DE102014101571B4 - INVERTER AND PROCESS FOR OPERATING AN INVERTER - Google Patents

Abstract

An inverter for feeding electric power of a DC power source (100, 300, 500, 600) to an AC power network (110, 310, 510, 610) is disclosed. A control of switches of the inverter bridge (130, 330, 530, 630) converts in the case of feeding reactive power during periods of a grid period of the AC voltage network (110, 310, 510, 610), in which the inverter electrical power from the AC mains (110, 310, 510, 610), converts the electrical power sourced from the AC mains (110, 310, 510, 610) into heat. For this purpose, the controller operates a first switch (T1, LS1) of the inverter bridge (130, 330, 530, 630) in a linear or a dissipatively clocked operating mode. The linear or dissipatively clocked mode of operation may be selected in response to a current flowing through the first switch (T1, LS1) such that power flow from the inverter bridge (130, 330, 530, 630) toward the DC source (100, 300, 500, 600) is prevented. A corresponding operating method is also described.

Description

TECHNICAL FIELD OF THE INVENTION

The invention relates to a dummy power inverter for converting direct current into alternating current and for feeding the alternating current into an alternating voltage network, as well as a method for operating a blind power inverter.

In the supply of electrical power, in particular those of renewable energy generation systems with photovoltaic or wind power generators, in an AC network normative framework conditions are observed. Until recently, grid-connected power plants had to drive the AC power they produced with a power factor (PF) close to one, ie. H. without phase shift to the mains voltage (cos phi about equals one) feed. However, more recent standards stipulate that decentralized power generation plants in particular must participate in network management and, in particular, provide both capacitive and inductive reactive power with power factors in the range 0.95 <PF <1, especially at a low voltage level. With increasing penetration, in particular of the low-voltage level with decentralized power generation plants, it is to be expected that the required range will be extended to at least 0.90 <PF <1.

In order to provide reactive power and to control it, an inverter must be able to temporarily supply electrical power to the AC mains during a period of the sinusoidal mains voltage, as well as to temporarily receive electrical power from the AC mains. However, many known inverter topologies are not set up, in particular with regard to the drawing of electrical power from the AC voltage network, so that particular effort is made to make such topologies perform blindly. As a result, both complexity and manufacturing costs increase significantly and there are also efficiency losses.

STATE OF THE ART

From the DE 10 2009 029 387 A1 a DC-AC inverter arrangement with a semiconductor bridge circuit is known, wherein a DC-DC converter for generating half-waves of an output-side AC voltage is provided and the bridge circuit is connected downstream of the DC-control plate and acts as a polarity reverser for the half-waves. In this arrangement, the DC-DC converter is designed as a four-quadrant controller, thus capable of being regenerated, and the inverter arrangement is thereby able to operate with blind power.

In Scripture DE 10 2011 017 601 A1 An inverter is described in which a bridge circuit is operated in phases as a polarity reverser and as a pulsed bridge to produce a desired output voltage waveform.

From the WO 2012/062375 A1 an inverter is known in which by means of a resonant DC-DC converter half-waves of an alternating current to be fed are generated, the rectified and fed by means of a network-controlled inverter bridge, which is designed as a Polwender, as a sinusoidal alternating current in an alternating voltage network.

When using such a combination of a DC-DC converter for forming the sine wave with a subsequent pole turner, there is the additional problem that the switching of the pole turner must be performed at a certain time and this at a, for example, by an output-side filter inductance inductively driven current to considerable Overvoltages can result. The Polwender can be destroyed if there are unexpected changes in the mains voltage and thus u. U. comes to a premature switching of the Polwenders. Additionally, at zero voltage crossing, "blanking" times occur, leading to increased total harmonic distortion (THD) distortion. Furthermore, during reactive power supply, the current of the DC-DC converter must also be reduced very quickly to zero in error-free operation in order to feed in a current form as long as possible according to a sine wave. In this case, however, there is the problem that in the region of the voltage zero crossing at an output-side inductance no high negative voltage time surface is available for lowering the current.

OBJECT OF THE INVENTION

The invention has for its object to provide an inverter with an inverter bridge, which is constructed with minimal circuit complexity and can provide reactive power, and to demonstrate an operating method for an inverter with an inverter bridge, a safe switching of the inverter bridge bridge is possible and at the same time distortions Harmonics are minimized with improved dynamics of current shaping.

SOLUTION

The object of the invention is achieved by an inverter having the features of independent claim 1 and a method for Operating an inverter with the features of independent claim 14 solved. Preferred embodiments are defined in the dependent claims.

DESCRIPTION OF THE INVENTION

In an embodiment according to the invention, an inverter for feeding electrical power of a DC power source into an AC voltage network comprises an inverter bridge which can be connected to the DC power source. A control of switches of the inverter bridge is set up such that during a supply of reactive power in periods of a grid period of the AC voltage network in which the inverter draws power from the AC mains, a first switch of the inverter bridge is operated such that the electric power related from the AC mains is converted into heat.

The invention is based on the realization that the advantages of a converter which is much simpler and less costly to construct than a bidirectionally operating inverter are its disadvantages, ie the conversion of the electrical power drawn from the AC voltage network into heat and thus the absence of recycling of the inverter current Energy, clearly outweigh.

In particular, the inverter bridge can be designed as a polarity reverser which feeds the input side applied half-wave modulated DC voltages or DC currents as AC currents in an AC voltage network, the switches of the inverter bridge are controlled by a controller. The half-wave modulated electrical power may be generated and provided from a DC link to the inverter bridge upstream modulation device, such as a DC-DC converter.

The inverter is preferably characterized in that the controller is set up to control at least one of the switches of the inverter bridge in a linear operating mode such that the at least one switch acts as a switchable, regulated resistor. As a result, return currents that occur at a power factor not equal to 1 in periods in which the inverter draws electrical power from the AC power network in which at least one switch is converted into heat, d. H. dissipated. As a result, these return currents and possibly occurring current and power peaks can be controlled in particular by inverters which have no or only a very small DC link memory.

As an inverter bridge in the context of this description, not only the parallel arrangement of two or more half-bridge branches is understood, each having at least two series-connected half-bridge switch. It can be added to these half-bridge switches other components, in particular switches, which may be part of the half-bridge branches, but also outside the half-bridge branches, in particular may be arranged parallel to the half-bridge branches and form part of the inverter bridge.

In one embodiment, the controller is configured to provide a switch of one of the half-bridge branches or a switch in one of two half-bridge branches forming an inverter bridge in the linear operating mode as switchable, controlled resistors in the periods when the inverter draws power from the AC grid to operate. The switches of the half-bridge branches connected to the negative pole of the input-side DC voltage are preferably operated in this way.

In an alternative embodiment, the inverter bridge comprises, in addition to the two half-bridge branches, a further switch which is arranged parallel to the half-bridge branches and thus between the poles of the input-side DC voltage. Preferably, the controller is adapted to operate this further switch in the periods in linear operation as a switchable, controlled resistance. Optionally, a current measurement can be arranged in series with the further switch, in particular in order to enable current-controlled operation with the aid of the switch.

In a further alternative embodiment, the inverter bridge comprises, in addition to the two half-bridges, a further switch which is arranged parallel to the bridge outputs and thus between the poles of the output-side AC voltage, ie parallel to a connected AC voltage network. Preferably, this switch is bidirectional, so that it can be controlled to each zero crossing of the mains voltage. Such bidirectional switches can be formed in a previously known manner by the antiparallel circuit of two unidirectional switches or by the series connection of two oppositely directed unidirectional switches with antiparallel diode. In this case, the advantage of the invention can also be achieved with a single unidirectional switch, in which case the switch only once per network period (ie only every other half-wave) is driven. In this case, the conversion takes place from the AC mains related power in heat only in periods of a predetermined polarity of the AC voltage network.

Preferably, the controller is configured to operate the other switch in the time periods in which the inverter draws power from the AC voltage network in the linear operating mode as a switchable, controlled resistance. Optionally, a current measurement can be arranged in series with the further switch, in particular in order to enable current-controlled operation with the aid of the switch. Optionally, the current measurement can also take place elsewhere in the inverter bridge and thus also enable a current-controlled operation of the switch.

The operation of a switch, such as a transistor, a MOSFET or an IGBT, in the linear operating mode as a switchable, controlled resistance can mean that the switch is not as usual in an inverter bridge from a conductive to a non-conductive state and back, which In normal operation of an inverter bridge for feeding pure active power to avoid switching losses should also happen as quickly as possible. Rather, the control electrode of the switch can be controlled analogously by means of a suitable driver circuit, so that the switch occupies a partially conductive intermediate state between conductive and non-conductive and thus represents an adjustable finite ohmic resistance.

Alternatively, the control electrode of the switch can be driven during a switching operation with a predetermined waveform, which leads to increased switching losses during the switching operation. In this case, the switching operation is repeated with a suitably selected frequency, which leads to a loss behavior corresponding to the finite ohmic resistance. When a sufficiently high switching frequency is used, the signal form which is designed to trigger the switch with a conventional signal form which is inherently designed for minimum switching losses can also generate sufficient losses.

Hereinafter, the temporary operation of a switch in a partially conductive state is also referred to as a linear mode of operation, while the lossy clocking falls under the concept of dissipative clocking.

The efficiency of an inverter according to the invention, which necessarily dissipates the electric power obtained from the AC mains in the provision of reactive power and feeds electric power exclusively unidirectionally into the AC mains from the DC or DC power source or a DC bus, is in many cases higher than that of other reactive power despite the dissipation losses Inverters that can exchange electrical power bidirectionally between source or DC link and AC mains. In addition, an inverter according to the invention is significantly cheaper to produce due to the small number of components.

In principle, any conventional inverter topology, in particular one that is not blind-performing, may be modified in accordance with the present invention by operating at least one of the switches of the at least one half-bridge branch in the periods when the inverter receives electrical power from the AC grid in a linear mode of operation is operated as a switchable, regulated resistor and converts the related power into heat. In the case of inverters which have no mains-frequency voltage half-waves generated by an upstream DC-DC converter in the DC link, but a largely constant DC link voltage, a power drain from the DC link into the one controlled, switchable circuit is achieved by suitable control of the remaining bridge switches or a switch located in one of the connections to the DC link Inhibited resistance switch in the linear operating mode. The control of the switch in one of the connections to the DC link takes place in such a way that the DC link and thus also the DC source is electrically separated from the inverter bridge during the periods in which the inverter draws power from the AC grid. During the remainder of the grid period, the switch connects the DC bus and thus also the DC power source to the inverter bridge, allowing power to flow from the DC source to the AC grid.

A special cost saving is achieved when already existing in the conventional inverter bridge switches are used so that only the controller is set up appropriately and suitable drivers for the switch, in particular for the operation of at least one switch in a linear operating mode configured drivers are provided.

The inverters according to the invention are suitable, in the event of a fault, to limit the voltage at the pole reverser by controlled resistive lowering of the voltage and to lower a current flowing through a network-side inductance to zero more quickly and thus to be able to perform a switching operation in a particularly efficient and trouble-free manner. For this purpose, with the help of the at least one switch in the linear operating mode for a short time one of the input side applied voltage of the DC source counter-directed counter-voltage can be generated.

For a more precise control of the current, in particular during the provision of reactive power, it is advantageous to arrange a current measuring device in series with the at least one switch operated in the linear operating mode. Preferably, the current measuring device is located on the input side in front of the at least one switch.

A method according to the invention for operating an inverter for feeding electrical power of a DC power source into an AC voltage network by means of an inverter bridge is characterized in that, during periods in which the inverter draws electrical power from the AC voltage network, this related electric power is converted into heat within the inverter bridge , Preferably, the method comprises driving switches of the inverter bridge, wherein the driving is operated in periods in which the inverter receives electrical power from the AC voltage network, at least one of the switches of the inverter bridge in a linear or dissipatively clocked operating mode, that the so operated at least a switch acts as a switchable, regulated resistor and thus converts the power sourced from the AC mains into heat.

In an alternative embodiment of the method, a switch, which is arranged parallel to the half-bridge branches and between the poles of an input-side DC voltage, is operated and operates in a linear or dissipatively clocked operating mode during the periods in which the inverter draws electrical power from the AC voltage network as a switchable, regulated resistor.

In a further alternative embodiment of the method, a switch of the inverter bridge, which is parallel to the outputs of the inverter bridge and thus parallel to the AC voltage network, is operated in a linear or dissipatively clocked operating mode in the periods in which the inverter receives electrical power from the AC mains and acts as a switchable, controlled resistance.

The conversion according to the invention of the electrical power flowing from the AC network into the inverter into heat can be used particularly advantageously for a transmission of the electrical power of the DC voltage source into the AC network, converting the electrical power of the DC power source into half-wave modulated electrical power and then feeding it the half-wave modulated electric power is used in the AC network by means of a Polwender forming inverter bridge. For this purpose, the inverter preferably comprises an input-side DC-DC converter for half-wave-wise modulation of the electrical power and an output-side inverter bridge, which half-wave-folds the polarity of the half-wave-modulated electrical power. In this embodiment, the inverter bridge is operated as a pole turner.

In addition, the method can be used in a conventional inherently blind power invertor topology, for example, in a single-phase H4, H5 or H6 bridge, or even in a three-phase B6 or NPC topology, both in single-stage and single-phase in multi-stage execution. In all of these topologies, conventionally, during the periods in which electrical power from the AC mains is flowing into the inverter, electrical currents are passed through one or more discrete intrinsic diodes included in the switches of the inverter bridge, which are electrically arranged in parallel with switches blocking those periods are. This results in either corresponding discrete diodes must be provided, which means increased component cost, or to the fact that the intrinsic diodes contained in the switches must meet special requirements, which limits the selection of usable switches and in particular the use optimized especially for low forward losses Can prevent switch. Again, the advantages that arise when using the solution according to the invention from the saving of components or the availability of optimal switch, the disadvantages that brings the waiver of the recycling of the energy flowing into the inverter with it.

BRIEF DESCRIPTION OF THE FIGURES

In the following the invention will be further explained and described with reference to embodiments shown in the figures.

1a shows a topology of a grid connected inverter known from the prior art,

1b shows a further topology of a grid-connected inverter known from the prior art,

2 shows an example of time courses of mains voltage, current and power at the output an inverter with a phase shift of 30 degrees,

3 shows an inverter bridge at two different times relative to the zero crossing of the mains voltage,

4 shows an inverter bridge according to 3 with detailed control circuit in two different variants,

5 shows an embodiment of an inverter bridge with additional switch, and

6 shows an inverter bridge according to 5 at two different times relative to the zero crossing of the mains voltage.

DESCRIPTION OF THE FIGURES

1a and 1b show topologies of grid-connected inverters known from the prior art, with a DC or DC source arranged on the left in each case 100 electrical power taken and in the right each arranged AC voltage network 110 is fed as alternating current.

The topology in 1a consists of a DC-DC converter 120 , which is designed here as a buck converter and high frequency (HF), for example a few kilohertz, one with the network frequency (NF) of the AC voltage network 110 modulated DC generated whose polarity of a power frequency (NF) operated inverter bridge 130 depending on the current polarity of the mains voltage of the alternating voltage network 110 is folded. The inverter bridge 130 works as a turner.

The topology according to 1b includes an inverter, which also has a DC-DC converter 120 includes, which is designed here as a high-frequency resonant converter and initially generates a high-frequency clocked inverter bridge a high-frequency alternating current, which is fed via a transformer and a rectifier bridge in a DC link capacitor. The high frequency clocked inverter bridge can be operated in such a way that the DC current flowing to the DC link capacitor or the voltage applied to the DC link capacitor voltage is modulated with the mains frequency, so that the downstream network-side inverter bridge 130 can be clocked as Polwender with the mains frequency.

In the 1a and 1b shown topologies are only very limited blind power, especially if the respective output side inverter bridges 130 be operated as a pole turner and no DC link exists ( 1a ), or if the DC link is made very small for cost reasons ( 1b ).

2 shows time courses of mains voltage V, current I and power P = V · I at the output of an inverter in normalized to their respective maxima V _peak , I _peak , P _peak values at a phase shift φ = 30 degrees. To facilitate the following discussion, time is represented as phase ωt within the network period. It is assumed that the inverter is connected to an AC mains 110 is connected with an ideal sinusoidal course of the mains voltage V. In the period 30 ° <ωt <180 ° current I and voltage V have the same polarity, so that in this period electric power P from the inverter into the AC mains 110 flows. The electrical energy E, which is fed in this period, corresponds to the integral of the power P over this period of time and is referred to below as E _source . In the period 0 <ωt <30 ° current I and voltage V have opposite polarity, so that in this period electric power P flows from the AC voltage network in the inverter. The electrical energy E, which flows into the inverter during this period, again corresponds to the integral of the power P over this period of time and is referred to below as E _sink .

The resulting energy E, in the period 0 <ωt <180 °, ie in a half-wave, in the alternating voltage network 110 is fed, E = E _source - E _sink . Assuming that the energy E _sink flowing into the inverter can not be recycled but is converted into heat, for example, the efficiency η of the inverter is reduced by the ratio of the integrals over the periods with the different power flow _directions :

Using the indefinite integral ∫sin (x) sin (x - φ) dx = x / 2cos (φ) + 1 / 4sin (φ - 2x) + C this equation can be solved and it results for the _{loss of} efficiency η _loss : η _loss = sin (φ) - φcos (φ) / sin (φ) + (π - φ) cos (φ)

Im in 2 shown with φ = 30 degrees, which corresponds to a power factor of approximately PF = 0.87, the maximum active power is about 87 percent of the apparent power, while the maximum reactive power Q = sin (30 °) is about 50 percent of the apparent power. On the other hand, the efficiency loss calculations given above show that the energy flowing into the inverter at a power factor of PF = 0.87 is only about 1.5 percent, at PF = 0.9 about 1 percent and at PF = 0.95 even only is about 0.35 percent of the energy fed into the AC grid.

This finding is surprising since an undesirable per se waiver of recycling in response to the supply of reactive power with power factors in the range of about 0.85 <PF <1 flowing into the inverter energy E only a marginal effect _sink on the overall efficiency η of Inverter has. In this case, the overall efficiency η due to other losses (switching losses, diode losses, resistive losses, etc.) are already a few percent below the theoretical optimum of 100 percent or already optimized by minimizing other losses such that the additional heat generated is easily dissipated. This results in the remarkable conclusion that the advantages of an inverter according to the invention, which is clearly simpler and less expensive to build compared to a bidirectionally operating inverter. its disadvantages, which entail the renouncement of the recycling of the energy flowing into the inverter, outweigh. This is especially true considering the fact that in many applications, the provision of a higher proportion of reactive power from the inverter is required only relatively rarely.

In 3 is an inverter bridge 330 This conclusion is made using the example of a Polwenders. The pole turner consists here by way of example of two half-bridge branches each with two switches HS1, HS2, LS1, LS2, the end points of the half-bridges being connected to a DC or DC source (DC source). 300 and the midpoints of the half bridges with the AC voltage network 310 are connected. The DC source 300 provides a half-wave sinusoidally modulated electrical power while the Polwender mains frequency reverses the polarity of the output voltage according to the mains voltage.

In principle, each of the switches of the half-bridge arms is adapted to convert the electrical power that flows into the inverter as part of the provision of reactive power, by converting it into heat during the respective periods of time (see FIG. 2 ) is operated in a linear mode of operation as a switchable, regulated resistor. In practice, it is preferable that the connected to the negative pole of the input-side DC source switch, ie the in 3 operate lower, so-called Lowside switch LS1, LS2 such.

In 3 are the current I from the DC source 300 that is in the AC mains 310 fed AC power _grid I, who entered through the left low-side switch LS1 flowing half-bridge current I _I and the current flowing through the right low-side switch LS2 half-bridge current I _R. In the left part of the 3 is a situation at a time just before ωt = 180 ° (cf. 2 ), that is, at a time when the grid current I _grid and grid voltage V still have the same polarity. The right low-side switch LS2 is completely conductive and conducts the grid current I _grid , ie I _R = I _grid . Provided the current I from the DC source 300 likewise corresponds to the mains current I _grid , the half-bridge current I _L = I-I _grid flowing through the left low-side switch LS1 is equal to zero. Should the current I from the DC source 300 which, in the ideal case, is already modulated in such a way that it forms sine half-waves, for some reasons be too high, for example higher than the grid current I _grid , the left low-side switch LS1 can be put into a linear operating mode (shown here by another switch symbol) and convert the excess current I _L = I - I _grid into heat.

In the right part of the 3 is a situation at a time shortly after ωt = 180 ° shown, ie at a time when the grid I _grid and grid voltage V have different polarity and electrical power from AC mains 310 flows into the inverter. Due to the now reversed polarity of the mains voltage, the polarity reverser is folded, and the left low-side switch LS1 is now completely conductive and conducts the current I from the DC source 300 , which is at best only slightly smaller than the situation previously described for the time just before ωt = 180 °; since the grid current I _grid only slightly decreased between the times shortly before and shortly after ωt = 180 °., I _L = - I _grid (under the above assumption that I _R = I _grid ). In this situation, therefore, the current I would be from the DC source 300 just opposite to the required mains current I _grid , if the Polwender would be operated conventionally and the right low-side switch LS2 would be in a locked state.

This contradiction is inventively resolved that the right lowside switch LS2 is operated in such a linear operating mode as a switchable, controlled resistance that it is the sum of the required grid current I _grid and the current I from the DC source 300 conducts, ie I _R = I + I _grid , and converts the associated electrical power into heat. It is recommended, now the current I from the DC source 300 as soon as possible to zero, and the DC source 300 only shortly before the end of the period in which line current I _grid and line voltage V have different polarity (zero crossing of the line current I _grid , see point in time ωt = 210 ° in 2 ) to start again. Any excess electric power between starting and zero crossing of the mains current I _grid can in turn be dissipated in the right low side switch LS2.

After the zero crossing of the mains current I _grid , current I is from the DC source 300 and the grid current I _grid no longer in opposite directions and the right low-side switch LS2 is completely disabled. The modulation of the mains current can then again by modulation of the electrical power of the DC source 300 be generated.

The inverter bridge described here 330 can also be used in an analog method to avoid distortions of the grid I _grid . Such distortions can occur in the region of the zero crossing of the grid current I _grid , if the DC source side of the inverter bridge 330 a bidirectional DC-DC converter stage is used, whereby the direction of the current I turn around and in principle also power from the AC mains 310 can flow into the inverter. Also in this case, the current from the DC-DC converter stage, which can have a significant value just before the zero crossing of the grid current I _grid , must be reduced to zero as quickly as possible, including one of the low-side switches (in the above example to 3 the right low-side switch LS2) in linear operating mode is particularly suitable as a switchable, regulated resistor.

4 shows two variants of a drive circuit for the switches of the inverter bridge, which are operated temporarily in the linear operating mode as a switchable, controlled resistance. The following statements refer to those in the left part of the 4 illustrated variant, where in the right part of 4 variant shown largely works analogously thereto.

At each of the control electrodes of the four switches HS1, HS2, LS1, LS2 of the inverter bridge (compare also 2 and 5 ) is a driver chip 440 connected. Here are the driver blocks 440 the upper so-called high-side switch HS1, HS2 substantially conventional design, as they are known to be used in inverters with digital switching switches, and can put the switches connected to them either in a conducting or in a blocking state.

• – der Schalter ist leitend, wenn am Eingang „on” eine logische 1 („high”) anliegt
• – der Schalter ist gesperrt, wenn am Eingang „on” eine logische 0 („low”) anliegt und am Eingang „Enable” (E) eine logische 0 („low”) anliegt
• – der Schalter ist in einem linearen Betriebsmodus, wenn am Eingang „on” eine logische 0 („low”) anliegt und am „Enable”-Eingang (E) eine logische 1 („high”) anliegt

The driver blocks 440 however, the low side switches LS1, LS2 are modified so that they can operate the low side switches LS1, LS2 in the following operating modes:

• - The switch is conductive if a logic 1 ("high") is present at the input "on"
• - The switch is disabled if a logic 0 ("low") is present at the input "on" and a logic 0 ("low") is present at the input "Enable" (E)
• - The switch is in a linear operating mode, if at the input "on" a logical 0 ("low") is applied and at the "Enable" input (E) is a logical 1 ("high")

In the latter linear operating mode, the respective switch is operated as a switchable, regulated resistor, wherein the current through the left switch LS1 to V _cL / R _s and the current through the right switch LS2 adjusts to V _cR / R _s . In this case, the control signals VcL and VcR are determined from the sum of the voltages dropping across the resistors Rs minus or plus the quotient of the instantaneously required mains current I _grid and the resistance value R _s .

The signal CD present at the "Enable" inputs E is set in such a manner in dependence on the control of the DC source that in the periods in which no control signal ("PWM from micro") is generated for the DC source logical 1 ("high") is present. These periods in which the DC source generates no electrical power, correspond in the case of providing reactive power just those periods in which power flows from the AC mains in the inverter. Conversely, the signal CD is always logic 0 ("low"), if no reactive power is provided, so that then all switches are either completely conductive or completely blocked.

5 shows an embodiment of an inverter bridge 530 , in addition to two conventional half-bridge branches 550 another switch T1, which is parallel to the half-bridge branches 550 and thus between the poles of the input DC source 500 is settled. A control of this inverter bridge is preferred 530 set up the further switch T1 in the periods during which electrical power from the AC mains 510 flows into the inverter to operate in linear operation as a switchable, regulated resistance. Optionally, this can also be an additional resistor 560 be used. For the details of the control will open 6 in which the different in the inverter bridge 630 flowing currents at two different times shortly before (left part) and shortly after (right part) the zero crossing of the mains current I _grid are shown, in combination with the descriptions to 3 and 4 The switch T1 optionally assumes the function of the left low side switch LS1 or the right low side switch LS2 in the linear operating mode. This embodiment has, inter alia, the advantage that the further switch T1 can be optimally designed for the conversion of electrical power into heat.

In an alternative embodiment, not shown, the switch T1 may also be between the outputs of the half-bridge branches 550 be arranged, ie parallel to the in 5 indicated AC output. In a further embodiment, the switch T1 may be part of a dissipation device that is located between the poles of the input-side DC source 500 and / or between the outputs of the half-bridge branches 550 can be arranged.

It will be understood that as an alternative to the previously described linear operation of a switch as a switchable, regulated resistor, alternative embodiments are possible in order to realize the inventive feature of converting the electrical power flowing from the AC mains into the inverter into heat. In particular, a dissipatively clocked operation of the switch is suitable, which may be a clocked operation with a capacitor connected in parallel and / or a resistor connected in series, or such a clocked operation that the switching losses of the switch by the clock to the desired heat conversion cumulate. The switching losses of the switch can be influenced by the frequency of the drive or the shape of the drive signal. In particular, for the embodiment of the inverter bridge with a separate switch in parallel with the half-bridge branches, the switch being used exclusively for converting the electrical power flowing from the AC grid into the inverter into heat, the switch can also be optimally designed for this conversion. This is in addition to the linear or dissipative clocked operation of the separate switch and a resistor chain with multiple part switches, the part switches depending on the currently required resistance one or more resistors of the resistor chain in the electrical path parallel to the half-bridge branches and thus between the poles of the input side DC Source can switch.

Advantageous developments of the invention will become apparent from the claims, the description and the drawings. The advantages of features and of combinations of several features mentioned in the introduction to the description are merely exemplary and can come into effect alternatively or cumulatively, without the advantages having to be achieved by embodiments according to the invention. Further features can be taken from the drawings. The combination of features of different embodiments of the invention or of features of different claims is also possible deviating from the chosen relationships of the claims and is hereby stimulated. This also applies to those features which are shown in separate drawings or are mentioned in their description. These features can also be combined with features of different claims. Likewise, in the claims listed features for further embodiments of the invention can be omitted.

LIST OF REFERENCE NUMBERS

100, 300, 500, 600

DC source

110, 310, 510, 610

AC power

120

DC converter

130, 330, 530, 630

Inverter bridge

440

driver blocks

550

Half-bridge branch

560

resistance

HS1, HS2, LS1, LS2, T1

switch

Claims (19)

Inverter for supplying electrical power to a DC power source ( 100 . 300 . 500 . 600 ) into an alternating voltage network ( 110 . 310 . 510 . 610 ) with one with the DC power source ( 100 . 300 . 500 . 600 ) connectable inverter bridge ( 130 . 330 . 530 . 630 ), characterized in that a control of switches of the inverter bridge ( 130 . 330 . 530 . 630 ) is set up such that during a supply of reactive power in periods of a grid period of the alternating voltage network ( 110 . 310 . 510 . 610 ), in which the inverter generates electrical power from the AC mains ( 110 . 310 . 510 . 610 ), a first switch (T1, LS1) of the inverter bridge ( 130 . 330 . 530 . 630 ) is operated such that the from the AC mains ( 110 . 310 . 510 . 610 ) related power is converted into heat. Inverter according to claim 1, characterized in that the inverter bridge ( 130 . 330 . 530 . 630 ) is executed as Polwender. Inverter according to one of claims 1 or 2, wherein the inverter bridge ( 130 . 330 . 530 . 630 ) a modulation device for providing a half-wave-wise modulated electrical power from the DC power source ( 100 . 300 . 500 . 600 ) is connected upstream. Inverter according to one of the preceding claims, wherein the controller is set up in such a way that the first switch (T1, LS1) for converting the voltage from the AC voltage network ( 110 . 310 . 510 . 610 ) is converted into heat in a linear operating mode or in a dissipatively clocked operating mode. Inverter according to one of the preceding claims, wherein an additional switch is provided, which the direct current source ( 100 . 300 . 500 . 600 ) from the inverter bridge ( 130 . 330 . 530 . 630 ) electrically disconnects during the periods in which the inverter electrical power from the AC mains ( 110 . 310 . 510 . 610 ). Inverter according to one of the preceding claims, wherein the first switch (T1, LS1) with a negative DC voltage input of the inverter bridge ( 130 . 330 . 530 . 630 ) connected is. Inverter according to one of the preceding claims, wherein a first half-bridge of the inverter bridge ( 130 . 330 . 530 . 630 ) comprises the first switch (LS1). Inverter according to claim 7, wherein a second half-bridge of the inverter bridge ( 130 . 330 . 530 . 630 ) comprises a second switch (LS2), and wherein the control of the switches of the inverter bridge ( 130 . 330 . 530 . 630 ) is set up in such a way that, in the case of the supply of reactive power during periods of a grid period of the alternating voltage network ( 110 . 310 . 510 . 610 ), in which the inverter generates electrical power from the AC mains ( 110 . 310 . 510 . 610 ), optionally the first switch (LS1) or the second switch (LS2) of the inverter bridge ( 130 . 330 . 530 . 630 ) for the conversion of the AC voltage network ( 110 . 310 . 510 . 610 ) is converted into heat in a linear operating mode or in a pulsed operating mode. Inverter according to one of Claims 1 to 6, in which the first switch (T1) is connected in parallel with half-bridge branches ( 550 ) of the inverter bridge ( 130 . 330 . 530 . 630 ) is arranged. Inverter according to one of claims 1 to 6, wherein the first switch is connected in parallel to the alternating voltage network ( 110 . 310 . 510 . 610 ) is arranged. An inverter according to any one of claims 9 to 10, wherein the first switch (T1) is a bidirectional switch. An inverter according to any one of the preceding claims, wherein the first switch (T1, LS1) is an IGBT or a MOSFET. An inverter as claimed in any one of the preceding claims, wherein the controller is adapted to convert the power into heat in a linear mode of operation or in a dissipatively clocked mode of operation of the first switch (T1, LS1) in response to a first switch (T1, T1, LS1). LS1) flowing current controls. Method for operating an inverter for supplying electrical power to a DC power source ( 100 . 300 . 500 . 600 ) into an alternating voltage network ( 110 . 310 . 510 . 610 ) by means of an inverter bridge ( 130 . 330 . 530 . 630 ), characterized in that during periods of a grid period of the alternating voltage network ( 110 . 310 . 510 . 610 ), in which the inverter generates electrical power from the AC mains ( 110 . 310 . 510 . 610 ) derived from the AC mains ( 110 . 310 . 510 . 610 ) related electrical power within the inverter bridge ( 130 . 330 . 530 . 630 ) is converted into heat. Method according to claim 14, wherein a first switch (T1, LS1) of the inverter bridge ( 130 . 330 . 530 . 630 ) for conversion from the AC mains ( 110 . 310 . 510 . 610 ) is converted into heat in a linear operating mode or in a dissipatively clocked operating mode. Method according to one of claims 14 to 15, wherein the inverter bridge ( 130 . 330 . 530 . 630 ) is operated as Polwender. Method according to one of Claims 14 to 16, in which, in the event of an error, the first switch (T1, LS1) is set in the linear operating mode in such a way that a DC-voltage on the inverter bridge ( 130 . 330 . 530 . 630 ) is reduced. Method according to one of Claims 14 to 17, in which, in the event of a fault, the first switch (T1, LS1) is set in the linear operating mode in such a way that one of the direct current source ( 100 . 300 . 500 . 600 ) into the inverter bridge ( 130 . 330 . 530 . 630 ) flowing stream is reduced. Method according to one of claims 14 to 18, wherein the time segments of a network period of the alternating voltage network ( 110 . 310 . 510 . 610 ), in which the inverter generates electrical power from the AC mains ( 110 . 310 . 510 . 610 ), only time periods of a predetermined polarity of the voltage of the alternating voltage network ( 110 . 310 . 510 . 610 ).

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