Tuesday, 27 September 2016
14:30 - 16:00 Power conversion
Turbine technology  
          

The session focuses on conversion systems for wind turbines based on power electronics, mainly for off-shore applications. The trend of higher power brings several challenges related to compliance with grid requirements. Harmonics and resonances become crucial issues that require proper analysis and innovative yet practical solutions. This session covers aspects related to power converter architectures and the modulation and control of the converter. We will look at the interaction with the electrical grid as well as practical aspects about how to optimise the primary control in different wind conditions and how to effectively test the control of large-scale wind systems in emulated conditions.

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Learning objectives

• To explain a number of ways that the different power converter systems (including their filter) impact the real operation of an offshore wind park;
• To identify the value of grid emulator test benches;
• To list possible advantages of frequency converters in wind energy;
• To explore methods that lead to reducing the cost reductions of integrating offshore wind energy.

This session will be chaired by:

Salvador Boleko Ribas Nordex and Acciona Windpower, Spain
Co-authors:
Boleko Ribas Salvador (1) ^F Azpillaga Alsasua Carmen (1) Beriain Guembe Guillermo (1) Cuesta Lerin David (1) Fernandez Garcia de Iturrospe Ana (1) Garcia Barace Alberto (1) Gonzalez Murua Alejandro (1) Luquin Hermoso de Mendoza Oscar (1)
(1) Nordex and Acciona Windpower, Sarriguren, Eguesibar, Spain

Enhanced active power control and primary frequency control through dynamic estimation of effective wind speed

Introduction

A functional implementation of the active power and primary frequency (droop) control (APC-PFC) demands a proper on-line, real-time estimation of the so-called available active power. Hereinafter we refer to available active power as the expected amount of power to be generated and sent to the grid by the wind-turbine at given environmental (e.g. wind speed, wind turbulence, air density, etc) and operating conditions (e.g. control settings or auxiliary control strategies, since some of them can involve increased or decreased power output w.r.t. the baseline controller), whilst assuming that the APC-PFC is turned off.

At wind farm control level active power set-point values are computed per wind turbine out of both the wind farm aggregate active power set-point imposed by the transmission system operator, the measured active power at the point of common coupling and the available active power estimates of each wind turbine. Poor estimates can negatively impact on any advanced power dispatch policy and thus impair the performance of the APC-PFC (e.g. by asking some wind turbines to yield more than they actually can).

The current work discloses an approach to estimate the available active power while showing how it is used to harmonise concurrent control strategies with the APC-PFC at wind turbine control level.

Approach

First we develop a real-time effective wind speed model-based estimator. The resulting estimate is evaluated out of rotor and generator angular speed readings, generator torque demand, air-density estimate and some aerodynamic data of the wind turbine.

Then, aeroelastic simulation is used as an oracle (black-box) to evaluate the average electrical power expected to be delivered to the grid for given conditions which are determined by the air-density and (average) effective wind speed values as well as by the control configuration settings. Thus, the oracle outcome is used to build preliminary available active power look-up tables. Once the forecast power values are proven right and corrected through real wind turbine measurements, the final version of the look-up tables is ready to be used online. Finally, whenever the APC-PFC is working, the effective wind observer and the lookup tables are jointly used to evaluate the corresponding available active power online.

Main body of abstract

The introduced approach provides to the APC-PFC a simple and efficient adaptation mechanism to handle variations in the air-density. Furthermore, it allows for an easy and straightforward harmonisation of the APC-PFC with formerly 'conflicting' control strategies, like for instance the acoustic noise reduction or some de-rating at high wind speeds aiming at load-mitigation. Both control strategies can't usually be realised without yielding a reduced output power w.r.t. the baseline control.

Additionally, since the approach adopted to estimate the effective wind speed also involves the evaluation of other relevant aerodynamic magnitudes like the power coefficient, and the tip speed ratio, it permits on the one hand to corroborate some design assumptions of our APC-PFC, like e.g. to check optimal TSR tracking below-rated, while on the other hand it also allows to provide further alternatives to eventually enhance the quality of the active power reserve tracking at wind-turbine level, e.g. by means of closed-loop control exploiting the feedback of the power coefficient estimate.

Conclusion

The discussed solution which characterises the available active power as a function of environmental and applied control strategies, benefits the realisation of APC-AFC at both (i) the wind farm control level by supplying the required input to advanced active (and reactive) power dispatch control algorithms, as well as (ii) the wind turbine control level by squarely harmonising the joint operation of concurrent control strategies affecting the expected output power w.r.t. the baseline wind turbine controller. Moreover, the availability of more information on aerodynamic magnitudes can be exploited to improve the APC-PFC behaviour in some instances.


Learning objectives
Alternative proven on the field approach to the APC-AFC problem.