Converter and method for operating a DC power supply network

Abstract

Method for operating a direct current supply network (50) which is connected to an alternating current supply network (10) via an active alternating current / direct current converter (20) and a star-grounded transformer (12), wherein at least one isolating element (11) is arranged between the alternating current supply network (10) and the transformer (12), and wherein at least one device for supplying direct current independently of the alternating current supply network (10) is present in the direct current supply network, characterized by the following steps: - stopping the conversion of alternating current to direct current by the converter (20); - opening the isolating device (11) to disconnect the transformer (12) from the AC power supply network (10); - supplying the direct current supply network (50) by the at least one device for supplying direct current; - Operating the converter (20) in a voltage-regulating mode and applying alternating voltage to at least two secondary windings of the transformer (12).

Description

The invention relates to a method for operating a direct current supply network which is connected to an alternating current supply network via an active alternating current / direct current converter and a transformer with a neutral point, wherein at least one separating element is arranged between the alternating current supply network and the transformer, and wherein at least one device for supplying direct current is present in the direct current supply network independently of the alternating current supply network. The invention further relates to an active AC / DC converter which is set up to carry out the method.In industrial plants, DC power supply networks are increasingly being used for supplying energy to components of the industrial plant. DC power supply networks have the advantage that energy stores buffering peak loads in the network are easier to integrate, as well as generating systems of regenerative energy, for example photovoltaic systems. In addition, DC converters, with which the usually higher voltage of the DC power supply system is lowered to an operating voltage of the component, can be implemented in the individual components with less outlay and overall volume than AC / DC converters ("power supplies"), which are arranged in each component or multiple times in each switchgear cabinet. Also, the efficiency of DC-DC converters is higher compared to power supplies.The publication DE 10 2021 113 205 A1 shows, for example, an arrangement with a converter in order to feed a direct current supply network through an alternating current supply network. In addition to the actual active AC / DC converter, the arrangement comprises a short-circuit protection device in order to protect the converter against short circuits in the DC power supply network.For feeding power into the direct current supply network, usually a neutral point grounded transformer is provided, which is connected on the primary side to a low or medium voltage network. TN networks are usually implemented in Europe by grounding the transformer neutral point.As is known, for example, from the publication DE 10 2020 129 918 A1, the ground potential can be transmitted to the DC voltage side when the DC power supply network is fed via the converter by the clocking of the switching elements of the converter. The DC voltage network then has potentials on its two power supply lines which are symmetrical with respect to the ground potential. This offers the advantage over a direct current supply network in which one of the two supply lines is connected to ground potential that the maximum voltage lying between the ground potential and one line of the direct current supply network is lower, specifically half as high. Accordingly, the requirements with regard to insulation strength in direct current supply networks are reduced.If the AC power supply system fails, alternative power supply sources are available in the DC power supply system of the type mentioned at the beginning in order to be able to bridge the power failure at least briefly and to be able to ensure that the components of the industrial installation can be shut down into a secured state before they are no longer supplied with power. Alternative power supply sources of this type are, for example, regenerative power generation systems, generators and / or energy stores. The publications WO 2022 / 017860 A1 and DE 10 2020 124 856 A1 describe bidirectionally operable converters with which an alternating current network can also be provided from direct current-side energy stores if the alternating current supply network fails.However, in the event of a failure of the AC power supply system, the problem arises that the reference to ground potential in the DC power supply system is no longer given, since the converter which has produced this ground reference is no longer operated. The DC power supply system thus becomes an insulated network that no longer has a secured ground reference. The existing protective concept is no longer sufficient for this resulting insulated network form, as a result of which protective concepts are required. For example, insulation monitors may become necessary at various locations in the DC power supply network. The aforementioned publication DE 10 2020 129 918 A1 describes the use of a DC-side ground circuit in order to connect the DC power supply system to ground potential in the event of a failure of the AC power supply system. The grounding circuit can be formed, for example, by a correspondingly controlled switch.It is an object of the present invention to provide a method for operating a DC power supply system of the type described at the beginning, in which reliable operation without additional protective concepts and without a separate DC-side grounding switch is possible even in the event of failure of the AC power supply system and disconnection from this AC power supply system.This object is achieved by a method and a converter having the respective features of the independent claims. Advantageous embodiments and developments are each the subject matter of the dependent claims.A method according to the invention for operating a direct current supply network is characterized by the following steps:The conversion from alternating current to direct current by the converter is stopped and the transformer is disconnected from the alternating current supply network, in particular after a fault is detected in the alternating current supply network, e.g. a failure of at least one phase of the alternating current supply network.The DC power supply system is then supplied by the at least one device for supplying DC power, and the converter is operated for converting DC power to AC power and applies AC power to at least two secondary windings of the transformer. All secondary windings of the transformer are preferably supplied with alternating voltage.By supplying the at least two secondary windings of the transformer with alternating voltage by the converter, which is fed from the direct current supply network, the direct current supply network and the transformer are coupled again, as a result of which a ground reference of the direct current supply network via the neutral point of the transformer is also re-established. From the DC power supply system operated briefly isolated, a TN power supply system with ground reference is again used. The center potential between lines of the DC power supply system is preferably brought to ground potential, as a result of which the potentials of the DC power lines are symmetrically about the neutral point of the transformer and therefore symmetrically about the ground potential. In this way, it is achieved that even in this operating mode a maximum potential difference between the DC lines and the ground potential is only half as large as the level of the voltage between the DC lines, i.e. the level of the voltage in the DC power supply network. The requirements for insulation in the DC power supply network thus do not increase.In an advantageous embodiment of the method, the converter is operated in a voltage-regulating manner for applying alternating voltage to the at least two secondary windings of the transformer, wherein a phase offset between the alternating voltages applied to the secondary windings is suitably selected in order to ensure a position of the potentials of the direct current lines which is as constant as possible with respect to the ground reference defined by the star point. In the case of two secondary windings to which alternating voltage is applied, a phase offset of 180° can be selected, and in the case of three secondary windings to which alternating voltage is applied, a phase offset of 120° can be selected. The converter preferably applies a voltage to the at least two secondary windings of the transformer, the amplitude of which is less than a minimum permissible secondary voltage in normal operation of the alternating current supply network and is particularly preferably less than approximately 50 volts (V). It has been found that voltages in the range from 10 V up to a few 10 V on the secondary side of the transformer are sufficient to establish the ground reference in the DC voltage network. In this way, magnetic reversal losses in the transformer are kept as low as possible.In a further advantageous embodiment of the method, the converter outputs a status signal which indicates the operation for supplying the at least one secondary winding of the transformer with alternating voltage. Consumers in the DC power supply system can react to this signal and set their operating mode depending on the status signal, e.g. switch over to an operating mode which requires less power.In a further advantageous embodiment of the method, an earth leakage detector arranged on the AC side is operated, which emits a signal when an earth leakage current is detected on the AC side, by means of which the DC power supply system is taken out of operation. By coupling the networks via the converter, a residual current detector which may already be present on the AC side can advantageously be used to detect insulation problems in the DC voltage network. To deactivate the DC power supply system, the operation of the at least one device for supplying DC power can be stopped, for example, in response to a detected fault current. In addition, a discharging device can be activated in order to reduce a voltage in the DC power supply network, which further increases operational reliability. It can be provided that fault current limits are set at the fault current detector, which differ from the fault current limits set at the fault current detector in normal operation, in order to be able to use it optimally for the detection of insulation faults in the DC power supply system.A converter according to the invention for converting alternating current to direct current has a device for monitoring a connection to an alternating current supply network via a transformer with a neutral point. It is characterized in that it is configured to convert direct current supplied via direct current lines to alternating current and to change it to a voltage-setting operation when the device detects a disconnection from the alternating current supply network, and is configured to set the alternating voltage in the voltage-setting operation via a connection to at least two secondary windings of the transformer in such a way that potentials of the direct current lines are symmetrically around the ground potential. With such a converter, the previously indicated method can be carried out with its advantages.The converter is preferably configured to generate an AC voltage whose amplitude is less than a minimum permissible secondary voltage in normal operation of the AC power supply system and in particular less than approximately 50 V. Furthermore, the converter preferably has an AC-side residual current detector which is configured to output a signal which is made available to external components when an AC-side residual current is detected. The advantages already mentioned with regard to the connection to the method are obtained in each case.The invention is explained in more detail below with reference to an exemplary embodiment with the aid of figures. The figures show: FIG. 1 shows a schematic block diagram of a direct current supply network connected to an alternating current supply network; and FIG. 2 shows a flow diagram of a method for operating a DC power supply network.FIG. 1 shows a schematic block diagram of a DC power supply network 50 coupled to an AC power supply network 10. The direct current supply network 50 is also referred to below for short as a DC (direct current) network 50. The alternating current supply network 10 is accordingly also referred to as an AC (alternating current) network 10.For coupling the DC grid 50 to the AC grid 10, a transformer 12 is provided, which is connected to the AC grid 10 via a separating element 11. Depending on the power to be provided in the DC grid 50, and depending on the availability, the AC grid 10 can be a medium-voltage grid and, correspondingly, the transformer 12 can be a medium-voltage transformer and the isolation element 11 can be a medium-voltage switching or isolation element. Alternatively, the AC network 10 can also be a low-voltage network.Typically, at the required powers, the AC grid 10 will be a three-phase grid and the transformer 12 will correspondingly be a three-phase transformer having at least three secondary side windings connected together at a node also called the "neutral point" of the transformer 12. The neutral point of the transformer 12 is connected to a ground point in the system shown in FIG. 1, so that the neutral point of the transformer 12 is at ground potential. This creates the TN network which is common in large parts in Europe, that is to say a network form in which the AC voltage low-voltage network is grounded in the region of the building installation. However, it is also conceivable for the transformer 12 to have only two secondary-side windings which are connected to one another in one node and are grounded there. In this case, a so-called split-phase network is produced. The method is also applicable in such a configuration.The transformer 12 is connected via at least one switching and / or safety element 13 and a residual current detector 14 to an active AC / DC converter 20, also referred to below as an AC / DC converter 20.This AC / DC converter 20 functions as an active rectifier in a normal operation in which the DC grid 50 is substantially supplied from the AC grid 10. It has correspondingly actively connected switching elements within converter bridge branches. In this way, losses can be minimized and the generated DC voltage can be regulated compared to passive rectifiers, wherein the output potentials on DC output lines (DC lines) of the DC grid 50 can be adjusted such that their potentials are symmetrically around the neutral point of the transformer 12 and thus symmetrically around the ground potential. In this way, it is achieved that a maximum potential difference between the DC lines and the ground potential is only half as great as the level of the voltage between the DC lines, i.e. the level of the voltage in the DC grid 50.The AC / DC converter 20 is connected to this DC grid 50 via at least one switching and / or safety element 22. In addition, the AC / DC converter 20 has an emergency-off input 21 which is coupled to the residual current detector 14 in order to go into a safe operating state when residual currents occur in the AC grid 10 and to disconnect the DC grid 50 from the AC grid 10.As a further energy source feeding into the DC grid 50, a DC-DC converter 30, also referred to below as DC / DC converter 30, is provided, which is connected on the input side to a photovoltaic generator (PV generator) 32. The PV generator 32 is symbolically represented by a plurality of individual PV cells. In one implementation of the arrangement shown, the PV generator 32 may consist of a plurality of PV modules connected in series and / or in parallel. It should be appreciated that other comparable DC / DC converters coupled to PV generators may be coupled to the DC grid 50. For the connection between the DC / DC converter 30 shown and the DC grid 50, at least one switching and / or safety element 33 is again provided. Comparable to the AC / DC converter 20, the DC / DC converter 30 also has an emergency-off input 31, which is coupled to the fault current detector 14 in order to be able to bring it into a safe operating state in the event of a fault currents occurring.As a further energy source, in particular for emergency power supply, a further DC-DC converter 40, also referred to below as DC / DC converter 40, is present, which is connected via at least one switching and / or safety element 42 to an energy store 43, in particular a storage battery. This further DC / DC converter 40 also has an emergency-off input 41, which, like the converters 20, 30, is coupled to the fault current detector 14 and is actuated by the latter in order to assume a safe operating state in the event of a fault.Two load arrangements 60 a, bare illustrated by way of example, each having a plurality of components which are operated with direct current. They are connected to the DC network 50 via sub-distributions 53, wherein separate safety and / or switching elements 61 a, bmay be connected upstream of each component or component group. The sub-distributions 53 are supplied with direct current via branches 51 of the DC network 50 and associated further safety and / or switching elements 52.A method according to the invention for operating a direct current supply network, for example the direct current supply network 50 illustrated in FIG. 1, is explained in more detail below with reference to FIG. 2. FIG. 2 shows a schematic flow diagram of an exemplary embodiment of the method, which is explained by way of example with reference to the arrangement according to FIG. 1.The method starts in a step S 1, in which the arrangement is operated into a normal operating state. In this context, normal operation means that all phases of the AC grid 10 are present and the AC grid 10 supplies the AC / DC converter 20 with alternating current via the transformer 12 and the interposed isolation or switching and safety elements 11, 13. This converts the alternating current to direct current and feeds it to the DC grid 50, from which the load arrangements 60 a, bare supplied. Other power sources, such as photovoltaic generator 32 with DC / DC converter 30, may assist in feeding DC grid 50. If the current sources feed more power into the DC grid 50 than the load arrangements 60 a, brecess, the AC / DC converter 20 can also be operated bidirectionally and feed the excess power back into the AC grid 10. The excess energy can, however, also be supplied to the energy store 43, or the current sources are regulated to such an extent that no excess energy is produced.In a next step S 2, a fault occurs in the AC network 10, which fault is detected by voltage monitors arranged on the input side in the AC / DC converter 20 or connected upstream thereof. The fault may relate to one or more phases of the AC network 10. After detecting the fault, the AC / DC converter 20 assumes a fault operating state in which it stops the clocking of its switching elements and thus stops the supply of the DC network 50.The DC grid 50 is then further supplied by the DC converter 30 of the PV system and / or the further DC converter 40 which is coupled to the DC energy store 43.In response to the detected fault in the AC network 10, in a next step S 3 the isolation element 11 opens, which is usually controlled by a fault detection circuit independent of the AC / DC converter 20.In a following step S 4, the AC / DC converter 20 queries whether the separating element 11 is open. If this is not the case, the AC / DC converter 20 remains in the fault operating state. It can be provided that the query in step S 4 is carried out repeatedly until it is determined that the separating element 11 is open. The method is then continued in a step S 5.In this step S 5, the AC / DC converter 20 is operated in a voltage-setting manner by DC voltage of the DC grid 50 being converted into AC voltage, which is then applied to the transformer 12 on the secondary side. This restores potential coupling of the DC grid 50 and of the transformer 12, whereby ground reference of the DC grid 50 via the neutral point of the transformer 12 is also re-established. From the DC network 50 operated in a briefly insulated manner, a TN network with ground reference is again obtained, in which the center potential between the DC lines of the DC network 50 is at ground potential.The AC / DC converter 20 is operated in a voltage-setting manner, i.e. it applies a predetermined voltage amplitude to the secondary-side windings of the transformer 12. In this voltage-setting operation, the voltage amplitude is preferably smaller than the minimum permissible secondary-side operating voltage of the transformer 12 in normal operation.Advantageously, a value is set which is less than 50% of the minimum permissible secondary-side operating voltage of the transformer 12 in normal operation, and in particular a value which is less than approximately 50 V is selected.In a following step S 6, the residual current detector 14 is used to monitor a residual current caused by the voltage-setting operation of the converter 20. The residual current detector 14 is in this case capable of detecting residual currents on the DC side. It can be provided in particular that fault current limit values for the voltage-setting operation of the AC / DC converter 20 are selected which differ from those of normal operation. It can also be provided to take into account both AC voltage components of the residual current and DC current components of the residual current in the voltage-generating operation of the AC / DC converter 20. The residual current detector 14 is preferably connected on the AC side of the converter 20, but can also be connected on the DC side.Subsequently, the AC / DC converter 20 remains in the voltage-setting operation as long as the AC grid 10 is still not available. This is monitored in a step S 7. If the AC network 10 is available again, the system is reset to normal operation again and the method branches back to step S 1.For this purpose, the AC / DC converter 20 is stopped and the separating element 11 is switched on again. Then, the AC / DC converter 20 can resume its normal operation.In addition to the check in step S 7 whether the AC grid 10 is available again, a check is made in a further step S 8 whether the residual current detector 14 detects an insulation fault in the DC grid 50. If not, the AC / DC converter 20 remains in the voltage-generating mode, whereby the DC grid 50 can continue to be operated as a TN grid, as long as the PV generators 32 or the energy store 43 can supply the load arrangements 60 a, b.If it is determined in step S 8 that the residual current detector 14 has triggered, an emergency stop signal is output in a following step S 9 to the connected devices, in the present case to the AC / DC converter 20 and the DC / DC converters 30 and 40, which then stop their operation. In this case, provision can be made for a signal for shutting down the load arrangements 60 a, bto be output beforehand in order to enable their components to end their operation reliably beforehand.It can be provided that the emergency signal is output only in voltage-generating operation, not in normal operation, or only after a predefined delay time. This can have the advantage that, in normal operation in the event of a fault current, the fault current which occurs can be used to trigger a fuse and thus to localize the fault location. If necessary, the DC network 50 can thereby be operated further, since the insulation fault has been removed by the triggering of the fuse.As an additional safety measure, in this case a discharging device, which may be present in the DC grid 50, may also be activated in order to bring the voltage in the grid as quickly as possible to a non-dangerous value, for example less than 50 V.List of reference characters10 AC power supply system (AC power supply system) 11 isolation element 12 transformer 13 switching and / or safety element 14 residual current detector 20 converter (AC / DC converter) 21 emergency off input 22 switching and / or safety element 30 DC converter (DC / DC converter) 31 emergency off input 32 photovoltaic generator (PV generator) 33 switching and / or safety element 40 further DC converter (DC / DC converter) 41 emergency off input 42 switching and / or safety element 43 energy storage device 50 DC power supply system (DC power supply system) 51 branch 52 switching and / or safety element 53 redistribution 60 a, b load arrangements 61 a, b switching and / or safety element

Claims

Method for operating a direct current supply network (50) which is connected to an alternating current supply network (10) via an active alternating current / direct current converter (20) and a transformer (12) with neutral point, wherein at least one isolation element (11) is arranged between the alternating current supply network (10) and the transformer (12), and wherein at least one device for supplying direct current is present in the direct current supply network independently of the alternating current supply network (10), characterized bythe following steps: - stopping the conversion of alternating current to direct current by the converter (20); - opening the isolation element (11) for isolating the transformer (12) from the alternating current supply network (10); - supplying the direct current supply network (50) by the at least one device for supplying direct current; operating the converter (20) in a voltage-setting operation and supplying at least two secondary windings of the transformer (12) with alternating voltage.Method according to Claim 1, in which the converter (20) applies a voltage to the at least two secondary windings of the transformer (12), the amplitude of which voltage is less than a minimum permissible secondary voltage in normal operation of the alternating-current power supply network (10).The method of claim 2, wherein the amplitude is less than about 50 volts.Method according to one of Claims 1 to 3, carried out after a failure of at least one phase of the alternating-current power supply network (10).Method according to one of Claims 1 to 4, wherein the converter (20) outputs a status signal which indicates the operation for supplying the at least one secondary winding of the transformer (12) with alternating voltage.Method according to claim 5, wherein consumers in the DC power supply network (50) set their operating mode depending on the status signal.Method according to one of Claims 1 to 6, wherein, in the voltage-generating mode, a residual current detector (14) monitors the converter (20) with regard to a residual current which occurs and, when a residual current is detected, emits a signal by means of which the DC power supply network (50) is taken out of operation.The method of claim 7, wherein the fault current detector monitors a fault current on the AC side of the inverter (20).Method according to claim 7 or 8, wherein to disable the DC power supply network (50), the operation of the at least one device for supplying DC power is stopped.Method according to claim 9, wherein additionally a discharging device is activated to decrease a voltage in the direct current supply network (50).Method according to one of Claims 8 to 10, wherein fault current limits which differ from the fault current limits which are set in the fault current detector (14) in a normal operation are set at the fault current detector (14) in the voltage-setting operation.Converter (20) for converting alternating current to direct current, having a device for monitoring a connection to an alternating current supply network (10) via a transformer (12) with neutral point, characterized in that the converter (20) is configured to convert direct current supplied via direct current lines to alternating current and to change it to a voltage-setting operation when the device detects a disconnection from the alternating current supply network (10), and the converter (20) is configured to set the alternating voltage in the voltage-setting operation via a connection to at least two secondary windings of the transformer (12) in such a way that potentials of the direct current lines are symmetrically around the ground potential.Converter (20) according to Claim 12, which is configured to generate an alternating voltage whose amplitude is smaller than a minimum permissible secondary voltage in normal operation of the alternating current supply network (10) and is in particular smaller than approximately 50 volts.Converter (20) according to Claim 12 or 13, having an AC-side residual current detector (14) which is configured to output a signal which is made available to external components when an AC-side residual current is detected.

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