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Tuning primary frequency controllers using robust control theory in a power system dominated by hydropower
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. (Hydropower)
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. (Hydropower)
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. (Hydropower)
2016 (English)Conference paper, (Refereed)
Abstract [en]

A deterioration of the grid frequency quality has been observed in the Nordic synchronous system, and an oscillation with period 40-90 seconds is often clearly visible in the frequency deviation. A working group, consisting of e.g. transmission system operators and power producers, has been initiated to improve and harmonise the technical requirements for primary frequency control in the Nordic countries. In this paper, a method to optimise the tuning of hydropower governors providing primary frequency control to the system is suggested. A model of the Nordic power system is set up, nominal values of the system parameters and their typical ranges of variation are presented, and a controller structure is defined. The objectives of primary frequency control are discussed and interpreted as requirements on the gain of the closed loop transfer functions of the system. These requirements are then used to define tuning goals for an optimisation of the controller parameters. The optimisation is carried out as a weighted minimisation of the closed loop system transfer functions in frequency domain. The result is evaluated in frequency domain and by time domain simulations of a system with added actuator non-linearities. The sensitivity to system parameter variation is analysed in terms of the performance of the optimised controllers in a system with changing parameters, but also in terms of how the optimisation result changes if the nominal system parameters are changed. The results show that compared to the governor tuning currently used in many hydropower plants in the system, retuned governors could reduce the amplitude of the 40-90 second oscillation considerably. A small, floating deadband on the controller output signal is discussed as a means to reduce the number of small movements in the actuators and turbines. The advantage of the presented method is that many different aspects and requirements on primary frequency control are taken into account and the trade-off between different aspects of the performance is visualised and can be controlled directly.

Place, publisher, year, edition, pages
2016.
Keyword [en]
frequency control, power system stability, governor, PID tuning, robust control, primary control
National Category
Engineering and Technology Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
URN: urn:nbn:se:uu:diva-308436OAI: oai:DiVA.org:uu-308436DiVA: diva2:1049861
Conference
CIGRE Session 2016
Available from: 2016-11-25 Created: 2016-11-25 Last updated: 2016-11-30
In thesis
1. The Frequency of the Frequency: On Hydropower and Grid Frequency Control
Open this publication in new window or tab >>The Frequency of the Frequency: On Hydropower and Grid Frequency Control
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Variations in the electricity consumption and production connected to the power system have to be balanced by active control. Hydropower is the most important balancing resource in the Nordic system, and will become even more important as the share of variable renewable energy sources increases. This thesis concerns balancing of active power, especially the real-time balancing called frequency control. The thesis starts in a description of the situation today, setting up models for the behaviour of hydropower units and the power system relevant to frequency control, and comparing the models with experiments on several hydropower units and on the response of the Nordic grid. It is found that backlash in the regulating mechanisms in hydropower units have a strong impact on the quality of the delivered frequency control. Then, an analysis of what can be done right now to improve frequency control and decrease its costs is made, discussing governor tuning, filters and strategies for allocation of frequency control reserves. The results show that grid frequency quality could be improved considerably by retuning of hydropower governors. However, clear technical requirements and incentives for good frequency control performance are needed. The last part of the thesis concerns the impact from increased electricity production from variable renewable energy sources. The induced balancing need in terms of energy storage volume and balancing power is quantified, and it is found that with large shares of wind power in the system, the energy storage need over the intra-week time horizon is drastically increased. Reduced system inertia due to higher shares of inverter connected production is identified as a problem for the frequency control of the system. A new, linear synthetic inertia concept is suggested to replace the lost inertia and damping. It is shown that continuously active, linear synthetic inertia can improve the frequency quality in normal operation and decrease wear and tear of hydropower units delivering frequency control.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. 105 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1460
Keyword
hydropower, frequency control, governors, power system stability, inertia, primary control
National Category
Engineering and Technology Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-308441 (URN)978-91-554-9769-9 (ISBN)
Public defence
2017-02-07, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:00 (English)
Opponent
Supervisors
Available from: 2017-01-13 Created: 2016-11-25 Last updated: 2017-01-17

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