ENERGY STORAGE SYSTEM AND METHOD FOR CONTROLLING AN ENERGY STORAGE SYSTEM

Abstract

An energy storage system is disclosed. The energy storage system includes at least one energy store, a power convertor for converting between a DC voltage present at the at least one energy store and an AC voltage, a transformer for transforming between the AC voltage and a line voltage of an energy supply network, and a control device for controlling the energy storage system. The transformer is switchable for setting a transformation ratio for converting between the AC voltage and the line voltage, wherein the control device is configured to set the transformation ratio of the transformer depending on the DC voltage present at the at least one energy store.

Claims

1. An energy storage system, comprising at least one energy store; a power convertor for converting between a DC voltage present at the at least one energy store and an AC voltage; a transformer for transforming between the AC voltage and a line voltage of an energy supply network; and a control device for controlling the energy storage system, wherein the transformer is switchable for setting a transformation ratio for converting between the AC voltage and the line voltage, wherein the control device is configured to set the transformation ratio of the transformer depending on the DC voltage present at the at least one energy store.

2. The energy storage system as claimed in claim 1, wherein the transformer is switchable in a stepwise manner for setting the transformation ratio.

3. The energy storage system as claimed in claim 1, wherein the transformer has a secondary winding, at which the AC voltage is present, a primary winding, at which the line voltage is present, and a switching device with a plurality of secondary taps at the secondary winding and/or with a plurality of primary taps at the primary winding.

4. The energy storage system as claimed in claim 3, the switching device is switchable for tapping the secondary winding via one of the secondary taps and/or the primary winding via one of the primary taps for setting the transformation ratio.

5. The energy storage system as claimed in claim 1, wherein the control device is configured to control the power convertor for converting between the DC voltage and the AC voltage, wherein the root-mean-square value of the AC voltage is dependent on the value of the DC voltage.

6. The energy storage system as claimed in claim 5, wherein the control device is configured to control the power convertor for setting the root-mean-square value of the AC voltage depending on the value of the DC voltage on the basis of a step function.

7. The energy storage system as claimed in claim 6, wherein different value ranges of the DC voltage are assigned to steps of the AC voltage.

8. The energy storage system as claimed in claim 5, wherein the power convertor is configured to set the root-mean-square value of the AC voltage by means of pulse width modulation.

9. The energy storage system as claimed in claim 8, wherein the control device predefines a modulation factor of the pulse width modulation on the basis of the value of the DC voltage.

10. The energy storage system as claimed in claim 5, wherein the control device is configured to set the transformation ratio of the transformer on the basis of a set step of the AC voltage.

11. The energy storage system as claimed in claim 1, wherein the energy storage system is configured as a battery storage power plant comprising at least one energy store in the form of a battery device.

12. The energy storage system as claimed in claim 1, wherein the control device is configured to control the power convertor for setting the maximum performance depending on the value of the AC voltage.

13. A method for controlling an energy storage system, wherein a power convertor converts between a DC voltage present at least one energy store and an AC voltage, and a transformer transforms between the AC voltage and a line voltage of an energy supply network, wherein the transformer is switched for setting a transformation ratio for converting between the AC voltage and the line voltage depending on the DC voltage present at the at least one energy store.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The concept underlying the proposed solution shall be explained in greater detail below on the basis of the exemplary embodiments illustrated in the figures.

[0029] FIG. 1 shows a schematic view of an energy storage system in the form of a

[0030] FIG. 2 shows a schematic view a branch of one exemplary embodiment of an energy storage system;

[0031] FIG. 3 shows a schematic view of a transformer.

[0032] FIG. 4 shows a view of a winding of the transformer with a switching device in the form of a tap switch for switching the transformation ratio of the transformer.

[0033] FIG. 5A shows a graphical view of the DC voltage available at an energy store in the form of a battery depending on the state of charge.

[0034] FIG. 5B shows a graphical view of a stepwise conversion of the DC voltage by means of a power convertor into an AC voltage with parallel stepping of a transformer.

[0035] FIG. 5C shows a graphical view of a resulting line voltage.

[0036] FIG. 6 shows a graphical view of the conversion by means of pulse width modulation.

DETAILED DESCRIPTION

[0037] FIG. 1 shows, in a schematic view, an energy storage system 1 in the form of a battery power plant having a plurality of energy stores 2 in the form of batteries. The energy storage system 1 is coupled to an energy supply network 6 and serves to temporarily store energy from renewable energy sources, for example, in order, even from a state of the energy supply network 6, to feed energy into the energy supply network 6 or to draw energy from the energy supply network 6 for temporary storage.

[0038] An energy store 2 in the form of a battery provides a DC voltage that can vary depending on the state of charge of the battery 2. Conversion is effected between the DC voltage of the energy store 2 in the form of the battery and the line voltage present on the part of the energy supply network 6 by means of power convertors 3 and a transformer 5, wherein in the exemplary embodiment in accordance with FIG. 1 separate power convertors 3 are assigned to different energy stores 2 and the AC voltage of the energy stores 2 that is obtained in this way is transformed to the line voltage of the energy supply network 6 by means of a common transformer 5.

[0039] The transfer chain between the energy supply network 6 and energy stores 2 is configured for feeding energy from the energy stores 2 into the energy supply network 6 and conversely also for feeding energy from the energy supply network 6 into the energy stores 2 in the form of the batteries. In the direction of feeding energy from the energy stores 2 into the energy supply network 6, the power convertors 3 in this case act as invertors for converting the DC voltage of each energy store 2 into an AC voltage, which is then transformed into the line voltage U.sub.Grid by means of the transformer 5. By contrast, in the direction of feeding energy from the energy supply network 6 into the energy stores 2, the power convertors 3 act as rectifiers for rectifying the AC voltage obtained after transformation at the transformer 5 into the DC voltage of the respective energy store 2.

[0040] In the case of the exemplary embodiment in accordance with FIG. 1, a generator transformer 4 serving for transformation and as galvanic isolation is additionally present in each path of an energy store 2.

[0041] In the case of the exemplary embodiment in accordance with FIG. 2 (which schematically illustrates a path assigned to an energy store 2), the conversion of the DC voltage U.sub.DC of the energy store 2 into the AC voltage U.sub.AC and the transformation of the AC voltage U.sub.AC into the line voltage U.sub.Grid of the energy supply network 6 are effected in a manner controlled by way of a control device 7. In this regard, the transformer 5 is switchable in a stepped manner for setting the transformation ratio, such that the transformer 5 does not have a constant transformation ratio, but rather is able to be switched over in a controlled manner.

[0042] Likewise, the AC voltage U.sub.AC between power convertor 3 and transformer 5 is not constant in terms of its root-mean-square value, but rather is settable in a variable manner, depending in particular on the DC voltage available at the energy store 2, said DC voltage being dependent on the state of charge of the energy store 2 in the form of the battery.

[0043] As is illustrated in FIG. 3, the transformer 5 has a secondary winding 50, at which the AC voltage U.sub.AC is present, and a primary winding 51, at which the line voltage U.sub.Grid is present, which are operatively connected to one another in a transformer-based manner via a transformer core 52 for guiding the magnetic flux. The transformation ratio of the transformer 5 results from the ratio of the numbers of turns of the windings 50, 51 in a manner known per se, wherein one or both of the windings 50, 51, as illustrated by way of example in FIG. 4, can be switched by switchover between different taps 530.

[0044] FIG. 4 illustrates by way of example a switching device 53 at the secondary winding 50, which can be used to switch over between different taps 530 of the secondary winding 50. By switching over between the taps 530, it is possible to set the effective winding length of the winding 50 and thus the effective number of turns of the winding 50 in order in this way to vary the transformation ratio of the transformer 5.

[0045] The switching device 53 has switches 531, 532, which can be used to switch over between the different taps 530. In the case of the example in accordance with FIG. 4, the switch 531 assigned to a step S4 (corresponding to a specific transformation ratio) is closed, such that in the case of the illustrated switching position of the switch 532, the winding 50 is tapped at the tap 530 assigned to the step S4. By means of the different taps 530, it is possible to switch between different steps S1 to S7 corresponding to different, discrete values of the transformation ratio in order in this way to set the transformation ratio in a stepped manner.

[0046] The switching device 53 can be configured as an on load tap changer or else as a no load tap changer for switching between the taps 530.

[0047] A switchover can additionally or alternatively also be effected at the primary winding 51.

[0048] The switchover between the taps 530 for setting a desired transformation ratio is effected depending on a DC voltage U.sub.DC available at the energy store 2, said DC voltage being dependent on the state of charge of the energy store 2. In this case, the switchover is controlled by way of the control device 7.

[0049] As is illustrated in FIG. 5A, the DC voltage U.sub.DC available at the energy store 2 in the form of the battery changes depending on the state of charge (SOC). In this regard, the DC voltage U.sub.DC is significantly lower when the battery is discharged or almost discharged (SOC close to 0%) compared with when the battery is fully charged (SOC at 100%). By way of example, the available DC voltage U.sub.DC can vary between 750 V and 900 V, but in a manner dependent on the specific design of the energy store 2.

[0050] In order to be able to utilize the capacity of the energy store 2 with high efficiency, provision can be made for setting the AC voltage U.sub.AC between power convertor 3 and transformer 5 in a variable manner, depending on the DC voltage U.sub.DC at the energy store 2, as is illustrated in FIG. 5B. In this case, the AC voltage U.sub.AC is set on the basis of a step function, wherein the AC voltage U.sub.AC is varied on the basis of predetermined, discrete steps assigned to the steps S1 to S7 of the transformation ratio of the transformer 5.

[0051] In this case, the setting of the transformation ratio at the transformer 5 and the setting of the root-mean-square value of the AC voltage U.sub.AC at the power convertor 3 are effected in a manner coordinated with one another and controlled by way of the control device 7, depending on the DC voltage U.sub.DC available at the energy store 2.

[0052] In this case, different steps of the AC voltage U.sub.AC are assigned to different value ranges of the DC voltage U.sub.DC at the energy store 2. In this regard, on the basis of the equation

[00001]$U_{\mathrm{AC},\max }=k_{M,\max }.Math.\frac{U_{D.Math.C}}{K.Math.\sqrt{2}}$

[0053] the root-mean-square value of the AC voltage U.sub.AC that can be maximally obtained from an available DC voltage U.sub.DC is calculated (K represents a safety factor); k.sub.M,max denotes the maximum modulation factor with a permissible maximal value of less than or equal to 1. Depending on this, the root-mean-square value of the AC voltage U.sub.AC is set by means of pulse width modulation in the power convertor 3 to the next lower step that is able to be set at the transformer 5:

[00002]$U_{\mathrm{AC},\mathrm{STEPm}}=U_{\mathrm{AC}}=k_M.Math.\frac{U_{D.Math.C}}{K.Math.\sqrt{2}},$

[0054] where k.sub.M represents a modulation factor (with a value of less than or equal to 1) of the pulse width modulation.

[0055] Depending on an available DC voltage U.sub.DC at the energy store 2, the pulse width modulation at the power convertor 3 is thus controlled so as to result in a root-mean-square value of the AC voltage U.sub.AC that corresponds to the respectively assigned step. Depending on the set step of the AC voltage U.sub.AC, the transformation ratio of the transformer 5 is then set so that the (AC) line voltage U.sub.Grid that is constant in terms of its root-mean-square value results after the transformation of the AC voltage U.sub.AC, as is illustrated in FIG. 5C.

[0056] In particular, an AC voltage U.sub.AC that is set on the basis of the step function illustrated as a solid line in FIG. 5B results on the output side of the power convertor 3. For a DC voltage value U.sub.DC=X, for example, by means of pulse width modulation on the output side of the power convertor 3 the AC voltage U.sub.AC is set in the manner corresponding to the step S3. By virtue of the fact that the transformation ratio at the transformer 5 is then set in such a way that precisely the line voltage U.sub.Grid is established on the output side of the transformer 5, a constant line voltage U.sub.Grid that is independent of the state of charge of the energy storage units 2 results on the part of the electrical network, as is illustrated in FIG. 5C.

[0057] This is effected in principle in this way both in the direction of feeding energy from the energy store 2 into the energy supply network 6 and conversely when feeding energy from the energy supply network 6 into the energy store 2.

[0058] One example of a pulse width modulation for converting the DC voltage U.sub.DC of the energy store 2 at the power convertor 3 to the energy supply network 6 is shown in FIG. 6. The DC voltage U.sub.DC of the energy store 2 is chopped into pulses in the context of the pulse width modulation, which pulses, in terms of their mean value, produce a sinusoidal profile of the AC voltage U.sub.AC. On the basis of the modulation factor of the pulse width modulation, in this case the root-mean-square value of the AC voltage U.sub.AC can be set in a desired manner.

[0059] In the context of the proposed procedure, the AC voltage U.sub.AC is thus set in a variable manner, depending on a DC voltage U.sub.DC available at the energy store 2. Depending on the AC voltage obtained, the transformation ratio of the transformer is set in a stepwise manner, such that transformation between the line voltage U.sub.Grid (which is constant in terms of its root-mean-square value) and the AC voltage U.sub.AC is effected in a desired manner.

[0060] This makes it possible to utilize the capacity of an energy store 2 in the form of a battery to a great extent, in particular even in the case of very low states of charge. Moreover, by means of the proposed procedure, it is possible to obtain a high efficiency during the operation of the energy storage system 1. By virtue of the fact that the performance of the power convertor is principally determined by the current-carrying capacity of the IGBTs used, the energy capacity of an energy store 2 can be utilized in a wider scope.

[0061] The concept underlying the proposed solution is not restricted to the exemplary embodiments outlined above, but rather can in principle also be realized in an entirely different form.

[0062] Although described above on the basis of an energy storage system in the form of a battery power plant, the proposed procedure is also usable for other kinds of energy stores, for example energy stores in the form of capacitors or electromechanical flywheels. In this respect, it is possible to use very different energy stores which make an electrical DC voltage available.

[0063] A switchable transformer of the type described here can have a large number of steps, for example 20 steps or more for a finely stepped switchover of the transformation ratio.

LIST OF REFERENCE SIGNS

[0064] 1 Energy storage system (energy storage power plant)

[0065] 2 Energy store

[0066] 3 Power convertor

[0067] 4 Generator transformer

[0068] 5 Transformer

[0069] 50 Primary winding

[0070] 51 Secondary winding

[0071] 52 Transformer core

[0072] 53 Switching device

[0073] 530 Tap

[0074] 531 Switch

[0075] 532 Switch

[0076] 6 Energy supply network

[0077] 7 Control device

[0078] S1-S7 Step

[0079] t Time

[0080] SOC State of charge

[0081] U.sub.DC DC voltage

[0082] U.sub.AC AC voltage

[0083] U.sub.grid Line voltage

[0084] X DC voltage value