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Wednesday, 12 March 2014
11:15 - 12:45 Advanced control concepts
Science & Research  


Room: Llevant
Session description

The application of advanced control can be utilised to improve turbine performance. The topics addressed in this session include wind turbine/farm control to provide frequency support including droop control, the collective control of a number of wind turbines through the use of a common bus bar and converter, and the stabilisation of floating wind turbines. The control design techniques used include model-predictive, nonlinear and robust control design.

Lead Session Chair:
William Leithead, University of Strathclyde, United Kingdom

Co-chair(s):
Marta Barreras, Gamesa, Spain
Asier Díaz de Corcuera IK4-IKERLAN, Spain
Co-authors:
Asier Díaz de Corcuera (1) F P Lluís Trilla (2) Aron Pujana-Arrese (1) Oriol Gomis-Bellmunt (3) Fernando Bianchi (2) Joseba Landaluze (1)
(1) IK4-IKERLAN, Arrasate-Mondragón , Spain (2) IREC, Barcelona, Spain (3) CITCEA-UPC, Barcelona, Spain

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Dr. Díaz de Corcuera is currently a researcher at IK4-IKERLAN (Department of Control Engineering and Power Electronics). He is European Doctor (Cum Laude Mention) by the University of Mondragon (The Basque Country, Spain) since April 2013. He studied Electrical Power Engineering and Automation at the University of The Basque Country. After his studies, he developed the thesis in the Control Engineering and Power Electronics department of IK4- IKERLAN. His research is focused on the design of robust controllers for load reduction in wind turbines.

Abstract

Mechanical load analysis of PMSG wind turbines in primary frequency regulation

Introduction

This paper analyzes the structural loads of wind turbines when they are performing Active Power Control (APC) for primary frequency regulation. A model of the wind turbine defined in the ‘Upwind’ European project completed with a detailed model of a PMSG electric machine is considered in GH Bladed. A collective pitch angle and generator torque H infinity robust controllers of the wind turbine are augmented with a droop curve and an active power control system in order to allow the frequency regulation in addition to track active power set-points required by the grid operator.

Approach

Different works in the literature [1][2][3][4] propose Active Power Control (APC) for wind turbines working with reserve power capability to help perform primary frequency control, but normally they use simplified models, especially from the aero-mechanical part of wind turbines. Also, the influence of APC on structural loads of wind turbines is not normally analyzed in the literature. In this case, a complete model of the ‘Upwind’ 5MW wind turbine in GH Bladed is used, where the power train has not been changed and a PMSG electrical machine has been designed instead of the initially defined ideal DFIG machine. A detailed model of the PMSG machine has been integrated in the model in GH Bladed. This model enables the analysis of the structural loads when different APC strategies are applied or different grid faults happen.

The first part of the paper deals with the description of the complete model developed in GH Bladed, especially the description of the detailed PMSG model and how it is integrated with the main wind turbine model.

The robust main controller of the wind turbine has been augmented with an active power controller in order to perform primary frequency regulation. The wind turbine works in deloaded conditions and the APC algorithm generates the generator torque and speed references for the main controller of the wind turbine according to the reserve input. The APC strategy changes the operation condition of the wind turbine due to the new power set-point taking into account the grid frequency, measured by the electrical machine, and the droop curve defined for the wind turbine. The operation points can be changed in different ways to increase or reduce the active power supplied in response to frequency dips. Some transition ways are analyzed and conclusions stated from the point of view of mechanical loads.

Finally, the paper proposes a pitch angle feed-forward contribution depending on the change of the active power set-point in order to avoid or limit the over-speed shown in some power transitions.


Main body of abstract

The PMSG interacts with the AC grid through a fully-rated back-to-back converter. This converter can be directly connected to the grid or it can incorporate a power transformer to adapt the output voltages. In Fig.1 a schematic view of the system under analysis is sketched. The simulation model used is depicted in Fig. 2 where the main signals are shown.


Figure 1: Schematic view of a wind energy conversion system


Figure 2: Simulation model scheme

The ‘speed controller’ block of Fig. 1 consists of the two H infinity robust main controllers [5]. They are augmented with an active power controller in order to perform primary frequency regulation, such as Fig. 3 shows. The wind turbine works in deloaded conditions and the APC algorithm generates the generator torque and speed references for the ‘speed controller’ of the wind turbine according to the input ‘%Reserve’. The APC strategy changes the operation condition of the wind turbine due to the new power set-point taking into account the grid frequency.


Figure 3: Block diagram of the APC command

Fig. 4 shows an example of droop curve as well as an example of frequency decrement defined in some European grid codes where the wind turbine can contribute to the primary frequency regulation according to the droop curve if it has reserve power.


Figure 4: Examples of droop curve and grid frequency dip

Fig. 5 shows the working zones of the ‘Upwind’ wind turbine model when it works without power reserve (red line) or with a power reserve of 15% (blue line). In above rated zone, the wind turbine works at point A with reserve and at point B without reserve. The power curves corresponding to 100%, 85% and 50% are shown as well. When the power reference should be decreased to 50% for a limit time period due to a grip frequency dip, the operation point of the wind turbine could shift from point A to point C, keeping the nominal speed by pitch actuation and decreasing the generator torque set-point value. If the power reference should be increased to 100% the operation point of the wind turbine could shift from point A to point B. However, other alternative operation points could be considered on the power curves, for instance points D, E, F, A’ and A’’. The decision of the operating point has influence in the working conditions of the generator and, especially, in the structural loads of the wind turbines during the operation point shift.


Figure 5: Operation zones of the upwind 5MM wind turbine

Some power transitions from operating point A are considered, with a decrease to 50% for 30 s and a increase from point A to 100% for 30 s. The power transition is made following the way ACB in one case and ADF in another case and the results obtained are compared. A constant wind of 19 m/s is considered.

Fig. 6 shows the mechanical power and the active power of the grid in the two power transitions mentioned. The active grid power is the mechanical power decreased by the losses in the electrical machine.


Figure 6: Mechanical power and active grid power during transitions ACABA and ADAFA


Figure 7: Generator speed during transitions ACABA and ADAFA


Figure 8: Generator torque during transitions ACABA and ADAFA


Figure 9: Pitch angle during transitions ACABA and ADAFA

The generator speed, generator torque and pitch angle signals can be observed in Fig. 7, Fig. 8 and Fig. 9. As a general comment, the transition ACABA is smoother than the transition ADAFA, because there is not speed change, which is penalized, as observed in the ADAFA case, with a high pitch action and it induces speed vibrations in the drive train, and activity in the demanded generator torque to compensate them.

As an example of the influence on the mechanical loads, Fig. 10 shows the Tower Base Mxy load. As it can be observed, the fatigue and extreme values increase a lot in the transition ADAFA. Similar results can be observed in other structural loads.


Figure 10: Tower Base Mxy during transitions ACABA and ADAFA


Conclusion

A complete wind turbine model integrating a detailed model of a PMSG electrical machine has been carried out in GH Bladed. At the same time, the generator torque and collective pitch angle H infinity robust controller of the wind turbine has been augmented with a droop curve and an active power control system in order to allow the automatic frequency regulation in addition to track active power set-points required by the grid operator. This model enables the analysis in simulation of the structural loads when different APC strategies are applied or different grid faults happen. Working the wind turbine in power reserve capacity, some power transitions have been performed as responses to grid frequency dips and the load implications analyzed. As a conclusion, in the working condition considered, the power transitions carried out changing only the torque reference and keeping the generator speed set-point are the best ones compared to others where the operation point speed changes. Extrapolating the results to all working zones, the best option is to keep the nominal speed in the above rates zone and to follow the corresponding maximum Cp curve in the below rated zone. However, working in reserve power capacity operation points with over-speed could be interesting from the point of view of an extra kinetic energy to be supplied to the grid when required. This has less load implications than other transitions analyzed and some benefits can be obtained because generator losses are lower. Different working operations of the electrical machine can be considered as well. For a preferred APC strategy, the control algorithm can be improved in order to limit over-speed during changes in power set-points.


Learning objectives
A complete wind turbine model, integrating aero-mechanical and electrical parts, enables the analysis in simulation of structural loads when different APC strategies are applied or different grid faults happen.

Working the wind turbine in power reserve capacity, power transitions performed changing only the torque reference and keeping the generator speed set-point are the best ones from the point of view of mechanical loads, but other transitions could be better for primary frequency regulation.




References
[1] Buckspan, A., J. Aho, L. Pao, P. Fleming, and Y. Jeong. 2012. Combining Droop Curve Concepts with Control Systems for Wind Turbine Active Power Control. IEEE Symposium on Power Electronics and Machines in Wind Applications, Denver, Colorado, July 16-18.
[2] Aho, J., Buckspan, A.L. Pao, J. Laks, and Y. Jeong. 2012. Tutorial of Wind Turbine Control for Supporting Grid Frequency through Active Power Control. American Control Conference, Montreal, Canada, June 27-29.
[3] Singh, M., V. Gevorgian, E. Muljadi, and E. Ela. 2013. Variable-Speed Wind Power Plant Operating with Reserve Power Capability. ECCE’2013, IEEE Energy Conversion Congress, September 15-19, Denver, Colorado, USA.
[4] Erlich, I. and M. Wilch. 2010. Primary Frequency Control by Wind Turbines. IEEE Power and Energy Society General Meeting, July.
[5] Diaz de Corcuera, A., A. Pujana-Arrese, J.M. Ezquerra, E. Segurola and J. Landaluze. 2012. H∞ Based Control for Load Mitigation in Wind Turbines. Energies 2012, 5(4), 938-967, ISSN 1996-1073.