Power converter and power supply method for transmitting power between AC and DC sides

Abstract

This application describes a power converter (10) for transferring power between an AC side (16) of the power converter (10) and a DC side (18) of the power converter (10), where the AC side (16) of the power converter (10) is connectable to a grounded three-phase AC power grid (12) and the DC side (18) of the power converter (10) is connectable to an ungrounded DC grid (14). The power converter (10) includes a bridge circuit (20), whose AC terminals (ACL1, ACL2, ACL3) are connectable to an AC side (16) of the power converter (10) via an AC switch (22), whose DC terminals (DCL+, DCL-) are connectable to a DC side (18) of the power converter (10) via circuit breakers (26.1, 26.2), whose DC link circuit is chargeable from an AC power grid (12) via a galvanically isolated AC pre-charge circuit (40), and the power converter (10) includes an isolation monitor (34) configured to measure an insulation resistance (50) of the DC side (18) of the power converter (10) when the AC pre-charge circuit (40) is connected to the DC link circuit. This application also describes a method for supplying power from a grounded three-phase AC power grid (12) to an ungrounded DC grid (14) using a transformerless power converter (10).

Description

Technical Field

[0001] A DC energy system preferably comprises at least one energy source enabling power to be available in the form of a DC current, i.e., a DC power source, such as a battery, a PV generator, or a fuel cell, and at least one load consuming power preferably in the form of a DC current, i.e., a DC sink, such as a consumer, and connections between these electrical components. The DC energy system may comprise a DC grid or a DC bus to which the electrical components are connected, and may comprise a power source, a storage, and / or a consumer. The transition between a DC bus having several connected components, such as only one power source and only one sink, and a DC grid having such a large number of components is fluid. In the present application, the term DC grid is also understood to mean a DC bus.

[0002] Such a DC grid can operate either grounded or ungrounded. Depending on the specific operating mode, in case of grounding, different monitoring and protection mechanisms may be required according to the standard.

[0003] An ungrounded DC grid in which the potentials DC+ and DC- of the respective DC lines DCL+ and DCL- do not have a fixed reference to the ground potential has the advantage that the first ground fault in the DC grid, for example, a poor insulation along one of the DC lines, still does not cause any damage. However, in order to detect the occurrence of a fault, for example, to enable the initiation of countermeasures as necessary when the first ground fault occurs, for example, to switch off the energy source, or to disconnect the energy source and / or the faulty location from the DC grid, insulation monitoring is required.

[0004] In a grounded DC grid, the potentials DC+ and DC- have a defined reference to the ground potential. Such a grounding reference can be easily implemented, for example, by a resistive connection between the ground potential and one of the DC potentials DC+ or DC-.

[0005] A DC grid may be connected via power converters to, for example, further DC grids or AC grids, such as AC power grids, and may exchange power with further energy grids, particularly to supplement or recharge the DC power supply of the DC grid. In principle, a DC grid may also be supplied with energy from further energy grids via power converters, permanently or sometimes entirely, and the energy source of the DC grid may be used to buffer power fluctuations as needed.

[0006] If the further energy grid has grounding in the form of a grounded neutral wire, for example, and the power converter is designed to be a transformerless device, i.e., there is no galvanic isolation between the AC side and the DC side, then the DC grid automatically has a fixed grounding reference via the power converter, i.e., it can be grounded. The specific position of the DC grid's potential relative to the grounding reference is predetermined in this case by the specific topology used for the power converter. For example, on the DC side, the power converter may have a split link circuit, whose center tap, i.e., intermediate potential, is connected to a neutral wire with a fixed grounding reference, and as a result, the DC potential of the link circuit is largely symmetric with respect to the ground potential.

[0007] In this regard, an ungrounded DC grid itself becomes a grounded grid through a connection to a grounded energy grid via a transformerless power converter. Therefore, this can result in two operating states for such a DC grid: an ungrounded "standalone" mode and a grounded mode when connected to a grounded energy grid. This should be taken into consideration when designing the system.

[0008] However, if a grounded DC grid is connected to a grounded AC grid, for example, via a power converter, this can result in such high, uncontrolled current flows between grids that damage components of the power converter or the grid itself. This is also likely to occur if there is an insulation fault in the ungrounded DC grid.

[0009] International Publication No. 2013 / 178654 describes DC grid insulation monitoring, which enables the measurement of the insulation resistance of a DC grid relative to the ground potential through a ground switch, using a ground resistor that connects the intermediate potential present between the switching elements of a half-bridge.

[0010] In this application, the abbreviation DC represents direct current or direct current voltage, and the abbreviation AC represents alternating current or alternating current voltage.

[0011] The object of the present invention is to disclose a power converter and method that facilitates handling or improves protection against ground currents that may flow when a DC grid is connected to a grounded AC power grid. [Overview of the Initiative]

[0012] The objective is achieved by a power converter having the features of independent claim 1. The objective is also achieved by a method having the features of independent claim 11. Advantageous embodiments of the method are claimed in the dependent claims.

[0013] In a power converter for transferring power between its AC side and DC side, the AC side of the power converter is connectable to a grounded three-phase AC power grid, and the DC side of the power converter is connectable to an ungrounded DC grid. The power converter has a bridge circuit, the AC terminal of the bridge circuit is connectable to the AC side of the power converter via an AC switch, and the DC terminal of the bridge circuit is connectable to the DC side of the power converter via a circuit breaker. The DC link circuit of the power converter is used to buffer and store energy during the operation of the power converter and may be part of the bridge circuit. The DC link circuit is chargeable from the AC power grid via a galvanically isolated AC precharge circuit. The power converter has an isolation monitor configured to measure the isolation resistance of the DC side of the power converter when the AC precharge circuit is connected to the DC link circuit.

[0014] In a bridge circuit, the AC current supplied to the terminals is converted to DC current supplied to the DC terminals, particularly through the clock drive of the semiconductor switch.

[0015] An AC precharge circuit allows the capacitors in a DC link circuit to be charged from the AC power grid when the AC side of the power converter may be disconnected from the AC power grid. The AC precharge circuit has galvanic isolation by providing a transformer to the AC precharge circuit, for example, in the form of a flyback converter. Another advantage is that the AC precharge circuit may be designed as an essential short-circuit protection circuit, thereby reducing the risk of fire in the event of a short circuit in the DC link circuit, for example.

[0016] The charging of the DC link circuit's capacitors from the AC power grid may be part of a protective concept where the DC link circuit's capacitors are charged from the AC power grid before the AC side of the power converter is connected to the AC power grid. By providing galvanic isolation to the AC precharge circuit, the DC grid connected to the DC side of the power converter can remain galvanically isolated from the AC power grid in such a situation, even if the DC link circuit's capacitors are being charged from the AC power grid, and thus may remain without a ground reference.

[0017] The isolation monitor includes measuring the isolation resistance of the DC link circuit, for example, as described in International Publication No. 2013 / 178654. Furthermore, the AC precharge circuit allows monitoring of the isolation resistance while the DC link circuit is being precharged from the AC power grid, in particular, before the AC side of the power converter is connected to the AC power grid. The isolation resistance may be monitored by an isolation monitor provided when the AC side of the power converter is disconnected from the AC power grid, the AC precharge circuit is inactive, and the DC grid is connected to the DC side of the power converter.

[0018] This enables compliance with standards that may require permanent isolation monitoring, particularly in IT systems. Specifically, the isolation resistance of the DC grid may be checked before the AC power grid is first connected to the DC grid.

[0019] In one embodiment, the AC and DC terminals of the bridge circuit are galvanically coupled, and the power transmission path between the AC and DC sides of the power converter is designed to be transformerless. This can provide cost advantages compared to a bridge circuit designed to have galvanic isolation.

[0020] One advantage of such a power converter having an AC precharge circuit is that it can be started from the AC side by the precharge process, even if it is designed to be a transformerless device and, for example, actively driven. This is particularly advantageous when starting from the DC side is undesirable or impossible, for example, if there is no energy source on the DC side, such as a solar power system or battery. In the second step of starting, a ground fault on the DC side can be checked, for example, by measuring the insulation resistance on the DC side. Once sufficient insulation on the DC side is established, it is often necessary to connect the AC side. Thus, the present invention enables starting from the AC side, especially when there is no DC power supply.

[0021] Furthermore, since the pre-charge circuit is designed to be galvanically isolated, the insulation resistance measurement will not be distorted by the pre-charge circuit.

[0022] In one embodiment, the isolation monitor is positioned between the AC side of the power converter and the AC terminal of the bridge circuit. The isolation monitor may include a grounding resistor that connects the intermediate potential between the switching elements of the half-bridge of the bridge circuit to the ground potential via a grounding switch.

[0023] In one embodiment, at least one of the AC switches functions as a ground switch. This makes it possible to reduce costs.

[0024] In one embodiment, isolation monitoring is provided by measuring fault current at the AC terminal using, for example, a DI converter.

[0025] During operation, and especially when supplying power to a DC load on a DC grid, for example, from a power converter that is a transformerless active rectifier, the ground reference for the DC grid may be the ground of the AC power grid on the AC side of the rectifier. The DC+ and DC- potentials may be, for example, symmetrical and have low resistance to ground via the connection to the AC side. The reference point for grounding in the DC grid may be advantageously selected such that a potential close to an intermediate potential is connected to the ground potential. The intermediate potential may preferably be midway between DC+ and DC-. A potential close to an intermediate potential may be achieved, for example, by symmetrically clocking the semiconductors of the bridge circuit in the middle of this bridge circuit. If the link circuit consists of a series connection of multiple capacitors, such a potential may also be provided to the center tap of this series connection, for example, through a connection to the neutral wire. This may limit the voltage of the DC+ and DC- potentials in the DC grid to ground to about half of the total DC voltage, and as a result, the isolation adjustment requirements are simplified, among other things, compared to grounding to DC+ or DC-. For example, if a DC grid is grounded to DC-, the isolation of the lines at DC+ potential must be designed for the overall system voltage relative to ground, and vice versa. This can lead to considerable costs if the system voltage is high.

[0026] When the AC power grid is disconnected from the power converter, the ground reference is lost. If the DC bus continues to operate, for example, by continuing to receive power from a DC power source on the DC bus, then ground faults need to be monitored during operation. This can be advantageously achieved through isolation monitoring using an isolation monitor.

[0027] The power converter allows, for example, the energy system to be checked by an insulation monitor each time the power converter is started. Furthermore, in order to permanently monitor the sufficient insulation state of the energy system, periodic insulation status measurements during operation are possible by measuring fault currents at the AC terminals.

[0028] In one embodiment, at least one circuit breaker having an additionally connectable precharge resistor is present for each DC terminal. In this case, the circuit breaker may be designed to have an additionally connectable precharge resistor that can be connected, in particular by bridging the circuit breaker or as a loop-shaped parallel connection composed of a semiconductor switch and a precharge resistor. By providing the precharge resistor, it becomes possible to monitor the insulation resistance using an insulation monitor when the DC grid is connected to the power converter, and the precharge resistor can protect against excessive current in this case - when a fault occurs. The DC terminals may be additionally connected individually or together, with or without initially providing the precharge resistor, and may be connected together without the precharge resistor if the insulation resistance of the DC grid is sufficiently high.

[0029] In a further embodiment, only one of the DC terminals is connected to the DC side of the power converter via a circuit breaker having a precharge resistor that can be connected in parallel. In this case, the other DC terminal is directly connected to the DC side. Thus, this provides only the monopole insulation of the DC terminal to which the DC grid is connected to the DC side, saving circuit breakers. Here, the insulation resistance when the circuit breaker is open may be determined.

[0030] In one embodiment, the power converter has a control unit configured to precharge the DC link circuit through an AC precharge circuit when the AC switch is open, connect the DC terminals to the DC grid by closing the circuit breaker, then measure the insulation resistance of the DC grid using an insulation monitor, and close the AC switch to set the voltage of the DC grid using the power converter if the insulation resistance is sufficiently high.

[0031] In one embodiment, the control unit is configured to charge the DC link circuit with a power significantly lower than the rated power of the power converter.

[0032] In one embodiment, the control unit is configured to selectively connect an additional precharge resistor to each DC line, particularly when measuring insulation resistance using an insulation monitor. This helps to avoid excessive current flow, for example, when connecting a DC link circuit to a DC grid.

[0033] In one embodiment, the power converter is configured to set the DC voltage while the DC side is connected to the DC terminal without the intervening of a precharge resistor. When the DC voltage is set, the power converter is configured to supply power to the DC side via a circuit breaker with a bridged charge resistor. This corresponds, for example, to the "normal mode" operating state of a power converter where the DC grid is powered from an AC power grid.

[0034] A method for supplying power from a grounded three-phase AC power grid to an ungrounded DC grid using a transformerless power converter, wherein the power converter comprises a bridge circuit in which its DC terminals are connectable to the DC grid by a circuit breaker and its AC terminals are connectable to the AC power grid via an AC switch, and a galvanically isolated AC precharge circuit for precharging the DC link circuit of the power converter from the AC power grid, the method being as follows: If the AC switch is open, the DC link circuit is pre-charged via the AC pre-charge circuit. The steps include: connecting the DC terminal to the DC grid by closing the circuit breaker, Subsequently, the insulation resistance of the DC grid is measured using an insulation monitor. If the insulation resistance is sufficiently high: the step of closing the AC switch, The procedure includes the step of setting the voltage of a DC grid using a power converter.

[0035] If the insulation resistance is too low, the power converter may be shut down for safety reasons.

[0036] The DC link circuit is preferably precharged at low power, i.e., at a power considerably lower than the power rating of the power converter. The AC precharge circuit is designed to be galvanically isolated, for example by providing a flyback converter in the AC precharge circuit, so that no potential reference is created between the DC link circuit and the AC power grid. The precharge resistor is preferably designed to have high ohm resistance to avoid excessively high fault currents in the event of a failure.

[0037] In one embodiment, the method further includes the step of connecting at least one DC terminal to a DC grid via a precharge resistor. In this embodiment, the precharge resistor is then considered when measuring the insulation resistance using an insulation monitor.

[0038] In one embodiment of the method, the step of measuring insulation resistance includes a first step of connecting a first DC terminal to a DC grid via a first precharge resistor, and a second DC terminal different from the first DC terminal to a DC grid via a second precharge resistor, wherein the insulation resistance is determined from the measurements obtained in the first and second steps.

[0039] In one embodiment of the method, the voltage of the DC grid is set by a power converter while the DC grid is connected to the DC terminal without the interposition of a precharge resistor. This corresponds, for example, to the "normal mode" operating state of a power converter where the DC grid is powered from an AC power grid.

[0040] In one embodiment, when the AC switch is closed, at least one DC terminal is connected to the DC grid via a precharge resistor. Thus, in this embodiment, the connection to the AC power grid is made at least via a precharge resistor of at least one of the DC terminals. This makes it possible to avoid excessive current flow when connecting the DC grid to the DC link circuit.

[0041] In one embodiment, the method is as follows: In the event of an AC power grid failure: This further includes the step of opening the AC switch and keeping the DC grid running via an energy source connected to the DC grid, The insulation resistance of the DC grid is measured continuously or repeatedly using an insulation monitor during continuous operation.

[0042] As an optional step, if an insulation fault, i.e., an excessively low insulation resistance of the DC grid to ground potential, is detected, the DC grid can be disconnected by opening a circuit breaker. If an insulation fault is detected, a fault signal is generated and transmitted in an appropriate manner.

[0043] In one embodiment of the method, after the AC switch is closed, i.e., when the AC power grid is connected to the DC grid and the DC grid is powered by the AC power grid, isolation monitoring of the DC grid may be performed by measuring fault currents at the AC terminals, for example, by a DI converter. This corresponds, for example, to the "normal mode" of the power converter.

[0044] In one embodiment of the method, the circuit breaker may be opened before the AC switch is closed. Then, when the AC switch is closed, the power converter may match the DC voltage on its DC side to the DC grid voltage, optionally balance the DC voltage with respect to ground potential, and then close the circuit breaker to connect to the DC grid and supply power from the AC power grid to the DC grid via the power converter. This makes it possible to reduce the inrush current when connecting the DC grid to the AC power grid.

[0045] The power converter and method, in particular via a transformerless power converter, enable variable connection of a DC energy system to a grounded AC power grid, while simultaneously making protective concepts available for such operation.

[0046] The present invention will be described in more detail below with reference to the drawings. [Brief explanation of the drawing]

[0047] [Figure 1] An exemplary embodiment of a power converter is schematically shown. [Figure 2] An exemplary embodiment of the method is schematically shown. [Modes for carrying out the invention]

[0048] Figure 1 schematically shows one embodiment of a power converter 10 having an AC side 16 and a DC side 18. A three-phase AC power grid 12, grounded at ground potential PE, is connected to the AC side 16. A DC grid 14 is connected to the DC side 18. The DC grid 14 has an insulation resistance 50 with respect to ground potential PE. A battery 42 can be connected to the DC grid 14 via a DC switch 46. The battery 42 may have an undesirable parasitic resistance 42.P with respect to ground potential PE. A load 44 can be connected to the DC grid via a DC switch 48. The load 44 may have an undesirable parasitic resistance 44.P with respect to ground potential PE. The load 44 may, in particular, comprise one or more consumers such as machinery, industrial plants, or electrolytic cells.

[0049] The power converter 10 has a bridge circuit 20 designed to convert the AC current or AC voltage of AC terminals ACL1, ACL2, ACL3 to the DC current or DC voltage of DC terminals DCL+, DCL-. Similarly, the bridge circuit 20 is designed to convert the DC current or DC voltage of DC terminals DCL+, DCL- to the AC current or AC voltage of AC terminals ACL1, ACL2, ACL3. In the illustrated exemplary embodiment, the conversion is performed by a control unit 30 that drives the semiconductor switches of the bridge circuit 20 in an appropriate manner. The bridge circuit 20 with semiconductor switches is generally a transformerless circuit, i.e., the AC terminals ACL1, ACL2, ACL3 and DC terminals DCL+, DCL- of such a bridge circuit are galvanically coupled. If the AC power grid has a grounding reference provided, for example, by a grounded neutral wire, the DC terminals DCL+, DCL- are therefore usually automatically assigned the grounding reference via the bridge circuit 20. In this case, the specific position of the potential of the DC terminals relative to the ground reference (from DC+ / DC- to PE) is predetermined by the topology particularly used in the bridge circuit 20. For example, the bridge circuit 20 may have a split DC link circuit on the DC side, the center tap of which is connected to the neutral wire of the AC power grid 12 at a fixed ground reference PE as an intermediate potential, and as a result the DC potential in the DC link circuit is set to be approximately symmetric with respect to the ground potential PE. In this respect, a DC grid 14 that is not grounded itself also becomes a grounded DC grid 14 through a connection to a grounded AC power grid 12 via a transformerless power converter 10. Thus, the DC grid 14 may have two operating states: an ungrounded "standalone mode" with power from the battery 42, and a grounded mode when connected to a grounded AC power grid 12.

[0050] The DC link circuit of the bridge circuit 20 should generally be precharged when the power converter 10 is started up, before the AC side 16 or DC side 18 is connected to the corresponding grids 12, 14. This is desirable, for example, to limit the charge current of the capacitor in the DC link circuit when it is first connected to the DC grid 14. The AC precharge circuit 40 of the power converter 10 is configured to perform such precharging from the AC power grid 12 and has galvanic isolation for this purpose. The DC link circuit of the bridge circuit 20 can be precharged directly from the AC power grid 12 via the AC precharge circuit 40. For this purpose, the AC side of the AC precharge circuit 40 is connected to the AC side 16 of the inverter 10. The AC precharge circuit is driven by the control unit 30. A semiconductor switch 38 is connected between the rectifier 36 and the DC terminal DCL-. The semiconductor switch 38 is also driven by the control unit 30.

[0051] The power converter 10 also has a fault current measurement value 32 based on differential current measurement values ​​of the three AC terminals ACL1, ACL2, and ACL3.

[0052] AC connections ACL1, ACL2, and ACL3 are connectable to AC side 16 via AC switch 22. An isolation monitor 34 is provided, which can be connected to the midpoint potential of the half-bridge of the bridge circuit 20 via ground switch 22.E. The isolation monitor 34 may be connected to the ground potential PE via ground switch SE. When connected, the isolation monitor 34 functions to monitor the isolation resistance 50 of the DC grid. For this purpose, the isolation monitor 34 has a ground resistance RE through which the midpoint potential is connected to the ground potential. Such an isolation monitor 34 is described, for example, in International Publication No. 2013 / 178654. In one embodiment shown in Figure 2, the isolation monitor may also have only one ground switch 22.E, which simultaneously represents part of a switchable connection between AC terminal ACL3 and AC side 16.

[0053] DC terminal DCL+ is connectable to DC side 18 via circuit breaker 26.1. DC terminal DCL- is connectable to DC side 18 via circuit breaker 26.2. An additional connectable precharge resistor 24.1 is provided for DC terminal DCL+. An additional connectable precharge resistor 24.2 is provided for DC terminal DCL-. Circuit breakers 26.1 and 26.2 having additional connectable precharge resistors 24.1 and 24.2 are designed to be connectable by bridging the circuit breakers 26.1 and 26.2, or as a loop-shaped parallel connection consisting of semiconductor switches 24.1 and 24.2 and precharge resistors 28.1 and 28.2.

[0054] Unlike the two-pole embodiment of the circuit breaker configuration shown in Figure 1 and described above, Figure 2 also shows a single-pole embodiment of the circuit breaker configuration. In this case, the DC terminal DCL- is directly connected to the DC side, i.e., it does not have a circuit breaker.

[0055] Figure 3 schematically illustrates a method for supplying power from a grounded three-phase AC power grid 12 to an ungrounded DC grid 14 using a transformerless power converter 10. In step S1, the DC link circuit is charged from the AC power grid 12 by the AC precharge circuit 40 when the AC switch 22 is open. In step S2, the DC terminals DCL+ and DCL- are connected to the DC grid 14 by closing circuit breakers 26.1 and 26.2. Then, in step S3, the insulation resistance 50 of the DC grid 14 is measured using an insulation monitor 34, and if the insulation resistance 50 is sufficiently high, the AC switch 22 is closed in step S4, and the voltage of the DC grid 14 is set by the power converter 10 in step S5. If, in step S3, it is determined that the insulation resistance 50 is not sufficiently high, i.e., the DC grid 14 does not have sufficiently high insulation against the ground potential PE, then step S3 is repeated.

[0056] The insulation resistance 50 is measured in step S3 with the switch 22.E closed, in order to connect the insulation monitor 34 to the midpoint potential of the bridge circuit 20.

[0057] Optionally, in step S2, at least one DC terminal DCL+, DCL- may be connected to the DC grid 14 via precharge resistors 28.1, 28.2. By providing precharge resistors 28.1, 28.2, when the DC grid 14 is connected to the power converter 10, it is possible to monitor the insulation resistance 50 using the insulation monitor 34 in step S2, and the precharge resistors 28.1, 28.2 can protect against overcurrent in this case—if a fault occurs. The DC terminals DCL+, DCL- may initially be connected individually and then connected together if the insulation resistance 50 of the DC grid 14 is sufficiently high. [Explanation of Symbols]

[0058] 10 Power Converters 12 AC power grids 14 DC grid 16 AC side 18 DC side 20 Bridge Circuits 22 AC switches 22.E Grounding switch 24.1, 24.2 switches 26.1, 26.2 Circuit breakers 28.1, 28.2 Precharge resistors 30 Control Units 32. Fault current measurement values 34. Insulation Monitor 36. Rectifier for AC precharge circuit 38 Semiconductor switches 40 AC Precharge Circuit 42 batteries 42.P Parasitic resistance 44 load 44.P Parasitic resistance 46 DC switches 48 DC switches 50 Insulation resistance ACL1, ACL2, ACL3 AC terminals DCL+, DCL- DC terminals PE ground potential RE earthing resistor SE Grounding Switch S1, S2, S3, S4, S5 Method Steps

Claims

1. A power converter (10) for transmitting power between the AC side (16) and the DC side (18) of the power converter (10), wherein the AC side (16) of the power converter (10) is connectable to a grounded three-phase AC power grid (12), and the DC side (18) of the power converter (10) is connectable to an ungrounded DC grid (14), the power converter (10) comprises a bridge circuit (20), the AC terminals (ACL1, ACL2, ACL3) of the bridge circuit (20) are connectable to the AC side (16) of the power converter (10) via an AC switch (22), and the DC terminals (DCL+, DCL-) of the bridge circuit (20) are connectable to the DC side (18) of the power converter (10) via circuit breakers (26.1, 26.2). The DC link circuit of the power converter (10) is chargeable from the AC power grid (12) via a galvanic isolation AC precharge circuit (40), and the power converter (10) has an isolation monitor (34) configured to measure the insulation resistance (50) of the DC side (18) of the power converter (10) when the AC precharge circuit (40) is connected to the DC link circuit. The control unit (30) When the AC switch (22) is open, the DC link circuit is pre-charged through the AC pre-charge circuit (40). By closing the circuit breakers (26.1, 26.2), the DC terminals (DCL+, DCL-) are connected to the DC grid (14). Subsequently, the insulation resistance (50) of the DC grid (14) is measured using the insulation monitor (34). A power converter characterized in that, when the insulation resistance (50) is sufficiently high, the AC switch (22) is closed and the power converter (10) is used to set the voltage of the DC grid.

2. The power converter according to claim 1, wherein the AC terminals (ACL1, ACL2, ACL3) and DC terminals (DCL+, DCL-) of the bridge circuit (20) are galvanically coupled, and the power transmission path between the AC side (16) and the DC side (18) of the power converter (10) is specifically designed to be a transformerless circuit.

3. The power converter according to claim 1 or 2, wherein the insulation monitor (34) is positioned between the AC side (16) of the power converter (10) and the AC terminals (ACL1, ACL2, ACL3) of the power converter (10).

4. The power converter according to claim 3, characterized in that the insulation monitor (34) has a grounding resistor (RE) that connects the potential between the switching elements of the half-bridge of the bridge circuit (20) to the ground potential (PE) via a grounding switch (SE, 22.E).

5. The power converter according to claim 4, characterized in that at least one of the AC switches (22) functions as a ground switch (22.E).

6. The power converter according to any one of claims 1 to 5, characterized in that there is at least one circuit breaker (26.1, 26.2) having additionally connectable precharge resistors (28.1, 28.2) for each DC terminal (DCL+, DCL-).

7. The power converter according to any one of claims 1 to 6, characterized in that at least one circuit breaker (26.1, 26.2) having an additionally connectable precharge resistor (28.1, 28.2) is present at exactly one of the DC terminals (DCL+, DCL-).

8. The power converter according to any one of claims 1 to 7, characterized in that insulation monitoring is provided by fault current measurement values ​​(32) at the AC terminals (ACL1, ACL2, ACL3).

9. The power converter according to claim 8, characterized in that the control unit (30) is configured to charge the DC link circuit with a power significantly lower than the rated power of the power converter (10).

10. The power converter (10) is configured to set the voltage of the DC side (18) while the DC side (18) is connected to the DC terminals (DCL+, DCL-) without the interposition of precharge resistors (28.1, 28.2), as described in claim 8 or 9.

11. A method for supplying power from a grounded three-phase AC power grid (12) to an ungrounded DC grid (14) using a transformerless power converter (10) equipped with a bridge circuit (20), wherein the DC terminals (DCL+, DCL-) of the bridge circuit (20) are connectable to the DC grid (14) by circuit breakers (26.1, 26.2), the AC terminals (ACL1, ACL2, ACL3) of the bridge circuit (20) are connectable to the AC power grid (12) via an AC switch (22), the power converter (10) includes a galvanically isolated AC precharge circuit (40) for precharging the DC link circuit of the power converter (10) from the AC power grid (12), and the method is as follows: When the AC switch (22) is open, the DC link circuit is pre-charged through the AC pre-charge circuit (40), The steps include: connecting the DC terminals (DCL+, DCL-) to the DC grid (14) by closing the circuit breakers (26.1, 26.2); Subsequently, the insulation resistance (50) of the DC grid (14) is measured using an insulation monitor (34), wherein the insulation monitor (34) is positioned between the AC power grid (12) and the AC terminals (ACL1, ACL2, ACL3), and the insulation monitor (34) has a grounding resistor (RE) that connects the potential between the switching elements of the half-bridge of the bridge circuit (20) to the ground potential (PE) via a grounding switch (SE, 22.E), and the measurement step is as follows: If the insulation resistance (50) is sufficiently high: a method comprising the steps of closing the AC switch (22) and setting the voltage of the DC grid (14) using the power converter (10).

12. The method according to claim 11, wherein at least one DC terminal (DCL+, DCL-) is connected to the DC grid (14) via precharge resistors (28.1, 28.2).

13. The method according to claim 12, wherein the step of measuring the insulation resistance (50) includes a first step of connecting a first DC terminal (DCL+, DCL-) to the DC grid (14) via a first precharge resistor (28.1, 28.2), and a second step of connecting a second DC terminal (DCL+, DCL-) different from the first DC terminal to the DC grid (14) via a second precharge resistor (28.1, 28.2), and the insulation resistance (50) is determined from the measurements obtained in the first and second steps.

14. The method according to any one of claims 12 or 13, wherein the voltage of the DC grid (14) is set by the power converter (10) while the DC grid (14) is connected to the DC terminals (DCL+, DCL-) without the precharge resistors (28.1, 28.2) interposed therebetween.

15. The method according to any one of claims 11 to 14, wherein when the AC switch (22) is closed, at least one DC terminal (DCL+, DCL-) is connected to the DC grid (14) via precharge resistors (28.1, 28.2).

16. The method according to any one of claims 11 to 15, further comprising the step of opening the AC switch (22) and continuing the operation of the DC grid (14) via an energy source (42) connected to the DC grid (14) in the event of a failure of the AC power grid (12), wherein the insulation resistance (50) of the DC grid (14) is measured continuously or repeatedly using the insulation monitor (34) during continuous operation.

17. The method according to any one of claims 11 to 16, wherein after the AC switch (22) is closed, insulation monitoring of the DC grid (14) is performed by fault current measurement values ​​(32) of the AC terminals (ACL1, ACL2, ACL3).

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