Abstract:
A pump-storage power plant, such as an electric unit, for this purpose can include a rotating electric synchronous machine and a frequency converter as well as a method for operating the electric unit. An exemplary method provides that the frequency converter is used to start the turbine and an output of the electric machine is fed directly into the current network after start-up, for example. Another exemplary method provides that the frequency converter is used to start the pump and the pump is started directly from rest and under load of a flooded pump or a water column, for example. Another exemplary method provides that the electric machine is synchronised with a frequency of the current network and is operated synchronously with the current network independent of the operating state of the pump or the turbine and supplies active power and reactive power.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to a pumped-storage power plant, in particular to an electric unit therefor, comprising a frequency converter and a rotating electric synchronous machine and method for operating the electric unit. 
       PRIOR ART 
       [0002]    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. 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. 
         [0003]    In this case, there are different systems 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. 
         [0004]    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. 
         [0005]    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. 
         [0006]    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. Thus, firstly a pump power is regulated and secondly the efficiency of the system can be increased, if required. 
         [0007]    In one embodiment for speed regulation of the pump or turbine, a synchronous machine whose stator is fed by means of a three-phase current with an adjustable frequency is used. The frequency conversion in this case is generated with the aid of a combination of a rectifier and an inverter, which are connected to one another via a voltage or current DC link. 
         [0008]    In order to operate a pumped-storage power plant, complex methods are required, for example, for runup in turbine operation, for runup in pump operation and for switching over from turbine operation to pump operation or from pump operation to turbine operation. 
         [0009]    For runup in turbine operation, first water is passed slowly from the storage basin to the turbine in order to run up said turbine, for example. Only when stator voltages of the machine are synchronous with the electric grid and have a correct phase angle can the machine feed power to the electric grid. 
         [0010]    For runup in pump operation, the pump is first drained, for example. For this purpose, additional auxiliary apparatuses are often used. This is necessary since in the prior art there is insufficient torque for running up the pump under load. Furthermore, it is necessary in the case of synchronous generators with a fixed speed, for example, to start the pump additionally with an auxiliary drive such as an auxiliary turbine or a power electronics starter. Only when the pump is in operation is water let out of the storage basin into the pump and a shutoff element opened. This furthermore represents a considerable load on the pump since, when the water is let in, a strong pulse is transmitted to the pump, as a result of which wear on the pump parts is increased. 
         [0011]    In order to switch over the types of operations such as from turbine operation to pump operation or from pump operation to turbine operation, pole switches are used in order to switch over the orientation of the rotating field in the electric machine. Said pole switches are involved and cost-intensive in terms of manufacture and maintenance. 
         [0012]    The operation of a pumped-storage power plant is therefore very complex and time-consuming. In particular for pumped stores, it is important to react quickly to changes between a surplus of electricity and a deficit of electricity and to change the operating mode. 
         [0013]    Against this background, the present invention is based on the object of simplifying the operation of a pumped-storage power plant and accelerating operation changes. 
       DESCRIPTION OF THE INVENTION 
       [0014]    This object is achieved by a method for runup in turbine operation as claimed in claim  1 , a method for runup in pump operation as claimed in claim  2 , a method for operating a pumped-storage power plant as claimed in claim  3  and an electric unit for a pumped-storage power plant as claimed in claim  4 . Further advantageous configurations result from the dependent claims, wherein the back-reference in the claims does not rule out any other expedient combinations of claims. 
         [0015]    In this case, the invention provides a method for runup in turbine operation of an electric unit for a pumped-storage power plant. The pumped-storage power plant comprises a rotating electric synchronous machine and a frequency converter, wherein the machine is connectable to a turbine and a pump or a combined pump turbine. Furthermore, the machine is connectable to an electric grid via the frequency converter. In this case, the method provides for the frequency converter to be used for runup of the turbine and for power from the electric machine to be fed into the electric grid directly after runup, for example. 
         [0016]    Furthermore, a method is provided for runup in pump operation of an electric unit for a pumped-storage power plant. The pumped-storage power plant comprises a rotating electric synchronous machine and a frequency converter, wherein the machine is connectable to a turbine and a pump or a combined pump turbine. Furthermore, the machine is connectable to an electric grid via the frequency converter. In this case, the method provides for the frequency converter to be used for runup of the pump and for the pump to be run up directly from standstill and under load, for example, of a fluted pump or a water column. 
         [0017]    Furthermore, a method for operating an electric unit for a pumped-storage power plant is provided. The pumped-storage power plant comprises a rotating electric synchronous machine and a frequency converter, wherein the machine is connectable to a turbine and a pump or a combined pump turbine. Furthermore, the machine is connectable to an electric grid via the frequency converter. The method provides for the electric machine to be operated synchronously with the electric grid independently of the operating state of the pump or turbine and to provide active power and reactive power. 
         [0018]    The invention furthermore relates to an electric unit for a pumped-storage power plant. The pumped-storage power plant comprises a rotating electric synchronous machine and a frequency converter, wherein the machine is connectable to a turbine and a pump or a combined pump turbine. Furthermore, the machine is connectable to an electric grid via the frequency converter. 
         [0019]    In an advantageous configuration of the electric unit, provision is made for the frequency converter to comprise at least two electrically connectable elements, wherein in each case one element is usable as rectifier and one element is usable as inverter, depending on the operation of the machine, and the frequency converter is in the form of a self-commutated converter with a voltage DC link or with a current DC link. Depending on the operation of the machine, in each case one element is usable as rectifier and one element is usable as inverter, wherein the machine-side element is also referred to as inverter unit INU, and the grid-side element is also referred to as active rectifier unit ARU. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    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. 
           [0021]    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 
           [0022]      FIG. 1  shows a schematic illustration of an electric unit comprising an electric synchronous machine and a frequency converter. 
       
    
    
       [0023]    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 
       [0024]      FIG. 1  shows a schematic illustration of an electric unit  1  comprising a rotating electric synchronous machine  2  and a frequency converter  3 . The machine  2  is in this case accommodated in a cavern, for example owing to local conditions or for protection purposes. The machine furthermore has a stator, which is fed by means of a three-phase current with an adjustable frequency. 
         [0025]    The operation of the machine  2  with the frequency converter  3  in pumped-storage power plants enables an improvement to be made in the dynamic response in order that start, stop and switchover times can be reduced. 
         [0026]    In this case, the invention provides a method for runup in turbine operation of the electric unit  1  for a pumped-storage power plant. The method provides for the frequency converter  3  to be used for running up the turbine and for power to be fed from the electric machine  2  into the electric grid  6  directly after runup, for example. 
         [0027]    In this case, no wait time until synchronous operation is required since the voltage on the grid side via the frequency converter  3  is always synchronous irrespective of the frequency of the voltage generated on the machine side. Therefore, power can be fed into the electric grid  6  immediately. During starting, there is no interruption to the synchronization of the machine between the start and operation as in solutions known from the prior art. 
         [0028]    Furthermore, a method for runup in pump operation of the electric unit  1  for a pumped-storage power plant is provided. The method provides for the frequency converter  3  to be used for running up the pump  5  and for the pump  5  to be run up directly from standstill and under load of, for example, a fluted pump or a water column. 
         [0029]    The frequency converter  3  can supply sufficient torque to the pump  5  in order to run up directly from standstill without any previous draining of the pump  5 . The pump  5  can be operated immediately without any delay and runup without any significant complexity is possible. For example, the power drawn from the electric grid  6  can increase in ramped fashion and interruption to the supply for synchronization is not necessary. 
         [0030]    Furthermore, a method for operating the electric unit  1  for a pumped-storage power plant is provided. The method provides for the electric machine  2  to be synchronized with a frequency of the electric grid  6  and to be operated synchronously with the electric grid  6  independently of the operating state of the pump  5  or the turbine  4  and to provide active power and reactive power. 
         [0031]    The method for the runup and switchover of operation are much quicker than in the prior art owing to the use of the frequency converter  3 . Furthermore, in the case of the electric unit  1 , no additional transformer is provided between the frequency converter  3  and the machine  2 , as a result of which the methods can be accelerated additionally in comparison with the prior art. 
         [0032]    For example, in the case of a combined pump  5  and turbine  4 , such as a pump turbine, the frequency converter  3  is used for switching over the direction of rotation of a rotating field of the machine  2 . Thus, a polarity reversal switch from the prior art is no longer required. The frequency converter  3  ensures that the power plant always remains at the electric grid  6  and synchronized therewith during the switchover operation. It is therefore possible to control the switchover time and the power gradient. In this case, the machine  2  can be fed over the total speed range in such a way that the speed reversal is supported via the torque of the machine  2 . Furthermore, switchover between pump operation and turbine operation can take place very quickly even when the water column needs to come to a standstill in the case of a pump turbine since gravitation additionally brakes the water column. The frequency converter  3  and therefore also the machine  2  do not need to be disconnected from the grid for this operation. For switchover from turbine operation to pump operation, the water column needs to be braked mechanically. In this case, the frequency converter  3  and the machine  2  remain connected to the electric grid  6 . 
         [0033]    Furthermore, magnetization of a generator transformer for connection to the electric grid  6  can take place via the frequency converter  3  for impact-free connection. 
         [0034]    The frequency converter  3  comprises, for example, two elements which are usable as inverter or rectifier depending on the mode of operation of the machine, for example in motor operation or generator operation. Speed regulation is enabled by virtue of the fact that the machine  2  has a stator, which is fed by means of a three-phase current with adjustable frequency. The machine-side element or inverter unit INU of the frequency converter  3  is operated as inverter in the pump mode and as rectifier in the turbine mode. The grid-side element or active rectifier unit ARU of the frequency converter  3  is operated as rectifier in the pump mode and as inverter in the turbine mode. 
         [0035]    The 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 DC link is provided between the elements and can be designed in this case in concentrated or distributed form. 
         [0036]    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  5  and a turbine  4  independently of one another in the optimum speed range of said pump and turbine. By virtue of the use of the synchronous machine  2 , 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  5  and the turbine  4 . 
         [0037]    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 frequency 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. 
       LIST OF REFERENCE SYMBOLS 
       [0000]    
       
           1  Electric unit 
           2  Machine 
           3  Frequency converter 
           4  Turbine 
           5  Pump 
           6  Electric grid