JP7808945B2 - Ground fault detection device - Google Patents

Description

The present invention relates to a ground fault detection device that monitors the insulation between an insulated electrical circuit and earth potential.

In a conventional ground fault detection device for detecting a ground fault between a high-voltage DC power supply and the vehicle body earth (chassis) provided in an electrically powered vehicle such as an electric vehicle or a hybrid vehicle, one end of a coupling capacitor (injection capacitor) is connected to either the positive or negative output end of the DC power supply (high-voltage circuit) to be detected for a ground fault, and a rectangular wave pulse voltage signal with a duty ratio of 50% output from an output unit is injected into the other end of the injection capacitor via a detection resistor, for example, as shown in Figure 1 of Patent Document 1. The injected pulse voltage signal is applied to an insulation resistor between the high-voltage circuit and the vehicle body earth, causing a ground fault current corresponding to the insulation resistor to flow.

This earth fault current is detected by measuring the voltage at the other end of the injection capacitor, which is the connection point with the detection resistor, using a voltage measurement circuit consisting of a resistor and a capacitor.The magnitude of the earth fault current is detected by measuring the voltage drop across the detection resistor caused by the earth fault current (current flowing through the insulation resistance) flowing through the detection resistor using the voltage measurement circuit.

The current flowing through the injection capacitor becomes transiently large at times when there is a large voltage change at the rise and fall of the injection pulse waveform of the rectangular wave pulse voltage signal, and this current also includes the charging current to the capacitance (which exists in parallel with the insulation resistance) between the high voltage circuit and the vehicle body earth.

For example, in Patent Document 1, the measurement of the earth fault current is performed by avoiding the time when this transient current flows, and by measuring twice: just before the end of the "H" level of the rectangular wave injection pulse voltage signal (measured value VH) and just before the end of the "L" level (measured value VL), and by using the difference between these values (VH-VL), the influence of the current flowing through the vehicle body-to-earth capacitance in the injected current flowing through the injection capacitor is reduced, and the current flowing through the insulation resistance (earth fault current) is measured with higher accuracy.

Examples of high-voltage circuits include an electric storage device (high-voltage battery), a motor drive device for an AC motor, and a DC power supply device for an electrically heated catalyst (hereinafter referred to as EHC) provided in the exhaust passage of an internal combustion engine, as shown in FIG. 1 of Patent Document 2.

When measuring the ground fault current of these high-voltage circuits, the DC voltage of the high-voltage circuits fluctuates due to voltage fluctuations in the high-voltage battery itself caused by charging and discharging the high-voltage battery, voltage fluctuations in the input voltage to the motor drive device caused by the on/off of a switch between the high-voltage battery and the motor drive device, and voltage fluctuations in the output voltage of the EHC DC power supply device used to control the EHC temperature, etc. These voltage fluctuations also cause fluctuations in the current flowing through the injection capacitor, which becomes a source of error in the ground fault current measurement.

For example, in Figure 1 of Patent Document 3, in order to reduce the influence on earth fault current measurement of voltage fluctuations in the high-voltage battery itself and voltage fluctuations in the input voltage to the motor drive device due to the on/off switching of a switch between the high-voltage battery and the motor drive device, one end of each of two coupling capacitors (injection capacitors) is connected to the positive and negative output terminals of the high-voltage battery, respectively, and the other two ends of the two injection capacitors are connected to one end of a detection resistor, a pulse voltage signal is injected via the detection resistor, and the connection point of the other two ends of the injection capacitors is used as the measurement point.

JP 2003-250201 A JP 2014-083943 A JP 2004-104923 A

As described above, in conventional ground fault detection devices, when measuring a ground fault current, the point where the voltage across the detection resistor is added to the output terminal voltage of the output section is set as the measurement point, the voltage at that measurement point is measured, and the voltage across the detection resistor is obtained from the difference between the voltage at that measurement point and the voltage that will be output from a predetermined output terminal of the output section. However, the voltage at this measurement point includes variations and fluctuations in the output terminal voltage of the output section, making it impossible to accurately obtain the voltage across the detection resistor (which is proportional to the ground fault current), and therefore making it impossible to measure the ground fault current with high accuracy.

Furthermore, because a voltage measurement circuit is also connected to the measurement point in addition to the injection capacitor, leakage current flows through the input impedance of the voltage measurement circuit (the input impedance consisting of the resistor, capacitor, and A/D converter) in addition to the ground-fault current flowing through the injection capacitor being measured. For example, if the injection pulse voltage is 5V and the input impedance is 500kΩ, this leakage current will be 10µA. This leakage current is also affected by variations and fluctuations in the injection pulse voltage, and will result in an error when calculating the ground-fault current from the voltage across the detection resistor. This posed a particular problem: minute ground-fault currents at the µA level could not be measured with high accuracy.

Since steady-state ground fault current = AC component of injected pulse voltage / insulation resistance, in the measurement range for monitoring signs of insulation degradation, where the insulation resistance is 10 MΩ to 100 kΩ, if the injected pulse voltage is 5 V (AC component ±2.5 V), for example, the measured ground fault current will be ±0.25 μA to ±25 μA. In this measurement range, the leakage current of 10 μA from the input impedance of the voltage measurement circuit described above has a significant effect on measurement error, making it difficult to detect signs of insulation degradation.

Furthermore, the earth fault current is measured twice, once just before the rectangular wave injection pulse voltage signal reaches the "H" level and once just before it reaches the "L" level, and the difference between these measurements is used. If the voltage across the injection capacitor in the injection path does not change between these two measurements, the difference in voltage applied to the insulation resistance in the two measurements will be the same as the voltage difference between the "H" and "L" levels of the injection pulse voltage signal, and the appropriate earth fault current that correlates with the insulation resistance value can be obtained from the difference between the two measurements.

However, the injection capacitor is charged and discharged by the earth-fault current flowing through it, and the voltage across the injection capacitor changes between the two measurements. When the voltage across the injection capacitor changes, this voltage change is added to the voltage difference between the "H" and "L" levels of the injection pulse voltage, changing the difference in voltage applied to the insulation resistance between the two measurements, and changing the earth-fault current obtained from the difference between the two measurements.

The smaller the insulation resistance (the more advanced the insulation degradation) and the greater the voltage change in the high-voltage circuit, the larger the charge/discharge current of the injection capacitor, and the larger the change in voltage across the injection capacitor between the two measurements. This also increases the change in the earth fault current obtained from the difference between the two measurements.

For example, when the insulation resistance is 100 kΩ, the steady-state earth fault current Ig1, when the voltage across the injection capacitor does not change between two measurements, is: Ig1 = injected pulse voltage AC component / insulation resistance = ±2.5 V / 100 kΩ = ±25 μA
The difference between 25 μA when the injection pulse voltage is at the “H” level and −25 μA when the injection pulse voltage is at the “L” level is 50 μA. By correlating this with the insulation resistance of 100 kΩ when this 50 μA is measured, an appropriate insulation resistance value can be obtained.

However, during a transient period when the voltage of the high-voltage circuit changes, for example, when a 400V high-voltage circuit is switched on and off and the voltage changes from 0V to 400V, a voltage obtained by adding the change in the high-voltage circuit voltage, 400V, to the injection pulse voltage is applied to the insulation resistance, and the earth fault current Ig2 for an insulation resistance of 100kΩ during this transient period is Ig2 = (AC injection pulse voltage + change in high-voltage circuit voltage) / insulation resistance
=(2.5V+400V)/100kΩ=4.025mA
This results in a current value of about 4 mA.

For example, if the injection capacitor capacity is 10 μF, the interval between two measurements is 0.1 seconds (1/2 the cycle of an injection pulse voltage signal of 5 Hz), and the change in earth fault current Ig2 during this time is not large (assuming that the change in Ig2 over 0.1 seconds is not large compared to the time constant of 1 second for an injection capacitor capacity of 10 μF and insulation resistance of 100 kΩ), the change in voltage ΔVc across the injection capacitor between two measurements is ΔVc = Earth fault current Ig2 × Measurement time interval / Injection capacitor capacity
=4mA×0.1sec/10μF=40V
Due to this ΔVc, the difference in voltage applied to the insulation resistance between the two measurements transiently changes by 8 (= 40 V / 5 V) times as much as during steady state. This creates a problem in that insulation resistance cannot be monitored properly (insulation resistance value changes significantly) during the transient period immediately after the voltage change in the high-voltage circuit (immediately after switching on and off).

Here, as shown in FIG. 1 of Patent Document 3, even if one end of each of two coupling capacitors (injection capacitors) is connected to the positive and negative output terminals of a high-voltage battery, the other two ends of the two injection capacitors are connected to one end of a detection resistor, a pulse voltage signal is injected via the detection resistor, and measurements are taken at the connection point between the other two ends of the injection capacitors as the measurement point, the voltage change in the high-voltage circuit (for example, 400 V) is still applied to the insulation resistance, so a large charge/discharge current flows and the change in voltage across the injection capacitor between the two measurements also becomes large. This does not solve the problem of a corresponding large change in the earth-fault current obtained from the difference between the two measurements, i.e., a large change in the insulation resistance value.

To improve this change in ground fault current and insulation resistance value, the voltage change ΔVc across the injection capacitor can be suppressed to 0.5 V, which is, for example, 10% of the injection pulse voltage of 5 V, so that the capacitance of the injection capacitor would need to be 800 μF (= 10 μF × 80) or more, which is the voltage ratio of 0.5 V to the aforementioned 40 V (= 40 V/0.5 V), which is 80 times this voltage, but this is not practical in terms of the size and cost of the capacitor.

Furthermore, because the time constant of an injection capacitor capacity of 10 μF and an insulation resistance of 100 kΩ is 1 (= 10 μF × 100 kΩ) second, a waiting time of several seconds is required for the transient phenomenon caused by switching the switch on and off to settle and the ground fault current to stabilize, and proper measurement is not possible during this time. Increasing the injection capacitor capacity can reduce the voltage change ΔVc across the injection capacitor, but the time constant of injection capacitor capacity × insulation resistance increases, lengthening the time during which proper measurement is not possible, which creates a problem.

This invention has been made to solve the above-mentioned problems, and aims to provide a ground fault detection device that can measure ground fault currents to minute values with high accuracy and can measure insulation resistance with high accuracy immediately after a voltage change in a high-voltage circuit.

The ground fault detection device of this invention comprises at least one or more electrical circuits insulated from the earth potential, a voltage output means that repeatedly outputs a first period in which a first voltage is output and a second period in which a second voltage is output, alternately, with a voltage reference point as the reference point, an injection means that injects the output of the voltage output means into the electrical circuit, a current measurement means that measures and outputs the injection current injected by the injection means, and a calculation means that calculates the insulation resistance between the electrical circuit and the earth potential based on the output of the current measurement means, wherein the injection means has at least one or more injection capacitors having one end connected directly or via a resistor to the output of the voltage output means and the other end connected directly or via a resistor to the electrical circuit, and a detection resistor having one end connected to the voltage reference point and the other end connected directly or via a resistor to the earth potential, and the current measurement means measures the voltage across the detection resistor.

According to this invention, one end of the detection resistor that detects the earth fault current is connected to the voltage reference point of the output of the voltage output means, and the current measurement means measures the voltage across the detection resistor, thereby enabling highly accurate earth fault current measurement with little influence from the output voltage of the voltage output means or leakage current to the measurement circuit.

1 is a circuit block diagram showing a ground fault detection device and a high-voltage circuit according to a first embodiment of the present invention; FIG. 4 is a diagram showing measurement points of an injection current in the first embodiment of the present invention. FIG. 10 is a diagram showing a simulation result according to the first embodiment of the present invention. FIG. 10 is a diagram showing a simulation result after 2 seconds have elapsed in the first embodiment of the present invention. FIG. 2 is a diagram showing an equivalent circuit of an injection circuit according to the first embodiment of the present invention. FIG. 10 is a circuit block diagram showing a ground fault detection device and a high-voltage circuit according to a second embodiment of the present invention. FIG. 10 is a diagram showing a simulation result in the second embodiment of the present invention. FIG. 10 is a diagram showing a simulation result after 2 seconds have elapsed in the second embodiment of the present invention.

Embodiment 1.
FIG. 1 is a circuit block diagram showing a ground fault detection device 100 and a high voltage circuit 50 according to a first embodiment of the present invention, and FIG. 2 is a diagram showing the measurement time points of the injected current.

In FIG. 1 , a high-voltage circuit 50 includes a DC power supply 51, a switch means 52 for switching the output of the DC power supply 51 on and off or for increasing or decreasing the voltage, a capacitor 53 connected in parallel to the output of the switch means 52, and a load 54. An insulation resistance Rga and a capacitance Cga are present between a positive electric circuit 54a of the load 54 and the vehicle earth (chassis) 60, and an insulation resistance Rgb and a capacitance Cgb are present between a negative electric circuit 54b of the load 54 and the vehicle earth 60.

The DC power supply 51 is a high-voltage battery or a power supply obtained by stepping up or down the output of a high-voltage battery, for example, a 400 V power supply; the switch means 52 is a mechanical or semiconductor switch that switches the current circuit on and off, or a power conversion device that varies the voltage to any voltage between 0 V and 400 V, for example; the capacitor 53 is a smoothing capacitor or an X capacitor (a capacitor connected between lines) for noise prevention; and the load 54 is, for example, a DC circuit section of a motor drive device or an electrically heated catalyst (EHC), etc.

The capacitances Cga and Cgb are the combined capacitances of stray capacitances present between the electric circuits 54a, 54b and the vehicle body earth 60, and noise-suppressing Y capacitors (capacitors connected between the line and the earth) connected to the electric circuits 54a, 54b of each device in the high-voltage circuit 50, and the insulation resistances Rga and Rgb are the insulation resistances between the electric circuits 54a, 54b and the vehicle body earth 60, which normally have a good insulation state of 20 MΩ or more, but may deteriorate to 100 kΩ or less in the event of an abnormality.

The ground fault detection device 100 measures the ground fault current flowing through this insulation resistance (the parallel combined resistance of Rga and Rgb) and monitors the insulation state of the high voltage circuit 50, and has a voltage output means 10, an injection means 20, a current measurement means 30, and a calculation means 40.

The voltage output means 10 includes a DC power supply 10b that supplies a predetermined voltage (e.g., 15 V) relative to a voltage reference point 10a, a DC power supply 10c that supplies a predetermined voltage (e.g., −15 V), a switching element Q1 having one end connected to the positive terminal of the DC power supply 10b, and a switching element Q2 having one end connected to the other end of the switching element Q1 and the other end connected to the negative terminal of the DC power supply 10c. By alternately turning on and off the switching elements Q1 and Q2 every predetermined period (e.g., 0.1 seconds), a rectangular wave pulse voltage of a predetermined voltage (e.g., ±15 V) and a predetermined frequency (e.g., 5 Hz) relative to the voltage reference point 10a is output from a voltage output point 10d (the connection point between the other end of the switching element Q1 and one end of the switching element Q2). This output becomes an injection pulse voltage signal with an “L” level (first voltage, e.g., −15 V) and an “H” level (second voltage, e.g., 15 V).

The injection means 20 injects the injection pulse voltage output by the voltage output means 10 into the high voltage circuit 50, and includes a resistor R1 having one end connected to the voltage output point 10d, an injection capacitor C1 having one end connected to the other end of the resistor R1, and a detection resistor R2 having one end connected to the voltage reference point 10a and the other end connected to the vehicle body earth 60, the other end of the injection capacitor C1 being connected to the negative side electric circuit 54b of the high voltage circuit 50. The other end of the injection capacitor C1 is connected to either the positive side electric circuit 54a or the negative side electric circuit 54b of the high voltage circuit 50. Note that resistors may be inserted between the other end of the detection resistor R2 and the vehicle body earth 60, and between the other end of the injection capacitor C1 and the electric circuit 54b.

The current measuring means 30 measures the injection current generated by the voltage injected by the injection means 20, and has an operational amplifier 31. The operational amplifier 31 has a positive terminal 31a connected to the other end of the detection resistor R2 (vehicle body earth 60) and a negative terminal 31b connected to an operational amplifier output 31c. A non-inverting amplifier circuit is formed with one end of the detection resistor R2 (voltage reference point 10a) as the reference, the other end of the detection resistor R2 (vehicle body earth 60) as the input, and the operational amplifier output 31c as the output. The operational amplifier output 31c, which is based on the voltage reference point 10a, is output to the calculation means 40 as a current measurement output 32.

The calculation means 40 measures the current measurement output 32 of the current measurement means 30 at a predetermined measurement time, calculates the insulation resistance, and detects an injection abnormality. First, the measurement time will be described.

2 is a diagram showing the measurement points of the injection current, and shows the waveform of the current measurement output 32 of the current measuring means 30, in which period TL (first period) is the period when the injection pulse voltage is at the "L" level (first voltage), and period TH (second period) is the period when the injection pulse voltage is at the "H" level (second voltage). Periods TL and TH are repeated alternately at predetermined intervals (e.g., 0.1 seconds), and FIG. 2 shows three periods, period TL, period TH, and period TL, as extracted from the diagram.

At the start of period TL of the first period (the point at which the injected pulse voltage changes from "H" level to "L" level), inrush current due to charging of capacitances Cga and Cgb flows from vehicle earth 60 to electrical circuits 54a and 54b, and flows through detection resistor R2 from voltage reference point 10a to vehicle earth 60, causing current measurement output 32 to swing sharply in the negative polarity direction (downward in the figure) and then decrease (transient change). Point 30a1, immediately after the start of period TL (third measurement point), indicates the measurement point in the middle of this transient change region.

When the charging of capacitances Cga and Cgb is completed and this transient change ends, a stable current flows through insulation resistors Rga and Rgb in the direction from vehicle earth 60 to electrical circuits 54a and 54b, and a current flows through detection resistor R2 in the direction from voltage reference point 10a to vehicle earth 60, and a stable voltage in the negative polarity (downward in the figure) is output from current measurement output 32. Point 30b1, just before the end of period TL (first measurement point), indicates the measurement point in this stable region.

At the beginning of the second period TH after the first period (when the injection pulse voltage changes from "L" to "H"), inrush current due to charging of capacitances Cga and Cgb flows from electrical paths 54a and 54b toward vehicle earth 60, and then flows through detection resistor R2 from vehicle earth 60 toward voltage reference point 10a, causing current measurement output 32 to temporarily swing significantly in the positive polarity direction (upward in the figure) and then decrease (transient change). Point 30c1, immediately after the beginning of period TH (fourth measurement point), indicates the measurement point in the middle of this transient change. When this transient change ends, current measurement output 32 stabilizes as it flows through insulation resistors Rga and Rgb, and point 30d1, immediately before the end of period TH (second measurement point), indicates the measurement point in this stable region.

The time point 30a2 of the period TL of the third period corresponds to the time point 30a1 of the first period TL, and the time point 30b2 corresponds to the time point 30b1.

Next, the measuring means, calculating means, and detecting means of the calculating means 40 will be described.
The calculation means 40 includes a first measurement means 41 that measures the current measurement output 32 just before the end of the period TL (time point 30b1, time point 30b2, etc.), divides it by the detection resistor R2, and holds and outputs it as output 41a (first measurement value) (as the current value flowing through the detection resistor R2); a second measurement means 42 that measures the current measurement output 32 just before the end of the period TH (time point 30d1, etc.), divides it by the detection resistor R2, and holds and outputs it as output 42a (second measurement value); a third measurement means 43 that measures the current measurement output 32 just after the start of the period TL (time point 30a1, time point 30a2, etc.), divides it by the detection resistor R2, and holds and outputs it as output 43a (third measurement value); and a fourth measurement means 44 that measures the current measurement output 32 just after the start of the period TH (time point 30c1, etc.), divides it by the detection resistor R2, and holds and outputs it as output 44a (fourth measurement value).

The calculation means 40 also includes first difference calculation means 45 which calculates the difference between the first measurement means output 41a and the second measurement means output 42a at a calculation point (first calculation point) after measurement at the measurement point immediately before the end of the period TL or a calculation point (second calculation point) after measurement at the measurement point immediately before the end of the period TH, and outputs the difference to an output 45a; insulation resistance calculation means 46 which calculates the insulation resistance from the first difference calculation means output 45a, the first measurement means output 41a, and the second measurement means output 42a; and injection abnormality detection means 47 which detects the presence or absence of an injection abnormality from the third measurement means output 43a and the fourth measurement means output 44a at a calculation point (third calculation point) after measurement at the measurement point immediately after the beginning of the period TL or a calculation point (fourth calculation point) after measurement at the measurement point immediately after the beginning of the period TH.

The insulation resistance calculation means 46 of the calculation means 40 includes a current difference value (first difference value) for period TL of the first difference calculation means 45, which is the difference calculation value between the first measurement means output 41a for period TL and the second measurement means output 42a for the immediately preceding period TH, a current difference value (second difference value) for period TH of the first difference calculation means 45, which is the difference calculation value between the second measurement means output 42a for period TH and the first measurement means output 41a for the immediately preceding period TL, second difference calculation means 46a (not shown), injection capacitor voltage change calculation means 46b (not shown), and resistance voltage drop calculation means 46c (not shown), and the second difference calculation means 46a calculates the difference between the current difference value for period TL and the current difference value for period TH.

The injection capacitor voltage change calculation means 46b of the insulation resistance calculation means 46 uses at least one of the first measurement means output 41a and the second measurement means output 42a to calculate the voltage change (difference) in the voltage across the injection capacitor immediately before the end of period TL and immediately before the end of period TH of the immediately previous period (time point 30b1 and time point 30d1, etc.), or immediately before the end of period TH and immediately before the end of period TL of the immediately previous period (time point 30d1 and time point 30b2, etc.).

The resistance voltage drop calculation means 46c of the insulation resistance calculation means 46 uses the first difference calculation means output 45a and the resistance values of resistor R1 and detection resistor R2, which are resistances of the injection current path of the injection means 20, to calculate the difference in voltage drop across resistor R1 and detection resistor R2 just before the end of period TL of the previous period and just before the end of period TH (time point 30b1 and time point 30d1, etc.), or just before the end of period TH of the previous period and just before the end of period TL (time point 30d1 and time point 30b2, etc.).

The insulation resistance calculation means 46 of the calculation means 40 calculates the insulation resistance using one of the first to third insulation resistance calculation methods described below.

The first insulation resistance calculation method determines the insulation resistance value from the output 45a of the first difference calculation means based on an approximate formula or a conversion table that has been set up in advance through experiments and simulations, and selects which of the periods TL and TH the output 45a of the first difference calculation means (current difference value) should be used.

In the second insulation resistance calculation method, the insulation resistance value is calculated from the calculation result of the second difference calculation means 46a based on an approximation formula or a conversion table that has been set up in advance through experiments and simulations.

The third insulation resistance calculation method calculates a parallel combined resistance value of the insulation resistances Rga and Rgb from the voltage difference between the “H” level and the “L” level of the voltage output point 10d, the calculation result of the injection capacitor voltage change calculation means 46b, the calculation result of the resistance voltage drop calculation means 46c, a capacitance-induced voltage 46d (described later) of the injection capacitor C1 caused by the capacitances Cga and Cgb, and a capacitance-induced current 46e (described later) caused by the capacitances Cga and Cgb.

The injection abnormality detection means 47 of the calculation means 40 determines that an injection abnormality has occurred when both the third measurement means output 43a and the fourth measurement means output 44a are within a predetermined judgment value range, or when the difference value between the third measurement means output 43a and the fourth measurement means output 44a is within a predetermined judgment value range.

In the ground fault detection device 100 configured as described above, when the voltage output means 10 outputs a voltage change of -30V from an "H" level, for example, 15V, to an "L" level, for example, -15V, during the period TL in Fig. 2, a charging current to the capacitances Cga and Cgb flows from the vehicle body earth 60 to the electrical circuits 54a and 54b as a current injected into the high voltage circuit 50 at the start of the period TL. This injected current is limited by the resistor R1 and the detection resistor R2, and if the combined series resistance value R10 of the two resistors is, for example, 12 kΩ, then a maximum current of -2.5 mA (=-30V/R10=-30V/12 kΩ) will flow, ignoring the effect of the injection capacitor C1.

If the parallel combined capacitance C10 of the electrostatic capacitances Cga and Cgb is, for example, 0.4 μF, charging occurs with a charging time constant of 4.8 ms (= R10 × C10 = 12 kΩ × 0.4 μF), and this charging current decreases exponentially. After 80 ms, which is 16 times the charging time constant, charging is complete and the injected current stabilizes at the ground-fault current value, which is the current flowing through the insulation resistances Rga and Rgb. Point 30b1 in FIG. 2 is, for example, 80 ms after the start of period TL, and the first measurement means 41 measures the current measurement output 32 corresponding to the current flowing through detection resistor R2 at this point.

Even when the voltage output means 10 outputs a voltage change from "L" level to "H" level during the period TH, the same operation occurs with the polarity of the current reversed, and the point 30d1 in Figure 2 is the point at which, for example, 80 ms have elapsed from the start of the period TH, and the second measurement means 42 measures the current measurement output 32 corresponding to the current flowing through the detection resistor R2 at this point.

If the parallel combined resistance R11 of the insulation resistances Rga and Rgb is, for example, 10 MΩ, the earth fault current will be ±1.5 μA (= ±15 V/R11 = ±15 V/10 MΩ), and this minute current will flow through the detection resistor R2. If the detection resistor R2 is, for example, 2 kΩ, the voltage across it will be ±3 mV (= ±1.5 μA × 2 kΩ), and the current measuring means 30 must measure this minute voltage with high precision.

The detection resistor R2, which detects the ground fault current, has one end connected to the voltage reference point 10a of the voltage output means 10, and the current measuring means 30 measures the other end of the detection resistor R2 based on the voltage reference point 10a, thereby being able to measure the voltage across the detection resistor R2, i.e., the ground fault current, with high accuracy.

In this configuration, the main cause of measurement error in the voltage across detection resistor R2 is the input offset voltage of operational amplifier 31. If a standard operational amplifier with a low input offset voltage (for example, an input offset voltage of 30 μV or less) is used for operational amplifier 31, the effect of that input offset voltage is small, at 1% or less (= 30 μV/3 mV) of the 3 mV voltage across detection resistor R2 mentioned above. Because there are no other unnecessary voltage variations or fluctuations that could cause errors, even μA-level ground fault currents can be measured with high accuracy.

The current measuring means 30 also uses a non-inverting amplifier circuit configuration, and only the positive terminal 31a of the operational amplifier 31 is connected to the other end of the detection resistor R2, which serves as the measurement point. The input impedance of the measurement circuit, which is another major cause of earth-fault current measurement errors due to the voltage across the detection resistor R2, exists only in the operational amplifier 31. If a standard operational amplifier with a high input impedance (for example, an input current of 500 pA or less) is used for the operational amplifier 31, the effect of that input impedance (input current) is small, at 0.033% (= 500 pA/1.5 μA) or less for the above-mentioned earth-fault current of 1.5 μA. Since no extraneous leakage current, which would cause earth-fault current errors, flows through the detection resistor R2, even earth-fault currents at the μA level can be measured with high accuracy.

The current measuring means 30 can also be configured as an inverting amplifier circuit. In this case, the input impedance of the measuring circuit corresponds to the input resistance of the inverting amplifier circuit, which is equivalent to being connected in parallel to the detection resistor R2. The influence of the input impedance can be reduced by using a high-precision resistor for the input resistance. However, since the input offset current of the operational amplifier in the inverting amplifier circuit multiplied by the combined parallel resistance of the input and output resistances is added to the input offset voltage, increasing the input and output resistances increases the influence of the input offset voltage. Furthermore, decreasing the input resistance increases the current that the operational amplifier must process, necessitating a larger control power supply for the operational amplifier.

Furthermore, here, one end of the detection resistor R2 was used as a reference and the other end of the detection resistor R2 was measured, but depending on how the control power supply for the operational amplifier is taken, one end of the detection resistor R2 may be used as a reference and one end of the detection resistor R2 may be measured.

Furthermore, resistor R1 can limit the injected current to protect ground fault current detection device 100 from failure due to overcurrent if electric paths 54a, 54b of high voltage circuit 50 are accidentally short-circuited to vehicle body earth 60. In this case, it is desirable to connect a Zener diode (not shown) in anti-series connection in parallel to detection resistor R2 to prevent unnecessary overvoltage from being applied to operational amplifier 31 of current measuring means 30.

Here, the operation of the ground fault detection device 100 when the voltage applied to the load 54 of the high voltage circuit 50 changes will be described.

The operation when the switch means 52 of the high voltage circuit 50 is turned on and the output voltage of the DC power supply 51 is applied to the load 54 will be described using waveforms simulated by a circuit simulator.
1, the simulation conditions are as follows: the output voltage of DC power supply 51 of high-voltage circuit 50 is 400 V, capacitor 53 is 20 μF, load 54 is 50 Ω, capacitances Cga and Cgb are each 0.2 μF, the output of voltage output means 10 of ground fault detection device 100 is a ±15 V 5 Hz rectangular wave pulse, the combined series resistance of resistor R1 and detection resistor R2 is 12 kΩ, injection capacitor C1 is 10 μF, insulation resistance Rga is 100 kΩ (insulation deterioration state), and Rgb is 1000 MΩ (insulation sound state). The measurement points of first measurement means 41 and second measurement means 42 are 0.08 seconds after the start of periods TL and TH.

3A and 3B show the results of the simulation. FIG. 3A shows the waveform at voltage output point 10d of voltage output means 10, which outputs -15V at "L" level for 0.1 seconds from time 0 seconds, then outputs 15V at "H" level for 0.1 seconds, and this is repeated thereafter.

3(b) is a diagram showing the waveform of the voltage applied to the load 54, in which the switch means 52 is turned on at time 0.05 seconds, and 400 V is applied to the load 54. In this simulation, a case where the voltage applied to the load 54 suddenly changes at this timing is examined.

FIG. 3( c) is a diagram showing the waveform (injection current waveform) of the current measurement output 32 of the current measuring means 30 and the measurement times. The measurement times are 0.08 seconds (time 30 b 1 in period TL) immediately before the end of the injection voltage pulse voltage “L” level (period TL), 0.18 seconds (time 30 d 1 in period TH) immediately before the end of the injection voltage pulse voltage “H” level (period TH), 0.28 seconds (time 30 b 2 in period TL), 0.38 seconds (time 30 d 2 in period TH), and 0.48 seconds (time 30 b 3 in period TL).

In the injection current waveform, at time 0.05 seconds when 400 V is applied to load 54, a large charging current flows in the positive polarity direction (upward in the figure) through capacitances Cga and Cgb, and thereafter, each time the injection pulse voltage changes from "L" to "H" and from "H" to "L," the charging current flows in the positive polarity direction (upward in the figure) and the negative polarity direction (downward in the figure) through capacitances Cga and Cgb. As injection capacitor C1 is charged by the injection current, the injection current decreases overall.

3(d) is a line graph showing the measured values of the injected current at each measurement point shown in FIG. 3(c), and shows the value of the current flowing through the detection resistor R2, which is the measured value of the current measurement output 32 divided by the resistance value of the detection resistor R2 (corresponding to the first measurement means output 41a and the second measurement means output 42a). The vertical axis is in μA, and the horizontal axis is in seconds.

FIG. 3(e) is a line graph showing the current difference value of the injected current at each measurement time point (each period) shown in FIG. 3(c). This graph shows the current difference value for the N period (corresponding to the first difference calculation means output 45a for the N period) obtained by dividing the difference value between the measured value of the current measurement output 32 at a measurement time point in a certain N period and the measured value of the current measurement output 32 at the measurement time point in the (N-1) period immediately preceding N by the resistance value of the detection resistor R2. The graph shows the current difference value from time point 30b1 at time point 30d1, the current difference value from time point 30d1 at time point 30b2, the current difference value from time point 30b2 at time point 30d2, and the current difference value from time point 30b3 at time point 30d2. The vertical axis is in μA, and the horizontal axis is in seconds. The current difference value at time point 30b1 is not plotted on the graph as it is an indefinite value. This indefinite value processing will be described later.

FIG. 3(f) is a line graph showing the difference in the injected current at each measurement time point (each period) shown in FIG. 3(c). It shows the difference (corresponding to the calculation result of the second difference calculation means 46a for the N period) between the current difference value for a certain N period (corresponding to the output 45a of the first difference calculation means for the N period) and the current difference value for the (N-1) period immediately preceding the N period (corresponding to the output 45a of the first difference calculation means for the (N-1) period). At time point 30b2, the difference value from time point 30d1 is shown. At time point 30d2, the difference value from time point 30b2 is shown. At time point 30b3, the difference value from time point 30d2 is shown. The vertical axis is in μA, and the horizontal axis is in seconds. Note that the calculations at time points 30b1 and 30d1 involve calculations involving the aforementioned indefinite current difference values, and therefore are not plotted on the graph.

Figure 3(g) is a line graph showing the calculated insulation resistance values at each measurement time point (each period) shown in Figure 3(c), with the vertical axis in kΩ and the horizontal axis in seconds. Note that time point 30b1 is not plotted on the graph because the calculation involves the indefinite value of the current difference value mentioned above.

Figure 4 shows the simulation results after 2 seconds have passed. As with Figure 3, Figure 4(a) shows the waveform at voltage output point 10d of voltage output means 10, Figure 4(b) shows the voltage waveform applied to load 54, and Figure 4(c) shows the waveform of current measurement output 32 (injected current waveform) of current measurement means 30 and the measurement times. The measurement times are 2.08 seconds (time 30b11 of period TL) just before the end of the injection voltage pulse voltage "L" level (period TL), 2.18 seconds (time 30d11 of period TH) just before the end of the injection voltage pulse voltage "H" level (period TH), 2.28 seconds (time 30b12 of period TL), 2.38 seconds (time 30d12 of period TH), and 2.48 seconds (time 30b13 of period TL).

4(d) is a line graph showing the measured values of the injected current at each measurement point shown in FIG. 4(c), and shows the value of the current flowing through the detection resistor R2, which is the measured value of the current measurement output 32 divided by the resistance value of the detection resistor R2 (corresponding to the first measurement means output 41a and the second measurement means output 42a). The vertical axis is in μA, and the horizontal axis is in seconds.

4(e) is a line graph showing the current difference value of the injected current at each measurement time point (each period) shown in FIG. 4(c), and shows the current difference value for period N (corresponding to output 45a of first difference calculation means for period N) obtained by dividing the difference value between the measured value of current measurement output 32 at a measurement time point in a certain period N and the measured value of current measurement output 32 at a measurement time point in the (N-1) period immediately before N by the resistance value of detection resistor R2. At time point 30b11, the current difference value from time 1.98 seconds is shown; at time point 30d11, the current difference value from time point 30b11; at time point 30b12, the current difference value from time point 30d11; at time point 30d12, the current difference value from time point 30b12; and at time point 30b13, the current difference value from time point 30d12. The vertical axis is in μA, and the horizontal axis is in seconds.

4(f) is a line graph showing the difference in the current difference value of the injected current at each measurement time point (each period) shown in FIG. 4(c), showing the difference (corresponding to the calculation result of the second difference calculation means 46a for the N period) between the current difference value for a certain N period (corresponding to the output 45a of the first difference calculation means for the N period) and the current difference value for the (N-1) period immediately preceding N (corresponding to the output 45a of the first difference calculation means for the (N-1) period). At time point 30b11, the difference value from time 1.98 seconds is shown; at time point 30d11, the difference value from time point 30b11; at time point 30b12, the difference value from time point 30d11; at time point 30d12, the difference value from time point 30b12; and at time point 30b13, the difference value from time point 30d12. The vertical axis is in μA, and the horizontal axis is in seconds.

FIG. 4(g) is a line graph showing the calculated insulation resistance values at each measurement time point (each period) shown in FIG. 4(c), with the vertical axis in kΩ and the horizontal axis in seconds.

Here, the simulation results will be explained.
As shown in Fig. 3(d), the injection current flowing through the detection resistor R2 is approximately 3.2 mA immediately after the switch means 52 is turned on. This current gradually decreases, and after about 2 seconds, as shown in Fig. 4(d), it is approximately 500 µA.

3(e), the current difference value of the injected current immediately after the switch means 52 is turned on is small at time point 30d1 and time point 30d2. This is because the injected current is large immediately after the switch means 52 is turned on, the injection capacitor C1 is charged quickly, the voltage change ΔVc between the voltage across the injection capacitor C1 and the measurement time point in the immediately preceding period is about 30 V, which cancels out the 30 V of the voltage change ΔV10d of the injected pulse voltage from the "L" level to the "H" level, and the difference in voltage applied to the insulation resistance Rga between the immediately preceding measurement time point 30b1 and measurement time point 30d1 and between the immediately preceding measurement time point 30b2 and measurement time point 30d2 is small.

Conversely, the current difference value of the injection current becomes large at time 30b2 and time 30b3. This is because the voltage change ΔVc between the voltage across injection capacitor C1 and the measurement time point in the previous period is added to the voltage change ΔV10d of −30 V from the “H” level to the “L” level of the injection pulse voltage, and the difference in voltage applied to insulation resistance Rga between the previous measurement time point 30d1 and measurement time point 30b2 and between the previous measurement time point 30d2 and measurement time point 30b3 becomes large.

4(e), when about two seconds have passed since the switch means 52 was turned on, the current difference values ΔId11 and ΔId12 at the measurement points (30d11, 30d12) of the period TH are 196 μA and 204 μA, and the current difference values ΔIb12 and ΔIb13 at the measurement points (30b12, 30b13) of the period TL are −290 μA and −283 μA, so the difference between the periods TH and TL becomes small. After a sufficient amount of time has passed, this difference disappears and the values stabilize, with the average value of the absolute values of ΔId12 and ΔIb13 being 244 μA (= (204 μA + 283 μA) / 2). An absolute value of 244 μA of the current difference value can be correlated with an insulation resistance value of 100 kΩ.

As shown in FIG. 3(e), immediately after the switch means 52 is turned on, the current difference values ΔId1 and ΔId2 at the measurement points (30d1, 30d2) of the period TH are −40 μA and 13 μA, and the current difference values ΔIb2 and ΔIb3 at the measurement points (30b2, 30b3) of the period TL are −499 μA and −458 μA. If the aforementioned 244 μA is used as the reference, the variation in absolute values between the periods TH is 27 (=40−13) μA. In comparison, it is 11% (= 27 μA / 244 μA), the variation in absolute values between periods TL is 41 (= 499 - 458) μA, which is 17% of the reference, the variation in absolute values between periods TH and TL is 459 (= 499 - 40) μA, which is 188% of the reference, and ΔIb2 at point 30b2 is -499 μA, which is 205% of the reference, so the variation between periods TH and TL is large and the error in the current difference value in period TL is also large.

At time 30b13, when more than two seconds have passed since the switch means 52 was turned on, ΔIb13 is −283 μA, which is 116% of the reference, and even at this time the error is not small at 16%.

Here, the insulation resistance calculation method and calculation results of the insulation resistance calculation means 46 of the calculation means 40 will be explained, but before that, the uncertain value processing of the current difference value mentioned in the explanation of FIG. 3(e) will be explained.

3(e), the current difference value at time 30b1 is set to an indefinite value, but because time 30b1 is the time point for measurement in the first period after switch means 52 is turned on, switch means 52 is still off at the time of measurement in the previous period, and when the current difference value is calculated by first difference calculation means 45 before and after the different states of switch means 52, the calculated value is 3090 μA, which is an abnormally excessive value. Since it is not desirable to use this excessive value in calculating the insulation resistance, this excessive current difference calculation value is set to an indefinite value, and calculations related to this indefinite value are not performed, and the immediately preceding calculated insulation resistance value is maintained.

The period for which the current difference value is to be determined as an indefinite value can be determined by determining a prominent period as an indefinite value based on the measured value of the current measurement output 32 and the transition of the current difference value, or by determining the control content and timing of the switch means 52 of the high voltage circuit 50, if these are known.

The insulation resistance calculation means 46 calculates the insulation resistance using one of the first to third insulation resistance calculation methods. First, the first insulation resistance calculation method and the calculation results will be described.

In the first insulation resistance calculation method, the correlation between the first difference calculation output 45a and the insulation resistance value is prepared in advance by experiment or simulation as an approximate formula or conversion table, and the insulation resistance value is calculated from the first difference calculation means output 45a based on the approximate formula or conversion table. However, instead of using all of the first difference calculation means outputs 45a, the first difference calculation means output 45a for either period TL or period TH is selected to be used. Also, the aforementioned indefinite value processing of the current difference value is performed.

In the conventional method (Patent Document 1), the difference between the "H" level and the "L" level of the injected pulse voltage is measured, and the aforementioned processing of uncertain values is not performed. Therefore, immediately after the switch means 52 is turned on, 3090 μA is measured at time 30b1 (0.08 seconds later), -499 μA at time 30b2 (0.28 seconds later), and -459 A at time 30b3 (0.48 seconds later), resulting in large ground fault currents (current difference values) with absolute values of 1266%, 205%, and 188% of the aforementioned reference value of 244 μA. Also, at time 30b13, 2.48 seconds later, -283 μA is measured, which is 116% of the aforementioned reference value of 244 μA, and is 16% larger even after more than 2 seconds have passed. A large earth fault current (current difference value) means that the insulation resistance is small and the degree of insulation deterioration has worsened, so when the switch means 52 is turned on, excessive insulation deterioration information is temporarily issued, which is a problem.

In order to prevent the temporary issuance of excessive insulation deterioration information when the switch means 52 is turned on, in addition to the aforementioned indefinite value processing of the current difference value, by selecting the output 45a of the first difference calculation means during the period TH when the current difference values ΔId1 and ΔId2 are small, rather than during the period TL when the current difference values ΔIb2 and ΔIb3 are large, the current difference values become −40 μA and 13 μA. In this case, the absolute values are 16% and 5% smaller than the aforementioned reference value of 244 μA, and the calculated insulation resistance value becomes a larger value accordingly, thereby preventing the issuance of excessive insulation deterioration information that was temporarily issued in the prior art (Patent Document 1).

The current difference value (first difference calculation means output 45a) becomes small when the polarity of the change in the charging voltage of injection capacitor C1 acts to cancel out the polarity of the change in the output voltage of the injection pulse voltage. In the case where injection capacitor C1 is connected to negative side electric circuit 54b and insulation resistance Rga is deteriorated (100 kΩ) as in this case, period TH becomes smaller when switch means 52 is turned on, and when switch means 52 is turned off, the polarity of the change in the charging voltage of injection capacitor C1 is reversed, so period TL becomes smaller. In the case where injection capacitor C1 is connected to positive side electric circuit 54a and insulation resistance Rgb is deteriorated (100 kΩ), the polarity of the change in the charging voltage of injection capacitor C1 is reversed, so period TL becomes smaller when switch means 52 is turned on, and period TH becomes smaller when switch means 52 is turned off. By selecting whether to use the current difference value (output 45a of the first difference calculation means) for the period TL or the period TH depending on the situation, it is possible to prevent excessive issuance of information about insulation deterioration.

Between the periods TL and TH, the period with the smaller absolute value of the current difference value (the smaller absolute value of the first difference calculation means output 45a) excluding the aforementioned indefinite value may be selected, or if the control content and timing of the switch means 52 of the high voltage circuit 50 etc. are known, the period may be selected based on the control content and timing.

Next, a second insulation resistance calculation method and calculation results performed by the insulation resistance calculation means 46 will be described. In the second insulation resistance calculation method, an approximation formula or conversion table is prepared in advance by experiment or simulation to show the correlation between the calculation results of the second difference calculation means 46a and the insulation resistance value, and the insulation resistance value is calculated from the calculation results of the second difference calculation means 46a based on the approximation formula or conversion table. In addition, the aforementioned indefinite value processing of the current difference value is performed.

The calculation results of the second difference calculation means 46a are shown in FIG. 3(f) and FIG. 4(f).
4(f), the calculation results of the second difference calculation means 46a approximately two seconds after the switch means 52 is turned on are -494 μA at time point 30d12 (2.38 seconds later) and 487 μA at time point 30b13 (2.48 seconds later), with the absolute values of the two approaching each other and stabilizing over time to an average value of 490 μA of the two absolute values. Therefore, when the calculation result of the second difference calculation means 46a is 490 μA in absolute value, it can be correlated with the insulation resistance Rga being 100 kΩ.

Using this 490 μA as the reference, the calculation results of the second difference calculation means 46a immediately after the switch means is turned on are as shown in FIG. 3(f), with the calculated value at time 30b2 (0.28 seconds later) being 458 μA, which is 93.5% of the reference value of 490 μA, and the calculated value at time 30d2 (0.38 seconds later) being −512 μA, which is 104.5% of the reference value, making it possible to calculate the insulation resistance with an accuracy of ±7% or less even immediately after the switch means 52 is turned on.

In the conventional method (Patent Document 1), the calculated value at time 30b2 (0.28 seconds later) immediately after the switch means 52 is turned on is 205% of the reference value, as described above. However, in the second insulation resistance calculation method, the calculated value is 93.5% of the reference value, enabling more accurate insulation resistance calculation.

Next, we will explain the third insulation resistance calculation method and the calculation results performed by the insulation resistance calculation means 46. First, we will explain the third insulation resistance calculation method using the equivalent circuits of the injection circuit at the three measurement points in Figure 3(c). Here, the equivalent circuit of the injection circuit is an equivalent representation of the path through which the injected current flows in Figure 1, where Rg is the parallel combined insulation resistance of the insulation resistances Rga and Rgb, and the electrostatic capacitances Cga and Cgb are excluded. The influence of the electrostatic capacitances Cga and Cgb will be described later.

5(a) shows the equivalent circuit of the injection circuit at time 30b1 in FIG. 3(c), FIG. 5(b) shows the equivalent circuit of the injection circuit at time 30d1, and FIG. 5(c) shows the equivalent circuit of the injection circuit at time 30b2.

In FIG. 5A, if the injection current is Ib1, the voltage across the injection capacitor C1 is Vcb1, the voltage drop across the resistor R1 and the detection resistor R2 is Vrb1, the DC power supply 51 is 400 V, and the DC power supply 10c is −15 V, then the voltage Vgb1 applied to the insulation resistor Rg is given by the following equation (1).

Furthermore, if the combined series resistance of the resistor R1 and the detection resistor R2 is R10, the voltage drop Vrb1 is given by the following equation (2).

In FIG. 5B, if the injected current is Id1, the voltage across the injection capacitor is Vcd1, the voltage drop across resistor R1 and detection resistor R2 is Vrd1, the DC power supply 51 is 400 V, and the DC power supply 10b is 15 V, then the voltage Vgd1 applied to the insulation resistance Rg is given by the following equation (3).

The voltage drop Vrd1 is expressed by the following formula (4).

Here, when the difference between Vgd1 and Vgb1, Vgd1-Vgb1, is calculated from equation (3)-equation (1), equation (2), and equation (4), the following equation (5) is obtained.

Furthermore, Vgb1 and Vgd1 are the products of the insulation resistance Rg and the injected current, and are expressed by the following equations (6) and (7).

Therefore, the difference Vgd1-Vgb1 between Vgd1 and Vgb1 is given by the following equation (8) from equation (7)-equation (6).

Furthermore, the following equation (9) is obtained from equations (5) and (8).

The difference in the injected current Id1-Ib1 and the difference in the voltage across the injection capacitor Vcd1-Vcb1 are

Then, from equation (9), the insulation resistance Rg is given by the following equation (12).

From the above, the insulation resistance Rg can be calculated using equation (12) at time point 30d1 of the period TH, and can also be calculated in the same manner for other periods TH.

The numerator on the right side of equation (12) corresponds to the difference in voltage applied to insulation resistance Rg, the denominator on the right side corresponds to the difference in ground-fault current flowing through insulation resistance Rg, 30V corresponds to the voltage difference between the "L" level and the "H" level at the start of period TH at voltage output point 10d, ΔVcd1 corresponds to the calculation result of the measurement value at time 30d1 by injection capacitor voltage change calculation means 46b (described later), R10 × ΔId1 corresponds to the calculation result of the measurement value at time 30d1 by resistance voltage drop calculation means 46c (described later), and ΔId1 corresponds to the output 45a of the first difference calculation means for the measurement value at time 30d1. Furthermore, no calculation is performed for the indefinite value of the current difference value described above, and the previous calculation value is maintained.

In addition, in FIG. 5(c), if the injection current is Ib2, the voltage across the injection capacitor C1 is Vcb2, the voltage drop across the resistor R1 and the detection resistor R2 is Vrb2, the DC power supply 51 is 400 V, and the DC power supply 10c is −15 V, then the voltage Vgb2 applied to the insulation resistance Rg is given by the following equation (13).

The voltage drop Vrb2 is expressed by the following equation (14).

Here, when the difference between Vgb2 and Vgd1, Vgb2-Vgd1, is calculated from equation (13)-equation (3), equation (4), and equation (14), the following equation (15) is obtained.

Furthermore, Vgb2 is the product of the insulation resistance Rg and the injected current, and is expressed by the following equation (16):

Therefore, the difference Vgb2-Vgd1 is given by the following equation (17) from equation (16)-equation (7).

Furthermore, the following equation (18) is obtained from equations (15) and (17).

The difference in the injected current Ib2-Id1 and the difference in the voltage across the injection capacitor Vcb2-Vcd1 are

Then, the insulation resistance Rg is expressed by the following equation (21).

From the above, the insulation resistance Rg can be calculated using equation (21) at time point 30b2 of period TL, and can also be calculated in the same manner during other periods TL.

The numerator on the right side of equation (21) corresponds to the difference in the voltage applied to the insulation resistance Rg, the denominator on the right side corresponds to the difference in the ground-fault current flowing through the insulation resistance Rg, -30 V corresponds to the voltage difference between the "H" level and the "L" level at the start of period TL at voltage output point 10d, ΔVcb2 corresponds to the calculation result of the measurement value at time point 30b2 by injection capacitor voltage change calculation means 46b (described later), R10 × ΔIb2 corresponds to the calculation result of the measurement value at time point 30b2 by resistance voltage drop calculation means 46c (described later), and ΔIb2 corresponds to the output 45a of the first difference calculation means for the measurement value at time point 30b2. Furthermore, no calculation is performed for the indefinite value of the current difference value described above, and the previous calculation value is maintained.

Here, we will explain the differences ΔVcd1 and ΔVcb2 in the voltage across the injection capacitor in the insulation resistance calculation formulas (12) and (21). These are calculated by the injection capacitor voltage change calculation means 46b.
From the capacitor characteristic equation (22) below,

The differences ΔVcd1 and ΔVcb2 in the voltages across the injection capacitor can be obtained by multiplying the charging current (injection current) of the injection capacitor C1 from time 30b1 to time 30d1 or from time 30d1 to time 30b2 over time and dividing the result by the capacitance of the injection capacitor C1. However, these can be simplified to the following first and second methods for calculating the change in the voltage across the injection capacitor, and it is sufficient to use either one of these methods to obtain the results.

Regarding these two calculation methods, the calculation method for the period TH will first be described using an example in which the measured value at time point 30d1 is calculated.

The first method for calculating the change in voltage across the injection capacitor is a method in which an injection current Ib1 at time point 30b1 flows as a charging current for injection capacitor C1 for the remaining time T1 of period TL (see FIG. 3, for example, 0.02 seconds), and an injection current Id1 at time point 30d1 flows for the elapsed time T2 of period TH (see FIG. 3, for example, 0.08 seconds). When the capacitance of injection capacitor C1 is C1, the difference ΔVcd1 is given by the following equation (23).

The second method for calculating the change in voltage across the injection capacitor is a method in which an injection current Id1 at time 30d1 flows as a charging current for the injection capacitor C1 for the entire time T3 (see FIG. 3 , for example, 0.1 seconds) of the period TH, and the difference ΔVcd1 when the capacitance of the injection capacitor C1 is C1 is given by the following equation (24).

Next, a method for calculating the period TL will be described using an example in which the measured value at time point 30b2 is calculated. In this case as well, ΔVcb2 is expressed by the following equation (25) in the first method for calculating the change in voltage across the injection capacitor, and by the following equation (26) in the second method for calculating the change in voltage across the injection capacitor.

In both the first and second methods for calculating the change in voltage across the injection capacitor, the differences ΔVcd1 and ΔVcb2 in the voltage across the injection capacitor can be calculated from the injection currents Ib1 and Ib2, which are the output 41a of the first measurement means at time 30b1 and time 30b2, and the injection current Id1, which is the output 42a of the second measurement means at time 30d1.

Furthermore, R10 × ΔId1 and R10 × ΔIb2 in the insulation resistance calculation formulas (12) and (21) are calculated by the resistance voltage drop calculation means 46c, and the difference in voltage drop values due to the resistance R1 and the detection resistance R2 can be calculated by multiplying the series combined resistance R10 of the resistance R1 and the detection resistance R2 by the current difference value (output 45a of the first differential current calculation means).

Up to this point, the explanation has been given excluding the electrostatic capacitances Cga and Ggb, but the error factors in the insulation resistance calculation caused by the electrostatic capacitances Cga and Ggb will be explained below.

When the injection pulse voltage changes from "H" level to "L" level or from "L" level to "H" level, the injection capacitor C1 is connected in series with the parallel combined capacitance Cg of the electrostatic capacitances Cga and Cgb and is charged and discharged, causing a change in the voltage across the injection capacitor C1. This change in the voltage across the injection capacitor ΔVc1 becomes an error factor.

ΔVc1 depends on the parallel combined capacitance Cg, and in the calculation for period TL, it is the voltage change ΔV10d (for example, −30 V) from the “H” level to the “L” level of the injection pulse voltage, and in the calculation for period TH, it is the voltage change ΔV10d (for example, 30 V) from the “L” level to the “H” level of the injection pulse voltage divided by the injection capacitor C1 and the electrostatic capacitance Cg, resulting in the following equation (27).

This ΔVc1 corresponds to the capacitance-induced voltage 46d, and by correctively adding this voltage change ΔVc1 to ΔVcd1 and ΔVcb2 in the above-mentioned equations (23) to (26), it is possible to calculate the insulation resistance with higher accuracy.

Furthermore, at the measurement point immediately before the end of periods TL and TH, when the voltage Vg applied across the insulation resistance Rg and capacitance Cg is stable, even if the change in the voltage Vg across the capacitance Cg is small, a charge/discharge current corresponding to that change flows through the capacitance Cg. This charge/discharge current flowing through the capacitance Cg is combined with the ground-fault current flowing through the insulation resistance Rg and flows through the detection resistor R2, which can be a source of error, especially when measuring a small ground-fault current.

The difference between the charge/discharge current of capacitance Cg at the measurement point immediately before the end of period TL and the measurement point immediately before the end of period TH corresponds to capacitance-induced current 46e, and is determined by the capacitance value of capacitance Cg and the resistance value of insulation resistance Rg. Therefore, by determining a correction value or correction formula for each insulation resistance value in advance through experiments or simulations and adding the correction value to ΔId and ΔIb in the denominators on the right-hand sides of equations (12) and (21), which are the insulation resistance calculation formulas, it is possible to calculate the insulation resistance with higher accuracy.

The results of calculations using the third insulation resistance calculation method described above are shown in Figures 3(g) and 4(g). In Figures 3(g) and 4(g), the first injection capacitor voltage change calculation method is used, and corrections are also made using the capacitance-induced voltage 46d and capacitance-induced current 46e.

As shown in Figure 3(g), the calculated insulation resistance value at time 30d1 (0.18 seconds later) immediately after switching on switch means 52 is 73.5 kΩ, which is smaller than the true value of 100 kΩ. However, from time 30b2 (0.28 seconds later) onwards, the calculated insulation resistance value can be calculated to be between 95.1 kΩ and 100.4 kΩ, with an error of 5% or less, making it possible to achieve more accurate measurements than with the second insulation resistance calculation method.

At time point 30d2 (0.38 seconds later), the resistance is 95.1 kΩ, an error of 5%, which is larger than the error at other times 30b2 and 30b3. This is because the current difference value ΔId2 during the aforementioned period TH is small at 13 μA, and both the numerator and denominator on the right-hand side of insulation resistance calculation formula (12) are small, which is the cause of the increase in error.

On the other hand, at time points 30b2 and 30b3 in period TL, the error is small at 0.4%, and more accurate calculations can be made by selecting the period with the larger absolute value of the current difference value depending on the situation and calculating the insulation resistance.

The period TL or TH having the larger absolute value of the current difference value (the larger absolute value of the first difference calculation means output 45a) may be selected, or if the control content and timing of the switch means 52 of the high voltage circuit 50 are known, the period may be selected based on the control content and timing.

The calculation results after about 2 seconds have passed since the switch means 52 was turned on show in FIG. 4(g), and highly accurate calculations were performed with an error of 1% or less.

Here, the detection of an injection abnormality will be described.

If an injection abnormality occurs in which the injection pulse voltage cannot be injected normally due to an abnormal connection between the ground fault detection device 100 and the vehicle body earth 60, an abnormal connection between one end of the injection capacitor C1 and the high-voltage circuit 50, an open circuit fault in the injection capacitor C1, an open circuit fault in the resistor R1 or the detection resistor R2, a fault in the voltage output means 10, a fault in the current measurement means 30, or the like, it will become impossible to properly monitor the ground fault current and insulation resistance. Here, a method for detecting these injection abnormalities will be described.

As shown in Figure 2, at time point 30a1 immediately after the start of period TL, the voltage at voltage output point 10d changes from the "H" level to the "L" level, causing an inrush current due to charging of capacitances Cga and Cgb to flow from vehicle body earth 60 to electrical circuits 54a and 54b.

If the effect of the injection capacitor C1 is ignored, the peak value of the inrush current will be the current value (e.g., -2.5 mA) obtained by dividing the voltage change (e.g., -30 V) at the voltage output point 10d by the combined series resistance value (e.g., 12 kΩ) of the resistor R1 and the detection resistor R2, and will change exponentially toward 0 A with a time constant of (R1 + R2) × (Cga + Cgb).

This inrush current flows through detection resistor R2 in the direction from voltage reference point 10a to vehicle body earth 60, causing a negative transient change in current measurement output 32. Third measurement means 43 measures current measurement output 32 at time point 30a1 during this transient change, and if the measured value exceeds a predetermined judgment value (for example, −1.25 mA, which is half the aforementioned peak current value of −2.5 mA) in the negative direction, the injection circuit can be determined to be healthy.

At time point 30c1 immediately after the start of period TH, the voltage at voltage output point 10d changes from "L" level to "H" level, causing inrush current due to charging of capacitances Cga and Cgb to flow from electrical paths 54a and 54b in the direction of vehicle earth 60. Current flows through detection resistor R2 in the direction from vehicle earth 60 to voltage reference point 10a, causing current measurement output 32 to transiently change in the positive direction. Fourth measurement means 44 measures current measurement output 32 at time point 30c1 during this transient change, and if the measured value exceeds a predetermined judgment value (e.g., 1.25 mA) in the positive direction, the injection circuit can be determined to be healthy.

The electrostatic capacitances Cga and Cgb are the floating capacitances between the Y capacitors in each device of the high-voltage circuit 50 and the vehicle body earth, but if there is an existing Y capacitor in the device, it will generally have a capacitance value sufficiently larger than the floating capacitance between the vehicle body earth and will be dominant for the aforementioned time constant (R1+R2)×(Cga+Cgb), making it possible to predict the inrush current waveform and also to predict the current measurement point 30a1 for capturing transient changes due to the inrush current.

If the third measuring means 43 or the fourth measuring means 44 has a low-pass filter for noise removal, it is desirable to set the measurement time points (time points 30a1 and 30c1) and the judgment value taking into account the effect of the filter. Furthermore, if the high-voltage circuit 50 does not have a Y capacitor in its internal circuit, the capacitances Cga and Cgb may be small, so it is desirable to adjust the measurement time points and the judgment value by verifying the actual device.

Furthermore, immediately after the switch means 52 is turned on, the current measurement output 32 shifts in the positive polarity direction (upward in the figure) as shown in FIG. 3(c), and at time point 30c1, the measured value exceeds in the positive polarity a predetermined judgment value of positive polarity (e.g., 1.25 mA). However, even if the injection circuit is healthy, there may be cases where the measured value does not exceed in the negative polarity a predetermined judgment value of negative polarity (e.g., −1.25 mA) at time point 30a2.

Conversely, immediately after the switch means 52 is turned off, the current measurement output 32 shifts in the negative polarity direction (downward in the figure), and even if the injection circuit is healthy, the measured value may not exceed the positive polarity predetermined judgment value (e.g., 1.25 mA) in the positive polarity direction.

Therefore, the injection abnormality detection means 47 determines that an injection abnormality has occurred when both the third measurement means output 43a and the fourth measurement means output 44a are within a predetermined range of determination values.

3(c), the absolute values of the difference between time 30c1 and time 30a2 and the difference between time 30a2 and time 30c2 will be approximately the same. Therefore, the injection abnormality detection means 47 may determine that an injection abnormality has occurred when the difference between the third measurement means output 43a and the fourth measurement means output 44a is within a predetermined determination value range.

In addition, if there is an abnormal connection between the ground fault detection device 100 and the vehicle body earth 60, an abnormal connection between one end of the injection capacitor C1 and the high voltage circuit 50, an open circuit failure in the injection capacitor C1, an open circuit failure in the resistor R1, an open circuit/short circuit failure in the detection resistor R2, an open circuit/short circuit failure in the voltage output means 10, or an open circuit/short circuit failure in the current measurement means 30, the injection current will not flow or cannot be measured, so the current measurement output 32 will be approximately 0 V, and the third measurement means output 43 a, the fourth measurement means output 44 a, and the difference between the two will all be approximately 0, and an injection abnormality will be determined.

The conditions under which an abnormality can be determined vary depending on the setting of the abnormality determination value for abnormalities such as an abnormal output voltage of the voltage output means 10, an abnormal resistance value of the resistor R1 or the detection resistor R2, and an abnormal measurement gain of the current measurement means 30. If the abnormality determination value is set to, for example, within ±1.25 mA, which is within half of the aforementioned peak current value ±2.5 mA, it is possible to determine abnormalities such as an insufficient (halved) voltage of the output voltage of the voltage output means 10, an abnormal increase (doubling) of the combined series resistance value of the resistor R1 and the detection resistor R2, and a decrease (halved) of the measurement gain of the current measurement means 30.

In this embodiment, the DC power supplies 10b and 10c of the voltage output means 10 are two dual power supplies of positive and negative polarity, but they may be a single power supply of either positive or negative polarity.

In this embodiment, the earth fault current is measured and the insulation resistance is calculated both when the switch means 52 is on and when it is off, but the earth fault current may be measured and the insulation resistance may be calculated only when the switch means 52 is off. When the switch means 52 is on, the voltage applied to the load 54 fluctuates and a lot of noise is generated because each device in the high voltage circuit 50 is in operation, but when the switch means 52 is off, the voltage applied to the load 54 is stable and the generated noise is small, allowing for more accurate earth fault current measurement and insulation resistance calculation.

Embodiment 2.
In the second embodiment, differences from the first embodiment will be described.
FIG. 6 is a circuit block diagram showing a ground fault detection device 101 and a high voltage circuit 50 according to a second embodiment of the present invention.

The ground fault detection device 101 includes a voltage output means 10, an injection means 21, a current measurement means 30, and a calculation means 40, and differs from the first embodiment in that it includes an injection means 21 instead of the injection means 20.

The injection means 21 has a resistor R1 having one end connected to the voltage output point 10d of the voltage output means 10, an injection capacitor C1a having one end connected to the other end of the resistor R1 and the other end connected to the positive side electric circuit 54a of the high voltage circuit 50, and an injection capacitor C1b having one end connected to the other end of the resistor R1 and the other end connected to the negative side electric circuit 54b of the high voltage circuit 50. The provision of two injection capacitors is different from the first embodiment.

In each of the calculation formulas of the third insulation resistance calculation method of the calculation means 40 in the second embodiment, the capacitance of the injection capacitor C1 in the first embodiment corresponds to the parallel combined capacitance of the injection capacitors C1a and C1b, and therefore the calculation is performed by replacing the capacitance C1 with the capacitance (C1a+C1b).

Here, the operation of the ground fault detection device 101 when the voltage applied to the load 54 of the high voltage circuit 50 changes will be described.

The operation when the switch means 52 of the high voltage circuit 50 is turned on and the output voltage of the DC power supply 51 is applied to the load 54 will be described using waveforms simulated by a circuit simulator.
As a condition for the simulation, in FIG. 6, the injection capacitor C1a is 5 μF and the injection capacitor C1b is 5 μF, which is different from the first embodiment, but the parallel combined capacitance of the injection capacitors C1a and C1b is 10 μF, the same as C1 in the first embodiment.

7A and 7B show the results of the simulation. As in FIG. 3, FIG. 7A shows the waveform at the voltage output point 10d of the voltage output means 10, FIG. 7B shows the voltage waveform applied to the load 54, and FIG. 7C shows the waveform (injected current waveform) of the current measurement output 32 of the current measurement means 30 and the measurement time point.

In the injection current waveform, even when 400 V is applied to the load 54 at time 0.05 seconds, a balance is maintained between the injection capacitors C1a and C1b and the capacitances Cga and Cgb, thereby preventing inrush current to the capacitances Cga and Cgb.

7(d) is a line graph showing the measured values of the injected current at each measurement time point shown in FIG. 7(c), similar to FIG. 3(d). FIG. 7(e) is a line graph showing the current difference values of the injected current at each measurement time point (each period) shown in FIG. 7(c), similar to FIG. 3(e). FIG. 7(f) is a line graph showing the difference values of the current difference values of the injected current at each measurement time point (each period) shown in FIG. 7(c), similar to FIG. 3(f). FIG. 7(g) is a line graph showing the calculated insulation resistance values at each measurement time point (each period) shown in FIG. 7(c), similar to FIG. 3(g).

FIG. 8 shows the simulation results after 2 seconds have elapsed. As with FIG. 4, FIG. 8(a) shows the waveform at voltage output point 10d of voltage output means 10, FIG. 8(b) shows the voltage waveform applied to load 54, and FIG. 8(c) shows the waveform (injected current waveform) of current measurement output 32 of current measurement means 30 and the measurement time point.

8(d) is a line graph showing the measured values of the injected current at each measurement time point shown in FIG. 8(c), similar to FIG. 4(d). FIG. 8(e) is a line graph showing the current difference values of the injected current at each measurement time point (each period) shown in FIG. 8(c), similar to FIG. 4(e). FIG. 8(f) is a line graph showing the difference values of the current difference values of the injected current at each measurement time point (each period) shown in FIG. 8(c), similar to FIG. 4(f). FIG. 8(g) is a line graph showing the calculated insulation resistance values at each measurement time point (each period) shown in FIG. 8(c), similar to FIG. 4(g).

The simulation results will be explained below.
As shown in Figure 7(d), the injected current flowing through detection resistor R2 is approximately 1.6 mA immediately after switching on switch means 52. This current is half that of embodiment 1, and the measurement current range of current measuring means 30 is halved, which is advantageous for high-precision current measurement. This current gradually decreases, and after about two seconds has passed, as shown in Figure 8(d), it is approximately 250 μA.

7(e), the current difference value of the injected current immediately after the switch means 52 is turned on is small at time point 30d1 and time point 30d2, but not as small as in embodiment 1. Because the injected current immediately after the switch means 52 is turned on is half that of embodiment 1, the voltage change ΔVc in the voltage across the injection capacitors C1a and C1b from the measurement time point in the immediately preceding period is about half, and the voltage change ΔV10d of 30 V from the "L" level to the "H" level of the injected pulse voltage is not completely canceled out, so the difference in voltage applied to the insulation resistance Rga between the immediately preceding measurement time point 30b1 and measurement time point 30d1 and between the immediately preceding measurement time point 30b2 and measurement time point 30d2 is not as small as in embodiment 1.

Furthermore, the current difference value of the injection current is large at time point 30b2 and time point 30b3, but not as large as in embodiment 1. This is because the voltage change ΔVc between the end-to-end voltage of injection capacitors C1a and C1b and the measurement time point in the immediately preceding period, which is added to the voltage change ΔV10d of −30 V when the injection pulse voltage changes from the “H” level to the “L” level, is 1/2, and therefore the difference in voltage applied to insulation resistance Rga between the immediately preceding measurement time point 30d1 and measurement time point 30b2 and between the immediately preceding measurement time point 30d2 and measurement time point 30b3, is not as large as in embodiment 1.

8(e), when about two seconds have passed since the switch means 52 was turned on, the current difference values ΔId11 and ΔId12 at the measurement points (30d11, 30d12) of the period TH are 220 μA and 224 μA, and the current difference values ΔIb12 and ΔIb13 at the measurement points (30b12, 30b13) of the period TL are −268 μA and −264 μA, so the difference between the periods TH and TL is small, as in embodiment 1. After a sufficient amount of time has passed, this difference disappears and the values stabilize, with the average value of the absolute values of ΔId12 and ΔIb13 being 244 μA (= (224 μA + 264 μA) / 2). When the absolute value of the current difference value is 244 μA, it can be correlated that the insulation resistance value is 100 kΩ.

Immediately after the switch means 52 is turned on, the current difference values ΔId1 and ΔId2 at the measurement points (30d1, 30d2) of the period TH are 106 μA and 127 μA, and the current difference values ΔIb2 and ΔIb3 at the measurement points (30b2, 30b3) of the period TL are −374 μA and −354 μA. If the aforementioned 244 μA is used as the reference, the variation in absolute values between the periods TH is 21 (=127−106) μA, which is 9% (=21 μA/244 μA) of the reference. The variation in the paired values is 20 (=374-354) μA, which is 8% of the reference, the variation in the absolute values of periods TH and TL is 268 (=374-106) μA, which is 110% of the reference, and ΔIb2 at point 30b2 is -374 μA, which is 153% of the reference. The variation in periods TH and TL is 1/9 (=10%/88%) smaller than in embodiment 1, and the error in the current difference value for period TL is 1/2 (=53%/105%) smaller than in embodiment 1.

At time 30b13, two seconds or more after the switch means 52 is turned on, ΔIb13 is −264 μA, which is 108% of the reference. The error at this time is 8%, which is half that of the first embodiment (=8%/16%).

Here, we will explain the calculation results of each insulation resistance calculation method of the insulation resistance calculation means 46 of the calculation means 40. First, we will explain the calculation results of the first insulation resistance calculation method.

In order to prevent the temporary issuance of excessive insulation deterioration information when the switch means 52 is turned on, in addition to the aforementioned processing of the indefinite value of the current difference value, by selecting the output 45a of the first difference calculation means during the period TH when the current difference values ΔId1 and ΔId2 are small, rather than during the period TL when the current difference values ΔIb2 and ΔIb3 are large, the current difference values become 106 μA and 127 μA. In this case, the absolute values are 43% and 52% smaller than the aforementioned reference value of 244 μA, and the calculated insulation resistance value becomes a larger value by that amount, so that the issuance of excessive insulation deterioration information can be prevented as in the first embodiment, and the calculated value is closer to the true value than in the first embodiment.

Next, we will explain the calculation results of the second insulation resistance calculation method performed by the insulation resistance calculation means 46. The calculation results of the second difference calculation means 46a are shown in Figures 7(f) and 8(f).

8(f), the calculation results of the second difference calculation means 46a approximately two seconds after the switch means 52 is turned on are -492 μA at time point 30d12 (2.38 seconds later) and 488 μA at time point 30b13 (2.48 seconds later), with the absolute values of the two approaching each other and stabilizing over time to an average value of 490 μA of the two absolute values. Therefore, when the calculation result of the second difference calculation means 46a is 490 μA in absolute value, it can be correlated with the insulation resistance Rga being 100 kΩ.

Using this 490 μA as the reference, the calculation results of the second difference calculation means 46a immediately after the switch means is turned on are as shown in FIG. 7(f), where the calculated value at time 30b2 (0.28 seconds later) is 480 μA, which is 98% of the reference value of 490 μA, and the calculated value at time 30d2 (0.38 seconds later) is −501 μA, which is 102% of the reference value, making it possible to calculate the insulation resistance with an accuracy of ±2% or less even immediately after the switch means 52 is turned on.

In the first embodiment, the calculated value at time 30b2 (0.28 seconds later) immediately after the switch means 52 is turned on is 93.5% of the reference value, as described above, whereas in the second embodiment, the calculated value is 98% of the reference value, enabling calculation of insulation resistance with higher accuracy.

Next, the calculation results of the third insulation resistance calculation method performed by the insulation resistance calculation means 46 will be described.

In each calculation formula, the capacitance C1 of the injection capacitor C1 in the first embodiment is replaced with the parallel combined capacitance (C1a+C1b) of the injection capacitors C1a and C1b.

The results of calculations using the third insulation resistance calculation method are shown in Figures 7(g) and 8(g). In Figures 7(g) and 8(g), the first injection capacitor voltage change calculation method is used, and corrections are also made using the capacitance-induced voltage 46d and capacitance-induced current 46e.

As shown in Figure 7(g), the calculated insulation resistance value at time 30d1 (0.18 seconds later) immediately after switching on the switch means 52 is 98.7 kΩ, which is closer to the true value of 100 kΩ than the 73.5 kΩ in embodiment 1. This is because two injection capacitors are used, reducing the effect of the charging current on the capacitances Cga and Cgb when the switch means 52 is on. From time 30b2 (0.28 seconds later), the calculated insulation resistance value can be calculated from 99.7 kΩ to 100.4 kΩ, with an error of less than 1%, allowing for calculations with higher accuracy than embodiment 1.

The calculation results after about 2 seconds have passed since the switch means 52 was turned on show in FIG. 8(g), and highly accurate calculations were performed with an error of 1% or less.

The effects of the second embodiment compared to the first embodiment cannot be obtained by connecting the other end of each injection capacitor to both ends of the high-voltage battery (DC power supply 51), as in the conventional method using two injection capacitors (for example, Patent Document 3), but can be obtained by connecting the other end of each injection capacitor to both ends of the load 54, as in the second embodiment.

So far, we have described a ground fault detection device installed in an electric vehicle, but the invention is not limited to this and can also be applied to ungrounded DC power distribution systems such as solar power generation systems and data centers, as well as other ungrounded power distribution systems. In such cases, the vehicle body ground corresponds to the chassis ground or earth ground.

10 Voltage output means 10a Voltage reference point 10b, 10c DC power supply 10d Voltage output point 20, 21 Injection means 30 Current measurement means 31 Operational amplifier 31a Operational amplifier + terminal 31b Operational amplifier - terminal 31c Operational amplifier output 32 Current measurement output 40 Calculation means 41 First measurement means 42 Second measurement means 43 Third measurement means 44 Fourth measurement means 45 First difference calculation means 46 Insulation resistance calculation means 46a Second difference calculation means 46b Injection capacitor voltage change calculation means 46c Resistance voltage drop calculation means 46d Capacitance-induced voltage 46e Capacitance-induced current 47 Injection abnormality detection means 50 High voltage circuit 51 DC power supply 52 Switch means 53 Capacitor 54 Load 54a Positive side circuit 54b Negative side circuit 60 Vehicle body earth 100, 101 Ground fault detection device R1 Resistor R2 Detection resistors Rg, Rga, Rgb Insulation resistors C1, C1a, C1b Injection capacitors Cg, Cga, Cgb Electrostatic capacitances Q1, Q2 Switching elements

Claims (4)

At least one electrical path insulated from earth potential;
a voltage output means for repeatedly outputting a first voltage during a first period and a second voltage during a second period, with a voltage reference point as a reference;
injection means for injecting the output of the voltage output means into the electrical path;
a current measuring means for measuring and outputting the injection current injected by the injection means;
A ground fault detection device having a calculation means for calculating an insulation resistance between the electric circuit and the earth potential based on an output of the current measurement means,
The injection means
at least one injection capacitor having one end connected to the output of the voltage output means directly or via a resistor and the other end connected to the electrical path directly or via a resistor, and connection means for connecting the voltage reference point to the earth potential directly or via a resistor;
The calculation means
a first measuring means for measuring an output of the current measuring means at a first measurement point in the first period and outputting the measured value as a first measurement value;
second measuring means for measuring the output of the current measuring means at a second measurement point in the second period and outputting the measured value as a second measurement value;
a first difference calculation means for calculating a difference between the first measurement value and the second measurement value at a first calculation time point after measurement at the first measurement time point or at a second calculation time point after measurement at the second measurement time point;
a first difference value of the first difference calculation means that calculates the difference between the first measurement value and the second measurement value of the immediately preceding period;
a second difference value of the first difference calculation means that calculates the difference between the second measurement value and the first measurement value in the immediately preceding period;
A ground fault detection device characterized by calculating the insulation resistance between the electric circuit and the earth potential based on one selected from the first difference value and the second difference value, or based on the difference between the first difference value and the second difference value. At least one electrical path insulated from earth potential;
a voltage output means for repeatedly outputting a first voltage during a first period and a second voltage during a second period, with a voltage reference point as a reference;
injection means for injecting the output of the voltage output means into the electrical path;
a current measuring means for measuring and outputting the injection current injected by the injection means;
A ground fault detection device having a calculation means for calculating an insulation resistance between the electric circuit and the earth potential based on an output of the current measurement means,
The injection means
at least one injection capacitor having one end connected to the output of the voltage output means directly or via a resistor and the other end connected to the electrical path directly or via a resistor; and connection means for connecting the voltage reference point to the earth potential directly or via a resistor;
The calculation means
a first measuring means for measuring an output of the current measuring means at a first measurement point in the first period and outputting the measured value as a first measurement value;
a second measuring means for measuring an output of the current measuring means at a second measurement point in the second period and outputting the measured value as a second measurement value;
Between the first measurement time point and the second measurement time point in the immediately preceding period, or between the second measurement time point and the first measurement time point in the immediately preceding period,
a first difference value that is a difference between the first measurement value and the second measurement value;
a second difference value that is a difference between the voltages across the injection capacitor calculated based on the first measurement value or the second measurement value;
a third difference value calculated based on the first difference value, which is a difference in voltage drop due to resistance of an injection current path of the injection means;
using a fourth difference value, which is the difference between the first voltage and the second voltage, of the voltage output means,
A ground fault detection device characterized by calculating the insulation resistance between the electric circuit and the earth potential by subtracting the second difference value and the third difference value from the fourth difference value and dividing the result by the first difference value. the electric path includes a first electric path and a second electric path connecting the switch means and the load;
the switch means changes the magnitude of the voltage applied to the load or turns the voltage on and off;
3. The ground fault detection device according to claim 1, wherein the injection capacitor includes a first capacitor connected to the first electrical path and a second capacitor connected to the second electrical path. The calculation means
a third measuring means for measuring an output of the current measuring means at a third measurement point in the first period and outputting the measured value as a third measurement value;
a fourth measuring means for measuring an output of the current measuring means at a fourth measurement point in the second period and outputting the measured value as a fourth measurement value;
At a third calculation point after the measurement at the third measurement point or a fourth calculation point after the measurement at the fourth measurement point,
3. The ground fault detection device according to claim 1, wherein an injection abnormality is determined when both the third measurement value and the fourth measurement value are within a predetermined judgment value, or when the difference between the third measurement value and the fourth measurement value is within a predetermined judgment value.