DE102020129918A1 - Device and method for grounding a DC voltage network - Google Patents

Abstract

The application describes an electrical system with a power converter (10) for power transfer between an AC side (16) of the power converter (10) and a DC side (18) of the power converter (10), the AC side (16) of the Converter (10) can be connected to a grounded three-phase AC supply network (12) and the DC side (18) of the converter (10) can be connected to an ungrounded DC network (14). The power converter (10) has a bridge circuit (20) whose AC connections (ACL1, ACL2, ACL3) can be connected to the AC side (16) of the power converter (10) and whose DC connections (DCL+, DCL-) can be connected to the DC side (18) of the power converter (10), the power converter (10) being designed in such a way that a ground reference through a connected AC supply network (12) leads to a ground reference on its DC side (18). . The electrical system has a grounding circuit connected to the DC grid (14) and a control unit (30), the control unit (30) being set up to detect and detect a disconnection of the power converter (10) from the AC supply grid (12). the grounding circuit establishes a ground reference for the DC network (14). The application also describes a method for operating an electrical system.

Description

Technical field of the invention

State of the art

A DC energy system comprises at least one energy source that preferably makes electrical power available as direct current, i.e. a DC source, e.g. a battery, a PV generator or a fuel cell, and at least one load that preferably consumes electrical power as direct current. i.e. a DC sink, e.g. B. a consumer, and connections between these electrical components. The DC power system may include a DC grid or bus where the electrical components are connected and may include other sources, storage, and/or loads. The transition between a DC bus with a few connected components, for example only one source and one sink, and a DC network with a large number of such components is fluid. In this application, the term DC network is also understood to mean a DC bus.

Such a DC network can be grounded or ungrounded. Depending on the specific operating mode, different monitoring and protection mechanisms may be required by standards in the event of a ground fault.

An unearthed DC network, in which the potentials DC+ and DC- of the respective DC lines DCL+ and DCL- have no fixed reference to the earth potential, has the advantage that any first earth fault in the DC network, for example an insulation fault along one of the DC lines, still no harm. However, insulation monitoring is required in order to detect the occurrence of any fault and, if necessary, to be able to initiate countermeasures in the event of a first ground fault, e.g. switch off the energy source or disconnect the energy source and/or the fault location from the DC network.

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

A DC network can be connected to another energy network, e.g. B. another DC network or an AC network, for example an AC supply network, and exchange electrical power with the other energy network, in particular to support or recharge the DC source in the DC network. In principle, the DC network can also be supplied permanently or at times completely with energy from the other energy network via the converter, with the energy source of the DC network being able to be used to buffer power fluctuations if necessary.

If the further energy network is grounded, e.g. in the form of a grounded neutral conductor and the power converter is designed without a transformer, i.e. has no galvanic isolation between the AC and DC side, the DC network can automatically be provided with a fixed earth reference via the power converter, i.e. grounded. The concrete position of the potentials of the DC network relative to the ground reference is specified by the concrete topology of the power converter used. For example, on the DC side, the converter can have a divided intermediate circuit whose midpoint, ie, middle potential, is connected to a neutral conductor with a fixed ground reference, so that the DC potentials on the intermediate circuit are largely symmetrical about ground potential.

In this respect, a DC network that is not grounded per se also becomes a grounded network through a connection to a grounded energy network via a transformerless converter. There can therefore be two operating states for such a DC network, unearthed "stand-alone" operation and grounded operation when connected to the grounded energy network. This must be taken into account when designing the system.

However, if a grounded DC network is connected to a grounded AC network, for example via a power converter, such high, uncontrolled current flows between the networks can occur that components of the power converter or components of the networks are damaged. This also applies if there is an insulation fault in a supposedly unearthed DC network.

In this application, the abbreviation DC stands for direct current or direct current and AC (alternating current) for alternating current or alternating voltage.

object of the invention

The invention is based on the object of demonstrating an electrical system and a method that enable safe operation of a DC energy system.

solution

The object is solved by an electrical system having the features of independent patent claim 1 . The task is completed by a Ver drive with the features of independent claim 8 solved. Advantageous embodiments of the method are claimed in the dependent claims.

Description of the invention

In an electrical system having a power converter for transferring power between an AC side of the power converter and a DC side of the power converter, the AC side of the power converter is connectable to a grounded three-phase AC utility grid and the DC side of the power converter is connected to an ungrounded DC -Mains connectable. The power converter has a bridge circuit whose AC connections can be connected to the AC side of the power converter and whose DC connections can be connected to the DC side of the power converter, the power converter being designed in such a way that a ground reference is provided by a connected AC supply network leads to an earth reference on its DC side. The electrical system has a grounding circuit connected to the DC grid and a control unit, the control unit being set up to detect a disconnection of the power converter from the AC supply grid and to establish a ground reference for the DC grid via the grounding circuit.

The direct ground reference of the DC network can therefore be switched on or off, depending on whether an indirect ground reference is established via the AC supply network via the converter or not. On the one hand, this can ensure that the DC network always has a ground reference, in particular a low-impedance ground reference, and that the potentials of the DC lines are always defined. The constant ground reference can be established either - indirectly - via the converter from the AC supply network or - directly - via the grounding circuit. The constant reference to earth can be desirable for safety reasons. On the other hand, a double ground reference can be avoided, which could lead to unwanted equalizing currents.

This enables the flexible operation of a DC network while fulfilling normative requirements. If a DC system is operated via the converter on the grounded AC supply system, e.g. on a TN system or a TT system as an AC supply system, the earth reference of the DC system can be defined via the converter, in particular by It is a transformerless power converter whose AC and DC connections are galvanically coupled, at least during operation. In this operating state, the DC network can be supplied with electrical power from the AC supply network via the converter. A DC source, e.g. B. Battery, in the DC grid can be used as a support and / or charged from the AC supply grid.

In the event that the power converter is disconnected from the AC supply network, e.g. B. by an AC network error, for reasons of device protection or the like, the electrical system according to the invention advantageously allows safe continued operation. is e.g. B. an energy store in the form of a DC source such. B. a battery, present in the DC network, the DC network can continue to be operated by being supplied with electrical power from the energy store. However, the ground reference of the DC grid is possibly no longer defined via the converter due to the separation of the converter from the AC supply grid and an isolated DC grid could form. This is undesirable because the DC voltages against ground potential in the DC network are no longer clearly defined. If necessary, establishing a direct ground reference via the grounding circuit can therefore advantageously and in accordance with the standard enable flexible continued operation of the DC network, since it can establish an alternative ground reference for the DC network in a defined manner when the converter is disconnected from the AC supply network. This is also advantageous compared to solutions in which z. B. via a voltage divider with high-impedance resistors between the DC lines and ground potential, a high-impedance earthing is established and a passive balancing of the DC network is effected by, particularly in the case of extensive DC networks with low parasitic conductor-earth resistances and/or a large number Comparatively fewer leakage currents occur at balancing resistors, less corrosion takes place and tolerances or aging of the balancing resistors are irrelevant.

In one embodiment of the electrical system, the control unit of the electrical system is set up to establish the ground reference of the DC network by driving a ground switch, via which the ground circuit is connected to a ground potential. For this purpose, the control unit is in communication with the grounding circuit and in particular with the grounding switch. The communication link can e.g. B. be wired or wireless.

In one embodiment of the electrical system, the grounding circuit has a DC/DC converter with a reference potential, it being possible to connect the reference potential to the ground potential by closing the grounding switch. The DC/DC controller is set up to establish the earth reference of the DC network via the connection of the reference potential to the earth potential via the earth switch. In particular, a midpoint of a half-bridge of the DC/DC converter can be the reference potential and be connectable to the earth potential via the earth switch. This allows the potentials of the DC conductors to be set symmetrically around the ground potential or to be kept there after the power converter has set these potentials symmetrically as long as it was connected to the grounded AC supply network. An otherwise impending asymmetry of the DC voltages compared to the ground potential after the indirect ground reference via the converter has been removed can cause adverse effects and, for example, lead to the limits for clearance and creepage distances in the generators and consumers in the DC network being exceeded and these being damaged . The electrical system is particularly advantageous compared to z. B. a "hard" grounding on one of the DC conductors of the DC network, because with a "hard" grounding on one of the DC conductors, voltages to ground on the DC conductors are about twice as high, which means that the insulation coordination is correspondingly more complex require and/or prevent the use of existing equipment and facilities.

In one embodiment of the electrical system, the DC/DC converter is connected to an electrical storage unit, e.g. B. DC source, connectable and set up to transfer electrical power between the storage unit and the DC grid. The control unit is set up to control the DC/DC controller in such a way that when the bridge circuit is detected as being separated from the AC supply network, the DC network can be operated via the DC/DC controller from the storage unit by using the Network required electrical power alternatively taken from the storage unit and the necessary ground reference of the DC network is alternatively established via the grounding circuit.

In one embodiment of the electrical system, the DC/DC converter is set up to supply the DC grid with DC power, with the control unit being set up to generate a DC voltage in the DC grid, e.g. B. via a measuring device, and to control the DC / DC controller so that the DC voltage is balanced by the DC potentials of the DC voltage are set symmetrically about the reference potential of the DC / DC controller. The reference potential of the DC/DC converter preferably has a connection to ground potential via the grounding circuit. This then leads to a balancing of the DC voltage around the ground potential.

Alternatively or additionally, the control unit can be set up, another DC / DC controller, z. B. can be located in the converter so that the DC voltage is balanced via the other DC / DC controller.

In one embodiment of the electrical system, the control unit is set up to detect a fault current by means of a fault current measurement and to disconnect the ground reference of the DC network when a fault current is detected. The error current measurement is preferably carried out by means of a residual current measurement at the AC terminals of the bridge circuit. The fault current measurement can be active in particular when the AC switches are open or closed, i.e. when the AC supply network is disconnected from or connected to the bridge circuit. The fault current measurement can also be carried out by monitoring the insulation of the DC network. For example, in one embodiment, the DC network can also be grounded in the event of a fault via a fuse, e.g. B. GFDI (ground fault detection interruption) can be provided. This serves to avoid impermissibly high leakage currents, e.g. in the event of insulation faults in the DC network.

• • Erfassen einer Trennung der Brückenschaltung vom AC-Versorgungsnetz und
• • bei erkannter Trennung: Herstellen eines Erdbezuges des DC-Netzes über die Erdungsschaltung.

A method of operating the electrical system includes the steps:

• • detecting a disconnection of the bridge circuit from the AC utility grid and
• • If disconnection is detected: establishment of a ground reference for the DC network via the grounding circuit.

If it is recognized that the bridge circuit is still connected to the AC supply network, the DC network itself remains ungrounded and in this respect retains its ground reference indirectly via the ground reference of the AC supply network via the converter.

The method thus enables flexible and safe operation of a DC network. The ground reference can be established flexibly via the converter and the grounded AC supply network or - if there is no connection to the grounded AC supply network - using the grounding circuit.

In one embodiment of the method, the ground reference of the DC network is established by closing the ground switch. This enables a particularly simple establishment of the ground reference. The control unit preferably controls the earth switch in order to cause it to close.

In one embodiment of the method, the closing of the grounding switch creates a connection between a reference potential of a DC/DC converter, the grounding circuit and a ground potential, with the DC/DC converter being designed in such a way that the reference potential is connected via the grounding switch to the ground potential establishes the ground reference of the DC network.

In one embodiment of the method, the DC network is supplied with electrical DC power via the DC/DC controller, with the DC/DC controller preferably setting the DC voltage of the DC network symmetrically about the reference potential, in particular when the bridge circuit is disconnected from the AC supply network. In one embodiment, the DC grid is supplied with electrical power from an electrical energy store connected to the DC/DC controller, e.g. B. in the form of a DC source. The DC/DC controller is connected to the electrical energy store on one of its two sides and is able to set a DC/DC output voltage on the other side. If the DC network is supplied with electrical power via the DC/DC controller and the bridge circuit is disconnected from the AC supply network, the DC/DC output voltage corresponds to the DC voltage of the DC network.

• • Erfassen eines Wiederverbindens der Brückenschaltung mit dem AC-Versorgungsnetz und
• • bei Wiederverbinden: Aufhebung des Erdbezugs des DC-Netzes über die Erdungsschaltung durch Öffnen des Erdschalters. Der Erdbezug des DC-Netzes ist wieder über die AC-Seite, d. h. das AC-Versorgungsnetz, gegeben. Bevorzugt steuert die Steuereinheit den Erdschalter an, um sein Schließen zu bewirken.

An embodiment of the method has the steps:

• • detecting a reconnection of the bridge circuit to the AC utility grid and
• • When reconnecting: Cancellation of the ground reference of the DC network via the ground circuit by opening the ground switch. The ground reference of the DC grid is again given via the AC side, ie the AC supply grid. The control unit preferably controls the earth switch in order to cause it to close.

If it is detected that the bridge circuit is still disconnected from the AC supply network, the DC network itself remains grounded via the grounding circuit.

character list

• 1 zeigt schematisch ein Ausführungsbeispiel eines elektrischen Systems;
• 2 zeigt schematisch ein Ausführungsbeispiel eines Verfahrens.

The invention is explained in more detail below with the aid of figures.

• 1 shows schematically an embodiment of an electrical system;
• 2 shows schematically an embodiment of a method.

character description

In 1 an embodiment of an electrical system is shown schematically. A power converter 10 has an AC side 16 and a DC side 18 . A three-phase AC supply network 12 grounded at ground potential PE is connected to the AC side 16 . A DC network 14 is connected to the DC side 18 . The DC grid 14 has a battery 42 , a load 44 and a DC/DC converter 40 . The load 44 can be connected to the DC grid via a DC switch 48 . The load 44 can in particular include one or more consumers such. B. a machine, an industrial plant, or an electrolyser. The battery 42 and the load 44 are connected to the power converter 10 via DC lines DC+, DC-.

The power converter 10 has a bridge circuit 20 which is designed to convert alternating current or alternating voltage at AC terminals ACL1, ACL2, ACL3 into direct current or direct voltage at DC terminals DCL+, DCL+. The bridge circuit 20 is also designed to convert direct current or direct voltage at the DC terminals DCL+, DCL+ into alternating current or alternating voltage at the AC terminals ACL1, ACL2, ACL3. In the exemplary embodiment shown, the conversion takes place in that a control unit 30 suitably controls the semiconductor switch of the bridge circuit 20 . The bridge circuit 20 with semiconductor switches is usually designed without a transformer, i. H. the AC connections ACL1, ACL2, ACL3 and the DC connections DCL+, DCL- of such a bridge circuit are galvanically coupled. If the AC supply network has an earth reference, e.g. B. by a grounded to PE neutral conductor, the DC connections DCL +, DCL- are thus automatically grounded via the bridge circuit 20 in regular operation of the power converter 10 .

The DC lines DC+, DC- are connected to the DC terminals DCL+, DCL-. The specific position of the potentials of the DC lines relative to the ground reference (DC+/DCagainst PE) is specified by the topology of the bridge circuit 20 that is specifically used. For example, the bridge circuit 20 can have a DC-side, divided DC intermediate circuit, the center point of which is connected to the neutral conductor of the AC supply network 12 with a fixed earth reference PE as the middle potential, so that the DC potentials in the DC intermediate circuit and thus to the Adjust the DC connections DCL+, DCL- largely symmetrically around the earth potential PE. In this respect, a DC network 14 that is not grounded per se also becomes a grounded DC network 14 through a connection to the grounded AC supply network 12 via the transformerless converter 10. It can thus be a grounded operation of the DC network 14 when connected to the grounded AC utility grid 12.

The AC connections ACL1, ACL2, ACL3 can be connected to the AC side 16 via the AC switch 22. The power converter 10 also has a fault current measurement 32 at the AC connections ACL1, ACL2, ACL3, which is based on a residual current measurement of the three AC connections ACL1, ACL2, ACL3 based. About the fault current measurement 32 z. B. a malfunction in the DC network can be detected.

The DC/DC converter 40 is arranged between the battery 42 and DC switches 46 . The DC/DC converter 40 can be connected to the DC grid 14 via the DC switches 46 and is set up to set a DC/DC output voltage on an output side of the DC/DC converter 40 . The DC/DC output voltage can be set, for example, by the control unit 30 driving semiconductor switches of the DC/DC converter 40 . An input side of the DC/DC converter 40 is connected to the terminals of the battery 42 . From the battery 42, the DC network 14, z. B. be supplied with electrical power when the power converter 10 is separated from the AC supply network 12 by the DC / DC converter 40 of the battery 42 draws a corresponding electrical power and feeds it into the DC network 42 with a suitable DC / DC output voltage .

The DC/DC converter 40 is part of a grounding circuit. The grounding circuit also includes a grounding switch 24 . A reference potential of the DC/DC converter 40 can be connected to ground potential PE via the ground switch 24 . When the grounding switch 24 is closed, the DC network 14 can be grounded via the grounding circuit. The grounding switch 24 can be controlled by the control unit 30 and can be opened and closed by a control signal from the control unit 30 .

The DC connections DCL+, DCL- of the bridge circuit 20 can be connected to the DC side 18 via isolating switches 26.1, 26.2. The DC network 14 has DC lines DC+ and DC-. A measuring device 38 is set up to measure the DC voltage U _DC between the two DC lines DC+, DC- of the DC network 14 . With the isolating switches 26.1, 26.2 closed, the DC voltage U _DC can be specified by the power converter 10 and, in particular, can correspond to the DC output voltage of the bridge circuit 20. When the DC switches 46 are closed and the bridge circuit 20 is disconnected from the AC supply network 12, the DC voltage U _DC can be set by the DC/DC controller 40 and, in particular, correspond to the DC/DC output voltage of the DC/DC controller 40 . The DC voltage U _DC can be determined on the respective DC lines DC+, DC- against earth potential PE. This means that each of the DC lines DC+, DC can be measured individually against ground potential PE.

In 2 an exemplary embodiment of a method for operating the electrical system is shown schematically.

In a step S1 it is checked whether the AC switches 22 are open - branch "yes" - or not - branch "no". If the AC switches are closed, then the AC connections ACL1, ACL2, ACL3 of the bridge circuit 20 are connected to the AC side 16 of the converter 10 and the DC network 14 is grounded via the converter 10. If the AC switches are open, the AC connections ACL1, ACL2, ACL3 of the bridge circuit 20 are separated from the AC side 16 of the converter 10 and the DC network 14 does not receive a ground reference via the converter 10. In step S1, if necessary be checked whether the ground reference of the DC network 14 has been omitted via the AC side 16 for other reasons.

If the DC network 14 is not grounded via the converter 10, the ground switch 24 is closed in a step S2 and the DC network 14 is thus grounded.

In a step S3, the DC voltage U _DC is balanced by the DC/DC converter 40 around the ground potential PE as the reference potential. The DC/DC output voltage can be set on the output side of the DC/DC controller 40 and corresponds to the DC voltage U _DC when the DC switches 46 are closed and the bridge circuit 20 is disconnected from the AC supply network 12 .

Optionally, the method can include the steps of detecting a reconnection of the bridge circuit 20 to the AC supply network 12 and removing the ground reference of the DC network 14 via the grounding circuit by opening the grounding switch 24 when a reconnection is detected. In this detected state, the ground reference of the DC network 14 is again via the AC side 16, i. H. the AC utility grid 12.

Reference List

10

power converter

12

AC utility grid

14

DC grid

16

AC side

18

DC side

20

bridge circuit

22

AC switch

24

earth switch

26.1, 26.2

circuit breaker

30

control unit

32

residual current measurement

38

tension meter

40

DC/DC controller

42

battery

44

load

46

DC switch

48

DC switch

ACL1, ACL2, ACL3

AC connection

DCL+, DCL-

DC connection

DC+, DC-

DC line

PE

earth potential

S1, S2, S3

process steps

U.D.C

DC voltage

Claims (12)

Electrical system with a power converter (10) for power transfer between an AC side (16) of the power converter (10) and a DC side (18) of the power converter (10), the AC side (16) of the power converter (10) can be connected to a grounded three-phase AC supply network (12) and the DC side (18) of the converter (10) can be connected to an ungrounded DC network (14), the converter (10) having a bridge circuit (20), whose AC connections (ACL1, ACL2, ACL3) can be connected to the AC side (16) of the converter (10) and whose DC connections (DCL+, DCL-) can be connected to the DC side (18) of the converter (10) are connectable, wherein the power converter (10) is designed in such a way that a ground reference through a connected AC supply network (12) leads to a ground reference on its DC side (18), wherein the electrical system has a grounding circuit connected to the DC grid (14) and a control unit (30), the control unit (30) being set up to detect a disconnection of the power converter (10) from the AC supply grid (12) and via the grounding circuit establishes a ground reference for the DC network (14). Electrical system after claim 1 , wherein the control unit (30) is set up to establish the earth reference of the DC network (14) in that an earth switch (24) is activated, via which the earth circuit is connected to an earth potential (PE). Electrical system after claim 1 or 2 , wherein the grounding circuit comprises a DC/DC controller (40) with a reference potential, wherein the reference potential can be connected to the ground potential (PE) by closing the grounding switch (24), the DC/DC controller (40) being set up for this purpose , if there is a connection between the reference potential and the ground potential (PE) via the grounding switch (24), establish the ground reference of the DC network (14). Electrical system after claim 3 , wherein a midpoint of a half-bridge of the DC/DC converter (40) forms the reference potential and can be connected to the ground potential (PE) via the grounding switch (24). Electrical system according to one of the preceding claims, in which the DC/DC converter (40) can be connected to a storage unit (42) and is set up for power transfer between the storage unit (42) and the DC network (14), the control unit ( 30) is set up to control the DC/DC controller (40) in such a way that when the bridge circuit (20) is separated from the AC supply network (12), the DC network (14) connected to the DC side (18) can be operated from the memory unit (42). Electrical system after claim 5 , wherein the DC / DC controller (40) is set up to supply the DC network (14) with a DC power, wherein the control unit (30) is set up, a DC voltage (U _DC ) in the DC network (14) and to control the DC/DC controller (40) in such a way that the DC voltage (U _DC ) is balanced by the DC potentials of the DC voltage (U _DC ) being balanced symmetrically around the reference potential of the DC/ DC controller (40) can be adjusted. Electrical system according to one of the preceding claims, wherein the control unit (30) is set up to detect a fault current by means of a fault current measurement (32) and to separate the ground reference of the DC network (14) when a fault current is detected. Method for operating an electrical system according to one of the preceding claims, with the steps: • detecting a separation of the bridge circuit (20) from the AC supply network (12) and • If disconnection is detected: establishment of a ground reference for the DC network (14) via the grounding circuit. procedure after claim 8 , wherein the earth reference of the DC network (14) is established by closing an earth switch (24). procedure after claim 9 , wherein the closing of the grounding switch (24) establishes a connection between a reference potential of a DC/DC controller (40), the grounding circuit and a ground potential (PE), the DC/DC controller (40) being designed such that a Connection of the reference potential via the ground switch (24) to the ground potential (PE) produces the ground reference of the DC network (14). procedure after claim 10 , wherein the DC network (14) is supplied with electrical DC power via the DC/DC controller (40), the DC/DC controller (40) supplying the DC voltage (U _DC ) of the DC network (14) adjusts symmetrically around the reference potential. Procedure according to one of Claims 8 until 11 , with the steps • detecting a reconnection of the bridge circuit (20) to the AC supply network (12) and • upon reconnection: canceling the ground reference of the DC network (14) via the grounding circuit by opening the grounding switch (24).

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