EP4150753A1

EP4150753A1 - 3-phase pv inverter with 2-phase isolated operation in the event of a network fault

Info

Publication number: EP4150753A1
Application number: EP21725131.3A
Authority: EP
Inventors: Alexander UNRU; Burkard Mueller
Original assignee: SMA Solar Technology AG
Current assignee: SMA Solar Technology AG
Priority date: 2020-05-11
Filing date: 2021-05-10
Publication date: 2023-03-22
Also published as: DE102020112723A1; DE102020112723B4; US20230070123A1; WO2021228769A1

Abstract

The invention relates to an inverter (10), comprising - a first bridge branch (11) with a first phase output (114), - a second bridge branch (12) with a second phase output (124), - a third bridge branch (13) with a third phase output (134), wherein the phase outputs (114, 124, 134) of the bridge branches (11, 12, 13) can each be connected to a phase conductor (L1, L2, L3) of a three-phase power distribution network (3). The inverter (10) is designed, - in a normal operating mode of the three-phase power distribution network (3) and/or of a higher-level power supply network (1) connected thereto, to connect the phase outputs (114, 124, 134) to the relevant phase conductor (L1, L2, L3) and, - in the event of a fault in the three-phase power distribution network (3) and/or in the higher-level power supply network (1) connected thereto, to disconnect the three-phase power distribution network (3) from the higher-level power supply network (1) by means of a network disconnector (2), to disconnect the first phase output (114) from the first phase conductor (L1) by means of a switching unit (15) and to connect same to a neutral conductor (N) of the three-phase power distribution network (3), and to establish a neutral potential for the neutral conductor (N) via the first bridge branch (11). The invention further relates to a method for operating an inverter (10) of this kind.

Description

3-PHASE PV INVERTER WITH 2-PHASE ISLAND OPERATION AT

MAINS FAILURE

The invention relates to an inverter with three bridge arms, each with a phase output, the three phase outputs each being connectable to a phase conductor of a three-phase network. The invention also relates to a method for operating such an inverter.

Such inverters are often used to convert direct voltage, e.g. from a photovoltaic (PV) system, into a grid-compliant alternating current for feeding into a three-phase alternating current network. Energy supply networks are generally designed to be three-phase at all voltage levels, with the voltage profile on the various phases being shifted by 120 ° in relation to a voltage profile on one of the other phases.

A three-phase inverter is known from the publication DE 102014 104216 B3, which can be operated in a single-phase emergency mode in the event of a failure of the power supply network. In this emergency operation, two of the three bridge branches are operated in such a way that a single-phase bridge current for emergency power supply, for example for a residential building, can be carried out on at least one phase conductor between these two bridge branches. The third branch of the bridge remains unused.

When converting direct current, during normal operation of a three-phase inverter, energy is drawn very evenly from the direct current source due to the symmetrical phase shift of 120 ° between the phases. A direct current intermediate circuit, which is connected upstream of the bridge arms, can therefore have a relatively small capacitance of its intermediate circuit capacitors in a three-phase inverter in relation to the power transmitted by the inverter. If only single-phase alternating current is provided via two of the bridge branches, this loads the intermediate circuit capacitors significantly more, which leads to pronounced periodic voltage fluctuations at twice the network frequency, also known as “low frequency ripple”. These place a heavy load on the intermediate circuit capacitors and lead to faster aging of the capacitors. According to the document DE 102017 131 042 A1, two of three bridge branches of an inverter are used to convert a direct current into a single-phase alternating current and the third bridge branch is used to exchange power between the direct current source and an energy store, for example a battery store. By suitably controlling the third bridge arm, the changing load on the direct current intermediate circuit can be compensated for by periodically exchanging energy between the direct current intermediate circuit and the energy store, which leads to a voltage smoothing in the intermediate circuit.

It is an object of the present invention to design an inverter that is three-phase in normal operation so that an emergency power supply of at least parts of the three-phase network with the lowest possible voltage fluctuations in the intermediate circuit is possible in emergency power operation, without using an additional energy store to compensate for the load on the intermediate circuit .

This object is achieved by an inverter and an operating method for such an inverter with the features of the respective independent claim. Advantageous refinements and developments are the subject of the dependent claims.

An inventive inverter of the type mentioned is set up to connect the phase outputs to the respective phase conductor in normal operation of the three-phase energy distribution network and / or of a higher-level energy supply network connected to the energy distribution network. In the event of a fault in the three-phase energy distribution network and / or the associated higher-level energy supply network, the inverter is also set up to disconnect the three-phase energy distribution network from the higher-level energy supply network by means of a network separation device, to separate the first phase output from the first phase conductor using a switching unit and to connect a To connect the neutral conductor of the three-phase power distribution network, to set a neutral potential for the neutral conductor via the first bridge branch, and to control the second bridge branch and the third bridge branch after the network disconnection in such a way that voltages between the second and third phase output and the neutral conductor in each case have a different phase position to one another. The bridge branches can for example a 2-level B6 or a multi-level (e.g. 3-level) NPC (Neutral Point Clamped), BSNPC (Bipolar Switch NPC), ANPC (Active NPC) or FLC (Flying Capacitor) topology .

A method according to the invention for operating such an inverter is characterized by the following steps:

- that in normal operation of the three-phase power distribution network and / or a higher-level power supply network connected to it, the phase outputs are connected to the respective phase conductor, and

- That in the event of a fault in the three-phase energy distribution network and / or in the associated higher-level energy supply network, the three-phase energy distribution network is separated from the higher-level energy supply network, the first phase output is separated from the first phase conductor and is connected to a neutral conductor of the three-phase energy distribution network that via the A neutral potential for the neutral conductor is set in the first bridge branch, and that after the mains disconnection, the second bridge branch and the third bridge branch are controlled in such a way that voltages between the second and third phase output and the neutral conductor (N) have a different phase position to one another.

The energy distribution network is switchably connected to the superordinate energy supply network via the network separation unit. The three-phase power distribution network usually has the three phase conductors and a neutral conductor. The higher-level energy supply network can also have four conductors, namely three phase conductors and one neutral conductor, in the area of the network separation unit. The neutral conductor of the higher-level power supply network can be connected to a grounding conductor (PE) in the area of the network separation unit, for example on a local network transformer. However, the higher-level power supply network does not necessarily have to have four conductors throughout. For example, it can also be designed as a three-wire network in some areas, which has the three phase conductors in the relevant areas, but not the neutral conductor.

The case that usually occurs in the event of a fault is such that a fault occurs primarily in the higher-level energy supply network, which, due to the galvanic connection of the higher-level energy supply network with the energy distribution network, spreads into the energy distribution network via the network separation unit. Hence a disorder that is primary in that Energy supply network is present, due to the galvanic connection of the two networks due to the initially closed network separation unit also present in the energy distribution network. By separating the energy distribution network from the energy supply network by means of the network separation unit by opening the network separation unit, however, the disturbance present in the higher-level energy supply network can be prevented from spreading to the energy distribution network. In a separate state of the two networks from one another, the energy distribution network can continue to be operated in island mode and its local loads can continue to be supplied despite a fault in the higher-level energy supply network.

The term “network separation” denotes the separation of the energy distribution network from the higher-level energy supply network by means of the separation unit. During the disconnection, the three phase conductors of the energy distribution network are separated from the phase conductors of the higher-level energy supply network assigned to them. It is not absolutely necessary that, in addition to the phase conductors, the neutral conductor of the power distribution network is also separated from the neutral conductor of the power supply network assigned to it. Nevertheless, within the scope of the invention it is also possible, and in individual cases also advantageous, that in addition to the three phase conductors, the neutral conductor of the energy distribution network is also separated from the respectively assigned conductors of the higher-level energy supply network. This type of separation is referred to below as "all-pole separation".

By providing two phase conductors opposite a neutral conductor, an island grid can continue to be supplied with an output of up to 2/3 of the nominal output of the inverter. Single-phase loads within a network installation are usually connected to the various phase conductors. Because two out of three phase conductors are operated in local island operation (emergency operation), more consumers in the island network remain operational than, for example, with the solution according to DE 10 2017 131 042 A1, in which only a single-phase island network is set up. Because the voltages between the second phase output and the neutral conductor, on the one hand, and between the third phase output and the neutral conductor, on the other hand, have a different phase position, the voltage fluctuations in the intermediate circuit of the inverter are minimized and, in particular, are lower than when the voltages are in the same phase position. as described, for example, in the publication DE 102017 131 042 A1 - would be the case. In an advantageous embodiment of the inverter or of the method, the first bridge branch is controlled in such a way that a center potential of a voltage of a DC voltage intermediate circuit is established at the first phase output. This is an easy-to-implement way of creating the neutral potential for the neutral conductor. This applies in particular when a 3-level bridge topology is used, in which only the inner switches of the first bridge branch then have to be switched on in order to apply the center potential to the first phase output. The setting of a center potential (which is constant over a period of time), however, requires an intermediate circuit voltage that is greater than twice the amplitude of the voltages provided by the inverter on the two phase conductors. If this is not the case, the first bridge branch can alternatively be activated after the separation from the first phase conductor in such a way that a periodically modulated voltage curve is set as the neutral potential at the first phase output.

When the energy distribution network is separated from the higher-level energy supply network, the connections between the three phase conductors of both networks are disconnected. A separation of the neutral conductor of the energy distribution network from the corresponding neutral conductor of the energy supply network is not absolutely necessary. Rather, within the scope of the invention, isolated operation of the power distribution network can also take place when the neutral conductors are or remain connected to one another. It is therefore possible for a corresponding mains disconnection unit to have only three switching contacts in this case. In a further advantageous embodiment of the inverter or the method, however, it is provided, in the event of a fault in the three-phase energy distribution network and / or the associated higher-level energy supply network, to disconnect the three-phase energy distribution network on all poles from the higher-level energy supply network by means of a network disconnection device. With all-pole disconnection, there is also a disconnection of the neutral conductors of the energy distribution network and the higher-level energy supply network. The mains disconnection device, which is usually arranged externally from the inverter, is controlled accordingly by the inverter. In the case of all-pole disconnection, for safety reasons it may be necessary to connect the neutral conductor of the power distribution network to a local ground potential (PE), for example a local ground anchor, as there is now one in the area of the The connection between the neutral conductor and the earth potential (PE) is ineffective for the energy distribution network due to the galvanic separation of the two networks. The connection of the neutral conductor of the power distribution network to the local earth potential (PE) can also be made by means of a suitable switching device, which is controlled by the inverter, for example.

In a further embodiment, the inverter is set up to control the second and third bridge branches after the mains disconnection in such a way that a phase shift of voltages between the second and third phase output and the neutral conductor deviates from one another by 120 °.

With a phase position of 120 °, the DC link is loaded unevenly over the course of a network period. This leads to large voltage ripples at twice the network frequency in the DC voltage intermediate circuit and loads capacitors in the DC voltage intermediate circuit. This load is advantageously reduced by changing the phase shift between the phase conductors and the neutral conductor in each case from 120 ° to a value at which the DC voltage intermediate circuit is loaded more evenly over the network period.

A transitional mode is preferably provided in which the second and third bridge branches are controlled in such a way that, after the system is disconnected, a voltage with a phase shift of 120 ° is initially output at the second and third phase output and then the phase shift between the second and third phase output the value deviating from 120 ° is changed. The transitional operation preferably only lasts for a few network periods, in particular less than 5 network periods, in order to keep the load on the intermediate circuit capacitors as low as possible.

In further advantageous refinements of the method, a phase shift between the voltages provided between the neutral conductor and the second and third phase output is set as a function of the topology of the bridge branches.

In the case of an inverter with a 3-level topology of the bridge branches, the phase shift that is set after the grid is disconnected is advantageous about 180 °. The exact value of the phase shift can preferably be regulated via a control loop in such a way that a magnitude of voltage ripples is minimized at twice the network frequency in a DC voltage intermediate circuit of the inverter.

In the case of an inverter with a 2-level bridge branch topology, a phase shift of at least 90 ° and less than 120 ° is set after disconnection from the grid. The phase shift is preferably 90 °, since minimal voltage ripples are observed in this case. However, the current at the first phase output increases when the phase shift is reduced from 120 ° towards 90 °. In a possible development, a maximum current is measured at the first phase output and the phase shift is reduced to 90 ° if the maximum current is below a predetermined threshold value and increased in the direction of 120 ° if the maximum current is above or at the predetermined threshold value. Thus, with regard to the load on the intermediate circuit, the best possible value of the phase shift is selected dynamically depending on the current load situation in the island network, which is still associated with an acceptable current load for the first phase output.

The invention is explained in more detail below on the basis of exemplary embodiments with the aid of figures. The figures show:

1 shows a schematic representation of a

Inverter connected to a local power distribution network;

2 shows a flow diagram of an operating method for a

Inverter; and

3 shows a schematic exemplary embodiment of an inverter in a second exemplary embodiment, connected to a local energy supply network.

1 shows a schematic circuit diagram of an arrangement with an inverter 10 in an exemplary embodiment. The inverter is coupled to a local energy distribution network 3, hereinafter also referred to as network 3, in a manner to be explained in more detail, which is connected to a superordinate energy distribution network via a network disconnection device 2 Power supply network 1 is connectable. Consumers 4, shown as resistors by way of example, are connected to the local network 3.

The local network 3, like the superordinate energy supply network 1, is a three-phase network comprising phase conductors L1, L2 and L3 and a neutral conductor N. A phase shift between the individual phase conductors L1, L2 or L3 is 120 °. In normal operation, the local network 3 is coupled to the higher-level energy supply network 1.

The inverter 10 comprises an inverter bridge with three bridge branches 11, 12, 13, which are basically designed in the same way and are referred to as first, second and third bridge branches 11, 12, 13 only for the sake of differentiation. Each of the bridge branches 11, 12, 13 comprises a series circuit of two semiconductor switching elements 111, 112 or 121, 122 or 131, 132 Intermediate circuit capacitor 141 is shown. It goes without saying that the intermediate circuit capacitor 141 in one implementation of the circuit shown can consist of a plurality of individual capacitors connected in parallel and / or in series.

In the example shown, the connections of the DC voltage intermediate circuit 14 also form the input connections of the inverter 10, to which a DC voltage source 5 is connected here.

The DC voltage source 5 is exemplified by the circuit symbol of a battery. In one implementation of the system shown in Fig. 1, it can be an interconnection of one or more rechargeable batteries and / or a photovoltaic (PV) generator, which in turn has a plurality of PV cells, arranged in a plurality of PV May include modules. Here, too, the PV modules can be connected in a series and / or parallel connection in order to form the PV generator. Likewise by way of example, in the case of the inverter 10, the direct current source 5 is connected directly to the direct voltage intermediate circuit 14. It is also conceivable to interpose a DC voltage converter in order to keep the DC voltage intermediate circuit 14 and the direct current source 5 at different voltage levels.

When the inverter 10 is in operation, the individual semiconductor switching elements 111, 112, 121, 122, 131 and 132 are switched from one not shown here Control unit controlled, preferably in a pulse width modulation method (PWM method) in order to convert the supplied direct current. In the example shown, IGBTs (Insulated Gate Bipolar Transistors) are used as semiconductor switching elements 111, 112, 121, 122, 131 and 132. In alternative configurations, bipolar transistors or MOSFETs (Metal Oxide Semi-Conductor Field Effect Transistors) can also be used.

In order to correctly determine the switching times in the PWM method, suitable current and / or voltage measured values are required at the bridge branches 11, 12, 13. Current measurements can be made, for example, with the help of shunts or Hall sensors that determine a current based on a measured magnetic field. Voltage measurements can be made with the help of voltage dividers. The measured current and / or voltage values are evaluated in the control unit. For the sake of clarity, it has been dispensed with in FIG. 1 to draw corresponding current and / or voltage measuring means.

The center taps of the bridge branches 11, 12, 13 are each led out of the inverter 10 as outputs via an output filter 113, 123, 133. These outputs represent phase outputs 114, 124 and 134 of the inverter 10. The output filters 113, 123, 133 are used to smooth the output voltage or the output current, so that they have a time curve that is as sinusoidal as possible.

In principle, other topologies can also be implemented in the inverter 10 than the so-called 2-level B6 topology shown, each with three bridge branches, each with two semiconductor switching elements and a center tap. An inverter can also be used in a three or multi-level topology such as "Neutral Point Clamped" (NPC), "Bipolar Switch Neutral Point Clamped" (BSNPC), "Active Neutral Point Clamped" (ANPC) or "Flying Capacitor" (FLC ) be constructed. These topologies generally require more semiconductor switching orange per bridge branch than the B6 topology, but can offer advantages in terms of their efficiency. An exemplary embodiment of an inverter 10 with a 3-level NPC topology is shown in the exemplary embodiment in FIG. 3.

Switching elements 115,

125 and 135 arranged, via which the phase outputs 114, 124, 134 can be connected to the corresponding conductors L1, L2 and L3 of the network 3. A special feature is the connection between the first Phase output 114 and the phase conductor L1 are also guided via a changeover switch 15, the function of which will be explained in more detail below.

A connection of the three phase outputs 114, 124, 134 with the three phase conductors L1, L2, L3 represents normal operation for the inverter 10, in which the power provided by the direct current source 5 is fed into the local network 3 in three phases and the consumer 4 is supplied or for feeding into the power supply network 1.

In connection with FIG. 2, a flowchart is used to describe an operating method according to the application, which can be carried out, for example, with the inverter 10 shown in FIG. 1 in the connection shown with the network 3. The method is explained below by way of example with reference to FIG. 1.

The method starts in a step S1, in which the inverter 10 feeds into the three-phase network 3 in the aforementioned normal mode in a flow or voltage-controlled manner. The network disconnection device 2 is closed and a connection to the energy supply network 1 is established. The switching element 15 is in a switching position in which the first phase output 114 is connected to the phase conductor L1.

In a next step S2, a fault in the local network 3 or the superordinate energy supply network 1 is detected, for example by a network monitoring device not shown in FIG. 1. In response to the network disturbance, the local network 3 is disconnected from the superordinate energy supply network 1 by opening the network disconnection device 2. The separation can, but does not necessarily have to be, all-pole.

In order to be able to continue to supply as many of the loads 4 as possible with energy in the event of network separation, the inverter 10 is used as a stand-alone inverter for the network 3. For this purpose, the switching device 15 is switched by the control device of the inverter 10 or a superordinate control device in such a way that the first phase output 114 is connected to the neutral conductor N of the network 3. It can be provided that a connection of the neutral conductor N of the local network 3 to a local ground potential PE, for example a local ground anchor is made. The first bridge branch 11 is controlled in such a way that the potential on the neutral conductor N represents a neutral potential for the consumers 4, which are connected to the phase conductors L2 and L3, whereby these consumers 4 can continue to be operated. A neutral potential represents, for example, a center potential in the DC voltage intermediate circuit 14, the use of the center potential as a neutral potential presupposing a sufficiently high intermediate circuit voltage. Specifically, a voltage in the intermediate circuit 14 is required for this, which corresponds to twice the amplitude of the phase voltage to be provided. If such a high intermediate circuit voltage is not present in normal operation, it can be provided to increase it accordingly after the mains disconnection, which is possible if a DC voltage converter is arranged between the direct current source 5 and the DC voltage intermediate circuit 14. Alternatively, provision can be made to provide an auxiliary voltage with a course that is modulated in turn, ie a value that is not constant over time, at the first phase output 114 as a neutral potential.

By providing two phase conductors opposite a neutral conductor, the network 3 continues to be supplied as an island network with an output of up to 2/3 of the nominal output of the inverter 10. In order to be able to operate the island grid, the inverter 10 is no longer operated in a flow- or voltage-controlled manner, but rather in a voltage-setting manner, so that it functions as a network generator.

The phase position of the phase conductors L2 and L3 that are still operated is initially adopted or retained when switching to island operation, so it has a phase difference of 120 °. However, this phase position leads to large voltage ripples at twice the network frequency in the DC voltage intermediate circuit 14 and represents a load for the DC voltage intermediate circuit 14 that fluctuates greatly over a period of the AC voltage.

In order to reduce this load, in a next step S3 the phase shift between the phase conductors L2 and L3 with respect to the neutral conductor N is changed from 120 ° to a value at which the DC voltage intermediate circuit 14 is loaded more evenly over the network period. In the case of an inverter with bridge branches in a 2-level topology (cf. FIG. 1), a load that is as uniform as possible is achieved with a phase shift of 90 °. In the case of an inverter with bridge branches in 3-level topologies (cf. FIG. 3) and in particular with the same load on the phase conductors L2 and L3, this is usually one Phase shift of 180 °. This means that the phase conductors L2 and L3, which are further supplied in the island network, are operated with respect to the neutral conductor in the manner of a so-called single-phase three-wire network, also known as a "split phase" network.

If the two phase conductors L2 and L3 are not evenly loaded - which is usually the case in practice - the optimal phase shift is not exactly 180 °, but deviates from it to smaller or larger values. In a development of the method it can be provided to determine the size of the voltage ripple at twice the network frequency in the DC voltage intermediate circuit 14 and to regulate the angle of the phase shift in a control loop so that the size of the voltage ripple is minimized.

In order to keep the effects, e.g. aging of the intermediate circuit capacitor 141, of the load on the DC voltage intermediate circuit 14 as low as possible with the original phase relationship of 120 °, the changeover to "split phase" operation takes place as quickly as possible after the detection of the network fault, for example within some network periods.

The “split phase” mode is retained until it is recognized in a next step S4 that the network fault in the energy supply network 1 has been eliminated.

In a next step S5, the phase conductors L2 and L3 are synchronized with the corresponding phase positions in the power supply network 1 with regard to their phase position by appropriate control of the second and third bridge branches 12, 13.

In a next step S6, the network 3 is reconnected to the power supply network 1 by switching the network disconnection device 2 on again. Furthermore, the first phase output 114 is separated from the neutral conductor N by opening the switching element 115.

In a subsequent step S7, the first phase output 114 is then synchronized to the phase position of the phase conductor L1 of the energy supply network 1.

In a final step S8, the switching element 15 is then switched over again in such a way that the first phase output 114, after the switching element 115 is subsequently switched on again, with the phase conductor L1 connected is. The arrangement is thus again in normal operation, which was also present in step S1.

FIG. 3 shows, in a manner comparable to FIG. 1, a further exemplary embodiment of an arrangement comprising an inverter 10 which is connected to a local network 3 which is coupled to an energy supply network 1 via a network isolating device 2. In this figure, the same reference symbols denote elements that are the same or have the same effect as in FIG. 1. The arrangement in FIG. 3 differs only in the topology of the inverter 10 from the arrangement in accordance with FIG. 1, the description of which is hereby explicitly referred to.

In the inverter 10 according to FIG. 3, the three bridge branches 11, 12,

13 constructed as 3-level NPC (Neutral Point Clamped) bridge branches. Instead of a series connection of the two semiconductor switching elements 111, 112 (or 121, 122 and 131, 132), two semiconductor switching elements 111 (or 121, 131) and two semiconductor switching elements 112 (or 122, 132) are connected in series, with a center tap between these two semiconductor switching elements is clamped to a neutral potential via a further semiconductor switching element 11T, 112 '(or 12T, 122' and 13T, 132 '). These further semiconductor switching elements 111 ‘, 112 '(or 12T, 122' and 13T,

132 ‘) are formed by diodes in the example shown.

As a further difference to the first exemplary embodiment in FIG. 1, the DC voltage intermediate circuit 14 is constructed as a divided intermediate circuit with two intermediate circuit capacitors 141, 142 connected in series. A center tap between the two intermediate circuit capacitors 141, 142 forms the neutral potential.

With this arrangement, too, the operating method described in connection with FIG. 2 can advantageously be carried out, in which, after the occurrence of a network fault, voltage is applied to the two phase conductors L2 and L3 by the inverter 10 with respect to the neutral conductor N, the neutral conductor N through the first Bridge branch 11 is held at a neutral potential. The advantage of a 3-level inverter is that a center potential can be generated as a neutral potential simply by switching on the inner ones of the switching elements 111 and 112 of the first bridge arm 11 on the neutral conductor N. In this exemplary embodiment, the subsequent setting of a phase position of approximately 180 ° leads to lower voltage ripples at twice the network frequency and thus to a lower load on the intermediate circuit capacitor 141, 142. Due to the divided design of the intermediate circuit 14, the inverter has dynamic and stable control behavior.

Reference number

1 superordinate energy supply network

Mains disconnection device

(local) network

consumer

DC power source

10 inverters

11 first branch of the bridge

111, 112 semiconductor switching device 111 ', 112' another semiconductor switching device

113 Filter arrangement

114 first phase output

115 switching element 12 second bridge branch

121, 122 semiconductor switching device 121 ', 122' another semiconductor switching device

123 filter arrangement

124 second phase output

125 switching element

13 third branch of the bridge

131, 132 semiconductor switching device 131 ', 132' another semiconductor switching device

133 Filter arrangement

134 third phase output

135 switching element

14 DC link

141, 142 intermediate circuit capacitor

15 switching device

L1, L2, L3 phase conductor N neutral conductor

Claims

Expectations

1. Inverter (10), comprising

- A first bridge branch (11) with a first phase output (114),

- A second bridge branch (12) with a second phase output (124),

- A third bridge branch (13) with a third phase output (134), the phase outputs (114, 124, 134) of the bridge branches (11, 12, 13) each having a phase conductor (L1, L2, L3) of a three-phase power distribution network (3 ) are connectable, characterized in that the inverter (10) is set up to

- To connect the phase outputs (114, 124, 134) to the respective phase conductor (L1, L2, L3) in normal operation of the three-phase energy distribution network (3) and / or a higher-level energy supply network (1) connected to it, and

- in the event of a fault in the three-phase energy distribution network (3) and / or the associated higher-level energy supply network (1), to separate the three-phase energy distribution network (3) from the higher-level energy supply network (1) by means of a network isolating device (2), the first phase output (114) to be separated from the first phase conductor (L1) by means of a switching unit (15) and to be connected to a neutral conductor (N) of the three-phase power distribution network (3), to set a neutral potential for the neutral conductor (N) via the first bridge branch (11), and after to control the second bridge branch (12) and the third bridge branch (13) in such a way that voltages between the second and third phase output (124, 134) and the neutral conductor (N) have a different phase relation to one another.

2. The inverter (10) according to claim 1, which is further configured to control the first bridge branch (11) in such a way that a center potential of a voltage of a DC voltage intermediate circuit (14) is established at the first phase output (114).

3. Inverter (10) according to claim 1 or 2, which is further set up in the event of a fault in the three-phase energy distribution network (3) and / or the associated higher-level energy supply network (1) by means of the network disconnection device (2), the three-phase energy distribution network (3) all poles to be disconnected from the higher-level power supply network (1).

4. Inverter (10) according to one of claims 1 to 3, which is further set up to control the second and third bridge branches (12, 13) after the mains disconnection so that a phase shift of voltages between the second and third phase output (124, 134) and the neutral conductor (N) deviates from one another by 120 °.

5. Inverter (10) according to claim 4, which is further set up to control the second and third bridge arm (12, 13) in a transitional mode so that after the mains disconnection at the second and third phase output (124, 134) initially voltages with a phase shift of 120 ° to each other and then the phase shift of the voltages between the second and third phase output (124, 134) and the neutral conductor in each case is changed to the value deviating from 120 °.

6. Inverter (10) according to claim 5, wherein the transitional operation lasts only a few network periods, in particular lasts less than 5 network periods.

7. Inverter (10) according to one of the preceding claims, wherein the bridge branches (11, 12, 13) have a 2-level B6 or a 3-level NPC, BSNPC, ANPC or FLC topology.

8. Method for operating an inverter (10) with three bridge branches (11, 12, 13) and three phase outputs (114, 124, 134), each of which can be connected to a phase conductor (L1, L2, L3) of a three-phase power distribution network (3) are characterized by the steps:

- In the event of a fault in the three-phase energy distribution network (3) and / or in the associated higher-level energy supply network (1), to separate the three-phase energy distribution network (3) from the higher-level energy supply network (1), the first phase output (114) from the first phase conductor ( L1) and to connect it to a neutral conductor (N) of the three-phase power distribution network (3), to set a neutral potential for the neutral conductor (N) via the first bridge branch (11), and after the network disconnection, the second bridge branch (12) and the third Control bridge arm (13) so that voltages between the second and third phase output (124, 134) and respectively the neutral conductor (N) to each other have a different phase position.

9. The method according to claim 8, wherein the first bridge branch (11) is controlled after the separation from the first phase conductor (L1) that a center potential of a voltage of a DC voltage intermediate circuit (14) is set as neutral potential at the first phase output (114) .

10. The method according to claim 8, wherein the first bridge branch (11) is controlled after the separation from the first phase conductor (L1) that a periodically modulated voltage curve is set as neutral potential at the first phase output (114).

11. The method according to any one of claims 8 to 10, in which after detection of the fault in the three-phase energy distribution network (3) and / or the associated higher-level energy supply network (1), the energy distribution network (3) is disconnected from the higher-level energy supply network (1) on all poles .

12. The method according to any one of claims 8 to 11, in which in the case of an inverter with a 3-level topology of the bridge branches (11, 12, 13) after the network separation, the second and third bridge branches (12, 13) are controlled in such a way that voltages are provided between the neutral conductor (N) and the second and third phase output (124, 134) which are phase-shifted by approximately 180 ° with respect to one another.

13. The method according to claim 12, in which the value of the phase shift is regulated via a control loop such that a magnitude of voltage ripples is minimized at twice the network frequency in a DC voltage intermediate circuit (14) of the inverter (10).

14. The method according to any one of claims 8 to 11, in which in the case of an inverter with a 2-level topology of the bridge branches (11, 12, 13) after the network separation, the second and third bridge branches (12, 13) are controlled in such a way that voltages are provided between the neutral conductor (N) and the second and third phase output (124, 134) which are at least 90 ° and less than 120 ° out of phase.

15. The method according to claim 14, wherein the phase shift is 90 °.

16. The method according to claim 14, wherein a maximum current is measured at the first phase output (114) and the phase shift is reduced to 90 ° if the maximum current is below a predetermined threshold and is increased in the direction of 120 ° if the maximum current is above or is at the predetermined threshold.