Patent application title: ELECTRIC UNIT FOR A PUMP-STORAGE POWER PLANT
Carl-Ernst Stephan (Lupfig, CH)
Claes Hillberg (Moehlin, CH)
Hanspeter Erb (Spiez, CH)
Peter Steimer (Ehrendingen, CH)
Peter Steimer (Ehrendingen, CH)
IPC8 Class: AF03B1306FI
Class name: Prime-mover dynamo plants electric control fluid-current motors
Publication date: 2015-10-15
Patent application number: 20150292469
A pumped-storage power plant is disclosed, such as an electric unit
having a converter and a rotating electric synchronous machine. The
converter is designed as a modular multilevel converter and the machine
is directly connectable to the converter, wherein the converter has an
1. An electric unit for a pumped-storage power plant, connectable to an
electric grid (2) and comprising: a frequency converter; and a rotating
electric synchronous machine, wherein the synchronous machine is
configured for connection to a water turbine and a water pump; wherein
the frequency converter is configured as a modular multilevel converter,
and the synchronous machine is directly connected to the frequency
converter; wherein the frequency converter has is configured to provide
an adjustable output voltage, to the synchronous machine which is
identical to an input voltage of the machine.
2. The electric unit as claimed in claim 1, in combination with a pumped-storage power plant, wherein at least the electric synchronous machine is provided in a cavern, in a shaft or in a closed area beneath a tailwater basin of the pumped-storage power plant.
3. The electric unit as claimed in claim 1, wherein the frequency converter comprises: plural series-connected unit cells for adjusting the output voltage by scaling, wherein the unit cells are separately controllable.
4. The electric unit as claimed in claim 3, comprising: a transformer, wherein the individual unit cells are configured to be controllably switched, an edge steepness and amplitude of a pulse generated by switching of the individual unit cells between an output of the frequency converter and the synchronous machine and the transformer being limitable to such an extent that, even without additional filter elements, no increased demands on isolation of the synchronous machine and the transformer exist.
5. The electric unit as claimed in claim 1, wherein the frequency converter is configured such that plural a voltage steps will modify an output voltage of the frequency converter.
6. The electric unit as claimed in claim 1, wherein the output voltage will have limited harmonic distortion such that grid connection conditions will be fulfilled without additional filtering and no differences emerge for a configuration of the synchronous machine with respect to harmonics when compared to a direct connection to the grid.
7. The electric unit as claimed in claim 1, comprising: a generator transformer which is connected on a grid-side of the electric unit to the frequency converter.
8. The electric unit as claimed in claim 2, wherein the frequency converter comprises: plural series-connected unit cells for adjusting the output voltage by scaling, wherein the unit cells are separately controllable.
9. The electric unit as claimed in claim 8, comprising: a transformer, wherein the individual unit cells are configured to be controllably switched, an edge steepness and amplitude of a pulse generated by switching of the individual unit cells between an output of the frequency converter and the synchronous machine and the transformer being limitable to such an extent that, even without additional filter elements, no increased demands on isolation of the synchronous machine and the transformer exist.
10. The electric unit as claimed in claim 9, wherein the frequency converter is configured such that plural voltage steps will modify an output voltage of the frequency converter.
11. The electric unit as claimed in claim 10, wherein the output voltage will have limited harmonic distortion such that grid connection conditions will be fulfilled without additional filtering and no differences emerge for a configuration of the synchronous machine with respect to harmonics when compared to a direct connection to the grid.
12. The electric unit as claimed in claim 8, comprising: a generator transformer which is connected on a grid-side of the electric unit to the frequency converter.
 The invention relates to a pumped-storage power plant, in particular to an electric unit therefor, comprising a converter and a rotating electric synchronous machine, wherein the machine is provided in a cavern.
 Regenerative energy sources such as, for example, wind energy and solar energy provide a continuously increasing proportion of the electricity demand. These energy sources do have discontinuous operating times, however. Therefore, a direct and permanent supply of electricity to consumers from these energy sources cannot be ensured. For this, energy stores need to be used which enable rapid changes between a surplus of electricity and a deficit of electricity and whose power and energy flow direction can be changed quickly and continuously.
 In this case, there are different system available as energy stores which are in each case particularly suitable for specific quantities of energy and application cases. For low quantities of energy up to approximately 20 MWh, kinetic stores (for example flywheels), electrochemical stores (batteries, redox flow cells) or electromagnetic stores (capacitors, supercapacitors, superconducting coils) are preferably used, depending on the application. For medium quantities of energy of up to a few 100 MWh, in principle thermodynamic stores (compressed-air stores, electrothermic stores) are particularly well suited. For large quantities of energy of typically over 100 MWh and usually over 1 GWh, pumped stores are used.
 Pumped stores or pumped-storage power plants are of particular interest owing to the large amount of energy that can be stored. In this case, with surplus electricity water is pumped from a first natural storage basin or storage basin set up artificially for this purpose into a second storage basin positioned higher. In the process, the electrical energy is converted into potential energy. In order to recover electricity, water is directed from the higher storage basin via a turbine back into the lower storage basin. For this system, minimization of the losses in the conversion processes is particularly important.
 Modern pumped stores have variable-speed drives. By decoupling the speed of the machines from a grid frequency, rotational speeds of the pumps and turbines can be set such that they are operated close to optimum efficiency. In addition, the variation in the speed during pump operation makes it possible to freely adjust the power consumption. In particular, systems with a variable speed can be connected to or synchronized with the grid quickly from a standstill.
 Pumped stores in accordance with the prior art have double-fed asynchronous machines and power electronics frequency converters, whereby speed regulation of a pump and a turbine is possible. In this case, a stator of the double-fed asynchronous machine is connected directly to the electric grid with a grid frequency. A rotor of the double-fed asynchronous machine is connected to the grid via a frequency transformer and can therefore have a variable frequency. Thus, firstly a pump power is regulated and secondly the efficiency of the system can be increased, if required.
 In the case of pumped stores, provision is often made for the machine equipment such as, for example, turbines, pumps and motor generators, to be accommodated in a cavern introduced into rock, in a cavity or in a closed area beneath the second storage basin, for example. Another form of pumped store provides pump, turbine and motor generator at the lower end of a shaft, wherein further power plant components are provided above ground in a building or likewise in the shaft, for example.
 This way of accommodating the machine equipment is a consequence of the circumstance whereby the turbines need to be positioned sufficiently below the lower storage basin in order to avoid cavitation. For space and geometry reasons, this is usually only possible by virtue of a powerhouse for accommodating the machine equipment being built underground. Furthermore, an underground design provides protection against environmental influences such as descents of avalanches, for example. Furthermore, for environmental protection or landscape protection reasons, an underground design is likewise often used.
 The converters according to the prior art used in this connection typically have an output voltage of 3 to 6 kV. In contrast, in the case of the electric machine, a voltage of 10 to 21 kV is usually used. Therefore, it is necessary to provide a power transformer between an output of the converter and terminals of the machine, which power transformer bypasses the respective voltage differences. The high currents in this case require that the transformer is provided in the direct vicinity of the machine, that is to say within the cavern.
 An essential disadvantage of this arrangement with a double-fed asynchronous machine is an increased requirement on space within the cavern, as a result of which costs for erecting the pump-storage power plant and digging the cavern are significantly increased. Moreover, the transformer has losses of the order of 1-2% of the transferred active power. Overall, the use of a transformer is disadvantageous owing to the high costs of the transformer. Furthermore, a transformer has a considerable risk of explosion owing to transformer oils and therefore represents a considerable safety risk.
 A further disadvantage of converters according to the prior art is that the voltage difference is overcome in one or two stages or steps. As a result of this, firstly, the high and steep voltage steps require smoothing or filtering in order not to overload an electrical isolation of the transformers which are directly coupled to the frequency converter, and the machine, and to avoid damage to the isolation. A smoothing or filtering arrangement is expensive, causes further losses of the active power and requires lots of space. Secondly, the transferred power has a large harmonic component which harmonics cause additional losses in the machine and in the connected transformers.
 Against this background, the present invention is based on the object of simplifying the erection, operation and maintenance of a pumped-storage power plant and increasing the efficiency.
DESCRIPTION OF THE INVENTION
 This object is achieved by an electric unit for a pumped-storage power plant as claimed in claim 1. Further advantageous configurations result from the dependent claims, wherein back-references in the claims do not exclude any other expedient combinations of claims.
 The invention relates to an electric unit for a pumped-storage power plant, wherein said electric unit is connectable to an electric grid. The electric unit in this case comprises at least one frequency converter and a rotating electric synchronous machine, which acts as motor or generator depending on a type of operation of the machine. The machine is mechanically connectable, for example, to a water turbine and a water pump or a reversible pump turbine. The converter is designed in this case as a modular multilevel converter MMC and is directly connected to the machine, that is to say without an intermediately connected transformer, wherein the converter has an adjustable voltage.
 In an advantageous configuration of the invention, at least the electric machine is provided in a cavern, in a shaft or in a closed area beneath the tailwater level.
 In another advantageous embodiment, the converter has a multiplicity of series-connected unit cells for scaling an output voltage. The unit cells each have, for example, a capacitor or an inductor. Furthermore, the unit cells are separately controllable, for example for adapting the output voltage to the electric machine.
 In a particularly advantageous embodiment, provision is made that edge steepness and amplitude of pulses between an output of the converter and the machine, for example terminals of the machine, is limitable to relieve the isolation.
 In another advantageous embodiment, a multiplicity of voltage steps, for example small steps, can be used to modify the output voltage.
 Another embodiment provides that the output voltage has low harmonic distortion, such that the grid connection conditions are fulfilled without additional filtering and no differences emerge in the case of configuration of the machine with respect to harmonics compared to a direct connection to the grid. Such grid connection conditions are defined, for example, in a grid code.
 In one embodiment of the invention, provision is made that the converter is connected on the grid-side to a generator transformer.
 Accordingly, the invention simplifies the erection of a pumped-storage power plant, in particular in the case of an underground construction, for example in a rock cavern, by sparing a transformer between machine and converter. Furthermore, active power losses of the omitted transformer are saved and the electric unit has a higher efficiency.
BRIEF DESCRIPTION OF THE DRAWINGS
 Further features, details and advantages of the invention result from the wording of the claims and from the description of exemplary embodiments on the basis of the figure.
 The invention will be explained in more detail on the basis of the following text with reference to preferred exemplary embodiments using the figure, in which
 FIG. 1 shows a schematic illustration of an electric unit comprising a converter, an electric synchronous machine and a generator transformer.
 The reference symbols and the significance thereof are summarized in the list of reference symbols. In general, the same reference symbols denote the same parts.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
 FIG. 1 shows a schematic illustration of an electric unit 1 connected to an electric grid 2. The electric unit 1 in this case comprises a modular multilevel converter 3 and a rotating electric synchronous machine 4. The machine 4 is in this case accommodated in a cavern. In addition a generator transformer 5 is provided in this embodiment to connect the electric unit to the electric grid 2.
 A frequency conversion is produced by means of a combination of a rectifier and an inverter, which are connected to one another via a concentrated or distributed voltage DC link or current DC link. The DC link in this case furthermore has units for energy storage, for example capacitors in the case of a voltage DC link and inductances in the case of a current DC link.
 The operation of the machine at a freely selectable speed has considerable advantages. In particular in the embodiment with a frequency converter and a synchronous machine, an established, reliable and low-maintenance generator technology can be used. Furthermore, there is the possibility of operating a pump and a turbine independently of one another in the optimum speed range of said pump and turbine. By virtue of the use of the synchronous machine 4, high speeds can be achieved for high drops, for example, in particular even at high powers. Furthermore, the speed range which can be achieved during operation continuously ranges from zero to the maximum speed and is only restricted by the operational limits of the pump and the turbine. The pump and the turbine can in principle be combined in one unit, for example a pump turbine. In particular, there is the possibility of retrofitting older systems for variable frequency operation without replacing the existing generator. A further advantage consists in very quick grid coupling and the possibility of generating positive and negative reactive power in the converter 3, in order that the generator can be operated exclusively with active power, as a result of which said generator has a more compact design. Furthermore, owing to the use of the converter 3, it is possible to switch over quickly from pump to turbine operation, for example.
LIST OF REFERENCE SYMBOLS
 1 Electric unit
 2 Electric grid
 3 Converter
 4 Electric synchronous machine
 5 Generator transformer
Patent applications by Hanspeter Erb, Spiez CH
Patent applications by Peter Steimer, Ehrendingen CH
Patent applications in class Fluid-current motors
Patent applications in all subclasses Fluid-current motors