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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
Agusti Egea-Alvarez CITCEA-UPC, Spain
Co-authors:
Agusti Egea-Alvarez (1) F P Oriol Gomis-Bellmunt (1) Antoni Sudria-Andreu (1)
(1) CITCEA-UPC, Barcelona, Spain (2) IREC, Barcelona, Spain

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Presenter's biography

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

Agusti Egea-Alvarez received the B.S. and the M.Sc Technical University of Catalonia (UPC), Barcelona, Spain in 2008 and 2010 respectively.Since 2008, he has been working in the CITCEA-UPC. Since 2010 he is enrolled in the electrical engineering Ph.D. programme of the UPC. He has done two PhD placements in the Catholic University of Leuven and Alstom Grid UK. His current research interests include control and operation of HVDC systems, renewable generation systems, electrical machines, electrical railway systems and power converters control.

Abstract

Sensorless control of a power converter for a cluster of small wind turbines

Introduction

This article presents a sensorless control for small wind farms with a unique power converter. This structure can be used in wind farms equipped with a squirrel cage induction generator wind turbine. The proposed system consists in a back-to-back power converter that connects the wind farm and the AC grid. The wind farm control system consists in an optimized control, based on a multimachine DTC (Direct Torque Control) system and a sensorless MPPT algorithm. Furthermore, the presented structure permits to reduce the number of converters and allows to accomplish the grid codes (fault ride through capability and reactive power support).

Approach

There are several wind farms based on fix speed wind turbines concept connected to the AC grid. These wind farms have been built during the eighties and nineties and represents the first generation of industrialized wind turbines. Around 20-25 years after the first connection, the investment is fully recovered and some components are worn down. But, these wind turbines can have a second live after an exhaustive revision and replacement of the damage components. This process is known as repowering.

However, these wind turbines cannot be reconnected directly to the grid due to the evolution of the grid codes. Nowadays, the electrical grid codes are requiring grid support, as fault right through capability or reactive power support. To achieve these objectives a power converter is needed. A standard solution is the installation of an individual power converter for each wind turbine. An economical and feasible alternative consists on the installation of a unique power converter, in back-to-back structure, for the entire wind farm [1]. As the wind turbines are close the wind speed can be assumed in the same range for the different wind turbines [3]. In this abstract the wind farm electrical scheme and the control of a centralized power converter is presented. The Wind Farm Converter (WFC) will control the optimum power extracted from the wind using a multimachine DTC-based algorithm only using the electrical measures on the power converter terminals. The Grid Side Converter (GSC) will control the power injected to the grid and will manage the grid support events. This hardware and control scheme is also valid for micro grids systems.


Main body of abstract

ANALAYSED SYSTEM AND PROPOSED CONTROL METHOD

The analysed system is a small wind farm composed by four wind turbines of a nominal power of 30 kW each one. There is a unique back-to-back power converter that links the wind turbines and the AC grid. Each wind turbine is connected to a common AC bus that collects the generated power. A sketch of the electrical and control system can be seen in figure 1

The WFC is in charge of the control of the wind turbines. The control algorithm is based on three main blocks: the multimachine estimator, the Sensorless Maximum Power Point Tracker (MPPT) and the multimachine DTC-based algorithms. The first one is the multimachine flux, speed and torque estimator that calculates the mentioned variables according the electrical measures. This block presents some modifications respect to the classical structures due to the multimachine system observability. The second is the sensorless MPPT algorithm that calculates the optimum torque reference only according to the estimated rotational speed. The DTC-based algorithm calculates the appropriate voltage vector according to the torque and flux reference values.

The GSC is in charge of the injected active power and the reactive power support. A DPC control has been used. Furthermore, GSC is responsible of the connection of a DC chopper in case of voltage sag. Using this DC chopper, the operation of the wind farm can remains as before during the voltage sag.

OPERATION POINTS AND MODES

During the normal operation, the system extracts the maximum power available. It means that the MPPT calculates the maximum power that can be extracted from the wind considering the aerodynamic design of the wind turbine and the DTC-based algorithm drives the wind turbine to this operational point. Consequently, the electrical frequency of the AC grid will change during this operation mode.

Once the nominal wind is reached the electrical frequency of the system is kept constant and the power reduction method of the wind turbine starts to act for higher wind speed values in order to not destroy the wind turbine. Big wind turbines are equipped with a pitch control system that regulates the blades angle in order to capture less power from the wind. But, usually, small wind turbine reduces the power extracted from the wind using the design of the blades. This concept is call passive stall. Consequently, the analysed wind turbines do not have an active power reduction method.

During AC voltage sag, probably, all the generated power might not be injected to the grid, due to the saturation of the GSC AC current. In that case, the energy starts to be stored in the DC bus. The presented topology is equipped with a DC chopper that dissipates this energy when the DC voltage is above a threshold. Using a DC chopper during perturbations the wind turbines do not feel the effects of the voltage sag. Once the AC grid is recovered the GSC returns to the previous operation point.

SIMULATION RESULTS
The final article will include some simulation studies as wind speed variation and an AC voltage sag.


Conclusion

A new sensorless control system for fixed-speed wind turbines has been presented. This control system is suitable for small wind farms equipped with wind turbines coming from repowering projects or low cost wind turbines (both based on Squirrel Cage induction generator). This structure can be also used in wind farms connected to a microgrid. The introduced wind farm control is based on the DTC algorithms, but an especial control effort has been done in order to adapt the estimators to the multimachine observability. In addition, the use of the back-to-back structure, with a DC chopper, provides immunity to the wind farm in case of an AC grid perturbation. The grid side converter is controlled using a DPC algorithm. The described operational points can be divided between the operation during normal, high wind speeds and fault operation. During normal operation, the system changes the rotational speed of the wind turbines in order to extract the maximum power. Once the nominal speed is reached, the wind turbine reduces the energy extracted from the wind and the electrical frequency is fixed. During a voltage sag, the current saturation of the GSC might be reach, and consequently this energy is stored in the DC bus. In order to avoid an overvoltage the DC chopper dissipates the excess of power. During a voltage sag the wind turbines produce the same amount of energy as before. Once the fault is cleared the GSC returns to the previous operation point. Some simulations results will show the correct performance of the system during normal operation (wind speed change) and fault conditions (AC voltage sag).


Learning objectives
Electrical part:
- Wind farm controlled using a unique power converter
- Fault ride through capability (and reactive power suport)

Control part:
. Multimachine DTC-based control
- Multimachine observers



References
[1] Oriol Gomis-Bellmunt, Adrià Junyent-Ferré, Andreas Sumper, Samuel Galceran-Arellano, Maximum generation power evaluation of variable frequency offshore wind farms when connected to a single power converter, Applied Energy, Volume 87, Issue 10, October 2010.
[2] Lascu, C.; Boldea, I.; Blaabjerg, F., "A modified direct torque control for induction motor sensorless drive," Industry Applications, IEEE Transactions on , vol.36, no.1, pp.122,130, Jan/Feb 2000
[3] Mikel de Prada Gil, Oriol Gomis-Bellmunt, Andreas Sumper, Joan Bergas-Jané, Power generation efficiency analysis of offshore wind farms connected to a SLPC (single large power converter) operated with variable frequencies considering wake effects, Energy, Volume 37, Issue 1, January 2012.