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Saarinen, L., Norrlund, P., Yang, W. & Lundin, U. (2018). Allocation of Frequency Control Reserves and its Impact on Wear and Tear on a Hydropower Fleet. IEEE Transactions on Power Systems, 33(1), 430-439
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
Bao, H., Yang, J., Zhao, G., Zeng, W., Liu, Y. & Yang, W. (2018). Condition of setting surge tanks in hydropower plants - A review. Renewable & sustainable energy reviews, 81, 2059-2070
Open this publication in new window or tab >>Condition of setting surge tanks in hydropower plants - A review
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2018 (English)In: Renewable & sustainable energy reviews, ISSN 1364-0321, E-ISSN 1879-0690, Vol. 81, p. 2059-2070Article, review/survey (Refereed) Published
Abstract [en]

Hydropower plays an important role in the safe, stable and efficient operation of power systems, especially with current trends toward renewable energy systems. The total global potential of gross, technical, economic, and exploitable hydropower are still enormous in the future, and the developments of new hydropower stations (HPSs) are of great importance. For constructions of new HPSs, the condition of setting surge tanks (CSST) is crucial for various perspectives, e.g. safety, stability and economy of HPSs. In this review, the CSST are summarized and analyzed from the three aspects: regulation assurance, operation stability, and the regulation quality, with an aim of providing a reference and guidance for research and engineering applications regarding surge tanks. Upstream and downstream surge tanks in conventional HPSs and pumped storage power stations are all included. Moreover, a comprehensive comparison of CSST under different conditions is conducted. One of the main focuses of this review is on Chinese studies, for introducing many meaningful results written in Chinese to more readers all over the world.

Keywords
Hydropower station, Surge tank, Regulation assurance, Stability, Regulation quality, Water inertia time constant
National Category
Energy Engineering
Identifiers
urn:nbn:se:uu:diva-347707 (URN)10.1016/j.rser.2017.06.012 (DOI)000417078200034 ()
Available from: 2018-04-06 Created: 2018-04-06 Last updated: 2018-04-06Bibliographically approved
Yang, W., Norrlund, P., Chung, C. Y., Yang, J. & Lundin, U. (2018). Eigen-analysis of hydraulic-mechanical-electrical coupling mechanism for small signal stability of hydropower plant. Renewable energy, 115, 1014-1025
Open this publication in new window or tab >>Eigen-analysis of hydraulic-mechanical-electrical coupling mechanism for small signal stability of hydropower plant
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2018 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 115, p. 1014-1025Article in journal (Refereed) Published
Abstract [en]

Hydropower shoulders important responsibility for regulation and control of power systems with intermittent renewable energy sources. The quality of regulation required for hydropower units has been increasing, and the interaction between hydropower plants (HPPs) and power systems is of great importance. This work aims to conduct a fundamental study on hydraulic-mechanical-electrical coupling mechanism for small signal stability of HPPs. The main focus is the impact of hydraulic-mechanical factors on the local mode oscillation in a Single-Machine-Infinite-Bus system. A twelfth-order state matrix is established for theoretical eigen-analysis as the core approach. Meanwhile, a simulation model based on Simulink/SimPowerSystems is built for validation. The influencing mechanisms of water column elasticity, governor mechanical component, and water inertia are studied under different control modes of the turbine governor. The results show considerable influence from hydraulic-mechanical factors, and the effect of turbine governor actions is no longer ignorable; also, the damping performance under power system stabilizers can be considerably affected. Insights into interactions among physical quantities in various conditions are obtained as well.

Keywords
Small signal stability, Hydropower plant, Turbine governor, Eigenvalue, Power system stabilizer
National Category
Energy Engineering
Identifiers
urn:nbn:se:uu:diva-340668 (URN)10.1016/j.renene.2017.08.005 (DOI)000413615500092 ()
Available from: 2018-02-05 Created: 2018-02-05 Last updated: 2018-02-05Bibliographically approved
Yang, W., Sundqvist, P., Lidenholm, J., Yang, J. & Lundin, U. (2018). Hydraulic damping mechanism of low frequency oscillations in power systems: Quantitative analysis using a nonlinear model of hydropower plants. Applied Energy, 212, 1138-1152
Open this publication in new window or tab >>Hydraulic damping mechanism of low frequency oscillations in power systems: Quantitative analysis using a nonlinear model of hydropower plants
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2018 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 212, p. 1138-1152Article in journal (Refereed) Published
Abstract [en]

As power systems grow reliant on an increasing mix of intermittent renewables, hydropower units are being called upon to provide more aggressive power-frequency control. This dynamic is enlarging the significance of interaction between hydraulic and electrical subsystems in hydropower plants (HPPs). The influence from HPPs on power system stability is of great importance, especially for hydro-dominant power systems like the Nordic power system and the China Southern Power Grid. This paper aims to quantify and reveal the influencing mechanism of the hydraulic damping of low frequency oscillations in power systems. An equivalent hydraulic turbine damping coefficient is introduced, and a nonlinear HPP model that combines electrical subsystems with a refined hydraulic-mechanical subsystem is established and verified. A novel quantifying methodology is proposed through simulations by two different models based on case studies on a Swedish HPP. Then, the quantification results of the damping coefficient are presented and the influencing mechanism behind is revealed, by studying three representative factors from the hydraulic-mechanical system: the delay in turbine governor systems, governor parameter and penstock length. Observations and discussions of on-site measurements are included to support the analysis. The results show that the damping effect from hydraulic turbines can be considerable. Based on the limited cases in the HPP, the damping coefficient can vary from + 3.0 to -2.3, while previously the contribution has been unclear and normally assumed to be positive. The phase shift in the mechanical power response with respect to the rotational speed deviation is an important reason for the different damping performance. Furthermore, the effect and significance of implementing the damping coefficient on cases with power system stabilizer (PSS) are demonstrated.

Place, publisher, year, edition, pages
ELSEVIER SCI LTD, 2018
Keywords
Hydraulic damping, Hydro-dominant power systems, Rotor angle stability, Low frequency oscillation, Nonlinear model, Numerical simulation
National Category
Environmental Engineering
Identifiers
urn:nbn:se:uu:diva-348101 (URN)10.1016/j.apenergy.2018.01.002 (DOI)000425200700085 ()
Available from: 2018-04-11 Created: 2018-04-11 Last updated: 2018-04-11Bibliographically approved
Saarinen, L., Norrlund, P., Yang, W. & Lundin, U. (2018). Linear synthetic inertia for improved frequency quality and reduced hydropower wear and tear. International Journal of Electrical Power & Energy Systems, 98, 488-495
Open this publication in new window or tab >>Linear synthetic inertia for improved frequency quality and reduced hydropower wear and tear
2018 (English)In: International Journal of Electrical Power & Energy Systems, ISSN 0142-0615, E-ISSN 1879-3517, Vol. 98, p. 488-495Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
ELSEVIER SCI LTD, 2018
Keywords
Synthetic inertia, Hydropower, Frequency control, Virtual synchronous machine, Wear and tear
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-350482 (URN)10.1016/j.ijepes.2017.12.007 (DOI)000425568100045 ()
Available from: 2018-05-17 Created: 2018-05-17 Last updated: 2018-06-05Bibliographically approved
Yang, W. (2017). Hydropower plants and power systems: Dynamic processes and control for stable and efficient operation. (Doctoral dissertation). Uppsala: Acta Universitatis Upsaliensis
Open this publication in new window or tab >>Hydropower plants and power systems: Dynamic processes and control for stable and efficient operation
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

As the largest global renewable source, hydropower shoulders a large portion of the regulation duty in many power systems. New challenges are emerging from variable renewable energy (VRE) sources, the increasing scale and complexity of hydropower plants (HPPs) and power grid. Stable and efficient operation of HPPs and their interaction with power systems is of great importance.

Theoretical analysis, numerical simulation and on-site measurement are adopted as main study methods in this thesis. Various numerical models of HPPs are established, with different degrees of complexity for different purposes. The majority of the analysis and results are based on eight HPPs in Sweden and China.

Stable operation (frequency stability and rotor angle stability) and efficient operation are two important goals. Regarding the stable operation, various operating conditions are analysed; the response time of primary frequency control (PFC) and the system stability of isolated operation are investigated. A fundamental study on hydraulic-mechanical-electrical coupling mechanisms for small signal stability of HPPs is conducted. A methodology is proposed to quantify the contribution to the damping of low frequency oscillations from hydraulic turbines. The oscillations, with periods ranging from less than one up to hundreds of seconds, are analysed.

Regarding the efficient operation, a description and an initial analysis of wear and tear of turbines are presented; a controller filter is proposed as a solution for wear reduction of turbines and maintaining the frequency quality of power systems; then the study is further extended by proposing a framework that combines technical plant operation with economic indicators, to obtain relative values of regulation burden and performance of PFC.

The results show that the coupling between the hydraulic-mechanical subsystem and the electrical subsystem can be considerable and should be considered with higher attention. Effectiveness and applicability of different numerical models are shown, supplying suggestions for further model optimization. For the influence from power systems on HPPs, the dynamic processes and corresponding control strategies of HPPs under diverse disturbances and requirements from power systems are addressed. For the influence from HPPs on power systems, quantifications of frequency quality and the hydraulic damping are conducted utilising proposed methodologies.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. p. 140
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1494
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-318470 (URN)978-91-554-9871-9 (ISBN)
Public defence
2017-05-19, Polhemsalen, Ångtröm 10134, Lägerhyddsvägen 1, Ångströmlaboratoriet, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2017-04-28 Created: 2017-03-24 Last updated: 2017-05-05
Yang, W., Norrlund, P., Saarinen, L., Yang, J., Zeng, W. & Lundin, U. (2017). Wear reduction for hydro power turbines considering frequency quality of power systems: a study on controller filters. IEEE Transactions on Power Systems, 32(2), 1191-1201
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
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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
Yang, W., Norrlund, P. & Yang, J. (2016). Analysis on regulation strategies for extending service life of hydropower turbines. In: : . Paper presented at 28th IAHR symposium on Hydraulic Machinery and Systems (IAHR2016). , Article ID 052013.
Open this publication in new window or tab >>Analysis on regulation strategies for extending service life of hydropower turbines
2016 (English)Conference paper, Published paper (Refereed)
Abstract [en]

Since a few years, there has been a tendency that hydropower turbines experience fatigue to a greater extent, due to increasingly more regulation movements of governor actuators. The aim of this paper is to extend the service life of hydropower turbines, by reasonably decreasing the guide vane (GV) movements with appropriate regulation strategies, e.g. settings of PI (proportional-integral) governor parameters and controller filters. The accumulated distance and number of GV movements are the two main indicators of this study. The core method is to simulate the long-term GV opening of Francis turbines with MATLAB/Simulink, based on a sequence of one-month measurements of the Nordic grid frequency. Basic theoretical formulas are also discussed and compared to the simulation results, showing reasonable correspondence. Firstly, a model of a turbine governor is discussed and verified, based on on-site measurements of a Swedish hydropower plant. Then, the influence of governor parameters is discussed. Effects of different settings of controller filters (e.g. dead zone, floating dead zone and linear filter) are also examined. Moreover, a change in GV movement might affect the quality of the frequency control. This is also monitored via frequency deviation characteristics, determined by elementary simulations of the Nordic power system. The results show how the regulation settings affect the GV movements and frequency quality, supplying suggestions for optimizing the hydropower turbine operation for decreasing the wear and tear.

Series
IOP Conference Series: Earth and Environmental Science, ISSN 1755-1307, E-ISSN 1755-1315 ; 49
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-310651 (URN)10.1088/1755-1315/49/5/052013 (DOI)000400156200054 ()
Conference
28th IAHR symposium on Hydraulic Machinery and Systems (IAHR2016)
Available from: 2016-12-17 Created: 2016-12-17 Last updated: 2018-04-04Bibliographically approved
Zeng, W., Yang, J. & Yang, W. (2016). Instability analysis of pumped-storage stations under no-load conditions using a parameter-varying model. Renewable energy, 90, 420-429
Open this publication in new window or tab >>Instability analysis of pumped-storage stations under no-load conditions using a parameter-varying model
2016 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 90, p. 420-429Article in journal (Refereed) Published
Abstract [en]

The S-shaped characteristics of a pump turbine make its rotational speed unstable when it starts up under no-load conditions with low head, affecting its ability to connect to the grid. Although advanced strategies for controlling the speed governor can alleviate this problem, they cannot fundamentally resolve the internal mechanisms that cause difficulties with a grid connection. Therefore, this study set out to theoretically explore the root cause of the instability and the dominant factors influencing it. A correlation fitting process was used to simplify the transcendental function for the pipe flow in elastic mode into a high-precision and low-order linear equation. Next, a detailed study of the two key factors affecting system stability (pump turbine S-shaped characteristics and water elasticity) was carried out based on the system model, and a comprehensive parameter that reflects the no-load characteristics was extracted. Furthermore, the Laplace transform and inverse transform decomposition were used to obtain a mathematical expression for the no-load oscillation in order to analyze the oscillation characteristics. Finally, simulations of no-load oscillations under various heads were performed to further validate the accuracy of the extracted comprehensive parameter for the no-load stability.

Keywords
Pumped-storage station, Pump turbine, No-load instability, Water elasticity, S-shaped characteristics
National Category
Energy Engineering
Identifiers
urn:nbn:se:uu:diva-280219 (URN)10.1016/j.renene.2016.01.024 (DOI)000370102400038 ()
Available from: 2016-03-10 Created: 2016-03-09 Last updated: 2017-11-30
Yang, W., Yang, J., Guo, W. & Norrlund, P. (2016). Response time for primary frequency control of hydroelectric generating unit. International Journal of Electrical Power & Energy Systems, 74, 16-24
Open this publication in new window or tab >>Response time for primary frequency control of hydroelectric generating unit
2016 (English)In: International Journal of Electrical Power & Energy Systems, ISSN 0142-0615, E-ISSN 1879-3517, Vol. 74, p. 16-24Article in journal (Refereed) Published
Abstract [en]

For evaluating the power quality in primary frequency control for hydroelectric generating units, the power response time is an indicator which is of main concern to the power grid. The aim of this paper is to build a suitable model for conducting reliable simulation and to investigate the general rules for controlling the power response time. Two huge hydropower plants with surge tank from China and Sweden are applied in the simulation of a step test of primary frequency control, and the result is validated with data from full scale measurements. From the analytical aspect, this paper deduces a time domain solution for guide vane opening response and a response time formula, of which the main variables are governor parameters. Then the factors which cause the time difference, between the power response time and the analytical response time of opening, are investigated from aspects of both regulation and water way system. It is demonstrated that the formula can help to predict the power response and supply a flexible guidance of parameter tuning, especially for a hydropower plant without surge tank.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-259529 (URN)10.1016/j.ijepes.2015.07.003 (DOI)000362309100003 ()
Funder
StandUp
Available from: 2015-08-07 Created: 2015-08-07 Last updated: 2017-12-04Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0003-1638-0792

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