Uninterruptible power supply for a low, medium or high voltage network

Abstract

An uninterruptible power supply (1) for a low-, medium- or high-voltage network (2, 3) and with an energy storage device (4) which has an energy store (5) and an inverter (6) connected to the energy store (5), which inverter is designed to extract energy from the energy store (5) and output it as alternating voltage, characterized by a transformer (7) which has first connections (8) for a supply network (2), second connections (9) for a load network (3) and third connections (10) connected or connectable to the energy storage device (4), wherein the transformer (7) has three transformer windings (14, 15, 16) arranged around a transformer core (17) for each phase, wherein a first transformer winding (14) of the three transformer windings (14, 15, 16) has the first terminals (8) of the transformer (7), a second transformer winding (15) of the three transformer windings (14, 15, 16) has the second terminals (9) of the transformer (7), and a third transformer winding (16) of the three transformer windings (14, 15, 16) has the third terminals (10) of the transformer (7), or wherein the transformer (7) has two transformer windings (14, 15) arranged around a transformer core (17) for each phase, wherein a first transformer winding (14) of the two transformer windings (14, 15) has the first terminals (8) of the transformer (7) at a first end and the third terminals (10) of the transformer (7) at a second end, and wherein a second transformer winding (15) of the two transformer windings (14, 15) has the second terminals (9) of the transformer (7), characterized by a first switching element (12) which is connected to a respective terminal of the first transformer winding (14) of each phase and is designed to connect the respective terminals of the first transformer windings (14) of each phase to one another to form a first star point in a control operating mode of the uninterruptible power supply (1).

Description

Technical FieldThe invention relates to an uninterrupted power supply for a low-voltage, medium-voltage or high-voltage power supply system and to an arrangement having a plurality of such uninterrupted power supplies.Background ArtUninterrupted power supplies, as are known, for example, from the laid-open specifications DE 198 50 886 A1, U.S. Pat. No. 4,556,802 A, JP 2005-151 688 A or DE 101 62 955 A1, are used in low-voltage, medium-voltage and high-voltage networks in order to protect critical loads arranged in a load network from interruption or disruption of the power supply by a supply network. For this purpose, the uninterrupted voltage supply contains an energy storage device, which can contain, for example, a battery, a supercapacitor, a fuel cell or another type of energy storage device. Since there is a need for synchronous feeding into the load grid, which is usually operated as an AC voltage grid, a converter is provided in energy storage means which provide a DC voltage in order to provide an AC voltage of the desired nature with respect to amplitude, frequency and phase position relative to the supply grid. Such an inverter also allows the operation of the uninterrupted voltage supply as an active grid filter, which can be used, for example, to damp harmonics of the grid frequency of the supply grid or of the load grid.A challenge in the implementation of an uninterrupted voltage supply is the necessary speed of the provision of energy from the energy store in the event of a fault. It is thus usually expected that the uninterrupted voltage supply can take over the supply of the load grid within a period of the grid frequency. A further challenge is a robustness of the uninterrupted voltage supply with respect to a fault on the supply network side, such as a short circuit. In such a case, the uninterrupted voltage supply should reliably feed the load grid despite the short circuit on the supply grid side, for which reason an outflow of energy from the energy store of the uninterrupted voltage supply into the supply grid should be prevented as much as possible.The invention therefore addresses the problem of introducing an improved uninterrupted voltage supply, preferably for a medium-voltage or high-voltage network. The invention achieves this object by a seamless power supply according to claim 1 and an arrangement according to claim 6.SUMMARY OF THE INVENTIONA first aspect of the invention relates to an uninterrupted voltage supply for a low-voltage, medium-voltage or high-voltage power supply system and having an energy storage device which has an energy storage device and an inverter connected to the energy storage device. The inverter is designed to draw energy from the energy store and output it as an alternating voltage. In this case, a transformer is provided which has first connections for a supply network, second connections for a load network and third connections which are connected or can be connected to the energy storage device.According to a first variant according to the invention, the transformer of the uninterrupted voltage supply has three transformer windings for each phase, which windings are arranged around a transformer core (often referred to as "three-winding transformer", wherein three windings, and therefore nine windings in a three-phase construction, are provided for each phase). In this case, a first transformer winding of the three transformer windings has the first connections of the transformer, a second transformer winding of the three transformer windings has the second connections of the transformer, and a third transformer winding of the three transformer windings has the third connections of the transformer.Such a structure with a three-winding transformer has the advantage that the supply network, the load network and the energy storage device are each galvanically isolated from one another. At the same time, the three-winding transformer limits the transmission of electrical energy between the instances mentioned, so that, in particular, a short circuit close to the transformer in the supply network cannot unduly impair the supply of the load network by the energy storage device.According to a second variant according to the invention, the transformer has two transformer windings arranged around a transformer core for each phase (often referred to as "two-winding transformer", wherein two windings, and thus six windings in a three-phase design, are provided for each phase). In this case, a first transformer winding of the two transformer windings has the first connections of the transformer at a first end and the third connections of the transformer at a second end. In addition, a second transformer winding of the two transformer windings has the second connections of the transformer.In such embodiments of the seamless power supply according to the invention, the supply network and the energy storage device are consequently connected to opposite ends of the same transformer winding. The transformer winding limits, as inductance, a current of the energy storage device flowing into a short circuit close to the transformer in the supply network, so that the supply of the load network can be continued without interruption or virtually without interruption from the energy storage device. In general, the supply network can be disconnected from the load network supplied by the uninterrupted voltage supply if the fault or short circuit continues. The embodiments with a dual-winding transformer advantageously reduce the outlay for the transformer, which can also be realized in a smaller structural volume.According to the invention, in both variants of the uninterrupted voltage supply according to the invention, a first switching element is provided, which is connected to a respective terminal of the first transformer winding of each phase and is designed to connect the respective terminals of the first transformer windings of each phase to one another to form a first neutral point in a regulating operating mode of the uninterrupted voltage supply. The first transformer windings can be activated electromagnetically with the first switching element by connecting them together to form a star circuit (regulating operation, i.e. supplying the load grid from the supply grid). Conversely, the star connection can be disconnected by the first switching element, whereby a current flow through the first transformer windings can be prevented and a disconnection of the load grid now supplied by the uninterrupted voltage supply from the supply grid can be effected.The seamless power supply of the invention has an advantage that it can be realized with a small number of components, which promotes low manufacturing costs and high reliability. Due to the inductance of the transformer windings, there is also no direct path between the output of the inverter and the supply grid, which is why in the case of a short circuit in the supply grid there is an impedance-associated separation between the energy store and the supply grid and the load grid can be supplied from the energy store. Since, in the closed-loop control mode, the supply network is connected to the transformer, the transformer is already magnetized in the event of a fault, with the result that the energy store can take over the supply of the load network without additional time losses for a required magnetization.The first and third terminals thereby denote different terminals of the transformer, for example terminals of different windings of the transformer or terminals at opposite ends of a winding of the transformer.Usually, the uninterrupted voltage supply, like the supply and load network, will be designed to be three-phase, so that the inverter and the transformer are correspondingly constructed to be three-phase.Voltages between 1 kV and 52 kV are usually considered as the medium voltage, high voltage all voltages lying above 52 kV. In order to adapt the output voltage of the inverter to the medium or high voltage of the supply and / or load network, the transformer can have a suitable winding ratio, wherein in most practical cases a relatively lower output voltage of the inverter is transformed up to a relatively higher voltage of the supply and / or load network.In a preferred embodiment of the uninterrupted voltage supply according to the invention with a three-winding transformer described above, the second transformer windings (i.e. the transformer winding to which the energy storage device is connected) can be connected to form a delta circuit. In combination with a star connection of the first and / or third transformer winding, a transformation ratio can result in order to step up the output voltage of the energy storage device to a rated voltage of the load grid. This is advantageous because inverters usually have a relatively low output voltage compared to a medium-voltage or high-voltage network.In addition to the first switching element, a second switching element can be provided, which is connected to respective opposite connections of the first transformer winding of each phase and is designed to connect the respective opposite connections of the first transformer windings of each phase to one another to form a second neutral point in the regulating operating mode of the uninterrupted voltage supply with the first connections of the transformer for the supply grid or in an exceptional operating mode of the uninterrupted voltage supply. The two switching elements thus permit switching between regulating and exception operating modes by switching the first transformer windings between two star circuits arranged at the opposite ends of the first transformer windings. In the regulating operating mode, the supply network thus feeds the first transformer windings which are interconnected from one of the switching elements to a star point on the side facing away from the supply network, and in the exceptional operating mode this star point is disconnected, so that the energy storage device can feed energy in this case. For this purpose, in the exceptional operating mode, the other switching element forms a star point of the first transformer windings on the supply network side thereof. In this case, the supply network is also disconnected from the first transformer windings by the other switching element, since otherwise the supply network would be short-circuited by the other switching element in the neutral point created by the switching element. For the reasons described, the first switching element is therefore preferably also designed to connect the respective terminals of the first transformer windings of each phase to the energy storage device in the exceptional operating mode. In principle, however, the connection to the energy storage device could also be made by another functional unit.The first switching element can also be designed as a residual current limiter. This makes it possible to limit the currents flowing in the neutral point formed by the first switching element, as a result of which, in the event of a short circuit in the supply network close to the transformer, the supply of the load network can take place without interruption or virtually without interruption from the energy storage device.The uninterrupted voltage supply can be equipped with a bypass which is designed to connect the supply network to the load network, bypassing the transformer, in response to a bypass signal. The bypass can be closed in the regulating operating mode, whereby the efficiency of the overall arrangement is increased because of the bypass of the transformer. In the exceptional operating mode, the bypass is opened accordingly, as a result of which the direct connection of the load grid to the supply grid is disconnected and the supply of the load grid can be taken over by the uninterrupted voltage supply.A second aspect of the invention relates to an arrangement having a plurality of uninterrupted voltage supplies according to the invention, wherein the uninterrupted voltage supplies are connected in parallel with one another. Such a modular construction makes it possible to interconnect a plurality of identical or else different embodiments of uninterrupted voltage supplies according to the invention, on the one hand, in accordance with the degree of protection desired and the size of the loads arranged in the load network. For example, loads in a supply grid with a low reliability can be protected against expected longer failure durations of the supply grid by interconnecting a plurality of uninterrupted voltage supplies according to the invention than would be necessary in a supply grid with a higher reliability. Also, by interconnecting larger loads can be secured than would be the case if only one uninterrupted voltage supply according to the invention were used.If a plurality of uninterrupted voltage supplies according to the invention are connected together to form an arrangement, then usually only one uninterrupted voltage supply with a bypass is required, since this one bypass can provide the corresponding function for the overall arrangement.Brief Description of the FiguresThe invention is explained in more detail with reference to figures of exemplary embodiments. The following are shown: FIG. 1 shows a first exemplary embodiment of an uninterrupted voltage supply according to the invention; FIG. 2 shows a second exemplary embodiment of an uninterrupted voltage supply according to the invention; FIG. 3 shows a third exemplary embodiment of an uninterrupted voltage supply according to the invention; FIG. 4 shows a fourth comparative example of a non-interrupted voltage supply according to the invention; FIG. 5 shows a fifth exemplary embodiment of an uninterrupted voltage supply according to the invention; FIG. 6 shows a sixth exemplary embodiment of an uninterrupted voltage supply according to the invention; and FIG. 7 shows an exemplary embodiment of an arrangement according to the invention with a plurality of uninterrupted voltage supplies.DETAILED DESCRIPTION OF THE FIGURESFIG. 1 shows a first exemplary embodiment of an uninterrupted voltage supply 1 according to the invention, which is connected to a supply network 2 via first connections 8. The uninterrupted voltage supply 1 receives electrical energy from the supply network 2, which it uses to supply loads in a load network 3 connected to second connections 9. Thus, the uninterrupted voltage supply 1 can store a portion of the electrical energy drawn from the supply network 2 in an energy storage device 4 (possibly after a physical conversion into another energy form such as rotational energy, chemical energy, pressure, etc.) and pass a further portion directly on to the load network 3. The energy stored in the energy storage device 4 can use the uninterrupted voltage supply 1 during a failure of the supply network 2 to continue supplying the load network 3, so that the critical loads arranged in the load network 3 can continue to be operated.In the example of the uninterrupted voltage supply 1 according to the invention shown in FIG. 1, the energy storage device comprises, merely by way of example, an energy storage device 5 designed as a battery, which is connected to third terminals 10 via a bidirectional inverter 6. The inverter 6 serves here for adapting between an alternating voltage from the supply network 2 and the load network 3 on the one hand and the direct voltage of the energy store 5.The first, second and third connections 8, 9 and 10 are arranged on a transformer 7 which advantageously connects the supply network 2, load network 3 and energy storage device 4 to one another. The transformer 7 can thus transfer electrical energy from the supply grid 2 to the load grid 3, from the supply grid 2 to the energy storage device 4 and from the energy storage device 4 to the load grid 3. At the same time, the inductance of the transformer 7 limits the rise speed of a current flowing from the uninterrupted voltage supply 1 into the supply grid 2 in the event of a fault (for example a short circuit), as a result of which, on the one hand, the uninterrupted supply of the load grid 3 is ensured and, on the other hand, sufficient time for disconnection of the supply grid 2 is gained.FIG. 2 shows a second exemplary embodiment of an uninterrupted voltage supply 1 according to the invention, which corresponds largely to the exemplary embodiment shown in FIG. 1, for which reason the discussion there can also be applied to the exemplary embodiment in question. The second exemplary embodiment of the uninterrupted voltage supply 1 according to the invention differs from that of FIG. 1 by a bypass 11 which is connected in parallel with the transformer 7 between the first terminals 8 and the second terminals 9. The bypass 11 allows the load network 3 to be connected to the supply network 2 directly and by bypassing the transformer 7, which can be useful for maintenance work on the transformer 7 or on the energy storage device 4 (switches, isolating switches and grounding switches can be provided for this purpose in order to reliably disconnect and ground these components). In addition, the efficiency of the direct supply of the load grid 3 via the bypass 11 can be higher than that of the indirect supply via the transformer 7 of the uninterrupted voltage supply 1, so that a standby operation of the uninterrupted voltage supply can be provided in which energy losses due to the presence of the uninterrupted voltage supply are reduced, which, however, usually requires a longer period of time until, in the event of a fault in the supply grid, the supply of the load grid from the uninterrupted voltage supply can take place.The bypass 11 can comprise, for example, a three-phase switch (not shown) and, if appropriate, a further transformer connected in series with the three-phase switch in the case of different voltage amplitudes in the load grid 3 and the supply grid 2.FIG. 3 shows a third exemplary embodiment of an uninterrupted voltage supply according to the invention in accordance with a first variant of the invention, in which the transformer 7 is constructed as a three-winding transformer and has first transformer windings 14, second transformer windings 15 and third transformer windings 16 on respective transformer cores 17 (or a common transformer core for all three phases). The first transformer windings 14 are connected at a first end to the first connections 8 and thus directly or indirectly to the supply network. At a second end, the first transformer windings 14 are connected to a first switching element 12 which, in a switching state, electrically connects the second ends of the first transformer windings 14 to one another to form a neutral point, such that electrical energy can flow from the supply network via the first connections 8 into the first transformer windings 14, as a result of which said windings magnetize the transformer core 17 and can in this way transmit the energy drawn from the supply network to the second transformer windings 15 (and, if appropriate, to the third transformer windings 16) for charging the energy store 5. In the second transformer windings 15, a voltage is induced in the transformer core 17 by the magnetic field, which voltage is output from the uninterrupted voltage supply to the load network.In a second switching state of the first switching element 12, the second ends of the first transformer windings 14 are disconnected, so that a current flow through the first transformer windings 14 is interrupted. This is useful in the case that the load grid is to be supplied from the energy storage device 4, for example if there has been a single-phase short circuit or a similar fault in the supply grid. In such a case, otherwise, the electrical energy provided by the energy storage device 4 could induce, via the magnetization of the transformer core 17, a current in the first transformer windings 14, which flowed into the supply grid and in this way was lost for the supply of the load grid.Because the transformer 7 is designed as a three-winding transformer, energy can be transmitted from the supply network both to the energy storage device 4 and to the load network, but also in the event of failure of the supply network from the energy storage device to the load network. All three are galvanically isolated from one another, in addition the transformer 7 advantageously limits the transmittable electrical power and the rate of rise of the current in the case of a short circuit close to the transformer. In addition, the uninterrupted voltage supply can be operated as an active grid filter if the first transformer windings 14 are conductively connected to one another, with the result that disturbing harmonics of the voltage provided by the supply grid can be attenuated.In the example shown, the first and second transformer windings 14 and 15 are shown connected together or connectable together in a star connection, while the third transformer windings 16 are shown connected together in a delta connection. In principle, however, a different constellation can also be selected in each case if the voltage levels of the supply network, load network and energy storage device 4 or aspects of the dimensioning of the transformer 7 which are not essential for the invention require it.FIG. 4 shows a fourth comparative example of an uninterrupted voltage supply, wherein, as in FIG. 3, an exemplary embodiment with three-winding transformer 7 is shown, so that the fundamental mode of operation corresponds to that of the exemplary embodiment shown above. Instead of a first switching element 12, however, a three-phase switch 18 is provided here in order to disconnect the supply network and interrupt the current flow through the first transformer windings 14.FIG. 5 shows a fifth exemplary embodiment of an uninterrupted voltage supply according to the invention according to a second variant of the invention, in which the transformer 7 is designed as a two-winding transformer in contrast to the exemplary embodiments of FIGS. 3 and 4. In this case, as in FIG. 3, a first switching element 12 is provided at the second ends of the first transformer windings, but this switching element is additionally connected to the energy storage device 4 and-depending on the switching state-connects the first transformer windings 14 together or connects them to the energy storage device 4. At the other ends of the first transformer windings 14, a second switching element 13 is provided, which can be constructed in accordance with the first switching element 12 and can electrically connect the first transformer windings 14 either to the supply network or to one another.The first and second switching elements 12, 13 make it possible to interconnect the first transformer windings 14 either at their one end or their other end, so that electrical energy can flow into them via their ends connected to the supply network or the energy storage device 4 and can be transmitted to the second windings 15 via the transformer core 17. In this way, electrical energy can be transmitted from the supply network or the energy storage device to the load network selectively and depending on the switching states of the first and second switching elements 12, 13. It is also possible to connect the first transformer windings 14 both to the supply grid and to the energy storage device 4, as a result of which the energy storage device 4 can be charged from the supply grid or the uninterrupted voltage supply can be operated as an active grid filter.Such an embodiment of the inventive uninterrupted voltage supply with a two-winding transformer has the advantage that the transformer 7 can be constructed in a simpler and material-saving and space-saving manner compared to embodiments with a three-winding transformer. This is opposed by the outlay for a further switching element.FIG. 6 shows a sixth exemplary embodiment of an uninterrupted voltage supply according to the invention according to the first variant of the invention, which corresponds largely to the exemplary embodiment of FIG. 3, so that the statements made there also apply to this exemplary embodiment. The first switching element 12 is designed here, however, as a residual current limiter, wherein the illustrated structure of the residual current limiter with semiconductor valves 19, diodes 20 and an inductance 21 is to be understood merely as an example. The residual current limiter, like the first switching element of FIG. 3, makes it possible to connect the first transformer windings 14 in a conducting manner or to interrupt a current flow through the first transformer windings 14. In addition, however, the current can be controlled by the fault current limiter by appropriate actuation thereof, which can be utilized in particular during operation as an active grid filter in that harmonics of the grid frequency are attenuated or amplified in a targeted manner. By means of the controllable current, for example, protective devices can also be triggered in a targeted manner, such that the fault in the supply network can be detected and switched off.FIG. 7 shows an exemplary embodiment of an arrangement according to the invention having a plurality of uninterrupted voltage supplies 1. the uninterrupted voltage supplies are connected in parallel, so that their output powers for supplying the load network 3 sum up. As a result, the arrangement can be constructed in modular fashion and adapted to the respective intended use by the number of interruption-free voltage supplies 1 connected in parallel being selected accordingly. It is also possible to retrofit further uninterrupted voltage supplies 1 if the power requirement in the load network 3 increases or faults in the supply network 2 prove to be more probable and time-consuming than originally assumed. Modularization enables transformers 7 and energy storage devices 4 to be manufactured in a uniform size, resulting in cost savings. In this case, only a bypass 11 or an uninterrupted voltage supply 1 with a bypass 11 need be provided, if desired, wherein said bypass is designed such that it can transmit the power required for supplying the load grid 3 from the supply grid 2.The invention has been explained in more detail with reference to exemplary embodiments. The exemplary embodiments are intended exclusively for illustrative purposes and are not intended to limit the invention, which is defined exclusively by the following patent claims.List of reference characters1 Uninterrupted voltage supply 2 Supply network 3 Load network 4 Energy storage device 5 Energy storage device 6 Inverter 7 Transformer 8 First connections 9 Second connections 10 Third connections 11 Bypass 12 First switching element 13 Second switching element 14 First transformer winding 15 Second transformer winding 16 Third transformer winding 17 Transformer core 18 Switch 19 Semiconductor valve 20 Diode 21 Inductance

Claims

An uninterrupted voltage supply (1) for a low-voltage, medium-voltage or high-voltage network (2, 3) and having an energy storage device (4) which has an energy storage device (5) and an inverter (6) connected to the energy storage device (5), which inverter is designed to draw energy from the energy storage device (5) and to output it as an alternating voltage, characterized bya transformer (7) which has first connections (8) for a supply network (2), second connections (9) for a load network (3) and third connections (10) connected or connectable to the energy storage device (4), wherein the transformer (7) has three transformer windings (14, 15, 16) arranged around a transformer core (17) for each phase, wherein a first transformer winding (14) of the three transformer windings (14, 15, 16) has the first connections (8) of the transformer (7), and a second transformer winding (15) of the three transformer windings (14, 15, 16), A transformer according to claim 16, wherein the transformer (16) has the second connections (9) of the transformer (7) and a third transformer winding (16) of the three transformer windings (14, 15, 16) has the third connections (10) of the transformer (7), or wherein the transformer (7) has two transformer windings (14, 15) arranged around a transformer core (17) for each phase, wherein a first transformer winding (14) of the two transformer windings (14, 15) has the first connections (8) of the transformer (7) at a first end and the third connections (10) of the transformer (7) at a second end, and wherein a second transformer winding (15) of the two transformer windings (14, 15) has the second connections (9) of the transformer (7), characterized bya first switching element (12) which is connected to a respective connection of the first transformer winding (14) of each phase and is designed to the effect that the transformer winding is connected to a respective connection of the first transformer winding (14) of each phase, connecting the respective terminals of the first transformer windings (14) of each phase to one another to form a first neutral point in a regulating operating mode of the uninterrupted voltage supply (1).The seamless power supply (1) of the preceding claim, comprising a second switching element (13) which is connected to respective opposite terminals of the first transformer winding (14) of each phase and is designed to connect the respective opposite terminals of the first transformer windings (14) of each phase to the first terminals (8) of the transformer for the supply grid (2) or to a second neutral point to one another in the regulating operating mode of the seamless power supply (1) or in an exceptional operating mode of the seamless power supply (1).The seamless power supply (1) of the preceding claim, wherein the first switching element (12) is further configured to connect the respective terminals of the first transformer windings (14) of each phase to the energy storage device (4) in the exceptional operation mode.The uninterrupted voltage supply (1) of one of the two preceding claims, in which the first switching element (12) is designed as a residual current limiter.The uninterrupted voltage supply (1) of one of the preceding claims, having a bypass (11) which is designed to connect the supply network (2) to the load network (3) in response to a bypass signal, bypassing the transformer (7).An arrangement comprising a plurality of uninterrupted voltage supplies (1), wherein the uninterrupted voltage supplies (1) are each formed according to one of the preceding claims and are connected in parallel with one another.

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