Power conversion device and power distribution system

Abstract

This power conversion device (5A) comprises: a full-bridge inverter (12A) that is connected to a positive-side DC bus (50A) and a negative-side DC bus (51A); a positive-side detection resistor (8A) that is disposed between the positive-side DC bus (50A) and a ground point (18A); a negative-side detection resistor (9A) that is disposed between the negative-side DC bus (51A) and a ground point (18B); and a control device (25A) that controls the connection between the output of the full-bridge inverter (12A) and a DC grid (1). The control device (25A) determines whether or not another power conversion device is connected to the DC grid (1) on the basis of the difference voltage between the absolute value of the voltage of the positive-side detection resistor (8A) and the absolute value of the voltage of the negative-side detection resistor (9A).

Description

Power Conversion Equipment and Power Distribution Systems

[0001] The present disclosure relates to a power conversion device and a power distribution system.

[0002] The power distribution system of Patent Document 1 includes a single-phase three-wire AC power supply line that supplies AC power to electrical equipment, a three-wire DC power supply line that supplies DC power to electrical equipment, and an AC-DC converter that converts AC power input from the AC power supply line to DC power output from the DC power supply line. The AC-DC converter has a path that commonly connects one of the three wires of the DC power supply line to the neutral wire of the AC power supply line, and applies DC voltages to a positive voltage line that is at a positive potential relative to the neutral wire and a negative voltage line that is at a negative potential relative to the neutral wire. Furthermore, the system employs a configuration using multiple resistors as a ground fault detector, and multiple switches can be used to disconnect each resistor from the DC power supply line. This prevents the resistors from being connected to the DC power supply line except when ground fault detection is required, eliminating the constant current loss that occurs when the resistors are connected, and reducing the power loss that is a drawback of resistor-based ground fault detectors.

[0003] JP 2011-15502 A

[0004] When a ground fault occurs in a case where multiple power conversion devices are connected to a DC grid, the ground fault current becomes larger than when only one power conversion device is connected. In Patent Document 1, it is not possible to detect whether or not another power conversion device having a ground fault detection circuit is connected to the DC grid, and when another power conversion device having a ground fault detection circuit is connected, the ground fault current becomes larger.

[0005] Therefore, an object of the present disclosure is to provide a power conversion device and a power distribution system that can detect whether or not another power conversion device having a ground fault detection circuit is connected to a DC grid.

[0006] A power conversion device according to the present disclosure includes a full-bridge inverter connected to a positive-side DC bus and a negative-side DC bus, a positive-side detection resistor disposed between the positive-side DC bus and a ground point, a negative-side detection resistor disposed between the negative-side DC bus and the ground point, and a control device that controls a connection between the output of the full-bridge inverter and a DC grid. The control device determines whether another power conversion device is connected to the DC grid based on a differential voltage between the absolute value of the voltage of the positive-side detection resistor and the absolute value of the voltage of the negative-side detection resistor.

[0007] According to the present disclosure, it is possible to detect whether or not another power conversion device having a ground fault detection circuit is connected to the DC grid.

[0008] 1 is a diagram showing the configuration of a DC power distribution system according to a first embodiment. It is a diagram showing the configurations of a first positive-side detection variable resistor 8A and a first negative-side detection variable resistor 9A according to the first embodiment. It is a diagram showing the detailed configuration of a first power conversion device 5A. It is a schematic diagram of a first voltage controller 104. It is a diagram showing an output voltage to ground (Vp) 64 of a positive-side DC grid 2, an output voltage to ground (Vn) 65 of a negative-side DC grid 3, and a voltage to ground (Vp-n) 66 of a grid neutral point potential. It is a diagram showing a current path returning from the first power conversion device 5A to the first power conversion device 5A via the positive-side DC grid 2 and a second ground point 18B in response to a change in the voltage to ground (Vp-n) 66 of the DC grid neutral point potential. 1 is a diagram showing a current path from the first power converter 5A via the negative-side DC grid 3 and the second ground point 18B and returning to the first power converter 5A due to a change in the voltage to ground (Vp-n) 66 of the DC grid neutral point potential. This diagram shows a first positive-side detection variable resistor voltage (Vpd) 26, a first negative-side detection variable resistor voltage (Vnd) 27, and a difference voltage (Vdif) 40 between their absolute values. This diagram shows the number of power converters electrically connected to the DC grid 1 and the difference voltage (Vdif) 40 detected by the first power converter 5A. This diagram is for explaining a ground fault current when the resistance value of the ground fault detection circuit is a standard resistance value. This diagram is for explaining a ground fault current when the resistance value of the ground fault detection circuit is a high resistance value. 1 is a diagram showing the magnitude of the ground fault current when the first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A are standard resistors and when they are high resistance values. FIG. 2 is a flowchart showing the control procedure of the first embodiment. FIG. 3 is a flowchart showing the procedure of the ground fault detection device confirmation operation. FIG. 4 is a diagram showing the configuration of the first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A in a first modified example of the first embodiment. FIG. 5 is a flowchart showing the control procedure of a second modified example of the first embodiment. FIG. 6 is a flowchart showing the control procedure of a third modified example of the first embodiment. FIG. 7 is a diagram explaining the operation when the absolute value of the differential voltage (Vdif) is equal to or less than the second threshold (TH2) 70. FIG. 8 is a diagram explaining the operation when the absolute value of the differential voltage (Vdif) 40 exceeds the second threshold (TH2) 70.

[0009] Embodiment 1. Fig. 1 is a diagram showing the configuration of a DC power distribution system according to embodiment 1. Fig. 1 shows a case where two power conversion devices are connected to a DC grid 1, but when one power conversion device is connected to the DC grid 1, three or more power conversion devices may be connected to the DC grid 1.

[0010] The DC power distribution system includes a DC grid 1, a first independent power source 4A such as a solar power generation system or a storage battery, a first positive side DC bus 50A, a first negative side DC bus 51A, a first power conversion device 5A that adjusts the voltage of the first independent power source 4A, a first control device 25A that controls the first power conversion device 5A, a first alarm device 71A, a second independent power source 4B, a second positive side DC bus 50B, a second negative side DC bus 51B, a second power conversion device 5B that adjusts the voltage of the second independent power source 4B, a second control device 25B that controls the second power conversion device 5B, and a second alarm device 71B.

[0011] The DC grid 1 includes a positive DC grid 2 that is at the positive side potential of the DC grid 1 and a negative DC grid 3 that is at the negative side potential of the DC grid 1. A load 90 is electrically connected to the DC grid 1.

[0012] The first power conversion device 5A includes a first positive-side bus capacitor 10A, a first negative-side bus capacitor 11A, a first full-bridge inverter 12A, a first positive-side filter reactor 13A arranged at the output of the first full-bridge inverter 12A, a first negative-side filter reactor 14A arranged at the output of the first full-bridge inverter 12A, a first positive-side filter capacitor 15A connected to the first positive-side filter reactor 13A, a first negative-side filter capacitor 16A connected to the first negative-side filter reactor 14A, a first connection relay 17A, a first positive-side inverter output terminal 6A, a first negative-side inverter output terminal 7A, a first positive-side detection variable resistor 8A, and a first negative-side detection variable resistor 9A.

[0013] The first positive inverter output terminal 6A is electrically connected to the positive DC grid 2. The first negative inverter output terminal 7A is electrically connected to the negative DC grid 3.

[0014] The first positive DC bus 50A electrically connects the first independent power source 4A and the first power conversion device 5A. The first negative DC bus 51A electrically connects the first independent power source 4A and the first power conversion device 5A.

[0015] The first positive side detection variable resistor 8A connects the first positive side DC bus 50A and the first ground point 18A. The first negative side detection variable resistor 9A connects the first negative side DC bus 51A and the first ground point 18A. The first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A constitute a first ground fault detection circuit.

[0016] The first positive side filter reactor 13A and the first positive side filter capacitor 15A form an LC filter, and the first negative side filter reactor 14A and the first negative side filter capacitor 16A form an LC filter.

[0017] The first connection relay 17A electrically connects or disconnects the first power conversion device 5A to or from the positive side DC grid 2 and the negative side DC grid 3 .

[0018] A voltage 26 across the first positive-side detection variable resistor 8A (hereinafter referred to as the first positive-side detection variable resistor voltage (Vpd)) is the potential difference from the first ground point 18A to the first positive-side DC bus 50A. A voltage 27 across the first negative-side detection variable resistor 9A (hereinafter referred to as the first negative-side detection variable resistor voltage (Vnd)) is the potential difference from the first ground point 18A to the first negative-side DC bus 51A.

[0019] The first control device 25A receives the first positive side detection variable resistor voltage (Vpd) 26 and the first negative side detection variable resistor voltage (Vnd) 27, and controls the first positive side detection variable resistor 8A, the first negative side detection variable resistor 9A, the first full-bridge inverter 12A, and the first connection relay 17A according to the results.

[0020] The first control device 25A outputs a first positive side detection resistance value switching signal 29. The first positive side detection resistance value switching signal 29 is a signal for changing the resistance value of the first positive side detection variable resistor 8A.

[0021] The first control device 25A outputs a first negative detection resistance value switching signal 30. The first negative detection resistance value switching signal 30 is a signal for changing the resistance value of the first negative detection variable resistor 9A.

[0022] The first control device 25A outputs a first inverter operation signal 31. The first inverter operation signal 31 controls the operation or stop of the first full-bridge inverter 12A.

[0023] The first control device 25A outputs a first connection relay control signal 32. The first connection relay control signal 32 controls the connection or disconnection of the first connection relay 17A.

[0024] The first virtual neutral point 28A is a node that electrically connects the first positive side filter capacitor 15A and the first negative side filter capacitor 16A, and is also a node that electrically connects the first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A.

[0025] The first notification device 71A issues a notification of the result determined by the first control device 25A to the outside.

[0026] The first power conversion device 5A includes a first positive-side bus capacitor 10A, a first negative-side bus capacitor 11A, a first full-bridge inverter 12A, a first positive-side filter reactor 13A arranged at the output of the first full-bridge inverter 12A, a first negative-side filter reactor 14A arranged at the output of the first full-bridge inverter 12A, a first positive-side filter capacitor 15A connected to the first positive-side filter reactor 13A, a first negative-side filter capacitor 16A connected to the first negative-side filter reactor 14A, a first connection relay 17A, a first positive-side inverter output terminal 6A, a first negative-side inverter output terminal 7A, a first positive-side detection variable resistor 8A, and a first negative-side detection variable resistor 9A.

[0027] The first positive inverter output terminal 6A is electrically connected to the positive DC grid 2. The first negative inverter output terminal 7A is electrically connected to the negative DC grid 3.

[0028] The first positive DC bus 50A electrically connects the first independent power source 4A and the first power conversion device 5A. The first negative DC bus 51A electrically connects the first independent power source 4A and the first power conversion device 5A.

[0029] The first positive side detection variable resistor 8A connects the first positive side DC bus 50A and the first ground point 18A. The first negative side detection variable resistor 9A connects the first negative side DC bus 51A and the first ground point 18A. The first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A constitute a first ground fault detection circuit.

[0030] The first positive side filter reactor 13A and the first positive side filter capacitor 15A form an LC filter, and the first negative side filter reactor 14A and the first negative side filter capacitor 16A form an LC filter.

[0031] The first connection relay 17A electrically connects or disconnects the first power conversion device 5A to or from the positive side DC grid 2 and the negative side DC grid 3 .

[0032] A voltage 26 across the first positive-side detection variable resistor 8A (hereinafter referred to as the first positive-side detection variable resistor voltage (Vpd)) is the potential difference from the first ground point 18A to the first positive-side DC bus 50A. A voltage 27 across the first negative-side detection variable resistor 9A (hereinafter referred to as the first negative-side detection variable resistor voltage (Vnd)) is the potential difference from the first ground point 18A to the first negative-side DC bus 51A.

[0033] The second power conversion device 5B includes a second positive side bus capacitor 10B, a second negative side bus capacitor 11B, a second full bridge inverter 12B, a second positive side filter reactor 13B arranged at the output of the second full bridge inverter 12B, a second negative side filter reactor 14B arranged at the output of the second full bridge inverter 12B, a second positive side filter capacitor 15B connected to the second positive side filter reactor 13B, a second negative side filter capacitor 16B connected to the second negative side filter reactor 14B, a second connection relay 17B, a second positive side inverter output terminal 6B, a second negative side inverter output terminal 7B, a second positive side detection variable resistor 8B, and a second negative side detection variable resistor 9B.

[0034] The second positive inverter output terminal 6B is electrically connected to the positive DC grid 2. The second negative inverter output terminal 7B is electrically connected to the negative DC grid 3.

[0035] The second positive-side DC bus 50B electrically connects the second independent power source 4B and the second power conversion device 5B. The second negative-side DC bus 51B electrically connects the second independent power source 4B and the second power conversion device 5B.

[0036] The second positive side detection variable resistor 8B connects the second positive side DC bus 50B and the second ground point 18B. The second negative side detection variable resistor 9B connects the second negative side DC bus 51B and the second ground point 18B. The second positive side detection variable resistor 8B and the second negative side detection variable resistor 9B constitute a second ground fault detection circuit.

[0037] The second positive side filter reactor 13B and the second positive side filter capacitor 15B form an LC filter, and the second negative side filter reactor 14B and the second negative side filter capacitor 16B form an LC filter.

[0038] The second connection relay 17B electrically connects or disconnects the second power conversion device 5B to or from the positive side DC grid 2 and the negative side DC grid 3 .

[0039] A voltage 36 across the second positive-side detection variable resistor 8B (hereinafter referred to as the second positive-side detection variable resistor voltage (Vpd2)) is the potential difference from the second ground point 18B to the second positive-side DC bus 50B. A voltage 37 across the second negative-side detection variable resistor 9B (hereinafter referred to as the second negative-side detection variable resistor voltage (Vnd2)) is the potential difference from the second ground point 18B to the second negative-side DC bus 51B.

[0040] The second control device 25B receives the second positive side detection variable resistor voltage (Vpd2) 36 and the second negative side detection variable resistor voltage (Vnd2) 37, and controls the second positive side detection variable resistor 8B, the second negative side detection variable resistor 9B, the second full-bridge inverter 12B, and the second connection relay 17B according to the results.

[0041] The second control device 25B outputs a second positive side detection resistance value switching signal 39. The second positive side detection resistance value switching signal 39 is a signal for changing the resistance value of the second positive side detection variable resistor 8B.

[0042] The second control device 25B outputs a second negative detection resistance value switching signal 40. The second negative detection resistance value switching signal 40 is a signal for changing the resistance value of the second negative detection variable resistor 9B.

[0043] The second control device 25B outputs a second inverter operation signal 41. The second inverter operation signal 41 controls the operation or stop of the second full-bridge inverter 12B.

[0044] The second control device 25B outputs a second connection relay control signal 42. The second connection relay control signal 42 controls the connection or disconnection of the second connection relay 17B.

[0045] The second virtual neutral point 28B is a node that electrically connects the second positive side filter capacitor 15B and the second negative side filter capacitor 16B, and is also a node that electrically connects the second positive side detection variable resistor 8B and the second negative side detection variable resistor 9B.

[0046] The second notification device 71B issues a notification of the result determined by the second control device 25B to the outside.

[0047] 2 is a diagram showing the configuration of the first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A according to embodiment 1. The second positive side detection variable resistor 8B and the second negative side detection variable resistor 9B also have the same configuration.

[0048] The first positive-side detection variable resistor 8A includes a positive-side detection resistor 81A, a positive-side detection resistor 82A, and a positive-side resistance switch 83A. The positive-side detection resistor 81A has a standard resistance value. The positive-side detection resistor 82A has a resistance value higher than the standard resistance value. The positive-side resistance switch 83A switches between the positive-side detection resistor 81A with the standard resistance value and the positive-side detection resistor 82A with the high resistance value. The positive-side resistance switch 83A is controlled by a first positive-side detection resistance value switching signal 29, but may also be manually switched.

[0049] The first negative-side detection variable resistor 9A includes a negative-side detection resistor 84A, a negative-side detection resistor 85A, and a negative-side resistance switch 86A. The negative-side detection resistor 84A has a standard resistance value. The negative-side detection resistor 85A has a resistance value higher than the standard resistance value. The negative-side resistance switch 86A switches between the negative-side detection resistor 84A with the standard resistance value and the negative-side detection resistor 85A with the high resistance value. The negative-side resistance switch 86A is controlled by a first negative-side detection resistance value switching signal 30, but may also be manually switched.

[0050] The resistance value of only one of the first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A may be variable, and the resistance value of only one of the second positive side detection variable resistor 8B and the second negative side detection variable resistor 9B may be variable.

[0051] 3 is a diagram showing a detailed configuration of the first power conversion device 5A. The detailed configuration of the second power conversion device 5B is also similar.

[0052] A first positive-side detection variable resistor 8A and a first negative-side detection variable resistor 9A are connected in series between a first positive-side DC bus 50A and a first negative-side DC bus 51A. A first positive-side bus capacitor 10A and a first negative-side bus capacitor 11A are connected in series between the first positive-side DC bus 50A and a first negative-side DC bus 51A. A first full-bridge inverter 12A is connected to the first positive-side DC bus 50A and the first negative-side DC bus 51A.

[0053] The first full-bridge inverter 12A includes a first arm reg1 and a second arm reg2. The first power conversion device 5A outputs an output voltage (Vout) 62. The first control device 25A outputs a first drive signal 60 for switching the first arm reg1 of the first full-bridge inverter 12A. The first control device 25A outputs a second drive signal 61 for switching the second arm reg2 of the first full-bridge inverter 12A. The first arm reg1 outputs an output voltage to ground (Vp) 64 referenced to the voltage of the first ground point 18A. The output voltage to ground (Vp) 64 is supplied to the positive-side DC grid 2. The second arm reg2 outputs an output voltage to ground (Vn) 65 referenced to the voltage of the first ground point 18A. The output voltage to ground (Vn) 65 is supplied to the negative-side DC grid 3. The voltage 63 to ground of the first virtual neutral point 28A with respect to the voltage of the first ground point 18A is represented as Ve.

[0054] In order to check whether another power conversion device having a ground fault detection circuit, such as the second power conversion device 5B, is connected to the DC grid 1 in addition to the first power conversion device 5A, it is necessary to shift the values ​​of the output voltage to ground (Vp) 64 of the positive side DC grid 2 and the output voltage to ground (Vn) 65 of the negative side DC grid 3 in the positive or negative direction relative to the ground potential. In this case, the output voltage (Vout) 62, which is the potential difference between the output voltage to ground (Vp) 64 of the positive side DC grid 2 and the output voltage to ground (Vn) 65 of the negative side DC grid 3, is kept constant.

[0055] The first control device 25A includes a first voltage controller 104 for shifting the values ​​of the output voltage to ground (Vp) 64 and the output voltage to ground (Vn) 65 in a positive or negative direction relative to the ground potential while keeping the output voltage (Vout) 62 constant.

[0056] In order to check whether any other power conversion device having a ground fault detection circuit, such as the second power conversion device 5B, is connected to the DC grid 1 in addition to the first power conversion device 5A, the first voltage controller 104 changes the ratio Ratio_P of the magnitude of the output voltage to ground (Vp) 64 and the ratio Ratio_N of the magnitude of the output voltage to ground (Vn) 65 from an equal state (both 1.0) while maintaining Ratio_P+Ratio_N=2.0.

[0057] 4 is a schematic configuration diagram of the first voltage controller 104. The command voltage generator 103 includes adders AD1 and AD2, subtractors SB1 to SB3, a voltage controller CT1, multipliers ML1 to ML4, a voltage controller CT1, a positive side current controller CT2, and a negative side current controller CT3.

[0058] The subtractor SB1 outputs the difference DV1 between the target output voltage Vout* and the output voltage Vout to the voltage controller CT1.

[0059] The voltage controller CT1 receives the output of the subtractor SB1, performs proportional control, or proportional control and integral control, and outputs the target current value Iout* of the current Ip flowing through the first positive side filter reactor 13A and the current In flowing through the first negative side filter reactor 14A.

[0060] The subtractor SB2 outputs a difference DV2 between Iout* and Ip to the positive side current controller CT2.

[0061] The positive side current controller CT2 receives the output of the subtractor SB2 and performs proportional control or proportional control and integral control.

[0062] The multiplier ML1 outputs a value obtained by multiplying Iout* by −1 to the subtractor SB3. The subtractor SB3 outputs a difference DV3 between −Iout* and In to the negative side current controller CT3.

[0063] The negative side current controller CT3 receives the output of the subtractor SB3 and performs proportional control or proportional control and integral control.

[0064] Multiplier ML2 outputs a value obtained by multiplying a voltage 1 / 2 times Vout by a positive side voltage to ground command Ratio_P to adder AD1. Multiplier ML3 outputs a value obtained by multiplying a voltage 1 / 2 times Vout by a negative side voltage to ground command Ratio_N to multiplier ML4. Multiplier ML4 multiplies a value obtained by multiplying a voltage 1 / 2 times Vout by the negative side voltage to ground command Ratio_N by -1 to adder AD2.

[0065] Adder AD1 adds the output of positive side current controller CT2 and a value obtained by multiplying a voltage that is 1 / 2 times Vout by a positive side voltage to ground command Ratio_P. The added value is input to drive signal generator 101 as an output voltage command Vp for first positive side inverter output terminal 6A of first power conversion device 5A.

[0066] Adder AD2 adds the output of negative-side current controller CT3 and a value obtained by multiplying a voltage that is −½ times Vout by a negative-side voltage-to-ground command Ratio_N. The added value is input to drive signal generator 101 as output voltage command Vn for first negative-side inverter output terminal 7A of first power conversion device 5A.

[0067] The drive signal generator 101 generates a first drive signal (VregP) 60 to be input to the first power conversion device 5A by comparing the output voltage command Vp at the first positive side inverter output terminal 6A with the modulated wave. The drive signal generator 101 generates a second drive signal (VregN) 61 to be input to the first power conversion device 5A by comparing the output voltage command Vn at the first negative side inverter output terminal 7A with the modulated wave.

[0068] With the above control configuration, the output voltage to ground (Vp) 64 of the positive side DC grid 2 and the output voltage to ground (Vn) 65 of the negative side DC grid 3 can be changed by changing the ratio (voltage to ground ratio) between the positive side voltage to ground command Ratio_P and the negative side voltage to ground command Ratio_N while keeping the output voltage Vout constant.

[0069] The reference values ​​of Ratio_P and Ratio_N are set to 1, and the respective values ​​are changed so that their total value is 2. The reason for setting the total value to 2 is that when the reference values ​​of Ratio_P and Ratio_N are 1 (1x) and the output voltage to ground is not changed, Vout / 2 is added to the output of the positive side current controller CT2 and the output of the negative side current controller CT3. During normal operation and when a ground fault is detected, Ratio_P=1.0 and Ratio_N=1.0 are set.

[0070] The ground fault detection device confirmation operation is an operation of confirming through control whether or not another power conversion device, such as the second power conversion device 5B, having a ground fault detection circuit equivalent to the first ground fault detection circuit consisting of the first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A is connected to the DC grid 1 in addition to the first power conversion device 5A.

[0071] During the ground fault detection device confirmation operation, Ratio_P and Ratio_N are set to values ​​other than 1.0, while satisfying Ratio_P+Ratio_N=2.0. For example, Ratio_P=1.5 and Ratio_N=0.5 are set. This makes it possible to make the grid neutral point potential voltage to ground (Vp-n) 66, which will be described later, more variable. During ground fault detection, Ratio_P=1.0 and Ratio_N=1.0 are set because the grid neutral point potential voltage to ground (Vp-n) 66 changes significantly when a ground fault occurs.

[0072] The control and operation for checking the ground fault detection device will be described below. Fig. 5 is a diagram showing the output voltage to ground (Vp) 64 of the positive side DC grid 2, the output voltage to ground (Vn) 65 of the negative side DC grid 3, and the voltage to ground (Vp-n) 66 of the grid neutral point potential.

[0073] The grid neutral potential voltage to ground (Vp-n) 66 represents the difference between the absolute value of the output voltage to ground 64 of the positive DC grid 2 and the absolute value of the output voltage to ground 65 of the negative DC grid 3 .

[0074] By changing Ratio_P and Ratio_N from the reference value "1", the output voltage to ground 64 of the positive side DC grid 2 is changed, for example, from 190V to 208.5V, and the output voltage to ground 65 of the negative side DC grid 3 is changed from -190V to -171.5V, and the value of the voltage to ground 66 of the grid neutral point potential changes from 0V to 37V.

[0075] FIG. 6 is a diagram showing a current path from the first power conversion device 5A through the positive side DC grid 2 and the second ground point 18B and back to the first power conversion device 5A due to a change in the DC grid neutral point potential to ground (Vp-n) 66.

[0076] In this example, when the voltage to ground 66 of the grid neutral point potential is controlled from 0 V to 37 V, the potential difference between the first positive side DC bus 50A and the first grounding point 18A (first positive side detection variable resistance voltage (Vpd) 26) changes from 345 V to 326.5 V, and the potential difference between the second positive side DC bus 50B and the second grounding point 18B (second positive side detection variable resistance voltage (Vpd2) 36) changes from 345 V to 363.5 V. As a result, the potential difference between the first positive side DC bus 50A and the second positive side DC bus 50B becomes 37 V.

[0077] A current is generated in a path that runs from the second positive-side detection variable resistor 8B of the second power conversion device 5B through the second ground point 18B, the first ground point 18A, and the first positive-side detection variable resistor 8A.

[0078] FIG. 7 is a diagram showing a current path from the first power conversion device 5A through the negative side DC grid 3 and the second ground point 18B and back to the first power conversion device 5A due to a change in the DC grid neutral point potential to ground (Vp-n) 66.

[0079] In this example, when the voltage to ground 66 of the DC grid neutral point potential is controlled from 0 V to 37 V, the potential difference between the first negative side DC bus 51A and the first grounding point 18A (first negative side detection variable resistance voltage (Vnd) 27) changes from −345 V to −363.5 V, and the potential difference between the second negative side DC bus 51B and the second grounding point 18B (second negative side detection variable resistance voltage (Vnd2) 37) changes from −345 V to −326.5 V. As a result, the potential difference between the first negative side DC bus 51A and the second negative side DC bus 51B becomes 37 V.

[0080] A current is generated in a path that runs from the second negative side detection variable resistor 9B of the second power conversion device 5B through the second ground point 18B, the first ground point 18A, and the first negative side detection variable resistor 9A.

[0081] As described above, when the first power conversion device 5A and the second power conversion device 5B are electrically connected to the DC grid 1, when the value of the grid neutral point potential to ground 66 is changed, for example, from 0 V to 37 V, the first positive side detection variable resistor voltage 26 (Vpd) changes from 345 V to 326.5 V, and the first negative side detection variable resistor voltage 27 (Vnd) changes from −345 V to −363.5 V.

[0082] FIG. 8 is a diagram showing the first positive-side detection variable resistor voltage (Vpd) 26, the first negative-side detection variable resistor voltage (Vnd) 27, and the difference voltage (Vdif) 40 between their absolute values.

[0083] 8, the vertical axis represents voltage and the horizontal axis represents change over time. Assume that a first power converter 5A is electrically connected to the DC grid 1, and the first power converter 5A controls the value of the grid neutral point potential voltage 66 to be 37 V. When a second power converter 5B is then connected to the DC grid 1, the first positive side detection variable resistor voltage (Vpd) 26 changes from 345 V to 326.5 V, and the first negative side detection variable resistor voltage (Vnd) 27 changes from −345 V to −363.5 V.

[0084] The differential voltage (Vdif) 40 between the absolute value of Vpd and the absolute value of Vnd changes from 0 V to −37 V. If the value of the grid neutral point potential to ground 66 is controlled to −37 V, the differential voltage (Vdif) 40 changes from 0 V to 37 V.

[0085] The above values ​​are based on the calculation results under the conditions that the first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A are 60 kΩ, the outputs of the first independent power source 4A and the second independent power source 4B are 690 V, and the DC grid 1 is 380 V.

[0086] FIG. 9 is a diagram showing the number of power conversion devices electrically connected to the DC grid 1 and the differential voltage (Vdif) 40 detected by the first power conversion device 5A.

[0087] When only the first power conversion device 5A, i.e., one device, is connected to the DC grid 1, the differential voltage (Vdif) 40 becomes 0V.

[0088] When the first power conversion device 5A and the second power conversion device 5B are connected to the DC grid 1, the differential voltage (Vdif) 40 becomes −37 V. As the number of connected power conversion devices increases, the differential voltage (Vdif) 40 decreases. The threshold value 91 is a differential voltage threshold value (Vdif_TH) that indicates a level for determining whether or not the DC grid 1 includes other power conversion devices that have ground fault detection resistors equivalent to the first positive-side detection variable resistor 8A and the first negative-side detection variable resistor 9A.

[0089] In FIG. 9 , when the grid neutral potential is controlled so that the voltage to ground 66 is a positive value, if a differential voltage (Vdif) 40 lower than the differential voltage threshold (Vdif_TH) 91 is detected, two or more units are present in the DC grid 1.

[0090] In addition, when it is detected that two or more power conversion devices are connected to the DC grid 1, the first notification device 71A may notify the outside using an image, color, sound, or light.

[0091] As described above, when the differential voltage (Vdif) 40 deviates from (below) the differential voltage threshold (Vdif_TH) 91, the first control device 25A determines that another power conversion device having a ground fault detection circuit other than the first power conversion device 5A is connected to the DC grid 1.

[0092] Next, changing the resistance value of the detection resistor will be described. The ground fault detection device checking operation is performed using a positive side detection resistor 81A with a standard resistance value and a negative side detection resistor 84A with a standard resistance value.

[0093] When the differential voltage (Vdif) 40 deviates from the differential voltage threshold (Vdif_TH) 91, the first control device 25A controls the positive-side resistor switch 83A and the negative-side resistor switch 86A using the first positive-side detection resistance value switching signal 29 and the first negative-side detection resistance value switching signal 30 to switch between the high-resistance positive-side detection resistor 82A and the high-resistance negative-side detection resistor 84A. This is because, if a ground fault occurs in a case where there is a ground fault detection circuit of another power conversion device, current flows through the ground fault detection circuit of the first power conversion device and the ground fault detection circuit of the other device, resulting in two currents flowing at the fault point. In this embodiment, the resistance value of the ground fault detection circuit of the first power conversion device is changed to a high resistance to reduce the amount of current flowing at the fault point. If only one of the first positive-side detection variable resistor 8A and the first negative-side detection variable resistor 9A has a variable resistance value, only the variable resistance may be switched to a high resistance value.

[0094] In normal operation, the grid neutral point potential to ground (Vp-n) 66, which is the difference between the output ground voltage 64 of the positive side DC grid 2 and the output ground voltage 65 of the negative side DC grid 3, is set to 0V.

[0095] The operation when a ground fault occurs will be described below. Fig. 10 is a diagram for explaining the ground fault current when the resistance value of the ground fault detection circuit is set to a standard resistance value.

[0096] With the first power conversion device 5A and the second power conversion device 5B electrically connected to the DC grid 1, a ground fault occurs at the ground fault point 38 in Figure 1, and the ground fault resistance (insulation resistance) 39 at that time is assumed to be 2 kΩ.

[0097] The following describes the changes in the short-circuit current and the differential voltage (Vdif) 40 when the first positive side detection variable resistor 8A, the first negative side detection variable resistor 9A, the second positive side detection variable resistor 8B, and the second negative side detection variable resistor 9B have standard resistance values.

[0098] A ground fault current 44 flows from the ground fault point 38 through a ground fault resistor (insulation resistor) 39. In the first power conversion device 5A, the first positive side detection variable resistor voltage 26, the first negative side detection variable resistor voltage 27, the differential voltage (Vdif) 40, and the ground fault current 44 are shown. When a ground fault occurs, the differential voltage (Vdif) 40 changes significantly, and a ground fault current 44 of 11 mA flows.

[0099] FIG. 11 is a diagram for explaining a ground fault current when the resistance value of the ground fault detection circuit is set to a high resistance value.

[0100] In this embodiment, the first control device 25A confirms the existence of the second power conversion device 5B through the ground fault detection device confirmation operation, and operates the first power conversion device 5A with the first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A replaced with a high resistance positive side detection resistor 82A and a high resistance negative side detection resistor 85A. In this embodiment, the ground fault current 44 is reduced to 6.5 mA.

[0101] FIG. 12 is a diagram showing the magnitude of the ground fault current when the first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A are set to standard resistances and when they are set to high resistances.

[0102] 1, a ground fault occurs at ground fault point 38, and the value of ground fault resistor (insulation resistor) 39 is set to 2 kΩ. The standard resistance values ​​of first positive side detection variable resistor 8A and first negative side detection variable resistor 9A are set to 60 kΩ, and the high resistance value is set to 600 kΩ.

[0103] 12 shows the ground fault current 44 versus the number of power conversion devices connected to the DC grid 1 when the first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A are set to standard resistors. From the second device onwards, the ground fault current 44 exceeds 10 mA and increases to nearly 25 mA.

[0104] Furthermore, the graph shows the ground fault current 44 relative to the number of power conversion devices connected to the DC grid 1 when the first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A are switched to high resistance values ​​after the ground fault detection device confirmation operation. The ground fault current 44 is 10 mA or less even when the sixth device is connected. From the above, it can be expected that this embodiment will have the effect of suppressing the ground fault current 44.

[0105] 13 is a flowchart showing the control procedure of the first embodiment. In step S101, a ground fault detection device checking operation is performed.

[0106] 14 is a flowchart showing the procedure for the ground fault detection device confirmation operation. In step S201, the first control device 25A sets the resistances of the first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A to the standard resistance values ​​of the positive side detection resistor 81A and the negative side detection resistor 84A.

[0107] In step S202, the first power converter 5A and, if any, other power converters are connected to the DC grid 1.

[0108] In step S203, the first voltage controller 104 shifts Ratio_P and Ratio_N from the reference value (1.0) under the condition that their total value becomes 2. As a result, while the output voltage (Vout) 62 is maintained constant, the values ​​of the output voltage to ground (Vpd) 64 of the positive side DC grid 2 and the output voltage to ground (Vnd) 65 of the negative side DC grid 3 shift in the positive or negative direction relative to the ground potential.

[0109] In step S204, the first control device 25A samples the first positive side detection variable resistance voltage (Vpd) 26 and the first negative side detection variable resistance voltage (Vnd).

[0110] In step S205, the first control device 25A calculates the differential voltage Vdif: Vdif = |Vpd| - |Vnd| (1) In step S206, the first control device 25A determines whether the differential voltage Vdif is equal to or greater than the threshold value (Vdif_TH) 91. If the differential voltage Vdif is less than the threshold value (Vdif_TH) 91, the process proceeds to step S207. If the differential voltage Vdif is equal to or greater than the threshold value (Vdif_TH) 91, the process proceeds to step S208.

[0111] In step S207, the first control device 25A determines that there is a power conversion device (for example, the second power conversion device 5B) having a ground fault detection circuit connected to the DC grid 1 other than the first power conversion device 5A.

[0112] In step S208, the first control device 25A determines that there is no ground fault detection circuit connected to the DC grid 1 other than the first power conversion device 5A.

[0113] In step S209, the first voltage controller 104 returns Ratio_P and Ratio_N to the reference value (1.0).

[0114] In step S102 in FIG. 13 , if there is a power conversion device other than the first power conversion device 5A that has a ground fault detection circuit connected to the DC grid 1, the process proceeds to step S103, and if there is no power conversion device other than the first power conversion device 5A that has a ground fault detection circuit connected to the DC grid 1, the process proceeds to step S107.

[0115] In step S103, the first control device 25A sets the resistances of the first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A to a high resistance positive side detection resistor 82A and a high resistance negative side detection resistor 85A.

[0116] In step S104, the first control device 25A samples the first positive side detection variable resistance voltage (Vpd) 26 and the first negative side detection variable resistance voltage (Vnd).

[0117] In step S105, the first control device 25A calculates the differential voltage Vdif: Vdif = |Vpd| - |Vnd| (1) In step S106, if the absolute value of the differential voltage Vdif is equal to or less than a predetermined first threshold value (TH1) 69, the first control device 25A returns to step S104, and if the differential voltage Vdif exceeds the predetermined first threshold value (TH1) 69, the first control device 25A proceeds to step S107.

[0118] In step S107, the first control device 25A samples the first positive side detection variable resistance voltage (Vpd) 26 and the first negative side detection variable resistance voltage (Vnd).

[0119] In step S108, the first control device 25A calculates the differential voltage Vdif: Vdif = |Vpd| - |Vnd| (1) In step S109, if the differential voltage Vdif is equal to or less than a predetermined first threshold (TH1) 69, the first control device 25A returns to step S109, and if the differential voltage Vdif exceeds the predetermined first threshold (TH1) 69, the first control device 25A proceeds to step S112.

[0120] In step S110, the first control device 25A determines that a ground fault has occurred. In step S111, the first control device 25A stops the operation of the first power conversion device 5A. That is, the first control device 25A stops the first full-bridge inverter 12A using the first inverter operation signal 31, and controls the first connection relay 17A using the first connection relay control signal 32 to electrically parallel-off the first power conversion device 5A and the DC grid 1.

[0121] 15 is a diagram showing the configuration of the first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A in Modification 1 of Embodiment 1. The second positive side detection variable resistor 8B and the second negative side detection variable resistor 9B have the same configuration.

[0122] The first positive side detection variable resistor 8A of this modification differs from the first positive side detection variable resistor 8A of the first embodiment in that it includes a positive side resistance switch 87A instead of the positive side resistance switch 83A.

[0123] The positive-side resistor switch 87A connects or disconnects the high-resistance positive-side detection resistor 82A and the standard-resistance positive-side detection resistor 81A in parallel.

[0124] The first negative side detection variable resistor 9A of this modification differs from the first negative side detection variable resistor 9A of the first embodiment in that it includes a negative side resistance switch 88A instead of the negative side resistance switch 86A.

[0125] The negative-side resistor switch 88A connects or disconnects the high-resistance negative-side detection resistor 85A and the standard-resistance negative-side detection resistor 84A in parallel.

[0126] In the detection device confirmation operation, by closing the positive side resistance switch 87A, the resistance value of the first positive side detection variable resistor 8A is set to the resistance value obtained by connecting in parallel a positive side detection resistor 81A with a standard resistance value and a positive side detection resistor 82A with a high resistance value, and by closing the negative side resistance switch 88A, the resistance value of the first negative side detection variable resistor 9A is set to the resistance value obtained by connecting in parallel a negative side detection resistor 84A with a standard resistance value and a negative side detection resistor 85A with a high resistance value.

[0127] When the absolute value of the differential voltage (Vdif) 40 exceeds the differential voltage threshold (Vdif_TH) 91, in the ground fault detection operation, the positive side resistance switch 87A is opened to set the resistance value of the first positive side detection variable resistor 8A to the high resistance value positive side detection resistor 82A, and the negative side resistance switch 88A is opened to set the resistance value of the first negative side detection variable resistor 9A to the high resistance value negative side detection resistor 85A.

[0128] Modification 2 of Embodiment 1 Fig. 16 is a flowchart showing the control procedure of Modification 2 of Embodiment 1. The flowchart of Modification 2 of Embodiment 1 differs from the flowchart of Embodiment 1 in that the flowchart of Modification 2 of Embodiment 1 includes step S103A instead of step S103, and step S111A instead of step S111.

[0129] In step S103A, the first control device 25A sets the resistances of the first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A to a high resistance value for the positive side detection resistor 82A and the high resistance value for the negative side detection resistor 85A. The first control device 25A further transmits a control command to the second control device 25B instructing the second control device 25B to switch the resistance values ​​of the ground fault detection circuit to a high resistance value. Upon receiving the control command, the second control device 25B switches the resistances of the second positive side detection variable resistor 8B and the second negative side detection variable resistor 9B to a high resistance value. If the resistance value of only one of the second positive side detection variable resistor 8B and the second negative side detection variable resistor 9B is variable, the second control device 25B may switch only the variable resistance value to a high resistance value.

[0130] In step S111A, the first control device 25A stops the operation of the first power conversion device 5A. That is, the first control device 25A stops the first full-bridge inverter 12A using a first inverter operation signal 31, and controls the first connection relay 17A using a first connection relay control signal 32 to electrically parallel-off the first power conversion device 5A and the DC grid 1. The first control device 25A further transmits a ground fault detection signal 35 to the second control device 25B. The second control device 25B, having received the ground fault detection signal 35, stops the operation of the second power conversion device 5B. That is, the second control device 25B stops the second full-bridge inverter 12B using a second inverter operation signal 41, and controls the second connection relay 17B using a second connection relay control signal 42 to electrically parallel-off the second power conversion device 5B and the DC grid 1.

[0131] Modification 3 of Embodiment 1 As described above, Fig. 10 shows the differential voltage (Vdif) 40 when the ground fault detection circuit of the first power conversion device 5A is set to the standard resistance value, and Fig. 11 shows the differential voltage (Vdif) 40 when the ground fault detection circuit of the first power conversion device 5A is set to the high resistance value.

[0132] With the standard resistance value, the differential voltage (Vdif) 40 is −335 V. With the high resistance value, the differential voltage (Vdif) 40 is −354 V. As the impedance of the ground fault detection circuit increases, the change in the differential voltage (Vdif) 40 increases. This raises concerns about malfunction due to noise.

[0133] Therefore, when the first positive-side detection variable resistor 8A and the first negative-side detection variable resistor 9A are set to high resistance values, the ground fault detection function of the first power converter 5A is stopped. Of the power converters connected to the DC grid 1, the ground fault detection circuit of one power converter is set to the standard resistance value, and the differential voltage threshold (Vdif_TH) 91 in FIG. 9 is set. The ground fault detection circuit of the second power converter 5B is set to the standard resistance value, and the second power converter 5B detects a ground fault. When a ground fault is detected, the second power converter 5B outputs a ground fault detection signal 45. Upon receiving the ground fault detection signal 45, the first power converter 5A recognizes the ground fault, stops operation, and disconnects itself from the DC grid 1.

[0134] 17 is a flowchart showing the control procedure of Modification 3 of Embodiment 1. The flowchart of Modification 3 of Embodiment 1 differs from the flowchart of Embodiment 1 in that the flowchart of Modification 3 of Embodiment 1 includes steps S401 to S407 instead of steps S104 to S106.

[0135] In step S103, the first control device 25A sets the first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A to high resistance values.

[0136] In step S401, the first power conversion device 5A stops the ground fault detection function, and the first control device 25A notifies the second control device 25B that the ground fault detection has been stopped.

[0137] In step S402, the second control device 25B sets the second positive side detection variable resistor 8B and the second negative side detection variable resistor 9B to the standard resistance values.

[0138] In step S403, the second control device 25B samples the second positive side detection variable resistance voltage (Vpd) 26 and the second negative side detection variable resistance voltage (Vnd).

[0139] In step S404, the second control device 25B calculates the differential voltage Vdif: Vdif = |Vpd| - |Vnd| (1) In step S405, if the differential voltage Vdif is equal to or less than the predetermined first threshold value (TH1) 69, the second control device 25B returns to step S403, and if the differential voltage Vdif exceeds the predetermined first threshold value (TH1) 69, the second control device 25B proceeds to step S406.

[0140] In step S406, the second control device 25B determines that a ground fault has occurred. In step S407, the second control device 25B stops operation of the second power conversion device 5B. That is, the second control device 25B stops the second full-bridge inverter 12B using the second inverter operation signal 41, and controls the second connection relay 17B using the second connection relay control signal 42 to electrically parallel-off the second power conversion device 5B and the DC grid 1. The second control device 25B further transmits a ground fault detection signal 45 to the first control device 25A. Upon receiving the ground fault detection signal 45, the first control device 25A stops operation of the first power conversion device 5A. That is, the first control device 25A stops the first full-bridge inverter 12A using a first inverter operation signal 31, and controls the first connection relay 17A using a first connection relay control signal 32 to electrically disconnect the first power conversion device 5A from the DC grid 1.

[0141] Embodiment 2. In Embodiment 1, when a power conversion device other than the first power conversion device 5A is connected to the DC grid 1, the first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A are set to high resistance values. When these resistors are set to high resistance values, the change in the differential voltage Vdif when a ground fault occurs is small, making the system more susceptible to noise. In this embodiment, in a first stage, the first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A are set to high resistance values ​​to determine the possibility of a ground fault. Then, in a second stage, the first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A are set to standard resistance values ​​to reduce the influence of noise and more accurately detect the occurrence of a ground fault.

[0142] 18 is a flowchart showing the control procedure of embodiment 2. The flowchart of embodiment 2 differs from the flowchart of embodiment 1 in that the flowchart of embodiment 2 includes steps S301 to S304.

[0143] If the answer is YES in step S106, it is determined that there is a possibility that a ground fault has occurred, and the process proceeds to step S301.

[0144] Through steps S301 to S304, it is determined whether a ground fault has occurred at the ground fault point 38 or whether a noise malfunction has occurred.

[0145] In step S301, the first control device 25A changes the resistance of the first positive side detection variable resistor 8A to a positive side detection resistor 81A having a standard resistance value, and changes the resistance of the first negative side detection variable resistor 9A to a negative side detection resistor 84A having a standard resistance value.

[0146] In step S302, the first control device 25A samples the first positive side detection variable resistance voltage (Vpd) 26 and the first negative side detection variable resistance voltage (Vnd).

[0147] In step S303, the first control device 25A calculates the differential voltage Vdif: Vdif = |Vpd| - |Vnd| (1) In step S304, if the absolute value of the differential voltage Vdif is equal to or less than a predetermined second threshold (TH2) 70, the first control device 25A returns to step S302, but if the absolute value of the differential voltage Vdif exceeds the predetermined second threshold (TH2) 70, the first control device 25A proceeds to step S110. The second threshold (TH2) 70 may be the same as or different from the first threshold (TH1) 69.

[0148] FIG. 19 is a diagram illustrating the operation when the absolute value of the differential voltage (Vdif) is equal to or less than the second threshold value (TH2) 70.

[0149] A first positive side detection variable resistor voltage (Vpd) 26, a first negative side detection variable resistor voltage (Vnd) 27, a differential voltage (Vdif) 40, and a ground fault current 44 are shown.

[0150] 19, it is assumed that spike noise is applied at point A. After point A, the absolute value of the differential voltage (Vdif) 40 exceeds the first threshold value (TH1) 69, so the resistances of the first positive-side detection variable resistor 8A and the first negative-side detection variable resistor 9A are changed to a positive-side detection resistor 81A with a standard resistance value and a negative-side detection resistor 84A with a standard resistance value.

[0151] Thereafter, at point B, the absolute value of the differential voltage (Vdif) 40 is equal to or less than the second threshold (TH2) 70, and therefore a malfunction is determined. The resistances of the first positive-side detection variable resistor 8A and the first negative-side detection variable resistor 9A are changed to a high-resistance positive-side detection resistor 82A and a high-resistance negative-side detection resistor 85A, and normal operation is resumed.

[0152] FIG. 20 is a diagram illustrating the operation when the absolute value of the differential voltage (Vdif) 40 exceeds the second threshold (TH2) 70.

[0153] A first positive side detection variable resistor voltage (Vpd) 26, a first negative side detection variable resistor voltage (Vnd) 27, a differential voltage (Vdif) 40, and a ground fault current 44 are shown.

[0154] 20, a ground fault occurs at point A. After point A, the differential voltage (Vdif) 40 exceeds the first threshold value (TH1) 69, so the resistances of the first positive-side detection variable resistor 8A and the first negative-side detection variable resistor 9A are changed to a positive-side detection resistor 81A with a standard resistance value and a negative-side detection resistor 84A with a standard resistance value.

[0155] Thereafter, the absolute value of the differential voltage (Vdif) 40 exceeds the second threshold value (TH2) 70 at point B, so it is determined that a ground fault has occurred, and processing to stop operation is performed.

[0156] In this embodiment, the first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A are set to high resistance values ​​to determine the possibility of a ground fault, and if there is a possibility of a ground fault, the first positive side detection variable resistor 8A and the first negative side detection variable resistor 9A are set to standard resistance values ​​to detect the ground fault, thereby making it possible to detect a ground fault without being affected by noise.

[0157] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present disclosure is defined by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims.

[0158] 1 DC grid, 2 positive side DC grid, 3 negative side DC grid, 4A first independent power source, 4B second independent power source, 5A first power conversion device, 5B second power conversion device, 6A first positive side inverter output terminal, 6B second positive side inverter output terminal, 7A first negative side inverter output terminal, 7B second negative side inverter output terminal, 8A first positive side detection variable resistor, 8B second positive side detection variable resistor, 9A first negative side detection variable resistor, 9B second negative side detection variable resistor, 10A first positive side bus capacitor, 10B second positive side bus capacitor, 11A first negative side bus capacitor, 11B second negative side bus capacitor, 12A first full bridge inverter, 12B second full bridge inverter, 13A first positive side filter reactor, 13B Second positive side filter reactor, 14A First negative side filter reactor, 14B Second negative side filter reactor, 15A First positive side filter capacitor, 15B Second positive side filter capacitor, 16A First negative side filter capacitor, 16B Second negative side filter capacitor, 17A First connecting relay, 17B Second connecting relay, 18A First grounding point, 18B Second grounding point, 25A First control device, 25B Second control device, 26 First positive side detection variable resistance voltage, 27 First negative side detection variable resistance voltage, 28A First virtual neutral point, 28B Second virtual neutral point, 29 First positive side detection resistance value switching signal, 30 First negative side detection resistance value switching signal, 31 First inverter operation signal, 32 First connecting relay control signal, 35, 45 Ground fault detection signal, 38 Ground fault point, 39 Second positive side detection resistance value switching signal, 40 Second negative side detection resistance value switching signal, 41 Second inverter operation signal, 42 Second connection relay control signal, 44 Ground fault current, 50A First positive side DC bus, 50B Second positive side DC bus, 51A First negative side DC bus, 51B Second negative side DC bus, 60 First drive signal, 61 Second drive signal, 63, 64, 65, 66 Ground voltage, 71A First alarm device, 71B Second alarm device, 81A, 82A Positive side detection resistor, 83A, 87A Positive side resistor switch, 84A, 85A Negative side detection resistor, 86A,88A negative side resistance switch, 90 load, 91 threshold, 101 drive signal generator, 103 command voltage generator, 104 first voltage controller, AD1, AD2 adder, CT1 voltage controller, CT2, CT3 current controller, DV1, DV2, DV3 difference, Iout target current value, Ip, In current, ML1, ML2, ML3, ML4 multiplier, Ratio_P, Ratio_N ground voltage command, SB1, SB2, SB3 subtractor, Vdif difference voltage, Vout output voltage, Vout* target output voltage, Vp, Vn output voltage command, reg1 first arm, reg2 second arm.

Claims

1. A full-bridge inverter connected to the positive DC bus and the negative DC bus, A positive-side detection resistor is placed between the positive-side DC bus and the ground point, A negative detection resistor is placed between the negative DC bus and the ground point, The system includes a control device that controls the connection between the output of the full-bridge inverter and the DC grid, The control device is a power converter that determines whether or not another power converter is connected to the DC grid based on the difference voltage between the absolute value of the voltage across the positive detection resistor and the absolute value of the voltage across the negative detection resistor.

2. The full-bridge inverter has a first arm and a second arm, The power converter according to claim 1, wherein the control device determines whether or not another power converter is connected to the DC grid by detecting the voltage of the positive detection resistor and the voltage of the negative detection resistor, while keeping the output voltage of the power converter constant and changing the output voltage of the first arm and the output voltage of the second arm.

3. The power conversion device according to claim 2, wherein the control device changes the ratio of the magnitudes of the output voltages of the first arm to ground and the ratio of the magnitudes of the output voltages of the second arm from an equal state while keeping the sum of the ratio of the magnitudes of the magnitudes of the output voltages of the first arm to ground and the ratio of the magnitudes of the output voltages of the second arm constant.

4. The power converter according to claim 1, wherein the control device determines that another power converter is connected to the DC grid when the differential voltage is less than a first threshold.

5. At least one of the resistance values ​​of the positive detection resistor and the negative detection resistor is variable. The power converter according to claim 1, wherein the control device sets at least one of the positive detection resistor and the negative detection resistor, whose resistance value is variable, to a standard resistance value, and detects the voltage of the positive detection resistor and the voltage of the negative detection resistor to determine whether or not another power converter is connected to the DC grid.

6. The power converter according to claim 5, wherein the control device determines that another power converter is connected to the DC grid, sets at least one of the positive detection resistors and the negative detection resistors, whose resistance value is variable, to a resistance value higher than the standard resistance value, and determines that a ground fault has occurred when the absolute value of the difference voltage between the absolute value of the voltage across the positive detection resistor and the absolute value of the voltage across the negative detection resistor exceeds a second threshold.

7. The power converter according to claim 5, wherein the control device determines that another power converter is connected to the DC grid, sets at least one of the positive detection resistors and negative detection resistors, whose resistance value is variable, to a resistance value higher than the standard resistance value, sets the positive detection resistor and the negative detection resistor to the standard resistance value when the absolute value of the difference voltage between the absolute value of the voltage of the positive detection resistor and the absolute value of the voltage of the negative detection resistor exceeds a second threshold, and determines that a ground fault has occurred when the absolute value of the difference voltage between the absolute value of the voltage of the positive detection resistor and the absolute value of the voltage of the negative detection resistor exceeds a third threshold.

8. The power converter according to claim 6 or 7, wherein the control device disconnects the connection between the output of the full-bridge inverter and the DC grid when it determines that the ground fault has occurred.

9. The power converter according to claim 2, wherein the control device, when determining whether or not another power converter is connected to the DC grid, deviates the ratio of the magnitudes of the output voltages of the first arm to ground and the ratio of the magnitudes of the output voltages of the second arm from equal, and when performing ground fault detection, equalizes the ratio of the magnitudes of the output voltages of the first arm to ground and the ratio of the magnitudes of the output voltages of the second arm.

10. A plurality of power conversion devices according to any one of claims 1 to 7 are provided, In the first power converter among the multiple power converters, if it is determined that the second power converter is connected to the DC grid, the control device of the first power converter transmits a command to the control device of the second power converter. The control device of the second power converter is a power distribution system that, upon receiving the command, switches the resistance value of at least one of the positive side detection resistors and the negative side detection resistors, the resistance value of which is variable.

11. The power distribution system according to claim 10, wherein the control device of the first power converter sets at least one of the positive side detection resistors and the negative side detection resistors, whose resistance value is variable, to a resistance value higher than the standard resistance value and stops ground fault detection, and the control device of the second power converter sets the positive side detection resistor and the negative side detection resistor to the standard resistance value and performs ground fault detection.