 2003.

Practical information: Only a limited number of these Ph.D. Thesis will be printed and distributed, except of the obligatory copies to the supervisors, the censors and the Danish National Library. The date of publication is XX June 2003 (no more details before this date). More information about the public presentation will be given about March-April 2003. List of publications: [1] H. Knudsen, V. Akhmatov, Induction generator models in dynamic simulation tools, Proc Int Conf Power System Transients IPST’99, June 1999, Budapest, Hungary, pp.253-258.

[2] V. Akhmatov, H. Knudsen, Dynamic modelling of windmills, Proc Int Conf Power Sy stem Transients IPST’99, June 1999, Budapest, Hungary, pp.289-294.

[3] V. Akhmatov, H. Knudsen, Modelling of windmill induction generators in dynamic simulation programs, Proc Int Conf IEEE Power Tech., Aug. 1999, Budapest, Hungary, Paper BPT99-243-12.

[4] V. Akhmatov, H. Knudsen, A. H. Nielsen, Advanced simulation of windmills in the electrical power supply, Int Journal Electrical Power & Energy Systems, vol.22, no. 6, 2000, pp. 421-434.

[5] V. Akhmatov, A. H. Nielsen, H. Knudsen, Electromechanical interaction and stability of power systems with windmills, Proc Int IASTED Conf Power and Energy Systems, Sept. 2000, Marbella, Spain, Paper 319-086.



4


[6] V. Akhmatov, H. Knudsen, M. Bruntt, A. H. Nielsen, J. K. Pedersen, N. K. Poulsen, A dynamic stability limit of grid-connected induction generators, Proc Int. IASTED Conf Power and Energy Systems, Sept. 2000, Marbella, Spain, Paper 319-087.

[7] V. Akhmatov, Some aspects of new windmill technologies on voltage stability, Seminar on Control Concepts of Wind Turbines, 23 Oct. 2000, Aalborg, Denmark.

[8] V. Akhmatov, H. Knudsen, A. H. Nielsen, J. K. Pedersen, N. K. Poulsen, Modelling and transient stability of large wind farms, 2d Int Workshop Transmission Networks for Offshore Wind Farms, March 2001, Royal Institute of Technology, Stockholm, Sweden, p. 11.

[9] H. Knudsen, V. Akhmatov, Evaluation of flicker level in a T&D network with large amount of dispersed windmills, Paper 226, Proc Int Conf CIRED-2001, June 2001, Amsterdam, The Netherlands.

[10] V. Akhmatov, H. Knudsen, A. H. Nielsen, J. K. Pedersen, N. K. Poulsen, Short-term stability of large-scale wind farms, Proc European Wind Energy Conf EWEC-2001, July 2001, Copenhagen, Denmark, Paper PG 3.56.

[11] V. Akhmatov, Note concerning the mutual effects of grid and wind turbine voltage stability control, Wind Engineering, vol. 25, no. 6, 2001, pp. 367 –371.

[12] V. Akhmatov, A.H. Nielsen, Fixed-speed active-stall wind turbines in offshore applications, Proc Topical Expert Meeting on Large scale integration into the grid, IEA R&D Wind Annex XI, Nov. 2001, Hexham, Near Newcastle, Great Britain.

[13] V. Akhmatov, Modelling of variable-speed wind turbines with doubly-fed induction generators in short-term stability investigations, 3d Int Workshop Transmission Networks for Offshore Wind Farms, April 2002, Royal Institute of Technology, Stockholm, Sweden, p. 23.

[14] V. Akhmatov, H. Knudsen, An aggregate model of grid-connected, large-scale, offshore wind farm for power stability investigations – importance of windmill mechanical system, Int Journal Electrical Power & Energy Systems, vol. 24, no. 9, 2002, pp. 709 –717.

[15] J. K. Pedersen, K. O. H. Pedersen, N. K. Poulsen, V. Akhmatov, A. H. Nielsen, Contribution to a dynamic wind turbine model validation from a wind farm islanding experiment, Electric Power Systems Research, vol. 64, no. 1, 2002, pp. 41 –51.

[16] V. Akhmatov, Variable-speed wind turbines with doubly-fed induction generators. Part I: Modelling in dynamic simulation tools, Wind Engineering, 2002, vol. 26, no. 2, pp. 85 –108.

[17] V. Akhmatov, Variable-speed wind turbines with doubly-fed induction generators. Part II: Power system stability, Wind Engineering, 2002, vol. 26, no. 3, pp. 171 –188.

[18] V. Akhmatov, H. Knudsen, A. H. Nielsen, J. K. Pedersen, N. K. Poulsen, Modelling and transient stability of large wind farms, Int Journal Electrical Power & Energy Systems, vol. 25, no. 2, 2003, pp. 123 –144.

+ 5 articles submitted or/and accepted for publication


Analysis of Dynamic Behaviour of Electric Power Systems With

Recommend Documents