Converter and method for operating a DC supply network
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2024-05-03
- Publication Date
- 2026-03-11
AI Technical Summary
Direct current supply networks become isolated and lose a secure earth connection when the active converter that establishes the ground reference is no longer operational, rendering existing protection concepts insufficient during AC power supply failures.
A method and converter that disconnect the transformer from the AC supply network, convert DC to AC, and apply alternating voltage to the transformer's secondary windings to re-establish a ground reference, allowing the DC network to operate as a TN network with earth connection, and include features like phase offset and residual current detection for enhanced safety and insulation monitoring.
Enables secure operation of the DC supply network without additional protection concepts, maintaining reduced insulation requirements and minimizing reversal magnetization losses, while allowing for efficient detection and response to faults and insulation issues.
Smart Images
Figure EP2024062169_14112024_PF_FP_ABST
Abstract
Description
[0001] Converter and method for operating a DC power supply network
[0002] The invention relates to a method for operating a direct current supply network that is connected to an alternating current supply network via an active AC / DC converter and a star-grounded transformer, wherein at least one isolating element is arranged between the AC supply network and the transformer, and wherein at least one device for supplying direct current independently of the AC supply network is present in the DC supply network. The invention further relates to an active AC / DC converter configured to implement the method.
[0003] In industrial plants, DC power grids are increasingly being used to supply power to components within the plant. DC power grids offer the advantage of being easier to integrate energy storage systems that buffer peak loads in the grid, as well as renewable energy generation systems, such as photovoltaic systems. Furthermore, DC converters, which reduce the usually higher voltage of the DC power grid to the operating voltage of the component, can be implemented in individual components with less effort and space than AC / DC converters ("power supplies"), which are installed in every component or multiple times in every control cabinet. The efficiency of DC converters is also greater than that of power supplies.
[0004] To feed power into the DC power grid, a transformer with a neutral grounding connection is typically used, which is connected on the primary side to a low- or medium-voltage grid. TN grids in Europe are typically implemented by grounding the transformer neutral point.
[0005] When the DC power supply network is fed via the converter, the earth potential is transferred to the DC side through the clocking of the converter's switching elements. The DC power network then has potentials on its two power supply lines that are symmetrical with respect to the earth potential. Compared to a DC power supply network, where one of the two supply lines is connected to earth potential, this offers the advantage that the maximum voltage between the earth potential and a line of the DC power supply network is lower—in fact, half as high. Accordingly, the insulation strength requirements for DC power supply networks are reduced.
[0006] If the AC power grid fails, alternative power sources are available in the DC power grid of the type mentioned above to bridge the power outage at least temporarily and ensure that the components of the industrial plant can be shut down to a safe state before they are no longer supplied with power. Such alternative power sources include, for example, renewable power generation plants, generators, and / or energy storage systems.
[0007] However, this poses the problem that the connection to ground potential in the DC power grid is no longer present, as the converter that established this ground connection is no longer operating. The DC power grid thus becomes an isolated grid that no longer has a secure ground connection. The existing protection concept is no longer sufficient for this resulting isolated grid configuration, which makes additional protection concepts necessary. For example, insulation monitors may be required at various points in the DC power grid.
[0008] It is an object of the present invention to provide a method for operating a direct current supply network of the type described above, in which secure operation is possible without additional protection concepts even in the event of a failure of the alternating current supply network and a disconnection from this alternating current supply network.
[0009] This object is achieved by a method and a converter having the respective features of the independent claims. Advantageous embodiments and further developments are the subject of the dependent claims.
[0010] A method according to the invention for operating a direct current supply network is characterized by the following steps:
[0011] The conversion of alternating current to direct current by the converter is stopped and the transformer is disconnected from the alternating current supply network, in particular after detection of a fault in the alternating current supply network, e.g. a failure of at least one phase of the alternating current supply network.
[0012] The DC power grid is then supplied by the at least one device for supplying DC power, and the converter is operated to convert DC power to AC power and supplies AC voltage to at least two secondary windings of the transformer. Preferably, all secondary windings of the transformer are supplied with AC voltage.
[0013] By applying alternating voltage from the DC power grid to at least two secondary windings of the transformer through the converter, the DC power grid and the transformer are recoupled, thereby re-establishing a ground reference of the DC power grid via the transformer's star point. The briefly isolated DC power grid becomes a TN network with a ground reference. Preferably, the center potential between lines of the DC power grid is brought to earth potential, whereby the potentials of the DC lines are symmetrical around the transformer's star point and thus symmetrical around the earth potential.This ensures that, even in this operating mode, the maximum potential difference between the DC lines and ground potential is only half the voltage between the DC lines, i.e., the voltage in the DC power grid. This does not increase the insulation requirements in the DC power grid.
[0014] In an advantageous embodiment of the method, the converter is operated in a voltage-regulating manner to apply 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 that the potentials of the direct current lines are as constant as possible with respect to the earth reference defined by the star point. With two secondary windings supplied with alternating voltage, a phase offset of 180° can be selected, and with three secondary windings supplied with alternating voltage, a phase offset of 120° can be selected. The at least two secondary windings of the transformer are preferably supplied with a voltage by the converter, the amplitude of which is smaller than a minimum permissible secondary voltage during normal operation of the alternating current supply network and is particularly preferably smaller than approximately 50 volts (V).It has been shown that voltages in the range of 10 V to several tens of V on the secondary side of the transformer are sufficient to establish the earth connection in the DC voltage network. This keeps core reversal losses in the transformer as low as possible.
[0015] In a further advantageous embodiment of the method, the converter outputs a status signal indicating the operation of applying alternating voltage to at least one secondary winding of the transformer. Consumers in the DC power supply network can respond to this signal and adjust their operating mode depending on the status signal, e.g., switch to an operating mode that requires less power.
[0016] In a further advantageous embodiment of the method, a residual current detector arranged on the AC side is operated, which, upon detection of an AC-side residual current, emits a signal that shuts down the DC power supply network. By coupling the networks via the converter, any residual current detector already present on the AC side can advantageously be used to detect insulation problems in the DC voltage network. To shut down the DC power supply network, for example, the operation of the at least one device for supplying DC power can be stopped in response to a detected residual current. In addition, a discharge device can be activated to reduce the voltage in the DC power supply network, which further increases operational reliability.It may be provided to set fault current limits on the fault current detector that differ from the fault current limits that are set on 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 network.
[0017] 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. It is characterized in that it is configured to convert direct current to alternating current and to switch to voltage-regulating operation when the device detects a disconnection from the alternating current supply network. Such a converter can be used to implement the aforementioned method with its advantages. Preferably, the converter is configured to generate an alternating voltage whose amplitude is smaller than a minimum permissible secondary voltage during normal operation of the alternating current supply network, and in particular, is smaller than approximately 50 V.Further preferably, the converter has an AC-side fault current detector configured to output a signal upon detection of an AC-side fault current, which is then made available to external components. This results in the advantages already mentioned in connection with the method.
[0018] The invention is explained in more detail below using an exemplary embodiment and figures. The figures show:
[0019] Fig. 1 is a schematic block diagram of a DC power supply network connected to an AC power supply network; and
[0020] Fig. 2 is a flowchart of a method for operating a DC power supply network.
[0021] Fig. 1 shows a schematic block diagram of a direct current supply network 50 coupled to an alternating current supply network 10. The direct current supply network 50 is also referred to below as a DC (direct current) network 50. The alternating current supply network 10 is correspondingly also referred to as an AC (alternating current) network 10.
[0022] To couple 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 device 11. Depending on the power to be provided in the DC grid 50 and depending on availability, the AC grid 10 can be a medium-voltage grid, and accordingly, the transformer 12 can be a medium-voltage transformer, and the separating device 11 can be a medium-voltage switching or isolating device. Alternatively, the AC grid 10 can also be a low-voltage grid.
[0023] As a rule, for the required power levels, the AC network 10 will be a three-phase network and the transformer 12 will be a corresponding three-phase transformer with at least three secondary windings connected to one another at a node, also called the "star point" of the transformer 12. In the system shown in Fig. 1, the star point of the transformer 12 is connected to an earthing point so that the star point of the transformer 12 is at earth potential. This creates the TN network common in many parts of Europe, i.e., a network type in which the AC low-voltage network is earthed in the area of the building installation. However, it is also conceivable for the transformer 12 to have only two secondary windings connected to one another at a node and earthed there. In this case, a so-called split-phase network is created. The method can also be used in such a configuration.
[0024] The transformer 12 is connected via at least one switching and / or safety device 13 and a fault current detector 14 to an active AC / DC converter 20, hereinafter also abbreviated as AC / DC converter 20.
[0025] This AC / DC converter 20 functions as an active rectifier during normal operation, in which the DC network 50 is primarily supplied from the AC network 10. It features correspondingly actively switched switching elements within converter bridge branches. Compared to passive rectifiers, this allows losses to be minimized and the generated DC voltage to be regulated, whereby the output potentials on the DC voltage output lines (DC lines) of the DC network 50 can be adjusted so that their potentials are symmetrical about the star point of the transformer 12 and thus symmetrical about 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 the voltage between the DC lines, i.e., the voltage in the DC network 50. As a result, the insulation requirements in the DC network 50, for example, are reduced.
[0026] The AC / DC converter 20 is connected to this DC network 50 via at least one switching and / or safety device 22. Furthermore, the AC / DC converter 20 has an emergency stop input 21, which is coupled to the fault current detector 14 in order to enter a safe operating state when fault currents occur in the AC network 10 and to disconnect the DC network 50 from the AC network 10.
[0027] A direct current converter 30, also referred to below as a DC / DC converter 30, is provided as a further energy source feeding into the DC grid 50. The input side of the converter is connected 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 can consist of a plurality of PV modules connected in series and / or parallel. It is understood that other comparable DC / DC converters coupled to PV generators can be coupled to the DC grid 50. At least one switching and / or safety device 33 is provided for the connection between the illustrated DC / DC converter 30 and the DC grid 50.Similar to the AC / DC converter 20, the DC / DC converter 30 also has an emergency stop 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 fault currents occurring.
[0028] As a further energy source, in particular for emergency power supply, there is a further DC / DC converter 40, hereinafter also referred to as a DC / DC converter 40, which is connected to an energy storage device 43, in particular a storage battery, via at least one switching and / or safety device 42. This further DC / DC converter 40 also has an emergency stop input 41, which, like the converters 20, 30, is coupled to the residual current detector 14 and is controlled by it in order to assume a safe operating state in the event of a fault.
[0029] Two load assemblies 60a, b are shown as examples, each comprising a plurality of components operated with direct current. They are connected to the DC network 50 via sub-distribution boards 53, with separate fuse and / or switching devices 61a, b being connected upstream of each component or component group. The sub-distribution boards 53 are supplied with direct current via branches 51 of the DC network 50 and associated additional fuse and / or switching devices 52.
[0030] A method according to the invention for operating a direct current supply network, for example the direct current supply network 50 shown in Figure 1, is explained in more detail below with reference to Figure 2. Figure 2 shows a schematic flow diagram of an embodiment of the method, which is explained by way of example with reference to the arrangement according to Figure 1.
[0031] The method starts in a step S1, in which the arrangement is operated in a normal operating state. Normal operation in this context means that all phases of the AC network 10 are present and the AC network 10 supplies the AC / DC converter 20 with alternating current via the transformer 12 and the intermediate isolating or switching and safety devices 11, 13. The AC / DC converter converts the alternating current to direct current and feeds it into the DC network 50, from which the load arrangements 60a, b are supplied. Additional power sources, such as the photovoltaic generator 32 with the DC / DC converter 30, can feed into the DC network 50 in a supporting manner. If the power sources feed more power into the DC network 50 than the load arrangements 60a, b require, the AC / DC converter 20 can also be operated bidirectionally and feed the excess power back into the AC network 10.However, the excess energy can also be fed to the energy storage device 43, or the power sources can be regulated to such an extent that no excess energy is produced.
[0032] In the next step S2, a fault occurs in the AC network 10, which is detected by voltage monitors located on the input side of the AC / DC converter 20 or connected upstream of it. The fault can affect one or more phases of the AC network 10. After detecting the fault, the AC / DC converter 20 enters a fault operating state in which it stops the timing of its switching elements, thus terminating the supply to the DC network 50.
[0033] 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 storage device 43.
[0034] In response to the detected fault in the AC network 10, the isolating element 11 opens in a next step S3, which is usually controlled by a fault detection circuit independent of the AC / DC converter 20.
[0035] In a subsequent step S4, the AC / DC converter 20 queries whether the isolating element 11 is open. If this is not the case, the AC / DC converter 20 remains in the fault operating state. It may be provided to repeat the query in step S4 until it is determined that the isolating element 11 is open. The method then continues in a step S5.
[0036] In this step S5, the AC / DC converter 20 is operated in a voltage-regulating manner by converting the direct voltage of the DC network 50 into alternating voltage, which is then applied to the secondary side of the transformer 12. This restores the potential coupling between the DC network 50 and the transformer 12, thereby re-establishing a ground connection of the DC network 50 via the star point of the transformer 12. The briefly isolated DC network 50 again becomes a TN network with a ground connection, in which the center potential between the DC lines of the DC network 50 is at ground potential.
[0037] The AC / DC converter 20 is operated in a voltage-regulating mode, i.e., it applies a predetermined voltage amplitude to the secondary windings of the transformer 12. In this voltage-regulating mode, the voltage amplitude is preferably smaller than the minimum permissible secondary operating voltage of the transformer 12 during normal operation. This reduces core losses in the transformer 12.
[0038] 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 is selected which is less than approximately 50 V.
[0039] In a subsequent step S6, the fault current detector 14 is used to monitor a fault current caused by the voltage-regulating operation of the converter 20. The fault current detector 14 is capable of detecting DC-side fault currents. In particular, it can be provided that fault current limit values are selected for the voltage-regulating operation of the AC / DC converter 20 that differ from those of normal operation. It can also be provided to consider both AC voltage components of the fault current and DC components of the fault current in the voltage-regulating operation of the AC / DC converter 20. The fault current detector 14 is preferably connected to the AC side of the converter 20, but can also be connected to the DC side.
[0040] Subsequently, the AC / DC converter 20 remains in voltage-regulating mode as long as the AC grid 10 remains unavailable. This is monitored in step S7. If the AC grid 10 becomes available again, normal operation is restored, and the method branches back to step S1.
[0041] To do this, the AC / DC converter 20 is stopped and the isolating element 11 is switched on again. The AC / DC converter 20 can then resume normal operation.
[0042] In addition to checking in step S7 whether the AC grid 10 is available again, a further step S8 checks whether the residual current detector 14 detects an insulation fault in the DC grid 50. If not, the AC / DC converter 20 remains in voltage-regulating mode, allowing the DC grid 50 to continue operating as a TN grid as long as the PV generators 32 or the energy storage device 43 can supply the load arrangements 60a, b.
[0043] If it is determined in step S8 that the residual current detector 14 has triggered, an emergency stop signal is issued in a subsequent step S9 to the connected devices, in this case to the AC / DC converter 20 and the DC / DC converters 30 and 40, which then stop their operation. Provision can be made for a signal to shut down the load assemblies 60a, b to be issued beforehand to allow their components to safely terminate their operation.
[0044] It can be specified that the emergency signal is only issued during voltage-regulating operation, not during normal operation, or only after a specified delay. This can have the advantage that in the event of a fault current during normal operation, the fault current occurring can be used to blow a fuse and thus locate the fault location. If necessary, the DC network 50 can continue to operate because the insulation fault has been eliminated by blowing the fuse.
[0045] As an additional safety measure, a discharge device, which may be present in the DC 50 network, can also be activated in this case in order to bring the voltage in the network down to a safe value, e.g. less than 50 V, as quickly as possible.
[0046] List of reference symbols
[0047] Alternating current supply network (AC network)
[0048] Separating organ
[0049] transformer
[0050] Switching and / or safety device
[0051] Residual current detector
[0052] Inverter (AC / DC converter)
[0053] Emergency stop input
[0054] Switching and / or safety device
[0055] DC-DC converters (DC / DC converters)
[0056] Emergency stop input
[0057] Photovoltaic generator (PV generator)
[0058] Switching and / or safety device of additional DC / DC converters
[0059] Emergency stop input
[0060] Switching and / or safety device
[0061] Energy storage
[0062] Direct current supply network (DC network)
[0063] branch
[0064] Switching and / or safety device Sub-distribution a, b Consumer arrangements a, b Switching and / or safety device
Claims
Claims 1. A 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).
2. Method according to claim 1, wherein the at least two secondary windings of the transformer (12) are supplied by the converter (20) with a voltage whose amplitude is smaller than a minimum permissible secondary voltage in normal operation of the AC power supply network (10).
3. The method of claim 2, wherein the amplitude is less than about 50 volts.
4. Method according to one of claims 1 to 3, carried out after a failure of at least one phase of the AC power supply network (10).
5. Method according to one of claims 1 to 4, wherein a status signal is output by the converter (20) which indicates the operation for applying alternating voltage to the at least one secondary winding of the transformer (12).
6. The method according to claim 5, wherein consumers in the DC power supply network (50) adjust their operating mode depending on the status signal.
7. Method according to one of claims 1 to 6, wherein in voltage-regulating operation a fault current detector (14) monitors the converter (20) for an occurring fault current and, upon detection of a fault current, outputs a signal by which the DC power supply network (50) is taken out of operation.
8. The method according to claim 7, wherein the fault current detector monitors a fault current on the AC side of the converter (20).
9. The method according to claim 7 or 8, wherein the operation of the at least one device for supplying direct current is stopped in order to decommission the direct current supply network (50).
10. The method according to claim 9, wherein a discharge device is additionally activated to reduce a voltage in the DC power supply network (50).
11. Method according to one of claims 8 to 10, wherein fault current limits are set on the fault current detector (14) in voltage-setting operation, which fault current limits differ from the fault current limits set on the fault current detector (14) in normal operation.
12. Converter (20) for converting alternating current to direct current, comprising a device for monitoring a connection to an alternating current supply network (10) via a transformer (12), characterized in that the converter (20) is designed to convert direct current to alternating current and to switch to a voltage-regulating operation when the device detects a separation from the alternating current supply network (10).
13. Converter (20) according to claim 12, which is designed 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 in particular is smaller than approximately 50 volts.
14. Converter (20) according to claim 12 or 13, comprising an AC-side fault current detector (14) which is configured to output a signal upon detection of an AC-side fault current, which signal is made available to external components.