JP6888456B2 - Power converter - Google Patents

Description

The present invention relates to a neutral point clamped (NPC: Neutral Point Clamped) type power conversion device, and a technique for alleviating a surge voltage in the power conversion device is disclosed herein.

The NPC type power conversion device is a three-level multi-level inverter, and has been used in recent years as a compact and highly efficient power conversion device. Such an NPC type power conversion device is described in Patent Document 1. The main circuit of the power conversion device is shown in FIG. Two capacitors C1 and C2 are placed between the high potential side positive potential terminal P to which a positive potential is applied from the DC power supply E and the low potential side negative potential terminal N to which a negative potential is applied from the DC power supply E. They are connected in series with the same polarity, and neutral point (intermediate potential point E / 2) terminals C are provided at the connection points of the capacitors C1 and C2, and these positive terminal P, negative negative terminal N, and neutral point terminal C are provided. A DC voltage source capable of applying a three-level potential is configured.

Two switching elements Q1 and Q2 such as Si-IGBT (Insulated Gate Bipolar Transistor) are connected in series between the positive terminal P and the negative terminal N of the DC voltage source with the same polarity, and the switching element Q1 Diodes D1 and D2 such as SiC-SBDs (Schottky Barrier Diodes) are connected in antiparallel to each of these and Q2, and the output terminal U connected to the load at the connection point between the switching element Q1 and the switching element Q2 Is provided (AC output). The upper arm is configured by the switching element Q1 provided on the positive electrode terminal P side and the diode D1 connected in antiparallel, and the lower arm is configured by the switching element Q2 provided on the negative electrode terminal N side and the diode D2 connected in antiparallel. There is. Further, two switching elements Q3 and Q4 such as Si-IGBT are connected in series between the neutral point terminal C and the output terminal U with opposite polarities, and with respect to each of the switching elements Q3 and Q4. Diodes D3 and D4 such as Si-FWD (Free Wheel Diode) are connected in antiparallel. These switching elements Q3 and Q4 and diodes D3 and D4 form an intermediate arm of a bidirectional switch.

If three sets of power conversion devices shown in FIG. 1 are connected in series to form a half-bridge circuit, a power conversion device for three-phase AC can be obtained (for example, FIG. 9 of Patent Document 1, Patent Document). 2 in FIG. 1).

In switching control of an NPC type power conversion device having a circuit configuration as disclosed in Patent Document 1, short circuits of capacitors C1 and C2 may occur when switching element states due to variations in switching elements and the like. In order to prevent this, a dead time (short circuit protection period) is set to a shorter time than the switching cycle (see paragraphs 0016 to 0020 of Patent Document 1). When the switching elements Q1 and Q2 of the upper and lower arms turn off at this dead time, a surge voltage is generated in which the voltage across the switching elements Q1 and Q2 exceeds the power supply voltage E due to the wiring inductance, which adversely affects the circuit element. There is a possibility (for example, paragraph 0007 of Patent Document 2).

Japanese Unexamined Patent Publication No. 2013-116020 Japanese Unexamined Patent Publication No. 2014-155395

In view of the above background, the present invention proposes an NPC type power conversion device having a circuit configuration capable of alleviating the surge voltage at the time of dead time.

According to the present invention
At least two capacitors connected in series between the positive and negative terminals,
At least two switching elements connected in series between the positive electrode terminal and the negative electrode terminal, and a diode connected in antiparallel to each of the switching elements.
At least two switching elements connected in series between the neutral point terminal provided at the connection point of the two capacitors and the output terminal provided at the connection point of the two switching elements, and antiparallel to each of the switching elements. A bidirectional switch that includes a connected diode,
In a power converter equipped with
The wiring inductance from the positive electrode terminal to the output terminal and the wiring inductance from the negative electrode terminal to the output terminal are smaller than the wiring inductance from the neutral point terminal to the output terminal.

In the power conversion device according to the present invention, when the switching elements of the upper and lower arms turn off in the above-mentioned dead time, the inductance of both wirings of the upper and lower arms is smaller than the inductance of the intermediate arm, so that the current flowing through the intermediate arm is small and The current flowing through the upper and lower arms increases. Then, the current change with respect to the intermediate arm becomes small, and the surge voltage generated by the current change of the intermediate arm and the wiring inductance is alleviated.

The circuit diagram which showed one form of the power conversion apparatus of the NPC system. The figure explaining the change of the voltage across a switching element in a dead time. Simulation results of voltage and current waveforms when the wiring inductance of each arm is the same. Simulation results of voltage and current waveforms when the wiring inductance of the upper and lower arms is smaller than the wiring inductance of the intermediate arm. Simulation results of voltage and current waveforms when the wiring inductance of the upper and lower arms is made smaller than that of FIG. 4 with respect to the wiring inductance of the intermediate arm. An example of a busbar structure that makes the wiring inductance different between the intermediate arm and the upper and lower arms.

FIG. 1 shows a circuit diagram of an NPC type power converter.
In the power conversion device according to this example, between the positive potential terminal P on the high potential side to which a positive potential is applied from the DC power supply E and the negative potential terminal N on the low potential side to which a negative potential is applied from the DC power supply E. Two capacitors C1 and C2 are connected in series with the same polarity, and a neutral point (intermediate potential point E / 2) terminal C is provided at the connection point of the two capacitors C1 and C2. A DC voltage source capable of applying a three-level potential is configured by a positive electrode terminal P, a negative electrode terminal N, and a neutral point terminal C. Two switching elements Q1 and Q2, which are Si-IGBTs in this example, are connected in series between the positive electrode terminal P and the negative electrode terminal N with the same polarity, and with respect to each of the switching elements Q1 and Q2. In this example, the diodes D1 and D2, which are SiC-SBDs of high-speed diodes, are connected in antiparallel. At the connection point of the two switching elements Q1 and Q2, an output terminal U connected to the load is provided (AC output). The upper arm is configured by the switching element Q1 provided on the positive electrode terminal P side and the diode D1 connected in antiparallel, and the lower arm is configured by the switching element Q2 provided on the negative electrode terminal N side and the diode D2 connected in antiparallel. There is.

Two switching elements Q3 and Q4, which are Si-IGBTs in this example, are connected in series between the neutral point terminal C and the output terminal U with opposite polarities, and are connected to each of the switching elements Q3 and Q4. On the other hand, in this example, the diodes D3 and D4 as Si-FWD are connected in antiparallel. These switching elements Q3 and Q4 and diodes D3 and D4 form an intermediate arm of a bidirectional switch.

In FIG. 1, the wiring inductance of the upper arm (wiring inductance from the positive terminal to the output terminal) is L1, the wiring inductance of the lower arm (wiring inductance from the negative terminal to the output terminal) is L2, and the wiring inductance of the intermediate arm (middle). The wiring inductance from the neutral terminal to the output terminal) is indicated by L3.

The power conversion device of this example is driven by switching control as described in Patent Document 1 described above, but in the switching control, for the purpose of preventing a short circuit of capacitors C1 and C2 that may occur when switching element states, the purpose is to prevent short circuits. The dead time for turning off both the switching elements Q1 and Q2 of the upper and lower arms is set to be shorter than the switching cycle. When the switching elements Q1 and Q2 of the upper and lower arms turn off at this dead time, a surge voltage is generated in which the voltage across the switching elements Q1 and Q2 exceeds the power supply voltage E due to the circuit inductance.

The operation at this time will be described as an example when the switching element Q2 of the lower arm turns off from on. In this case, as the voltage between UN of the lower arm changes from 0 to the intermediate potential E / 2 due to the turn-off of the switching element Q2, the voltage between PU of the upper arm changes from the power supply voltage E to the intermediate potential E / 2. (Switching element Q1 is off). Then, the current flowing in the lower arm is commutated to the intermediate arm (switching element Q4 is turned on). Further, since the voltage due to the wiring inductance L3 of the intermediate arm and the current change (-di / dt) is added here, the voltage between UN and N becomes higher than the intermediate potential E / 2 and P-. The U-to-U voltage is lower than the intermediate potential E / 2. Then, when the voltage between U and N tries to exceed the power supply voltage E, the voltage between P and U becomes 0, so that the diode D1 of the upper arm conducts and the reflux current flows through both the intermediate arm and the upper arm. become. At this time, since the conducting diode D1 normally plays the role of a clipping diode, the NU-U voltage should be clamped to the power supply voltage E, but in reality, the parasitic capacitance and wiring inductance of the diode D1 Since resonance occurs due to L1 and L3, the resonance voltage is superimposed on the power supply voltage E, resulting in a waveform that generates a surge voltage (see FIG. 2).

Therefore, the power conversion device according to the embodiment of the present invention is designed so that the wiring inductances L1 and L2 of the upper arm and the lower arm are smaller than the wiring inductance L3 of the intermediate arm. In the above operating condition, if the wiring inductance L1 of the upper arm is small, the amount of current flowing to the upper arm through the diode D1 is large (the current tends to flow to the lower impedance). As the current flowing through the upper arm increases, the current flowing through the intermediate arm decreases, so that -di / dt in the intermediate arm decreases, and the surge voltage generated by this -di / dt and the wiring inductance L3 is reduced by that amount. Can be done. Regarding this effect, the waveforms of the simulation results are shown in FIGS. 3 to 5.

FIG. 3 shows a waveform when L1 = L2 = L3 = 30 nH according to the conventional design. The power supply voltage E used in the simulations of FIGS. 3 to 5 is 600 V, and the intermediate potential E / 2 is 300 V. At the turn-off of the switching element Q2, the lower arm voltage rises and the lower arm current decreases. The lower arm voltage is finally focused on the intermediate potential E / 2, but a surge voltage of 920 V is generated in the dead time. At this time, the upper arm voltage once drops to 0 V and then focuses on the intermediate potential E / 2. Along with this, a current commutating to the intermediate arm and a current flowing to the upper arm are generated, and the current flowing to the upper arm becomes 45A.

FIG. 4 shows a waveform when the present invention is applied and L1 = L2 = 20 nH and L3 = 40 nH. The circuit as a whole behaves in the same manner as in FIG. 3, but as a result of the wiring inductance L1 of the upper arm being smaller than the wiring inductance L3 of the intermediate arm, the current flowing to the upper arm increases to 65A and the surge voltage is 880V. It has dropped to.
FIG. 5 is also a simulation to which the present invention is applied, and is a waveform when L1 = L2 = 10 nH and L3 = 50 nH. This also behaves in the same manner as in FIG. 3 as a whole circuit, but as a result of the wiring inductance L1 of the upper arm being smaller than that in the case of FIG. 4 with respect to the wiring inductance L3 of the intermediate arm, the current flowing to the upper arm is further increased. It has increased to 100A and the surge voltage has decreased to 820V.
From these results, it can be seen that the effect of surge voltage mitigation can be obtained by reducing the wiring inductances L1 and L2 of the upper and lower arms to 1/2 or less of the wiring inductance L3 of the intermediate arm.

In order to design the wiring inductances L1 and L2 of the upper and lower arms to be smaller than the wiring inductance L3 of the intermediate arm, specifically, the bus bar design as shown in FIG. 6 is possible as an example. In the illustrated example, the bus bars 3 forming the intermediate arm are laminated between the bus bars 1 and 2 forming the upper arm and the lower arm, and the shapes of the bus bars 1 and 2 forming the upper arm and the lower arm are formed. The shape of the bus bar 3 constituting the intermediate arm is different. Specifically, the copper plate area (volume) of the bus bar 3 constituting the intermediate arm is increased with respect to the copper plate area (volume) of the bus bars 1 and 2 constituting the upper arm and the lower arm, and the cutout portion of the copper plate is increased. It is made smaller. The wiring inductance can be easily adjusted by the area ratio of the cutout portion. For example, the area of the cutout portion of the intermediate arm can be easily adjusted by making the area of the cutout portion of the upper arm and the lower arm more than twice the area of the cutout portion of the upper arm and the lower arm.

Regarding the adjustment of the wiring inductance by the bus bar, that is, the wiring shape, in addition to this, the wiring thickness of the intermediate arm that reduces the wiring length of the upper arm and the lower arm to 1/2 or less with respect to the wiring length of the intermediate arm. It is also possible to adjust the wiring thickness of the upper arm and the lower arm by doubling or more.

Although the above description has focused on the case of the switching element Q2, those skilled in the art can easily understand from the present specification that the same action and effect can be obtained even at the turn-off of the switching element Q1.

Q1 to Q4 Switching elements D1 to D4 Diode E Power supply voltage E / 2 Intermediate potential P Positive terminal N Negative terminal C Neutral point terminal U Output terminal L1 Upper arm wiring inductance L2 Lower arm wiring inductance L3 Intermediate arm wiring inductance

Claims (7)

Two capacitors connected in series between the positive and negative terminals of the DC power supply,
Two switching elements connected in series between the positive electrode terminal and the negative electrode terminal, and a diode connected in antiparallel to each of the switching elements.
At least two switching elements connected in series between the neutral point terminal provided at the connection point of the two capacitors and the output terminal provided at the connection point of the two switching elements, and antiparallel to each of the switching elements. A bidirectional switch that includes a connected diode,
In a power converter equipped with
A power conversion device, characterized in that the wiring inductance from the positive electrode terminal to the output terminal and the wiring inductance from the negative electrode terminal to the output terminal are smaller than the wiring inductance from the neutral point terminal to the output terminal. The first aspect of claim 1, wherein the wiring inductance from the positive electrode terminal to the output terminal and the wiring inductance from the negative electrode terminal to the output terminal are 1/2 or less with respect to the wiring inductance from the neutral point terminal to the output terminal. Power converter. The electric power according to claim 1 or 2, wherein the wiring from the neutral point terminal to the output terminal is laminated between the wiring from the positive electrode terminal to the output terminal and the wiring from the negative electrode terminal to the output terminal. Conversion device. Claim 1 in which the wiring length from the positive electrode terminal to the output terminal and the wiring length from the negative electrode terminal to the output terminal are 1/2 or less with respect to the wiring length from the neutral point terminal to the output terminal. Or the power conversion device according to 2. Claim 1 or claim 1, wherein the wiring thickness from the positive electrode terminal to the output terminal and the wiring thickness from the negative electrode terminal to the output terminal are twice or more the wiring thickness from the neutral point terminal to the output terminal. 2. The power conversion device according to 2. The wiring from the positive electrode terminal to the output terminal, the wiring from the negative electrode terminal to the output terminal, and the wiring from the neutral point terminal to the output terminal have cutout portions, respectively.
The first or second claim, wherein the cut-out portion of the wiring from the positive electrode terminal to the output terminal and the wiring from the negative electrode terminal to the output terminal is smaller than the cut-out portion of the wiring from the neutral point terminal to the output terminal. Power converter. The area of the cut-out portion of the wiring from the neutral point terminal to the output terminal is twice or more the area of the cut-out portion of the wiring from the positive electrode terminal to the output terminal and the wiring from the negative electrode terminal to the output terminal. Item 6. The power conversion device according to Item 6.

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