Electric leakage detection circuit

Abstract

One end of a coupling capacitor C1 is connected to a live part of a high-voltage system 20 installed in a state of being insulated from the ground. A voltage application unit 12 generates an AC voltage and applies the AC voltage to the other end of the coupling capacitor C1 via a sense resistor R1. A voltage measurement unit 13 measures a voltage at a voltage divider point between the coupling capacitor C1 and the sense resistor R1. An electric leakage detection unit 14 calculates a value of insulation resistance between the live part and the ground on the basis of the measured voltage at the voltage divider point, and if the calculated value of insulation resistance is less than or equal to a threshold value, determines that electric leakage is occurring between the live part and the ground.

Description

Leakage detection circuit

[0001] The present disclosure relates to a leakage detection circuit for detecting leakage in a high-voltage system.

[0002] In recent years, EVs have become popular, and high-voltage batteries are mounted on EVs. In addition, electrification of construction machinery, ships and the like has been progressing, and high-voltage batteries are also mounted on electrified construction machinery and the like. If leakage occurs between a high-voltage system including a high-voltage battery and a vehicle body, there is a risk of electric shock, so it is necessary to install a leakage detection circuit for detecting leakage between the high-voltage system and the body ground.

[0003] As one form of leakage detection circuit, there is known a circuit that outputs an AC voltage, measures the voltage applied to the combined impedance of an insulation resistance between a high-voltage system and ground and a coupling capacitor, calculates the insulation resistance value, and determines that leakage has occurred when the calculated insulation resistance value is equal to or less than a threshold value.

[0004] Stray capacitance exists between a high-voltage system including a high-voltage battery, a motor, an inverter, a wire harness and the like and ground such as a chassis and a vehicle body. Since this stray capacitance exists in parallel with the insulation resistance, in the method of detecting leakage using an AC voltage as described above, if the stray capacitance increases due to aging degradation or the like, the accuracy of leakage detection decreases. When the stray capacitance increases, the detection voltage decreases and the insulation resistance value is calculated as a lower value, so there occurs a case where a state where no leakage actually occurs is erroneously determined as leakage having occurred.

[0005] Patent Document 1 discloses a method of calculating a stray capacitance based on a rising gradient (time constant) of a voltage detected by a detection unit when a rectangular wave is output to a leakage detection circuit, and calculating an insulation resistance in consideration of the stray capacitance. In this method, it is necessary to map the relationship between the rising gradient and the stray capacitance in advance. Since it is necessary to conduct tests in advance and create a mapping of stray capacitance with respect to the rising gradient, extra development man-hours increase.

[0006] Patent Document 2 discloses a method in which a table is created in advance that defines the relationship between the stray capacitance value and the detected voltage value at a certain frequency, based on experimental results under known conditions such as insulation resistance. When the detected voltage drops, the stray capacitance value is estimated by referring to the table, and the threshold is corrected by subtracting a correction value corresponding to the estimated stray capacitance value from the threshold. However, this method requires prior testing and creation of the table, which increases development man-hours. Furthermore, errors may occur in the estimation of stray capacitance when the insulation resistance decreases.

[0007] Japanese Patent Publication No. 2021-047107, International Publication No. 19 / 130717

[0008] This disclosure is made in view of these circumstances, and its purpose is to provide a low-cost technology for eliminating the effects of stray capacitance in a leakage current detection method using AC voltage.

[0009] To solve the above problems, a leakage current detection circuit in one aspect of the present disclosure includes: a coupling capacitor installed in an insulated state from earth, with one end connected to a live part of a high-voltage system including a power storage unit and a load; a voltage application unit that generates an AC voltage and applies it to the other end of the coupling capacitor via a detection resistor; a voltage measurement unit that measures the voltage at a voltage divider point between the coupling capacitor and the detection resistor; and a leakage current detection unit that calculates the value of the insulation resistance between the live part and the earth based on the measured voltage at the voltage divider point, and determines that a leakage current has occurred between the live part and the earth if the calculated insulation resistance value is less than or equal to a threshold value. The leakage current detection unit calculates the value of the insulation resistance based on the measured voltage at the voltage divider point when an AC voltage of a first frequency is applied to the detection resistor, and the measured voltage at the voltage divider point when an AC voltage of a second frequency different from the first frequency is applied to the detection resistor.

[0010] According to this disclosure, a leakage current detection method using AC voltage can be used at low cost and the effects of stray capacitance can be eliminated.

[0011] This diagram illustrates the configuration of a high-voltage system and a leakage current detection circuit according to an embodiment. This diagram shows the leakage current path of the high-voltage system in Figure 1 rewritten as an equivalent circuit. This diagram shows an example of the relationship between insulation resistance and voltage at the voltage divider point. This diagram illustrates the concepts of AC voltage at a first frequency and AC voltage at a second frequency.

[0012] Figure 1 is a diagram illustrating the configuration of a high-voltage system 20 and a leakage current detection circuit 10 according to an embodiment. In this embodiment, an example is assumed in which the high-voltage system 20 is mounted on an electric vehicle. The high-voltage system 20 includes a drive battery 21, a switch SW1, an inverter 22, and a motor 23. The high-voltage system 20 is installed on the vehicle in an insulated state from the chassis ground and body ground of the electric vehicle.

[0013] The drive battery 21 is installed inside the vehicle separately from the auxiliary battery (usually a 12V output lead-acid battery is used). The voltage of the drive battery 21 installed in a typical electric vehicle is often set to 400V to 500V. The drive battery 21 includes multiple cells connected in series or in series-parallel. Lithium-ion battery cells, nickel-metal hydride battery cells, etc., can be used as cells. Hereinafter, this specification assumes the use of lithium-ion battery cells (nominal voltage: 3.6-3.7V).

[0014] The high-voltage system 20 includes an inverter 22 and a motor 23 as high-voltage loads. The positive terminal of the drive battery 21 is connected to one end of the inverter 22 by a positive wire harness, and the negative terminal of the drive battery 21 is connected to the other end of the inverter 22 by a negative wire harness. Switches SW1 are inserted into the positive and negative wire harnesses, respectively. Switches SW1 can be SSRs (Solid State Relays), IGBTs (Insulated Gate Bipolar Transistors), mechanical relays, MOSFETs (Metal Oxide Semiconductor Field Effect Transistors), etc. Switches SW1 function as contactors that control the conduction / interruption between the drive battery 21 and the high-voltage loads.

[0015] The inverter 22 is a bidirectional inverter connected between the drive battery 21 and the motor 23. During acceleration, the inverter 22 converts the DC power supplied from the drive battery 21 into AC power and supplies it to the motor 23. During regeneration, it converts the AC power supplied from the motor 23 into DC power and supplies it to the drive battery 21. For example, a three-phase AC motor is used for the motor 23. During acceleration, the motor 23 rotates in accordance with the AC power supplied from the inverter 22. During regeneration, it converts the rotational energy due to deceleration into AC power and supplies it to the inverter 22.

[0016] The leakage current detection circuit 10 includes a control unit 11, a first low-pass filter 15, a second low-pass filter 16, a detection resistor R1, and a coupling capacitor C1. The control unit 11 includes a voltage application unit 12, a voltage measurement unit 13, and a leakage current detection unit 14. For example, a microcontroller can be used for the control unit 11.

[0017] One end of the coupling capacitor C1 is connected to the live part of the high-voltage system 20. In the example shown in Figure 1, it is connected to the negative wire harness between the drive battery 21 and the inverter 22. Note that one end of the coupling capacitor C1 may be connected to any position on the current path through which the current supplied from the drive battery 21 flows. The other end of the coupling capacitor C1 is connected to the detection resistor R1.

[0018] The voltage application unit 12 generates an AC voltage and applies the generated AC voltage to the other end of the coupling capacitor C1 via the detection resistor R1. In this embodiment, the voltage application unit 12 generates a PWM (Pulse Width Modulation) voltage. The high level of the PWM voltage is set to the power supply voltage VDD (e.g., 5V) of the control unit 11, and the low level of the PWM voltage is set to the ground voltage GND (e.g., 0V) of the control unit 11.

[0019] A first low-pass filter 15 is connected between the output terminal of the control unit 11, which is connected to the voltage application unit 12, and the detection resistor R1. The first low-pass filter 15 can be configured as an RC circuit. The rectangular wave PWM voltage output from the output terminal of the control unit 11 passes through the first low-pass filter 15, attenuating the high-frequency components and converting it into a sinusoidal voltage. Alternatively, a D / A converter that converts the PWM voltage into a sinusoidal voltage may be used instead of the first low-pass filter 15. Furthermore, a sinusoidal oscillator may be provided outside the microcontroller without the first low-pass filter 15.

[0020] The voltage measurement unit 13 measures the voltage at the voltage division point N between the coupling capacitor C1 and the detection resistor R1. A second low-pass filter 16 is connected between the input terminal of the control unit 11, which is connected to the voltage measurement unit 13, and the voltage division point N. The second low-pass filter 16 removes noise components contained in the voltage at the voltage division point N.

[0021] Figure 2 is a diagram showing the leakage current path of the high-voltage system 20 in Figure 1 rewritten as an equivalent circuit. The leakage current detection unit 14 calculates the value of the insulation resistance R2 between the live part of the high-voltage system 20 and the ground based on the measured voltage Vout at the voltage division point N measured by the voltage measurement unit 13. When calculating the value of the insulation resistance R, the leakage current detection unit 14 uses the peak value of the measured voltage Vout. Alternatively, the peak-to-peak value may be used instead of the peak value.

[0022] The leakage current detection unit 14 calculates the value of the insulation resistance R2 using the following equation (Equation 1): Vout = Vin * {((1 / jωC1) + R2) / (R1 + (1 / jωC1) + R2)} ... (Equation 1)

[0023] In (Equation 1), the value of the detection resistor R1, the impedance value of the coupling capacitor C1, and the test voltage Vin applied to the series circuit of the detection resistor R1, the coupling capacitor C1, and the insulation resistor R2 are known. Therefore, the value of the insulation resistor R2 can be calculated by measuring the voltage Vout at the voltage divider point N. The leakage current detection unit 14 determines that a leakage current has occurred between the live part of the high-voltage system 20 and the ground if the calculated insulation resistance R2 value is below a threshold.

[0024] If the high-voltage system 20 is installed in the vehicle in an insulated state from the chassis earth and body earth, it can be assumed that an insulation resistance R2 exists in the inspection path of the leakage current detection circuit 10 via the coupling capacitor C1. The voltage measurement unit 13 measures the voltage Vout, which is obtained by dividing the inspection voltage Vin by the combined impedance of the coupling capacitor C1 and the insulation resistance R2. The leakage current detection unit 14 calculates the value of the insulation resistance R2 based on the peak value of the measured voltage Vout and determines whether or not there is a leakage current based on the value of the insulation resistance R2.

[0025] The above equation (Equation 1) assumes an ideal state in which there is no stray capacitance between the live part of the high-voltage system 20 and the ground. However, in reality, a stray capacitance C2 exists in parallel with the insulation resistance R2. Since the stray capacitance C2 acts to underestimate the value of the insulation resistance R2, the leakage detection unit 14 may incorrectly determine that a leakage has occurred even though there is no actual leakage.

[0026] Figure 3 shows an example of the relationship between insulation resistance R2 and the voltage Vout at the voltage division point N. Figure 3 shows an example in which leakage current is determined to have occurred when the value of insulation resistance R2 falls below the resistance threshold. The threshold voltage for leakage current detection is set to the voltage Vout at the voltage division point N when the value of insulation resistance R2 is at the resistance threshold value, assuming that there is no influence from stray capacitance C2.

[0027] If the stray capacitance C2 of the high-voltage system 20 increases due to aging or other factors, the voltage Vout at the voltage division point N decreases, causing the leakage current detection unit 14 to mistakenly perceive a decrease in insulation resistance R2. For example, even if the value of insulation resistance R2 is sufficiently large, an increase in stray capacitance C2 can cause the voltage Vout at the voltage division point N to fall below the threshold voltage. In that case, the leakage current detection unit 14 will mistakenly determine that a leakage current has occurred, even though there is no actual leakage current. To improve the accuracy of leakage current detection, it is necessary to eliminate the influence of stray capacitance C2.

[0028] Therefore, in this embodiment, the leakage current detection unit 14 calculates the value of the insulation resistance R2 based on the measured voltage Vout1 at the voltage division point N when an AC voltage of a first frequency f1 is applied to the detection resistor R1, and the measured voltage Vout2 at the voltage division point N when an AC voltage of a second frequency f2, which is different from the first frequency f1, is applied to the detection resistor R1.

[0029] The leakage current detection unit 14 calculates the value of the insulation resistance R2 using the following equations (Equation 2) and (Equation 3): Vout1 = Vin1 * {((1 / jω １ C1)+(R2 / (jω １ C2・R2+1))) / (R1+(1 / jω １ C1)+(R2 / (jω １ C2・R2+1)))} ...(Formula 2) Vout2=Vin2・{((1 / jω ２ C1)+(R2 / (jω ２ C2・R2+1))) / (R1+(1 / jω ２ C1)+(R2 / (jω ２ C2・R2+1)))} ...(Formula 3)

[0030] As shown in (Equation 2) and (Equation 3) above, we can define an equation that shows that the measured voltage Vout at the voltage division point N is equal to the voltage obtained by multiplying the test voltage Vin applied to the detection resistor R1 by the impedance division ratio. The impedance division ratio is defined as the ratio of (b) the impedance of the coupling capacitor C1 and the sum of the parallel combined impedances to (a) the sum of the parallel combined impedances of the impedance of the detection resistor R1, the impedance of the coupling capacitor C1, the impedance of the insulation resistor R2, and the impedance of the stray capacitance C2.

[0031] The leakage current detection unit 14 solves for the impedance of the stray capacitance C2 from the above (Equation 2) in the first state where an AC voltage Vin1 of the first frequency f1 is applied, and substitutes the solved equation for the impedance of the stray capacitance C2 into the above (Equation 3) in the second state where an AC voltage Vin2 of the second frequency f2 is applied, thereby eliminating the variable for the impedance of the stray capacitance C2 from the above (Equation 3).

[0032] The value of the detection resistor R1, the impedance value of the coupling capacitor C1, the first test voltage Vin1 at the first frequency f1, and the second test voltage Vin2 at the second frequency f2 are known. The first measurement voltage Vout1 when the first test voltage Vin1 is applied and the second measurement voltage Vout2 when the second test voltage Vin2 is applied can be obtained as measured values. Therefore, the leakage current detection unit 14 can calculate the value of the insulation resistance R2 from which the effect of stray capacitance C2 has been removed, by eliminating the variable of stray capacitance C2 from (Equation 3). Furthermore, by substituting the calculated value of insulation resistance R2 into (Equation 2) or (Equation 3) above, the impedance value of stray capacitance C2 can be calculated.

[0033] Figure 4 shows an image of the AC voltage Vout1 with a first frequency f1 and the AC voltage Vout2 with a second frequency f2. Different frequencies can be set for the first frequency f1 and the second frequency f2, which can be determined in advance from experiments or simulations. The voltage application unit 12 can generate the AC voltage Vout1 with the first frequency f1 and the AC voltage Vout2 with the second frequency f2 by changing the duty cycle of the PWM voltage.

[0034] As described above, this embodiment allows for the elimination of the influence of stray capacitance in a leakage current detection method using AC voltage at low cost. Specifically, by applying AC voltages with multiple different frequencies to the detection resistor R1 and obtaining the peak values ​​of the AC voltages with multiple different frequencies from the voltage division point N, a system of simultaneous equations defining the relationship between the test voltage Vin and the measured voltage Vout can be established. By establishing this system of simultaneous equations, the variable of stray capacitance C2 can be eliminated, and the value of the insulation resistance R2 can be calculated with high accuracy. This is particularly effective for systems with large stray capacitance C2. Even when the stray capacitance C2 changes due to aging, the value of the insulation resistance R2 can be calculated with high accuracy.

[0035] Furthermore, since the control unit 11 generates a PWM signal and the first low-pass filter 15 generates a sine wave, multiple sine waves of different frequencies can be generated by changing the duty cycle. Therefore, there is no need to install multiple signal processing units (filter circuits) to generate multiple sine waves of different frequencies, nor is there a need to install a sine wave oscillator. Consequently, multiple sine waves of different frequencies can be generated at low cost.

[0036] Furthermore, since a high-voltage switch circuit and simulated capacitance are not required to obtain the correction value, the increase in circuit size and cost can be suppressed. Also, because there is no timing required to interrupt the normal path and connect to the simulated capacitance path in the high-voltage switch circuit to obtain the correction value, leakage current detection can be performed continuously.

[0037] Furthermore, since it is not necessary to create a mapping of stray capacitance to the rising slope of the detected voltage in advance, the increase in development man-hours can be suppressed. In addition, in the leakage current detection method according to this embodiment, the effect is small even if the voltage of the drive battery 21 fluctuates due to deterioration or the like.

[0038] The present disclosure has been described above based on embodiments. The embodiments are illustrative, and it will be understood by those skilled in the art that various modifications are possible in combinations of their components and processing processes, and that such modifications are also within the scope of the present disclosure.

[0039] In the above-described embodiment, it was assumed that an AC voltage Vout1 with a first frequency f1 and an AC voltage Vout2 with a second frequency f2 were applied to the detection resistor R1 in a time-division manner. In this regard, two voltage application units 12 and first low-pass filters 15 may be provided to apply the AC voltage Vout1 with a first frequency f1 and the AC voltage Vout2 with a second frequency f2 to the detection resistor R1 in a superposition manner. In that case, it is necessary to change the amplitude of the AC voltage Vout1 with a first frequency f1 and the AC voltage Vout2 with a second frequency f2. In order to measure the first voltage Vout1 and the second voltage Vout2 at the voltage division point N with high accuracy, it is desirable to provide two A / D converters between the voltage division point N and the voltage measurement unit 13.

[0040] In the embodiment described above, an example in which a driving storage battery 21 is used as the power supply unit of the high-voltage system 20 has been described. In this regard, capacitors such as electric double-layer capacitors and lithium-ion capacitors may be used as the power supply unit of the high-voltage system 20.

[0041] Note that the embodiment may be specified by the following items.

[0042] [Item 1] A coupling capacitor (C1) is installed in an insulated state from the earth, with one end connected to the live part of a high-voltage system (20) including a power storage unit (21) and loads (22, 23); a voltage application unit (12) generates an AC voltage and applies it to the other end of the coupling capacitor (C1) via a detection resistor (R1); a voltage measurement unit (13) measures the voltage at the voltage division point between the coupling capacitor (C1) and the detection resistor (R1); and a leakage detection unit (14) calculates the value of the insulation resistance (R2) between the live part and the earth based on the measured voltage at the voltage division point, and determines that leakage current has occurred between the live part and the earth if the calculated value of the insulation resistance (R2) is below a threshold; The leakage current detection unit (14) is a leakage current detection circuit (10) that calculates the value of the insulation resistance (R2) based on the measured voltage at the voltage divider point when an AC voltage of a first frequency is applied to the detection resistor (R1) and the measured voltage at the voltage divider point when an AC voltage of a second frequency different from the first frequency is applied to the detection resistor (R1). This allows for the elimination of the influence of stray capacitance at low cost. [Item 2] An equation is defined showing that the measured voltage at the voltage divider point is equal to the voltage obtained by multiplying the voltage applied to the detection resistor (R1) by the impedance division ratio, the impedance division ratio is defined as the ratio of (a) the impedance of the coupling capacitor (C1) and the sum of the parallel combined impedances of the impedance of the coupling capacitor (C1), the impedance of the coupling capacitor (C1), the impedance of the insulation resistor (R2), and the impedance of the stray capacitance (C2), to (b) the ratio of the impedance of the coupling capacitor (C1) and the sum of the parallel combined impedances, the leakage current detection unit (14) solves for the impedance of the stray capacitance (C2) from the equation for the AC voltage of the first frequency, and substitutes the solved equation for the impedance of the stray capacitance (C2) into the equation for the AC voltage of the second frequency, thereby eliminating the variable for the impedance of the stray capacitance (C2) from the equation, as described in Item 1.[Item 3] A leakage current detection circuit (10) according to Item 1, further comprising a low-pass filter (15) connected between the voltage application unit (12) and the detection resistor (R1), wherein the voltage application unit (12) outputs a PWM voltage. This allows for the generation of multiple sine waves with different frequencies at low cost.

[0043] This disclosure can be used for detecting electrical leakage in EV batteries.

[0044] 10 Leakage current detection circuit, 11 Control unit, 12 Voltage application unit, 13 Voltage measurement unit, 14 Leakage current detection unit, 15 First low-pass filter, 16 Second low-pass filter, R1 Detection resistor, C1 Coupling capacitor, 20 High-voltage system, 21 Drive battery, 22 Inverter, 23 Motor, SW1 Switch, R2 Insulation resistor, C2 Stray capacitance.

Claims

1. A leakage current detection circuit comprising: a coupling capacitor installed in an insulated state from earth, with one end connected to a live part of a high-voltage system including a power storage unit and a load; a voltage application unit that generates an AC voltage and applies it to the other end of the coupling capacitor via a detection resistor; a voltage measurement unit that measures the voltage at a voltage divider point between the coupling capacitor and the detection resistor; and a leakage current detection unit that calculates the value of the insulation resistance between the live part and the earth based on the measured voltage at the voltage divider point, and determines that leakage current has occurred between the live part and the earth if the calculated value of the insulation resistance is less than or equal to a threshold value, wherein the leakage current detection unit calculates the value of the insulation resistance based on the measured voltage at the voltage divider point when an AC voltage of a first frequency is applied to the detection resistor and the measured voltage at the voltage divider point when an AC voltage of a second frequency different from the first frequency is applied to the detection resistor.

2. An equation is defined that shows that the measured voltage at the voltage divider point is equal to the voltage obtained by multiplying the voltage applied to the detection resistor by the impedance division ratio, the impedance division ratio is defined as the ratio of (a) the impedance of the detection resistor, the impedance of the coupling capacitor, and the sum of the parallel combined impedances of the impedance of the insulation resistor and the impedance of the stray capacitance to (b) the ratio of the impedance of the coupling capacitor and the sum of the parallel combined impedances, the leakage current detection unit solves for the impedance of the stray capacitance from the equation for the AC voltage of the first frequency, and substitutes the solved equation for the impedance of the stray capacitance into the equation for the AC voltage of the second frequency, thereby eliminating the variable for the impedance of the stray capacitance from the equation, according to claim 1.

3. The leakage current detection circuit according to claim 1, further comprising a low-pass filter connected between the voltage application unit and the detection resistor, wherein the voltage application unit outputs a PWM (Pulse Width Modulation) voltage.

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