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  • 1.
    Bladh, Johan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Wallin, Mattias
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Saarinen, Linn
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Lundin, Urban
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Standstill Frequency Response Test on a Synchronous Machine Extended With Damper Bar Measurements2016In: IEEE transactions on energy conversion, ISSN 0885-8969, E-ISSN 1558-0059, Vol. 31, no 1, p. 46-56Article in journal (Refereed)
    Abstract [en]

    Standstill Frequency Response (SSFR) test data from a salient-pole synchronous machine with reconfigurable damper winding is presented. In addition to the regular measurements, the damper bar currents are measured and used to obtain the stator-to-damper transfer functions. The test is performed three times with physically different damper winding configurations. An extension to the standard SSFR test analysis scheme is suggested where the stator-to-damper transfer functions are included. The validity of the identified models is substantiated by comparison of the simulated and measured machine response to a drive torque step disturbance. It is found that the damper winding measurements can be incorporated in the analysis scheme to isolate the effect of the damper circuits. However, for a machine of the type studied, also the standard SSFR test produce yields models that are accurate enough for power system studies.

  • 2. Dahlborg, Elin
    et al.
    Saarinen, Linn
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Norrlund, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Primary Frequency Control - Pay for PerformanceManuscript (preprint) (Other academic)
  • 3.
    Saarinen, Linn
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    A hydropower perspective on flexibility demand and grid frequency control2014Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The production and consumption of electricity on the power grid has to balance at all times. Slow balancing, over days and weeks, is governed by the electricity market and carried out through production planning.  Fast balancing, within the operational hour, is carried out by hydropower plants operating in frequency control mode. The need of balancing power is expected to increase as more varying renewable energy production is connected to the grid, and the deregulated electricity market presents a challenge to the frequency control of the grid.

    The first part of this thesis suggests a method to quantify the need for balancing or energy storage induced by varying renewable energy sources. It is found that for high shares of wind and solar power in the system, the energy storage need over a two-week horizon is almost 20% of the production.

    The second and third part of the thesis focus on frequency control. In the second part, measurements from three Swedish hydropower plants are compared with the behaviour expected from commonly used power system analysis hydropower models. It is found that backlash in the guide vane and runner regulating mechanisms has a large impact on the frequency control performance of the plants.

    In the third part of the thesis, the parameters of the primary frequency control in the Nordic grid are optimised with respect to performance, robustness and actuator work. It is found that retuning of the controller parameters can improve the performance and robustness, with a reasonable increase of the actuator work. A floating deadband in the controller is also discussed as a means to improve performance without increasing the actuator work.

    List of papers
    1. Power system flexibility need induced by wind and solar power intermittency on time scales of 1-14 days
    Open this publication in new window or tab >>Power system flexibility need induced by wind and solar power intermittency on time scales of 1-14 days
    2015 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 83, p. 339-344Article in journal (Refereed) Published
    Abstract [en]

    This article describes a method to assess the needed production flexibility to adapt the power system to the production from variable renewable energy sources such as wind power and photovoltaics over time horizons of 1-14 days. Load and production data from the German power system is used to quantify the flexibility need in terms of power and energy storage requirement due to higher shares of renewable energy (20-80%). It is found that with an 80% variable renewable energy share in the German system, the average power need from flexible sources decreases by 31 GW (59%) while the peak power need only decreases by 3 GW (4%). In terms of energy, the storage need over a two week horizon increases by 2.6 TWh, which is 14% of the average load per day. If the European plans for 100 GW wind power in the North Sea region are realised, this would mean an increase of the energy storage need in the region with 2.2 TWh over a two week horizon.

    Keywords
    Energy storage, wind power, solar power, dispatching, balancing
    National Category
    Electrical Engineering, Electronic Engineering, Information Engineering
    Research subject
    Engineering Science with specialization in Science of Electricity
    Identifiers
    urn:nbn:se:uu:diva-237789 (URN)10.1016/j.renene.2015.04.048 (DOI)000358455100031 ()
    Available from: 2014-12-05 Created: 2014-12-05 Last updated: 2017-12-05
    2. Field Measurements and System Identification of Three Frequency Controlling Hydropower Plants
    Open this publication in new window or tab >>Field Measurements and System Identification of Three Frequency Controlling Hydropower Plants
    2015 (English)In: IEEE transactions on energy conversion, ISSN 0885-8969, E-ISSN 1558-0059, Vol. 30, no 3, p. 1061-1068Article in journal (Refereed) Published
    Abstract [en]

    The dynamic behaviour of hydropower plants participating in primary frequency control is investigated in this paper through frequency response, step response and setpoint change tests on three Swedish hydropower plants. Grey-box system identification is used to estimate the parameters of simple linear models suitable for power system analysis and the major shortcomings of the linear models are discussed. It is found that frequency response tests with sinusoidal input signals give more reliable information about the dynamics of the plants than step response tests. It is also shown that backlash in the runner and guide vane regulating mechanisms are of great importance for the dynamic behaviour of the plants, and that the incremental gain from guide vane opening to power varies considerably with the operation point.

    Keywords
    hydropower, system identification, frequency control, field measurements
    National Category
    Electrical Engineering, Electronic Engineering, Information Engineering
    Research subject
    Engineering Science with specialization in Science of Electricity
    Identifiers
    urn:nbn:se:uu:diva-237793 (URN)10.1109/TEC.2015.2425915 (DOI)000360439300026 ()
    Available from: 2014-12-05 Created: 2014-12-05 Last updated: 2017-12-05Bibliographically approved
    3. Robust primary frequency control in a system dominated by hydropower
    Open this publication in new window or tab >>Robust primary frequency control in a system dominated by hydropower
    (English)Manuscript (preprint) (Other academic)
    Abstract [en]

    Concerns about deteriorating quality of the Nordic grid frequency, including frequently occurring oscillations with a period of 40-90~s, has called for improved primary frequency control of the Nordic hydropower. In this paper, the parameters of the currently used PI controller with droop are optimised using a robust control approach. The trade-off between performance, actuator work and robustness is analysed in frequency domain and time domain, and the sensitivity to disturbances and model errors is discussed. It is found that the low-frequency oscillation of the system can be reduced by almost half if the parameters of the primary control are retuned.

    Keywords
    Load frequency control, primary frequency control, hydraulic turbine, robust control, PID control
    National Category
    Electrical Engineering, Electronic Engineering, Information Engineering
    Research subject
    Engineering Science with specialization in Science of Electricity
    Identifiers
    urn:nbn:se:uu:diva-237795 (URN)
    Available from: 2014-12-05 Created: 2014-12-05 Last updated: 2015-10-08
    Download full text (pdf)
    fulltext
  • 4.
    Saarinen, Linn
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    The Frequency of the Frequency: On Hydropower and Grid Frequency Control2017Doctoral 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.

    List of papers
    1. Field Measurements and System Identification of Three Frequency Controlling Hydropower Plants
    Open this publication in new window or tab >>Field Measurements and System Identification of Three Frequency Controlling Hydropower Plants
    2015 (English)In: IEEE transactions on energy conversion, ISSN 0885-8969, E-ISSN 1558-0059, Vol. 30, no 3, p. 1061-1068Article in journal (Refereed) Published
    Abstract [en]

    The dynamic behaviour of hydropower plants participating in primary frequency control is investigated in this paper through frequency response, step response and setpoint change tests on three Swedish hydropower plants. Grey-box system identification is used to estimate the parameters of simple linear models suitable for power system analysis and the major shortcomings of the linear models are discussed. It is found that frequency response tests with sinusoidal input signals give more reliable information about the dynamics of the plants than step response tests. It is also shown that backlash in the runner and guide vane regulating mechanisms are of great importance for the dynamic behaviour of the plants, and that the incremental gain from guide vane opening to power varies considerably with the operation point.

    Keywords
    hydropower, system identification, frequency control, field measurements
    National Category
    Electrical Engineering, Electronic Engineering, Information Engineering
    Research subject
    Engineering Science with specialization in Science of Electricity
    Identifiers
    urn:nbn:se:uu:diva-237793 (URN)10.1109/TEC.2015.2425915 (DOI)000360439300026 ()
    Available from: 2014-12-05 Created: 2014-12-05 Last updated: 2017-12-05Bibliographically approved
    2. Full-scale test and modelling of the frequency control dynamics of the Nordic power system
    Open this publication in new window or tab >>Full-scale test and modelling of the frequency control dynamics of the Nordic power system
    Show others...
    2016 (English)Conference paper, Published paper (Refereed)
    Abstract [en]

    The grid frequency quality in the Nordic power system has been deteriorating during the last decade. To improve the situation, a better understanding of the system is needed. In this paper, a model of the Nordic power system dynamics with respect to normal operation frequency control is set up and compared with full-scale measurements on the system. The "60 s oscillation" of the grid frequency is measured and explained by the system model.

    Keywords
    frequency control, hydropower, power system stability, field measurements, primary contol, frequency response
    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-308435 (URN)10.1109/PESGM.2016.7741711 (DOI)
    Conference
    2016 IEEE Power and Energy Society General Meeting (PESGM)
    Available from: 2016-11-25 Created: 2016-11-25 Last updated: 2016-11-30
    3. A Mathematical Model and Its Application for Hydro Power Units under Different Operating Conditions
    Open this publication in new window or tab >>A Mathematical Model and Its Application for Hydro Power Units under Different Operating Conditions
    Show others...
    2015 (English)In: Energies, E-ISSN 1996-1073, Vol. 8, no 9, p. 10260-10275Article in journal (Refereed) Published
    Abstract [en]

    This paper presents a mathematical model of hydro power units, especially the governor system model for different operating conditions, based on the basic version of the software TOPSYS. The mathematical model consists of eight turbine equations, one generator equation, and one governor equation, which are solved for ten unknown variables. The generator and governor equations, which are different under various operating conditions, are presented and discussed in detail. All the essential non-linear factors in the governor system (dead-zone, saturation, rate limiting, and backlash) are also considered. Case studies are conducted based on one Swedish hydro power plant (HPP) and three Chinese plants. The simulation and on-site measurements are compared for start-up, no-load operation, normal operation, and load rejection in different control modes (frequency, opening, and power feedback). The main error in each simulation is also discussed in detail. As a result, the model application is proved trustworthy for simulating different physical quantities of the unit (e.g., guide vane opening, active power, rotation speed, and pressures at volute and draft tube). The model has already been applied effectively in consultant analyses and scientific studies.

    National Category
    Energy Engineering
    Identifiers
    urn:nbn:se:uu:diva-262746 (URN)10.3390/en80910260 (DOI)000362553000064 ()
    Note

    Correction in: Energies 9(6) Article number: 477 DOI: 10.3390/en9060477

    Available from: 2015-09-18 Created: 2015-09-18 Last updated: 2023-08-28Bibliographically approved
    4. Wear and tear on hydro power turbines: influence from primary frequency control
    Open this publication in new window or tab >>Wear and tear on hydro power turbines: influence from primary frequency control
    Show others...
    2016 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 87, p. 88-95Article in journal (Refereed) Published
    Abstract [en]

    Nowadays the importance and need of primary frequency control of hydro power units are significantly increasing, because of the greater proportion of intermittent renewable energy sources and more complex structure of power systems. It brings a problem of increasing wear and tear of turbines. This paper studies this problem by applying numerical simulation and concise theoretical derivation, from the point view of regulation and control. Governor models under opening and power feedback mode are built and validated by measurement data. The core index, guide vane movement, is analyzed based on ideal sinusoidal frequency input and real frequency records. The results show the influences on wear and tear of different factors, e.g. governor parameters, power feedback mode and nonlinear governor factors.

    National Category
    Ocean and River Engineering
    Identifiers
    urn:nbn:se:uu:diva-262747 (URN)10.1016/j.renene.2015.10.009 (DOI)000367759500008 ()
    Available from: 2015-09-18 Created: 2015-09-18 Last updated: 2017-12-04Bibliographically approved
    5. Tuning primary frequency controllers using robust control theory in a power system dominated by hydropower
    Open this publication in new window or tab >>Tuning primary frequency controllers using robust control theory in a power system dominated by hydropower
    2016 (English)Conference paper, Published 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.

    Keywords
    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:nbn:se:uu:diva-308436 (URN)
    Conference
    CIGRE Session 2016
    Available from: 2016-11-25 Created: 2016-11-25 Last updated: 2016-11-30
    6. Wear reduction for hydro power turbines considering frequency quality of power systems: a study on controller filters
    Open this publication in new window or tab >>Wear reduction for hydro power turbines considering frequency quality of power systems: a study on controller filters
    Show others...
    2017 (English)In: IEEE Transactions on Power Systems, ISSN 0885-8950, E-ISSN 1558-0679, Vol. 32, no 2, p. 1191-1201Article in journal (Refereed) Published
    Abstract [en]

    Nowadays, the wear and tear of hydropower turbines is increasing, due to more regulation movements caused by the increasing integration of intermittent renewable energy sources. In this paper, a controller filter is proposed as a solution to the tradeoff between reducing the wear of turbines and maintaining the regulation performance and thereby the frequency quality of the power systems. The widely used dead zone is compared with a floating dead zone and a linear filter, by time-domain simulation and frequency-domain analysis. Simulink models are built and compared with onsite measurement. Then, the time-domain simulation is used to investigate the guide vane movement, the load disturbance and the power system frequency, based on a one-day grid frequency datameasured in this study. In the theoretical analysis, the describing functions method and the Nyquist criterion are adopted to examine the stability of the system with different filters. The results show that the floating dead zone, especially the one after the controller, has a better performance than the dead zone on both the wear reduction and frequency quality. The linear filter has a relatively weak impact on both guide vane movements and the frequency quality. Other related conclusion and understandings are also obtained.

    National Category
    Ocean and River Engineering
    Identifiers
    urn:nbn:se:uu:diva-262748 (URN)10.1109/TPWRS.2016.2590504 (DOI)000395865900033 ()
    Available from: 2015-09-18 Created: 2015-09-18 Last updated: 2017-04-27Bibliographically approved
    7. Allocation of Frequency Control Reserves and its Impact on Wear and Tear on a Hydropower Fleet
    Open this publication in new window or tab >>Allocation of Frequency Control Reserves and its Impact on Wear and Tear on a Hydropower Fleet
    2018 (English)In: IEEE Transactions on Power Systems, ISSN 0885-8950, E-ISSN 1558-0679, Vol. 33, no 1, p. 430-439Article in journal (Refereed) Published
    Abstract [en]

    Power systems are making a transition from purely technical, centrally planned systems to market based, decentralized systems. The need for balancing power and frequency control reserves are increasing, partially due to variable renewable production, which gives an opportunity for new incomes but also a challenge in terms of changed modes of operation with risk for reduced lifetime for controllable power plants. This paper investigates how the allocation of a sold volume of frequency control reserves within a large hydropower production fleet can affect the costs of providing primary and secondary reserves, in terms of its impact on wear and fatigue, production losses, and the quality of the delivered frequency control. The results show that for primary control, low static gain in the governors results in poor quality and a large amount of load cycles of the units. High static gain, on the other hand, increases the production losses. The control work of the fleet can be reduced by using a proper balance of primary and secondary control gain on each unit, although the intuitive results from linear models exaggerate this effect. Automatic secondary control improves the system frequency quality but also increases the wear.

    Keywords
    hydropower, frequency control, primary control, reserve allocation, wear and tear
    National Category
    Electrical Engineering, Electronic Engineering, Information Engineering
    Research subject
    Engineering Science with specialization in Science of Electricity
    Identifiers
    urn:nbn:se:uu:diva-308437 (URN)10.1109/TPWRS.2017.2702280 (DOI)000418776400038 ()
    Note

    Title of this article in dissertation reference lists: Allocation of Frequency Control Reserves and its Impact on Wear on a Hydropower Fleet

    Available from: 2016-11-25 Created: 2016-11-25 Last updated: 2018-02-07Bibliographically approved
    8. Primary Frequency Control - Pay for Performance
    Open this publication in new window or tab >>Primary Frequency Control - Pay for Performance
    (English)Manuscript (preprint) (Other academic)
    Keywords
    frequency control, hydropower, remuneration, market, primary control, power system stability
    National Category
    Electrical Engineering, Electronic Engineering, Information Engineering
    Research subject
    Engineering Science with specialization in Science of Electricity
    Identifiers
    urn:nbn:se:uu:diva-308439 (URN)
    Available from: 2016-11-25 Created: 2016-11-25 Last updated: 2016-11-29
    9. Power system flexibility need induced by wind and solar power intermittency on time scales of 1-14 days
    Open this publication in new window or tab >>Power system flexibility need induced by wind and solar power intermittency on time scales of 1-14 days
    2015 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 83, p. 339-344Article in journal (Refereed) Published
    Abstract [en]

    This article describes a method to assess the needed production flexibility to adapt the power system to the production from variable renewable energy sources such as wind power and photovoltaics over time horizons of 1-14 days. Load and production data from the German power system is used to quantify the flexibility need in terms of power and energy storage requirement due to higher shares of renewable energy (20-80%). It is found that with an 80% variable renewable energy share in the German system, the average power need from flexible sources decreases by 31 GW (59%) while the peak power need only decreases by 3 GW (4%). In terms of energy, the storage need over a two week horizon increases by 2.6 TWh, which is 14% of the average load per day. If the European plans for 100 GW wind power in the North Sea region are realised, this would mean an increase of the energy storage need in the region with 2.2 TWh over a two week horizon.

    Keywords
    Energy storage, wind power, solar power, dispatching, balancing
    National Category
    Electrical Engineering, Electronic Engineering, Information Engineering
    Research subject
    Engineering Science with specialization in Science of Electricity
    Identifiers
    urn:nbn:se:uu:diva-237789 (URN)10.1016/j.renene.2015.04.048 (DOI)000358455100031 ()
    Available from: 2014-12-05 Created: 2014-12-05 Last updated: 2017-12-05
    10. Linear Synthetic Inertia for Improved Frequency Quality and Reduced Hydropower Wear and Tear
    Open this publication in new window or tab >>Linear Synthetic Inertia for Improved Frequency Quality and Reduced Hydropower Wear and Tear
    (English)In: IEEE Transactions on Sustainable Energy, ISSN 1949-3029, E-ISSN 1949-3037Article in journal (Refereed) Submitted
    Keywords
    frequency control, hydropower, inertia, synthetic inertia, damping, power system stability, wear and tear
    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-308440 (URN)
    Available from: 2016-11-25 Created: 2016-11-25 Last updated: 2017-11-29
    11. Standstill Frequency Response Test on a Synchronous Machine Extended With Damper Bar Measurements
    Open this publication in new window or tab >>Standstill Frequency Response Test on a Synchronous Machine Extended With Damper Bar Measurements
    2016 (English)In: IEEE transactions on energy conversion, ISSN 0885-8969, E-ISSN 1558-0059, Vol. 31, no 1, p. 46-56Article in journal (Refereed) Published
    Abstract [en]

    Standstill Frequency Response (SSFR) test data from a salient-pole synchronous machine with reconfigurable damper winding is presented. In addition to the regular measurements, the damper bar currents are measured and used to obtain the stator-to-damper transfer functions. The test is performed three times with physically different damper winding configurations. An extension to the standard SSFR test analysis scheme is suggested where the stator-to-damper transfer functions are included. The validity of the identified models is substantiated by comparison of the simulated and measured machine response to a drive torque step disturbance. It is found that the damper winding measurements can be incorporated in the analysis scheme to isolate the effect of the damper circuits. However, for a machine of the type studied, also the standard SSFR test produce yields models that are accurate enough for power system studies.

    National Category
    Other Electrical Engineering, Electronic Engineering, Information Engineering
    Research subject
    Engineering Science with specialization in Science of Electricity
    Identifiers
    urn:nbn:se:uu:diva-182155 (URN)10.1109/TEC.2015.2450994 (DOI)000372024100005 ()
    External cooperation:
    Available from: 2012-10-04 Created: 2012-10-04 Last updated: 2022-01-28Bibliographically approved
    Download full text (pdf)
    fulltext
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  • 5.
    Saarinen, Linn
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Vattenfall AB, Älvkarlebylaboratoriet, Älvkarleby, Sweden.
    Dahlbäck, Niklas
    Vattenfall AB, Älvkarlebylaboratoriet, Älvkarleby, Sweden.
    Lundin, Urban
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Power system flexibility need induced by wind and solar power intermittency on time scales of 1-14 days2015In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 83, p. 339-344Article in journal (Refereed)
    Abstract [en]

    This article describes a method to assess the needed production flexibility to adapt the power system to the production from variable renewable energy sources such as wind power and photovoltaics over time horizons of 1-14 days. Load and production data from the German power system is used to quantify the flexibility need in terms of power and energy storage requirement due to higher shares of renewable energy (20-80%). It is found that with an 80% variable renewable energy share in the German system, the average power need from flexible sources decreases by 31 GW (59%) while the peak power need only decreases by 3 GW (4%). In terms of energy, the storage need over a two week horizon increases by 2.6 TWh, which is 14% of the average load per day. If the European plans for 100 GW wind power in the North Sea region are realised, this would mean an increase of the energy storage need in the region with 2.2 TWh over a two week horizon.

  • 6.
    Saarinen, Linn
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Lundin, Urban
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Robust primary frequency control in a system dominated by hydropowerManuscript (preprint) (Other academic)
    Abstract [en]

    Concerns about deteriorating quality of the Nordic grid frequency, including frequently occurring oscillations with a period of 40-90~s, has called for improved primary frequency control of the Nordic hydropower. In this paper, the parameters of the currently used PI controller with droop are optimised using a robust control approach. The trade-off between performance, actuator work and robustness is analysed in frequency domain and time domain, and the sensitivity to disturbances and model errors is discussed. It is found that the low-frequency oscillation of the system can be reduced by almost half if the parameters of the primary control are retuned.

  • 7.
    Saarinen, Linn
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Norrlund, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Lundin, Urban
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Field Measurements and System Identification of Three Frequency Controlling Hydropower Plants2015In: IEEE transactions on energy conversion, ISSN 0885-8969, E-ISSN 1558-0059, Vol. 30, no 3, p. 1061-1068Article in journal (Refereed)
    Abstract [en]

    The dynamic behaviour of hydropower plants participating in primary frequency control is investigated in this paper through frequency response, step response and setpoint change tests on three Swedish hydropower plants. Grey-box system identification is used to estimate the parameters of simple linear models suitable for power system analysis and the major shortcomings of the linear models are discussed. It is found that frequency response tests with sinusoidal input signals give more reliable information about the dynamics of the plants than step response tests. It is also shown that backlash in the runner and guide vane regulating mechanisms are of great importance for the dynamic behaviour of the plants, and that the incremental gain from guide vane opening to power varies considerably with the operation point.

  • 8.
    Saarinen, Linn
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Norrlund, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Lundin, Urban
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Tuning primary frequency controllers using robust control theory in a power system dominated by hydropower2016Conference 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.

  • 9.
    Saarinen, Linn
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Norrlund, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Lundin, Urban
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Agneholm, Evert
    Gothia Power, Gothenburg, Sweden..
    Westberg, Andreas
    Svenska Kraftnat, Sundbyberg, Sweden..
    Full-scale test and modelling of the frequency control dynamics of the Nordic power system2016In: 2016 IEEE POWER AND ENERGY SOCIETY GENERAL MEETING (PESGM), New York: IEEE, 2016Conference paper (Refereed)
    Abstract [en]

    The grid frequency quality in the Nordic power system has been deteriorating during the last decade. To improve the situation, a better understanding of the system is needed. In this paper, a model of the Nordic power system dynamics with respect to normal operation frequency control is set up and compared with full-scale measurements on the system. The "60 s oscillation" of the grid frequency is measured and explained by the system model.

  • 10.
    Saarinen, Linn
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Norrlund, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Lundin, Urban
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Agneholm, Evert
    Westberg, Andreas
    Full-scale test and modelling of the frequency control dynamics of the Nordic power system2016Conference paper (Refereed)
    Abstract [en]

    The grid frequency quality in the Nordic power system has been deteriorating during the last decade. To improve the situation, a better understanding of the system is needed. In this paper, a model of the Nordic power system dynamics with respect to normal operation frequency control is set up and compared with full-scale measurements on the system. The "60 s oscillation" of the grid frequency is measured and explained by the system model.

  • 11.
    Saarinen, Linn
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Norrlund, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Yang, Weijia
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Linear Synthetic Inertia for Improved Frequency Quality and Reduced Hydropower Wear and TearIn: IEEE Transactions on Sustainable Energy, ISSN 1949-3029, E-ISSN 1949-3037Article in journal (Refereed)
  • 12.
    Saarinen, Linn
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Vattenfall Hydropower AB, SE-81470 Alvkarleby, Sweden.
    Norrlund, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Alvkarlebylaboratoriet, Vattenfall Res & Dev, SE-81426 Alvkarleby, Sweden.
    Yang, Weijia
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Lundin, Urban
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Allocation of Frequency Control Reserves and its Impact on Wear and Tear on a Hydropower Fleet2018In: IEEE Transactions on Power Systems, ISSN 0885-8950, E-ISSN 1558-0679, Vol. 33, no 1, p. 430-439Article in journal (Refereed)
    Abstract [en]

    Power systems are making a transition from purely technical, centrally planned systems to market based, decentralized systems. The need for balancing power and frequency control reserves are increasing, partially due to variable renewable production, which gives an opportunity for new incomes but also a challenge in terms of changed modes of operation with risk for reduced lifetime for controllable power plants. This paper investigates how the allocation of a sold volume of frequency control reserves within a large hydropower production fleet can affect the costs of providing primary and secondary reserves, in terms of its impact on wear and fatigue, production losses, and the quality of the delivered frequency control. The results show that for primary control, low static gain in the governors results in poor quality and a large amount of load cycles of the units. High static gain, on the other hand, increases the production losses. The control work of the fleet can be reduced by using a proper balance of primary and secondary control gain on each unit, although the intuitive results from linear models exaggerate this effect. Automatic secondary control improves the system frequency quality but also increases the wear.

  • 13.
    Saarinen, Linn
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Vattenfall Hydropower AB, Älvkarleby, Sweden..
    Norrlund, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Älvkarlebylaboratoriet, Vattenfall Res & Dev, Älvkarleby, Sweden..
    Yang, Weijia
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Wuhan Univ, State Key Lab Water Resources & Hydropower Engn S, Wuhan 430072, Hubei, Peoples R China..
    Lundin, Urban
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Linear synthetic inertia for improved frequency quality and reduced hydropower wear and tear2018In: International Journal of Electrical Power & Energy Systems, ISSN 0142-0615, E-ISSN 1879-3517, Vol. 98, p. 488-495Article in journal (Refereed)
    Abstract [en]

    The power system inertia is decreasing in many electrical grids as the share of production from directly connected synchronous generators decreases. Lower inertia increases the frequency deviations in normal operation, which leads to increased wear and tear in hydropower turbines and other units providing frequency control to the system. The predominant concepts for synthetic inertia from for example wind power does not address the frequency quality in normal operation, only the acute problem of frequency stability during large disturbances. This paper investigates how the frequency quality and frequency controlling hydropower units are affected by decreasing inertia and damping, using the Nordic power system as a case study. A new type of synthetic inertia (SI), which is linear and continuously active, is suggested as a means to mitigate the impacts on these units. It is shown that the suggested linear SI controller can effectively replace synchronous inertia and damping, improving frequency quality and reducing hydropower wear and tear. The controller includes an energy recovery feedback loop, to avoid depletion of the energy source behind the controller. The power and energy needed to provide linear SI is quantified, and the impact of the SI energy recovery integration time constant is investigated.

  • 14.
    Yang, Weijia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Norrlund, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Saarinen, Linn
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Yang, Jiandong
    Guo, Wencheng
    Zeng, Wei
    Wear and tear on hydro power turbines: influence from primary frequency control2016In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 87, p. 88-95Article in journal (Refereed)
    Abstract [en]

    Nowadays the importance and need of primary frequency control of hydro power units are significantly increasing, because of the greater proportion of intermittent renewable energy sources and more complex structure of power systems. It brings a problem of increasing wear and tear of turbines. This paper studies this problem by applying numerical simulation and concise theoretical derivation, from the point view of regulation and control. Governor models under opening and power feedback mode are built and validated by measurement data. The core index, guide vane movement, is analyzed based on ideal sinusoidal frequency input and real frequency records. The results show the influences on wear and tear of different factors, e.g. governor parameters, power feedback mode and nonlinear governor factors.

  • 15.
    Yang, Weijia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Wuhan Univ, State Key Lab Water Resources & Hydropower Engn S, Wuhan 430072, Peoples R China..
    Norrlund, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Vattenfall R&D, SE-81426 Alvkarleby, Sweden..
    Saarinen, Linn
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Vattenfall R&D, SE-81426 Alvkarleby, Sweden..
    Yang, Jiandong
    Wuhan Univ, State Key Lab Water Resources & Hydropower Engn S, Wuhan 430072, Peoples R China..
    Zeng, Wei
    Wuhan Univ, State Key Lab Water Resources & Hydropower Engn S, Wuhan 430072, Peoples R China..
    Lundin, Urban
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Wear reduction for hydro power turbines considering frequency quality of power systems: a study on controller filters2017In: IEEE Transactions on Power Systems, ISSN 0885-8950, E-ISSN 1558-0679, Vol. 32, no 2, p. 1191-1201Article in journal (Refereed)
    Abstract [en]

    Nowadays, the wear and tear of hydropower turbines is increasing, due to more regulation movements caused by the increasing integration of intermittent renewable energy sources. In this paper, a controller filter is proposed as a solution to the tradeoff between reducing the wear of turbines and maintaining the regulation performance and thereby the frequency quality of the power systems. The widely used dead zone is compared with a floating dead zone and a linear filter, by time-domain simulation and frequency-domain analysis. Simulink models are built and compared with onsite measurement. Then, the time-domain simulation is used to investigate the guide vane movement, the load disturbance and the power system frequency, based on a one-day grid frequency datameasured in this study. In the theoretical analysis, the describing functions method and the Nyquist criterion are adopted to examine the stability of the system with different filters. The results show that the floating dead zone, especially the one after the controller, has a better performance than the dead zone on both the wear reduction and frequency quality. The linear filter has a relatively weak impact on both guide vane movements and the frequency quality. Other related conclusion and understandings are also obtained.

  • 16.
    Yang, Weijia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Wuhan Univ, State Key Lab Water Resources & Hydropower Engn S, Wuhan, Hubei, Peoples R China; Wuhan Univ, State Key Lab Water Resources & Hydropower Engn S, Wuhan, Hubei, Peoples R China; Oak Ridge Natl Lab, Div Environm Sci, Oak Ridge, TN USA.
    Sundqvist, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Vattenfall R&D, Älvkarleby, Sweden.
    Saarinen, Linn
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Vattenfall R&D, Älvkarleby, Sweden.
    Witt, Adam
    Oak Ridge Natl Lab, Div Environm Sci, Oak Ridge, TN USA.
    Smith, Brennan
    Oak Ridge Natl Lab, Div Environm Sci, Oak Ridge, TN USA.
    Yang, Jiandong
    Wuhan Univ, State Key Lab Water Resources & Hydropower Engn S, Wuhan, Hubei, Peoples R China.
    Lundin, Urban
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Burden on hydropower units for short-term balancing of renewable power systems2018In: Nature Communications, E-ISSN 2041-1723, Vol. 9, article id 2633Article in journal (Refereed)
    Abstract [en]

    There is a general need to change hydropower operational regimes to balance the growing contribution of variable renewable energy sources in power systems. Quantifying the burden on generation equipment is increasingly uncertain and difficult. Here, we propose a framework combining technical and economic indicators to analyze primary frequency control (PFC) on a timescale of seconds. We develop a model integrating hydraulic, mechanical, and electrical subsystems to characterize efficiency loss, wear and fatigue, regulation mileage, and frequency quality. We evaluate burden relief strategies under three idealized remuneration schemes for PFC, inspired by those used in Sweden, the USA, and China, respectively. We show how burden and compensation vary under future scenarios of renewable power systems. Our framework can be used by producers to develop favorable operation strategies that reduce burden and increase economic value, and by transmission system operators to provide insights on the relation between incentive structures and regulating performance.

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  • 17.
    Yang, Weijia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Yang, Jiandong
    Guo, Wencheng
    Zeng, Wei
    Wang, Chao
    Saarinen, Linn
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Norrlund, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    A Mathematical Model and Its Application for Hydro Power Units under Different Operating Conditions2015In: Energies, E-ISSN 1996-1073, Vol. 8, no 9, p. 10260-10275Article in journal (Refereed)
    Abstract [en]

    This paper presents a mathematical model of hydro power units, especially the governor system model for different operating conditions, based on the basic version of the software TOPSYS. The mathematical model consists of eight turbine equations, one generator equation, and one governor equation, which are solved for ten unknown variables. The generator and governor equations, which are different under various operating conditions, are presented and discussed in detail. All the essential non-linear factors in the governor system (dead-zone, saturation, rate limiting, and backlash) are also considered. Case studies are conducted based on one Swedish hydro power plant (HPP) and three Chinese plants. The simulation and on-site measurements are compared for start-up, no-load operation, normal operation, and load rejection in different control modes (frequency, opening, and power feedback). The main error in each simulation is also discussed in detail. As a result, the model application is proved trustworthy for simulating different physical quantities of the unit (e.g., guide vane opening, active power, rotation speed, and pressures at volute and draft tube). The model has already been applied effectively in consultant analyses and scientific studies.

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    fulltext
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