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US-12627243-B2 - Method for operating an inverter, and inverter

US12627243B2US 12627243 B2US12627243 B2US 12627243B2US-12627243-B2

Abstract

The disclosure is a method for operating an inverter with a DC input and an AC output. The DC input is connected to a DC source, bridge branches of a bridge circuit are connected to the AC output via power chokes, and the AC output is connected to an AC grid via isolating switches. The method includes opening the isolating switches, and controlling semiconductor switches of at least two bridge branches of the bridge circuit that are connected downstream of at least one power choke at the AC side, such that a DC source connected to the DC input is loaded. The sum of the currents flowing out of at least one of the at least two bridge branches on the AC side corresponds to the sum of the currents flowing into at least one other bridge branch of the at least two bridge branches on the AC side.

Inventors

  • Uwe Stickelmann
  • Alexander Unru

Assignees

  • SMA SOLAR TECHNOLOGY AG

Dates

Publication Date
20260512
Application Date
20240124
Priority Date
20210730

Claims (15)

  1. 1 . A method for operating an inverter with a DC input and an AC output, in which the DC input is configured to be connected to a DC source, and bridge branches of a bridge circuit of the inverter are connected to the AC output via power chokes, wherein the bridge branches are associated with different phases of the inverter, respectively, and the AC output is configured to be connected to an AC grid via isolating switches, wherein the inverter is configured to feed electrical power to the AC grid, comprising: opening the isolating switches, and controlling semiconductor switches of at least two bridge branches of the bridge circuit, the at least two bridge branches associated with different phases of the inverter being connected downstream of at least one power choke at an AC side of the inverter, such that a DC source connected to the DC input is loaded, wherein a sum of a totality of currents flowing out of at least one of the at least two bridge branches on the AC side corresponds to a sum of a totality of currents flowing into at least one other bridge branch of the at least two bridge branches on the AC side.
  2. 2 . The method according to claim 1 , wherein the inverter is configured to feed electrical power into a single-phase or multi-phase AC grid, wherein the inverter has for each phase at least two parallel bridge branches that are connected on the AC side downstream of the at least one power choke, and whose semiconductor switches are controlled in such a way that the DC source connected to the DC input is loaded, wherein the bridge branches are configured to operate in an interleaving mode.
  3. 3 . The method according to claim 1 , wherein a connection on the AC side of the at least two bridge branches of the bridge circuit downstream of the at least one power choke is effected by closing a relay.
  4. 4 . The method according to claim 3 , wherein the inverter is configured to feed electrical power into a three-phase AC grid, wherein the inverter has at least one bridge branch for each phase, and a first bridge branch from a first phase and a second bridge branch from a second phase are connected with each other on the AC side downstream of the at least one power choke, and the semiconductor switches are controlled in such a way that a current flowing out of the first bridge branch flows into the second bridge branch.
  5. 5 . The method according to claim 3 , wherein the inverter is configured to feed electrical power into a three-phase AC grid, wherein the inverter has at least one bridge branch for each phase, and three bridge branches are connected with each other on the AC side downstream of the at least one power choke, and the semiconductor switches are controlled in such a way that a current flowing out of at least one bridge branch of one phase is divided and flows into at least one bridge branch of each of the other two phases or that currents flowing out of at least one bridge branch of each of two phases flow in total into the at least one bridge branch of a third phase.
  6. 6 . The method according to claim 1 , wherein the inverter is configured to feed electrical power into a single-phase AC grid.
  7. 7 . The method according to claim 1 , wherein the semiconductor switches for loading the DC source connected to the DC input are controlled in such a way that the inverter is operated in a voltage-controlled operation with a voltage of zero at the AC output.
  8. 8 . The method according to claim 1 , wherein a degree of load of the DC source connected to the DC input is adjustable by a control of the semiconductor switches.
  9. 9 . The method according to claim 4 , wherein the bridge circuit is a B6 bridge circuit.
  10. 10 . The method according to claim 5 , wherein the bridge circuit is a B6 bridge circuit.
  11. 11 . The method according to claim 6 , wherein the bridge circuit is an H4 bridge circuit or an H5 bridge circuit.
  12. 12 . The method according to claim 1 , wherein the bridge branches each have a TNPC (T-type neutral-point clamped), INPC (I-type neutral-point clamped) or ANPC (active neutral-point clamped) topology.
  13. 13 . The method according to claim 4 , wherein the bridge branches each have a TNPC (T-type neutral-point clamped), INPC (I-type neutral-point clamped) or ANPC (active neutral-point clamped) topology.
  14. 14 . The method according to claim 5 , wherein the bridge branches each have a TNPC (T-type neutral-point clamped), INPC (I-type neutral-point clamped) or ANPC (active neutral-point clamped) topology.
  15. 15 . An inverter with a DC input and an AC output and a bridge circuit with controllable semiconductor switches, in which the DC input is configured to be connected to a DC source, and bridge branches of the bridge circuit of different phases of the inverter are connected via power chokes to the AC output and selectively coupled together downstream of the power chokes via switching circuitry, and the AC output is configured to be connected to an AC grid via isolating switches, wherein the inverter is configured to feed electrical power into the AC grid and wherein the inverter has a control circuit that is configured to carry out the method according to claim 1 .

Description

REFERENCE TO RELATED APPLICATIONS This application is a Continuation of International Application number PCT/EP2022/068782, filed on Jul. 6, 2022, which claims the benefit of German Application number 10 2021 119 899.2, filed on Jul. 30, 2021. The contents of the above-referenced patent applications are hereby incorporated by reference in their entirety. FIELD The application relates to a method for operating an inverter with a DC input and an AC output as well as an inverter and its use. The inverter can be used to connect photovoltaics or other specific DC voltage sources to an AC grid. BACKGROUND In some situations, when operating an inverter, it is necessary to load a DC source from which the inverter feeds electrical power into an AC grid. The term AC grid can be used, for example, to refer to an alternating current grid or an alternating voltage grid. The term DC can be used to refer to direct current or direct voltage, a DC source can accordingly refer to a direct current source or a direct voltage source. As an example, there is a requirement to discharge the DC source, for example, a hydrogen fuel cell or an input intermediate circuit in a PV (photovoltaic) system, within a specified time in the event of an AC grid failure or a controlled shutdown of the system. For this purpose, resistors can be connected to the output side of the DC source for discharging. These resistors convert the residual energy from the fuel cell or the intermediate circuit into heat. As the residual energy is usually quite high, the resistors must be able to convert high amounts of energy and are therefore very large and expensive. In particular, these resistors represent additional components which mean an increased complexity of the control and operation of the overall system. A PV inverter usually starts up in the morning as soon as the DC voltage of the PV generator is sufficiently high and the inverter is connected to the AC grid. It is problematic if the DC voltage is sufficiently high but the irradiation is so low that a grid connection on the AC side would cause the DC voltage to collapse again immediately and cause an immediate AC grid disconnection. This continuous switching operation is detrimental to the service life of the grid disconnection point, as it is usually only designed for a certain number of switching cycles. At this point, it makes sense to only initiate an AC grid connection if not only sufficient DC voltage but also sufficient DC power is available. The same problem can also occur in wind generators if sufficient power is not yet available. In two-stage inverter topologies that have a DC/DC converter upstream of the inverter bridge, the available power of the DC source can be determined by loading it and thus charging the intermediate circuit of the two-stage inverter to a certain potential. The energy content of the charging process can be used to draw direct conclusions about the power availability of the DC source. In single-stage topologies, the DC voltage of the DC source is mainly used as an indicator. However, this indicator can be unreliable as the information about the rated current is missing in this case. SUMMARY One object of the disclosure is to provide an improved method and an improved inverter for loading a DC source that can be connected to the inverter. An inverter has a DC input for connecting to a DC source and an AC output for connecting to an AC grid via isolating switches, wherein bridge branches of a bridge circuit are connected to the AC output via power chokes. The AC output of the inverter can be connected to the AC grid via the isolating switches and disconnected from the AC grid via the isolating switches. The inverter is configured to feed the electrical power provided by the DC source into the AC grid. A method for operating the inverter comprises: a) opening the isolating switches, andb) controlling semiconductor switches of at least two bridge branches of the bridge circuit, wherein the bridge branches are connected downstream of at least one power choke at the AC side, such that a DC source connected to the DC input of the inverter is loaded. A totality of currents flowing out of at least one of the at least two bridge branches on the AC side of the inverter corresponds in its sum to a totality of currents flowing into at least one other bridge branch of the at least two bridge branches on the AC side. This means that the sum of the currents flowing out of the bridge branches on the AC side corresponds to the sum of the currents flowing into the bridge branches on the AC side. Energy can therefore be converted between two or more bridge branches of the inverter in order to discharge the DC side due to the resulting conversion losses. Such an inverter and an inverter operated in such a way enables a load of the DC source from which the inverter can feed power into an AC grid. The load can be applied without the use of additional resistive components. This enables a