DC Insulation Resistance Monitoring System

Abstract

This system provides a DC insulation resistance monitoring system whose measurement results are less susceptible to external noise. [Solution] A DC insulation resistance monitoring system for a high-voltage distribution system, comprising: a neutral point switch for a grounded instrument transformer; an insulating capacitor switched on and off by the neutral point switch; a DC power supply for charging the insulating capacitor; a switch for switching the connection between the DC power supply and the insulating capacitor; and a voltage measuring unit for the insulating capacitor. The voltage measuring unit measures and holds the drift voltage Vd of the insulating capacitor, measures and holds the peak voltage Vp of the insulating capacitor, measures and holds the capacitor voltage Vt after a certain time t has elapsed since the DC power supply was turned off after charging the insulating capacitor with the DC power supply, calculates the voltage discharge time constant of the insulating capacitor as (Vp-Vd) / (Vp-Vt)×t, and calculates the insulation resistance R as T / c using the capacitance C of the insulating capacitor.

Description

Technical Field

[0001] The present invention relates to a DC insulation resistance monitoring system.

Background Art

[0002] As a conventional method for measuring the DC insulation resistance of a power distribution system, a method of interrupting the power supply of the power distribution system, connecting a DC high-voltage power source, and measuring the leakage current has been common. As a DC insulation resistance monitoring system for improving this, for example, there is one described in Patent Document 1. In that system, the primary side neutral point of the grounding type instrument transformer connected to the power distribution system is DC-insulated by an insulating capacitor, a DC power source is connected to the neutral point, and the leakage current flowing is measured, thereby measuring the DC insulation resistance of the power distribution system in the live line state.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the DC insulation resistance monitoring system described in Patent Document 1, when measuring the time change of the voltage of the insulating capacitor, there has been a problem that the measurement result is easily affected by external noise. <000002​​​​​​​A DC insulation resistance monitoring system for a high-voltage distribution system, comprising an ungrounded high-voltage distribution system having a low-voltage winding of an extra-high-voltage transformer, a grounded instrument transformer, a high-voltage winding of a high-voltage transformer, a distribution board, and a receiving board, wherein the system includes a neutral point switch for the grounded instrument transformer, an insulating capacitor switched on and off by the neutral point switch, a DC power supply for charging the insulating capacitor, a switch for switching the connection between the DC power supply and the insulating capacitor, and a voltage measuring unit for the insulating capacitor, wherein the voltage measuring unit measures and holds the drift voltage Vd of the insulating capacitor, measures and holds the voltage peak value Vp of the insulating capacitor, measures and holds the capacitor voltage Vt after a certain time t has elapsed after the DC power supply has been shut off after charging the insulating capacitor with the DC power supply, calculates the voltage discharge time constant of the insulating capacitor as (Vp-Vd) / (Vp-Vt)×t, and calculates the insulation resistance R as T / c using the capacitance C of the insulating capacitor. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a DC insulation resistance monitoring system in which the measurement results are less susceptible to external noise.

[0008] Further means and effects of the present invention will become apparent throughout the entire specification below. [Brief explanation of the drawing]

[0009] [Figure 1] This is a DC insulation resistance monitoring system according to Example 1. [Figure 2] Figure 1 is an internal diagram of the voltage measurement unit 15. [Figure 3] This is the internal circuit of the voltage measurement unit in Example 1. [Figure 4] This is the output voltage waveform of the voltage measurement unit in Example 1. [Figure 5] This is a flowchart for calculating the discharge time constant of the isolation capacitor voltage in the DC insulation resistance monitoring system of Example 1. [Figure 6] This is a second flowchart for calculating the discharge time constant of the isolation capacitor voltage in the DC insulation resistance monitoring system of Example 2. [Figure 7] This is the internal circuit of the handover circuit of the DC insulation resistance monitoring system of Example 3. [Figure 8] This is the second internal circuit of the handover circuit in the DC insulation resistance monitoring system of Example 4. [Modes for carrying out the invention]

[0010] The embodiments of the present invention will be described below from the perspective of explaining the technical concept.

[0011] Embodiments of the present invention will be described below with reference to the drawings. [Examples]

[0012] Example 1 will be explained using Figures 1 to 4.

[0013] First, Figure 1 shows the DC insulation resistance monitoring system of Example 1. The power distribution system for measuring DC insulation resistance consists of an extra-high voltage transformer 1, a high-voltage distribution panel 2, a high-voltage receiving panel 3, a low-voltage panel 4, and a high-voltage transformer 5. The high-voltage distribution panel 2 is equipped with a grounding instrument transformer 10.

[0014] In Example 1, an insulating capacitor 12 is inserted between the neutral point of the grounding instrument transformer 10 and the ground, and the neutral point switch 11 is connected to the insulating capacitor 12. A DC power supply 13 is also connected to the insulating capacitor 12 via a DC power supply switch. A voltage measuring unit 15 is provided to measure the voltage of the insulating capacitor 12. 100 is the voltage input unit.

[0015] Figure 2 shows the internal configuration of the voltage measurement unit 15 of the DC insulation resistance monitoring system in Figure 1. The voltage of the insulation capacitor 12 is input to the transfer circuit 60, where impedance conversion and other operations are performed, and then input to the CPU 61. In the CPU 61, the input voltage is measured and held at the timings determined by the first voltage holding unit 62, the second voltage holding unit 63, and the third voltage holding unit 64. The held voltage signal is sent to the time constant calculation unit 65, and the discharge time constant T of the insulation capacitor 12 is calculated using an algorithm described later. The calculated discharge time constant T is sent to the determination unit 66, where it is compared with a preset value for determination. If there is an abnormality, an abnormality reporting signal is sent to the abnormality reporting unit 67. The abnormality reporting unit may be inside the voltage measurement unit 15 or arranged in an external control panel or the like.

[0016] Figure 3 shows the internal circuit of the voltage measurement unit 15 of the DC insulation resistance monitoring system in Figure 1. It can also be regarded as an example of the configuration within the transfer circuit 60 in Figure 2.

[0017] The voltage of the insulation capacitor 12 is divided by a voltage divider composed of the high-voltage side resistor 20 and the low-voltage side resistor 21 of the voltage divider, and then input to the voltage follower 22. The output of the voltage follower 22 is input to an A / D converter (not shown) or the like to measure the time change of the voltage of the insulation capacitor 12. The measured value digitized by the A / D converter is input to the CPU 61 in Figure 2 as an example. Since the voltage divider is connected in parallel with the main circuit resistor to be measured, it causes measurement errors. Therefore, by setting the high-voltage side resistor 20 of the voltage divider to a value larger than the main circuit resistor to be measured, the error can be reduced.

[0018] Figure 4 shows the output voltage waveform when the voltage of the insulation capacitor 12 is input to the voltage measurement unit 15 shown in Figures 1 to 3. It is the output voltage waveform when the neutral point switch 11 is opened, the insulation capacitor 12 is charged to a certain voltage from the DC power supply 13 through the DC power supply switch 14, and then the DC power supply switch 14 is opened.

[0019] When the power supply of the voltage follower 22 of the voltage measurement unit 15 is turned on at time t0, the output voltage of the voltage follower 22 rises to Vd0. This is due to the offset voltage inside the operational amplifier.

[0020] Next, when a voltage is applied to the main circuit at time t1, the leakage magnetic flux generated from a transformer or the like acts on the voltage measurement unit 15, and the output voltage of the voltage measurement unit 15 further rises to Vd1. Vd1 is not a constant value and varies depending on the relative position between the main circuit and the voltage measurement unit 15.

[0021] Next, when the insulating capacitor 12 is charged by the DC power supply 13 at time t2, the output voltage of the voltage measurement unit 15 rises to Vp. Then, when the DC power supply 13 is disconnected from the insulating capacitor 12, the charge of the insulating capacitor 12 discharges through the DC ground resistance of the main circuit, so the output voltage of the voltage measurement unit 15 decreases, and at time t3 when a time Δt has elapsed, the output voltage of the voltage measurement unit 15 reaches Vt(t3).

[0022] FIG. 5 shows a flowchart for calculating the discharge time constant T of the insulating capacitor voltage. First, in step 1 indicated by reference numeral 31, Vd1 in FIG. 4 is held. Next, in step 2 indicated by reference numeral 32, Vp in FIG. 4 is held. Next, in step 3 indicated by reference numeral 33, Vt(t3) in FIG. 4 is held. Next, in step 4 indicated by reference numeral 34, the time constant T is calculated by the following formula. Note that T is intended by the Greek letter tau and is used in the specification.

[0023] T = (Vp - Vd1) / (Vp - Vt(t3)) × Δt Based on this flowchart, by holding the four quantities of Vd1, Vp, Vt(t3), and Δt and only performing subtraction, multiplication, and division, the discharge time constant T of the voltage of the insulating capacitor 12 can be calculated with a small amount of calculation.

[0024] Then, by using the capacitance C of the insulating capacitor 12 and calculating the resistance R as T / C, the value of the DC insulation resistance can be derived.

[0025] Furthermore, the discharge time constant T can be determined by eliminating the influence of Vd0, which is an offset voltage inside the operational amplifier, and Vd1, which includes Vd0. Therefore, since calculation results that eliminate the influence of external noise can be obtained, it is possible to provide a DC insulation resistance monitoring system in which the measurement results are less susceptible to external noise. [Examples]

[0026] Example 2 will be explained using Figure 6. This example shows additional processing in addition to the flowchart shown in Figure 5. This processing is performed using the circuits and configurations shown in Figures 1 to 3.

[0027] In step 5, indicated by symbol 35, Vt at time t3 + Δt is stored, and in step 6, indicated by symbol 36, Vt at time t3 + 2 × Δt is stored. Then, in step 7, indicated by symbol 37, the ratios R1, R2, and R3 are calculated using the following three formulas.

[0028] R1 = (Vp - Vd1) / (Vt(t3) - Vd1) R2=(Vt(t3)-Vd1) / (Vt(t3+Δt)-Vd1) R3=(Vt(t3+Δt)-Vd1) / (Vt(t3+2×Δt)-Vd1) Note that R1, R2, and R3 are calculated values ​​and do not correspond to actual resistors. Next, we calculate the variance S2 of R1, R2, and R3.

[0029] In step 8, indicated by reference numeral 38, if the variance S2 falls below the threshold value, it is determined that the voltage of the insulating capacitor 12 is being measured correctly, and the process proceeds to step 39. If the variance S2 exceeds the threshold value, it is determined that the voltage of the insulating capacitor 12 is not being measured correctly, and the process returns to step 31.

[0030] In step 9, indicated by reference numeral 39, the DC insulation resistance value is calculated by dividing the discharge time constant T by the capacitance C of the insulating capacitor 12. If this value falls below a certain threshold, for example, 6 M ohms, an abnormality is detected in step 10, indicated by reference numeral 40. Various methods can be used to detect the abnormality, such as communication to external devices, notification by sound or light, or notification by displaying text or graphics. If the value does not fall below 6 M ohms, the process ends, or the process restarts from step 1.

[0031] In this configuration, it is possible to determine whether or not the voltage of the insulating capacitor 12 has been measured correctly, thereby eliminating erroneous measurements of the DC insulation resistance and improving reliability. [Examples]

[0032] Example 3 will be explained using Figure 7. In this figure, the neutral switch 11 and the isolation capacitor 12 in the circuit of Figure 2 in Example 1 are placed near the grounding instrument transformer 10. In the circuit of Figure 2, the lead connecting the isolation capacitor 12 and the DC power switch 14 on the upper side of the paper is shown as the core wire, and the lead connecting the isolation capacitor 12 and the negative terminal of the DC power supply 13 is shown as the sheath, connected by a coaxial cable.

[0033] In this embodiment, a second voltage follower 22b is added to the first voltage follower 22a in the transfer circuit 60. The output signals of these are input to the differential amplifier 50 and differentially amplified. With this configuration, if external noise enters the low-voltage side resistor 21 of the resistor divider in common mode, it can be canceled out, thereby improving the signal-to-noise ratio.

[0034] This makes it possible to provide a DC isolation resistance monitoring system that reduces the effects of noise and improves the signal-to-noise ratio through its circuit configuration. [Examples]

[0035] Embodiment 4 will be explained using Figure 8. Similar to Figure 7, the neutral switch 11 and the isolation capacitor 12 in the circuit of Figure 2 are placed near the grounding instrument transformer 10. In the circuit of Figure 2, the lead connecting the isolation capacitor 12 and the upper terminal of the DC power switch 14 is shown as the core wire, and the lead connecting the isolation capacitor 12 and the negative terminal of the DC power supply 13 is shown as the sheath, and they are connected by a coaxial cable.

[0036] In this embodiment, a dummy resistor 51 is added to the transfer circuit 60, and the voltage across it is input to the second voltage follower 22b. The low-voltage side resistor 21 of the resistor divider and the dummy resistor 51 are arranged in parallel. With this configuration, even if external noise caused by leakage flux enters the voltage measurement section, it can be canceled out, thereby improving the signal-to-noise ratio.

[0037] This makes it possible to provide a DC isolation resistance monitoring system that reduces the effects of noise and improves the signal-to-noise ratio through its circuit configuration.

[0038] Furthermore, the technical concepts disclosed in Examples 1 to 4 are particularly suitable for systems that measure the DC insulation resistance of ungrounded high-voltage power distribution systems in a live state.

[0039] On the other hand, while the above explanations have focused on the technical concept of a DC insulation resistance monitoring system, it is also possible to add a voltage measurement unit 15 only during measurement to measure DC insulation resistance. One example of this is to have the part below the voltage input unit 100 in Figure 1 as a DC insulation resistance measuring device, and to connect the voltage input unit 100 to the upper part of Figure 1 only during measurement. Even in such a case, as long as it has the technical concept disclosed in Examples 1 to 4, namely the voltage measurement unit 15, its internal configuration, circuitry, and processing algorithm, the DC insulation resistance measuring device is also included in the scope of the present invention. Furthermore, even the voltage measurement unit 15 alone is included in the scope of the present invention as long as it has its internal configuration, circuitry, and processing algorithm.

[0040] The idea and concept of the present invention have been described above using various embodiments. Of course, examples realized by combining these embodiments are also included within the scope of the present invention. Furthermore, modifications and similar examples thereof, as long as they utilize the disclosed ideas and concepts, are also included within the scope of the present invention.

[0041] Furthermore, an example of the present invention described using the above embodiments can also be expressed as follows.

[0042] <Part 1> In an ungrounded high-voltage distribution system having a low-voltage winding for an extra-high-voltage transformer, a grounded instrument transformer, a high-voltage winding for a high-voltage transformer, a distribution board, and a receiving board, The system comprises a neutral point switch for the grounding type instrument transformer, an insulating capacitor that is switched on and off by the neutral point switch, a DC power supply for charging the insulating capacitor, a switch for switching the connection between the DC power supply and the insulating capacitor, and a voltage measuring unit for the insulating capacitor. The voltage measuring unit measures and holds the drift voltage Vd of the insulating capacitor. The voltage peak value Vp of the aforementioned isolation capacitor is measured and held. After charging the isolation capacitor with the DC power supply, the capacitor voltage Vt is measured and held after a certain time t has elapsed since the DC power supply was cut off. A DC insulation resistance monitoring system for a high-voltage distribution system, which calculates the voltage discharge time constant of the insulating capacitor as (Vp-Vd) / (Vp-Vt)×t and calculates the insulation resistance R as T / c using the capacitance C of the insulating capacitor. <Part 2> The DC insulation resistance monitoring system described in <Part 1> that triggers an alarm when the insulation resistance falls below a specified value. <Part 3> A DC insulation resistance monitoring system as described in <Part 2>, which measures and holds Vt at time t3, time t3+Δt, and time t3+2×Δt, and determines that the voltage of the insulating capacitor is being measured normally if the variance of these values ​​is below a judgment value. <Part 4> The DC insulation resistance monitoring system described in <Part 3> comprises a first voltage follower and a second voltage follower in the voltage measuring unit, and a differential amplifier circuit that differentially amplifies each output, wherein the high-voltage side potential of the low-voltage side resistor of the resistor divider is input to the first voltage follower and the low-voltage side potential of the low-voltage side resistor of the resistor divider is input to the second voltage follower. <Part 5> The DC insulation resistance monitoring system described in <Part 3> comprises a first voltage follower and a second voltage follower in the voltage measuring unit, and a differential amplifier circuit that differentially amplifies each output, wherein the high-voltage side potential of the low-voltage side resistor of a resistor divider is input to the first voltage follower and the high-voltage side potential of a dummy resistor is input to the second voltage follower. <Part 6> The system comprises a voltage input unit for receiving voltage from the device to be measured, a neutral point switch, an insulating capacitor that is switched on and off by the neutral point switch, a DC power supply for charging the insulating capacitor, a switch for switching the connection between the DC power supply and the insulating capacitor, and a voltage measurement unit for the insulating capacitor. The voltage measuring unit measures and holds the drift voltage Vd of the insulating capacitor. The voltage peak value Vp of the aforementioned isolation capacitor is measured and held. After charging the isolation capacitor with the DC power supply, the capacitor voltage Vt is measured and held after a certain time t has elapsed since the DC power supply was cut off. A DC insulation resistance measuring device that calculates the voltage discharge time constant of the insulating capacitor as (Vp-Vd) / (Vp-Vt)×t and calculates the insulation resistance R as T / c using the capacitance C of the insulating capacitor. <Part 7> The DC insulation resistance measuring device described in <Part 6> that triggers an alarm when the insulation resistance falls below a specified value. <Part 8> A DC insulation resistance measuring device as described in <Part 7>, which measures and holds Vt at time t3, time t3+Δt, and time t3+2×Δt, and determines that the voltage of the insulating capacitor is being measured normally if the variance of these values ​​is below a judgment value. <Part 9> The DC insulation resistance measuring device described in <Part 8> comprises a first voltage follower and a second voltage follower in the voltage measuring unit, and a differential amplifier circuit that differentially amplifies each output, wherein the high-voltage side potential of the low-voltage side resistor of the resistor divider is input to the first voltage follower and the low-voltage side potential of the low-voltage side resistor of the resistor divider is input to the second voltage follower. <Part 10> The DC insulation resistance measuring device described in <Part 8> comprises a first voltage follower and a second voltage follower in the voltage measuring unit, and a differential amplifier circuit that differentially amplifies each output, wherein the high-voltage side potential of the low-voltage side resistor of a resistor divider is input to the first voltage follower and the high-voltage side potential of a dummy resistor is input to the second voltage follower. <Part 11> Measure and hold the drift voltage Vd of the isolation capacitor. The voltage peak value Vp of the aforementioned isolation capacitor is measured and held. After charging the isolation capacitor with the DC power supply, the capacitor voltage Vt is measured and held after a certain time t has elapsed since the DC power supply was cut off. A DC insulation resistance measuring device that calculates the voltage discharge time constant of the insulating capacitor as (Vp-Vd) / (Vp-Vt)×t and calculates the insulation resistance R as T / c using the capacitance C of the insulating capacitor. <Part 12> The DC insulation resistance measuring device described in <Part 10> that triggers an alarm when the insulation resistance falls below a specified value. <Part 13> A DC insulation resistance measuring device as described in <Part 12>, which measures and holds Vt at time t3, time t3+△t, and time t3+2×△t, and determines that the voltage of the insulating capacitor is being measured normally if the variance of these values ​​is below a judgment value. <Part 14> A DC insulation resistance measuring device as described in <Part 13>, comprising a first voltage follower, a second voltage follower, and a differential amplifier circuit that differentially amplifies each output, wherein the high-voltage side potential of the low-voltage side resistor of a resistor divider is input to the first voltage follower, and the low-voltage side potential of the low-voltage side resistor of the resistor divider is input to the second voltage follower. <Part 15> A DC insulation resistance measuring device as described in <Part 13>, comprising a first voltage follower, a second voltage follower, and a differential amplifier circuit that differentially amplifies each output, wherein the high-voltage side potential of the low-voltage side resistor of a resistor divider is input to the first voltage follower, and the high-voltage side potential of a dummy resistor is input to the second voltage follower. [Explanation of Symbols]

[0043] 1: Extra-high voltage transformer 2: High-voltage distribution panel 3: High-voltage receiving panel 4: Low-voltage panel 5: High-voltage transformer 10: Grounded instrument transformer 11: Neutral switch 12: Isolation Capacitor 13:DC power supply 14:DC power switch 15: Voltage measurement section 16: Overvoltage suppression means 20: Resistor divider, high-voltage side resistor 21: Resistor divider low-voltage side resistor 22, 22a, 22b: Voltage Follower 23: Measuring instrument 50 Differential Amplifier Section 51: Dummy resistor 60: Handover Circuit 61:CPU 62: First voltage holding unit 63: Second voltage holding section 64: Third voltage holding unit 65: Time constant calculation section 66: Judgment section 67: Abnormality reporting department 100: Voltage input section

Claims

1. In an ungrounded high-voltage distribution system having a low-voltage winding for an extra-high-voltage transformer, a grounded instrument transformer, a high-voltage winding for a high-voltage transformer, a distribution board, and a receiving board, The system comprises a neutral point switch for the grounding type instrument transformer, an insulating capacitor that is switched on and off by the neutral point switch, a DC power supply for charging the insulating capacitor, a switch for switching the connection between the DC power supply and the insulating capacitor, and a voltage measuring unit for the insulating capacitor. The voltage measuring unit measures and holds the drift voltage Vd of the insulating capacitor. The voltage peak value Vp of the aforementioned isolation capacitor is measured and held. After charging the isolation capacitor with the DC power supply, the capacitor voltage Vt is measured and held after a certain time t has elapsed since the DC power supply was cut off. A DC insulation resistance monitoring system for a high-voltage distribution system, which calculates the voltage discharge time constant of the insulating capacitor as (Vp - Vd) / (Vp - Vt) × t, and calculates the insulation resistance R as T / c using the capacitance C of the insulating capacitor.

2. The DC insulation resistance monitoring system according to claim 1, which triggers an alarm when the insulation resistance falls below a specified value.

3. A DC insulation resistance monitoring system according to claim 2, which measures and holds Vt at time t3, time t3 + Δt, and time t3 + 2 × Δt, and determines that the voltage of the insulating capacitor is being measured normally if the variance of these values ​​is below a determination value.

4. The DC insulation resistance monitoring system according to claim 3, wherein the voltage measuring unit comprises a first voltage follower, a second voltage follower, and a differential amplifier circuit for differentially amplifying each output, the first voltage follower receives the high-voltage side potential of the low-voltage side resistor of a resistor divider, and the second voltage follower receives the low-voltage side potential of the low-voltage side resistor of a resistor divider.

5. The DC insulation resistance monitoring system according to claim 3, wherein the voltage measuring unit comprises a first voltage follower, a second voltage follower, and a differential amplifier circuit for differentially amplifying each output, the first voltage follower receives the high-voltage side potential of the low-voltage side resistor of a resistor divider, and the second voltage follower receives the high-voltage side potential of a dummy resistor.

6. The system comprises a voltage input unit for receiving voltage from the device to be measured, a neutral point switch, an insulating capacitor that is switched on and off by the neutral point switch, a DC power supply for charging the insulating capacitor, a switch for switching the connection between the DC power supply and the insulating capacitor, and a voltage measurement unit for the insulating capacitor. The voltage measuring unit measures and holds the drift voltage Vd of the insulating capacitor. The voltage peak value Vp of the aforementioned isolation capacitor is measured and held. After charging the isolation capacitor with the DC power supply, the capacitor voltage Vt is measured and held after a certain time t has elapsed since the DC power supply was cut off. A DC insulation resistance measuring device that calculates the voltage discharge time constant of the insulating capacitor as (Vp - Vd) / (Vp - Vt) × t, and calculates the insulation resistance R as T / c using the capacitance C of the insulating capacitor.

7. The DC insulation resistance measuring device according to claim 6, which issues an alarm when the insulation resistance falls below a specified value.

8. A DC insulation resistance measuring device according to claim 7, which measures and holds Vt at time t3, time t3 + Δt, and time t3 + 2 × Δt, and determines that the voltage of the insulating capacitor is being measured normally if the variance of these values ​​is below a determination value.

9. The DC insulation resistance measuring device according to claim 8, wherein the voltage measuring unit comprises a first voltage follower, a second voltage follower, and a differential amplifier circuit for differentially amplifying each output, the first voltage follower receives the high-voltage side potential of the low-voltage side resistor of a resistor divider, and the second voltage follower receives the low-voltage side potential of the low-voltage side resistor of a resistor divider.

10. The DC insulation resistance measuring device according to claim 8, comprising a first voltage follower and a second voltage follower in the voltage measuring unit, and a differential amplifier circuit for differentially amplifying each output, wherein the high-voltage side potential of the low-voltage side resistor of a resistor divider is input to the first voltage follower and the high-voltage side potential of a dummy resistor is input to the second voltage follower.

11. Measure and hold the drift voltage Vd of the isolation capacitor. The voltage peak value Vp of the aforementioned isolation capacitor is measured and held. After charging the isolation capacitor with the DC power supply, the capacitor voltage Vt is measured and held after a certain time t has elapsed since the DC power supply was cut off. A DC insulation resistance measuring device that calculates the voltage discharge time constant of the insulating capacitor as (Vp - Vd) / (Vp - Vt) × t, and calculates the insulation resistance R as T / c using the capacitance C of the insulating capacitor.

12. The DC insulation resistance measuring device according to claim 10, which issues an alarm when the insulation resistance falls below a specified value.

13. A DC insulation resistance measuring device according to claim 12, which measures and holds Vt at time t3, time t3 + Δt, and time t3 + 2 × Δt, and determines that the voltage of the insulating capacitor is being measured normally if the variance of these values ​​is below a determination value.

14. The DC insulation resistance measuring device according to claim 13, comprising a first voltage follower, a second voltage follower, and a differential amplifier circuit for differentially amplifying each output, wherein the first voltage follower is input to the high-voltage side potential of the low-voltage side resistor of a resistor divider, and the second voltage follower is input to the low-voltage side potential of the low-voltage side resistor of a resistor divider.

15. A DC insulation resistance measuring device according to claim 13, comprising a first voltage follower, a second voltage follower, and a differential amplifier circuit for differentially amplifying each output, wherein the first voltage follower is input to the high-voltage side potential of the low-voltage side resistor of a resistor divider, and the second voltage follower is input to the high-voltage side potential of a dummy resistor.

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