1. INTRODUCTION The control system often appears to play a key role during commissioning of prototype or modified wind turbines. It is often a hard task to get the control system operating properly. This may finally result in disappoin-ting results concerning time, costs, damage or even (personal) accidents. Problems during commissioning should be minimised but are inherent to technical inno-vation, cost reducing modifications or system integration effects like design failures or unexpected system beha-viour. The problems are usually difficult to solve, mainly caused by the poor working circumstances at the remote site. Up-scaling, offshore locations and future wind power plants will deteriorate this, while just high reliability, safety and progress is needed. Moreover, extreme site conditions are uncommon and extreme system behaviour can be risky. As in other industrial branches (automotive, chemical, oil), real-time process simulation is aimed to avoid many of these problems. Preceding to turbine commissioning, the turbine controller will interact in real-time with the process simulator, which simulates the behaviour of relevant windturbine components and environmental influences. Handling of abnormal conditions, like failures of components, emergency situations and extreme wind gusts, can be verified on beforehand at factory (factory acceptance test) instead on the site. Currently, process simulation for wind turbine control systems is applied barely, vendor specific or at subsystem level only. This project aims to build a generic process simulator, which shall be configurable for the most prevailing modern turbines and have an open architecture for future innovations. The results of project phase 1 are described: the context of process simulation (section 2), a proven project approach (section 3) and simulator specifications as inventoried from industrial turbines (section 4). Phase 2, concerning modelling and implementation (section 5) is in execution at present. 2. SIMULATOR CONTEXT AND ACTORS The process simulator consists of a real-time process computer, a host PC and libraries of modular simulation software.

As depicted in figure 1, the process simulator exchanges data with three actors: the wind turbine control system, the operator and real turbine systems and components.

Figure 1: Context of process simulation The wind turbine control system is here defined as a system that provides all control for safe, stable and reliable operation under all conditions (component failures, emergency situations, extreme wind gusts etc). The process simulator should provide status- and measured values in real-time to the turbine controller, while simulating turbine behaviour with just enough detail of system dynamics and events. Preceding to simulation cases, an operator configures and parameterises the turbine simulator models. During simulation the operator can force specific behaviour on demand, e.g. a wind gust, failure of component(s). Effects will be on-line monitored and logged for detailed analysis afterwards. 'Hardware in the loop' facilitates in direct connection of physical turbine systems of components, like actuators and sensors, instead of simulate them. It supports in testing suspicious or crucial (safety) systems

REAL TIME PROCESS SIMULATION

FOR EVALUATION OF WIND TURBINE CONTROL SYSTEMS E.L. van der Hooft1, vanderhooft@ecn.nl, Tel. +31 224 564913 T.W. Verbruggen1, verbruggen@ecn.nl, Tel. +31 224 564046

P. Schaak1, schaak@ecn.nl, Tel. +31 224 564278 T.G. van Engelen1, vanengelen@ecn.nl, Tel. +31 224 564141

1ECN Wind Energy, P.O. Box 1, 1755 ZG Petten, The Netherlands, Fax. +31 224 568214

ABSTRACT: This paper describes intermediate results of the development of a generic real-time process simulator for verification of wind turbine control systems. The objective of the project is to reduce commissioning time and increase safety and reliability of prototype and modified industrial turbines. The simulator is aimed to simulate the behaviour of windturbine components and environmental influences in real-time, in particular for abnormal and extreme situations. Proper acting of turbine controls can then be verified easily by connecting the controller direct to the configurable process simulator. ‘Hardware in the loop’ is added to evaluate real components instead of simulating them. The first project phase has been finished and was focussed on exploration and analysis. A development methodology has been established based on a case with a simplified turbine. Inventorying two different industrial wind turbines, as well as external influences, specifies general turbine components. The second phase involves modelling and implementation and is currently in execution. The simulator is foreseen to be available for industrial use before end 2005.

3. PROJECT APPROACH Building a generic real-time process simulator with good prospects for industrial use, requires: - detailed component specifications of prevailing

industrial turbines; - a conscientious project approach; - a broad knowledge of modelling physical processes. To cover this, a methodology as shown in figure 2 has been set up. The first project phase focuses on explora-tion and analysis. It has been resulted in a case proven methodology [1] and a comprehensive description of turbine systems and components behaviour [2] (section 4). The second project phase focuses on synthesis of the process simulator using the methodology as found in phase 1 (section 5).

Figure 2: Project approach Figure 2 is the formalised top level result of a preceding task ‘Problem analysis’. The succeeding tasks 1 through 5, were derived and by using a simplified fictive turbine and turbine control system. Each task has been broken down into small well-ordered activities. The process to be simulated, has been divided into different simulation items: - external turbine influences or ‘media’, viz. wind,

waves, electrical grid and soil; - wind turbine. The smallest parts of these items are defined as compo-nents. Due to the complexity of a turbine, this item has two intermediate layers: turbine parts (e.g. rotor, drive train) and systems (e.g. pitch system, electric conver-sion). Examples of components are generator, gearbox, shaft bearing, but also wind speed and wind direction. Furthermore, an unambiguous nomenclature has been defined also, to assign clear semantics to general terms like process, system, component, model, module etc.

4. SYSTEM ANALYSIS During the second task of phase 1, the tuning with industrial control and turbine systems and components has been realised. Therefore, the systems of two conceptual different industrial turbines were analysed: - a variable speed, electric pitch to vane, direct drive

turbine [2][3]; - a two speed, hydraulic pitch to stall turbine with

gearbox transmission [2][4]. The system analysis task consists of two tasks: inven-tarisation and definition. Both were applied on each system of the two turbines. The distinguished systems and accompanying processes are listed in table 1. Table 1: Turbine systems break down Turbine parts Systems Processes

Blade system Aerodynamic conversion and blade related facilities

Meteo system Measurements of meteo-rological quantities Rotor

Pitch system Control and adjustment of pitch angles

Transmission system

Mechanical conversion and conditioning of shaft, bearings, gearbox and brake

Drive train Electric conversion system

Electrical conversion by generator and converter; grid connection by transformer and main switch

Tower- and foundation system

Tower vibration and bending modes caused by excitations Support

Yaw- and retwist system

Nacelle alignment and rotation, retwisting of power cable

Safety system Emergency interventions to stop turbine General Power supply

system Power supply for active systems and components

During inventarisation, the following property groups of each system were described: - construction, working principle and control strategy; - related measurements and detections; - safety facilities; - I/O and interactions. In order to extract specific properties and behaviour for the modelling task (paragraph 5.1), the definition part has resulted in the following specifications of each system: - division into sub-systems and components; - internal system interactions between sub-systems and

components; - external system interactions to other systems and

media; - composition or constant properties; - phenomena or continuous behaviour; - events or discrete behaviour; - ‘hardware in the loop’ relationships. In particular, the power supply and safety systems are highly interactive with other turbine systems. Similar to the wind turbine, the media were also analysed and specified. These results are shown in table 2.

Table 2: Media break down Media Components Processes

Rotor effective wind speed

Stochastical that causes axial force and aerodyna-mic torque

Blade effective wind speed

Stochastical that causes yaw-/ tilt moments and blade bending moments

Measured wind speed

Stochastical for wind speed at fixed point

Wind

Measured wind direction

Stochastical for wind direction at fixed point

Waves Force effective wave acceleration

Stochastical that causes tower excitation (diffraction)

Soil none Passive medium

Grid none

Power exchange between turbine and grid, including variation effects forced by grid

Supplementary, two additional system analyses for modern multi-megawatt turbines with offshore potentials shall be done during project phase 2: N80 (Nordex) and NM92 (NEG Micon) turbine. This should guarantee sufficient industrial input and involvement to start the modelling task. 5. SYNTHESIS According to figure 2, synthesis of the process simulation tool will be done in three tasks: - modelling; - implementation; - real-time implementation. 5.1 Modelling The modelling task will answer the question on how the components and interactions of each system will be realised. The proposed methodology of Ward-Mellor [5] uses transformation schemes to describe system behaviour. A transformation schema comprises interactions between data-processes and control-processes, which respectively represent continuous and discrete behaviour. Each data-process can be broken down into a more detailed transformation schema consisting of lower level data-processes (data flow diagram). Finally this will result in an unambiguous description like differential equations, block schemes or an algebraic function. Similar, each control-process can be broken down into more control-processes. Events will be defined unam-biguously by using state transition diagrams. As shown in figure 2, industrial involvement will input practical issues, while scientific knowledge from recent developments on project [6][7] and design codes [8][9] will cover the theoretical needs. 5.2 Implementation The specified ‘component models’ will be converted to ‘component modules’. Component modules consist of program code and shall run non real-time to verify their performance and behaviour. Based on experience and seamless connection to the real-time environment (paragraph 5.3), the computing

products of The Mathworks Inc. will be used. The Simulink1 toolbox facilitates in a graphic environment for time domain simulations and supports easy implemen-tation of data-processes. The Stateflow1 toolbox will be used to implement control-processes. A process of a wind turbine realisation can be configured at the host-PC, from libraries comprising ‘component modules’. Software libraries will also be useful to hide confidential knowledge and experience of manufacturers. 5.3 Real time implementation and case The industrial accepted real-time environment for rapid prototyping of dSpace GmbH2 would be used to run process simulations. Before downloading to the process computer, the process of a typical windturbine must be converted to C-code (using Real Time Workshop1) and compiled automa-tically for use by this dedicated hardware. During process simulation the host-PC is destined to be a user interface for operating, monitoring, logging and evaluating purposes. Finally, a case will be set up for a typical turbine and accompanying control system to demonstrate and prove the possibilities of real-time process simulation.

Operator

Process simulator

Turbinecontroller

Hardwarein the loop

Windturbine

Signal I/O Signal I/O

Host computer

Operation

Monitoring

Figure 3: Process simulation 6. CONCLUSION Real-time process simulation of turbine control systems seems to be a powerful solution to improve control performance and achieve quick and safe commissioning of modern wind turbines. The first project phase has resulted in a development methodology and detailed system specifications of industrial turbines. The obtained results as well as the industrial involvement shall be a good base to fulfil the second synthesis project phase. The simulator is foreseen to be available for industrial use before end 2005.

1 Simulink, Stateflow and Real Time Workshop are products of The Mathworks Inc., see www.mathworks.com 2 See www.dspace.com

ACKNOWLEDGEMENT The work presented in this paper is an initiative of ECN Wind Energy and is partly funded by the BSE-program of the Dutch Ministry of Economic Affairs as represented by Novem. REFERENCES [1] Engelen, T.G. van; Schaak, P.: “Processimulator

voor Windturbinebesturingen: Volume I Probleem-analyse”; ECN-C--01-112; ECN Windenergie, Petten, November 2001.

[2] Hooft, E.L. van der; Verbruggen, T.W.; Engelen, T.G. van: “Processimulator voor Windturbinebestu-ringen: Volume II Systeemanalyses”; ECN-C--02-082; ECN Windenergie, Petten, Oktober 2002.

[3] Hooft, E.L. van der; Verbruggen, T.W.: “Proces-simulator voor Windturbinebesturingen: Annex A Systeeminventarisatie LW50-750/B2a”; ECN-CX--02-090 (confidential); ECN Windenergie, Petten, Oktober 2002.

[4] Verbruggen, T.W.; Hooft, E.L. van der: “Proces-simulator voor Windturbinebesturingen: Annex B Systeeminventarisatie NW62/3/1000”; ECN-CX--02-043 (confidential); ECN Windenergie, Petten, Oktober 2002.

[5] Ward, P.T.; Mellor, S.J.: “Structured Development for real-time systems - Volume I: Introduction and Tools”; Yourdan Press; Prentice Hall, Engelwood Cliffs; 1985.

[6] Engelen, T.G. van; Hooft, E.L. van der; Schaak, P.: “Development of wind turbine control algorithms for industrial use”; Proceedings of EWEC p1098-1101, Copenhagen, Denmark, 2-6 July 2001.

[7] Pierik, J.T.G. et al: “Steady state electrical design, power performance and economic modelling of offshore wind farms”; Proceedings of Special Topic Conference Offshore Wind Energy, Brussels, Belgium, 2001.

[8] Lindenburg C; Hegberg, T.: “PHATAS IV, Program for HAWT analysis and simulation”; ECN-C--99-093; ECN Windenergie, Petten, 2000.

[9] Engelen T.G. van; Hofland, L.D.; Vugts, J.H.: “TURBU Offshore, computerprogramma voor frequentiedomein analyse van HAWT; fase I: modelbeschrijving”; ECN Windenergie, Petten, ECN-C--02-073; 2002.