JP7628062B2 - Power Conversion Equipment - Google Patents

Description

The present invention relates to a power conversion device.

Figure 3 shows the power conversion device 10C described in Patent Document 1. The power conversion device 10C is a hybrid power storage device that can be connected to solar cells 21, 22, and includes step-up DC/DC converters 11, 12, a storage battery 13, a bidirectional DC/DC converter 14, a capacitor C1, a bidirectional inverter 15, a relay circuit 16, and a control unit 17C.

Figure 4 shows a circuit diagram of the bidirectional inverter 15. The bidirectional inverter 15 includes DC terminals T11 and T12, switching elements Q1 to Q4, diodes D1 to D4 connected in parallel in the reverse direction to each of the switching elements Q1 to Q4, a filter circuit consisting of coils L1 and L2 and a capacitor C2, and AC terminals T13 and T14. The DC terminals T11 and T12 are connected to the capacitor C1, and the AC terminals T13 and T14 are connected to the relay circuit 16. The connection point between the switching elements Q2 and Q4 is designated as a, and the connection point between the switching elements Q1 and Q3 is designated as b.

The driving method of the bidirectional inverter 15 is generally the bipolar driving method shown in FIG. 5, and the unipolar driving method shown in FIG. 6. FIGS. 5(A) and 6(A) show the carrier signal and the command signal (the first command signal and the second command signal in FIG. 6). FIGS. 5(B) and 6(B) show the output voltage between a and b of the bidirectional inverter 15, and FIGS. 5(C) and 6(C) show the output current between a and b.

In the case of the bipolar drive method, the control unit 17C compares the carrier signal with the command signal to generate a PWM signal, and turns on/off the switching elements Q1 to Q4 based on the generated PWM signal and its inverted signal. On the other hand, in the case of the unipolar drive method, the control unit 17C compares the carrier signal with the first command signal to generate a first PWM signal, turns on/off the switching elements Q2 and Q4 based on the first PWM signal and its inverted signal, and compares the carrier signal with the second command signal to generate a second PWM signal, and turns on/off the switching elements Q1 and Q3 based on the second PWM signal and its inverted signal.

For example, in the case of power conversion operation (DC/AC conversion operation) from DC terminals T11, T12 to AC terminals T13, T14 in the unipolar drive system, when the negative side of a sine wave is output from AC terminals T13, T14, step-down chopper operation is performed by switching element Q1 and diode D3, switching element Q2 is always off and switching element Q4 is always on. When the positive side of a sine wave is output from AC terminals T13, T14, step-down chopper operation is performed by switching element Q2 and diode D4, switching element Q1 is always off and switching element Q3 is always on.

When performing power conversion operation (AC/DC conversion operation) from AC terminals T13, T14 to DC terminals T11, T12 in the unipolar drive method, when the positive side of a sine wave is input to AC terminals T13, T14, the switching element Q4 and diode D2 perform boost chopper operation, and switching elements Q1, Q3 are always off. When the negative side of a sine wave is input to AC terminals T13, T14, the switching element Q3 and diode D1 perform boost chopper operation, and switching elements Q2, Q4 are always off.

Figure 7 (A) shows the output voltage, voltage to ground, and leakage current of the bidirectional inverter 15 in the bipolar drive system. Figure 7 (B) shows the output voltage, voltage to ground, and leakage current of the bidirectional inverter 15 in the unipolar drive system.

The bipolar drive method has lower power conversion efficiency than the unipolar drive method because switching losses occur in the switching elements Q1 to Q4, but the AC component of the voltage to ground is smaller than in the unipolar drive method, so leakage current can be reduced. The unipolar drive method has higher voltage to ground and leakage current than the bipolar drive method, but the number of switching elements Q1 to Q4 that are turned on and off at high frequency is halved, so power conversion efficiency can be improved.

The capacitance component of the solar cells 21, 22 connected to the power conversion device 10C with respect to the ground varies depending on the type and structure of the solar cells 21, 22, and may also increase rapidly when they get wet due to rain or the like. If the capacitance component is large, the leakage current will be large, causing a problem that the earth leakage breaker installed in the distribution board in the home will be activated, cutting off the power supply to the loads in the home connected via the distribution board.

By using solar cells 21 and 22 that have a small capacitance to the ground or whose capacitance does not increase easily even when wet with rain, the bidirectional inverter 15 can be driven using a unipolar drive method, improving the power conversion efficiency without operating the earth leakage breaker.

However, a typical power conversion device is required to be able to connect to a variety of solar cells. For this reason, in the conventional power conversion device 10C, in order to be able to handle solar cells 21 and 22 with large leakage currents, the bidirectional inverter 15 is always driven in a bipolar drive method, and the earth leakage breaker is not operated even at the expense of power conversion efficiency.

JP 2019-134602 A

The present invention was made in consideration of the above circumstances, and its objective is to provide a power conversion device that can improve power conversion efficiency without operating a ground fault circuit interrupter.

In order to solve the above problems, a power conversion device according to the present invention comprises:
A power conversion device that is connected to at least one power generation means and supplies power to a load via a switchboard,
a DC/DC converter connected to the power generating means;
an inverter that converts a DC voltage input from the DC/DC converter side into an AC voltage and outputs the AC voltage to the switchboard side;
a leakage current detection unit that detects a leakage current in a power line that connects the inverter and the switchboard;
a control unit that switches the drive method of the inverter between a bipolar drive method and a unipolar drive method,
The control unit is
when the detection value of the leakage current detection unit is smaller than a first threshold value while the inverter is driven in the bipolar drive mode, the drive mode is switched from the bipolar drive mode to the unipolar drive mode;
When the inverter is driven in the unipolar drive mode, if the detection value of the leakage current detection unit is equal to or greater than a second threshold value that is greater than the first threshold value, the inverter switches from the unipolar drive mode to the bipolar drive mode.

In this configuration, a leakage current detection unit is provided for detecting leakage current, and when the leakage current is small, the inverter is driven using a unipolar drive method, and when the leakage current is large, the inverter is driven using a bipolar drive method. Therefore, with this configuration, it is possible to improve the power conversion efficiency without operating the earth leakage breaker.

In the power conversion device,
The control unit is
while driving the inverter in the bipolar driving mode, controlling the DC/DC converter so that an input voltage of the inverter is maintained at a first voltage value;
When the inverter is driven in the unipolar drive mode, the DC/DC converter may be controlled so that the input voltage of the inverter is maintained at a second voltage value smaller than the first voltage value.

In the power conversion device,
It is preferable that the control unit drives the inverter in the bipolar drive mode when starting the operation of the inverter.

The power conversion device is
A storage means;
a bidirectional DC/DC converter for charging and discharging the power storage means;
Further equipped with
The inverter may be a bi-directional inverter and may be configured to be connected to the DC/DC converter and the bi-directional DC/DC converter.

The power conversion device is
an opening/closing means for opening and closing the power generating means and the DC/DC converter, the opening/closing means being disposed between the power generating means and the DC/DC converter and configured to electrically disconnect the power generating means and the DC/DC converter when the opening/closing means is in an open state;
the switching means is provided on both a positive power line and a negative power line connecting the power generation means and the DC/DC converter,
The control unit is
When the power generating means is not generating power, the switching means may be set to the open state and the inverter may be driven in the unipolar drive mode.

The present invention provides a power conversion device that can improve power conversion efficiency without operating a ground fault circuit interrupter.

1 is a block diagram of a power conversion device according to a first embodiment. FIG. 11 is a block diagram of a power conversion device according to a second embodiment. FIG. 1 is a block diagram of a conventional power conversion device. FIG. 2 is a circuit diagram of a bidirectional inverter. 4A and 4B are waveform diagrams of various waveforms in a bipolar drive system, in which (A) is a waveform diagram of a carrier signal and a command signal, (B) is a waveform diagram of an output voltage, and (C) is a waveform diagram of an output current. 5A and 5B are waveform diagrams of various waveforms in a unipolar drive system, in which (A) is a waveform diagram of a carrier signal and first and second command signals, (B) is a waveform diagram of an output voltage, and (C) is a waveform diagram of an output current. 1A and 1B are diagrams showing output voltage, voltage to ground, and leakage current, in which (A) is a diagram for a bipolar drive system, and (B) is a diagram for a unipolar drive system.

Below, an embodiment of a power conversion device according to the present invention will be described with reference to the attached drawings.

[First embodiment]
1 shows a power conversion device 10A according to a first embodiment of the present invention. The power conversion device 10A is a hybrid power storage device that can be connected to solar cells 21, 22 (corresponding to the "power generation means" of the present invention), and includes terminals T1 to T8, step-up DC/DC converters 11, 12, a storage battery 13, a bidirectional DC/DC converter 14, a capacitor C1, a bidirectional inverter 15, a relay circuit 16, a control unit 17A, and a leakage current detection unit 18.

Terminals T1 and T2 are connected to the positive and negative sides of the solar cell 21, respectively. Terminals T3 and T4 are connected to the positive and negative sides of the solar cell 22, respectively. Terminals T5 and T6 are connected to the commercial power system and are also connected to household loads via a distribution board (including a switchboard) (not shown) equipped with a ground fault circuit interrupter. Terminals T7 and T8 are connected to an independent output line. A household load (e.g., a home appliance such as a refrigerator) that is to be operated preferentially in the event of a power outage in the commercial power system is connected to the independent output line.

The DC/DC converter 11 is connected to terminals T1 and T2 via breaker BR1. Breaker BR1 is normally on (closed). Under the control of the control unit 17A, the DC/DC converter 11 boosts the DC generated voltage input from the solar cell 21 and outputs it to the bidirectional inverter 15.

The DC/DC converter 12 is connected to terminals T3 and T4 via breaker BR2. Breaker BR2 is normally on (closed). Under the control of the control unit 17A, the DC/DC converter 12 boosts the DC generated voltage input from the solar cell 22 and outputs it to the bidirectional inverter 15.

The storage battery 13 corresponds to the "power storage means" of the present invention, and includes at least one battery unit and a battery management system (BMS) attached to the battery unit. The battery management system acquires battery information (e.g., the amount of stored power) of the battery unit and transmits it to the control unit 17A. The battery management system may be included in the control unit 17A.

The storage battery 13 is connected to one DC end of the bidirectional DC/DC converter 14, and the bidirectional inverter 15 is connected to the other DC end via the capacitor C1. The bidirectional DC/DC converter 14 performs charging and discharging operations of the storage battery 13 under the control of the control unit 17A.

The bidirectional inverter 15 has DC/DC converters 11 and 12 and a bidirectional DC/DC converter 14 connected to its DC end via a capacitor C1, and a relay circuit 16 connected to its AC end. Under the control of the control unit 17A, the bidirectional inverter 15 switches between a bipolar drive method and a unipolar drive method to perform DC/AC conversion and AC/DC conversion.

As shown in Figure 4, the bidirectional inverter 15 includes DC terminals T11 and T12, switching elements Q1 to Q4, diodes D1 to D4 connected in parallel in the reverse direction to each of the switching elements Q1 to Q4, a filter circuit consisting of coils L1 and L2 and a capacitor C2, and AC terminals T13 and T14. The DC terminals T11 and T12 are connected to the capacitor C1, and the AC terminals T13 and T14 are connected to the relay circuit 16.

The relay circuit 16 includes relays S1 to S6. Relays S1 and S2 are installed in the power line connecting the bidirectional inverter 15 and terminals T5 and T6. Relays S3 and S4 are installed in the power line connecting the bidirectional inverter 15 and terminals T7 and T8 via relays S1 and S2. Relays S5 and S6 are installed in the power line connecting the bidirectional inverter 15 and terminals T7 and T8 without via relays S1 and S2.

Relays S1 to S6 are switched between an on state (closed state) and an off state (open state) under the control of the control unit 17A. For example, when the commercial power system is energized, relays S1 to S4 are in the on state and relays S5 and S6 are in the off state, whereas during a power outage, relays S1 to S4 are in the off state and relays S5 and S6 are in the on state.

The leakage current detection unit 18 is provided on the power line connecting the bidirectional inverter 15 to the terminals T5 and T6 (the power line connecting the bidirectional inverter 15 to the switchboard) and detects leakage current on the power line. The leakage current detection unit 18 outputs the detection value (current value) of the detected leakage current to the control unit 17A.

The control unit 17A controls the DC/DC converters 11 and 12, the bidirectional DC/DC converter 14, the bidirectional inverter 15, and the relay circuit 16. The control unit 17A may be configured with an analog control circuit, a digital control circuit using a microcontroller or the like, or a circuit that combines an analog control circuit and a digital control circuit.

The control unit 17A switches the drive method of the bidirectional inverter 15 between a bipolar drive method and a unipolar drive method. The bipolar drive method and the unipolar drive method are similar to conventional technology (see, for example, Figures 5 to 7), so a description thereof will be omitted here.

When the control unit 17A starts the operation of the bidirectional inverter 15, it drives it using the bipolar drive method. The bipolar drive method has a smaller AC component of the voltage to ground than the unipolar drive method, and can reduce leakage current.

When the bidirectional inverter 15 is driven in the bipolar drive mode, if the detection value of the leakage current detection unit 18 is smaller than the first threshold value, the control unit 17A switches the drive mode of the bidirectional inverter 15 from the bipolar drive mode to the unipolar drive mode. If the detection value of the leakage current detection unit 18 is equal to or greater than the first threshold value, the control unit 17A maintains the bipolar drive mode.

When the bidirectional inverter 15 is driven in the unipolar drive mode, if the detection value of the leakage current detection unit 18 is equal to or greater than a second threshold value that is greater than the first threshold value, the control unit 17A returns the drive mode of the bidirectional inverter 15 to the bipolar drive mode. If the detection value of the leakage current detection unit 18 is smaller than the second threshold value, the control unit 17A maintains the unipolar drive mode.

For example, in the above-mentioned switchboard, the earth leakage breaker operates when the leakage current reaches 30 [mA]. In the power conversion device 10A, the difference in leakage current between when the bidirectional inverter 15 is driven by the bipolar drive method and when it is driven by the unipolar drive method during evaluation (e.g., during evaluation before product shipment) is 15 [mA]. In this case, for example, the first threshold can be set to 10 [mA], and the second threshold can be set to 27 [mA] (= 10 [mA] + 15 [mA] + α). It is preferable that the second threshold is a value larger than the value obtained by adding the difference (15 [mA]) to the first threshold, taking into account the increase (+α) in leakage current due to the capacitance component of the solar cells 21, 22 to the ground, and smaller than the operating voltage value (30 [mA]) at which the earth leakage breaker operates.

When the bidirectional inverter 15 is driven in the unipolar drive mode, if the solar cells 21 and 22 get wet due to rain or the like, the capacitance component of the solar cells 21 and 22 with respect to the ground increases, the leakage current increases, and the detection value of the leakage current detection unit 18 reaches a second threshold value (e.g., 27 [mA]), the control unit 17A switches the drive mode of the bidirectional inverter 15 to the bipolar drive mode. If the solar cells 21 and 22 then dry, the capacitance component of the solar cells 21 and 22 with respect to the ground decreases, the leakage current decreases, and the detection value of the leakage current detection unit 18 becomes smaller than the first threshold value (e.g., 10 [mA]), the control unit 17A switches the drive mode of the bidirectional inverter 15 to the unipolar drive mode.

When the bidirectional inverter 15 is driven by the bipolar drive method, the control unit 17A controls the DC/DC converters 11, 12 and the bidirectional DC/DC converter 14 so that the input voltage of the bidirectional inverter 15 (the voltage across the capacitor C1) is maintained at a predetermined first voltage value. On the other hand, when the bidirectional inverter 15 is driven by the unipolar drive method, the control unit 17A controls the DC/DC converters 11, 12 and the bidirectional DC/DC converter 14 so that the input voltage of the bidirectional inverter 15 is maintained at a predetermined second voltage value that is smaller than the first voltage value.

For example, in the unipolar drive method and the bipolar drive method, if the input voltage (voltage across capacitor C1) of the bidirectional inverter 15 is fixed to the same value (e.g., 200 [V]) and the bidirectional inverter 15 is driven with the same duty (e.g., 90 [%]), the resulting output voltage is as follows: In the unipolar drive method, it is 200 [V] x 0.9 = 180 [V], whereas in the bipolar drive method, it is (200 [V] x 0.9) - (200 [V] x 0.1) = 160 [V].

When driving the bidirectional inverter 15 with the bipolar drive method, the input voltage of the bidirectional inverter 15 needs to be 225 [V] to obtain the same output voltage (180 [V]) as with the unipolar drive method. However, if the input voltage is fixed at 225 [V] and the bidirectional inverter 15 is driven with the unipolar drive method, the difference between the input voltage and the output voltage becomes large, and the power conversion efficiency decreases.

Therefore, in this embodiment, the first voltage value is set to the lowest voltage value (e.g., 225 [V]) at which the difference between the input voltage and the output voltage of the bidirectional inverter 15 is minimized in the bipolar drive system and the desired output voltage can be obtained. The second voltage value is set to the lowest voltage value (e.g., 200 [V]) at which the difference between the input voltage and the output voltage of the bidirectional inverter 15 is minimized in the unipolar drive system and the desired output voltage can be obtained.

In the end, in the power conversion device 10A according to this embodiment, the leakage current is monitored by the leakage current detection unit 18, and the drive method of the bidirectional inverter 15 is switched according to the magnitude of the leakage current. This improves the power conversion efficiency of the bidirectional inverter 15 compared to the conventional fixed bipolar drive method, and also makes it possible to prevent the earth leakage breaker installed in the switchboard from operating.

Furthermore, in the power conversion device 10A according to this embodiment, the input voltage of the bidirectional inverter 15 (the voltage across the capacitor C1) is controlled to the minimum voltage value at which the difference between the input voltage and the output voltage of the bidirectional inverter 15 is minimized and the desired output voltage can be obtained, depending on the drive method of the bidirectional inverter 15, thereby improving the power conversion efficiency.

[Second embodiment]
2 shows a power conversion device 10B according to a second embodiment of the present invention. The power conversion device 10B differs from the first embodiment in that it includes a first switching means RL1 and a second switching means RL2 and that a control unit 17B controls the first switching means RL1 and the second switching means RL2, but is the same as the first embodiment in other respects.

The first switching means RL1 is, for example, a relay, and is interposed in both the positive power line and the negative power line that connect the solar cell 21 and the DC/DC converter 11. The first switching means RL1 is switched between an open state (off state) and a closed state (on state) simultaneously under the control of the control unit 17B, and when in the open state, it cuts off the electrical connection between the solar cell 21 and the DC/DC converter 11.

The second switching means RL2 is, for example, a relay, and is installed on both the positive power line and the negative power line that connect the solar cell 22 and the DC/DC converter 12. The second switching means RL2 is switched between an open state (off state) and a closed state (on state) simultaneously under the control of the control unit 17B, and when in the open state, it cuts off the electrical connection between the solar cell 22 and the DC/DC converter 12.

When the solar cells 21, 22 have not generated power for a predetermined period of time (when the voltage generated by the solar cells 21, 22 is not applied to the terminals T1, T2), the control unit 17B opens the first switching means RL1 and the second switching means RL2 and drives the bidirectional inverter 15 in the unipolar drive mode. In this case, regardless of the detection value of the leakage current detection unit 18, the drive mode of the bidirectional inverter 15 is fixed to the unipolar drive mode.

In the power conversion device 10B according to this embodiment, the solar cells 21, 22 are disconnected when they have not generated power for a predetermined period of time, thereby preventing leakage current from being input from the solar cells 21, 22. For example, even if the capacitance component of the solar cells 21, 22 to ground increases and the leakage current increases when the solar cells 21, 22 are not generating power, the drive method of the bidirectional inverter 15 does not switch to the bipolar drive method (the unipolar drive method is maintained), so a decrease in power conversion efficiency can be avoided.

[Modification]
Although the embodiment of the power conversion device according to the present invention has been described above, the present invention is not limited to the above embodiment.

The power conversion device of the present invention is a power conversion device that is connected to a power generation means and supplies power to a load via a switchboard, and can be modified as appropriate as long as it includes a DC/DC converter, an inverter that converts a DC voltage input from the DC/DC converter side into an AC voltage and outputs it, a leakage current detection unit that detects leakage current, and a control unit that switches the inverter drive method between a bipolar drive method and a unipolar drive method. For example, the power conversion device of the present invention may be a solar power generation device.

The control unit of the present invention can be appropriately configured to switch from the bipolar drive system to the unipolar drive system if the detection value of the leakage current detection unit is smaller than a first threshold value when the inverter is driven in the bipolar drive system, and to switch from the unipolar drive system to the bipolar drive system if the detection value of the leakage current detection unit is equal to or greater than a second threshold value that is larger than the first threshold value when the inverter is driven in the unipolar drive system.

The first and second thresholds can be set appropriately as long as they are smaller than the operating voltage value at which the earth leakage breaker of the switchboard operates. It is preferable that the control unit performs switching control with hysteresis so that switching between the bipolar drive method and the unipolar drive method does not occur frequently in a short period of time near the first and second thresholds.

10A, 10B Power conversion device 11, 12 DC/DC converter 13 Storage battery 14 Bidirectional DC/DC converter 15 Bidirectional inverter 16 Relay circuit 17A, 17B Control unit 18 Leakage current detection unit 21, 22 Solar cell

Claims (5)

A power conversion device that is connected to at least one power generation means and supplies power to a load via a switchboard,
a DC/DC converter connected to the power generating means;
an inverter that converts a DC voltage input from the DC/DC converter side into an AC voltage and outputs the AC voltage to the switchboard side;
a leakage current detection unit that detects a leakage current in a power line that connects the inverter and the switchboard;
a control unit that switches the drive method of the inverter between a bipolar drive method and a unipolar drive method,
The control unit is
When the inverter is driven in the bipolar drive mode, the DC/DC converter is controlled so that an input voltage of the inverter is maintained at a first voltage value, and when a detection value of the leakage current detection unit is smaller than a first threshold value, the DC/DC converter is switched from the bipolar drive mode to the unipolar drive mode;
a second voltage value that is smaller than the first voltage value when the inverter is driven in the unipolar drive mode, and a second voltage value that is larger than the first threshold value when the detection value of the leakage current detection unit is equal to or greater than a second threshold value that is larger than the first threshold value, the power conversion device is characterized in that: 2. The power conversion device according to claim 1 , wherein the control unit drives the inverter in the bipolar drive mode when starting the operation of the inverter. A storage means;
a bidirectional DC/DC converter for charging and discharging the power storage means;
Further equipped with
3. The power conversion device according to claim 1, wherein the inverter is a bidirectional inverter and is connected to the DC/DC converter and the bidirectional DC/DC converter. an opening/closing means for opening and closing the power generating means and the DC/DC converter, the opening/closing means being disposed between the power generating means and the DC/DC converter and configured to electrically disconnect the power generating means and the DC/DC converter when the opening/closing means is in an open state;
the switching means is provided on both a positive power line and a negative power line connecting the power generation means and the DC/DC converter,
The control unit is
4. The power conversion device according to claim 1 , wherein, when the power generation means is not generating power, the switching means is set to the open state and the inverter is driven by the unipolar drive method. A power conversion device that is connected to at least one power generation means and supplies power to a load via a switchboard,
a DC/DC converter connected to the power generating means;
an inverter that converts a DC voltage input from the DC/DC converter side into an AC voltage and outputs the AC voltage to the switchboard side;
a leakage current detection unit that detects a leakage current in a power line that connects the inverter and the switchboard;
a control unit that switches the drive method of the inverter between a bipolar drive method and a unipolar drive method,
The control unit is
when the detection value of the leakage current detection unit is smaller than a first threshold value while the inverter is driven in the bipolar drive mode, the drive mode is switched from the bipolar drive mode to the unipolar drive mode;
when the detection value of the leakage current detection unit is equal to or greater than a second threshold value that is greater than the first threshold value while the inverter is driven in the unipolar driving mode, the driving mode is switched from the unipolar driving mode to the bipolar driving mode;
an opening/closing means for opening and closing the power generating means and the DC/DC converter, the opening/closing means being disposed between the power generating means and the DC/DC converter and configured to electrically disconnect the power generating means and the DC/DC converter when the opening/closing means is in an open state;
the switching means is provided on both a positive power line and a negative power line connecting the power generation means and the DC/DC converter,
The control unit is
When the power generating means is not generating power, the switching means is opened and the inverter is driven in the unipolar drive mode.
A power conversion device comprising:

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