Voltage-buck auxiliary split-source inverter

Abstract

A step-down auxiliary split-source inverter comprising: a DC link having two voltage rails; at least two pairs of series connected switches; a first connection point and a second connection point for receiving voltage terminals of a fuel cell, one of the two voltage rails forming the first connection point; a series connected switch component and an inductor having a first end formed by a terminal of the switch component and a second end formed by a terminal of the inductor, the first end forming the second connection point for receiving voltage terminals of a fuel cell. The inverter further comprises: at least two first diodes; a second diode having a first terminal and a second terminal, wherein the switch component is adapted to be controlled to be conductive when any one of the lower switches connected to the voltage rail forming the first connection point is controlled to be conductive.

Description

Technical Field

[0001] This invention relates to inverters, and more particularly to split-source inverters that enable fuel cells as an energy source. Background Technology

[0002] Integrating low-voltage (LV) energy sources such as photovoltaics (PV) and fuel cells (FC) into AC grids has been a research topic for the past few years. Beyond this, integrating these energy sources with medium-voltage (MV) grids is receiving even greater attention due to the advantages seen behind MV integration at higher power levels. PV applications typically stack solar panels to achieve voltages up to 1.5kV; however, this is not the case for FC, which still has limitations in output voltage, with the highest voltage range available in the market being 400V–700V. Therefore, integrating FC into LV or MV grids requires the use of boost DC-DC converters and inverters to connect the FC to the AC grid, with high-voltage boost converters or transformers being essential for MV grids.

[0003] Typically, FC systems require two power conditioning stages, with a DC-DC converter used before the DC-AC converter to achieve, for example... Figure 1 The correct voltage level for the LV or MV AC mains grid integration is shown. Alternatively, an isolation or step-up transformer can be used.

[0004] Another approach is to use a single-level solution, such as Figure 2 The split-source inverter (SSI) shown integrates boost capability into inverter operation. This integrated solution achieves boost characteristics using inverter switches with additional diodes connected to the common inductor; that is, it avoids the use of additional active switches and gate drive circuitry. The operation of the SSI is described in the following: A. Abdelhakim, P. Mattavelli, and G. Spiazzi, “Three-Phase Split-Source Inverter (SSI): Analysis and Modulation,” IEEE Transactions on Power Electronics, Vol. 31, No. 11, pp. 7451-7461, November 2016. The inverter uses at least one of the lower semiconductor switches to charge the input inductor, while only one state is used to discharge the inductor on the inverter DC link when all upper switches are on. The operation of the three-phase SSI does not require generating any special pulses for its basic operation or modification of the standard modulation scheme of the voltage source inverter (VSI). Therefore, the same modulation scheme of the VSI can be applied to the SSI.

[0005] When using an SSI topology with a voltage source such as a fuel cell, it is important to maximize the utilization of the available voltage. Summary of the Invention

[0006] The object of this invention is to provide a buck-assisted split-source converter to mitigate the aforementioned disadvantages related to voltage utilization. This object is achieved through an inverter circuit according to an embodiment of the invention.

[0007] According to an embodiment of the present invention, a buck-assisted split-source inverter is provided. The buck-assisted split-source inverter includes: a DC link having two voltage rails; at least two pairs of switches connected in series, wherein anti-parallel diodes are connected between the voltage rails of the DC link, and the center point of the series-connected switches forms the phase output of the inverter; a first connection point and a second connection point for receiving voltage terminals of a fuel cell, one of the two voltage rails forming the first connection point; a switching component and an inductor connected in series, having a first end formed by terminals of the switching component and a second end formed by terminals of the inductor, the first end forming the second connection point for receiving voltage terminals of the fuel cell; at least two first diodes, wherein the at least two first diodes cause their corresponding terminals to... A terminal is connected together and connected to the second end of the series connection of the switching element and the inductor, and the other terminals of at least two first diodes are connected to separate phase outputs of the inverter; and a second diode has a first terminal and a second terminal, the first terminal being connected to a point between the switching element and the inductor, and the second terminal being connected to a voltage rail forming a first connection point, the polarity of the second diode corresponding to the polarity of at least two first diodes, such that a current path is formed through the second diode and at least two first diodes, wherein the switching element is adapted to be controlled to conduct when any of the lower switches connected to the voltage rail forming the first connection point is controlled to conduct.

[0008] According to an embodiment of the present invention, the first connection point is formed by the negative voltage rail of the DC link, the anodes of at least two first diodes are connected together, and the anode of the second diode is connected to the negative voltage rail of the DC link.

[0009] According to an embodiment of the present invention, the first connection point is formed by the positive voltage rail of the DC link, the cathodes of at least two first diodes are connected together, and the cathode of a second diode is connected to the positive voltage rail of the DC link.

[0010] According to an embodiment of the present invention, when any of the switches connected to the voltage rail forming the first connection point is controlled to be turned on and when the control signal is valid, the switching component is adapted to be controlled to be turned on.

[0011] According to an embodiment of the present invention, the DC link of the buck-assisted split-source inverter may include capacitors connected between the voltage rails of the DC link.

[0012] According to an embodiment of the present invention, the DC link of the buck-assisted split-source inverter may include a battery connected between the voltage rails of the DC link.

[0013] According to an embodiment of the present invention, a buck-assisted split-source inverter may include a boost converter connected to a DC link, the boost converter receiving power from a battery or supercapacitor.

[0014] According to an embodiment of the present invention, the DC link may include a battery connected between the voltage rails of the DC link, wherein the battery is connected to the output of the boost converter.

[0015] The present invention is based on the concept of modifying the configuration of the SSI circuit using a minimal number of additional components and without affecting the modulation of the inverter circuit.

[0016] Compared to known SSI configurations, the circuitry of this invention reduces voltage stress on the inverter switching components while simultaneously increasing DC link utilization. These advantages, along with standard inverter modulation, enable a more efficient architecture in use. Particularly concerning low-voltage sources such as fuel cells, the circuitry of this invention provides highly efficient DC-to-AC conversion. Attached Figure Description

[0017] In the following description, the invention will be illustrated in more detail with reference to the accompanying drawings, in which:

[0018] Figure 1 A conventional fuel cell integrated with an AC load is shown;

[0019] Figure 2 A known split-source inverter structure for integration with an AC load is shown; and

[0020] Figure 3 , Figure 4 , Figure 5 , Figure 6 and Figure 7 An embodiment of the buck-assisted split-source inverter of the present invention is shown. Detailed Implementation

[0021] Figure 1 A configuration for feeding electrical energy from a fuel cell to the grid is shown. In this configuration, a boost converter is first used to boost the DC voltage V from a voltage source (e.g., a fuel cell). dcThe voltage is then boosted. In the second stage, a voltage source inverter converts the boosted voltage into a three-phase AC voltage. A filter is also used to smooth the voltage generated by the inverter before feeding it to the grid.

[0022] Figure 2 A known split-source inverter with fuel cell 21 is shown. The split-source inverter has a voltage source inverter bridge connected between the voltages of the DC link. Figure 2 The DC link has capacitor 22 to maintain the voltage of the DC link. The voltage of the DC link is used to generate AC voltage using the switches of the inverter bridge.

[0023] In such Figure 2 In the split-source inverter shown, the inverter's switching components and additional inductor 23 are used to boost the voltage from voltage source 21. Furthermore, an additional diode 24 is required to prevent the DC link from being short-circuited. Although Figure 2 The operation of the split-source inverter is known, but the following description of the operation is provided.

[0024] exist Figure 2 In the example, a fuel cell is provided as a voltage source 21, and the voltage source 21 has a voltage V. FC The inverter bridge, formed by three parallel inverter branches, is modulated to activate either the upper or lower semiconductor switch of a phase. That is, the output voltage v is generated from the DC link voltage using the upper or lower semiconductor switch. a v b v c The upper switch is connected to the positive rail 25 of the DC link, and the lower switch is connected to the negative rail 26 of the DC link.

[0025] When the inverter bridge switches are modulated, current flows from fuel cell 21. When any of the lower switching components in the current switching components are turned on, inductor 23 is charged from voltage source 21. That is, a current path is formed from the voltage source through inductor 23, the conducting switch, and the corresponding diode to the negative rail 26 of the DC link and back to source 21. When one or more of the lower switching components are turned on, the voltage of the capacitor decreases as current is supplied to the load through the operating upper switch.

[0026] When all the upper switches are selected to be on during the modulation of the inverter switches, the inductor discharges and pushes current through the switching components to capacitor 22, thereby charging the capacitor. This state is typically achieved during normal modulation and no additional capacitor charging is required.

[0027] In a split-source inverter, during the modulation of the switching components, the voltage of voltage source 21 is effectively increased by the switching components of the inverter bridge.

[0028] Figure 3 The main circuit of the present invention is shown. The buck-assisted split-source inverter includes: a DC link having two voltage rails 31, 32; and at least two pairs of switches connected in series, wherein there are diodes connected in anti-parallel between the rails of the DC link, and the center point of the series-connected switches forms the phase output of the inverter.

[0029] exist Figure 3 In the diagram, a three-phase inverter bridge is shown with an upper switching component S. a,u S b,u S c,u and lower switch component S a,l S b,l S c,l It's important to note that each switching element in the inverter bridge also has diodes connected in parallel, allowing current to flow in both directions. The center point of the switch forms the phase output of the inverter. The output voltage is expressed as V. a v b and v c .

[0030] Furthermore, according to the present invention, the buck-assisted split-source inverter includes a first connection point 33 and a second connection point 34 for receiving voltage terminals of the fuel cell, with one of the two voltage rails forming the first connection point 33. Figure 3 The diagram shows a buck-assisted split-source inverter with a fuel cell 35 connected to a circuit. Connection points 33 and 34 are provided in the circuit for receiving the fuel cell and are therefore terminals for connecting the voltage output of the fuel cell.

[0031] exist Figure 3 In one embodiment, the first connection point is shown as DC voltage rail 32, which serves as the negative voltage rail of the DC link. In another embodiment discussed below, the first connection point is positive voltage rail 31.

[0032] like Figure 3 As shown, the buck-assisted split-source inverter also includes a switching component 36 and an inductor 37 connected in series. This series connection has a first end formed by the terminals of the switching component 36 and a second end formed by the terminals of the inductor 37. The first end of the series connection forms a second connection point 34 for receiving the voltage terminals of the fuel cell.

[0033] Furthermore, the inverter of the present invention includes at least two first diodes 39, wherein one terminal of each of the at least two first diodes 39 is connected together and connected to a second terminal of the series connection between the switching component and the inductor. The other terminals of the at least two first diodes 39 are connected to separate phase outputs of the inverter. Figure 3An embodiment with three phases and therefore three first diodes 39 is shown. The diodes are connected such that their anodes are connected to the inductor, allowing current to flow from the inductor to the inverter bridge.

[0034] Figure 3 A second diode 40 having a first terminal and a second terminal is also shown. The first terminal is connected to the point between the switching element 36 and the inductor 37. The second terminal of the second diode 40 is connected to a voltage rail 32 forming a first connection point 33. The polarity of the second diode corresponds to the polarity of at least two first diodes, such that a current path is formed through the second diode and the at least two first diodes. The polarity of the diode blocks the voltage at the point between the switching elements 36. The switching element 36 of the present invention is adapted to be controlled to conduct when any of the switches connected to the voltage rail forming the first connection point 33 is controlled to conduct. Figure 3 In the current implementation, switch S a,l S b,l S c,1 When any one of them is controlled to be turned on, the switching component 36 is also controlled to be turned on. Figure 3 In this context, this is shown as an "OR" circuit 41, which receives the control signal of the lower switch and outputs a logical "OR" of the input signal as the control signal of the switching component 36.

[0035] When Figure 2 and Figure 3 Upon comparison, it can be seen that the circuit of the present invention differs from known structures in that the present invention includes a switching component 36 and a second diode 40. Except that the switching component 36 is controlled to conduct when any switch connected to the first connection point is controlled to conduct, the operation of the circuit of the present invention corresponds to a known structure. Typically, a three-phase inverter can generate eight different switching states, which can also be referred to as the output voltage vector. From these states, the two states in which all of the upper switches or all of the lower switches are conducting are zero states, and the voltage between the output phases of the inverter is zero. Combined Figure 3 In this embodiment, when all upper switches are controlled to be on, current from inductor 37 is driven through the upper switch components to charge the DC link. According to this embodiment, since all the semiconductor switches connected to the first connection point are not on, switch component 36 is also not on, and current flows through the second diode 40.

[0036] Since the switching element 36 is used in conjunction with the inverter bridge, the voltage conversion gain can be further modified due to the additional degree of freedom in modulating the switching element 36. This can result in a reduction in voltage stress across the DC link in order to obtain the same AC voltage.

[0037] Figure 4 Another embodiment of the invention is shown. In this embodiment, the control of switch 36 is influenced by an additional control signal, Signal. An AND gate 45 is shown to provide control of switch 36, and the input to the AND gate is the additional control signal and the output of an OR gate 41. Figure 3 The implementation is similar; the "OR" gate receives control information from the lower switch. In other words, in Figure 4 In this implementation, switch 36 is controlled to be turned on if any of the lower switch components is controlled to be turned on and if the additional control signal is in a logic high state.

[0038] Using the control signal Signal, even though one of the lower switches is controlled to be on, the switching element 36 can also be set to the blocking state. When the control signal remains active, control information from the lower semiconductor switch is transmitted to the switching element. Using the control signal, switch 36 can be controlled to the blocking state. As described above, the current path through the lower switching element charges the inductor 37 according to the voltage of the fuel cell 35. If the current path from the fuel cell is disconnected from the switching element when current flows through the lower semiconductor switch, the current path changes to diode 40, and current is driven from inductor 37. When the inductor discharges through diode 40, the energy for the next charging cycle of the DC link is reduced. Therefore, the additional signal Signal enables a reduction in the DC link voltage. A reduction in the DC link voltage allows for higher utilization of the DC link voltage because a higher modulation index can be used in the control of the inverter bridge.

[0039] exist Figure 3 and Figure 4 In this embodiment, the voltage of the DC voltage link is supported by capacitor 42, which is charged by the fuel cell during operation.

[0040] Figure 5 Another embodiment is shown, in which the battery or battery cell is connected to a DC link. During operation, the battery is charged and discharged.

[0041] Figure 6 Another embodiment of the invention is illustrated. In this embodiment, a separate boost converter is connected to a DC link. The voltage to the boost converter is obtained from a battery or battery cell. The boost converter is operated in a known manner to control the voltage of the DC link. Preferably, the boost converter is operated such that a constant DC voltage is obtained in the DC link. In one embodiment, a supercapacitor is used instead of a battery or battery cell to maintain the DC link voltage. In such an embodiment, a supercapacitor is used instead of... Figure 6 The battery or battery cell.

[0042] Figure 7 Another embodiment of the invention is shown, wherein, like Figure 6 The DC link voltage is controlled using a boost converter, just like before. Figure 7 In, there is a voltage V B The battery or battery cell is connected to the output of the boost converter, and thus also to the input of the split-source inverter. The connection has a voltage V. SC The supercapacitors are used to feed power to the boost converter. Figure 7 In this structure, fuel cells, batteries, and supercapacitors can be integrated with minimal converter requirements.

[0043] exist Figures 3 to 7 In the illustrated embodiment, an input terminal for the fuel cell is provided such that one of the input terminals is a negative DC link. In such an embodiment, a first connection point is formed by the negative voltage rail of the DC link, the anodes of at least two first diodes are connected together, and the anode of a second diode is connected to the negative voltage rail of the DC link.

[0044] In another embodiment, the first connection point is formed by the positive voltage rail of the DC link, the cathodes of at least two first diodes are connected together, and the cathode of a second diode is connected to the positive voltage rail of the DC link. In such an embodiment, the fuel cell is adapted to be connected from its positive output terminal to the positive voltage rail. In this case, the output voltage state in which the energy storage device of the DC link is charged is when all the lower semiconductor switches of the inverter bridge are controlled to be turned on. Furthermore, when any upper switch of the upper switches of the inverter bridge is controlled to be turned on, the inductor is charged and the switching components are controlled to be turned on.

[0045] Figure 6 and Figure 7 The embodiments of the invention presented herein enable the integration of different energy sources into the converter structure. Different control schemes can be used to control the additional boost converter at the input of the buck-assisted split-source converter. The high-level control of the split-source converter and the additional boost converter can be synchronized to account for the different energy sources used in the structure.

[0046] It will be apparent to those skilled in the art that the inventive concept can be implemented in various ways as technology advances. The invention and its embodiments are not limited to the examples described above, but can be varied within the scope of the claims.

Claims

1. A step-down auxiliary split-source inverter, comprising: A DC link with two voltage rails; At least two pairs of switches connected in series, wherein anti-parallel diodes are connected between the voltage rails of the DC link, the series-connected switches have a lower switch connected to one of the two voltage rails, and the center point of the series-connected switches forms the phase output of the inverter; A first connection point and a second connection point for receiving the voltage terminals of the fuel cell, wherein one of the two voltage rails forms the first connection point; A switching component and an inductor connected in series have a first end formed by the terminals of the switching component and a second end formed by the terminals of the inductor, the first end forming a second connection point for receiving the voltage terminal of the fuel cell; At least two first diodes, wherein one terminal of each corresponding diode is connected together and connected to a second terminal of the series connection between the switching element and the inductor, and the other terminals of the at least two first diodes are connected to separate phase outputs of the inverter; and A second diode has a first terminal and a second terminal. The first terminal is connected to a point between the switching component and the inductor, and the second terminal is connected to a voltage rail forming the first connection point. The polarity of the second diode corresponds to the polarity of the at least two first diodes, such that a current path is formed through the second diode and the at least two first diodes. Wherein, when any of the lower switches connected to the voltage rail forming the first connection point is controlled to be turned on, the switching component is adapted to be controlled to be turned on.

2. The step-down auxiliary split-source inverter according to claim 1, wherein, The first connection point is formed by the negative voltage rail of the DC link, the anodes of the at least two first diodes are connected together, and the anode of the second diode is connected to the negative voltage rail of the DC link.

3. The step-down auxiliary split-source inverter according to claim 1, wherein, The first connection point is formed by the positive voltage rail of the DC link, the cathodes of the at least two first diodes are connected together, and the cathode of the second diode is connected to the positive voltage rail of the DC link.

4. The step-down auxiliary split-source inverter according to claim 1, 2 or 3, wherein, The switching component is adapted to be controlled to conduct when any of the switches connected to the voltage rail forming the first connection point is controlled to conduct and when the control signal is valid.

5. The buck-assisted split-source inverter according to any one of claims 1 to 3, wherein, The DC link of the buck-assisted split-source inverter includes a capacitor connected between the voltage rails of the DC link.

6. The buck-assisted split-source inverter according to any one of claims 1 to 3, wherein, The DC link of the buck-assisted split-source inverter includes a battery connected between the voltage rails of the DC link.

7. The buck-assisted split-source inverter according to any one of claims 1 to 3, wherein, The buck-assisted split-source inverter includes a boost converter connected to the DC link, which receives power from a battery or supercapacitor.

8. The step-down auxiliary split-source inverter according to claim 7, wherein, The DC link includes a battery connected between the voltage rails of the DC link, wherein the battery is connected to the output of the boost converter.

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