Title of Invention	APPARATUS FOR FEEDING ELECTRICAL ENERGY INTO AN ENERGY SUPPLY SYSTEM AND DC VOLTAGE TRANSFORMER FOR SUCH AN APPARATUS
Abstract	The invention describes an apparatus for feeding electrical energy into an energy supply system (8) with a DC voltage transformer (2), which is intended to be connected to a DC voltage generator (1), and an inverter (3) which is connected to said DC voltage transformer (2) and is intended to be connected to the energy supply system (8), with a bipolar voltage intermediate circuit. The DC voltage transformer (2) is constructed according to the invention in such a way that the DC voltage generator (1) can be grounded either at its negative, positive or any central output. For this purpose, a storage inductor (16) is provided which is switched both in a charging cycle and in a discharging cycle in such a way that no currents flow via a grounding line (19) during normal operation.

Title of Invention

APPARATUS FOR FEEDING ELECTRICAL ENERGY INTO AN ENERGY SUPPLY SYSTEM AND DC VOLTAGE TRANSFORMER FOR SUCH AN APPARATUS

Abstract

The invention describes an apparatus for feeding electrical energy into an energy supply system (8) with a DC voltage transformer (2), which is intended to be connected to a DC voltage generator (1), and an inverter (3) which is connected to said DC voltage transformer (2) and is intended to be connected to the energy supply system (8), with a bipolar voltage intermediate circuit. The DC voltage transformer (2) is constructed according to the invention in such a way that the DC voltage generator (1) can be grounded either at its negative, positive or any central output. For this purpose, a storage inductor (16) is provided which is switched both in a charging cycle and in a discharging cycle in such a way that no currents flow via a grounding line (19) during normal operation.

Full Text	APPARATUS FOR FEEDING ELECTRICAL ENERGY INTO A POWER GRID AND DC VOLTAGE CONVERTER FOR SUCH AN APPARATUS The invention relates to an apparatus of the type recited in the preamble of claim 1 and to a DC voltage converter suited therefor. To feed electrical energy generated with DC voltage generators such as photovoltaic or fuel cell plants into an AC grid, in particular into the utility grid (50/60 Hz), various inverters are used. Between the DC voltage generator and the inverter there is mostly provided a DC voltage converter (DC-DC chopper) that serves the purpose of converting the DC voltage supplied by the DC voltage generator into a DC voltage needed by the inverter or adapted thereto. For different reasons, it is desired to ground one of the outputs of the DC voltage generator. The reason for the desired grounding is on the one side that there are countries which prescribe such grounding. On the other side, different disadvantages arise in operation when grounding is missing. One of the problems encountered is that of the high-frequency leakage currents. Due to inevitable, parasitic capacitances between the DC voltage generator and the ground, considerable equalizing currents creating an intolerable safety risk may occur in case of potential fluctuations so that complex monitoring measures using fault current sensors or the like are needed for contact protection or for establishing the electromagnetic compatibility (EMC) and said equalizing currents can only be securely avoided by grounding. Moreover, it is known that photovoltaic generators behave very differently with respect to degradation, depending on which technology is used to manufacture them. Generators with crystalline and polycrystalline cells or certain thin film modules are preferably grounded with the negative terminal, whilst backside-contact cells are preferably grounded at the positive terminal. A grounding of the type described, through which the disadvantages mentioned could be avoided, is readily possible using DC voltage converters with transformers which cause the DC voltage side to galvanically separate from the AC voltage side. Irrespective of whether grid transformers or high-frequency transformers are being used, transformers result i.a. in a reduction of efficiency, in parts in considerable weight and overall size and/or in an additional control expense, which is the reason why transformerless voltage converters are in principle preferred. However, the usual topologies of transformerless DC voltage transformers either make the desired grounding impossible to perform since this grounding would lead to a short-circuit of needed switches, capacitances or the like or they result in increased circuit expense and other disadvantages. Therefore, numerous attempts have been made to avoid the mentioned disadvantages in another way. Circuits are known in particular, which serve the purpose of reducing the undesirable leakage currents (e.g., DE 10 2004 037 466 A1, DE102 21 592 A1, DE 10 2004 030 912 B3). In these circuits, a solar generator e.g., is operated isolated from the grid in certain phases of internal electrical energy transport. When the solar generator is periodically electrically reconnected to the grid, its parasitic capacitances are only slightly reconverted so that the potential of the solar generator changes with grid frequency, sinusoidally and at a voltage amplitude that corresponds to half the grid voltage. High-frequency currents then form through the low voltage differences of the solar generator between two switching cycles only and through asymmetries during switching. Capacitive leakage currents can thus be strongly minimized but cannot be avoided completely as a matter of principle. A circuit arrangement is further 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 solar generator have a fixed potential and capacitive leakage currents cannot flow in principle. Since the two direct current sources have a different yield, a circuit for compensating the power differences and the voltages is additionally provided. In this proposed circuit, the disadvantage lies in the high voltage differences in the solar generator and at the switches, in the additional losses in the compensation circuit and in the fact that at least four high-frequency pulsed switches are needed. Besides, circuit arrangements have already been known by means of which a solar generator can be grounded on one side, in spite of the lack of a transformer. As a result, capacitive leakage currents are prevented as a matter of principle. One of these circuit arrangements (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 average output current. With this circuit, which is also referred to as "Flying Inductor", the efficiency is therefore affected by the high number of components participating simultaneously in series in the current flow. Another disadvantage of this circuit is that discontinuous current pulses are impressed upon the grid, which call for a capacitive mains filter which, as inherent to its functional principle, degrades the power factor but also the efficiency of the circuit in the part load range because of its own need for idle power. Although such a capacitive mains filter can be avoided with another known circuit (DE 197 32 218 C1), nine active switches are needed therefor, of which at least two 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 apparatus negatively affected. The topology of a Flying Inductor also has the disadvantage that the voltage load of the switches depends on the grid voltage and is sensitive to mains power failures and that it can only be operated in the three-phase mode of operation by three-fold use with the help of three inverters. Irrespective thereof, inverters with current source characteristics are needed, which is undesirable in many cases. Finally, apparatus are known (US 2007/0047277 A1), which are intended for inverters having a bipolar voltage intermediate circuit containing two series-connected capacitors connected together at a ground terminal. Such type inverters, which are nowadays mainly used for the purposes of interest herein, can be configured as half-bridge inverters in 3-level circuits and at need as inverters for one-phase or three-phase grid supply. In all of these cases, the connection node between the two capacitors forms a ground terminal that is associated with the zero or neutral conductor of the respective grid and is connected therewith. The DC voltage converter of this known apparatus contains one choke, two diodes and one switch. In this case, the ground terminal of the inverter can be connected to the negative output of the DC voltage generator. This is made possible using a storage choke that is composed of two magnetically coupled coils. The two coils of this storage choke are galvanicaliy connected together at one end in such a manner that on the one side, when the switch is closed, one of the two coils is charged by the DC voltage generator and the other coil via the first coil by virtue of the magnetic coupling and that, on the other side, when the switch is open, the two coils are discharged via a respective one, associated, of the two capacitors and via a diode belonging thereto. The advantage that this apparatus makes it possible to ground the DC voltage generator with relatively simple means, in particular without any transformer and with only one switch, is offset by the disadvantage that the ground terminal can only be connected to the negative output of the DC voltage converter. Further, this apparatus does not allow for monitoring the ground line leading from the ground terminal to the DC voltage generator with respect to fault currents since, as a matter of principle, operating currents also flow in this ground line. A circuit with a power storage choke and two switches connected in series therewith is known from JP 11 235024 A1. On the output voltage side, there are provided two diodes in order to decouple the input and the output. A DC-AC converter with a negative and a positive input and a three-phase AC output is used. Grounding is provided neither at the input nor at the output of the DC-AC converter. It is not mentioned whether the DC-AC converter is transformerless. At the output of the DC-DC circuit, there is only provided one single capacitor. Through this circuit, bidirectional operation of a DC- DC converter can be provided. In view of this prior art, the technical problem of the invention is to configure the apparatus of the type mentioned herein above, and in particular a DC voltage converter suited therefor, in such a manner that it is possible to ground the DC voltage generator at any terminal and that this can be realized with relatively simple construction means. In accordance with the invention the solution to this problem is achieved with the characterizing features of the claims 1 and 16. The invention allows for grounded operation of the DC voltage generator by using a DC voltage converter which, in the simplest case, only needs one storage choke, two diodes and two switches. As a result, in spite of only slightly increased expense, the advantage is obtained that the DC voltage generator can be grounded almost anywhere. Other advantageous features of the invention will become apparent from the dependent claims. The invention will be understood better upon reading the exemplary description accompanying the appended drawings wherein Fig. 1 through 3 show a first exemplary embodiment of an apparatus of the invention for feeding electrical energy into an energy supply system with three different grounding possibilities for a DC voltage generator; Fig. 4 shows the signals for controlling two switches of the apparatus shown in Fig. 1 through 3 and the current curves resulting therefrom; Fig. 5 shows an apparatus as shown in the Figs. 1 through 3, but with a slightly modified DC voltage converter; Fig. 6 and 7 show a second exemplary embodiment of an apparatus of the invention with two different grounding possibilities for a DC voltage generator; Fig. 8 through 10 schematically show the DC voltage converter shown in the Figs. 6 and 7 as a component part having a structure that may be selected through plug contacts; and Fig. 11 through 13 different types of inverters, which may be operated with the DC voltage converter of the invention as an alternative to the inverter shown in the Figs. 1 through 3. According to Fig. 1, an apparatus for generating electrical energy contains a DC voltage generator 1, a DC voltage converter 2 and an inverter 3. The DC voltage generator 1 consists e.g., of a photovoltaic or fuel cell plant and comprises at the outputs 4 (+) and 5 (-) a capacitor C that is connected in parallel therewith. A preferred inverter 3 within the scope of the present application comprises two outputs 6 and 7 which serve herein for single phase supply of electrical energy into a grid 8 the phase L of which is connected with the output 6 and the zero or neutral conductor N of which is connected with the output 7. The inverter 3 moreover contains three inputs E1, E2 and E3. Between the inputs E1 and E2, there are disposed two series connected capacitors C1 and C2 the connection node of which lies at the input E3. The capacitors C1 and C2 form a usual bipolar voltage intermediate circuit of the inverter 3. As shown in Fig. 1, the inverter 3 is configured to be a half bridge inverter and is provided for this purpose with two switches S1 and S2 the one terminal of which is respectively connected with a respective one of the inputs E1 and E2 and the other terminal of which leads to a common connection node 9 and from there to the output 6, via a smoothing or grid choke L1. A diode D1, D2 is additionally connected in parallel with a respective one of the two switches S1, S2; the diode D1 can thereby be made conductive from the connection node 9 in the direction of the input E1 and the diode D2 from the input E3 in the direction of the connection node 9, said diodes closing respectively in the opposite direction. Finally, the input E3 is directly connected with the output 7, grounded on the other side and configured to be a ground terminal as a result thereof. The inverter 3 substantially operates as follows: if the switches S1, S2 are alternately switched on and off, the side (input E1) which is positive with respect to E3 of the capacitor C1 is connected to phase L via the connection node 9 and the grid choke L1 e.g., during the positive half wave of the switch signal (switch S1 at first closed, switch S2 open). When the switch S1 opens next, the current can continue to flow through the grid choke L1, the capacitor C2 and the diode D2. During the negative half wave of the grid 8 (switch S1 open, switch S2 at first closed), the side (input E2) of the capacitor C2, which is negative with respect to E3, is connected to the phase L via the connection node 9 and the choke L1, the current being allowed to continue to flow through the diode D1 and the capacitor C1 after the switch S2 has closed. The two capacitors C1, C2 are discharged alternately as a result thereof, they being recharged in a known way with the help of any suited DC voltage converter. Apparatus of the type described are generally known (e.g., US 2007/0047277 A1, Fig. 10) and need therefore not be described in detail to those skilled in the art. Referring to Fig. 1, a DC voltage converter 2 of the invention contains two inputs 10 and 11, which are connected to the two outputs 4 and 5 of the DC voltage generator, as well as three outputs 12, 13 and 14 which are connected to the inputs E1, E2 and E3 of the inverter 3. At the input 10 there is connected a switch S3, which leads to a connection node 15. The one terminal of a storage choke 16 is connected to this connection node 15, the other terminal of said storage choke being located at a connection node 17 that is connected with the input 11 via a second switch S4. Moreover, the connection node 17 is connected to the output 12 via a first diode D3 whilst the output 13 leads to the connection node 15 via a second diode D4. The diode D3 can be made conductive in the direction of the output 12, the diode D4 in the direction of the connection node 15, whilst both are closing in the respective opposite direction. As a result, the functioning principle of the DC voltage converter 2 is as follows: When the switches S3 and S4 are closed at the same time, the storage choke 16 is recharged by the DC voltage generator 1 or by its capacitor C. The switch S3, the storage choke 16 and the switch S4 form a first series electric circuit that serves for storing electrical energy in the storage choke 16. At this time, the diodes D3 and D4 prevent the current flow to or from the capacitors C1 and C2. If, by contrast, the two switches S3 and S4 are opened simultaneously, the storage choke 16 discharges via the diode D3, the series-connected capacitors C1 and C2 and the diode D4. In this phase, the storage choke 16 forms, together with the parts D3, C1, C2 and D4, a second series electric circuit intended for discharging the storage choke 16 or for accordingly recharging the capacitors C1, C2. If the two capacitors C1, C2 have the same capacitance, they are charged to the same voltage UC1 = UC2. In their opened condition, the voltage load of the switches S3, S4 is relatively small. When the diodes D3 and D4 are conductive, the voltage at the switch S3 is US3 = UC + UC2 at the most, wherein UC is the output voltage of the DC voltage generator 1. The voltage at the switch S4, by contrast, is US4 = UC1 at the most. Irrespective thereof, the DC voltage converter 2 described offers the advantage that the DC voltage generator 1 can be operated with a relatively large range of output voltages. If the DC voltage converter 2 were missing, it should be made certain that the DC voltage generator 1 always supplies the inputs E1 and E2, even under unfavourable conditions, with such a high output voltage that the capacitors C1 and C2 are charged to a voltage that is higher than the grid amplitude (usually about ± 325 V). If, by contrast, a boost DC voltage converter 2 is provided, the voltages at the capacitors C1, C2 can be set to the desired height by selecting the pulse-duty factor at which the switches S3 and S4 are operated even if the output voltage of the DC voltage generator 1 is lower than the voltage at least needed by the inverter 3 (or by the grid 8). Further, the apparatus described is very flexible in utilization. This results from the fact that the voltages at C1 and C2 may be both higher and lower than the input voltage at the capacitor C, depending on the selected pulse duty factor for S3 and S4. If the pulse duty factor is more than 0.5, the DC voltage converter operates in the boost mode of operation. If the pulse duty factor is less than 0.5, the DC voltage converter 2 operates in the buck mode of operation. A pulse duty factor of 0.5 practically results in the voltage applied at the output of the DC voltage generator 1 being fed directly. The maximum voltage load of the inverter switches S1 and S2 is about 2 • UC1, wherein UC1 is the maximum voltage at the capacitor C1. In the simplest case, it is also possible to always have only one of these switches switched at high-frequency for each half mains period, whilst the other one remains switched off. Moreover, a continuous current flow into the grid 8 is possible on the inverter side. A major advantage of the invention is finally obtained in that the grounding point E3 can be connected optionally with the input 11 of the DC voltage converter 2 and as a resuit thereof with the negative output 5 (Fig. 1), with the input 10 of the DC voltage converter 2 and as a result thereof with the positive output 5 (Fig. 2) or with any other terminal 18 (Fig. 3) of the DC voltage generator 1, as this also applies for the neutral conductor N of the grid 8. During normal operation, no current flows through a grounding line 19 (Fig. 1) or 20 (Fig. 2) or 21 (Fig. 3), which is respectively shown in a dashed line and which connects the grounding point E3 with the corresponding input of the DC voltage converter 2 or with the corresponding output of the DC voltage generator 1. This results in particular from the fact that, together with the parts E3, C1, C2 and D4, the storage choke 16 forms an electric circuit, which is closed in itself and does not contain the lines 19, 20 or 21. As a result, it may be concluded that there is a fault in the plant if current still flows in the line 19, 20 or 21. In accordance with the invention, a monitoring element in the form of a circuit breaker or the like is preferably disposed in the line 19, 20 or 21 for automatically switching off the plant when a preselected tolerable current peak is exceeded. This function is independent on which input of the DC voltage converter 2 or which output of the DC voltage generator 1 the ground terminal E3 is connected to. In a known way, the switches S1 through S4 are practically configured to be semiconductor switches that may be switched on and off periodically when operated with control units that have not been illustrated herein (microcontrollers, PWM controls and so on), the switch frequency being e.g., 16 kHz or more. The signals for activating the switches S3 and S4 and the current path in the storage choke 16 are illustrated by way of example in Fig. 4. It can be seen therefrom that the two switches S3, S4 are always switched on and off simultaneously. Fig. 5 shows an exemplary embodiment that has been modified over Fig. 1 through 3 insofar as the storage choke 16 is divided into two coil parts W11 and W12 through a central terminal or a coil tap 23. In this case, the arrangement is such that the connection node 15 is connected to the tap 23 and that, as a result thereof, only the part W11 of the storage choke 16, which is fixed by the tap 23, lies in the first electric circuit which serves to charge the storage choke 16, whilst the second electric circuit contains the entire storage choke 16 located between the diodes D4 and D3 or the part W11 + W12 thereof. As a result, one may tap another optimization potential of the arrangement of the invention for the relationship between input voltage and output voltage, the load of the switch S3 and the diodes D3 and D4. If the transmission ratio is higher, one has the possibility, beside the pulse duty factor for S3 and S4, to influence the effective current and voltage load of the component parts via the relation (W12 + W11): W11. In principle, the location of the tap 23 can be chosen ad lib. A particular advantage of the tap 23 is that the maximum voltage load at the switch S3 in the open condition is only given by the voltage US3 = UC + [- n/(n + 1)] • UC1 + UC2, wherein n = W12/W11 and W11 and W12 simultaneously refer to the number of windings of the coils W11 and W12. The voltage load at the switch S4 is US4 = UC1. Alternatively, it is also possible to connect the tap 23 with the switch S4 in analogous fashion. For the rest, the apparatus shown in Fig. 5 corresponds to that shown in the Figs. 1 through 3, this being the reason why the output 14 of the DC voltage converter 2 may be connected optionally with the output 4 or 5 or with any other output of the DC voltage generator 1. Another exemplary embodiment of the invention is illustrated in the Figs. 6 and 7. This embodiment differs from the one shown in the Figs. 1 through 5 in particular in that the advantages described are obtained herein with a coupled storage choke 24, which is known per se but which is electrically connected in a hitherto unknown way. The storage choke 24 contains a first coil W1 and a second one W2 which are magnetically coupled together and are for example wound on a common core 25 for this purpose. Like the choke coil 16 in Fig. 1, the first coil W1 is electrically interposed between the two switches S3, S4 or between the two connection nodes 15 and 17. Moreover, the connection node 17 is connected to the output 12 via the diode D3 like in Fig. 1. By contrast, the input 13 of the DC voltage converter 2 is connected via a diode D5 to a terminal of the coil W2 the other terminal of which leads to the connection node 15 via a connection node 26 and a diode D6. Moreover, the connection node 26 is connected to the output 14. With this provision, the functioning is as follows: The first coil W1 of the storage choke 24 forms, together with the two switches S3, S4, a first series electric circuit that is placed in parallel with the outputs 4,5 of the DC voltage generator 1 and serves for charging the coil W1 with electrical energy when the switches S3, S4 are closed. Since the two coils W1, W2 are magnetically coupled, the coil W2 is also charged in this phase via the coil W1. The sense of winding of the two coils W1, W2 is thereby chosen so as to obtain the same voltage polarities at terminals which are shown by dots in Fig. 6. In the open condition of the switches S3, S4, the two coils W1, W2 lie in a second series electric circuit that leads from one of the terminals of the coil W1 (connection node 17), via the diode D3, the series-mounted capacitors C1 and C2, the diode D5, the coil W2, the connection node 26 and the diode D6 back to the other terminal of the coil W1 (connection node 15). Like in the case shown in Fig. 1, this second electric circuit is an electric circuit that is closed in itself and serves to jointly discharge the coils W1, W2 or to jointly charge the capacitors C1, C2. Moreover, the two coils W1, W2 are galvanically connected together through this electric circuit. As a result of this arrangement, it is possible to optionally connect the output 14 of the DC voltage converter 2 or the output E3 of the inverter 3 through the line 19 (Fig. 6), or the line 20 (Fig. 7) to the input 11 or 10 of the DC voltage converter 2 and, as a result thereof, also optionally to the output 5 or 4 of the DC voltage generator 1, in order to ground it at the negative output 5 (Fig. 6) or at the positive output 4 (Fig. 7). Moreover, the input E3 could be connected, analogous to Fig. 3, to any central output of the DC voltage generator 1. In all the cases described, these lines 19, 20 and, if applicable, 21 are not in use in normal operation since no current is allowed to flow through these lines 19 through 21 neither during charging nor during discharging of the storage choke 16. As a result, like in the case illustrated in the Figs. 1 through 5, a still measured current flow in these lines 19 through 21 or between the grounding point E3 and one of the terminals 4, 5 or 18 would be indicative of a fault in the plant or in the DC voltage converter 2 and could be used to switch off the plant. An advantage of the apparatus shown in Fig. 6 over the apparatus shown in the Figs. 1 through 3 results from the lower voltage load of the switch S3. Since the diode D6 is conductive during the blocking phase of the switches S3 and S4, the maximum voltage applied at switch S3 is the voltage UC, whilst the voltage UC1 is applied at S4 since the diode D3 is also conductive. In the apparatus shown in Fig. 7, by contrast, the voltage load at the switch S3 is equal to zero and at the switch S4 to UC + UC1. According to another exemplary embodiment of the invention that has not been illustrated separately the coil W1 of the choke coil 16 can be divided into two parts by a tap, in analogous fashion to Fig. 5. Like in Fig. 5, it is thereby possible to connect the tap to one of the connection nodes 15, 17 while disposing the two coil parts in the second electric circuit. At need, the voltage load of the switch S3 of the exemplary embodiment shown in the Figs. 6 and 7 is further reduced as a result thereof. The magnetic coupling of the coils W1, W2 in the Figs. 6 and 7 is preferably obtained by the fact that they are wound on one common core, above each other or behind each other according to need. Preferably, they have the same number of windings and are practically wound on the core 25 in opposing senses of winding in the arrangement schematically shown in the Figs. 6 and 7 in order to obtain the right directions of the current flow during charging and discharging. The Figs. 8 through 10 show how the DC voltage converter 2, herein specially the DC voltage converter 2 shown in the Figs. 6 and 7, can be configured to be a component part 27 that is provided with a plurality of terminals configured to be plug contacts or the like. As shown in Fig. 8, the DC voltage converter 2 has, unlike in the Figs. 6 and 7, in addition to the inputs 10, 11 and the outputs 12, 13, four additional outputs 28, 29, 30 and 31 and no output 14. The terminal 28 is directly connected to the input 10, the terminal 31, to input 11. Further, the terminal 29 is connected to the terminal of the coil W2 that is remote from the diode D5 and the terminal 30, to the connection node 26, this terminal not communicating with point 26 in Fig. 8. Through suited connections, the grounding of the DC voltage generator 1 may now be optionally provided at the negative terminal 5 (Fig. 9) or at the positive terminal 4 (Fig. 10). If grounding is desired to occur at the negative output 5, the terminal 31 is grounded as shown in Fig. 9 and is connected to the input E3 of the inverter 3 and as a result thereof to the neutral conductor N of the grid 8 via a monitoring element 32. Moreover, the terminals 29 and 30 are connected together. As a result, one obtains the arrangement shown in Fig. 6 if, to utilize the component 27, one connects the outputs 4, 5 of the DC voltage generator 1 to its inputs 10 and 11, its outputs 12 and 13 to the inputs E1, E2 of the inverter 3 and its terminals 29, 30 jointly to the input E3 of the inverter 3. If, by contrast, grounding is desired to occur at the positive output of the DC voltage generator 1, the terminal 28 is grounded as shown in Fig. 10 and is connected to the input E3 of the inverter 3 via the monitoring element 32. The other connections occur like in Fig. 9. By merely re-plugging the terminals 28, 31 of the component part 27 or of the DC voltage converter 2 located therein one has the option to choose to ground the DC voltage generator 1 at the positive or at the negative output 4, 5. Other outputs of the component part 27 could serve to ground also central terminals of the DC voltage generator 1. The same procedure is followed when using the DC voltage converter shown in the Figs. 1 through 5. Although the description given herein above only refers to the inverter 3 configured to be a half bridge inverter, it is clear to those skilled in the art that other inverters with a bipolar voltage intermediate circuit can be connected to the DC voltage converter 2 of the invention. This is schematically shown in the Figs. 11 through 13. Fig. 11 shows a half bridge inverter in a 3-level circuit, Fig. 12 another inverter in a 3-level circuit with center point (each in a single-phase implementation) and Fig. 13 shows an inverter for 3-phase grid 8 supply. All the three inverters have a bipolar voltage intermediate circuit, the inputs E1 through E3 and the outputs 6, 7 correspond to the above description. Since such type inverters are known per se, it seems that they need not be discussed further herein. The invention is not limited to the exemplary embodiments described, which can be varied in various ways. This applies in particular insofar as the inverters 3 and the DC voltage converters 2 are preferably manufactured and sold as a finished structural unit as shown in the drawings, but they can also be manufactured and sold as separate component parts. The embodiments described referring to the Figs. 8 through 10 are particularly suited therefor since they make it possible to mass-produce universally utilizable DC voltage converters which are independent on the kind of grounding of the DC voltage converter 1 desired in a particular case. Accordingly, the invention not only relates to the combination of a DC voltage converter 2 and of an inverter 3, but also to the DC voltage converter 2 alone. It is further clear that in the above specification only those component parts were described that are needed to garner an understanding of the invention, and that in particular the required and actually known control units, MPP controls and so on can be additionally provided. Also, it is understood that the various features can also be used in other combinations as those described and illustrated. Amended Claims: 1. An apparatus for feeding electrical energy into a power grid (8) with a DC voltage converter (2) intended for connection to a DC voltage generator (1) and with an inverter (3) connected thereto and intended for connection to said power grid (8), said inverter containing a bipolar voltage intermediate circuit with two capacitors (C1, C2) that are placed in series and are connected together at a ground terminal (E3) intended for connection to a terminal of said DC voltage generator (1), said DC voltage converter (2) comprising at least two diodes (D3, D4), one switch and one storage choke (16) which is charged by the DC voltage generator (1) when the switch is closed and is discharged via the capacitors (C1, C2) and the diodes (D3, D4) when the switch is open, characterized in that, on the one side, the storage choke (16) forms, together with two switches (S3, S4) disposed on either side of said storage choke, a first electric circuit intended for charging said storage choke (16), said electric circuit being connected to said DC voltage generator (1) by closing the switches (S3, S4) whilst at the same time the blocking diodes (D3, D4) decouple the storage choke (16) from the inverter (3) in terms of potential, and that, on the other side, said storage choke (16) lies, together with the two diodes (D3, D4) and the two capacitors (C1, C2), in a second electric circuit intended for simultaneously discharging the storage choke (16) via both capacitors (C1, C2) and diodes (D3, D4), said second electric circuit being made operative by opening the switches (S3, S4) so that, when the storage choke (16) is discharged, the DC voltage generator (1) is galvanically connected to the inverter (3) via only one ground line (19, 20, 21) connected to the ground terminal (E3). 2. The apparatus as set forth in claim 1, characterized i n that the first electric circuit is a series circuit in which the storage choke (16) is electrically interposed between the two switches (S3, S4). 3. The apparatus as set forth in claim 1 or 2, characterized i n that the second electric circuit is a series circuit that leads from a first terminal of the storage choke (16), via a first diode (D3), the two capacitors (C1, C2) and a second diode (D4), to a second terminal of the storage choke (16). 4. The apparatus as set forth in any one of the claims 1 through 3, characterized i n that the storage choke (16) is divided and comprises a coil tap (23) that is connected to one of the two switches (S3, S4) in such a way that only one first part (W11) of the storage choke (16), which is fixed by said tap (23), lies in the first electric circuit, whilst a second part (W11 + W12) of the storage choke (16) is arranged in the second electric circuit. 5. The apparatus as set forth in claim 1, characterized i n that the storage choke (16) contains a first coil (W1) and a second coil (W2), said two coils (W1, W2) being magnetically coupled together and galvanically connected together, said first coil (W1) forming the first electric circuit together with the two switches (S3, S4) and both coils (W1, W2) lying together in the second electric circuit. 6. The apparatus as set forth in claim 5, characterized i n that the second electric circuit is a series circuit that leads from a first terminal of the first coil (W1), via the first diode (D3), the two capacitors (C1, C2), a second diode (D5), the second coil (W2) and a third diode (D6), back to a second terminal of the first coil (W1). 7. The apparatus as set forth in claim 5 or 6, characterized i n that the first coil (W1) is divided and comprises a coil tap which is connected to one of the two switches (S3, S4) in such a manner that only one first part of the first coil, which is fixed by said tap, lies in the first electric circuit, whilst a second part of the first coil is disposed in the second electric circuit. 8. The apparatus as set forth in any one of the claims 5 through 7, characterized i n that the two coils (W1, W2) are wound on one common core (16). 9. The apparatus as set forth in claim 8, characterized i n that the coils (W1, W2) are wound on the core (16) in opposite senses of winding. 10. The apparatus as set forth in any one of the claims 5 through 9, characterized i n that the two coils (W1, W2) have the same number of windings. 11.The apparatus as set forth in any one of the claims 1 through 10, characterized i n that the DC voltage converter (2) is configured to be a component part (27) that comprises a plurality of terminals (12, 13, 28 through 31) configured to be plug contacts by means of which it may be connected to associated inputs of the inverter (3), depending on the grounding desired for the DC voltage generator (1). 12. The apparatus as set forth in any one of the claims 1 through 11, characterized i n that the DC voltage converter (2) is provided with a ground line (19, 20, 21) that connects an input (10, 11) to be connected to the DC voltage generator (1) to an output (14) to be connected to the ground terminal (E3) of the inverter (3). 13. The apparatus as set forth in claim 12, characterized i n that a monitoring element (32) for sensing fault currents is connected in the ground line (19, 20, 21). 14. The apparatus as set forth in any one of the claims 1 through 13, characterized in that the inverter (3) is configured to be an inverter comprising a half bridge. 15.The apparatus as set forth in any one of the claims 1 through 13, characterized i n that the inverter (3) is configured to be an inverter having a half bridge in 3-level circuit (Fig. 7). 16. The apparatus as set forth in any one of the claims 1 through 13, characterized i n that the inverter (3) is configured to be an inverter having a half bridge in 3-level circuit with center point (Fig. 8). 17. The apparatus as set forth in any one of the claims 1 through 13, characterized i n that the inverter (3) is configured for single or three-phase supply of electrical energy into the power grid (8) (Fig. 2 through 6 or Fig. 7). 18. The apparatus as set forth in any one of the previous claims that the DC voltage converter (2) is combined with the inverter (3) to form a structural unit. The invention describes an apparatus for feeding electrical energy into an energy supply system (8) with a DC voltage transformer (2), which is intended to be connected to a DC voltage generator (1), and an inverter (3) which is connected to said DC voltage transformer (2) and is intended to be connected to the energy supply system (8), with a bipolar voltage intermediate circuit. The DC voltage transformer (2) is constructed according to the invention in such a way that the DC voltage generator (1) can be grounded either at its negative, positive or any central output. For this purpose, a storage inductor (16) is provided which is switched both in a charging cycle and in a discharging cycle in such a way that no currents flow via a grounding line (19) during normal operation.

Full Text

APPARATUS FOR FEEDING ELECTRICAL ENERGY
INTO A POWER GRID AND DC VOLTAGE
CONVERTER FOR SUCH AN APPARATUS
The invention relates to an apparatus of the type recited in the preamble of
claim 1 and to a DC voltage converter suited therefor.
To feed electrical energy generated with DC voltage generators such as
photovoltaic or fuel cell plants into an AC grid, in particular into the utility
grid (50/60 Hz), various inverters are used. Between the DC voltage
generator and the inverter there is mostly provided a DC voltage converter
(DC-DC chopper) that serves the purpose of converting the DC voltage
supplied by the DC voltage generator into a DC voltage needed by the
inverter or adapted thereto.
For different reasons, it is desired to ground one of the outputs of the DC
voltage generator. The reason for the desired grounding is on the one side
that there are countries which prescribe such grounding. On the other side,
different disadvantages arise in operation when grounding is missing. One
of the problems encountered is that of the high-frequency leakage currents.
Due to inevitable, parasitic capacitances between the DC voltage generator
and the ground, considerable equalizing currents creating an intolerable
safety risk may occur in case of potential fluctuations so that complex
monitoring measures using fault current sensors or the like are needed for
contact protection or for establishing the electromagnetic compatibility
(EMC) and said equalizing currents can only be securely avoided by
grounding. Moreover, it is known that photovoltaic generators behave very

differently with respect to degradation, depending on which technology is
used to manufacture them. Generators with crystalline and polycrystalline
cells or certain thin film modules are preferably grounded with the negative
terminal, whilst backside-contact cells are preferably grounded at the
positive terminal.
A grounding of the type described, through which the disadvantages
mentioned could be avoided, is readily possible using DC voltage
converters with transformers which cause the DC voltage side to
galvanically separate from the AC voltage side. Irrespective of whether grid
transformers or high-frequency transformers are being used, transformers
result i.a. in a reduction of efficiency, in parts in considerable weight and
overall size and/or in an additional control expense, which is the reason
why transformerless voltage converters are in principle preferred. However,
the usual topologies of transformerless DC voltage transformers either
make the desired grounding impossible to perform since this grounding
would lead to a short-circuit of needed switches, capacitances or the like or
they result in increased circuit expense and other disadvantages.
Therefore, numerous attempts have been made to avoid the mentioned
disadvantages in another way. Circuits are known in particular, which serve
the purpose of reducing the undesirable leakage currents (e.g., DE 10 2004
037 466 A1, DE102 21 592 A1, DE 10 2004 030 912 B3). In these circuits,
a solar generator e.g., is operated isolated from the grid in certain phases
of internal electrical energy transport. When the solar generator is
periodically electrically reconnected to the grid, its parasitic capacitances
are only slightly reconverted so that the potential of the solar generator
changes with grid frequency, sinusoidally and at a voltage amplitude that
corresponds to half the grid voltage. High-frequency currents then form
through the low voltage differences of the solar generator between two
switching cycles only and through asymmetries during switching. Capacitive

leakage currents can thus be strongly minimized but cannot be avoided
completely as a matter of principle.
A circuit arrangement is further 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 solar generator have a fixed potential and capacitive
leakage currents cannot flow in principle. Since the two direct current
sources have a different yield, a circuit for compensating the power
differences and the voltages is additionally provided. In this proposed
circuit, the disadvantage lies in the high voltage differences in the solar
generator and at the switches, in the additional losses in the compensation
circuit and in the fact that at least four high-frequency pulsed switches are
needed.
Besides, circuit arrangements have already been known by means of which
a solar generator can be grounded on one side, in spite of the lack of a
transformer. As a result, capacitive leakage currents are prevented as a
matter of principle. One of these circuit arrangements (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 average output current.
With this circuit, which is also referred to as "Flying Inductor", the efficiency
is therefore affected by the high number of components participating
simultaneously in series in the current flow. Another disadvantage of this
circuit is that discontinuous current pulses are impressed upon the grid,
which call for a capacitive mains filter which, as inherent to its functional
principle, degrades the power factor but also the efficiency of the circuit in
the part load range because of its own need for idle power. Although such a
capacitive mains filter can be avoided with another known circuit (DE 197
32 218 C1), nine active switches are needed therefor, of which at least two
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 apparatus negatively affected. The topology of a
Flying Inductor also has the disadvantage that the voltage load of the
switches depends on the grid voltage and is sensitive to mains power
failures and that it can only be operated in the three-phase mode of
operation by three-fold use with the help of three inverters. Irrespective
thereof, inverters with current source characteristics are needed, which is
undesirable in many cases.
Finally, apparatus are known (US 2007/0047277 A1), which are intended
for inverters having a bipolar voltage intermediate circuit containing two
series-connected capacitors connected together at a ground terminal. Such
type inverters, which are nowadays mainly used for the purposes of interest
herein, can be configured as half-bridge inverters in 3-level circuits and at
need as inverters for one-phase or three-phase grid supply. In all of these
cases, the connection node between the two capacitors forms a ground
terminal that is associated with the zero or neutral conductor of the
respective grid and is connected therewith.
The DC voltage converter of this known apparatus contains one choke, two
diodes and one switch. In this case, the ground terminal of the inverter can
be connected to the negative output of the DC voltage generator. This is
made possible using a storage choke that is composed of two magnetically
coupled coils. The two coils of this storage choke are galvanicaliy
connected together at one end in such a manner that on the one side, when
the switch is closed, one of the two coils is charged by the DC voltage
generator and the other coil via the first coil by virtue of the magnetic
coupling and that, on the other side, when the switch is open, the two coils
are discharged via a respective one, associated, of the two capacitors and
via a diode belonging thereto.

The advantage that this apparatus makes it possible to ground the DC
voltage generator with relatively simple means, in particular without any
transformer and with only one switch, is offset by the disadvantage that the
ground terminal can only be connected to the negative output of the DC
voltage converter. Further, this apparatus does not allow for monitoring the
ground line leading from the ground terminal to the DC voltage generator
with respect to fault currents since, as a matter of principle, operating
currents also flow in this ground line.
A circuit with a power storage choke and two switches connected in series
therewith is known from JP 11 235024 A1. On the output voltage side, there
are provided two diodes in order to decouple the input and the output. A
DC-AC converter with a negative and a positive input and a three-phase AC
output is used. Grounding is provided neither at the input nor at the output
of the DC-AC converter. It is not mentioned whether the DC-AC converter is
transformerless. At the output of the DC-DC circuit, there is only provided
one single capacitor. Through this circuit, bidirectional operation of a DC-
DC converter can be provided.
In view of this prior art, the technical problem of the invention is to configure
the apparatus of the type mentioned herein above, and in particular a DC
voltage converter suited therefor, in such a manner that it is possible to
ground the DC voltage generator at any terminal and that this can be
realized with relatively simple construction means.
In accordance with the invention the solution to this problem is achieved
with the characterizing features of the claims 1 and 16.
The invention allows for grounded operation of the DC voltage generator by
using a DC voltage converter which, in the simplest case, only needs one
storage choke, two diodes and two switches. As a result, in spite of only

slightly increased expense, the advantage is obtained that the DC voltage
generator can be grounded almost anywhere.
Other advantageous features of the invention will become apparent from
the dependent claims.
The invention will be understood better upon reading the exemplary
description accompanying the appended drawings wherein
Fig. 1 through 3 show a first exemplary embodiment of an apparatus of the
invention for feeding electrical energy into an energy supply
system with three different grounding possibilities for a DC
voltage generator;
Fig. 4 shows the signals for controlling two switches of the apparatus
shown in Fig. 1 through 3 and the current curves resulting
therefrom;
Fig. 5 shows an apparatus as shown in the Figs. 1 through 3, but with
a slightly modified DC voltage converter;
Fig. 6 and 7 show a second exemplary embodiment of an apparatus of the
invention with two different grounding possibilities for a DC
voltage generator;
Fig. 8 through 10 schematically show the DC voltage converter shown in
the Figs. 6 and 7 as a component part having a structure that
may be selected through plug contacts; and
Fig. 11 through 13 different types of inverters, which may be operated with
the DC voltage converter of the invention as an alternative to
the inverter shown in the Figs. 1 through 3.
According to Fig. 1, an apparatus for generating electrical energy contains
a DC voltage generator 1, a DC voltage converter 2 and an inverter 3. The
DC voltage generator 1 consists e.g., of a photovoltaic or fuel cell plant and

comprises at the outputs 4 (+) and 5 (-) a capacitor C that is connected in
parallel therewith.
A preferred inverter 3 within the scope of the present application comprises
two outputs 6 and 7 which serve herein for single phase supply of electrical
energy into a grid 8 the phase L of which is connected with the output 6 and
the zero or neutral conductor N of which is connected with the output 7. The
inverter 3 moreover contains three inputs E1, E2 and E3. Between the
inputs E1 and E2, there are disposed two series connected capacitors C1
and C2 the connection node of which lies at the input E3. The capacitors
C1 and C2 form a usual bipolar voltage intermediate circuit of the inverter 3.
As shown in Fig. 1, the inverter 3 is configured to be a half bridge inverter
and is provided for this purpose with two switches S1 and S2 the one
terminal of which is respectively connected with a respective one of the
inputs E1 and E2 and the other terminal of which leads to a common
connection node 9 and from there to the output 6, via a smoothing or grid
choke L1. A diode D1, D2 is additionally connected in parallel with a
respective one of the two switches S1, S2; the diode D1 can thereby be
made conductive from the connection node 9 in the direction of the input E1
and the diode D2 from the input E3 in the direction of the connection node
9, said diodes closing respectively in the opposite direction. Finally, the
input E3 is directly connected with the output 7, grounded on the other side
and configured to be a ground terminal as a result thereof.
The inverter 3 substantially operates as follows: if the switches S1, S2 are
alternately switched on and off, the side (input E1) which is positive with
respect to E3 of the capacitor C1 is connected to phase L via the
connection node 9 and the grid choke L1 e.g., during the positive half wave
of the switch signal (switch S1 at first closed, switch S2 open). When the
switch S1 opens next, the current can continue to flow through the grid
choke L1, the capacitor C2 and the diode D2. During the negative half wave

of the grid 8 (switch S1 open, switch S2 at first closed), the side (input E2)
of the capacitor C2, which is negative with respect to E3, is connected to
the phase L via the connection node 9 and the choke L1, the current being
allowed to continue to flow through the diode D1 and the capacitor C1 after
the switch S2 has closed. The two capacitors C1, C2 are discharged
alternately as a result thereof, they being recharged in a known way with
the help of any suited DC voltage converter.
Apparatus of the type described are generally known (e.g., US
2007/0047277 A1, Fig. 10) and need therefore not be described in detail to
those skilled in the art.
Referring to Fig. 1, a DC voltage converter 2 of the invention contains two
inputs 10 and 11, which are connected to the two outputs 4 and 5 of the DC
voltage generator, as well as three outputs 12, 13 and 14 which are
connected to the inputs E1, E2 and E3 of the inverter 3. At the input 10
there is connected a switch S3, which leads to a connection node 15. The
one terminal of a storage choke 16 is connected to this connection node 15,
the other terminal of said storage choke being located at a connection node
17 that is connected with the input 11 via a second switch S4. Moreover,
the connection node 17 is connected to the output 12 via a first diode D3
whilst the output 13 leads to the connection node 15 via a second diode D4.
The diode D3 can be made conductive in the direction of the output 12, the
diode D4 in the direction of the connection node 15, whilst both are closing
in the respective opposite direction. As a result, the functioning principle of
the DC voltage converter 2 is as follows:
When the switches S3 and S4 are closed at the same time, the storage
choke 16 is recharged by the DC voltage generator 1 or by its capacitor C.
The switch S3, the storage choke 16 and the switch S4 form a first series
electric circuit that serves for storing electrical energy in the storage choke

16. At this time, the diodes D3 and D4 prevent the current flow to or from
the capacitors C1 and C2. If, by contrast, the two switches S3 and S4 are
opened simultaneously, the storage choke 16 discharges via the diode D3,
the series-connected capacitors C1 and C2 and the diode D4. In this
phase, the storage choke 16 forms, together with the parts D3, C1, C2 and
D4, a second series electric circuit intended for discharging the storage
choke 16 or for accordingly recharging the capacitors C1, C2. If the two
capacitors C1, C2 have the same capacitance, they are charged to the
same voltage UC1 = UC2.
In their opened condition, the voltage load of the switches S3, S4 is
relatively small. When the diodes D3 and D4 are conductive, the voltage at
the switch S3 is US3 = UC + UC2 at the most, wherein UC is the output
voltage of the DC voltage generator 1. The voltage at the switch S4, by
contrast, is US4 = UC1 at the most.
Irrespective thereof, the DC voltage converter 2 described offers the
advantage that the DC voltage generator 1 can be operated with a relatively
large range of output voltages. If the DC voltage converter 2 were missing,
it should be made certain that the DC voltage generator 1 always supplies
the inputs E1 and E2, even under unfavourable conditions, with such a high
output voltage that the capacitors C1 and C2 are charged to a voltage that
is higher than the grid amplitude (usually about ± 325 V). If, by contrast, a
boost DC voltage converter 2 is provided, the voltages at the capacitors C1,
C2 can be set to the desired height by selecting the pulse-duty factor at
which the switches S3 and S4 are operated even if the output voltage of the
DC voltage generator 1 is lower than the voltage at least needed by the
inverter 3 (or by the grid 8).
Further, the apparatus described is very flexible in utilization. This results
from the fact that the voltages at C1 and C2 may be both higher and lower

than the input voltage at the capacitor C, depending on the selected pulse
duty factor for S3 and S4. If the pulse duty factor is more than 0.5, the DC
voltage converter operates in the boost mode of operation. If the pulse duty
factor is less than 0.5, the DC voltage converter 2 operates in the buck
mode of operation. A pulse duty factor of 0.5 practically results in the
voltage applied at the output of the DC voltage generator 1 being fed
directly. The maximum voltage load of the inverter switches S1 and S2 is
about 2 • UC1, wherein UC1 is the maximum voltage at the capacitor C1. In
the simplest case, it is also possible to always have only one of these
switches switched at high-frequency for each half mains period, whilst the
other one remains switched off. Moreover, a continuous current flow into
the grid 8 is possible on the inverter side.
A major advantage of the invention is finally obtained in that the grounding
point E3 can be connected optionally with the input 11 of the DC voltage
converter 2 and as a resuit thereof with the negative output 5 (Fig. 1), with
the input 10 of the DC voltage converter 2 and as a result thereof with the
positive output 5 (Fig. 2) or with any other terminal 18 (Fig. 3) of the DC
voltage generator 1, as this also applies for the neutral conductor N of the
grid 8. During normal operation, no current flows through a grounding line
19 (Fig. 1) or 20 (Fig. 2) or 21 (Fig. 3), which is respectively shown in a
dashed line and which connects the grounding point E3 with the
corresponding input of the DC voltage converter 2 or with the
corresponding output of the DC voltage generator 1. This results in
particular from the fact that, together with the parts E3, C1, C2 and D4, the
storage choke 16 forms an electric circuit, which is closed in itself and does
not contain the lines 19, 20 or 21. As a result, it may be concluded that
there is a fault in the plant if current still flows in the line 19, 20 or 21. In
accordance with the invention, a monitoring element in the form of a circuit
breaker or the like is preferably disposed in the line 19, 20 or 21 for
automatically switching off the plant when a preselected tolerable current

peak is exceeded. This function is independent on which input of the DC
voltage converter 2 or which output of the DC voltage generator 1 the
ground terminal E3 is connected to.
In a known way, the switches S1 through S4 are practically configured to be
semiconductor switches that may be switched on and off periodically when
operated with control units that have not been illustrated herein
(microcontrollers, PWM controls and so on), the switch frequency being
e.g., 16 kHz or more.
The signals for activating the switches S3 and S4 and the current path in
the storage choke 16 are illustrated by way of example in Fig. 4. It can be
seen therefrom that the two switches S3, S4 are always switched on and off
simultaneously.
Fig. 5 shows an exemplary embodiment that has been modified over Fig. 1
through 3 insofar as the storage choke 16 is divided into two coil parts W11
and W12 through a central terminal or a coil tap 23. In this case, the
arrangement is such that the connection node 15 is connected to the tap 23
and that, as a result thereof, only the part W11 of the storage choke 16,
which is fixed by the tap 23, lies in the first electric circuit which serves to
charge the storage choke 16, whilst the second electric circuit contains the
entire storage choke 16 located between the diodes D4 and D3 or the part
W11 + W12 thereof. As a result, one may tap another optimization potential
of the arrangement of the invention for the relationship between input
voltage and output voltage, the load of the switch S3 and the diodes D3 and
D4. If the transmission ratio is higher, one has the possibility, beside the
pulse duty factor for S3 and S4, to influence the effective current and
voltage load of the component parts via the relation (W12 + W11): W11. In
principle, the location of the tap 23 can be chosen ad lib. A particular
advantage of the tap 23 is that the maximum voltage load at the switch S3

in the open condition is only given by the voltage US3 = UC + [- n/(n + 1)] •
UC1 + UC2, wherein n = W12/W11 and W11 and W12 simultaneously refer
to the number of windings of the coils W11 and W12. The voltage load at
the switch S4 is US4 = UC1. Alternatively, it is also possible to connect the
tap 23 with the switch S4 in analogous fashion. For the rest, the apparatus
shown in Fig. 5 corresponds to that shown in the Figs. 1 through 3, this
being the reason why the output 14 of the DC voltage converter 2 may be
connected optionally with the output 4 or 5 or with any other output of the
DC voltage generator 1.
Another exemplary embodiment of the invention is illustrated in the Figs. 6
and 7. This embodiment differs from the one shown in the Figs. 1 through 5
in particular in that the advantages described are obtained herein with a
coupled storage choke 24, which is known per se but which is electrically
connected in a hitherto unknown way. The storage choke 24 contains a first
coil W1 and a second one W2 which are magnetically coupled together and
are for example wound on a common core 25 for this purpose.
Like the choke coil 16 in Fig. 1, the first coil W1 is electrically interposed
between the two switches S3, S4 or between the two connection nodes 15
and 17. Moreover, the connection node 17 is connected to the output 12 via
the diode D3 like in Fig. 1. By contrast, the input 13 of the DC voltage
converter 2 is connected via a diode D5 to a terminal of the coil W2 the
other terminal of which leads to the connection node 15 via a connection
node 26 and a diode D6. Moreover, the connection node 26 is connected to
the output 14. With this provision, the functioning is as follows:
The first coil W1 of the storage choke 24 forms, together with the two
switches S3, S4, a first series electric circuit that is placed in parallel with
the outputs 4,5 of the DC voltage generator 1 and serves for charging the
coil W1 with electrical energy when the switches S3, S4 are closed. Since

the two coils W1, W2 are magnetically coupled, the coil W2 is also charged
in this phase via the coil W1. The sense of winding of the two coils W1, W2
is thereby chosen so as to obtain the same voltage polarities at terminals
which are shown by dots in Fig. 6.
In the open condition of the switches S3, S4, the two coils W1, W2 lie in a
second series electric circuit that leads from one of the terminals of the coil
W1 (connection node 17), via the diode D3, the series-mounted capacitors
C1 and C2, the diode D5, the coil W2, the connection node 26 and the
diode D6 back to the other terminal of the coil W1 (connection node 15).
Like in the case shown in Fig. 1, this second electric circuit is an electric
circuit that is closed in itself and serves to jointly discharge the coils W1,
W2 or to jointly charge the capacitors C1, C2. Moreover, the two coils W1,
W2 are galvanically connected together through this electric circuit.
As a result of this arrangement, it is possible to optionally connect the
output 14 of the DC voltage converter 2 or the output E3 of the inverter 3
through the line 19 (Fig. 6), or the line 20 (Fig. 7) to the input 11 or 10 of the
DC voltage converter 2 and, as a result thereof, also optionally to the output
5 or 4 of the DC voltage generator 1, in order to ground it at the negative
output 5 (Fig. 6) or at the positive output 4 (Fig. 7). Moreover, the input E3
could be connected, analogous to Fig. 3, to any central output of the DC
voltage generator 1. In all the cases described, these lines 19, 20 and, if
applicable, 21 are not in use in normal operation since no current is allowed
to flow through these lines 19 through 21 neither during charging nor during
discharging of the storage choke 16. As a result, like in the case illustrated
in the Figs. 1 through 5, a still measured current flow in these lines 19
through 21 or between the grounding point E3 and one of the terminals 4, 5
or 18 would be indicative of a fault in the plant or in the DC voltage
converter 2 and could be used to switch off the plant.

An advantage of the apparatus shown in Fig. 6 over the apparatus shown in
the Figs. 1 through 3 results from the lower voltage load of the switch S3.
Since the diode D6 is conductive during the blocking phase of the switches
S3 and S4, the maximum voltage applied at switch S3 is the voltage UC,
whilst the voltage UC1 is applied at S4 since the diode D3 is also
conductive. In the apparatus shown in Fig. 7, by contrast, the voltage load
at the switch S3 is equal to zero and at the switch S4 to UC + UC1.
According to another exemplary embodiment of the invention that has not
been illustrated separately the coil W1 of the choke coil 16 can be divided
into two parts by a tap, in analogous fashion to Fig. 5. Like in Fig. 5, it is
thereby possible to connect the tap to one of the connection nodes 15, 17
while disposing the two coil parts in the second electric circuit. At need, the
voltage load of the switch S3 of the exemplary embodiment shown in the
Figs. 6 and 7 is further reduced as a result thereof.
The magnetic coupling of the coils W1, W2 in the Figs. 6 and 7 is preferably
obtained by the fact that they are wound on one common core, above each
other or behind each other according to need. Preferably, they have the
same number of windings and are practically wound on the core 25 in
opposing senses of winding in the arrangement schematically shown in the
Figs. 6 and 7 in order to obtain the right directions of the current flow during
charging and discharging.
The Figs. 8 through 10 show how the DC voltage converter 2, herein
specially the DC voltage converter 2 shown in the Figs. 6 and 7, can be
configured to be a component part 27 that is provided with a plurality of
terminals configured to be plug contacts or the like. As shown in Fig. 8, the
DC voltage converter 2 has, unlike in the Figs. 6 and 7, in addition to the
inputs 10, 11 and the outputs 12, 13, four additional outputs 28, 29, 30 and
31 and no output 14. The terminal 28 is directly connected to the input 10,

the terminal 31, to input 11. Further, the terminal 29 is connected to the
terminal of the coil W2 that is remote from the diode D5 and the terminal
30, to the connection node 26, this terminal not communicating with point
26 in Fig. 8. Through suited connections, the grounding of the DC voltage
generator 1 may now be optionally provided at the negative terminal 5 (Fig.
9) or at the positive terminal 4 (Fig. 10).
If grounding is desired to occur at the negative output 5, the terminal 31 is
grounded as shown in Fig. 9 and is connected to the input E3 of the inverter
3 and as a result thereof to the neutral conductor N of the grid 8 via a
monitoring element 32. Moreover, the terminals 29 and 30 are connected
together. As a result, one obtains the arrangement shown in Fig. 6 if, to
utilize the component 27, one connects the outputs 4, 5 of the DC voltage
generator 1 to its inputs 10 and 11, its outputs 12 and 13 to the inputs E1,
E2 of the inverter 3 and its terminals 29, 30 jointly to the input E3 of the
inverter 3.
If, by contrast, grounding is desired to occur at the positive output of the DC
voltage generator 1, the terminal 28 is grounded as shown in Fig. 10 and is
connected to the input E3 of the inverter 3 via the monitoring element 32.
The other connections occur like in Fig. 9. By merely re-plugging the
terminals 28, 31 of the component part 27 or of the DC voltage converter 2
located therein one has the option to choose to ground the DC voltage
generator 1 at the positive or at the negative output 4, 5. Other outputs of
the component part 27 could serve to ground also central terminals of the
DC voltage generator 1.
The same procedure is followed when using the DC voltage converter
shown in the Figs. 1 through 5.
Although the description given herein above only refers to the inverter 3

configured to be a half bridge inverter, it is clear to those skilled in the art
that other inverters with a bipolar voltage intermediate circuit can be
connected to the DC voltage converter 2 of the invention. This is
schematically shown in the Figs. 11 through 13. Fig. 11 shows a half bridge
inverter in a 3-level circuit, Fig. 12 another inverter in a 3-level circuit with
center point (each in a single-phase implementation) and Fig. 13 shows an
inverter for 3-phase grid 8 supply. All the three inverters have a bipolar
voltage intermediate circuit, the inputs E1 through E3 and the outputs 6, 7
correspond to the above description. Since such type inverters are known
per se, it seems that they need not be discussed further herein.
The invention is not limited to the exemplary embodiments described, which
can be varied in various ways. This applies in particular insofar as the
inverters 3 and the DC voltage converters 2 are preferably manufactured
and sold as a finished structural unit as shown in the drawings, but they can
also be manufactured and sold as separate component parts. The
embodiments described referring to the Figs. 8 through 10 are particularly
suited therefor since they make it possible to mass-produce universally
utilizable DC voltage converters which are independent on the kind of
grounding of the DC voltage converter 1 desired in a particular case.
Accordingly, the invention not only relates to the combination of a DC
voltage converter 2 and of an inverter 3, but also to the DC voltage
converter 2 alone. It is further clear that in the above specification only
those component parts were described that are needed to garner an
understanding of the invention, and that in particular the required and
actually known control units, MPP controls and so on can be additionally
provided. Also, it is understood that the various features can also be used
in other combinations as those described and illustrated.

Amended Claims:
1. An apparatus for feeding electrical energy into a power grid (8) with a
DC voltage converter (2) intended for connection to a DC voltage
generator (1) and with an inverter (3) connected thereto and intended for
connection to said power grid (8), said inverter containing a bipolar
voltage intermediate circuit with two capacitors (C1, C2) that are placed
in series and are connected together at a ground terminal (E3) intended
for connection to a terminal of said DC voltage generator (1), said DC
voltage converter (2) comprising at least two diodes (D3, D4), one
switch and one storage choke (16) which is charged by the DC voltage
generator (1) when the switch is closed and is discharged via the
capacitors (C1, C2) and the diodes (D3, D4) when the switch is open,
characterized in
that, on the one side, the storage choke (16) forms, together with two
switches (S3, S4) disposed on either side of said storage choke, a first
electric circuit intended for charging said storage choke (16), said
electric circuit being connected to said DC voltage generator (1) by
closing the switches (S3, S4) whilst at the same time the blocking
diodes (D3, D4) decouple the storage choke (16) from the inverter (3) in
terms of potential, and that, on the other side, said storage choke (16)
lies, together with the two diodes (D3, D4) and the two capacitors (C1,
C2), in a second electric circuit intended for simultaneously discharging
the storage choke (16) via both capacitors (C1, C2) and diodes (D3,
D4), said second electric circuit being made operative by opening the
switches (S3, S4) so that, when the storage choke (16) is discharged,
the DC voltage generator (1) is galvanically connected to the inverter (3)
via only one ground line (19, 20, 21) connected to the ground terminal
(E3).

2. The apparatus as set forth in claim 1,
characterized i n
that the first electric circuit is a series circuit in which the storage choke
(16) is electrically interposed between the two switches (S3, S4).
3. The apparatus as set forth in claim 1 or 2,
characterized i n
that the second electric circuit is a series circuit that leads from a first
terminal of the storage choke (16), via a first diode (D3), the two
capacitors (C1, C2) and a second diode (D4), to a second terminal of
the storage choke (16).
4. The apparatus as set forth in any one of the claims 1 through 3,
characterized i n
that the storage choke (16) is divided and comprises a coil tap (23) that
is connected to one of the two switches (S3, S4) in such a way that only
one first part (W11) of the storage choke (16), which is fixed by said tap
(23), lies in the first electric circuit, whilst a second part (W11 + W12) of
the storage choke (16) is arranged in the second electric circuit.
5. The apparatus as set forth in claim 1,
characterized i n
that the storage choke (16) contains a first coil (W1) and a second coil
(W2), said two coils (W1, W2) being magnetically coupled together and
galvanically connected together, said first coil (W1) forming the first
electric circuit together with the two switches (S3, S4) and both coils
(W1, W2) lying together in the second electric circuit.

6. The apparatus as set forth in claim 5,
characterized i n
that the second electric circuit is a series circuit that leads from a first
terminal of the first coil (W1), via the first diode (D3), the two capacitors
(C1, C2), a second diode (D5), the second coil (W2) and a third diode
(D6), back to a second terminal of the first coil (W1).
7. The apparatus as set forth in claim 5 or 6,
characterized i n
that the first coil (W1) is divided and comprises a coil tap which is
connected to one of the two switches (S3, S4) in such a manner that
only one first part of the first coil, which is fixed by said tap, lies in the
first electric circuit, whilst a second part of the first coil is disposed in the
second electric circuit.
8. The apparatus as set forth in any one of the claims 5 through 7,
characterized i n
that the two coils (W1, W2) are wound on one common core (16).
9. The apparatus as set forth in claim 8,
characterized i n
that the coils (W1, W2) are wound on the core (16) in opposite senses of
winding.
10. The apparatus as set forth in any one of the claims 5 through 9,
characterized i n
that the two coils (W1, W2) have the same number of windings.

11.The apparatus as set forth in any one of the claims 1 through 10,
characterized i n
that the DC voltage converter (2) is configured to be a component part
(27) that comprises a plurality of terminals (12, 13, 28 through 31)
configured to be plug contacts by means of which it may be connected
to associated inputs of the inverter (3), depending on the grounding
desired for the DC voltage generator (1).
12. The apparatus as set forth in any one of the claims 1 through 11,
characterized i n
that the DC voltage converter (2) is provided with a ground line (19, 20,
21) that connects an input (10, 11) to be connected to the DC voltage
generator (1) to an output (14) to be connected to the ground terminal
(E3) of the inverter (3).
13. The apparatus as set forth in claim 12,
characterized i n
that a monitoring element (32) for sensing fault currents is connected in
the ground line (19, 20, 21).
14. The apparatus as set forth in any one of the claims 1 through 13,
characterized in
that the inverter (3) is configured to be an inverter comprising a half
bridge.
15.The apparatus as set forth in any one of the claims 1 through 13,
characterized i n
that the inverter (3) is configured to be an inverter having a half bridge in
3-level circuit (Fig. 7).

16. The apparatus as set forth in any one of the claims 1 through 13,
characterized i n
that the inverter (3) is configured to be an inverter having a half bridge in
3-level circuit with center point (Fig. 8).
17. The apparatus as set forth in any one of the claims 1 through 13,
characterized i n
that the inverter (3) is configured for single or three-phase supply of
electrical energy into the power grid (8) (Fig. 2 through 6 or Fig. 7).
18. The apparatus as set forth in any one of the previous claims
that the DC voltage converter (2) is combined with the inverter (3) to
form a structural unit.

The invention describes an apparatus for feeding electrical energy into an energy supply system (8) with a DC
voltage transformer (2), which is intended to be connected to a DC voltage generator (1), and an inverter (3) which is connected to
said DC voltage transformer (2) and is intended to be connected to the energy supply system (8), with a bipolar voltage intermediate
circuit. The DC voltage transformer (2) is constructed according to the invention in such a way that the DC voltage generator (1)
can be grounded either at its negative, positive or any central output. For this purpose, a storage inductor (16) is provided which is
switched both in a charging cycle and in a discharging cycle in such a way that no currents flow via a grounding line (19) during
normal operation.

Documents:

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Patent Number

272872

Indian Patent Application Number

4237/KOLNP/2009

PG Journal Number

19/2016

Publication Date

06-May-2016

Grant Date

29-Apr-2016

Date of Filing

07-Dec-2009

Name of Patentee

SMA SOLAR TECHNOLOGY AG

Applicant Address

SONNENALLEE 1, 34266 NIESTETAL GERMANY

Inventors:

#	Inventor's Name	Inventor's Address
1	SAHAN, BENJAMIN	FRIEDENSTRASSE 17B, 34121 KASSEL GERMANY
2	ZACHARIAS, PETER	HEINRICH-ALBERT-STRASSE 3, 34131 KASSEL GERMANY

PCT International Classification Number

H02J3/38; H02M3/158; H02M7/48

PCT International Application Number

PCT/DE2008/000620

PCT International Filing date

2008-04-12

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	10 2007 028 078.7	2007-06-15	Germany