Adjoint wind turbine modeling with ADAMS, Simulink and PSCAD/EMTDC

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1 NORDIC WIND POWER CONFERENCE, 1- MARCH, 4, CHALMERS UNIVERSITY OF TECHNOLOGY 1 Adjoint wind turbine modeling with ADAMS, Simulink and PSCAD/EMTDC Sanna Uski, Bettina Lemström, Juha Kiviluoma, Simo Rissanen and Petteri Antikainen VTT Processes, Energy Production, P.O. Box 166, FIN-44 VTT, Finland sanna.uski@vtt.fi Abstract Wind turbine dynamics are usually studied with a fairly simple model of the power system - and vice versa, aeroelastic properties of the wind turbine itself are fairly simple or non-existent in network studies. This paper describes how three commercial programs are used together for continuous, simultaneous and adjoint simulation. A multibody system code ADAMS is used for modeling the turbine, the control system of the turbine is modeled in Matlab/Simulink and the electrical components in PSCAD/EMTDC. A simulation example is shown and possible usage of the system is discussed. Index Terms wind power, modeling, ADAMS, Simulink, PSCAD/EMTDC C I. INTRODUCTION OMMERCIAL tools for aerodynamic and mechanical modeling as well as for electromagnetic modeling are nowadays in common use. In case of wind power modeling, however, turbine dynamics is usually studied with a fairly simple model of the power system - and vice versa aeroelastic properties of the wind turbine are fairly simple or non-existent in network studies. As an alternative to these traditional approaches, this report brings forth a solution in which the best parts of different tools are used jointly. The approach and the codes developed can naturally be used for other electromechanical systems as well. The mechanics of the turbine are modeled with multibody systems code ADAMS. This complex model includes 3D-wind field and flexible parts such as blades, tower, drive-train etc. The control system of the turbine that regulates blade pitching, yawing and generator characteristics is modeled in Matlab/Simulink. The electrical components of the turbine and the network are modeled in PSCAD/EMTDC. A. ADAMS II. THE SIMULATION TOOLS The dynamic model of the wind turbine is created using graphical modeling program ADAMS from MSC.Software. ADAMS is widely used in different mechanical engineering problems. Wind turbine design is assisted with a special NREL produced package ADAMS/WT [1]. After the prototype is build, ADAMS is used to simulate the model and analyze the results. ADAMS models are usually constructed of flexible main components such as blades, tower and drive train. Typically a model consists of a few hundred degrees of freedom. The model used in this project consists of about 5 DOF. The effect of the wind on the blades is added into the simulation with Aerodyn from NREL []. Aerodyn runs as a separate program, which takes blade angles as input from ADAMS and sends calculated output forces back. Aerodyn uses three dimensional wind field to calculate the forces created by the wind and the blade profile. B. Simulink Simulink is very suitable for modeling complex control systems. Simulink makes it possible to add blade pitch control, yawing, vibration control, generator control and other systems to the dynamic turbine model. Additional benefit of using Simulink is getting to use the functionality of Matlab also. ADAMS and Simulink are very suitable for creating an accurate mechanical model of the turbine including different control systems. For appropriate power system modeling it is practical to use software which is developed specifically for that purpose. Simulink has a power system blockset named SimPowerSystems that can be purchased separately. In this work, however, a separate program, namely PSCAD/EMTDC, has been chosen because of the good experience of it at VTT. Simulink works then as a platform for connecting the electromagnetic simulation into the mechanical simulation. C. PSCAD/EMTDC PSCAD/EMTDC is one of the foremost commercial electromagnetic transient simulation tools. It has been developed by the Manitoba HVDC Research Center since PSCAD is a graphical front-end to EMTDC for creating models and analyzing results. In PSCAD one combines blocks to form a power network. These blocks are actually FORTRAN code, which call for EMTDC code library to combine them into executable file. Running this file, runs the simulation and the results can be picked up by PSCAD on the run. In this study the electrical parts of the generator, frequency converter, transformer and network as well as network faults and system disturbances are modeled in PSCAD/EMTDC. Measurements and protection relays can also be conveniently modeled.

2 NORDIC WIND POWER CONFERENCE, 1- MARCH, 4, CHALMERS UNIVERSITY OF TECHNOLOGY A. Principle III. COMBINING SIMULATIONS An example of cause-effect chain describes the principle of the adjoint simulation. The chain starts from the power system as a network fault created in PSCAD/EMTDC. This affects the generator and through a change in the electrical torque, the mechanical side of the turbine. PSCAD/EMTDC passes the change in torque to ADAMS through Simulink. ADAMS returns the new shaft speed to Simulink, which sends it to PSCAD/EMTDC. The dependence between the electrical and mechanical parts works similarly the other way around. For example wind speed or wind direction change can be seen in the network simulation as power fluctuations. The ADAMS Wind Turbine model is part of the Simulink model as an integrated block. Simulink communicates with ADAMS through ADAMS/Controls. Simulink is also used to control turbine behavior. ADAMS and Simulink run always on the same computer. PSCAD/EMTDC can be run on the same or, if desired, on another computer through local area network. Data analysis is possible in all three programs. B. PSCAD/EMTDC Simulink connection PSCAD v4. has an interface to Simulink, but it is not suitable for continuous and simultaneous simulations. PSCAD/EMTDC calls Simulink, which runs a whole simulation and then returns the result to PSCAD/EMTDC. For this reason new tools were developed at VTT for continuous and simultaneous simulations. Both Simulink and PSCAD/EMTDC have now block libraries, which hold the components of the connection. C. Simulation setup and runtime It is likely a good idea to set the connection time-step to be the same as the time-step of the mechanical model because the time-step in the electromagnetic simulation is usually considerably lower. A 3-second simulation, similar to one presented in the following chapter, takes less than a half an hour to complete when using two computers. The ADAMS model uses a time-step of 5 milliseconds and has above degrees of freedom. The time-step used in PSCAD is microseconds and the model tested is quite simple. The two computers used in the simulations both have WinXP installed. The one with ADAMS and PSCAD has dual 8 MHz Pentium processors with one gigabyte of memory. The computer with just PSCAD is a GHz Pentium with 51 megabytes of memory. IV. FIXED SPEED TURBINE To demonstrate the adjoint simulation, an example of fixed speed wind turbine model is presented along with simulation results. The models in both Simulink and PSCAD are presented schematically in Appendix I. A. ADAMS The turbine modeled is a Bonus 6 kw Mark IV with arctic equipment (e.g. blade heating). The model contains of three blades divided into ten parts each, low speed shaft, gearbox, high speed shaft, generator, tower and nacelle. These elements are connected to each other with some flexibility. When simulating with ADAMS alone the electrical countertorque is described by a single equation. For an induction generator the countertorque is: pmr su T = ω, (1) R + s X where s is slip, U voltage, p number of pole pairs, m number of phases, R rotor resistance and X stator and rotor leakage inductance. In the new set-up the electrical countertorque comes from PSCAD/EMTDC which provides a proper generator model and network calculation. B. Simulink Since the turbine in question is a fixed speed turbine, there is no pitch control or active speed control. This simplifies the Simulink model considerably. In the case of fixed speed wind turbines Simulink is more like a link between ADAMS and PSCAD/EMTDC. Yawing and brakes could certainly be modelled and controlled in Simulink, when regarded interesting. C. PSCAD/EMTDC There is a fully developed squirrel cage induction machine available in EMTDC among some other machine models. [3] All EMTDC machine models are programmed in state variable form and are based on generalized machine theory. The squirrel cage machine is modeled as a double-cage machine to account for the deep bar effect of the rotor cage. The direct and quadrature axes are equal. Saturation on magnetizing and leakage inductances can be disabled or enabled in the machine model. If enabled, up to ten saturation curve points can be given. The machine model is set as a motor, and therefore positive terminal power and shaft torque indicate motor operation. The machine parameter values provided by a machine manufacturer, should to be transformed to correspond to the machine set-up in EMTDC. D. Example - Voltage dip Very rapid increase of wind power in many countries has led to tightened requirements for the turbines in case of grid disturbances. The most demanding requirements require the wind turbines to ride-through voltage dips down to even 15-5 % of the nominal value. Squirrel cage generators are susceptible to fast voltage changes. A three-phase voltage dip close to the generator in a weak network induces a large transient current on the generator due to change in the magnetic fluxes of the generator. This in turn generates a large torque, which affects the mechanical parts of the turbine. To show this effect, a moderate symmetrical voltage dip of 5 milliseconds is simulated. The graphs of the simulation are presented in Appendix II.

3 NORDIC WIND POWER CONFERENCE, 1- MARCH, 4, CHALMERS UNIVERSITY OF TECHNOLOGY 3 When comparing the countertorque from the generator with the torque in the low speed shaft, one can see that a major part of the torque is smoothed out by the gearbox between the shafts. The gear teeth suffer and the lifetime of the gearbox shortens. The voltage dip affects also the shaft. The twist angle of the low-speed shaft turns even slightly negative at the end of the voltage dip. The effect of the voltage dip can be seen as far as the base of the tower (Fig. 1). Former oscillation is interrupted by a new shake, which is a lot faster. The direction of the initial torque is same as the generator s direction of rotation. T orqu e in tow e r b ase (N m ) 8 x Fig. 1. Torque at the base of the tower. Voltage dip occurs at the period s. [3] Manitoba HVDC Research Centre, EMTDC User s guide, Winnipeg, Canada,, pp V. DISCUSSION This paper describes how ADAMS, Simulink and PSCAD/EMTDC can be used together for continuous, simultaneous and adjoint simulation. Different models and tools working simultaneously enables to study - what mechanical phenomena are transferred to the electrical side - the influence of network disturbances to the mechanical side - the impact of control actions and development of new control strategies - indicators that can be used for condition monitoring purposes - new technical solutions and materials in order to reduce harmful forces and events on e.g. drive train. Further, the purpose of the study is not solely in the integration of the three programs, as it is not always convenient or even necessary to model both the electromagnetic and the aerodynamic and mechanical parts very detailed. The three tools working together can be used to support development of mechanical models in PSCAD/EMTDC as well as better modeling of for example network disturbances in ADAMS-Simulink. ACKNOWLEDGMENT The article is a mutual accomplishment under several projects. The authors show their appreciation for the sponsors of these projects; VTT s Strategic Technology Theme Intelligent Products and Systems, Nordic Energy Research and Finnish companies. The turbine model was originally made in NewIcetools-project NNE The financial support from EU and the data received from Bonus are acknowledged. REFERENCES [1] []

4 NORDIC WIND POWER CONFERENCE, 1- MARCH, 4, CHALMERS UNIVERSITY OF TECHNOLOGY 4 APPENDIX I The upper diagram of Fig. A1 shows a simple Simulink model with no controls implemented. ADAMS is inside the adams_sub submodel and it communicates with Simulink in discrete time steps. ADAMS itself changes the time-step of its simulation according to derivative changes in the variables. ADAMS gets the wind input from Aerodyn, which runs as a separate program and is not visible in the figure. ADAMS sends rotation speed of the high speed shaft to Simulink through the hand-drawn arrow, which depicts the connection between the simulations. The generator model uses shaft speed and network parameters as input for calculating the electrical torque, which PSCAD sends back to Simulink through another arrow. Simulink feeds the torque into ADAMS, which uses it alongside the wind input in the next time-step. Fig. A1. Simulink model with ADAMS submodel and PSCAD model with a generator and network. The blue arrows transfer shaft speed and electrical torque between the models.

5 NORDIC WIND POWER CONFERENCE, 1- MARCH, 4, CHALMERS UNIVERSITY OF TECHNOLOGY 5 APPENDIX II Voltage (kv) Current [ka] Torque (Nm) Shaft speed (rpm) Torque in LSS (Nm) x Torque in LSS LSS twist angle Fig. A. The effect of a voltage dip on different variables of the generator and the turbine: generator voltage, output current, counter torque created by the generator, high speed shaft speed, and torque and twist angle at low speed shaft LSS twist angle (deg)