Ac inverter and insulation resistance drop detection method

Abstract

To improve the accuracy of detecting a decrease in insulation resistance between the output line of an AC inverter and ground.SOLUTION: An AC inverter 1 comprises an inverter unit 3 that converts DC power supplied from a battery B into AC power and outputs it, and output lines Lh and Lc. The AC inverter 1 also includes an output unit 2 that outputs the output of the inverter unit 3 to a load Lo, and an insulation monitoring unit 4 that detects a decrease in insulation resistance between at least one of the output lines Lh, Lc and the ground PE by comparing the average or integrated value of a monitored voltage Vop, which is the voltage between the higher of the voltages of the output lines Lh, Lc referenced to a ground GND and the voltage of the ground PE referenced to the ground GND, over N cycles of AC power (N: natural number), with a threshold Vth.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a technique for detecting a decrease in insulation resistance between an output line of an AC inverter and ground. [Background technology]

[0002] Some AC inverters detect a decrease in insulation resistance between an output line and ground based on the voltage between the output line and ground. Related technology is disclosed in Patent Document 1.

[0003] However, due to variations in the capacitance between the output line and ground (for example, the Y capacitor capacitance or parasitic capacitance between the output line and ground), variations in the voltage between the output line and ground may occur, which may result in an erroneous detection of a decrease in insulation resistance. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-047181 Summary of the Invention [Problem to be solved by the invention]

[0005] An object according to one aspect of the present invention is to improve the accuracy of detecting a decrease in insulation resistance between an output line of an AC inverter and ground. [Means for solving the problem]

[0006] An AC inverter according to one embodiment of the present invention comprises an inverter unit that converts DC power supplied from a battery into AC power and outputs it; an output unit having at least two output lines that outputs the output of the inverter unit to a load; and an insulation monitoring unit that detects a decrease in insulation resistance between at least one of the output lines and the ground by comparing a threshold value with an average or integrated value of a monitored voltage, which is a voltage between the higher of the voltages of the output lines referenced to ground and the voltage of the earth referenced to ground, over N cycles of the AC power (N: a natural number).

[0007] This makes it possible to detect a decrease in insulation resistance without being affected by variations in capacitance between the output line and ground, thereby improving the accuracy of detecting a decrease in insulation resistance.

[0008] The insulation monitoring unit may be configured to lower the threshold value in accordance with the limit on the AC power.

[0009] This makes it possible to prevent a decrease in insulation resistance from being erroneously detected due to a limit on AC power, thereby further improving the accuracy of detecting a decrease in insulation resistance.

[0010] The insulation monitoring unit may also be configured to include: a first diode having an anode terminal connected to a first output line; a second diode having an anode terminal connected to a second output line; an operational amplifier having a voltage output from a connection point between the cathode terminals of the first and second diodes input to one of its positive input terminal or its negative input terminal and having the ground voltage input to the other of its positive input terminal or its negative input terminal, and outputting the monitored voltage from its output terminal; and a detection unit that detects a decrease in the insulation resistance by comparing an average value or an integrated value of the monitored voltage with the threshold value.

[0011] In this way, the voltage to be monitored can be obtained using a relatively simple configuration such as a diode and an operational amplifier, which simplifies the configuration of the AC inverter.

[0012] Furthermore, one embodiment of the present invention provides a method for detecting a decrease in insulation resistance in an AC inverter that converts DC power supplied from a battery into AC power and outputs the AC power to a load via at least two output lines, and the method is a method for detecting a decrease in insulation resistance between at least one of the output lines and ground, in which the detection unit detects a decrease in insulation resistance between at least one of the output lines and ground, and the detection unit detects a decrease in insulation resistance between at least one of the output lines and ground by comparing a threshold value with an average or integrated value of a monitored voltage, which is the voltage between the higher of the voltages of the output lines relative to ground and the voltage of the ground relative to ground, over N cycles of the AC power (N: natural number).

[0013] This makes it possible to detect a decrease in insulation resistance without being affected by variations in capacitance between the output line and ground, thereby improving the accuracy of detecting a decrease in insulation resistance. [Effects of the Invention]

[0014] According to the present invention, it is possible to improve the accuracy of detecting a decrease in insulation resistance between the output line of an AC inverter and the ground. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 2 is a diagram illustrating an example of an AC inverter according to an embodiment. [Figure 2] 10A and 10B are diagrams illustrating voltages on output lines and voltages input to operational amplifiers. [Figure 3] 10A and 10B are diagrams illustrating information indicating a relationship between a voltage to be monitored and a threshold value, and examples of the voltage to be monitored and the threshold value. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, the embodiments will be described in detail with reference to the drawings.

[0017] FIG. 1 is a diagram illustrating an example of an AC inverter according to an embodiment.

[0018] The AC inverter 1 shown in FIG. 1 includes an output unit 2, an inverter unit 3, and an insulation monitoring unit 4.

[0019] For example, AC inverter 1 is mounted on a vehicle such as an electric vehicle or a plug-in hybrid vehicle, converts DC power supplied from battery B into AC power, and outputs it to load Lo. The load Lo may be an electrical appliance such as a microwave oven or a vacuum cleaner. Battery B is formed from a lithium-ion secondary battery or the like, and supplies power to a traction motor mounted on the vehicle.

[0020] Furthermore, the AC inverter 1 may be provided with a DC-DC converter between the inverter unit 3 and the battery B, such as a DAB (Dual Active Bridge) circuit.

[0021] The AC inverter 1 may also have a function of charging the battery B by converting AC power supplied from a system power supply (not shown) into DC power and outputting the DC power to the battery B.

[0022] The output unit 2 includes a hot-side output line Lh (first output line) and a cold-side output line Lc (second output line), and outputs AC power input to the output lines Lh and Lc from the inverter unit 3 to a load Lo. It also includes a ground line Lpe that is connected to the ground PE, which is the ground, via, for example, the body or tires of the vehicle.

[0023] The output unit 2 includes inductors L1 and L2, capacitors C1 to C3, choke coils L3 and L4, and a voltage sensor V, and suppresses normal mode noise and common mode noise contained in the AC power. In the example shown in FIG. 1, one terminal of the inductor L1 is connected to one terminal of the choke coil L3 via an output line Lh, and the other terminal of the choke coil L3 is connected to one terminal of the load Lo via the output line Lh. One terminal of the inductor L2 is connected to one terminal of the choke coil L4 via an output line Lc, and the other terminal of the choke coil L4 is connected to the other terminal of the load Lo via the output line Lc. One terminal of the capacitor C1 is connected to the output line Lh between the inductor L1 and the choke coil L3, and the other terminal of the capacitor C1 is connected to the output line Lc between the inductor L2 and the choke coil L4. One terminal of the capacitor C2 is connected to the output line Lh between the choke coil L3 and one terminal of the load Lo, and the other terminal of the capacitor C2 is connected to ground PE (earth) via a ground line Lpe. Furthermore, one terminal of capacitor C3 is connected to output line Lc between choke coil L4 and the other terminal of load Lo, and the other terminal of capacitor C3 is connected to ground PE via ground line Lpe. That is, normal mode noise contained in the AC power is suppressed by inductors L1, L2 and capacitor C1, and common mode noise contained in the AC power is suppressed by choke coils L3, L4 and capacitors C2, C3. Note that the circuit configuration of output unit 2 is not particularly limited as long as output unit 2 is provided with at least output lines Lh and Lc.

[0024] The inverter unit 3 includes a capacitor C4, switching elements Q1 to Q4, and a control unit CNT. The inverter unit 3 converts DC power supplied from a battery B into AC power and outputs it to output lines Lh and Lc. For example, each of the switching elements Q1 to Q4 is configured with an IGBT (Insulated Gate Bipolar Transistor) and a diode connected in parallel to the IGBT. In the example shown in FIG. 1, one terminal of the capacitor C4 is connected to the positive terminal of the battery B and to the collector terminals of the switching elements Q1 and Q3. The other terminal of the capacitor C4 is connected to the negative terminal of the battery B and to ground GND, which is the reference potential of the AC inverter 1, and to the emitter terminals of the switching elements Q2 and Q4. The junction between the emitter terminal of the switching element Q1 and the collector terminal of the switching element Q2 is connected to the output line Lh. The junction between the emitter terminal of the switching element Q3 and the collector terminal of the switching element Q4 is connected to the output line Lc. The potentials of the ground GND and the ground PE are assumed to be different from each other. The circuit configuration of the inverter unit 3 is not particularly limited as long as it has at least the function of converting DC power supplied from the battery B into AC power and outputting it to the output lines Lh and Lc.

[0025] The control unit CNT is configured by a processor or a programmable device (such as an FPGA (Field Programmable Gate Array) or a PLD (Programmable Logic Device)), and controls the operation of the switching elements Q1 to Q4. For example, the control unit CNT turns on and off the switching elements Q1 to Q4 so that AC power of a predetermined frequency (for example, 50 Hz) is output from the AC inverter 1 to the load Lo.

[0026] The insulation monitoring unit 4 includes a diode D1 (first diode), a diode D2 (second diode), an operational amplifier OP, resistors R1 to R5, and a detection unit DTC.

[0027] In the example shown in Figure 1, the anode terminal of diode D1 is connected to output line Lh between capacitor C1 and choke coil L3, and the anode terminal of diode D2 is connected to output line Lc between capacitor C1 and choke coil L4. The positive input terminal of operational amplifier OP is connected to the connection point between the cathode terminal of diode D1 and the cathode terminal of diode D2 via resistor R1 and to ground GND via resistor R2. The negative input terminal of operational amplifier OP is connected to ground PE via resistor R3 and to the output terminal of operational amplifier OP via resistor R4. The output terminal of operational amplifier OP is connected to ground GND via resistor R5 and to the input terminal of detection unit DTC.

[0028] That is, diodes D1 and D2 form a rectifier circuit, and the higher of the voltage of output line Lh with respect to ground GND and the voltage of output line Lc with respect to ground GND is applied to the connection point of the cathode terminals of diodes D1 and D2. Also, operational amplifier OP and resistors R1 to R5 form a differential amplifier circuit (subtraction circuit), in which the voltage of the connection point of the cathode terminals of diodes D1 and D2 is input to the positive input terminal of operational amplifier OP, the voltage of ground PE with respect to ground GND is input to the negative input terminal of operational amplifier OP, and the monitored voltage Vop, which is the voltage obtained by subtracting the voltage of ground PE from the voltage of the connection point of the cathode terminals of diodes D1 and D2, is output from the output terminal of operational amplifier OP.

[0029] The circuit configuration of the insulation monitoring unit 4 is not particularly limited as long as it has the rectifier circuit and the differential amplifier circuit (subtractor circuit).

[0030] In addition, the insulation monitoring unit 4 may be configured by a microcontroller consisting of a processor, memory, etc., and functions equivalent to the above-mentioned rectifier circuit and the above-mentioned differential amplifier circuit (subtraction circuit) may be realized by software by executing a program stored in the memory by the processor.

[0031] The detection unit DTC is configured by a processor or a programmable device, calculates the average or integrated value of the monitored voltage Vop output from the operational amplifier OP over one cycle, and when the average or integrated value becomes equal to or less than a threshold Vth, detects a decrease in at least one of the insulation resistance between the output line Lh and the ground PE and the insulation resistance between the output line Lc and the ground PE. For example, the threshold Vth is set to the maximum value of the voltage output from the operational amplifier OP when at least one of the insulation resistance between the output line Lh and the ground PE and the insulation resistance between the output line Lc and the ground PE decreases.

[0032] In the example shown in FIG. 1, the operational amplifier OP receives the voltage applied to the junction between the cathode terminals of the diodes D1 and D2 (the higher of the voltages on the output lines Lh and Lc), and the operational amplifier OP receives the voltage applied to the ground PE (the negative input terminal). Alternatively, the operational amplifier OP may receive the voltage applied to the ground PE (the higher of the voltages on the output lines Lh and Lc) and the voltage applied to the junction between the cathode terminals of the diodes D1 and D2 (the negative input terminal). In this configuration, the monitored voltage Vop output from the output terminal of the operational amplifier OP increases as the insulation resistance between the output lines Lh and Lc and the ground PE decreases. Therefore, in this configuration, when the average value (integrated value) of the monitored voltage Vop over one cycle exceeds the threshold value Vth′, a decrease in the insulation resistance of at least one of the insulation resistance between the output line Lh and the ground PE and the insulation resistance between the output line Lc and the ground PE is detected. For example, the threshold value Vth' is set to the minimum value of the voltage output from the operational amplifier OP when at least one of the insulation resistance between the output line Lh and the ground PE and the insulation resistance between the output line Lc and the ground PE is reduced. That is, the detection unit DTC may be configured to compare the average value or integrated value of the monitored voltage Vop over N cycles of the AC power (N: natural number) with the threshold value Vth or the threshold value Vth'.

[0033] Furthermore, the detection unit DTC may be configured to, upon detecting a decrease in at least one of the insulation resistance between the output line Lh and the ground PE and the insulation resistance between the output line Lc and the ground PE, send a notification to that effect to the control unit CNT. In this configuration, the control unit CNT may be configured to, upon receiving the notification that the insulation resistance has decreased, stop the switching elements Q1 to Q4 and notify the user that the insulation resistance has decreased.

[0034] Here, for example, assume that the insulation resistance between the output line Lh and the ground PE decreases. FIG. 2(a) shows an example of the output voltage of the AC inverter 1 (the difference between the potential of the output line Lh and the potential of the output line Lc), FIG. 2(b) shows an example of the voltage of the output line Lh, FIG. 2(c) shows an example of the voltage of the output line Lc, FIG. 2(d) shows an example of the voltage at the connection point between the cathode terminals of the diodes D1 and D2, FIG. 2(e) shows an example of the voltage of the ground PE, and FIG. 2(f) shows an example of the monitored voltage Vop output from the operational amplifier OP. Note that the voltages shown in FIGS. 2(b) to 2(f) are based on the potential of the ground GND. The horizontal axis of the two-dimensional coordinate system shown in FIGS. 2(b) to 2(f) represents time, and the vertical axis represents voltage. Assume that the insulation resistance between the output line Lh and the ground PE decreases at time t.

[0035] In this case, as shown in Figure 2(d), the voltage waveform input to the positive input terminal of the operational amplifier OP remains unchanged before and after the insulation resistance between the output line Lh and ground PE decreases. However, as shown in Figure 2(e), the voltage waveform input to the negative input terminal of the operational amplifier OP increases after the insulation resistance between the output line Lh and ground PE decreases compared to before the insulation resistance between the output line Lh and ground PE decreases. Therefore, as shown in Figure 2(f), the monitored voltage Vop output from the output terminal of the operational amplifier OP decreases after the insulation resistance between the output line Lh and ground PE decreases compared to before the insulation resistance between the output line Lh and ground PE decreases. Therefore, it can be seen that, for example, the average or integrated value of the monitored voltage Vop per AC voltage cycle can be compared with the threshold value Vth.

[0036] Thus, if at least one of the insulation resistances between the output line Lh and ground PE and between the output line Lc and ground PE decreases, the voltage of the output lines Lh and Lc relative to ground GND does not change, but the voltage of ground PE relative to ground GND increases. Therefore, if the difference between the voltage of the output lines Lh and Lc relative to ground GND and the voltage of ground PE relative to ground GND is calculated and the difference becomes relatively large, it can be determined that at least one of the insulation resistances between the output line Lh and ground PE and the insulation resistance between the output line Lc and ground PE has decreased.

[0037] In the example shown in Figure 1, there is a Y capacitor consisting of capacitors C2 and C3. However, because the amount of charge charged to capacitors C2 and C3 per AC power cycle is equal to the amount of charge discharged from capacitors C2 and C3, the average current flowing through capacitors C2 and C3 per AC power cycle can be considered zero. The same is true for the parasitic capacitance between output lines Lh and Lc and ground PE. Therefore, variations in the capacitance and parasitic capacitance of capacitors C2 and C3 do not affect the average value of the voltage across output lines Lh and Lc per AC power cycle (integral value (area below the voltage per cycle shown in Figure 2(b) and Figure 2(c))) or the average value of the voltage across the connection point of the cathode terminals of diodes D1 and D2 (integral value (area below the voltage per cycle shown in Figure 2(d))). On the other hand, the average value (integral value (area below the voltage per cycle shown in Figure 2(e))) of the voltage applied to ground PE per AC power cycle is not affected by variations in the capacitance or parasitic capacitance of capacitors C2 and C3, but increases as the insulation resistance between output lines Lh and Lc and ground PE decreases. Therefore, the monitored voltage Vop (average or integrated value of monitored voltage Vop per AC power cycle), which is the voltage obtained by subtracting the average value (integral value) of the voltage applied to ground PE per AC power cycle from the average value (integral value) of the voltage applied to the connection point of the cathode terminals of diodes D1 and D2 per AC power cycle, does not change due to variations in the capacitance or parasitic capacitance of capacitors C2 and C3, but decreases as the insulation resistance between output lines Lh and Lc and ground PE decreases. In this embodiment, when the average or integrated value of the monitored voltage Vop falls below the threshold Vth, it is detected that at least one of the insulation resistance between the output line Lh and the ground PE and the insulation resistance between the output line Lc and the ground PE has decreased. In this way, in this embodiment, a decrease in insulation resistance can be detected without being affected by variations in the capacitance and parasitic capacitance of the capacitors C2 and C3, thereby improving the accuracy of detecting a decrease in insulation resistance.

[0038] <Theoretical calculation example of voltage input / output to operational amplifier OP> The average (integrated) voltage per cycle of the AC power input to the positive input terminal of the operational amplifier OP is V. IN+ can be calculated using the following formula 1. Also, the average (integrated) voltage of the ground PE per one cycle of AC power, V PE can be calculated using the following formula 2. Furthermore, the monitored voltage Vop per cycle of AC power can be calculated using the following formula 3. Although there is a Y capacitor consisting of capacitors C2 and C3, the average current flowing through the Y capacitor is assumed to be zero because the amount of charge and discharge per cycle is equal. Furthermore, by solving the simultaneous equations of the following formulas 4, 5, and 6 obtained from the circuit shown in Figure 1, the following formulas 7 and 8 can be calculated. Furthermore, A is the amplitude of the AC voltage output from AC inverter 1, VH is the voltage of battery B, D is the voltage drop of diodes D1 and D2, Voffset is the offset voltage of operational amplifier OP, and V IN- is the voltage input to the negative input terminal of the operational amplifier OP. r1 to r4 are the resistance values ​​of the resistors R1 to R4, respectively. r6 is the resistance value of the insulation resistance between the output line Lh and ground PE, and r7 is the resistance value of the insulation resistance between the output line Lc and ground PE. i6 is the current flowing through the insulation resistance between the output line Lh and ground PE, i7 is the current flowing through the insulation resistance between the output line Lc and ground PE, and i3 is the current flowing through resistor R3. Vs1 is the input offset voltage of the operational amplifier OP. D and Voffset are relatively small values ​​and can be omitted.

[0039]

number

[0040] V PE =V IN- +r3×i3...Equation 2

[0041] Vop=V IN- +r4×i3...Equation 3

[0042] i6+i7=i3...Equation 4

[0043] r6×i6=r7×i7...Equation 5

[0044]

number

[0045]

number

[0046]

number

[0047] As described above, the above equations 1 to 3 do not include the capacitance of capacitors C2 and C3 or the parasitic capacitance between output lines Lh and Lc and ground PE, and therefore, even if there is variation in the capacitance or parasitic capacitance of capacitors C2 and C3 due to manufacturing variations in AC inverter 1, it is understood from theoretical calculations that the monitored voltage Vop can be calculated without being affected by such capacitance variations.

[0048] The AC inverter 1 of this embodiment is configured to detect a decrease in insulation resistance between at least one of the output lines Lh, Lc and the ground PE by comparing the average value (integrated value) of the monitored voltage Vop, which is between the higher of the voltages of the output lines Lh, Lc referenced to the ground GND and the voltage of the ground PE referenced to the ground GND, over N cycles of AC power (N: natural number), with a threshold value Vth or a threshold value Vth'.

[0049] This makes it possible to detect a decrease in insulation resistance between at least one of the output lines Lh, Lc and the ground PE using the monitored voltage Vop, which is not affected by variations in the capacitance of the capacitors C2, C3 or the parasitic capacitance between the output lines Lh, Lc and the ground PE, thereby preventing erroneous detection of a decrease in insulation resistance and improving the accuracy of detecting a decrease in insulation resistance.

[0050] Furthermore, the AC inverter 1 of this embodiment can obtain the monitored voltage Vop with a relatively simple configuration including the diodes D1 and D2 and the operational amplifier OP, so that the configuration of the AC inverter 1 can be simplified.

[0051] The present invention is not limited to the above-described embodiments, and various improvements and modifications are possible without departing from the spirit and scope of the present invention.

[0052] <Variation 1> For example, if the AC power is about to exceed the rated power and the control unit CNT limits the AC power, the voltage of the output lines Lh and Lc drops, causing the monitored voltage Vop to drop. Therefore, even if the insulation resistance between the output lines Lh and Lc and the ground PE has not decreased, the monitored voltage Vop may fall below the threshold Vth, resulting in a false detection of a decrease in insulation resistance. Therefore, the insulation monitoring unit 4 (detection unit DTC) may be configured to lower the threshold Vth in response to the limit on the AC power.

[0053] FIG. 3(a) is a diagram illustrating an example of information indicating the correspondence relationship between the monitored voltage Vop and the threshold value Vth. The horizontal axis in FIG. 3(a) indicates the clamp voltage Vlim [V] (e.g., the average value per cycle of the absolute value (full-wave rectified value) of the output voltage waveform of the output-limited AC inverter 1) indicating the output clamp voltage of the AC power, and the vertical axis indicates the threshold value Vth [V]. The solid line in FIG. 3(a) represents information D indicating the correspondence relationship between the clamp voltage Vlim and the threshold value Vth. The smaller the clamp voltage Vlim (the stronger the restriction), the smaller the threshold value Vth corresponding to that clamp voltage Vlim. For example, when the clamp voltage Vlim is Vlim1, the threshold value Vth is Vth1, and when the clamp voltage Vlim is Vlim2, the threshold value Vth is Vth2. It is assumed that Vlim1>Vlim2 and Vth1>Vth2.

[0054] The voltage sensor V outputs the voltage between the output lines Lh and Lc to the detection unit DTC. In the first modification, the insulation monitoring unit 4 (detection unit DTC) calculates Vlim according to the value of the voltage sensor V, and refers to the information D at predetermined timings (e.g., at each processor clock cycle or each AC cycle) to set the threshold value Vth corresponding to the current clamp voltage Vlim as the current threshold value Vth. For example, as shown in FIG. 3(b), if the clamp voltage Vlim drops from Vlim1 to Vlim2 at time t, the insulation monitoring unit 4 (detection unit DTC) changes the current threshold value Vth from Vth1 to Vth2 at a predetermined timing after time t.

[0055] The insulation monitoring unit 4 (detection unit DTC) may be configured to lower the threshold value Vth to a predetermined value when the control unit CNT sends a notice that the AC power will be limited to the insulation monitoring unit 4 (detection unit DTC). Even with this configuration, the threshold value Vth can be lowered in accordance with the limit on the AC power.

[0056] As described above, according to the AC inverter 1 of the first modification, the threshold value Vth can be reduced in accordance with the limit on the AC power, and therefore, it is possible to prevent the monitored voltage Vop from falling below the threshold value Vth even when the insulation resistance between the output lines Lh, Lc and the ground PE has not decreased. In other words, it is possible to prevent an erroneous detection that the insulation resistance between at least one of the output lines Lh, Lc and the ground PE has decreased even when the insulation resistance between the output lines Lh, Lc and the ground PE has not decreased, and it is possible to further improve the accuracy of detecting a decrease in insulation resistance.

[0057] <Variation 2> In the above embodiment, the output unit 2 is configured as a single-phase, two-wire system using two output lines Lh and Lc. However, the output unit 2 may be configured as a power distribution system using three or more output lines, such as a single-phase, three-wire system using three output lines or a three-phase, three-wire system using three output lines. When the output unit 2 is configured in this manner, the voltages to be monitored are calculated in the same manner for the two phases for which insulation monitoring is performed. When monitoring all phases in a three-phase AC system, the voltages to be monitored are calculated for each of the three combinations of U-phase, V-phase, V-phase, W-phase, and W-phase, U-phase. When the average (integrated value) of the voltages to be monitored Vop falls below the threshold value Vth, a decrease in insulation resistance is detected. Even with this configuration, the accuracy of detecting a decrease in insulation resistance can be improved, as in the above embodiment. In the embodiment and the first and second modifications, the insulation monitoring unit 4 and the control unit CNT are described as being configured separately, but the control unit CNT may be configured as a microcomputer having the functions of the insulation monitoring unit 4, for example. [Explanation of symbols]

[0058] 1 AC inverter 2 Output section 3 Inverter section 4. Insulation monitoring section Lh, Lc output lines B Battery Lo load PE grounding GND Ground D1, D2 diodes OP operational amplifier R1~R5 Resistors DTC detection unit V Voltage sensor

Claims

1. an inverter unit that converts DC power supplied from the battery into AC power and outputs the AC power; an output unit having at least two output lines and outputting an output of the inverter unit to a load; an insulation monitoring unit that detects a decrease in insulation resistance between at least one of the output lines and the ground by comparing an average value or an integrated value of a monitored voltage, which is a voltage between a higher voltage of the output lines relative to the ground and a voltage of the earth relative to the ground, over N cycles of the AC power (N: a natural number), with a threshold value; An AC inverter comprising:

2. 2. The AC inverter according to claim 1, The insulation monitoring unit reduces the threshold value in accordance with the limit on the AC power. AC inverter.

3. 2. The AC inverter according to claim 1, The insulation monitoring unit a first diode having an anode terminal connected to the first output line; a second diode having an anode terminal connected to the second output line; an operational amplifier to which a voltage output from a connection point between the cathode terminals of the first and second diodes is input to one of a positive input terminal or a negative input terminal, to which the ground voltage is input to the other of the positive input terminal or the negative input terminal, and which outputs the monitored voltage from an output terminal; a detection unit that detects a decrease in the insulation resistance by comparing an average value or an integrated value of the monitored voltage with the threshold value; An AC inverter comprising:

4. 1. A method for detecting a decrease in insulation resistance in an AC inverter that converts DC power supplied from a battery into AC power and outputs the AC power to a load via at least two output lines, the method comprising: detecting a decrease in insulation resistance between at least one of the output lines and ground; The detection unit detects a decrease in insulation resistance between at least one of the output lines and the ground by comparing an average value or an integrated value of a monitored voltage, which is a voltage between a higher voltage of the output lines relative to the ground and a voltage of the ground relative to the ground, over N cycles of the AC power (N: natural number), with a threshold value. A method for detecting a decrease in insulation resistance.

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