WO2007036374A2

WO2007036374A2 - Power inverter for two direct current sources and a method for the operation thereof

Info

Publication number: WO2007036374A2
Application number: PCT/EP2006/064250
Authority: WO
Inventors: Harald Schweigert
Original assignee: SIEMENS AKTIENGESELLSCHAFT öSTERREICH
Priority date: 2005-09-28
Filing date: 2006-07-14
Publication date: 2007-04-05
Also published as: DE102005046379B4; EP1929617A2; CN101278471A; JP2009510986A; JP4705682B2; KR20080066701A; DE102005046379A1; CN101278471B; WO2007036374A3

Abstract

The invention relates to a power inverter for supplying power from first and second direct current sources with a common reference potential (0) in an alternating current network, wherein said power converter is connected, on the output side thereof, to the conductor (L1Netz) and to the neutral conductor (NNetz) of the alternating current network, the first direct current source has a potential (1) positive with respect to the reference potential (0), the second direct current source has a potential (2) negative with respect to the reference potential (0) and the reference potential (0) is connected to the neutral conductor (NNetz), the power inverter comprises a first step-down controller by which the positive potential (1) is connected to the conductor (L1Netz) of the alternating current network and a second step-down controller by which the negative potential (2) is connected to the conductor (NNetz) of the alternating current network. The inventive device reduces the number of necessary components to a minimum, thereby minimising power dissipation and raises the efficiency.

Description

Inverter for two DC sources and method of inverter operation

description

The invention relates to an inverter for feeding electrical energy from a first and a second DC power source having a common reference potential in an AC network, the inverter being connected on the output side to a conductor and a neutral conductor of the AC mains. Furthermore, the invention relates to a method for operating the inverter.

To feed the energy from DC sources into an AC grid, inverters with different topologies are known. DC power sources such as photovoltaic cells, fuel cells, batteries, etc., generally have the discharge current dependent voltage characteristics. Due to external influences, such as changing lighting conditions in photovoltaic cells, the maximum extraction power, also called maximum power point (MPP), is constantly changing. The regulation of an inverter must take into account such dynamic operating conditions.

In addition, AC grid operators demand that inverters feed a sinusoidal current into an AC grid, whether it be a grid or a stand-alone grid.

A simple way of controlling an inverter is given in US Pat. No. 6,914,418 B2. It is described a so-called MPP tracking, in which continuously the extraction current is slightly changed and multiplied by the measured voltage of the DC power source. The resulting extraction capacity is compared with the immediately before measured. Likewise, the voltage is compared with that measured immediately before. According to the Changes in extraction capacity and voltage in the next step, a higher or lower extraction current is specified.

Alternatively, US Pat. No. 4,390,940 A1 specifies an inverter with which photovoltaic cells can be operated with a maximum removal power.

The selection of a known inverter topology for a particular application depends, among others, on the

Voltage level of the connected DC sources from. If the voltage level of the DC power sources is below the peak voltage of the AC mains to which power is supplied, the inverter typically includes a boost converter stage and an inverter stage. The

US 2004/0165408 A1 describes such an inverter, in which case two DC sources are connected to a common reference potential.

Since a Hochsetzerstufe is not lossless and thus reduces the efficiency of the inverter, so you also know inverters that include only one Weselrichterstufe. Then the voltage of the connected DC sources must always be above the peak voltage of the AC mains. According to the state of the art, DC sources are therefore combined into so-called strings, resulting in multiple output voltages of the individual DC sources as string output voltages.

Especially alternative DC sources such as

Photovoltaic systems or fuel cells require a high efficiency of the inverter for their economic use. The object of the invention is therefore to provide a comparison with the prior art improved inverter with high efficiency.

According to the invention, this object is achieved with a

Inverter for feeding electrical energy from a first and a second DC power source with a common reference potential in an AC network, the inverter being connected on the output side to a conductor and a neutral conductor of the AC network, and wherein

the first DC power source has a positive potential with respect to the reference potential,

the second DC power source has a negative potential with respect to the reference potential, the reference potential of the two DC power sources is connected to the neutral conductor,

- The inverter comprises a first buck converter with which the positive potential is connected to the conductor of the AC mains, - The inverter comprises a second buck converter with which the negative potential is connected to the conductor of the AC mains.

This arrangement reduces the necessary components to a minimum, whereby the power loss compared to known inverter circuits is reduced and the efficiency is optimized. In this case, the voltages of the DC sources are at least equal to or higher than the recurring maximum expected peak voltage of the AC network.

This arrangement is advantageous, above all, if the first and the second DC power source are designed as so-called strings of a photovoltaic system. Each string delivers a voltage above the peak voltage of the AC voltage network. Since photovoltaic systems are usually mounted on roofs of buildings, the connection of the reference potential of the two strings causes with the Neutral conductor, that in the building there is no disturbing electrical power frequency alternating field with earth.

An advantageous embodiment provides that the first step-down converter comprises a first capacitor, a first switching element, a first diode connected in series with a first diode and a choke circuit and the input side with the positive potential and the reference potential of the first DC power source and the output side via a filter capacitor The second step-down converter comprises a second capacitor, a second switching element, a second diode connected in series with a second diode and the choke circuit and the input side with the negative potential and the reference potential of the second DC power source and the output side via the filter capacitor is connected to the conductor of the AC mains and that the neutral is continuously connected to the reference potential of the first and the second DC power source. This topology forms in a particularly simple manner, the two buck converter for the connection of two DC power sources to an AC network.

It is advantageous if the throttle circuit is divided into a first throttle element and a second throttle element, when the first step-down divider comprises the first throttle element and when the second step-down divider comprises the second throttle element. This significantly reduces the load on the switching elements.

A further increase in the efficiency is achieved when the positive potential of the first DC power source and the negative potential of the second DC power source are connected to each other via a balance converter. This is especially important when the AC power fed current must not have a direct current component. Differences in the maximum possible power output of the two DC power sources can then be compensated for by transferring the excess power of a DC power source to the power path and thus to the potential of the other DC power source by means of a balance transformer.

An advantageous arrangement further provides that the compensation converter comprises a third switching element and a fourth switching element connected in series, and that a

Connection point between the third switching element and the fourth switching element via a third throttle and a means for measuring current is connected to the reference potential. This gives a simple topology of the balance converter with few components and high efficiency.

It is advantageous if the means for measuring the current comprises a shunt resistor. The current can then be measured easily. Other ways of measuring current, such as by means of a DC-compensated magnetic transducer, are also possible.

It is also advantageous if antiparallel to the first switching element, a third diode is arranged and when antiparallel to the second switching element, a fourth diode is arranged. When the inverter is disconnected from the AC mains, the energy stored in the choke circuit can then be dissipated via these diodes. In addition, these diodes form elements of a protection circuit to protect the Weselrichters against voltage spikes in the AC network.

To protect the components in the inverter from voltage spikes in the AC network, it is also advantageous if a connection point between the second switching element and the second throttle element via a fifth diode and a parallel circuit of a first resistor and a third capacitor to the reference potential is connected and when a connection point between the first switching element and the first throttle element via a sixth diode and a parallel circuit of a second resistor and a fourth capacitor is connected to the reference potential. About this and the antiparallel to the first and second switching element arranged diodes are then constructed at voltage peaks from the AC mains current paths to the capacitors, whereby the short-term overvoltages drop across the choke circuit and thus do not burden the switching elements. The switching elements must therefore not be oversized and there are no additional complex filters necessary.

It is also advantageous if the inverter has a

Control unit comprising suitable means for controlling the switching elements and auxiliary switching elements. The control signals are then generated in the inverter itself, whereby an integrated design of the inverter is possible.

Operates the inverter according to the invention by the two buck converters are driven alternately in such a way that a feed current in the form of whole sine waves results and that the positive half sine waves are trained by means of the first buck converter from the input side applied positive potential and that the negative half sine waves by means of second buck converter be formed from the input side negative potential. Thus, the energy from two DC sources is fed into an AC network in a simple manner.

It is advantageous if continuously the voltage and the extraction current of the first DC power source is measured and when the first step-down is controlled in such a way that continuously the product of extraction current and voltage of the first DC source of the momentarily to be delivered maximum Power of the first DC power source is approximated and when continuously the voltage and the Entnahmeström the second DC power source is measured that the second buck converter is controlled in such a way that continuously the product of bleed current and voltage of the second

DC source of the momentarily maximum output power of the second DC power source is approximated. The two DC sources are then always taken the maximum output power and thus optimizes the overall efficiency.

In order to prevent a DC component of the feed, it is advantageous if the DC component of the feed current is measured continuously and if the lower set divider a lower feed-in power is given at positive DC component and if the negative feed DC share a lower feed-in power is given to the second buck converter.

In this case, an advantageous embodiment provides that, as of a predetermined reduction in the feed-in power of a buck converter, energy is transferred from the potential to which this buck converter is connected to the other potential by means of a compensation converter. The potential at which the first buck converter is connected corresponds to the positive connection of the first DC source. The potential at which the second buck converter is connected corresponds to the negative terminal of the second DC power source. The extraction currents of the step-down dividers can then continue to be specified in such a way that the maximum possible power is taken from the DC sources.

Advantageously, the control unit gives the step-down divider, which is connected to the potential to which the compensating transformer transfers energy, a higher one Feed-in performance, even before an adjustment of the performance specifications takes place via MPP tracking.

The invention will now be described by way of example with reference to the accompanying drawings. In a schematic representation:

Fig. 1 Topology of a basic circuit Fig. 2 circuit with two throttle elements Fig. 3 circuit with compensating converter

Fig. 4 circuit with compensating transformer and protection circuit against voltage spikes

FIG. 1 shows an exemplary circuit topology for an inverter according to the invention with two buck converters arranged in parallel. To the first step-down converter, consisting of a first capacitor Cl, a first switching element Sl (eg transistor), a first diode Dl and a choke as a choke circuit L is the input side, the first DC power source with its positive terminal as positive potential 1 and its negative terminal as a reference potential 0 connected. The second DC power source is connected with its positive terminal as reference potential 0 and with its negative terminal as a negative potential 2 to the second buck converter, which consists of a second capacitor C2, a second switching element S2, a second diode D2 and the throttle as a throttle circuit L.

So that no network short circuit arises, there are two more

Auxiliary switching elements HSl and HS2 arranged. In this case, the first auxiliary switching element HSL is arranged in series with the first diode D1 of the first step-down converter. It is off when the second buck converter is working. The second auxiliary switching element HS2 is arranged in series with the second diode D2 of the second buck converter and is turned off when the first buck converter is operating. The circuit shown in Figure 2 differs from that shown in Figure 1 only in that are arranged as a choke circuit, two throttle elements Ll and L2, wherein the first buck converter the first

Throttle element Ll and the second step-down divider comprises the second throttle element L2. This arrangement causes a lower load on the switching elements Sl and S2.

On the output side, the inverter is connected to an AC network, wherein the reference potential 0 is continuously connected to the neutral conductor N _network and the output of the choke circuit L to a conductor Ll _{network of} the AC _mains . On the output side, a filter capacitor CF is additionally arranged between the neutral conductor N _network and the conductor Ll _network .

To the switching elements Sl and S2 antiparallel diodes D3 and D4 are arranged, which keep a current path for demagnetizing the choke circuit L open at a separation of the inverter from the AC mains.

The two DC sources are formed for example by two strings of a photovoltaic system. The inverter then feeds energy from the strings into the connected AC grid when the string voltage is higher than the peak of the maximum expected AC voltage (e.g., 230V + 10% x 1.414 = 358V).

In addition, both strings are made of the same size panel surfaces, since the first panel surface only the first

Downsampler supplied, which feeds in the positive half-wave energy and the second panel area only supplies the second buck converter, which feeds energy in the negative half-wave. In order not to feed any DC component into the AC grid, the same amount of energy is taken from both panel surfaces. The two capacitors C 1 and C 2 must be dimensioned sufficiently large since each of the two step-down dividers feeds energy into the alternating current network only during the half-cycles assigned to it and no energy is emitted therebetween. However, the capacitors C1 and C2 continue to be charged by the DC power sources during periods of lack of power output, and the voltage limits set for the circuit must not be reached.

The inverter is operated as follows:

On the output side, the DC component of the current supplied to the AC mains is measured. This can be done for example by means of a current transformer with Hall converter. The residual DC current thus measured forms an input variable for the regulation of the two buck converter.

The step-down dividers are regulated with a current setpoint specification. AC grid operators are required to have the current supplied to the AC grid sinusoidal, i. without power harmonics, must be. In order to achieve this, there are two possibilities for basic target current formation: a) By means of a voltage divider, a sine half-wave is derived from the mains voltage and used as a model for the current form. b) In a memory (for example EPROM), a sine half-cycle is stored as a table and converted into a analog signal by a DA converter during read-out in the 50 Hz rhythm. This solution requires the generation of a synchronizing pulse from the mains voltage to indicate the beginning of the respective half-wave and to start the read-out process from the memory. Although this is more complicated than solution a), but allows to adapt with an adaptation of the table on circuit-induced distortions of the current and to compensate for them. The result is in both cases a sequence of

Half sine waves. In this case, a basic nominal current signal is generated for the first step-down divider, the positive Sinushalbwellen exists and for the second buck converter one which consists of negative sine half-waves. An addition of the two basic nominal current signals thus results in a complete sine wave signal.

In a next step, a load value is generated for each of the two DC sources. In each case, the value is to be determined at which each DC power source outputs the maximum power (Maximum Power Point, MPP). For a panel surface of a photovoltaic system, a characteristic for the current as a function of solar radiation can be specified, in which a maximum of power is fed into the grid. The solar radiation can then be measured and the corresponding current value can be specified. However, disturbing factors such as partial shading or soiling of panel surfaces are not taken into account.

It is therefore more expedient to carry out the so-called MPP tracking, in which the current drawn from a string and the corresponding string voltage are continuously measured and multiplied together. By slight variation of the load, it can then be determined whether an increase in output tends to be possible or the maximum has already been reached.

As a result of the MPP tracking, there is an output signal that either describes the setpoint current from a string or the setpoint voltage. If the setpoint voltage is specified, the inverter must increase the current until the string voltage drops to the specified value. In the case of the invention, a setpoint voltage is more appropriate as a regulation input. Due to the connected capacitors Cl and C2, this does not change so quickly and the control becomes more stable than with current setpoints. For each of the two DC sources, MPP tracking therefore specifies a setpoint voltage as the setpoint. In this case, the voltage across the capacitor C1 or C2 is compared with the respective setpoint value for each DC power source by means of a differential amplifier. The

The control characteristic requires an integral component (for example PI controller) in order to act slowly and to keep the control deviations low.

To limit the power of each buck converter to protect the power components, it is expedient to provide a maximum value for the output signal of each of the two differential amplifiers. Thereby, the target current can be limited without generating current distortions and harmonics in the AC line current.

The set current formation for controlling each buck converter is effected by multiplication of the output signals of the differential amplifier with the respective basic nominal current signal. The output signal of the differential amplifier of the first

Step-down converter is multiplied by the basic setpoint current signal, which is formed from the sequence of positive half sine waves. The output signal of the differential amplifier of the second buck converter is multiplied by the basic nominal current signal, which is formed from the sequence of negative sine half-waves.

It is expedient to control the buck converters in so-called current mode. In this case, the switching frequency of the switching elements Sl and S2 is determined by a clock generator (for example 3OkHz).

The individual switching operations are then the following for the variant shown in FIG. 2 (in which case the first step-down divider is initially considered): The switching element Sl is turned on at the beginning of a period. As a result, current flows into the throttle element Ll and increases according to the relationship:

Δl (current increase) =

= [U (voltage) / L (inductance Ll)] x Δt (switch-on time)

By means of a comparator current flowing through the throttle element Ll current is compared with the target current value. When the inductor current reaches the setpoint current value, the comparator switches off the switching element S1 and the inductor current commutates to the first diode D1 and the auxiliary switching element SH1 connected in series.

To protect against rapidly rising overcurrents (as a result of mains voltage pulses, external electronics faults, etc.), it is expedient to use a comparator for current measurement with a fixed limit value which immediately switches off the switching elements S1 and S2 in the event of current violations.

The current is best to measure in the drain line of the first switching element Sl; by shunt resistor or DC capable (compensated) current transformer. Due to the one-sided connection to the first capacitor Cl, the measured current signal is relatively trouble-free.

It is also possible to measure the current in the demagnetization phase of the first throttle element Ll. Here is the cheapest measurement point in the

Drain line of the first auxiliary switching element HSl. This current is then phase-shifted with respect to the current through the first switching element Sl. It can also be measured only the last flowed through the first switching element Sl current. The throttle element Ll prevents abrupt current changes, so that after switching off the first switching element Sl and the commutation of the inductor current to the first diode Dl and the first auxiliary switching element HSL at the first moment nor the last flowed through the first switching element Sl current continues to flow. Depending on the detected current value must then be intervened in the turn-on of the first switching element Sl by influencing the clock generator.

The control of the first switching element Sl then no longer takes place according to the known current mode, since the increasing inductor current does not directly lead to switching off of the switching element Sl, but a detected at a later time current value is used.

The second step-down divider comprising the capacitor C2, the second switching element S2, the second choke element L2, the second diode D2 and the second auxiliary switching element HS2 operate in the corresponding manner during the negative line half-cycles.

Since it is an inverter for two DC sources, it makes sense to disable the control stage of the not in use Tiefsetzstellers. This is especially important in the transition from positive to negative half-wave. There may easily be disturbances in the AC network around the grid neutral point (e.g., by ripple control pulses, switching operations). If both buck-boosters were then activated, current would flow between the two buck-boosters and lead to significant losses.

As already explained above, it is regulated in the feed-in regulations of AC grid operators that no or only a very small proportion of direct current may be fed into the AC grid. To meet these requirements, the following is appropriate: A current sensor (transducer or shunt resistor) in the inverter's line feed line measures the AC current that is being fed. About an integrator, which consists in the simplest case of an RC element with a time constant well above the 50Hz mains frequency, a direct current flowing into the network can be detected. Alternatively, the stream of each sine half-wave may be digitized, integrated into a processor and subtracted from each other.

From the DC signal, a correction signal is derived. This correction signal intervenes as an additional signal in the current control of the buck converter and acts for the Tiefsetzsteiler power-limiting, which feeds the DC component in the AC mains. It is always possible to reduce the feed-in power of a buck converter, since a DC power source already running in the MPP operating point is used to compensate for

Inequalities of the feed streams can no longer deliver power.

Thus, the current drain from that DC power source having the higher possible feed-in power (e.g., higher efficiency fuel cell, larger panel area or different roof pitch in photovoltaic systems, different line losses, etc.) is reduced so as not to supply DC power. This increases the voltage of this DC power source, which thus no longer runs at the optimum operating point. Therefore, it is important to use two identical DC sources with the same maximum power output.

If the requirement for two DC power sources with the same maximum power output can not be met, the circuit is replaced by a compensating converter AW

Energy transfer between the first and second capacitors Cl and C2 added. Both DC sources can then deliver the maximum possible power. At a

Power surplus of the first DC power source is then half of this excess from the first capacitor Cl transmit the second capacitor C2 of the second buck converter. At a power surplus of the second DC power source, the transmission is reversed accordingly.

FIG. 3 shows this circuit, whereby a choke inverter with the following elements is added by way of example to the basic circuit shown in FIG.

The inductor inverter comprises a third inductor L3, which is connected to a first terminal via a shunt resistor RS for current measurement to the neutral conductor N _network . The second terminal of the third choke L3 is connected via a third switching element S3 to the drain line of the first switching element Sl and via a fourth switching element S4 to the drain line of the second switching element S2. Antiparallel to the two switching elements S3 and S4 of the inverter freewheeling diodes are optionally arranged (eg MOSFETs or IGBTs with freewheeling diodes).

Alternatively, a bidirectional transducer (e.g., flyback converter) may also serve as the equalizing transformer AW.

It is expedient that the regulation of the equalizing transformer AW works independently of the rest of the regulation in order not to make the control loops too complex.

For each step-down divider, a separate differential amplifier is arranged in addition to the differential amplifier, which defines the feed-in current

Control deviation between the MPP setpoint voltage and the actual voltage determined. If this difference reaches a predefinable maximum value (eg 2% of the nominal MPP voltage), the compensation converter AW is started. The balance transducer AW then converts the excess energy from one capacitor to the other capacitor. The difference signal serves the compensating transducer AW as a target current specification. The regulation of the compensating transducer AW counteracts the DC control. Detect the

Direct current control DC component in the feed, this initially causes a reduction in the current drain from the more powerful DC power source. This causes the DC source to move away from the MPP operating point as the voltage increases, causing the compensating converter to respond and boost current. By means of balancing transformer AW, the other buck converter is then supplied, whereby the DC component in the feed current drops while the power increases at the same time.

There are two possibilities, the increase in the transmitted power of the additionally supplied

To induce buck converter. Without additional intervention in the control, this is done by MPP tracking, but only after a relatively large time constant, as an MPP tracker usually has a slow control dynamics.

In order to bring about a faster increase in the feed current of the additionally supplied buck converter, it is expedient to make the setpoint current of the buck converter directly dependent on the power of the compensation converter AW.

Since the compensating transducer AW can transmit energy in both directions, its activation must check during the startup of the inverter or in the case of strongly fluctuating power outputs of the DC sources (eg photovoltaic panels in the case of very variable cloud cover) another criterion. If the request for energy transfer from both buck-boosters comes to the other, the control must block the compensation transformer AW. In this case, there is no stable state and both buck converters must first increase the feed-in power until one has reached the MPP operating point. For systems with technical differences in the two DC sources, the use of a digital controller leads to a further improvement of the control dynamics. This is the case, for example, if a direct current source formed by a string has smaller panel areas in a photovoltaic system. The digital control then enables a detection of the power difference of the two DC power sources over several hours and the formation of an average value, which dictates the equalization power to be transmitted from the equalizing converter AW when the system is switched on again from the beginning. Reaching the control equilibrium takes less time in this way.

FIG. 4 shows a basic circuit with a compensating converter and additional circuit elements for deriving voltage peaks occurring in the AC network. Voltage peaks are triggered, for example, by switching operations and reach pulse voltage levels of up to a few kilovolts on a 230V / 400V line. In order to protect the electronics of an inverter against such voltage spikes, over-dimensioning of the circuit elements is usually required. In addition, elaborate filters are necessary.

In the present circuit, additional current paths are provided with two additional capacitors C3 and C4. This ensures that the voltage loading of the circuit elements does not increase significantly above the maximum operational load by passing the current through the inductors Ll and L2 into the four capacitors Cl, C2, C3 and C4. It is important to ensure that the time constants of the LC elements formed from the throttle elements Ll and L2 with the capacitors Cl, C2, C3, C4 are greater than the maximum expected duration of a network overvoltage pulse. The additional circuit elements are arranged so that a connection point between the second switching element S2 and the second throttle element L2 via a fifth diode D5 and a parallel circuit of a first resistor Rl and a third capacitor C3 is connected to the reference potential 0. Furthermore, the reference potential is 0 via a parallel connection of a fourth capacitor C4 and a second resistor R2 and further via a sixth diode D6 with a connection point between the first

Switching element Sl and the first throttle element Ll connected.

In addition, it is necessary that the anti-parallel divider D3 and D4 are arranged to the switching elements Sl and S2, e.g. through MOSFETs with integrated diodes.

In the case of a positive voltage pulse from the alternating current network, the following happens through this circuit arrangement: The first capacitor C1 is charged to the peak value of the mains voltage by the preceding mains period. (Since the capacitor Cl has a high-resistance discharge resistance, there is almost no recharging during normal operation and thus harmonic distortion of the sinusoidal current.) Through a positive voltage pulse current then flows through the first inductor Ll and the third diode D3 arranged antiparallel to the first switching element Sl first capacitor Cl, whereby the voltage at the Verschaltungspunkt between the first switching element Sl and the first diode Dl is limited to the voltage of the first capacitor Cl plus the diode threshold of the third diode D3. At the same time, a current flows through the second throttle element L2, via the fifth diode D5 in the third capacitor C3, whereby the voltage at the connection point between the second switching element S2 and the second diode D2 to the voltage of the third capacitor C3 plus

Diode threshold of the fifth diode D5 is limited. If the mains voltage peak fades again, the signals are demagnetized both throttle elements Ll and L2 continue in the capacitors Cl and C3 and are finally de-energized.

If the voltage pulse from the AC mains is negative, the same process takes place via the fourth diode D4 and the second capacitor C2 as well as the sixth diode D6 and the fourth capacitor C4.

Care must be taken to ensure that the capacitors C1, C2, C3 and C4 are dimensioned such that they are not charged to an unacceptably high level for the maximum expected line overvoltage with a maximum expected duration. After the current consumption due to the overvoltages, the third and fourth capacitors C3 and C4 discharge again via the first and second resistors R1 and R2 connected in parallel.

The main advantage of this design is that the function of this protection circuit does not depend on the switching speed of monitoring and, if necessary, of

Power switching elements depends, since the throttle elements Ll and L2 behave like power sources and can build up any voltages in a very short time (a few 10ns). Furthermore, after demagnetization, parasitic capacitances (e.g., winding capacitances) may cause oscillations, which may overburden the control of power switching elements in an active protection circuit. In the present protection circuit therefore only diodes are provided as limiting elements.

As an alternative to the above-described arrangement of a protective circuit, it is also possible to provide an arrangement in which low-resistance discharge resistors can be connected to the capacitors C 1, C 2, C 3 and C 4 via additional switching elements. By means of comparators, the voltages at the capacitors Cl, C2, C3 and C4 are then monitored and switched on when reaching upper limits the discharge resistors.

In a further protection circuit variant voltage-limiting varistors are provided, with which the throttle elements Ll and L2 are connected via diodes. In doing so, the differential internal resistance of the Varisotren or suppressor diodes must be taken into consideration, as a result of which high currents can occur, which would then significantly increase the limiting voltage.

The control of the first switching element S1 and the second switching element S2 is to be set such that when an overvoltage occurs (when the mains voltage exceeds the maximum peak voltage, eg [240V + 10%] x crest factor = 373V) both in the positive and in the negative direction the switching elements S1 and S2 remain switched off for a fixed time in order to give the pulse voltages and the currents in the throttle elements L1 and L2 time to decay (for example 500 μs).

Claims

claims

1. Inverter for feeding electrical energy from a first and a second DC source with a common reference potential (0) in an AC network, the inverter on the output side connected to a conductor (Ll _network ) and a neutral conductor (N _network ) of the AC network, characterized in that - the first DC power source has a positive potential (1) with respect to the reference potential (0),

The second DC power source has a negative potential (2) with respect to the reference potential (0),

The reference potential (0) of the two DC sources is connected to the neutral conductor (N _network ),

- the inverter comprises a first buck converter with which the positive potential (1) is connected to the conductor (L1 _network ) of the AC _mains ,

- The inverter comprises a second buck converter with which the negative potential (2) to the

Head (Ll _network ) of the AC _mains is turned on.

2. The inverter according to claim 1, characterized in that the first and the second DC power source are designed as so-called strings of a photovoltaic system.

3. Inverter according to claim 1 or 2, characterized in that the first step-down converter a first capacitor (Cl), a first switching element (Sl), a first auxiliary switching element (SHl) connected in series with the first diode (Dl) and a choke circuit ( L) and input side with the positive potential (1) and the reference potential (0) of the first DC power source and the output side via a filter capacitor (CF) to the

Head (LL _network ) of the AC network is connected and that the second buck converter a second capacitor (C2), a second switching element (S2), a second diode (D2) connected in series with a second auxiliary switching element (HS2) and the choke circuit (L) and having the negative potential (2) and the reference potential (0) on the input side second DC power source and the output side via the filter capacitor (CF) to the conductor (Ll _network ) of the AC network is connected and that the neutral conductor ( ^N _network ) is continuously connected to the reference potential (0) of the first and the second DC power source.

4. Inverter according to claim 3, characterized in that the throttle circuit (L) in a first throttle element (Ll) and a second throttle element (L2) is divided, that the first buck converter comprises the first throttle element (Ll) and that the second

Step down converter comprises the second throttle element (L2).

5. Inverter according to one of claims 1 to 4, characterized in that the positive potential (1) of the first DC power source and the negative potential (2) of the second DC power source via a balance transducer (AW) are interconnected.

6. An inverter according to claim 5, characterized in that the balance transducer (AW) comprises a third switching element (S3) and a series-connected fourth switching element (S4), and that a connection point between the third switching element (S3) and the fourth switching element ( S4) is connected to the reference potential (0) via a third choke (L3) and a current measuring means.

7. Inverter according to claim 6, characterized in that the means for measuring current comprises a shunt resistor (RS).

8. Inverter according to one of claims 3 to 7, characterized in that anti-parallel to the first switching element

(Sl) a third diode (D3) is arranged and that anti-parallel to the second switching element (S2), a fourth diode (D4) is arranged.

9. Inverter according to claim 8, characterized in that a connection point between the second switching element (S2) and the second throttle element (L2) via a fifth diode (D5) and a parallel circuit of a first resistor (R1) and a third capacitor (C3 ) is connected to the reference potential (0) and that a connection point between the first switching element (Sl) and the first throttle element (Ll) via a sixth diode (D6) and a parallel circuit of a second resistor (R2) and a fourth capacitor (C4 ) is connected to the reference potential (0).

10. Inverter according to one of claims 1 to 9, characterized in that the inverter has a

Control unit with suitable means for controlling the switching elements (Sl, S2, S3, S4) and auxiliary switching elements (HSl, HS2).

11. A method for operating an inverter according to one of claims 1 to 10, characterized in that the two buck converters are driven alternately in such a way that a feed current in the form of whole sine waves results and that the positive half sine waves by means of the first buck converter from the input side positive potential (1) and that the negative sine half-waves are formed by means of a second step-down divider from the negative potential (2) present on the input side.

12. The method according to claim 11, characterized in that continuously the voltage and the extraction current of the first DC source is measured and that the first step-down divider is driven in such a way that continuously the product of extraction current and voltage of the first DC power of the momentarily maximum output power of the first DC power source is approached and that continuously the voltage and the extraction current of the second DC power source is measured that second step-down divider is driven in such a way that the product of extraction current and voltage of the second DC power source of the currently maximum output power of the second DC power source is continuously approximated.

13. The method according to claim 12, characterized in that continuously the DC component of the feed current is measured and that at a positive DC component of the first step-down divider, a lower feed power is specified and that at a negative DC component, the second step-down converter a lower feed power is specified.

14. A method for operating an inverter according to one of claims 5 to 10, characterized in that from a predetermined reduction of the feed power of a buck converter energy from the potential (1 or 2) to which this buck converter is connected by means of

Equalization transformer (AW) is transferred to the other potential (2 or 1).

15. The method according to claim 14, characterized in that the Tiefsetzsteiler, which is connected to the potential (1 or 2) to which the balance transducer (AW) transmits energy, a higher feed-in power is specified.