Power distribution system

Abstract

This power distribution system comprises: a power converter (4) that converts the voltage of a primary-side DC bus (1) into AC or DC and outputs the AC or DC to power feeding lines (9); and a voltage controller (104) that controls the power converter (4) so as to change the output voltage to the ground of each line of the power feeding lines (9) while keeping the output voltage of the power converter (4) constant.

Description

Power Distribution System

[0001] The present disclosure relates to electrical power distribution systems.

[0002] In recent years, DC power distribution systems that supply DC power to DC equipment in buildings, factories, train stations, and the like have been attracting attention.

[0003] In a DC power distribution system, the positive and negative poles of a DC bus are grounded via resistors with the same resistance to equalize the magnitude of the ground voltages of the positive and negative poles of the DC bus. Furthermore, a method of detecting ground faults is used that measures the voltage across the resistor and uses the difference in the voltage across the positive and negative ground resistors (the voltage between the positive pole and the ground and the voltage between the negative pole and the ground) when the insulation resistance to the ground (hereinafter simply referred to as insulation resistance) drops due to a ground fault or other reason.

[0004] However, this detection method faces the problem of difficulty in detecting a voltage difference between the positive and negative grounding resistances when both the positive and negative electrodes have a low insulation resistance. If the positive and negative grounding resistances were to be set to different values ​​to detect this anomaly, the voltages to ground between the positive and negative electrodes would be unbalanced, resulting in a continuous application of a higher voltage to one pole of equipment connected to the DC power distribution system than to the other pole. This would continuously stress the equipment and potentially cause deterioration due to migration. While reducing the difference between the positive and negative grounding resistances would reduce the stress on connected equipment caused by the voltage imbalance to ground, it would be difficult to set a threshold value while avoiding the effects of noise and measurement errors.

[0005] In Patent Document 1, in addition to the ground resistor, multiple detection resistors and switches are provided, and the resistance values ​​are switched using the switches to individually detect decreases in insulation resistance on the positive electrode side and decreases in insulation resistance on the negative electrode side.

[0006] JP 2011-015502 A

[0007] However, the method of Patent Document 1 requires a plurality of resistors and switches, which increases the scale and cost due to the increased hardware.

[0008] An object of the present disclosure is to provide a power distribution system that can detect a decrease in insulation resistance on the positive electrode side and a decrease in insulation resistance on the negative electrode side without adding hardware.

[0009] The power distribution system of the present disclosure includes a power converter that converts the voltage of a primary DC bus into AC or DC and outputs the converted voltage to a power feeder, and a voltage controller that controls the power converter to change the output voltage to ground of each line of the power feeder while keeping the output voltage of the power converter constant.

[0010] According to the present disclosure, it is possible to provide a power distribution system that can detect a decrease in insulation resistance on the positive electrode side and a decrease in insulation resistance on the negative electrode side without adding hardware.

[0011] 1 is a schematic configuration diagram of a DC power distribution system according to a first embodiment. FIG. 1 is a schematic configuration diagram of an insulation resistance decrease determiner 102. FIG. 2 is a diagram showing an example of changes in Vpe, Vne, and Ie when a ground fault abnormality occurs in which the insulation resistance decreases to approximately 0 ohms. FIG. 2 is a schematic configuration diagram of a non-isolated step-down DC / DC converter. FIG. 3 is a schematic configuration diagram of a voltage controller 104. FIG. 4 is a diagram showing Vse and Vte when the voltage-to-ground ratio is changed. FIG. 5 is a flowchart showing a procedure for detecting an insulation resistance decrease using changes in the output voltage-to-ground in the first embodiment. FIG. 6 is a diagram showing operating waveforms in a normal state and when the insulation resistance of both the S pole 7 and the T pole 8 decreases when the voltage-to-ground ratio is changed. FIG. 7 is a diagram showing operating waveforms when the voltage-to-ground is changed when the insulation resistance of the S pole 7 decreases. FIG. 8 is a schematic configuration diagram of a DC power distribution system according to a second embodiment. FIG. 9 is a diagram showing operating waveforms when the voltage-to-ground is changed in a normal state and when the insulation resistance of both the S pole 7 and the T pole 8 decreases. 10 is a flowchart showing a procedure for detecting a decrease in insulation resistance in embodiment 3. FIG. 11 is a diagram showing operating waveforms in a normal state and when the insulation resistance of both the south pole 7 and the temperature pole 8 decreases when the voltage to ground is changed. FIG. 12 is a diagram showing operating waveforms when the voltage to ground is changed when the insulation resistance of the south pole 7 decreases. FIG. 13 is a diagram showing operating waveforms when the voltage to ground is changed when the insulation resistance of the temperature pole 8 decreases.

[0012] Hereinafter, embodiments will be described with reference to the drawings. Embodiment 1 (Explanation of Overall Configuration) Fig. 1 is a schematic configuration diagram of a DC power distribution system according to embodiment 1. The DC power distribution system includes a DC bus 1, a power feeder 9, a power converter 4, a voltage controller 104, a voltage-to-ground change command unit 113, a detector 5, and an insulation resistance decrease determiner 102.

[0013] The power feeder 9 has a south pole 7 and a tenth pole 8. The south pole 7 and the tenth pole 8 are connected to one or more DC devices (not shown) that operate on DC power. Examples of DC devices include lighting equipment, air conditioners, elevators, and other appliances; power storage devices such as lithium-ion batteries and supercapacitors; solar power generation devices; DC / DC converters that convert DC voltages to other voltages; and DC / AC inverters. There is no limit to the number of DC devices that can be connected, and multiple load facilities can be connected as long as the power supply capacity of the power feeder 9 is within the capacity. Furthermore, the distance to these DC devices can be extended by taking into consideration the impedance of the power feeder. For example, the power feeder 9 may be connected to DC devices installed in a separate building or other location away from the power converter 4.

[0014] The DC bus 1 has a positive electrode 2 and a negative electrode 3. Similar to the power supply line 9, the DC bus 1 is connected to one or more DC devices (not shown) that operate by receiving DC power.

[0015] The power converter 4 converts the DC power of the DC bus 1 into DC power having a voltage that is the same as or different from the voltage of the DC bus 1, and outputs the converted DC power from an output terminal 6 to a power feeder 9. The power converter 4 includes a plurality of switching elements and the output terminal 6.

[0016] The multiple switching elements are operated by drive signals from a voltage controller 104. The output terminal 6 has an output terminal S connected to the south pole 7 and an output terminal T connected to the east pole 8. The voltage between the output terminal S and the output terminal T is defined as an output voltage Vout. The voltage to ground at the output terminal S is defined as an output voltage to ground Vse. The voltage to ground at the output terminal T is defined as an output voltage to ground Vte.

[0017] The voltage controller 104 includes a command voltage generator 101 and a drive signal generator 103. The command voltage generator 101 generates output voltage commands Vs and Vt based on an output voltage target value.

[0018] The drive signal generator 103 outputs a drive signal VregS based on a comparison between the output voltage command Vs and a modulated wave such as a triangular wave. The drive signal generator 103 outputs a drive signal VregT based on a comparison between the output voltage command Vt and a modulated wave such as a triangular wave. The drive signals VregS and VregT are input to the power converter 4.

[0019] (Explanation of ground fault detection) In a DC power distribution system, in order to prevent electric shock accidents due to leakage current that flows as the insulation resistance value decreases, it is necessary to detect a decrease in the insulation resistance value of the positive pole 2 and negative pole 3 of the DC bus 1 and a decrease in the insulation resistance value of the S pole 7 and T pole 8 of the power supply line 9.

[0020] A detector 5 is provided to detect a decrease in the insulation resistance value. The detector 5 includes a detection resistor RP connected between the positive electrode 2 and the ground E, and a detection resistor RN connected between the negative electrode 3 and the ground E.

[0021] The resistance values ​​of the detection resistors RP and RN are set to be the same to equalize the magnitude of the voltages to ground of the positive pole 2 and negative pole 3 of the DC bus 1, but different resistance values ​​may be selected. However, if different resistance values ​​are selected, the voltages to ground will be unbalanced, which may cause continuous stress on the equipment and lead to deterioration due to migration or the like.

[0022] Increasing the resistance values ​​of the detection resistors RP and RN can limit the leakage current that flows when the insulation resistance of the DC bus 1 and the power feeder 9 decreases. Therefore, the resistance values ​​of the detection resistors RP and RN may be set to several tens of kilohms or more so as to suppress the leakage current to a level that does not affect the human body even in the event of a ground fault in which the insulation resistance values ​​of the DC bus 1 and the power feeder 9 become zero.

[0023] The voltage Vpe across the detection resistor RP, the current Ipe flowing through the detection resistor RP, the voltage Vne across the detection resistor RN, and the current Ine flowing through the detection resistor RN are measured by a measuring device such as a sensor or a transformer. The measured values ​​are used to detect a decrease in insulation resistance. One end of the detection resistor RP and one end of the detection resistor RN may be connected at a connection point D, which may be connected to ground E. To directly measure the current Ie flowing between connection point D and ground E, a resistor E and a measuring device may be installed between connection point D and ground E.

[0024] 2 is a schematic diagram of the insulation resistance degradation determiner 102. The insulation resistance degradation determiner 102 includes a processor 111 and a determiner 112.

[0025] The processor 111 processes the voltage or current value measured by the detector 5 and outputs the detected value. The processor 111 performs filtering to remove noise, averaging to take the average over a certain period of time, or calculation of the effective value.

[0026] The determiner 112 determines whether the insulation resistance has decreased based on the detection value output from the processor 111. The determiner 112 determines that the insulation resistance has decreased when the detection value exceeds a preset threshold value.

[0027] 3 shows an example of changes in Vpe, Vne, and Ie when a ground fault occurs, causing the insulation resistance to drop to approximately 0 ohms. Fig. 3 shows an example in which the detection resistors RP and RN have the same resistance value.

[0028] If a ground fault occurs in any one of the positive electrode 2, negative electrode 3, south electrode 7, and trend electrode 8, Vpe, Vne, and Ie will each change from their normal values, making it possible to detect an abnormality from any of the measured values.

[0029] However, in cases where the insulation resistance is simply reduced, rather than when the insulation resistance becomes 0 ohms as in a ground fault, it is difficult to distinguish between an abnormality in either the positive electrode 2 or the S electrode 7, or between an abnormality in either the negative electrode 3 or the T electrode 8. Furthermore, when the insulation resistances of both the positive electrode 2 and the negative electrode 3 are reduced, or when the insulation resistances of both the S electrode 7 and the T electrode 8 are reduced, Vpe, Vne, and Ie do not change, and therefore cannot be detected.

[0030] To deal with these, the voltage-to-ground change command unit 113 commands the command voltage generator 101 to change the output voltage-to-ground of the power converter 4. More specific steps will be described later.

[0031] (Configuration of Power Converter) The power converter 4 in this embodiment is configured as a non-insulated step-down DC / DC converter capable of bidirectional power interchange. Fig. 4 is a schematic configuration diagram of a non-insulated step-down DC / DC converter.

[0032] The power converter 4 includes an input capacitor section 23 , an S-reg 20 , a T-reg 21 , and an output filter section 24 .

[0033] The input capacitor section 23 is connected to the DC bus 1 to store energy and smooth the voltage. The input capacitor section 23 includes a capacitor C1 and a capacitor C2 connected in series. The capacitors C1 and C2 are connected to the virtual neutral point 22.

[0034] The S-reg 20 and the T-reg 21 are connected in parallel with the input capacitor unit 23. The S-reg 20 includes two switching elements SW1 and SW2 connected in series. For example, the on / off state of the switching element SW1 is controlled in accordance with a drive signal VregS, and the on / off state of the switching element SW2 is controlled in accordance with an inverted signal of the drive signal VregS.

[0035] The T-reg 21 includes two switching elements SW3 and SW4 connected in series. For example, the switching element SW3 is controlled to be turned on or off in accordance with a drive signal VregT, and the switching element SW4 is controlled to be turned on or off in accordance with an inverted signal of the drive signal VregT.

[0036] The output filter section 24 is connected to a midpoint 2S of the two switching elements of the S-leg 20 and a midpoint 2T of the two switching elements of the T-leg 21 .

[0037] The output filter unit 24 includes an output reactor Ls, an output reactor Lt, an output capacitor Cs, and an output capacitor Ct.

[0038] The output reactor Ls has a first end connected to the midpoint 2S and a second end connected to the south pole 7 of the power feeder 9. The output reactor Lt has a first end connected to the midpoint 2T and a second end connected to the treasure pole 8 of the power feeder 9.

[0039] The output capacitor Cs has a first end connected to the south pole 7 and a second end connected to the virtual neutral point 22. The output capacitor Ct has a first end connected to the east pole 8 and a second end connected to the virtual neutral point 22.

[0040] The purpose of connecting the output capacitor Cs and the output capacitor Ct to the input side in this manner is to determine the reference potential of the output of the power converter 4. Because the DC bus 1, which is the input of the power converter 4, is fixed by a resistor connected to the detector 5, the ground potential of the positive pole 2 and the negative pole 3 is fixed. Therefore, by connecting the output capacitor Cs and the output capacitor Ct to the positive pole 2 or the negative pole 3 of the DC bus 1, rather than to the virtual neutral point 22, the reference potential of the voltage output from the power converter 4 to the feeder 9 can be determined. In this embodiment, the output capacitor Cs and the output capacitor Ct are configured to be connected to the virtual neutral point 22 so that the voltages applied to the output capacitor Cs and the output capacitor Ct are uniform during normal operation.

[0041] With the above configuration, the power converter 4 can vary the voltages to ground of the south pole 7 and the north pole 8 of the power feeder 9 .

[0042] In the above description, the power converter 4 is described as being a so-called full-bridge step-down DC / DC converter having two legs, but other circuit configurations may be used as long as the voltage of the power supply line to ground can be changed as desired, such as a step-up DC / DC converter or a DC / AC inverter.

[0043] (Configuration of Power Converter) FIG. 5 is a schematic configuration diagram of the voltage controller 104. As shown in FIG.

[0044] The command voltage generator 101 includes adders AD1 and AD2, subtractors SB1 to SB3, a voltage controller 31, multipliers ML1 to ML4, a voltage controller 31, an S-side current controller 32, and a T-side current controller 33.

[0045] The subtractor SB1 outputs the difference DV1 between the target output voltage Vout* and the output voltage Vout to the voltage controller 31.

[0046] The voltage controller 31 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 Is flowing through the output reactor Ls and the current It flowing through the output reactor Lt.

[0047] The subtractor SB2 outputs the difference DV2 between Iout* and Is to the S-side current controller 32.

[0048] The S-side current controller 32 receives the output of the subtractor SB2 and performs proportional control or proportional control and integral control.

[0049] The multiplier ML1 outputs the value obtained by multiplying Iout* by −1 to the subtractor SB3. The subtractor SB3 outputs the difference DV3 between −Iout* and It to the T-side current controller 33.

[0050] The T-side current controller 33 receives the output of the subtractor SB3 and executes proportional control or proportional control and integral control.

[0051] Multiplier ML2 multiplies a voltage 1 / 2 times Vout by the S-pole voltage to ground command Ratio_S and outputs the result to adder AD1. Multiplier ML3 multiplies a voltage 1 / 2 times Vout by the T-pole voltage to ground command Ratio_T and outputs the result to multiplier ML4. Multiplier ML4 multiplies a voltage 1 / 2 times Vout by the T-pole voltage to ground command Ratio_T by -1 and outputs the result to adder AD2.

[0052] The adder AD1 adds the output of the S-side current controller 32 and a value obtained by multiplying a voltage half that of Vout by the S-pole voltage-to-ground command Ratio_S. The sum is input to the drive signal generator 103 as the output voltage command Vs for the output terminal S of the power converter 4.

[0053] The adder AD2 adds the output of the T-side current controller 33 and a value obtained by multiplying a voltage that is −½ times Vout by the T-pole voltage-to-ground command Ratio_T. The added value is input to the drive signal generator 103 as the output voltage command Vt for the output terminal T of the power converter 4.

[0054] The drive signal generator 103 generates a drive signal VregS to be input to the power converter 4 by comparing the output voltage command Vs at the output terminal S with the modulated wave. The drive signal generator 103 generates a drive signal VregT to be input to the power converter 4 by comparing the output voltage command Vt at the output terminal T with the modulated wave.

[0055] Fig. 6 is a diagram showing Vse and Vte when the voltage-to-ground ratio is changed. With the above control configuration, as shown in Fig. 6, the output voltage Vout is kept constant, and the ratio (voltage-to-ground ratio) between the south pole voltage-to-ground command Ratio_S and the east pole voltage-to-ground command Ratio_T is changed, thereby making it possible to change the output voltages-to-ground Vse and Vte of the power converter 4.

[0056] The reference values ​​of Ratio_S and Ratio_T 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_S and Ratio_T are 1 (1x) and the voltage to ground is not changed, Vout / 2 is added to the output of S-side current controller 32 and −Vout / 2 is added to the output of T-side current controller 33. For example, when Ratio_S>Ratio_T, it is possible to set Ratio_S=1.5 and Ratio_T=0.5.

[0057] (Explanation of Procedure for Detecting Insulation Resistance Decrease) FIG. 7 is a flowchart showing a procedure for detecting insulation resistance decrease using a change in output voltage to ground in the first embodiment.

[0058] In step S1, the processor 111 receives Vpe and Vne measured by the detector 5 and calculates Vdiff = |Vpe| - |Vne|. As described above, the decrease in insulation resistance can be determined using any one of the measured values ​​of Vpe, Vne, and Ie. However, in this description, we use a method using Vpe and Vne, which does not require additional circuitry to detect Ie and allows for easier setting of a threshold for the decrease in insulation resistance. This is because the amount of change in insulation resistance is greater than when using either Vpe or Vne alone, making it more resistant to noise and errors and easier to set the threshold. The determiner 112 compares Vdiff with a first threshold Vth1 to determine whether the insulation resistance of any one of the positive electrode 2, negative electrode 3, south electrode 7, and south electrode 8 has decreased. If the insulation resistance of any one of the positive electrode 2, negative electrode 3, south electrode 7, and south electrode 8 has decreased, Vdiff is greater than or equal to Vth1, and processing proceeds to step S6 to identify the location of the decrease. When the insulation resistance of both the positive electrode 2 and the negative electrode 3 has decreased (a), when the insulation resistance of both the south electrode 7 and the temperature electrode 8 has decreased (b), or when there is no decrease in insulation resistance and the system is normal (c), Vdiff<Vth1, and processing proceeds to step S2. Referring to FIG. 3 , in cases (a), (b), and (c), Vdiff=0 and Vdiff<Vth1. The threshold value Vth1 may be set to, for example, a voltage obtained by multiplying a current value that affects the human body by the value of the detection resistor. Alternatively, the value of Vth1 may be changed for each time period by setting a time period.

[0059] In step S2, the voltage to ground change commander 113 issues a command to change the voltage to ground to the voltage controller 104. The south pole voltage to ground command Ratio_S and the east pole voltage to ground command Ratio_T are both normally set to 1. For example, in step S2, the voltage to ground change commander 113 changes Ratio_S to 1.5 and Ratio_T to 0.5. The relationship between Ratio_S and Ratio_T may be set to Ratio_S > Ratio_T or Ratio_S < Ratio_T, provided that the condition Ratio_S + Ratio_T = 2 is satisfied.

[0060] In step S3, the processor 111 recalculates Vdiff = |Vpe| - |Vne| to confirm the effect of issuing the voltage-to-ground change command in step S2. The processor 111 calculates |ΔVdiff1|, the absolute value of the change in Vdiff calculated in step S1 and Vdiff calculated in step S3. The determiner 112 compares |ΔVdiff1| with a second threshold value Vth2. If |ΔVdiff1| ≥ Vth2, the process proceeds to step S4, and if |ΔVdiff1| < Vth2, the process proceeds to step S5. The second threshold value Vth2 is determined arbitrarily depending on the amount of insulation resistance decrease to be detected.

[0061] In step S4, the determiner 112 determines that the insulation resistance of both the south pole 7 and the temperature pole 8 has decreased.

[0062] In step S5, the determiner 112 determines that the operation is normal. Figure 8 is a diagram showing the operating waveforms under normal conditions and when the insulation resistance of both the S pole 7 and the T pole 8 is reduced when the voltage to ground is changed. As shown in Figure 8, under normal conditions, |ΔVdiff1| < Vth2, and when the insulation resistance of both the S pole 7 and the T pole 8 is reduced, |ΔVdiff1| ≥ Vth2.

[0063] In step S6, the determiner 112 determines whether Vdiff is positive or negative. If Vdiff is positive, the process proceeds to step S8, and if Vdiff is negative, the process proceeds to step S7. This is because Vdiff becomes negative when the insulation resistance of the south electrode 7 or the positive electrode 2 decreases, and Vdiff becomes positive when the insulation resistance of the temperature electrode 8 or the negative electrode 3 decreases.

[0064] In step S7, the voltage to ground change command unit 113 issues a command to change the voltage to ground to the voltage controller 104 in the same manner as in step S2.

[0065] In step S9, the processor 111 recalculates Vdiff = |Vpe| - |Vne| to check the effect of issuing the ground voltage change command in step S7. The processor 111 calculates |ΔVdiff2|, the absolute value of the change in Vdiff calculated in step S1 and Vdiff calculated in step S9. The determiner 112 compares |ΔVdiff2| with a third threshold value Vth3. If |ΔVdiff2| ≥ Vth3, the process proceeds to step S10, and if |ΔVdiff2| < Vth3, the process proceeds to step S11. The threshold value Vth3 is determined taking into account the change in ground voltage and the input / output voltage of the power converter 4. For example, if the voltage of the DC bus 1 is 690V, Vout is 380V, and Ratio_S is set to 1.5 and Ratio_T is set to 0.5 in step S7, the threshold value Vth3 can be set to 50V, for example.

[0066] In step S10, the determiner 112 determines that there is a decrease in insulation resistance of the south pole 7. Fig. 9 is a diagram showing the operating waveforms when the voltage to ground is changed when the insulation resistance of the south pole 7 has decreased. As shown in Fig. 9, when the insulation resistance of the south pole 7 has decreased, |ΔVdiff2| ≥ Vth3 holds.

[0067] In step S11, the determiner 112 determines that there is a decrease in insulation resistance of the positive electrode 2. In step S8, the voltage-to-ground change commander 113 issues a command to change the voltage to ground to the voltage controller 104 in the same manner as in step S2.

[0068] In step S12, the processor 111 recalculates Vdiff = |Vpe| - |Vne| to check the effect of issuing the ground voltage change command in step S8. The processor 111 calculates |ΔVdiff3|, which is the absolute value of the change in Vdiff calculated in step S1 and Vdiff calculated in step S12. The determiner 112 compares |ΔVdiff3| with a fourth threshold value Vth4. If |ΔVdiff3| ≥ Vth4, the process proceeds to step S13, and if |ΔVdiff3| < Vth4, the process proceeds to step S14.

[0069] In step S13, the determiner 112 determines that the insulation resistance of the T electrode 8 has decreased. In step S14, the determiner 112 determines that the insulation resistance of the negative electrode 3 has decreased.

[0070] The above detection flowchart is usually repeated at regular intervals. If it is desired to detect an abnormality early, it is effective to constantly perform the determination in step S1 and, once Vdiff≧Vth1 is detected, proceed to step S6 by interrupt processing. The magnitude relationships among the thresholds Vth1, Vth2, Vth3, and Vth4 are not particularly limited.

[0071] As described above, according to the first embodiment, the DC power distribution system includes the power converter 4 that converts power from the DC bus 1 and outputs it to the power feeder 9, and the detector 5 and insulation resistance decrease determiner 102 that detect insulation resistance decrease. A decrease in the insulation resistance of the DC bus 1 and the power feeder 9 is detected using the voltage or current measured by the detector 5. A voltage-to-ground change commander 113 issues a command to the voltage controller 104 that controls the power converter 4, thereby changing the voltage to ground that the power converter 4 outputs to the power feeder 9. The change in the voltage to ground makes it possible to detect whether or not insulation resistance has decreased at both poles of the power feeder 9, and, when insulation resistance has decreased at one point of either the DC bus 1 or the power feeder 9, to identify the location of the decrease.

[0072] Second Embodiment (Configuration Description) Fig. 10 is a schematic configuration diagram of a DC power distribution system according to a second embodiment.

[0073] The DC power distribution system of the second embodiment differs from the DC power distribution system of the first embodiment in that a detector 200 composed of capacitors CP and CN is provided instead of the detector 5 in the first embodiment. By configuring the detector 200 with capacitors CP and CN, it is possible to limit the current Ie, which is a DC current, more than when the detector 200 is configured with resistors RP and RN.

[0074] (Explanation of Operation) The operation of the second embodiment will be explained with respect to differences from the first embodiment.

[0075] In the first embodiment, when the voltage-to-ground change commander 113 sets Ratio_S and Ratio_T to constant values ​​in order to change the voltage to ground in step S2, a bias occurs in the voltage charged to the capacitor of the detector 200. As a result, a change occurs in |ΔVdiff1| even though the condition is normal, and |ΔVdiff1| exceeds Vth2, resulting in an erroneous determination that the insulation resistance of both the south pole 7 and the temperature pole 8 has decreased. Similarly, in steps S9 and S12, the location of the decreased insulation resistance is erroneously determined.

[0076] Therefore, in the second embodiment, the voltage-to-ground change commander 113 does not set Ratio_S and Ratio_T to constant values ​​in order to change the voltage to ground, but instead changes them continuously. The change applied to the set value is made so that the value changes from the initial value and returns to the original value, like a half wave of an AC waveform. Specifically, when not changing the voltage to ground, the voltage-to-ground change commander 113 sets both Ratio_S and Ratio_T to 1. When changing the voltage to ground, the voltage-to-ground change commander 113 sets Ratio_S to a value of an AC waveform for a half cycle with an offset of 1 and an amplitude of less than 1, and sets Ratio_T to a value of an AC waveform for a half cycle with an offset of 1, an amplitude of less than 1, and a phase that is 180 degrees ahead of Ratio_S. The interval of the AC waveform can be set arbitrarily.

[0077] As a result, the voltage controller 104 uses the ratio when the voltages to ground of each line of the power supply line are equal as a reference ratio, and controls the ratio of the voltages to ground output by the power converter 4 to continuously change from the reference ratio and return to the reference ratio within a certain period of time.

[0078] This allows correct determination in steps S3, S9, and S12. Note that, during determination, some measures are taken, such as comparing the peak value of |ΔVdiff| with a threshold value.

[0079] Figure 11 shows the operating waveforms when the voltage to ground is changed during normal operation and when the insulation resistance of both the south pole 7 and the temperature pole 8 has decreased. In Figure 11, a half-cycle sine wave is used to change the set value, but any AC waveform such as a square wave, a triangular wave, or a sawtooth wave may also be used.

[0080] According to the second embodiment, in a DC power distribution system equipped with a detector 200, when changing the voltage to ground, the setting signals of Ratio_S and Ratio_T given from the voltage to ground change commander 113 to the voltage controller 104 are changed to have AC waveforms.

[0081] This prevents erroneous determination of a decrease in insulation resistance caused by uneven voltage charging of the capacitor of the detector 200 when the voltage to ground changes, making it possible to detect a decrease in insulation resistance and identify the location of the decrease.

[0082] Third Embodiment (Configuration) In the first embodiment, the power converter 4 is configured as a non-insulated DC / DC converter. In the third embodiment, the power converter 4 is configured as a non-insulated DC / AC converter. The non-insulated DC / AC converter outputs AC power to the power feeder 9. The output can be single-phase AC or three-phase AC, but the third embodiment will be described using a single-phase AC output as an example. When the output is a single-phase AC output, the configuration of the power converter 4 is the same as that of the non-insulated step-down DC / DC converter of the first embodiment. When a three-phase AC output is used, one leg is added, and accordingly, one output reactor, one output capacitor, and one drive signal are also added to the output filter section. The detector 5 may be configured as a resistor, or as in the second embodiment, as a capacitor. Here, the case where the detector 5 is configured as a resistor will be described as an example.

[0083] (Explanation of Operation) In the third embodiment, the output Vout has a sine wave waveform, so a sine wave waveform is given to the voltage command value Vout*.

[0084] FIG. 12 is a flowchart showing a procedure for detecting a decrease in insulation resistance in the third embodiment.

[0085] In step S41, the processor 111 receives Vpe and Vne measured by the detector 5 and calculates Vdiff = |Vpe| - |Vne|. The determiner 112 compares Vdiff with a first threshold Vth1 to determine whether the insulation resistance of any one of the positive electrode 2, negative electrode 3, S electrode 7, and T electrode 8 has decreased. If the insulation resistance of any one of the positive electrode 2, negative electrode 3, S electrode 7, and T electrode 8 has decreased, Vdiff≧Vth1, and the process proceeds to step S46 to identify the location of the decrease. If the insulation resistance of both the positive electrode 2 and negative electrode 3 has decreased (a), if the insulation resistance of both the S electrode 7 and T electrode 8 has decreased (b), or if the insulation resistance is normal and no decrease has occurred (c), then Vdiff<Vth1, and the process proceeds to step S42.

[0086] When the insulation resistance of the positive electrode 2 or the negative electrode 3 of the DC bus 1 decreases, Vdiff has a DC component. On the other hand, when the insulation resistance of the S pole 7 or the T pole 8 of the power feeder 9 decreases, Vdiff has an AC component. Therefore, it is also effective for the processor 111 to calculate the average value and the effective value of Vdiff, and for the determiner 112 to compare the average value with a DC threshold value Vth1_dc and the effective value with an AC threshold value Vth1_ac.

[0087] In step S42, the voltage to ground change command unit 113 issues a command to the voltage controller 104 to change the voltage to ground.

[0088] In step S43, the processor 111 recalculates Vdiff = |Vpe| - |Vne| to check the effect of issuing the ground voltage change command in step S42. The processor 111 calculates |ΔVdiff1|, which is the absolute value of the change in Vdiff calculated in step S41 and Vdiff calculated in step S43. The determiner 112 compares |ΔVdiff1| with a second threshold value Vth2. If |ΔVdiff1| ≥ Vth2 at at least one point in time, the process proceeds to step S44. If |ΔVdiff1| < Vth2 at all times, the process proceeds to step S45.

[0089] In step S44, the determiner 112 determines that the insulation resistance of both the south pole 7 and the temperature pole 8 has decreased.

[0090] In step S45, the determiner 112 determines that the operation is normal. Figure 13 is a diagram showing operating waveforms under normal conditions and operating waveforms when the insulation resistance of both the S pole 7 and the T pole 8 is reduced when the voltage to ground is changed. As shown in Figure 13, under normal conditions, |ΔVdiff1| < Vth2 is always satisfied, and when the insulation resistance of both the S pole 7 and the T pole 8 is reduced, |ΔVdiff1| ≥ Vth2 is satisfied at least at one point in time.

[0091] In step S46, determiner 112 determines whether Vdiff is an AC waveform. If Vdiff is an AC waveform, the process proceeds to step S47. If Vdiff is not an AC waveform, i.e., if Vdiff is a DC waveform, the process proceeds to step S52. Methods for determining whether Vdiff is an AC waveform include, for example, detecting the zero-crossing points of Vdiff or detecting the amount of change in the waveform within a certain period of time by differentiation or the like.

[0092] In step S47, the voltage to ground change command unit 113 issues a command to change the voltage to ground to the voltage controller 104 in the same manner as in step S42.

[0093] In step S49, the processor 111 recalculates Vdiff = |Vpe| - |Vne| to check the effect of issuing the ground voltage change command in step S47. The processor 111 calculates a change amount ΔVdiff2 from Vdiff calculated in step S41 to Vdiff calculated in step S49. The determiner 112 compares ΔVdiff2 with a third threshold value Vth3. If ΔVdiff2 ≥ Vth3, the process proceeds to step S50, and if ΔVdiff2 < Vth3, the process proceeds to step S51.

[0094] In step S50, the determiner 112 determines that the insulation resistance of the south pole 7 has decreased. In step S51, the determiner 112 determines that the insulation resistance of the temperature pole 8 has decreased.

[0095] 14 is a diagram showing the operating waveforms when the voltage to ground is changed when the insulation resistance of the south pole 7 drops. For example, if the threshold value Vth3 is set to 0, and Ratio_S is set to be greater than Ratio_T when the insulation resistance of the south pole 7 drops, the amplitude of Vdiff changes to become larger (ΔVdiff2 changes to a positive value), so that ΔVdiff2 is equal to or greater than the threshold value Vth3, and it is determined that the insulation resistance of the south pole 7 has dropped.

[0096] 15 is a diagram showing the operating waveforms when the voltage to ground is changed when the insulation resistance of the T pole 8 drops. When the insulation resistance of the T pole 8 drops, setting Ratio_S > Ratio_T causes the amplitude of Vdiff to change to be smaller (ΔVdiff2 changes to negative), so that ΔVdiff2 becomes less than the threshold value Vth3, and it is determined that the insulation resistance of the T pole 8 has dropped.

[0097] In this way, it is possible to identify the location of the drop in insulation resistance. If Vdiff is positive in step S52, the process proceeds to step S53, and if Vdiff is 0 or negative, the process proceeds to step S54.

[0098] In step S53, the determiner 112 determines that the insulation resistance of the negative electrode 3 has decreased. In step S54, the determiner 112 determines that the insulation resistance of the positive electrode 2 has decreased.

[0099] According to the third embodiment, even if the power converter 4 is a non-insulated DC / AC converter, it is possible to detect a decrease in the insulation resistance of the DC bus 1 and the power feeder 9.

[0100] 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.

[0101] 1 DC bus, 2 Positive pole, 2S, 2T Midpoint, 3 Negative pole, 4 Power converter, 5, 200 Detector, 6 Output terminal, 9 Power feeder, 20, 21 Leg, 22 Virtual neutral point, 23 Input capacitor section, 24 Output filter section, 31 Voltage controller, 32, 33 Current controller, 101 Drive signal generator, 102 Insulation resistance drop determiner, 103 Command voltage generator, 104 Voltage controller, 111 Processor, 112 Determinator, 113 Ground voltage change commander, AD1, AD2 Adder, C1, C2, CN, CP, Cs, Ct Capacitor, D Connection point, RP, RN Resistor, ML1, ML2, ML3, ML4 Multiplier, Ls, Lt Output reactor, SB1, SB2, SB3 Subtractor, SW1, SW2, SW3, SW4 switching elements.

Claims

1. A power distribution system comprising: a power converter that converts the voltage of a primary DC bus into AC or DC and outputs the converted voltage to a power feeder; and a voltage controller that controls the power converter to change the output voltage to ground of each line of the power feeder while keeping the output voltage of the power converter constant.

2. The power distribution system according to claim 1, wherein the voltage controller changes the ratio of the magnitude of the output voltage to ground of each line of the power supply line while keeping the total value of the ratios set for each line constant at all times.

3. A power distribution system as claimed in claim 1 or 2, wherein the voltage controller includes: a command voltage generator that generates an output voltage command by receiving a voltage to ground change command and adding a value obtained by multiplying the output voltage of the power converter by a voltage to ground ratio to an output signal of a current controller; and a drive signal generator that generates a drive signal for the power converter based on the output voltage command.

4. The power distribution system according to any one of claims 1 to 3, wherein the power converter is configured as a non-isolated power converter and includes legs and output reactors in the same number as the number of wires of the power feeder, and the voltage controller controls the current flowing through each output reactor.

5. The power distribution system according to any one of claims 1 to 4, further comprising: a detector connected between the DC bus and a ground potential; and an insulation resistance degradation determiner for determining degradation of the insulation resistance of the power feeder and the DC bus based on a change before and after changing a voltage to ground for at least one of the voltages or currents detected by the detector.

6. The power distribution system of claim 5, wherein the detector includes one or both of a resistor and a capacitor.

7. A power distribution system according to any one of claims 1 to 6, wherein the voltage controller uses a ratio when the voltages to ground of each line of the power supply line are equal as a reference ratio, and controls the ratio of the voltages to ground output by the power converters to be continuously changed from the reference ratio and returned to the reference ratio within a fixed period of time.

8. The power distribution system according to any one of claims 1 to 7, wherein the power converter is a non-isolated DC / DC converter.

9. The power distribution system according to any one of claims 1 to 7, wherein the power converter is a non-isolated DC / AC converter.

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