Converter and converter system

Abstract

Provided is a converter with which the amount of capacitors can be reduced. A converter 1 is provided with: a DC-side circuit 3 connectable to a DC power supply 5 and including a plurality of DC-side switches Sp, Sn, Vn, Vp; an AC-side circuit 4 connectable to an AC power supply 7 and including a plurality of AC-side switches Up1, Up2, Un1, Un2, Vp1, Vp2, Vn1, Vn2 different from the DC-side switches Sp, Sn, Vn, Vp; and a transformer 2 that magnetically couples the DC-side circuit 3 and the AC-side circuit 4. The converter 1 enables power conversion between DC and AC. In the converter 1, the capacity of capacitors included in the DC-side circuit 3 is greater than the capacitive components of the AC-side circuit 4.

Description

Converter and converter system

[0001] The present invention relates to a converter and a converter system.

[0002] In recent years, as data centers have become larger, efforts to reduce the size of various devices have been underway, including the miniaturization of converters that perform AC / DC conversion. For example, Patent Document 1 discloses a power return device that achieves miniaturization by connecting a DC side circuit and an AC side circuit via a transformer and causing the two circuits to perform predetermined operations to directly perform AC / DC conversion.

[0003] Patent No. 6660579

[0004] Converters that connect two circuits via a transformer, such as those described above, contain parasitic inductance. Therefore, when the current changes when switching the converter, a sudden high voltage (a surge voltage) may occur due to the current change. This may result in a voltage higher than the specified level, which could cause the device to malfunction due to overvoltage. This results in the problem of requiring a large amount of capacitors to stabilize operation.

[0005] The present invention has been made to solve such problems, and has an object to provide a converter and a converter system that reduce the amount of capacitors.

[0006] A converter according to one aspect of the present invention is a converter capable of converting power between DC and AC, comprising: a DC-side circuit connectable to a DC load or a DC power source and having a plurality of DC-side switches; an AC-side circuit connectable to an AC power source and having a plurality of AC-side switches different from the DC-side switches; and a transformer magnetically connecting the DC-side circuit and the AC-side circuit. In this converter, the capacitance of a capacitor included in the DC-side circuit is larger than the capacitance of the AC-side circuit.

[0007] A converter according to one aspect of the present invention includes a DC-side circuit connectable to a DC load or a DC power source and having a plurality of DC-side switches, an AC-side circuit connectable to an AC power source and having a plurality of AC-side switches different from the DC-side switches, a transformer magnetically connecting the DC-side circuit and the AC-side circuit, and a control unit capable of converting power between DC and AC by operating the DC-side switches and the AC-side switches. In this converter, the capacitance of a capacitor in the DC-side circuit is larger than the capacitance of the AC-side circuit.

[0008] A converter system according to one aspect of the present invention includes the above-described plurality of DC side circuits and the above-described plurality of AC side circuits connected to the plurality of DC side circuits, respectively, via the above-described plurality of transformers, and the plurality of AC side circuits are connected in series and configured to be connectable to an AC power source.

[0009] A voltage conversion system for a data center power supply, a solar PCS, a battery PCS, and a railway system according to one aspect of the present invention includes the above converter or the above converter system.

[0010] In the converter and converter system of the present invention, the capacitance of the capacitor in the DC side circuit is larger than the capacitance component of the AC side circuit, so no capacitance component is actively introduced into the AC side circuit. Even if the capacitance component of the AC side circuit is small, the surge voltage can be suppressed by operating the AC side circuit while the DC side circuit is short-circuited. Therefore, the present invention makes it possible to realize a converter and converter system that requires a small amount of capacitor.

[0011] FIG. 1 is a circuit diagram of a converter according to the present embodiment. FIG. 2 is an explanatory diagram of the operation of the converter shown in FIG. 1. FIG. 3 is a diagram illustrating transitions between modes shown in FIG. 2. FIG. 4 is an explanatory diagram of the timing of transition from mode 0B to mode 4. FIG. 5 is an explanatory diagram of the converter in the state of mode 0B. FIG. 6 is an explanatory diagram of the converter in the transition process from mode 0B to 4-1. FIG. 7 is an explanatory diagram of the converter in the transition process from mode 0B to 4-2. FIG. 8 is an explanatory diagram of the converter in the transition process from mode 0B to 4-3. FIG. 9 is an explanatory diagram of the converter in the transition process from mode 0B to 4-4. FIG. 10 is an explanatory diagram of the converter in the state of mode 4. FIG. 11 is an explanatory diagram of the timing of transition from mode 2 to mode 0A. FIG. 12 is an explanatory diagram of the converter in the state of mode 2. FIG. 13 is an explanatory diagram of the converter in the transition process from mode 2 to 0A-1. FIG. 14 is an explanatory diagram of the converter in the transition process from mode 2 to 0A-2. FIG. 15 is an explanatory diagram of the converter in the transition process from mode 2 to 0A-3. FIG. 16 is an explanatory diagram of the converter in the transition process from mode 2 to 0A-4. FIG. 17 is an explanatory diagram of the converter in the state of mode 0A. FIG. 18 is a diagram illustrating changes in the voltage and current of a transformer in a comparative example. FIG. 19 is a schematic diagram illustrating the effect of increasing the gate resistance of each switch. FIG. 20 is a schematic configuration diagram of a converter system according to another embodiment. FIG. 10 is a schematic configuration diagram of a converter system according to still another embodiment.

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0013] 1 is a circuit diagram of a converter 1 capable of AC / DC (Alternating Current / Direct Current) power conversion according to an embodiment of the present invention. In the converter 1, an AC side circuit 4 and a DC side circuit 3 are magnetically connected via a transformer 2. The DC side circuit 3 has four switches, and the AC side circuit 4 has eight switches. The configuration of each switch is shown at the bottom of the figure, and the detailed configuration will be described later.

[0014] A DC power supply 5 and a capacitor 6 are connected in parallel to the DC side circuit 3. The DC power supply 5 is not an essential component. Instead of or in addition to the DC power supply 5, a DC load, a renewable energy device, etc. may be connected. The DC side circuit 3 has a two-leg configuration, and switches constituting upper and lower arms are connected in series to each leg. The connection points of the upper and lower arms of the two legs are connected to the DC side coil of the transformer 2.

[0015] The arm of the AC side circuit 4 functions as a bidirectional switch, and as will be described later, can be set to any of four states depending on the gate conditions: bidirectional voltage conduction (bidirectional conduction), unidirectional voltage blocking (other-directional conduction), other-directional voltage blocking (one-directional conduction), and bidirectional voltage blocking.

[0016] For ease of explanation, the side above the transformer 2 in the figure will be referred to as the high side, and the side below the figure will be referred to as the low side. Furthermore, the leg on the transformer 2 side, which is composed of the upper and lower arms, will be referred to as the S phase, and the leg on the DC power supply 5 side will be referred to as the T phase. The S phase high-side switch will be referred to as Sp, the S phase low-side switch as Sn, the T phase high-side switch as Tp, and the T phase low-side switch as Tn.

[0017] The bottom of the figure shows the detailed configuration of switches Sn, Sp, Tn, and Tp. Each switch is configured by connecting a MOS-FET (Metal-Oxide-Semiconductor Field-Effect Transistor) and a diode in parallel. Note that a MOS-FET is an example of a semiconductor switch, and any type of switch can be used as long as the switching speed changes depending on the resistance of the gate terminal, as described below.

[0018] The diode is arranged with its cathode (C) at the top of the diagram and its anode (A) at the bottom, with the forward direction running from bottom to top. The MOS-FET in the illustrated example is an N-type, with the source (S) of the MOS-FET connected to the anode of the diode and the drain (D) connected to the cathode of the diode. The MOS-FET conducts when a high-level signal is input to its gate (G). However, since FETs, including MOS-FETs, also function as diodes, the diode can be omitted. The transistor configuration is similar even if a GaN-HEMT is used instead of a MOS-FET.

[0019] The switches Sp, Sn, Tp, and Tn are configured in this manner so that when in a conductive state, they allow current to flow in both directions, and when in a non-conductive state, they are conductive from the low side to the high side, but non-conductive from the high side to the low side.

[0020] The AC side circuit 4 has a two-leg configuration similar to the DC side circuit 3, and is connected to the AC side coil of the transformer 2. The connection points of the upper and lower arms of the two legs are connected to an AC power source 7. The AC power source 7 is connected to the AC side circuit 4 via an interconnection reactor 8 that can generate current intermittently. The AC side circuit 4 outputs a square wave voltage (V1) to the AC power source 7 side. The current flowing through the interconnection reactor 8 corresponds to the integral value of the product of the voltage applied to the interconnection reactor 8 and the current of the interconnection reactor 8.

[0021] For the sake of explanation, the leg on the AC power supply 7 side will be referred to as the U-phase, and the leg on the transformer 2 side will be referred to as the V-phase. Furthermore, in each of the U-phase and V-phase legs, the upper and lower arms each have two switches connected in series. Therefore, if the switches in the upper and lower arms of each phase are shown in order from top to bottom in the figure, the U-phase high-side switches will be referred to as Up1 and Up2, the U-phase low-side switches as Un1 and Un2, the V-phase high-side switches as Vp1 and Vp2, and the V-phase low-side switches as Vn1 and Vn2.

[0022] In the illustrated example, in the upper arm of the U-phase, switch Up1 is arranged in the forward direction from the high side to the low side, and switch Up2 is arranged in the forward direction from the low side to the high side. In each arm of the AC side circuit 4 configured in this manner, bidirectional current conduction, forward conduction from the high side to the low side, forward conduction from the low side to the high side, and a bidirectional blocking state can be realized between the low side and the high side. These controls can be performed by changing the gate voltage applied to the gate. That is, depending on the gate signal, each arm is in one of the following states: bidirectional voltage conduction (bidirectional conduction), unidirectional voltage blocking (bidirectional conduction), bidirectional voltage blocking (unidirectional conduction), and bidirectional voltage blocking. Note that the switches in each arm are arranged so that the forward directions of the diodes face each other, but this is not limited to this. The switches in each arm may also be arranged so that the reverse directions of the diodes face each other.

[0023] The control unit 10 controls the conduction / non-conduction of the switches Sp, Sn, Tp, and Tn (also referred to as DC side switches) in the DC side circuit 3 and the switches Up1, Up2, Un1, Un2, Vp1, Vp2, Vn1, and Vn2 (also referred to as AC side switches) in the AC side circuit 4 by changing the gate voltage of the MOS-FET.

[0024] An example of a method for controlling the converter 1 by the control unit 10 is shown in Fig. 2. In the explanation of Fig. 2, the voltage Vtx1 at the DC side circuit 3 side terminal of the transformer 2, the voltage Vtx2 at the AC side circuit 4 side terminal of the transformer 2, the voltage V1 at the AC power source 7 side terminal of the AC side circuit 4, and a voltage command value for the voltage V1, all of which are shown in Fig. 1, are used. The voltage command value is a sine wave indicated by a bold line in Fig. 2(a), and will hereinafter be referred to as a virtual command value.

[0025] 2 is an explanatory diagram of the operation of the converter 1 of this embodiment. The operation of the converter 1 shown in this figure is performed by the control unit 10, and power conversion between AC and DC is possible by the operation of the control unit 10. An overview of the control of power conversion performed by the control unit 10 is as follows.

[0026] 1 , the DC side circuit 3 operates in accordance with a desired AC waveform, thereby applying a voltage V1 having a desired AC waveform to the AC side circuit 4 via the transformer 2. Then, the AC side circuit 4 can pass an AC current in accordance with the phase difference and amplitude difference between the AC power generated by the DC side circuit 3 and the AC power generated by the AC power supply 7. Below, the overall operation of each leg of the converter 1 in accordance with a virtual command value for each of any virtual legs will be described with reference to FIG.

[0027] 2, (a) shows the virtual command value (sine wave) and carrier wave (triangular wave) for each virtual leg indicating the desired AC waveform, and (b) shows the voltage V1 at the terminal on the AC power source 7 side of the AC side circuit 4. As shown by the voltage V1, a pulse voltage waveform is output whose width is adjusted so as to have an AC fundamental wave component that is approximately the same as the virtual command value. Also, a current corresponding to the integral value of the product of the voltage applied to the interconnection reactor 8 and the inductance of the interconnection reactor 8 flows through the interconnection reactor 8.

[0028] (c) shows the voltage Vtx2 on the AC side circuit 4 side of the transformer 2, and (d) shows the voltage Vtx1 on the DC side circuit 3 side of the transformer 2. The AC side circuit 4 operates so as to ultimately generate the voltage V1 shown in (b) using the voltages shown in (c) and (d). (e) shows modes 0 to 4 as operating modes of the DC side circuit 3, and the operation of each mode will be described later using FIG. 3 and other figures. (a) to (e) will be described in detail below.

[0029] 1A shows a method for determining the control timing of the switches of converter 1 when the converter operates with a desired sine wave as a command value. The example in this figure is the same as that showing the voltage command values ​​and carrier waves of each leg in a full-bridge converter. Note that in the following control, a voltage command value (dotted line) obtained by inverting the positive and negative signs of a virtual voltage command value (solid line) is used.

[0030] The voltage V1 output from the AC side circuit 4 to the AC power supply 7 shown in (b) changes at the timing determined in (a). The voltage V1 is positive in the first half of the carrier wave and negative in the second half. Therefore, the voltage V1 is determined by comparing the carrier wave with the voltage command value indicated by the solid line and the voltage command value indicated by the dotted line.

[0031] Specifically, when the relationship is solid line voltage command > carrier wave > dotted line voltage command, voltage V1 becomes a positive voltage. When the relationship is carrier wave > solid line voltage command > dotted line voltage command, or when the relationship is solid line voltage command > dotted line voltage command > carrier wave, V1 becomes zero voltage. When the relationship is dotted line voltage command > carrier wave > solid line voltage command, V1 becomes a negative voltage. With this operation, an output with a width according to the magnitude of the sine wave, which is the command value, is generated on the AC power supply 7 side of the AC side circuit 4, and is reproduced as a square wave voltage V1 whose width changes according to the sine wave command value.

[0032] In this operation, first, the DC side circuit 3 is controlled, and thereby a voltage Vtx1 is generated at the terminal of the transformer 2 on the DC side circuit 3 side, and a voltage Vtx2 is generated at the terminal on the AC side circuit 4 side via the transformer 2. The voltages Vtx1 and Vtx2 generated in the transformer 2 alternate between positive and negative voltages, with zero voltage interposed between them.

[0033] When the DC side circuit 3 is controlled, the voltage stored in the capacitor 6 on the DC side circuit 3 side is applied to the terminal of the transformer 2 on the DC side circuit 3 side, exciting the transformer 2. Therefore, by alternately applying a positive voltage and a negative voltage to the transformer 2, it is possible to prevent biased magnetization of the transformer 2. In order to prevent biased magnetization, it is more preferable that the pulse widths of a pair of positive and negative voltages are equal, so the pulse widths of adjacent pulse waveforms are controlled to be approximately equal.

[0034] Additionally, in the section where the voltage V1 shown in (b) is zero, the AC side circuit 4 is operated with the terminals on the DC side circuit 3 side of the transformer 2 shorted. To short the terminals on the DC side circuit 3 side of the transformer 2, there are two methods: one is to turn on the upper arms of both legs of the DC side circuit 3 and turn off the lower arms, and the other is to turn on the lower arms of both legs and turn off the upper arms.

[0035] The voltage Vtx1 generated at the terminal on the AC side circuit 4 side of the transformer 2 is the voltage Vtx2 at the DC side circuit 3 side of the transformer 2 multiplied by the turns ratio. In order to generate the voltage V1 at the terminal on the AC power supply 7 side of the AC side circuit 4 as shown in (b), the voltage Vtx2 at the terminal on the AC side circuit 4 side of the transformer 2 as shown in (c) must be inverted within the AC side circuit 4 as needed. Therefore, the AC side circuit 4 generates the voltage V1 as shown in (b) by inverting the polarity of the voltage Vtx2 at the terminal on the transformer 2 side as shown in (c).

[0036] That is, the polarities of the voltages Vtx1 and Vtx2 of the transformer 2 are determined according to the state of the DC side circuit 3. Then, the polarities of the voltages Vtx1 and Vtx2 are either left as is or inverted according to the state of the AC side circuit 4, and output as voltage V1 to the terminal on the AC power supply 7 side of the AC side circuit 4. Therefore, the operating mode of the AC side circuit 4 is determined according to the voltage Vtx2 on the transformer 2 side of the DC side circuit 3 so that voltage V1 can be obtained.

[0037] For example, when voltage Vtx2 in (c) is a positive voltage and voltage V1 in (b) is a positive voltage, the operating mode of AC side circuit 4 in (e) becomes mode 1, Vtx positive mode (hatched with a mixture of thin and thick lines sloping downward to the right). In mode 1, the S-phase upper arm (switch Sp) and T-phase lower arm (switch Tn) of DC side circuit 3 are turned on, and the U-phase upper arm (switches Up1 and Up2) and V-phase lower arm (switches Vn1 and Vn2) of AC side circuit 4 are turned on. The bidirectional conduction states of the other arms of DC side circuit 3 and AC side circuit 4 are turned off.

[0038] When voltage Vtx2 in (c) is negative and voltage V1 in (b) is positive, the operating mode of AC side circuit 4 in (e) becomes mode 2 Vtx negative mode (hatched with a mixture of thin and thick lines sloping upward to the right). In mode 2, the S-phase lower arm (switch Sn) and T-phase upper arm (switch Tn) of DC side circuit 3 are turned on, and the U-phase lower arm (switches Un1 and Un2) and V-phase upper arm (switches Vp1 and Vp2) of AC side circuit 4 are turned on. The bidirectional conduction states of the other arms of DC side circuit 3 and AC side circuit 4 are turned off.

[0039] When the voltage Vtx2 in (c) is a positive voltage and the voltage V1 in (b) is a negative voltage, the operating mode of the AC side circuit 4 in (e) becomes the Vtx positive mode of mode 3 (thick hatching sloping downward to the right). In mode 3, the S-phase upper arm (switch Sp) and the T-phase lower arm (switch Tn) of the DC side circuit 3 are turned on, and the U-phase lower arm (switches Un1 and Un2) and the V-phase upper arm (switches Vp1 and Vp2) of the AC side circuit 4 are turned on. The other arms of the DC side circuit 3 and the AC side circuit 4 are turned off.

[0040] When the voltage Vtx2 in (c) is a negative voltage and the voltage V1 in (b) is a negative voltage, the operating mode of the AC side circuit 4 in (e) becomes the Vtx negative mode of mode 4 (shown by thick hatching sloping upward to the right). In mode 4, the S-phase lower arm (switch Sn) and the T-phase upper arm (switch Tn) of the DC side circuit 3 are turned on, and the U-phase upper arm (switches Up1 and Up2) and the V-phase lower arm (switches Vn1 and Vn2) of the AC side circuit 4 are turned on.

[0041] Thus, when the voltage V1 in (b) is a positive voltage, i.e., when the command value is positive, mode 1 or 2 is selected. When the voltage V1 is a negative voltage, i.e., when the command value is negative, mode 3 or 4 is selected. When the polarity of the voltage Vtx2 in (c) and the voltage V1 in (b) is the same, mode 1 or 3, which is a positive polarity mode, is selected, and when the polarity is negative (opposite polarity), mode 2 or 4 is selected.

[0042] Furthermore, in this embodiment, the DC side circuit 3 is controlled so that the terminals of the transformer 2 on the DC side circuit 3 side are short-circuited in the section (b) where the voltage V1 output from the AC side circuit 4 is zero voltage. That is, the AC side circuit 4 is operated in a state where the terminals of the transformer 2 on the DC side circuit 3 side are short-circuited. This state of the transformer 2 in the section (b) where the voltage V1 is zero voltage is shown as Mode 0. By operating the AC side circuit 4 when the terminals of the transformer 2 on the DC side circuit 3 side are short-circuited, each element of the AC side circuit 4 is switched in a zero DC state, thereby suppressing voltage spikes.

[0043] 2, the changes in modes 0 to 4 of converter 1 were described as switching AC side circuit 4 and DC side circuit 3 simultaneously, but in reality, the state of converter 1 changes through a transition state as shown in Fig. 3. Furthermore, even in mode 0, in which the terminals on the DC side circuit 3 side of transformer 2 are short-circuited, there are mode 0A when the command value is positive and mode 0B when the command value is negative.

[0044] 3 is a diagram showing state transitions of the converter 1. The upper part of this diagram shows the case where the command value, voltage V1, is positive (the first half of FIG. 2), and the lower part shows the case where voltage V1 is negative (the second half of FIG. 2).

[0045] When the command value is positive, there is a mode OA in which the command value is zero voltage between mode 1 in which the transformer 2 has positive polarity and mode 2 in which the transformer 2 has negative polarity.

[0046] When the command value is negative, there is a mode 0B in which the command value is zero voltage between mode 3 in which the transformer 2 has positive polarity and mode 4 in which the transformer 2 has negative polarity.

[0047] Mode 0A and Mode 0B have different configurations. Specifically, in Mode 0A, the upper arms of the switches for all phases (S, T, U, and P phases) in both the DC side circuit 3 and the AC side circuit 4 are turned on and the lower arms are turned off. In contrast, in Mode 0B, the lower arms of the switches for all phases (S, T, U, and P phases) in both the DC side circuit 3 and the AC side circuit 4 are turned on and the upper arms are turned off.

[0048] Transitions between modes are made via transition modes. In this embodiment, there are limitations on mode transitions, and transitions possible are from mode 0A to mode 1, from mode 2 to mode 0A, from mode 3 to mode 0B, from mode 0B to mode 4, from mode 0A to mode 3, from mode 1 to mode 0B, from mode 0B to mode 2, and from mode 4 to mode 0B. Furthermore, transition modes exist between these mode transitions; for example, transition from mode 0A to mode 1 is made via transition mode 0A to 1.

[0049] In each transition mode, first, in order to reduce the surge peak voltage when the AC side circuit 4 is turned off, the DC side circuit 3 is operated to short-circuit the terminals of the transformer 2 on the DC side circuit 3 side, and then switching of the AC side circuit 4 is performed. Furthermore, in the transition mode, the switching state is transitioned so that the current originating from the current source of the interconnection reactor 8 is not interrupted and a PN short circuit does not occur.

[0050] 4 to 7F, switches in the ON state are surrounded by dotted lines, and switches in the OFF state are not surrounded by dotted lines.

[0051] FIG. 4 and FIGS. 5A to 5F are diagrams showing detailed operations of the converter 1 when transitioning from mode 0B to mode 4. FIG. 4 shows the timing of transition from mode 0B to mode 4. When a transition to mode 4 is instructed in mode 0, the converter transitions to mode 4 via transition states (modes 0B to 4-1 to 4-4). The conduction states of the switches of the converter 1 in these modes are shown in FIGS. 5A to 5F. As an example, modes 0B to 4-1 to 3 are each controlled every 0.2 microseconds.

[0052] 5A, the upper arms are turned off and the lower arms are turned on in both the DC side circuit 3 and the AC side circuit 4. That is, the switches Sn and Tn in the DC side circuit 3 and the switches Un1, Un2, Vn1, and Vn2 in the AC side circuit 4 are turned on, and the other switches are turned off. In this state, the terminal of the transformer 2 on the DC side circuit 3 side is short-circuited.

[0053] Then, when the state becomes Mode 0Bto4-1 as shown in Fig. 5B, the switch Up1 of the AC side circuit 4 is turned on. In the state of Mode 0Bto4-2 as shown in Fig. 5C, the switch Un2 of the AC side circuit 4 is turned off. In the state of Mode 0Bto4-3 as shown in Fig. 5D, the switches Up2 and Vp1 of the AC side circuit 4 are turned on.

[0054] In this way, the switch on the AC side circuit 4 side is operated while the terminals on the DC side circuit 3 side of the transformer 2 are short-circuited. This operation can suppress PN short-circuiting of the terminals on the AC side circuit 4 side of the transformer 2, and further suppress the peak voltage when the arms of the AC side circuit 4 are turned off, as will be described later. Furthermore, the switching of the AC side circuit 4 is performed so as to maintain a state in which two or more current paths are secured in the AC side circuit 4. This makes it possible to prevent intermittent current from flowing from the interconnection reactor 8 as a current source.

[0055] Thereafter, in the mode 0B to 4-4 state as shown in Fig. 5E, the switch Tn of the DC side circuit 3 is turned off. Then, in the mode 4 state as shown in Fig. 5F, the switch Tp is turned on. In this way, the short circuit of the terminals on the DC side circuit 3 side of the transformer 2 is released, and the transition to mode 4 is completed.

[0056] Next, Figures 6 and 7A to 7F show detailed operations of converter 1 when transitioning from mode 2 to mode 0A. Figure 6 shows the timing of transition from mode 0A to mode 2. When a transition from mode 2 to mode 0A is instructed, the converter transitions to mode 0A via transition states (mode 2 to 0A-1 to 4). The conduction states of the switches of converter 1 in these modes are shown in Figures 7A to 7F.

[0057] In detail, first, in mode 2 of FIG. 7A, the switches Sn and Tp of the DC side circuit 3 and the switches Up1, Un1, Un2, Vp1, Vp2, and Vn1 of the AC side circuit 4 are turned on, and the other switches are turned off.

[0058] When the mode 2to0A-1 state is entered as shown in Fig. 7B, the switch Sn of the DC side circuit 3 is turned off. When the mode 2to0A-2 state is entered as shown in Fig. 7C, the switch Sp of the DC side circuit 3 is turned on. This shorts the terminals of the transformer 2 on the DC side circuit 3 side.

[0059] Thereafter, in the mode 2to0A-3 state as shown in Fig. 7D, the switch Un2 of the AC side circuit 4 is turned off. Then, in the mode 2to0A-4 state as shown in Fig. 7E, the switch Up2 of the AC side circuit 4 is turned on. Finally, in the mode 0A state as shown in Fig. 7F, the switches Un1 and Vn1 are turned off, thereby transitioning to mode 0A.

[0060] In this way, the switch on the AC side circuit 4 side is operated while the terminals on the DC side circuit 3 side of the transformer 2 are short-circuited. As a result, PN short-circuiting of the terminals on the AC side circuit 4 side of the transformer 2 can be suppressed, and further, as will be described later, peak voltages can be suppressed when the arms of the AC side circuit 4 are turned off. Furthermore, the switching of the AC side circuit 4 is performed so as to maintain a state in which two or more current paths are secured in the AC side circuit 4. This makes it possible to prevent intermittent current from flowing from the interconnection reactor 8 as a current source.

[0061] 8 is a diagram showing the changes in voltage and current on the AC side circuit 4 of the transformer 2 in the comparative example. In this diagram, the voltage (V) is shown by a thick line, and the current (i) is shown by a thin line. Note that this diagram is intended to explain the effects of the present invention, and the positive and negative voltages and specific switch operations are merely examples.

[0062] A voltage equal to the turns ratio multiplied by the DC capacitor voltage is applied to the arm in the OFF state of the AC side circuit 4 via the transformer 2 in response to the operation of the DC side circuit 3. In the illustrated example, a voltage equal to the turns ratio multiplied by the DC capacitor voltage is applied to the arm in the OFF state of the AC side circuit 4 in section A. No voltage is generated in section B, and a voltage equal to the turns ratio multiplied by the DC capacitor voltage is applied to the arm in the OFF state in section C. Then, in the AC side circuit 4, an operation to turn on the switch is performed in section B where no voltage is generated.

[0063] In this case, at the timing of the transition from section B to section C, the application of voltage resumes and the current changes from a predetermined value to zero. Here, a jump voltage (L·di / dt) occurs on the AC side circuit 4 of the transformer 2 in response to the change in current. Therefore, in section C, the jump voltage (L·di / dt) caused by the switch operation is added to the applied voltage, which may result in a voltage exceeding the element's breakdown voltage being applied to the arm.

[0064] In contrast to this, in the present invention, as shown in the center of the bottom of the figure, the terminals on the DC side circuit 3 side of the transformer 2 are short-circuited to create a "zero voltage," thereby limiting the voltage value generated in section C to just a jump voltage. As a result, the maximum value of the voltage generated on the AC side circuit 4 side of the transformer 2 can be suppressed.

[0065] In this embodiment, the capacitance of the capacitor 6 in the DC side circuit 3 is larger than the capacitance component consisting of the parasitic capacitance of the AC side circuit 4. Specifically, the capacitance of the capacitor in the DC side circuit 3 is configured to be 100 times or more, preferably 1000 times or more, larger than the capacitance of the capacitor in the AC side circuit 4. Generally, a small capacitance can cause instability in circuit operation during switching. However, even in a configuration in which the capacitance component of the AC side circuit 4 is extremely small, a voltage pulse having a sine wave fundamental wave can be output to the terminal voltage (V1) on the AC side circuit 4 side by performing the switch operation shown in FIG. 2. Note that although this embodiment has been described using an example of a circuit in which the AC side circuit 4 does not include a capacitor, the AC side circuit 4 may also include a capacitor. If the AC side circuit 4 includes a capacitor, the capacitance component of the AC side circuit 4 includes the capacitor included in the AC side circuit 4 and the parasitic capacitance of the AC side circuit 4. The capacitance of the capacitor 6 in the DC side circuit 3 is larger than the capacitance component of the AC side circuit 4.

[0066] As mentioned above, the jump voltage (ΔV) can be expressed as follows: ΔV=L·di / dt Therefore, the jump voltage can also be suppressed by extending the interruption time.

[0067] In the example of this embodiment, the rate of change of the current is further slowed down by "slowing down the switching speed", and thus the jump voltage (L·di / dt) caused by the switching operation is reduced.

[0068] Such speedup can be achieved by connecting a clamp element such as a Zener diode between the drain and source of the MOS-FET to suppress di / dt and reduce the jump-up voltage.As another example, di / dt can be suppressed and the jump-up voltage can be reduced by resistively dividing the voltage between the drain and source of the MOS-FET and electrically connecting the voltage division point and the gate via a buffer circuit or an amplifier circuit.

[0069] This reduction in the switching speed is achieved by "increasing the resistance" of the gate resistance of the MOS-FET of each switch in the AC side circuit 4. Increasing the resistance of the gate resistance will be explained with reference to FIG.

[0070] 9 is a schematic diagram showing the effect of increasing the gate resistance of each switch in the AC side circuit 4. This diagram shows the results of a simulation of the change in switch operation depending on the gate resistance of the MOS-FET when the inductance is 100 nH. The gate resistance is smallest in (a) shown at the top, followed by (b) shown in the middle and (c) shown at the bottom.

[0071] In the illustrated example, in (a), a voltage of 400 V is applied and the current changes from 22 A to 0 A in 5.5 ns. In (b), a voltage of 200 V is applied and the current changes from 22 A to 0 A in 11 ns. In (c), a voltage of 100 V is applied and the current changes from 22 A to 0 A in 22 ns.

[0072] Here, the voltage change (L di / dt) caused by the switching operation is determined according to the rate of change of the current, so if the current value changes from 22 A to 0 A in all three examples, the rate of change (di / dt) will be smaller if the switching operation is performed over a longer period of time, and the jump-up voltage can be reduced further. Therefore, increasing the resistance of the gate resistor as in (c) reduces the jump-up voltage.

[0073] On the other hand, the loss occurring in the inductance is the integral value of "VI", and since V is "L di / dt", it is "1 / 2 L i ２", which shows that the total loss is not affected by the rate of change of the current. In other words, since it is the same in cases (a) to (c), increasing the resistance does not increase the loss. In this way, even if the gate resistance is increased, the jump voltage can be reduced without increasing the loss.

[0074] According to the converter 1 of this embodiment, as shown in FIGS. 5A to 5F and 7A to 7F, the switch of the AC side circuit 4 is operated while the DC side terminal of the transformer 2 connected to the DC side circuit 3 is short-circuited.

[0075] When the switch of the AC side circuit 4 is operated, a voltage jump occurs due to a change in current. However, because the switch of the DC side circuit 3 is operated and the DC side of the transformer 2 is short-circuited, the overall voltage can be reduced. As a result, the risk of unstable operation of the converter 1 due to the generation of a voltage higher than the specified level can be reduced.

[0076] By operating the AC side circuit 4 with the DC side of the transformer 2 short-circuited as in this embodiment, the elements of the AC side circuit 4 are switched at zero DC voltage, and the peak voltage during switching can be kept low.

[0077] The converter 1 of this embodiment is configured to have a relatively slow switching speed of the switches in the AC side circuit 4. This is achieved, for example, by increasing the gate resistance of the MOS-FET. Slowing the switching speed reduces the amount of change in current per unit time, making it possible to reduce the surge voltage, which is determined according to the time differential value of the current.

[0078] 10 is a schematic diagram of a converter system according to another embodiment. According to this diagram, a converter system 100 includes three converters 1A, 1B, and 1C. A DC-side circuit 3A of converter 1A, a DC-side circuit 3B of converter 1B, and a DC-side circuit 3C of converter 1C are provided independently of each other and are connected to AC-side circuits 4A, 4B, and 4C via transformers 2A, 2B, and 2C, respectively.

[0079] The AC side circuit 4A of converter 1A, the AC side circuit 4B of converter 1B, and the AC side circuit 4C of converter 1C are connected in series at their connection points to an AC power supply 7 (not shown). Specifically, terminal 12A connected to the upper and lower arms of the V phase of AC side circuit 4A is connected to terminal 11B connected to the upper and lower arms of the U phase of AC side circuit 4B, and terminal 12B connected to the upper and lower arms of the V phase of AC side circuit 4B is connected to terminal 11C connected to the upper and lower arms of the U phase of AC side circuit 4C. The AC power supply (not shown) is connected between terminal 11A connected to the upper and lower arms of the U phase of AC side circuit 4A and terminal 12C connected to the upper and lower arms of the V phase of AC side circuit 4C.

[0080] With this configuration, the sum of the power converted by the multiple converters 1A, 1B, and 1C can be output between the terminals 11A and 12C. Since the power output in this manner is the sum of the output voltages of the three converters 1A, 1B, and 1C, the output voltage of the converter system 100 can be increased. In other words, the converter system 100 can be connected to a high-voltage power system or power source.

[0081] Furthermore, since the switching timing of each series converter can be freely set regardless of the current value, harmonics in the AC output voltage of the series body can be suppressed by appropriately changing the switching timing of each series converter. Specifically, harmonics can be suppressed by shifting the phase of the carrier wave of converters 1A to 1C.

[0082] Fig. 11 is a schematic diagram of a converter system according to another embodiment of the present invention. In a converter system 101 shown in Fig. 11, all outputs from the DC side circuit 3 shown in Fig. 10 are connected together.

[0083] In recent years, a converter called an SST (Solid State Transformer) has been attracting attention. Since an SST has a DC capacitor in the AC side bridge, DC capacitor voltage balance control is required. However, the converter system 101 shown in Fig. 11 does not have a DC capacitor, and therefore has the advantage of not requiring DC capacitor balance control of the series-connected AC side bridge.

[0084] Furthermore, when an SST is made three-phase, balance control of the average voltage or total voltage of the AC-side DC voltage for each phase is required. However, even when the converter system 101 (converter) in FIG. 11 is made three-phase, there is an advantage that phase-to-phase balance control is not required because there is no DC voltage in the AC-side bridge.

[0085] The converter 1 and converter system 100 shown in this embodiment are expected to be applied to reducing the size and weight of converters / inverters for railway vehicles that can operate as power sources for data centers, solar power PCSs (power conditioning systems), battery PCSs, and voltage conversion systems for railway systems. In particular, when connected in series as shown in Figures 10 and 11, high voltages can be achieved, making commercial transformers unnecessary and suitable for weight reduction. In addition, various balance controls are not required, so various sensors such as voltage sensors for DC capacitors are not required, and the control device can also be simplified, making it suitable for the above-mentioned applications. Furthermore, if the iron cores of the transformers 2A to 2C in Figure 11 are all made common, some high-order harmonics will be generated, but individual bias magnetization suppression control will also be unnecessary.

[0086] 10 and 11, the three AC-side circuits 4A to 4C and the three DC-side circuits 3A to 3C are connected by three transformers 2A to 2C, respectively, but at least some of the transformers 2A to 2C may be common. That is, the connection between one of the AC-side circuits 4A to 4C and two or more of the DC-side circuits 3A to 3C, the connection between two or more of the AC-side circuits 4A to 4C and one of the DC-side circuits 3A to 3C, or the connection between two or more of the AC-side circuits 4A to 4C and two or more of the DC-side circuits 3A to 3C may be made via any one of the transformers 2A to 2C.

[0087] The present invention allows various embodiments and modifications without departing from the broad spirit and scope of the present invention. Furthermore, the above-described embodiments are intended to explain the present invention and do not limit the scope of the present invention. That is, the scope of the present invention is defined by the claims, not the embodiments. Various modifications made within the scope of the claims and the meaning of the invention equivalent thereto are considered to be within the scope of the present invention.

[0088] The following additional notes are provided regarding the above-described embodiments.

[0089] (Supplementary Item 1) A converter capable of power conversion between DC and AC, comprising: a DC side circuit connectable to a DC load or a DC power supply and having a plurality of DC side switches; an AC side circuit connectable to an AC power supply and having a plurality of AC side switches different from the DC side switches; and a transformer that magnetically connects the DC side circuit and the AC side circuit, wherein the capacitance of a capacitor included in the DC side circuit is larger than the capacitance component of the AC side circuit.

[0090] (Supplementary Item 2) The converter according to (Supplementary Item 1), wherein the capacitance component of the AC side circuit includes a parasitic capacitance of the AC side circuit, and when the AC side circuit includes a capacitor, the capacitance component of the AC side circuit further includes a capacitor included in the AC side circuit.

[0091] (Supplementary Item 3) The converter according to (Supplementary Item 1) or (Supplementary Item 2), wherein the DC side switch of the DC side circuit is operated so that the DC side of the transformer connected to the DC side circuit is short-circuited, and the AC side switch of the AC side circuit is operable while the DC side of the transformer is short-circuited.

[0092] (Supplementary Item 4) The converter according to any one of (Supplementary Item 1) to (Supplementary Item 3), wherein the AC side circuit is connected to the AC power supply via a clamp element.

[0093] (Supplementary Item 5) The converter according to any one of (Supplementary Item 1) to (Supplementary Item 4), further including a control unit capable of converting power between direct current and alternating current by operating the DC side switch and the AC side switch.

[0094] (Supplementary Item 6) The converter according to any one of (Supplementary Item 1) to (Supplementary Item 5), wherein the control unit is configured to operate the DC side switch of the DC side circuit so that a DC side of the transformer connected to the DC side circuit is short-circuited, and to operate the AC side switch of the AC side circuit while the DC side of the transformer is short-circuited.

[0095] (Supplementary Item 7) A converter system having a plurality of converters according to any one of (Supplementary Item 1) to (Supplementary Item 6), comprising a plurality of the DC side circuits and a plurality of the AC side circuits connected to the plurality of DC side circuits, respectively, via a plurality of the transformers, and configured to be connectable to the AC power supply while the plurality of AC side circuits are connected in series.

[0096] (Supplementary Item 8) The converter system according to (Supplementary Item 7), wherein at least one of the connections between one of the AC side circuits and two or more of the DC side circuits, the connections between two or more of the AC side circuits and one of the DC side circuits, and the connections between two or more of the AC side circuits and two or more of the DC side circuits is performed using one of the transformers.

[0097] (Supplementary Item 9) The converter system according to (Supplementary Item 7) or (Supplementary Item 8), wherein the plurality of DC side circuits are connected in parallel.

[0098] (Supplementary Item 10) A converter system according to any one of (Supplementary Item 7) to (Supplementary Item 9), wherein the DC side switches of the plurality of DC side circuits are operated so that DC sides of the plurality of transformers connected to the plurality of DC side circuits are short-circuited, and the AC side switches of the plurality of AC side circuits are operated while the DC sides of the transformers are short-circuited.

[0099] (Appendix 11) A voltage conversion system for a data center power supply, a solar PCS, a battery PCS, or a railway system, comprising a converter described in any one of (Appendix 1) to (Appendix 6) or a converter system described in any one of (Appendix 7) to (Appendix 10).

[0100] REFERENCE SIGNS LIST 1, 1A, 1B, 1C Converter 2, 2A, 2B, 2C Transformer 3, 3A, 3B, 3C DC side circuit 4, 4A, 4B, 4C AC side circuit 5 DC power supply 6 Capacitor 7 AC power supply 8 Interconnection reactor 10 Control unit 100, 101 Converter system Sn, Sp, Tn, Tp, Un1, Un2, Up1, Up2, Vn1, Vn2, Vp1, Vp2 Switch

Claims

1. A converter capable of power conversion between DC and AC, comprising: a DC side circuit connectable to a DC load or a DC power source and having a plurality of DC side switches; an AC side circuit connectable to an AC power source and having a plurality of AC side switches different from the DC side switches; and a transformer that magnetically connects the DC side circuit and the AC side circuit, wherein the capacitance of a capacitor included in the DC side circuit is larger than the capacitance component of the AC side circuit.

2. A converter as claimed in claim 1, wherein the capacitance component of said AC side circuit includes a parasitic capacitance of said AC side circuit, and when said AC side circuit includes a capacitor, the capacitance component of said AC side circuit further includes a capacitor included in said AC side circuit.

3. A converter as claimed in claim 1, configured to operate the DC side switch of the DC side circuit so that the DC side of the transformer connected to the DC side circuit is short-circuited, and to operate the AC side switch of the AC side circuit while the DC side of the transformer is short-circuited.

4. A converter as claimed in claim 3, wherein the DC side circuit and the AC side circuit comprise upper and lower arms, and when the polarity of the DC side circuit and the AC side circuit is changed, the upper arms of the DC side circuit and the AC side circuit are short-circuited, or the lower arms of the DC side circuit and the AC side circuit are short-circuited, depending on the states of the upper and lower arms of the DC side circuit after the change in polarity.

5. The converter according to claim 3 or 4, wherein the AC side circuit is connected to the AC power supply via a clamp element.

6. The converter according to claim 1, further comprising a control unit capable of converting power between DC and AC by operating the DC side switch and the AC side switch.

7. A converter as described in claim 6, wherein the control unit is configured to operate the DC side switch of the DC side circuit so that the DC side of the transformer connected to the DC side circuit is short-circuited, and to operate the AC side switch of the AC side circuit while the DC side of the transformer is short-circuited.

8. A converter system having a plurality of the converters according to claim 1, comprising a plurality of the DC side circuits and a plurality of the AC side circuits connected to each of the plurality of DC side circuits via a plurality of the transformers, and the plurality of AC side circuits are connected in series and are configured to be connectable to the AC power source.

9. The converter system according to claim 8, wherein at least one of the connections between one of the AC side circuits and two or more of the DC side circuits, the connections between two or more of the AC side circuits and one of the DC side circuits, and the connections between two or more of the AC side circuits and two or more of the DC side circuits is performed using one of the transformers.

10. The converter system according to claim 8, wherein the plurality of DC side circuits are connected in parallel.

11. A converter system according to claim 8, wherein the DC side switches of the plurality of DC side circuits are operated so that the DC sides of the plurality of transformers connected to the plurality of DC side circuits are short-circuited, and the AC side switches of the plurality of AC side circuits are operated while the DC sides of the transformers are short-circuited.

12. A voltage conversion system for a data center power supply, a solar PCS, a battery PCS, or a railway system, comprising the converter according to claim 1 or the converter system according to claim 8.

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