Effects of Increased Solar and Wind Energy on Hydro Plant Operation

In many countries, needs to reduce greenhouse gas emissions have led to increased installation of intermittent renewables such as wind and solar. It is important to take into account how integration of these technologies changes the operations of and affects maintenance needs at hydroelectric plants.

By Eduard Doujak

Europe is undergoing a strong effort to fulfill its obligation under the Kyoto Protocol, which entered into force in February 2005, to reduce greenhouse gas emissions. (The Kyoto Protocol is an international agreement linked to the United Nations Framework Convention on Climate Change, which commits its parties by setting internationally binding emission reduction targets.) Worldwide, representatives of 150 nations signed this contract and agreed to reduce their emissions on the basis of 1990 levels. Austria in particular committed to reduce these emissions by 13% as related to 1990 levels.

The increase in installed capacity after 2002 can be attributed to incentives designed to boost development of solar and wind facilities4

Austria signed the contract to join the European Union in 1995. Part of this contract was the requirement to liberalize the country’s electricity market within 10 years. Austria decided to open up its electricity market in 1998. As a member of the EU, Austria also agreed to the target rules for the climate policy by 2020, which are known as “20-20-20-targets”:

— 20% fewer greenhouse gas emissions compared with 1990;
— 20% share of renewable energy sources related to the entire energy production; and
— 20% increase in energy efficiency.

As a result of these commitments, the share of renewable energy sources in the country should rise to 34% by 2020. Knowing that Austria already produces 60% of its electricity from hydropower, this significant growth in renewable energy sources is only possible by installing other renewables, such as wind, solar and biomass facilities. However, it was recognized that something would have to be done to foster investment in small hydro and other renewable sources. To install these new technologies, a subsidy system was implemented in 2002, embedded in a so-called feed-in tariff (FIT).

Figure 1 shows how much wind and solar capacity is being installed in Austria. It is remarkable that after 2002 the installed capacity of these technologies increased considerably, which correlates to implementation of the FIT system.

These renewable energy sources cause some technical problems. Because they are non-dispatchable and fluctuating, the consequences for dispatchable sources like hydropower are evident in faster regulating and servicing of ancillary sources. Hydropower stations can generally be quickly regulated and are therefore well-suited to deliver frequency control. Thus, electric power utilities can make better use of their hydroelectric assets by selling ancillary services to the grid operator.

Hydropower stations must go through pre-qualification tests and need to be re-assessed with respect to the updated grid codes that define the conditions for ancillary services and specify the time constants for primary and secondary control, e.g. Swiss grid code (www.swissgrid.ch) or ENTSOE code www.entsoe.eu). Primary control power needs to be fully available within 30 seconds, while half of the output needs to be delivered after only 15 seconds. Secondary control power needs to be fully available after 2 minutes for 15 minutes’ duration.

Taking these time restrictions into account, the water turbines have to be operated far from best efficiency point. Also, the layout of old hydropower stations is not appropriate to run under these conditions. Therefore, special considerations have to be undertaken to increase the operational range of the hydro turbines. Questions of fast and frequent changes of the operating condition, wide operating ranges and the issues of dynamic loads, cavitation purposes and hydraulic stability need to be investigated and researched. Operators have to adjust their maintenance intervals according to these new requirements.

These new demands will be discussed below, using the examples of low- and high-head runners.

Low-head runners
Run-of-river plants in Austria are normally equipped with Kaplan runners. In the past, these hydropower plants and the runners were designed for discharge control, meaning low actuating frequency of the regulation mechanism. By changing the operation to frequency control, the actuating frequency of the regulation mechanism is increased to a much higher level.

Austrian utility Verbund, together with Andritz Hydro, investigated the consequences of such an operational change on some units. As Figures 2 and 3 show, the measured pressure signals of the runner servomotor are increased by a factor of 60.1

Run-of-river plants in Austria, equipped with Kaplan runners, were designed for discharge control. By changing to frequency control, the measured pressure signals of the runner servomotor are increased by a factor of 60.1

The biggest question in these cases is calculation of the residual life of the turbine runner components. Therefore, Verbund and Andritz Hydro performed new load spectrum analyses and combined them with Miner’s rule to assess the residual life for the investigated components. (Miner’s rule is a proven method to investigate cycling stresses and residual lifetime.) As a consequence, appropriate maintenance strategies must be adopted for those units.

In some cases, the existing runner unit would not withstand the new dynamic loading from frequency control without design corrections. Therefore, it is strongly advisable to check case-by-case if the existing unit is appropriate for an operational change. Nevertheless, it is questionable how these new requirements will act on the old machines, and without further research in this area a common rule cannot be derived.

High-head runners
In high-head runners, the situation is much more difficult because the operational range is much lower than that of Kaplan or Pelton turbine runners. Francis or pump turbines are single-regulated and therefore restricted in their off-design operation. This ensures smooth operation with longer life and lower maintenance costs.

Operating at zero discharge condition leads to a pre-rotation at the draft tube cone, influencing the flow pattern at this area and causing higher frequencies at the power spectrum of pressure.

By changing the operational point into the direction of low-load or speed-no-load conditions for Francis runners in turbine mode, new phenomena appear and the dynamic loads on the units rise. On the way to very low part-load operation, some effects occur, such as draft tube vortex rope, cavitational effects, and swirl-dominated flow regime leading to unsteady pressure fields. These result in higher stresses for the machines and lead to a more complex task in the case of determining component behavior, as well as in terms of maintenance strategies and life time prediction. Data has been published on the tasks to be fulfilled and outcomes.2

The Institute for Energy Systems and Thermodynamics has performed several studies in the field of low-load operation of pump turbines over the past few years. One aspect is the investigation of draft tube pre-rotation in pumping mode. It has been shown that operating at zero discharge condition leads to a pre-rotation at the draft tube cone, influencing the flow pattern at this area and causing higher frequencies at the power spectrum of pressure (see Figure 4).

The peaks at very low frequencies, especially at f/fΩ < 1, can be interpreted as slow-moving vortex structures in the draft tube. These unfavorable flow conditions cause additional stresses to the turbine unit and should be taken into account in the consideration of life time assessments.

In some cases, the injection of stabilization air through the runner or seal can help to suppress the draft tube vortex at part load operation in turbine mode. Plant measurements with acceleration sensors mounted at the guide vanes and draft tube cone showed the influence of stabilization aeration at different operation points. At about 50% part load of the machine, the unit runs in turbine mode with maximum available head.

Both examples show that it is not easy to determine these unsteady flow phenomena. It can be done either by laboratory investigations or site measurements. Computational fluid dynamics in this case can support the outcomes but has several difficulties and has to be calibrated by measurements. Both approaches combined can help to understand the flow pattern at this low-load field better and will provide a basis for the subsequent stress calculations by finite element analysis. The goal of these investigations is the life time assessment of the machine unit components to provide operators the needed information for their maintenance strategies. Nevertheless, this field is still under extensive research by suppliers and universities.

Variable speed units could change the situation, but at this time the author is unaware of any retrofit implementing variable-speed machines instead of conventional ones. Newer plants are equipped with variable-speed machines, which are better at part load operation but are not flexible enough to run at very low load. For the purpose of this article, variable machines will have a lesser influence because the effects being investigated are out of the range of the speed variation.

Conclusion
The increasing demands of more flexible operation of hydraulic units due to deregulation of the electricity market and stronger supply of non-dispatchable contributors cause an unfavorable off-peak operation. Often the electromechanical equipment of a hydropower station has to be operated for a longer period under these conditions with more transient events, such as start/stops or power changes. This situation is often overlapped by new grid stability requirements from the load dispatching operator.

Taking all these facts into account, we arrive at totally new specifications for newly installed water turbines and special challenges for old units. Kaplan turbines have been designed for base load operation with low frequency at the actuating mechanism. The design operational points of Francis or pump turbines have been traditionally at peak or close to peak efficiency, resulting in a smoother operation and thus fewer transients.

From the hydropower plant operator’s point of view, it is extremely important to know more about these flow phenomena resulting in machine wear effects and strongly applying to the residual life time of the components. As a result of changing the operating scheme at existing hydropower units, additional maintenance and control can be derived.

Although it would be nice to include a case study illustrating this topic, the European electricity market is a work in progress and these new demands on the machines are now starting. All involved parties are looking for solutions and answers to overcome the barriers. I am sure that we will hear and read more about such case studies in the future.  

Notes

1. Benda, S., and E. Wurm, Lebensdauer von Laufradteilen bei unterschiedlichen Betriebsarten von Kaplanturbinen, Presentation at Praktikerkonferenz, Graz, Austria, 2011.

2. Sick, M., et al., Flexible Turbine Operation Enabling Frequency Control, HYDRO 2012 conference, Bilbao, Portugal, 2012.

3. Edinger, G., S. Erne, E. Doujak, and C. Bauer, Flow Determination of a Pump-turbine at Zero Discharge, Proceedings of International Association for Hydro-Environment Conference, Montreal, Quebec, Canada, 2014.

4. Bacher, A., “Impact of Increased Feed-in of Solar and Wind Energy to the Electricity Grid on the Operating Conditions of Hydro Power Plants,” Masters thesis, Vienna University of Technology, Austria, 2013.

Eduard Doujak is an assistant professor with the Institute for Energy Systems and Thermodynamics, Department of Fluid-Flow Machinery at the Vienna University of Technology.

● Peer Reviewed This article has been evaluated and edited in accordance with reviews conducted by two or more professionals who have relevant expertise. These peer reviewers judge manuscripts for technical accuracy, usefulness, and overall importance within the hydroelectric industry.

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