By Thomas Beyer
The 1,060-mw Goldisthal pumped-storage plant on the Schwarza River is the biggest hydroelectric project in Germany and the most modern in Europe. It is an important component of Vattenfall Europe Generation AG & Co. KG’s generation capacity. Construction on the project began in September 1997, and the plant started commercial operation in October 2004.
The Goldisthal project is unique in that two of the four vertical Francis pump-turbine units feature variable-speed (asynchronous) motor-generators. This arrangement provides several benefits for Vattenfall Europe, including: power regulation during pumping operation, improved efficiency at partial load conditions, and high dynamic control of the power delivered, for stabilization of the grid.
Developing the Goldisthal project
In 1965, scientists with the German Democratic Republic performed a ranking study that identified the current site on the Schwarza River in Thuringia State as the best location for a large pumped-storage hydroelectric project. Beginning in 1972, geologists with Baugrund Dresden of Germany performed intensive geological investigations and began such infrastructure work as the transformer substation and bypass road. In 1975, Schachtbau Nordhausen GmbH of Germany built several 4-kilometer-long investigation tunnels to explore the site geology.
The upper reservoir (top right) of the 1,060-mw Goldisthal pumped-storage project is the largest artificial upper reservoir in Germany. The project is producing as much as 7,500 megawatt-hours of electricity a day.
Work on the Goldisthal project was suspended in 1980, primarily for economic reasons. The increase in energy demand in the country was not as high as expected, and financial problems delayed the construction schedule for the plant.
Nine years later, as a result of the political unification of Germany and restructuring of the East German power supply, investigations to develop Goldisthal resumed. To conduct the investigation, the government established Vereinigte Energiewerke AG (VEAG), the predecessor of Vattenfall Europe.
In 1990, VEAG began the planning and permission procedures to develop Goldisthal. The utility proposed to install four turbine-generating units with a total capacity of about 1,000 mw. VEAG determined that the most advantageous arrangement for this plant involved combining conventional pumped-storage (synchronous) technology (represented by two single-speed motor-generators) with variable-speed (asynchronous) technology (represented by two variable-speed motor-generators).
Variable-speed motor-generators allow operation of the pump-turbine unit over a wider range of head and flow, making them economically advantageous for a pumped-storage facility. VEAG conducted considerable investigations to determine how many variable-speed units to install. The company eventually decided to install two variable-speed units and two conventional units.
The decision was based on a variety of reasons. First, calculations indicated the company need about 200 mw of controlled power in pumping operation, which was within the control range of two variable-speed units. Second, asynchronous machines are not able to restart the system during a power outage – they need support from the grid to begin operating. Consequently, the conventional units would be required in the event of a power outage. Third, because nobody in Europe had any experiences in operating large variable-speed units, VEAG considered installing only variable speed as too risky.
VEAG received the building permit for Goldisthal in 1996. Construction began in September 1997, with work on the access tunnel to the underground powerhouse and transformer caverns, as well as such surface infrastructure work as providing site access.
Main features of the project are:
- Upper reservoir with a useable capacity of 12 million cubic meters. This is the largest artificial upper reservoir in Germany, covering 55 hectares under full storage conditions. The rockfill dam impounding this reservoir is 3,370 meters long;
- Two 6.2-meter-diameter headrace tunnels with steel-armored lining, with a total length of about 870 meters;
- Underground powerhouse cavern containing four 265-mw vertical Francis pump-turbines, four motor-generators, and auxiliary systems. This cavern is accessed via a 1-kilometer-long tunnel from the operation building complex;
- Underground transformer cavern containing four unit transformers, the 10-kilovolt (kv) transformers of the internal electrical supply, switchgear, and starting frequency converters;
- Two 8.2-meter-diameter tailrace tunnels with concrete-armored lining, each 275 meters long;
- Lower reservoir with a capacity of 18.9 million cubic meters, impounded by a rockfill dam; and
- Operational buildings with control room and staff facilities, in a side valley. All operational processes of Goldisthal are supervised from this location with the help of an S7 programmable logic controller provided by Voith Siemens Hydro Power Generation of Germany. This also is the location of the central control of all hydro plants belonging to Vattenfall Europe.
A consortium of VA Tech Escher Wyss of Germany, Voith Siemens, and CBE Blansko Engineering a.s. of the Czech Republic supplied the four draft tube flap gates, the pump-turbines, and compressed air systems. Suppliers for the motor-generators and converter were a consortium of Alstom Energietechnik of Germany, VA Tech Elin (now Andritz VA Tech Hydro of Austria), and VEM Sachsenwerk Dresden of Germany. Andritz VA Tech Hydro designed, manufactured, erected, and commissioned all four motor-generators.
Choosing variable-speed machines
The most important innovation at the Goldisthal project is the first-ever application of variable-speed motor-generators of this size in a hydro plant in Europe. In essence, turbines have one optimum operating point in terms of head, flow, unit size, and speed. But when these units are coupled with a variable-speed motor-generator, operating speed can be varied over a certain range of the nominal synchronous speed of the turbine-generating unit. As head and flow vary, the unit is able to increase or decrease its speed to operate closer to peak efficiency for this unique set of conditions.
The difference between synchronous and asynchronous machines is the rotor. While classical synchronous generators have salient poles, variable-speed generators have a three-phase winding on the rotor. And while the synchronous rotor is energized by a direct current (DC) to create a rotating magnetic field, the asynchronous rotor is energized by a low-frequency alternating current (AC). A direct frequency converter in the rotor circuit is used to control the frequency. If the frequency is changed, so too is the speed of the unit.
The rotor can be retarded or accelerated opposite the stator field, from 90 to 104 percent of the synchronous speed. The variable frequency of the asynchronous generators at Goldisthal ranges from 5 Hertz (Hz) opposite the stator field of 333 revolutions per minute (rpm) (which provides 300 rpm) to 0.01 Hz (which is nearly the rated speed of the unit) to 2 Hz additional to the stator field (which provides 340 rpm).
Asynchronous motor-generators provide several advantages, including:
- More flexibility in their operation;
- Higher efficiency over a wide range of operations at partial load conditions;
- A wide range of controllable and optimized power consumption in pump operation;
- Additional and faster features for grid control, such as fast power outlet regulation;
- Better use of the reservoir because higher water level variations can be allowed; and
- Better contribution to grid stability because of the high moment of inertia of the rotating masses.
Asynchronous machines make it possible to regulate power not only in turbine mode but also in pumping mode. The range of control at Goldisthal amounts to 190 mw to 290 mw.
The powerplant at Goldisthal is arranged to be split in half, with each side being a mirror image of the other. Each half of the plant contains one synchronous and one asynchronous machine working together at one headrace tunnel. This allows operators to take half of the plant off line at any one time for maintenance while the other half continues operating.
Operations to date
The Goldisthal plant was commissioned in October 2004. Since that time, Vattenfall Europe’s expectations have been completely fulfilled with regard to operations of the facility. The variable-speed machines operate an average of 19 hours a day in both primary and secondary regulation. When operating under conditions of partial load, these units have an efficiency advantage of about 10 percent when compared with the single-speed units.
An automatic controller on the two variable-speed machines constantly calculates and adjusts the units for optimal production, based on the momentary head and the power output required.
The asynchronous machines can be started more quickly than the synchronous units because no fixed rotation speed is necessary for synchronization of the variable-speed units. Starting from 95 percent of full synchronous speed, the frequency converter regulates its parameters to the current speed and releases the unit to synchronization.
Vattenfall Europe expects overall maintenance expenditures to be lower for variable-speed units because of the smaller starting and brake load operation, which is helped by the starting frequency converter. However, periods between maintenance of the variable-speed machines are expected to be somewhat shorter than that of the conventional machines. These units are inspected every four weeks, compared with every six weeks for the synchronous units. In addition, inspection of the variable-speed units requires ten hours, compared with eight hours for the synchronous machines.
The greater frequency of inspection and greater time required results to a large extent from the larger quantity of auxiliary systems associated with the asynchronous generator. For example, because the rotor of the asynchronous machine needs more power and the voltage and current are much higher than in a synchronous rotor, the slip ring system is much bigger.
With regard to ancillary services, the asynchronous units at Goldisthal have been quite valuable. Because of the large capacity of the units, a large regulation range is available. This is used daily for the grid frequency control. The asynchronous machines can be regulated from 40 mw up to 265 mw, while the synchronous machines can only be regulated from 100 mw to 265 mw Thus, the asynchronous machines provide 60 mw more for regulation. This allows Vattenfall Europe to take advantage of the lower basic power output of 40 mw, saving water to be used for later generation.
In addition, the asynchronous machines have the ability to respond very quickly. If fast power is needed in the grid, the asynchronous machines can retard their speed and supply additional braking energy to the grid (for a few seconds). In early November 2006, parts of Europe experienced a large blackout. In the eastern part of Germany, frequency on the grid was 50.6 Hz when it normally is 50 Hz. Vattenfall Europe used the Goldisthal units to take energy out of the grid, and the asynchronous units were used for regulation in pumping operation.
On average, about 70 percent of the work ability of the power plant is used each day. That results in daily production during turbine operation of 5,500 to 7,500 megawatt-hours. s
Mr. Beyer may be reached at Vattenfall Europe Generation, Operations-Hydropower Plants, Am Rotseifenbach, Goldisthal 98746 Germany; (49) 36781-332322; E-mail: thomas1. email@example.com.
Thomas Beyer is the head of the Goldisthal pumped-storage power plant, owned by Vattenfall Europe Generation AG & Co. KG.
Pumped-Storage Construction in Europe
Construction of pumped-storage hydroelectric projects is experiencing a significant upswing in central Europe. The following examples provide a snapshot of the development that is occurring.
Avce in Slovenia
This 178-mw project, being developed on the Soca River in Kanal, Slovenia, is the country’s first pumped-storage project. Slovenian utility Slovenske Elektrarne and its subsidiary Soske Elektrarne Nova Gorica d.o.o. (SENG) are developers. Slovenia will use its nighttime electricity surplus, that otherwise is exported at low prices, to pump water into an upper reservoir for release through turbines during the day for generation when electricity prices are high.
The European Investment Bank provided a 43 million euro (US$51 million) loan for the project. The Slovenia consortium of Primorje d.d Ajdovscina and SCT Ljubljana is performing civil construction, while a consortium of Melco, Rudis, and Sumitomo is supplying the pump-turbine and variable-speed motor-generator equipment.
Avce is expected to be on line by November 2008.
Kopswerk 2 in Austria
This 540-mw project, under development by Vorarlberger Illwerke AG and EnBW AG, is on the Ill River in Vorarlberg State, at the extreme western tip of Austria. The project will adjoin the existing conventional 247-mw Kopswerk 1 hydro project; the existing Koppsee and Rifabecken reservoirs will serve as Kopswerk 2’s upper and lower reservoirs. All major project works are underground. The plant will feature three 150-mw pump-turbines operating under a head of 800 meters.
Voith Siemens Hydro Power Generation is supplying the pump-turbines and transducers, six globe valves, and two butterfly valves. Andritz VA Tech Hydro is providing the generators, piping, penstock, surge tank lining, and distribution piping. ABB AG is providing hardware and software for instrumentation, controls, and automation. Other companies working on the project include: consortium Arge Drukstollen Kops II (Swietelsky Bau Tunnelbau GesmbH, Torno SA, and Torno S.p.A.); consortium Arge Kavernenkrafthaus Kops II (Jager Bau GmbH, Beton, und Monierbau, Zublin, and Alpine Mayreder); Hans Kunz GmbH, VAM GmbH, COWA Remscheid GmbH, and Adams Schweig AG.
Kopswerk 2 is to begin operating in March 2008.
Limberg 2 in Austria
Verbund Austrian Hydro Power is developing this 480-mw project, part of the Glockner Kaprun scheme, near the 240-mw Kaprun project. Two existing storage reservoirs, Mooserboden and Wasserfallboden, will serve as the upper and lower reservoirs; the powerhouse will be underground.
Jakko Poyry Group Oyj is responsible for detailed design and site supervision of construction. Voith Siemens Hydro Power Generation is providing two 240-mw reversible vertical Francis pump-turbines and governors. Andritz VA Tech Hydro is designing, manufacturing, and installing the excitation systems and two motor-generators, which will feature variable-speed capability. Siemens AG Osterreich is supplying the 420-kilovolt gas-insulated switchgear.
Limberg 2’s first unit is to be commissioned in 2011, with all units on line the following year.
Nestil in Switzerland
This 141-mw facility, being built by Kraftwerke Linth-Limmern AG in Linth Valley, Switzerland, is designed to boost output from the existing 386.4-mw Tierfehd facility. The scheme will use the existing Limmernboden Reservoir as the upstream reservoir and the equalizing basin of the Tierfehd project as the downstream reservoir. When this pumped-storage project is completed in 2009, output at Tierfehd will be nearly 700 million kilowatt-hours (kwh), compared with current output of 400 million kwh.
Andritz VA Tech Hydro is supplying, installing, and commissioning electro-mechanical equipment, including a four-phase reversible pump-turbine, motor-generator, and auxiliary systems. DSD-Noell GmbH is supplying a penstock extension. This work includes design, fabrication, and installation of an underground high-pressure pump steel liner, bifurcation, and suction liner to link the lower lake with the high-pressure radial pump.
By Elizabeth A. Ingram