Inverter

Abstract

To provide an inverter which can separate a smoothing capacitor from an interconnection reactor and suppress generation of noise and leak current.SOLUTION: An inverter 1 converts DC power of a DC power source 2 into AC power so as to be interconnected with an AC power system comprising an AC power source 7, and comprises: two serially-connected smoothing capacitors C1, C2 connected in parallel with the DC power source to divide its DC voltage to a half; a bridge circuit 3 having at least four switching elements and consisting of two legs each having upper and lower arms, for converting the DC power into the AC power; an interconnection reactor 5 arranged on two AC output lines for connecting the bridge circuit with the AC power system; and a separation circuit 6 comprising switching elements S5, S6 for separation for separating the smoothing capacitors from the interconnection reactor by return current, in which a mid-point P1 between the two smoothing capacitors is connected with connection points of the upper and lower arms of each leg via the switching elements for separation.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an inverter that converts DC power to AC power with high efficiency for connection to a commercial power grid or a load. [Background technology]

[0002] It has been known in the past to convert DC power generated by a distributed power source such as a solar cell into AC power suitable for a commercial power system using an inverter circuit and supply it to the power system. As shown in Fig. 18, for example, a transformerless inverter circuit has a smoothing capacitor C on the input side connected in parallel to a solar cell 2, four switching elements S101 to S104, and four free wheel diodes D101 to D104 connected in anti-parallel, a bridge circuit (H-bridge) 103 that converts DC power into AC power by switching, an interconnection reactor 5 on the output side that converts the output current into a sine wave current, and an AC power source 7 or a load 8 that is interconnected to the power system.

[0003] Furthermore, in order to prevent energy loss caused by current return between the smoothing capacitor C and the interconnection reactor 5, which would result in a decrease in power conversion efficiency, an isolation circuit is provided to isolate the smoothing capacitor from the interconnection reactor during reflux (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] U.S. Patent No. 7,046,534 Summary of the Invention [Problem to be solved by the invention]

[0005] Here, when the H-bridge inverter operates with unipolar pulse-width modulation, it is expected that power conversion efficiency will be further improved and the filter will be more compact. However, while operating the H-bridge with unipolar pulse-width modulation makes it possible to reduce the size of the inverter, it creates problems with leakage current and the resulting EMI (electromagnetic interference) noise, which must be suppressed.

[0006] Furthermore, when an unbalanced load is connected in a single-phase three-wire inverter system, an imbalance is likely to occur in the output voltages on both sides of the neutral point due to factors such as the difference in the size of the loads on both sides of the neutral point.

[0007] An object of the present invention is to provide an inverter that can suppress energy loss due to current return by isolating a smoothing capacitor from an interconnection reactor, and can also suppress the generation of noise and leakage current.

[0008] It also aims to eliminate imbalances in the output voltages on either side of the AC neutral when inverters operating with unipolar pulse width modulation are used in single-phase, three-wire systems. [Means for solving the problem]

[0009] In order to achieve the above object, an inverter according to the present invention is an inverter that converts DC power of a DC power source into AC power so as to be connected to an AC power system including an AC power source or a load, The power converter comprises: two smoothing capacitors connected in series, the smoothing capacitors being connected in parallel to a DC power source and dividing the DC voltage by half; a bridge circuit having at least four switching elements and consisting of two legs, each having an upper and lower arm, for converting DC power to AC power; an interconnection reactor connecting the bridge circuit to the AC power system; and an isolation circuit including an isolation switching element for isolating the smoothing capacitor and the interconnection reactor by reflux; The midpoint between the two smoothing capacitors is connected to the connection point of the upper and lower arms of each leg via the isolation switching element.

[0010] According to this configuration, the isolation circuit isolates the smoothing capacitor and the interconnection reactor, improving power conversion efficiency. Furthermore, the midpoint between the two smoothing capacitors is connected to the connection points of the upper and lower arms of each leg via the isolation switching element, thereby bypassing common-mode noise and preventing the generation of noise and leakage current.

[0011] Preferably, the bridge circuit has a free wheel diode connected in anti-parallel to the switching element of the lower arm of each leg, the separation circuit is configured by connecting two separation switching elements in series in the reverse direction between connection points of the upper arm and the lower arm of each leg, the switching elements each having a free wheel diode connected in reverse parallel to the switching element, A free wheel diode is provided between the connection point of the isolation switching element and the positive connection point of each upper arm, and The midpoint between the two smoothing capacitors and the connection point of the two isolation switching elements are connected via a clamp switching element having a switching element and a freewheeling diode connected in inverse parallel to the switching element.

[0012] According to this configuration, by adding a clamp switching element to a so-called H6.5 inverter, it is possible to suppress the generation of EMI (electromagnetic interference) noise and leakage current at low cost.

[0013] Preferably, the midpoint between the two smoothing capacitors is connected to point O, which is the neutral point of the AC power system, to form a single-phase three-wire system. Therefore, the smoothing capacitor and the interconnection reactor are separated in the single-phase three-wire system, and the generation of EMI (electromagnetic interference) noise and leakage current can be suppressed. Furthermore, even in an unbalanced load in which the loads on both sides of point O are different in magnitude, a stable AC output can be obtained in which the balance of the voltage amplitude of each phase is not lost. This is because the clamp switching element connects the midpoint between the two smoothing capacitors to point O, which is the neutral point of the load or AC power source, and the voltage of point O is always held at zero level, thereby maintaining balance.

[0014] A distributed power supply system according to the present invention includes the inverter, and the DC power supply is a distributed power supply. With this configuration, the DC power of the distributed power supply is converted to AC power by the inverter with high efficiency and low noise. Effect of the Invention

[0015] In the present invention, the smoothing capacitor and the interconnection reactor are separated by a separation circuit through reflux, and the midpoint between the two smoothing capacitors is connected to the connection points of the upper and lower arms of each leg via a separation switching element, thereby isolating the smoothing capacitor and the interconnection reactor and preventing the generation of EMI (electromagnetic interference) noise and leakage current. [Brief description of the drawings]

[0016] [Figure 1] 1 is a circuit diagram showing a three-level neutral point clamped inverter for a single-phase three-wire system according to a first embodiment of the present invention. [Diagram 2] FIG. 2 is a modulation logic diagram of the inverter of FIG. [Diagram 3] FIG. 2 is a modulation logic diagram of the inverter of FIG. [Figure 4] 2 is a waveform diagram showing each mode of the inverter of FIG. 1. [Diagram 5] 2 is a circuit diagram showing the operation of the inverter of FIG. 1. [Figure 6] 2 is a circuit diagram showing the operation of the inverter of FIG. 1. [Figure 7] 2 is a circuit diagram showing the operation of the inverter of FIG. 1. [Figure 8] 2 is a circuit diagram showing the operation of the inverter of FIG. 1. [Figure 9] 2 is a circuit diagram showing the operation of the inverter of FIG. 1. [Figure 10] 2 is a circuit diagram showing the operation of the inverter of FIG. 1. [Figure 11] FIG. 2 is a circuit diagram showing an inverter for simulation in FIG. 1. [Figure 12] FIG. 12 is a characteristic diagram showing the simulation results of FIG. [Figure 13] FIG. 1 is a circuit diagram showing a conventional H6.5 inverter. [Figure 14] FIG. 14 is a characteristic diagram showing a simulation result of the circuit of FIG. [Figure 15] 5A and 5B are circuit diagrams showing a modified example of the first embodiment. [Figure 16] 6(A) to 6(C) are circuit diagrams showing other modified examples of the first embodiment. [Figure 17] FIG. 6 is a circuit diagram showing an inverter for a single-phase two-wire system according to a second embodiment of the present invention. [Figure 18] FIG. 1 is a circuit diagram showing a conventional high-efficiency inverter. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] 1 is a circuit diagram showing a three-level neutral point clamped inverter 1 according to a first embodiment of the present invention. This inverter 1 is a transformerless inverter that converts DC power from a DC power source 2, such as a solar cell, which is a distributed power source, into AC power so as to be connected to an AC power system including an AC power source 7 or a load 8.

[0018] The inverter 1 is equipped with two series-connected smoothing capacitors C1, C2 that are connected in parallel to a DC power source 2 and store electricity intermediately to smooth and stabilize the DC power, and divide the DC voltage Ed to 1 / 2·Ed, a bridge circuit 3 having four switching elements that converts the DC power to AC power, interconnection reactors 5, 5 that connect the bridge circuit 3 and an AC power source 7, and an isolation circuit 6 that isolates the smoothing capacitors C1, C2 from the interconnection reactors 5, 5. Transistors such as IGBTs are used for the switching elements (switches), but MOSFETs or the like may also be used.

[0019] The bridge circuit 3 consists of two legs, each having an upper arm and an lower arm of the switches S1 to S4. The switches S2 and S4 of the lower arm have freewheeling diodes D2 and D4 connected in anti-parallel, respectively, while the switches S1 and S3 of the upper arms each do not have a freewheeling diode like the lower arms.

[0020] The separation circuit 6 has two separation switches S5, S6 connected in series in the opposite direction between connection points P2, P3 of the upper and lower arms of each leg, and each switch S5, S6 has a free wheel diode D5, D6 connected in anti-parallel, respectively.

[0021] A freewheeling diode D7 is provided between the connection point P4 of the isolating switches S5 and S6 and the positive connection point P5 of each upper arm. This configures an H6.5 inverter with six switches and five diodes.

[0022] In the inverter 1, the midpoint P1 between the two smoothing capacitors C1 and C2 is further connected to a connection point P4 of the two isolating switches S5 and S6 via a neutral point clamp switch Sclamp. A diode D8 is connected in anti-parallel to the neutral point clamp switch Sclamp.

[0023] That is, in this inverter 1, a midpoint P1 between smoothing capacitors C1, C2 is connected to connection points P2, P3 between the upper and lower arms of each leg, and further, output terminals U, W of AC power systems 7, 8 are connected to these connection points P2, P3 via interconnection reactors 5, 5, respectively, forming a three-level neutral point clamp inverter with output potentials of +Ed, 0, -Ed.

[0024] The inverter 1 is further connected to a midpoint P1 between the two smoothing capacitors C1 and C2 and point O of the AC power systems 7 and 8, forming a single-phase three-wire system having, for example, two loads of 100 V and one load of 200 V. The operation of the inverter 1 will be described below.

[0025] As shown in the PWM (Pulse Width Modulation) logic diagram in Figure 2, switches S1 to S4 are turned on and off using a switching signal obtained by comparing a sine-wave voltage command signal (Modulating Signal) with a triangular carrier signal (Carrier). By changing the voltage command signal to a sine wave, an AC output signal can be obtained, and the inverter is used as a single-phase PWM inverter. The logic in Figure 2 is created by a controller (not shown).

[0026] In Figures 3(A) and 3(B), the on signal is indicated by a thick line and the pulse signal is indicated by a dashed line. Figure 3(A) shows the modulation logic when the voltage command signal is in a positive half cycle, and Figure 3(B) shows the modulation logic when the voltage command signal is in a negative half cycle.

[0027] In Fig. 3(A), the switches S1 and S2 are turned on and off, the switch S5 is on, and the switch S6 is turned on by the on-pulse of the neutral point clamp switch Sclamp. There is no AC output. In Fig. 3(B), the switches S3 and S4 are turned on and off, the switch S6 is on, and the switch S5 is turned on by the on-pulse of the neutral point clamp switch Sclamp.

[0028] The output voltage V and output current I obtained by the modulation logic of Figures 3(A) and (B) are shown in Figure 4. Modes I to VI are determined by the positive and negative states of the voltage and current.

[0029] 5 to 10 are circuit diagrams showing the operation of the inverter 1 which outputs various combinations of AC voltage V and AC current I by controlling the switches S1 to S6 in response to output signals from the control circuit in FIG.

[0030] In Fig. 5, the switches are controlled based on the modulation logic in Fig. 3(A), the switches S1 and S2 in Fig. 5 are turned on and off at a predetermined duty ratio, and current flows as shown by the arrows. When both switches S1 and S2 are turned off, the neutral point clamp switch Sclamp is turned on, and the switch S6 is also turned on. At this time, the output potential is 0 (corresponding to mode I in Fig. 4).

[0031] In Fig. 6, the isolation switch S5 is turned on, and a current flows as shown by the arrow through this switch S5 and the freewheeling diode D6. This causes conduction from the interconnection reactor 5 to the freewheeling diode D6 to the isolation switch S5 to the interconnection reactor 5, and during the freewheeling period of this active power, the smoothing capacitors C1 and C2 are isolated from the interconnection reactors 5 and 5 (corresponding to mode II in Fig. 4).

[0032] As a result, the isolation circuit 6, which isolates the smoothing capacitors C1, C2 and the interconnection reactors 5, 5 during reflux, acts to prevent energy loss due to the return of current between the smoothing capacitors C1, C2 and the interconnection reactors 5, 5, thereby improving the power conversion efficiency of the inverter.

[0033] In Fig. 7, the switches are controlled based on the modulation logic in Fig. 3(B), and the electromagnetic energy in the interconnection reactors 5, 5 is returned to the DC side through the AC power systems 7, 8, the isolation diode D6, the diode D1, and the diode D4 of the switch S4, as shown by the arrows in Fig. 7. If the electromagnetic energy in the current-smoothing interconnection reactors 5, 5 decreases during this reactive power return period, the circuit will no longer be energized (corresponding to mode III in Fig. 4).

[0034] In Fig. 8, during the on-period of the switches S3 and S4 that are turned on and off at a predetermined duty ratio, current flows through the switches S3 and S4 to the AC power systems 7, 8 as shown by the arrows, thereby conducting current from the DC side to the AC side. The output potential at this time is zero (corresponding to mode IV).

[0035] In Fig. 9, as shown by the arrows, conduction occurs in the order of interconnection reactor 5 → freewheel diode D5 → isolation switch S6 → interconnection reactor 5, and the smoothing capacitors C1, C2 are isolated from the interconnection reactors 5, 5 during the freewheeling period of active power. At this time, the neutral point clamp switch Sclamp and switch S6 are turned on, and the midpoint P1 between the smoothing capacitors C1, C2 and point O on the AC output side are clamped (corresponding to mode V in Fig. 4).

[0036] In Fig. 10, the isolation switches S5 and S6 are off. At this time, as shown by the arrows, the electromagnetic energy in the interconnection reactors 5 and 5 is returned to the DC side through the AC power systems 7 and 8, the isolation diodes D6, the diode D1, and the diode D2 of the switch S2 (corresponding to mode VI in Fig. 4).

[0037] In this way, the separation circuit 6 separates the smoothing capacitors C1, C2 from the interconnection reactors 5, 5 by reflux, and the output potential has three levels {+Ed, -Ed, 0} by neutral point clamping. Furthermore, the midpoint P1 between the two smoothing capacitors C1, C2 is connected to point O of the AC power source 7, forming a single-phase three-wire system. In other words, the inverter 1 constitutes a three-level neutral point clamped inverter for a single-phase three-wire system.

[0038] FIG. 11 shows a circuit diagram for simulating the three-level neutral-point clamped inverter 1 of the single-phase three-wire system in FIG. 1. In order to reduce the effect of unbalanced loads mainly due to differences in load magnitudes, the load 8 constituting the AC power system is divided into three parts. Between 0.1 and 0.414 seconds, a 5Ω load resistor L1 is connected between the 100V UO terminals. Between 0.247 and 0.747 seconds, a 10Ω load resistor L2 is connected between the 200V UW terminals. Between 0.58 and 0.914 seconds, a 5Ω load resistor L3 is connected between the 100V OW terminals. After that, the simulation is in an unloaded state until the end of the simulation.

[0039] Fig. 12 shows the simulation results of the three-level neutral point clamped inverter of this single-phase three-wire system. Fig. 13 shows the conventional inverter H6.5 without the neutral point clamp switch Sclamp of Fig. 11, and Fig. 14 shows the simulation results.

[0040] Figure 14(a) shows the load current waveform, (b) shows the waveforms of the voltage Vuo between the UO terminals and the voltage Vow between the OW terminals, and (c) shows the root mean square (RMS) values ​​of the same voltages. As can be seen from Figure 14(c), there is an imbalance in the voltage Vuo between the UO terminals and the voltage Vow between the OW terminals that deviates significantly above or below 100V. As mentioned above, this is thought to be mainly due to variations in the characteristics of the elements.

[0041] In contrast, as shown in Figures 12(b) and (c), in the three-level neutral-point clamped inverter of this single-phase three-wire system, point O (neutral point) on the output side is clamped to the midpoint P1 between the smoothing capacitors C1 and C2 by the neutral-point clamp switch Sclamp, so that even if there is an unbalanced load on the AC line or there is variation in the characteristics of the elements, the voltage balance of each phase can be achieved compared to Figures 14(b) and (c). In particular, as can be seen from Figure 12(c), the voltage Vuo between the UO terminals and the voltage Vow between the OW terminals are stable at around 100V.

[0042] In this way, the inverter 1 is a three-level neutral point clamp type having an isolation circuit 6, and is a single-phase three-wire system in which the midpoint P1 between the smoothing capacitors C1, C2 is connected to point O of the AC power supply 7. By isolating the smoothing capacitors C1, C2 from the interconnection reactors 5, 5, it is possible to prevent current return between the smoothing capacitors C1, C2 and the interconnection reactors 5, 5, as well as to suppress the generation of EMI (electromagnetic interference) noise and leakage current. Furthermore, the voltage of each phase can be balanced even with an unbalanced load.

[0043] Figures 15 and 16 show modified examples of the first embodiment. Both have an isolation circuit 6 as shown in Figure 1, and are three-level neutral-point clamped inverters for a single-phase three-wire system. As with the first embodiment, these modified examples also have the effect of isolating the smoothing capacitors C1, C2 from the interconnection reactors 5, 5, suppressing the generation of EMI (electromagnetic interference) noise and leakage current, and providing a stable AC output even with an unbalanced load.

[0044] In Fig. 15(A), unlike Fig. 1, the switches S10 and S11 of the upper arms of the bridge circuit have free wheel diodes D10 and D11 connected in anti-parallel, respectively, and do not have a free wheel diode D7. The other configurations are the same as in Fig. 1.

[0045] In Fig. 15(B), like Fig. 15(A), the switches S10 and S11 of the upper arms of the bridge circuit have free wheel diodes D10 and D11 connected in anti-parallel, respectively, and do not have the free wheel diode D7 of Fig. 1. Furthermore, instead of two separation switches, in Fig. 15(B), a separation switch S12 and a free wheel diode D12 connected in anti-parallel to the separation switch S12 are connected in series with a diode D14. In addition, a separation switch S13 and a free wheel diode D13 connected in anti-parallel to the separation switch S13 are connected in series with a diode D15, and these two series-connected circuits are connected in parallel. A neutral point clamp switch Sclamp is connected to the series connection point of the diode D15 and the separation switch S13.

[0046] In Fig. 16(A), like Fig. 15(A), the switches S10 and S11 of the upper arms of the bridge circuit have free wheel diodes D10 and D11 connected in anti-parallel, respectively, and do not have the free wheel diode D7 (Fig. 1). The separation switch S12 and the free wheel diode D12 connected in anti-parallel to it are connected in series with the diode D14, and the separation switch S13 and the free wheel diode D13 connected in anti-parallel to it are connected in series with the diode D15. These two series circuits are connected in parallel. Furthermore, instead of the neutral point clamp switch Sclamp of Fig. 1, two diodes D17 and D18 are connected to the series connection points of the above two series circuits, respectively.

[0047] In Fig. 16(B), like Fig. 16(A), the switches S10 and S11 of the upper arms of the bridge circuit have free wheel diodes D10 and D11 connected in anti-parallel, respectively, and do not have a free wheel diode D7. Unlike Fig. 15(A), diodes D21 and D25 are connected in series, diodes D22 and D26 are connected in series, and these two series circuits are connected in parallel. A separation switch S23 and a free wheel diode D23 connected in anti-parallel thereto are connected to the series connection point of the two series circuits, and further, two diodes D27 and D28 are connected to the two series connection points, respectively.

[0048] In Fig. 16(C), unlike Fig. 1, the switches S10 and S11 of the upper arms of the bridge circuit have free wheel diodes D10 and D11 connected in anti-parallel, respectively, and do not have the free wheel diode D7. Furthermore, instead of the two separation switches connected in series in Fig. 1, a bidirectional switch S15 is provided, and instead of the neutral point clamp switch, two switches S16 and S17 arranged in parallel are connected to the input and output points of the bidirectional switch S15, respectively.

[0049] The present invention is not limited to inverters for single-phase three-wire systems. Fig. 17 is a circuit diagram showing an inverter in a single-phase two-wire system according to a second embodiment. The first embodiment is a single-phase three-wire system in which the midpoint P1 between the smoothing capacitors C1 and C2 in Fig. 1 is connected to the point O of the AC power source 7, whereas the second embodiment is a single-phase two-wire system in which the midpoint P1 and the point O are not connected, and the operation is almost the same as that of the first embodiment, so a description thereof will be omitted.

[0050] In this case, similarly to the first embodiment, the smoothing capacitors C1, C2 are separated from the interconnection reactors 5, 5 by a separation circuit 6 through reflux, and the midpoint between the two smoothing capacitors C1, C2 is connected to the connection point of the separation switches S5, S6. This provides isolation between the smoothing capacitors C1, C2 and the interconnection reactors 5, 5, and the midpoint between the two smoothing capacitors C1, C2 is connected to the connection points of the upper and lower arms of each leg via the isolation switching elements S5, S6, Sclamp, so that common mode noise is bypassed and the occurrence of EMI (electromagnetic interference) and leakage current can be suppressed.

[0051] As described above, in the present invention, the smoothing capacitor and the interconnection reactor are separated by the isolation circuit through reflux, and the midpoint between the two smoothing capacitors is connected to the connection points of the upper and lower arms of each leg via the isolation switching element, thereby isolating the smoothing capacitor and the interconnection reactor, and the generation of EMI (electromagnetic interference) noise and leakage current can be suppressed by using the unipolar pulse width modulation method.

[0052] In addition, if the midpoint between the two smoothing capacitors is connected to point O of the AC power system to form a three-level neutral point type single-phase three-wire system, the voltage balance of each phase can be achieved even with an unbalanced load.

[0053] In the above-described embodiments, solar cells are used as distributed power sources, but fuel cells or the like may also be used.

[0054] The inverter may also be operated as a converter.

[0055] The present invention is not limited to the above-described embodiment, and various additions, modifications, and deletions are possible without departing from the scope of the present invention. Therefore, such modifications are also included in the scope of the present invention. [Explanation of symbols]

[0056] 1: Inverter 2:DC power supply 3: Bridge circuit 5: Interconnection reactor 6: Separation circuit 7: AC power supply 8: Load 10: Control unit C1, C2: Smoothing capacitors D2, D4: Freewheeling diodes of the bridge circuit D5, D6: Freewheeling diode D7: Freewheeling diode P1: Midpoint of smoothing capacitor P2, P3: Connection points of the upper and lower arms of each leg of the bridge circuit S1 to S4: Switching elements (switches) of the bridge circuit S5, S6: Isolation switching elements (isolation switches) Sclamp: Clamp switching element (clamp switch)

Claims

1. An inverter that converts DC power of a DC power source into AC power so as to be connected to a power system including an AC power source, a bridge circuit having at least four switching elements and consisting of two legs each having an upper and lower arm, for converting DC power to AC power; an interconnection reactor connecting the bridge circuit and the AC power supply; and an isolation circuit including an isolation switching element for isolating the smoothing capacitor and the interconnection reactor during a period in which current is returned from the interconnection reactor; an inverter, the midpoint between the two smoothing capacitors being connected to a connection point between the upper and lower arms of each leg via the isolation switching element;

2. In claim 1, the bridge circuit has a free wheel diode connected in anti-parallel to the switching element of the lower arm of each leg, the separation circuit is configured by connecting two separation switching elements in series in the reverse direction between connection points of the upper arm and the lower arm of each leg, the switching elements each having a free wheel diode connected in reverse parallel to the switching element, A freewheel diode is provided between the connection point of the isolation switching element and the positive connection point of each upper arm, and an inverter in which a midpoint between the two smoothing capacitors and a connection point of the two isolation switching elements are connected via a neutral point clamp switching element having a switching element and a free wheel diode connected in inverse parallel to the switching element.

3. In claim 1, further comprising: The inverter is a single-phase three-wire system in which a midpoint between the two smoothing capacitors is connected to point O of the AC power system.

4. A distributed power supply system comprising the inverter according to claim 1 , wherein the DC power supply is a distributed power supply.

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