Inverter

An inverter (1) for feeding electric power into a utility grid (7) or into a load is described. The inverter (1) contains direct voltage inputs (2, 3), one first intermediate circuit (8) connected thereto and comprising two series connected capacitors (C1, C2) that are connected together at a ground terminal (14), two alternating voltage outputs (5, 6) of which one at least is provided with a grid choke (L1) and one bridge section (10). In accordance with the invention, the inverter (1) contains only two switches (S1, S2), which are disposed in the bridge section (10) and are to be switched at high frequency, as well as, between the first intermediate circuit (8) and the bridge section (10), a second intermediate circuit (9) that is devised at least for selectively boosting or bucking the direct voltage and intended for supplying said bridge section (10) with positive and negative voltage, said second intermediate circuit comprising an internal freewheeling (D5, D6) for maintaining the currents flowing through the grid choke (L1) in opposite directions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims Priority from German Application No. DE 10 2007 038 960.6 filed on 14 Aug. 2007

FIELD OF THE INVENTION

The invention relates to an inverter of the type mentioned in the preamble of claim1.

In order to feed electric power generated with direct voltage generators such as photovoltaic or fuel cell plants into an alternating current grid, in particular into the utility grid (50/60 Hz), inverters of various types are used. In most cases, a direct voltage converter (DC-DC controller) is provided between the direct voltage generator and the inverter and serves the purpose of converting the direct voltage delivered by the direct voltage generator into a direct voltage needed by the inverter or adapted thereto.

For different reasons, it is desired to ground one of the outputs of the direct voltage generator or to fix the potential of the direct voltage generator with respect to ground potential. The reason therefor is on the one side that such grounding is prescribed in some countries. On the other side, in the absence of grounding, diverse disadvantages occur during operation. One of them is the problem of high frequency leakage currents. Unavoidable, parasitic capacitances between the direct voltage generator and ground may give rise to considerable equalizing currents in case of potential fluctuations, said equalizing currents constituting an intolerable safety risk which make it necessary to provide for complex monitoring provisions with the help of error current sensors or the like for contact protection or for achieving electromagnetic compatibility (EMC) and which can only be securely avoided through grounding. Potential fluctuations at the direct voltage generator can further lead to permanent destruction of certain solar modules such as thin film modules or the like.

In principle, leakage currents of the type described can be readily avoided if direct voltage converters with transformers are being used to galvanically separate the direct voltage side from the alternating voltage side. However, independent on whether grid transformers or high frequency transformers are being used, transformers result in a reduction of efficiency, in parts considerable weight and size and/or an additional control expense, this being the reason why transformerless voltage converters are being preferred. The usual topologies of transformerless voltage converters however make the desired grounding either impossible since it would cause needed switches, capacitances or the like to short-circuit or result in an increased switching expense and in other drawbacks.

DESCRIPTION OF THE PRIOR ART

Therefore, numerous tests have already been conducted in order to avoid in another way the occurrence of the disadvantages mentioned. Circuits are known in particular, which serve the purpose of reducing the undesired leakage currents (e.g., DE 10 2004 037 466 A1, DE 102 21 592 A1, DE 10 2004 030 912 B3). In these circuits, a solar generator is e.g., operated in certain phases of the inner electric power transport, isolated from the grid. As the solar generator is periodically electrically connected to the grid, the charge of its parasitic capacitances is only slightly reversed so that the potential of the solar generator changes sinusoidally at grid frequency and at a voltage amplitude that corresponds to half the grid voltage. High frequency currents then only form through the slight voltage differences of the solar generator between two switching cycles and through asymmetries during switching. Capacitive leakage currents can thus be strongly minimized but cannot be completely avoided.

Further, a circuit arrangement is known (DE 102 25 020 A1), which uses a divided solar generator the center point of which is grounded. As a result, all the parts of the generator have a fixed potential and capacitive leakage currents cannot flow in principle. Since the two direct current sources have different outputs, a circuit for compensating the output differences and the voltages is moreover provided. In this proposed circuit, the disadvantages encountered are the high voltage differences in the solar generator and at the switches, the additional losses in the compensation circuit and the fact that at least four switches timed at high frequency are needed.

Besides, circuit arrays are already known by means of which a solar generator can be grounded on one side, despite the absence of a transformer. As a result, capacitive leakage currents are prevented as a matter of principle. One of these circuit arrays (DE 196 42 522 C1) however needs five active switches, one or two switches having to switch simultaneously at high frequency and to provide the mean output current. On this circuit, which is also referred to as a “flying inductor”, the efficiency is negatively affected by the high number of components simultaneously participating in series in the current flow. The disadvantage of this circuit is that intermittent current pulses are impressed upon the grid, said pulses requiring a capacitive grid filter which, as a matter of principle, degrades not only the power factor but also the efficiency of the circuit in the part load range due to its own freewheeling power need. Although such a capacitive grid filter can be avoided with another known circuit (DE 197 32 218 C1), nine active switches are needed for this purpose of which two at least must be switched simultaneously at high frequencies so that the expense in terms of construction is even further increased and both the robustness and the efficiency of the overall device is negatively affected. The topology of a flying inductor further has the disadvantage that the voltage load of the switches depends on the grid voltage and is sensitive to grid failures.

Finally, devices are known (US 2007/0047277 A1) that are configured with two stages and that comprise, beside the actual inverter (DC-AC converter), a direct voltage or a DC-DC converter. The inverters are provided with a bipolar voltage intermediate circuit containing two series-connected capacitors that are connected together at a ground terminal associated with the neutral conductor of the respective grid and connected therewith. In this case, the ground terminal of the inverter can moreover be connected with the negative output of the direct voltage generator. This is made possible using a special storage choke composed of two magnetically coupled windings.

The advantage that this device allows for grounding the direct voltage generator with relatively simple means, in particular without transformer, is opposed by the disadvantage that it needs at least three active switches clocked at high frequency and that it is formed with two stages, which increases the expense in terms of controlling, results in unavoidable losses and impairs efficiency.

BRIEF SUMMARY OF THE INVENTION

In view of this prior art, the technical problem of the invention is to devise the inverter of the type mentioned above in such a manner that the potential of the direct voltage generator can be fixed with respect to ground potential not only with relatively simple means in terms of construction but also with a small number of components and with relatively small loads at least for the switches that must be switched at high frequency.

The characterizing features of claim1serve to solve this problem.

The invention proposes an inverter in a one-stage construction, i.e., an inverter in which the DC-DC part and the DC-AC part are combined into a combined circuit array with the possibility of boosting and bucking the input voltage. As a result, a common control is made possible in one single stage. Moreover, an inverter is provided in which all the functions are performed by only two switches that are to be switched at high frequency. Finally, the potential of the direct voltage generator can be fixed with respect to ground potential and power can be fed into the grid with a non intermittent current. By virtue of the relatively small number of components one further achieves high reliability and long useful life of the inverter.

Further advantageous features of the invention will become apparent from the dependent claims.

The invention will be best understood from the following description when read in conjunction with the accompanying drawings. In said drawings:

DETAILED DESCRIPTION OF THE INVENTION

According toFIG. 1, a one-phase inverter1of the invention contains two inputs2(positive) and3(negative) intended to apply a direct voltage, said inputs being connected e.g., to the corresponding outputs of a direct voltage generator4in the form of a photovoltaic or fuel cell plant, a capacitor that has not been illustrated can be connected in parallel to said outputs as usual.

The inverter1further contains two outputs5and6that serve for delivering an alternating voltage and for connection e.g., to a schematically illustrated, one-phase utility grid7or to a load. A smoothing or grid choke L1can be connected upstream of at least one of the outputs, this applying in the exemplary embodiment for the output5that is connected to phase L of the grid7.

As contrasted with most of the known circuit arrays, no additional and separate DC-DC converter is interposed between the direct voltage generator4and the inverter1. Instead, an inverter is proposed in accordance with the invention, which not only performs a DC-AC conversion as shown inFIG. 1, but is also suited for boosting or bucking the input voltage to a desired level, i.e., which also has the properties of the DC-DC converter with boosting-bucking function. Both functions are brought together in the inverter1of the invention. For this purpose, the inverter1comprises a first intermediate circuit8, a second intermediate circuit9and a bridge section10.

The first intermediate circuit8is connected to the two inputs2and3and contains two series-connected capacitors C1and C2that are connected with their one terminals11,12to a respective one of the inputs2and3and with their other terminal to each other. This connecting point is at the same time a ground terminal14that has to be brought to ground potential. The intermediate circuit8thus is an actually known, bipolar intermediate voltage circuit that is devised for the bridge section10to be fed from a positive source C1(top connection11, positive to ground) and from a negative source C2(lower connection12, negative to ground). By grounding to ground terminal14and by using relatively high capacitances C1and C2, one moreover achieves that the potential of the direct voltage generator4is relatively constant and that, even if there are parasitic capacitances to ground, no substantial stray currents are obtained.

The second intermediate circuit9contains a first series member consisting of a first diode D3connected to terminal11and of a first storage choke L3connected in series therewith, as well as a second series member consisting of a second diode D4connected to terminal12and of a second storage choke L2connected in series therewith. The other terminal of the storage choke L3is connected to a first input15, the other terminal of the storage choke L2to a second input16of the bridge section10.

Moreover, the second intermediate circuit9contains a capacitor C4that is connected on the one side with a connection point17between the diode D3and the storage choke L3of the first series member and on the other side with the input16, as well as a capacitor C3that is connected on the one side with a connecting point18between the diode D4and the storage choke L2of the second series member and on the other side with the input15of the bridge section10.

The bridge section10is substantially formed by two series-connected switches S1and S2, each comprising one first terminal connected with the input15or16and one second terminal. The two second terminals are connected together at a common connecting point19that is located between the two switches S1, S2and that is connected to the one output5of the inverter1through the grid choke L1thus leading, through the grid choke L1, to phase L of the grid7. The normal conductor N of the grid7is connected to the output6of the inverter1and from there through a line20to the connecting point14between the two capacitors C1and C2and is brought to ground potential like this connecting point14.

In the exemplary embodiment, the second intermediate circuit9further comprises two freewheeling paths that are each formed by one additional diode D5and D6. The freewheeling path with the diode D5lies between the grounding terminal14and the input15of the bridge section10, the diode D5being conductive in the direction of the input15. The second freewheeling path with the diode D6, by contrast, lies between the grounding terminal14and the other input16of the bridge section10, but here, the diode D6can only be made conductive in the opposite direction, meaning in the direction of connecting point14. Moreover,FIG. 1shows that the cathode of the diode D5and the anode of the diode D6are connected to the ground terminal14.

Finally, one diode D1and D2can be connected in parallel with a respective one of the switches S1, S2in the bridge section, the diode D1being conductive in the direction of the input15and the diode D2, in the opposite direction toward the connecting point19between the two switches S1, S2.

The switches S1, S2are switched at high frequency, e.g., at a frequency of 16 kHz or more, e.g., of 100 kHz, i.e., they are brought alternately into a conductive and into a non-conductive state. Further switches are not needed in the inverter1of the invention.

Generally, the basic idea of the invention is to provide two intermediate circuits8and9that are joined together. The first intermediate circuit8is connected to the DC generator4on the input side and substantially only consists of a storage configured to be a capacitive voltage splitter the connecting point14of which is connected to ground potential. As a result, the potential of the DC generator4is fixed with respect to ground potential, preferably symmetrically, with C1being chosen to equal C2. By contrast, as will be discussed in closer detail herein after, the second intermediate circuit9serves on the one side to supply the bridge section10with the necessary electrical voltages which are positive or negative with respect to ground potential and on the other side e.g., to boost the output voltage of the first intermediate circuit8to the value desired for grid or load feeding by setting its output voltage by selecting the ratio at which the two switches S1and S2are switched on simultaneously. In the exemplary embodiment, the second intermediate circuit9moreover has the function of providing a freewheeling path for the current flowing through the grid choke L1.

All this is achieved by an integrated circuit array that only needs a small number of switches, provides for a small voltage load on the switches and allows for non-intermittent, continuous current feed into the grid7. Moreover, the components D3, L3, C4, D5on the one side and D4, L2, C3and D6on the other side are configured to be preferably completely symmetrical so that identical conditions are achieved for the current flows during the positive and negative half waves. Generally, an inverter1is thus obtained which comprises only two switches S1, S2that are connected at high frequency and are subjected to relative low load.

The functioning of the inverter1described will be discussed in closer detail herein after with respect to theFIGS. 2 through 5for the case in which one has a positive half wave, i.e., a positive output voltage is applied at the connecting point17and in which positive current is fed into the grid7through the grid choke L1.

We first assume that all the two switches S1and S2are in the open state (FIG. 1). Then, after a short equalization, the operating condition becomes stationary as soon as the two capacitors, if C1=C2, are charged to the half generator voltage and the capacitors C3and C4are charged to full generator voltage via the current paths D3, C4, L2, D4and D3, L3, C3and D4respectively. Now, no current flows through the storage chokes L2and L3and inFIG. 1, the capacitor C4has its positive side on the left and the capacitor C3on the right.

In order to allow for the boosting function, the two switches S1and S2are switched on simultaneously (overlapping), as is shown inFIGS. 2 and 5for a time interval t31and a phase A. Closing the switches S1, S2results in the components C4and L3on the one side and C3, L2on the other side to be connected in parallel or short-circuited. As a result, the capacitor C4is uncharged via a current path from C4via L3, S1, S2and back to C4through the storage choke L3and L3is charged accordingly at the same time. Moreover, the capacitor C3is uncharged via a current path from C3via S1, S2, L2and back to C3so that the storage choke L2is charged. As a result, the currents S1and S2increase progressively in accordance withFIG. 5. Finally, a current flowing through the grid choke L1toward the grid7and having been generated in L1in a previous phase can continue to flow via a freewheeling path enforced through diode D5and back to L1, said current not having to flow through the uncoupling or coupling diodes D3, D4. The respective freewheeling D5and D6described would not be necessary if L1were missing, which could be envisaged in principle. However, the grid choke L1offers the advantage that it participates in smoothing the current to be delivered into the grid7and that it prevents this current from growing excessively. As further shown inFIG. 5, phase A leads to a different increase of the currents flowing through the switches S1, S2since through S1and only therethrough also flows the freewheeling current from L1, whilst through S2only flow the charge reversal currents from C3, C4into L3, L2. The power flow in the second intermediate circuit9is prevented from being reversed by the locking action of the diodes D3and D4.

In a subsequent phase B (FIGS. 3 and 5), the switch S1is brought into the open condition during a time interval t2whilst switch S2remains closed. As a result, a negative voltage is applied to the connecting point19of the inverter1. Now, the current flowing in L1can only be decreased via the path L1,7,20,14, C1, D3, C4, S2,19and back to L1, this current flowing in the opposite direction through S2, as shown inFIG. 5. Moreover, by opening switch S1, the short-circuit of S3, C4or L2, C3is abolished so that the storage choke L2now transfers the power stored therein via D4, C2, C1and D3to C4and that L3now transfers the power stored therein via D4, C2, C1and D3to C4. The two high frequency switches S1, S2suffice to build an inverter capable of feeding into the grid7from a voltage source the output voltage of which is higher or lower than the peak value of the grid voltage.

As shown inFIG. 5, the current flowing through D3in phase B is composed of the current flowing through the grid choke L1, which remains almost constant, and of the current effected by charge reversal, whilst through diode D4there only flows the current effected by charge reversal.

In another phase C, during time interval t32, the switch S1is closed again (FIG. 2). Now, the same conditions prevail like during phase A; this is best seen inFIG. 5.

During a first part t1* of a time interval t1, a phase D follows phase C; in this phase D, switch S1is closed according toFIGS. 4 and 5, whilst switch S2is open. As a result, a positive voltage is applied to the connecting point19of the bridge section10. During this phase, power is transferred on the one side from the first intermediate circuit8into the grid7, commencing at C1via D3, L3,15, S1,19, L1,7,20and back to C1. At the same time, the storage chokes L2, L3deliver the power stored therein to the capacitors C3, C4(L3, C3, D4, C2, C1, D3and back to L3and L2, D4, C2, C1, D3, C4and back to L2) respectively. Accordingly, currents decreasing until complete discharge of L2and L3flow in D3and D4, current D3being however increased by the current delivered to the grid7via L1, whilst D4only carries the charge reversal currents.

In a last phase E in which, like inFIG. 4, S1is closed and S2remains open and which extends over a time interval t1-t1*, power is then only transferred into the grid7from C1via D3in analogous fashion to phase D, whilst the charge reversal currents have become zero.

When a negative current is fed into the grid during the negative half waves, the inverter1described functions substantially in the same way, although circumstances complementingFIGS. 2 through 5are obtained. This more specifically means that switch S1is kept closed and switch S2is opened instead in phase B. Hence, a freewheeling path leading from L1via S2, D6and7back to C2is active during phase C. By contrast, during the phases D and E, power is transferred into the grid7starting from C2, via7, L1, S2, L2, D4and back to C2.

During the negative half waves also, the capacitors C3, C4are boosted to a preselected value by the high switching frequencies and by a preselected duty cycle determining the boosting degree. Theoretically, the voltage C3and C4would continue to increase both during the positive and during the negative half wave. Since power is permanently drawn from the capacitors C3, C4by feeding power into the grid7or through a resistive load, the voltages at C3, C4are prevented from increasing above a preselected value.

Irrespective thereof, the storage choke L3is more strongly charged during the positive half waves and storage choke C2, during the negative half waves.

Generally, one thus obtains an inverter1capable of delivering an output voltage that is variable within wide limits without a usual DC-DC converter and with only two high frequency switches S1, S2. The boosting function is thereby achieved by reversing the charge of the magnetic stores L2, L3and the capacitive stores C3, C4and substantially by the fact that the power stored in the storage chokes L2, L3when the switches S1and S2are closed is delivered to the capacitors C1, C4by opening at least one of the two switches S1, S2in order to charge said capacitors to an intermediate circuit voltage that is higher than the input voltage.

The control signals for the switches S1, S2are generated appropriately during the time span t31, t2and t32and t1shown inFIG. 5, using the usual means used for PWM controls, the objective being to approximate as close as possible the output current of the inverter1to a sinus shape. For this purpose, a reference signal in the form of a triangular or a sawtooth signal can be compared for example with a target value signal, the reference signal being generated separately for each polarity of the output voltage.

FIG. 6schematically shows the structure of a three-phase inverter24. This structure is obtained by the fact that for each phase of the grid7a separate second intermediate circuit22a,22band22cis provided, which is provided with one internal freewheeling diode each, as shown inFIG. 1. Moreover, a bridge section10a,10band10cconfigured in accordance with theFIGS. 1 and 6is connected to each second intermediate circuit22a,22band22c, said bridge section supplying one of the three phases of the grid with current. A line25leads from the connecting point14to the neutral conductor of the grid7that has not been illustrated herein.

The invention is not limited to the exemplary embodiments described, which can be varied in many ways. It is clear in particular that in the description given herein above only those components were described that were necessary to garner an understanding of the invention, and that in particular the necessary and actually known controls, MPP controls and so on can be provided additionally. Moreover, it is understood that the various components can also be used in other combinations than those described and illustrated.