High-precision insulation resistance detection circuit, monitoring method and vehicle-mounted charger

Abstract

The invention provides a high-precision insulation resistance detection circuit, a monitoring method and a vehicle-mounted charger, the high-precision insulation resistance detection circuit is applied to an alternating current power supply with a power conversion module, and the insulation resistance detection circuit comprises a controllable switch, a bus impedance module, a live wire impedance module, a zero line impedance module, a voltage sampling module and a processing module. The controllable switch is connected in series with the bus impedance module, the voltage sampling module is used for collecting voltage between the output end of the power conversion module and the grounding end, a first input port of the processing module is used for receiving an insulation resistance voltage sampling signal output by the voltage sampling module, and a second input port of the processing module is used for receiving a bus voltage sampling signal. According to the scheme, the insulation resistance measurement completely avoids the influence of a Y capacitor of a power frequency alternating current output bus, and a test signal is equivalent to a smooth direct current voltage, so that the insulation resistance measurement result is accurate and stable, and the error is small.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a high-precision insulation resistance detection circuit, monitoring method, and on-board charger. Background Technology

[0002] Electric new energy vehicles (EVs) are now widely used, and mid-to-high-end models are equipped with bidirectional on-board chargers (OBCs). When working in the forward direction, the bidirectional OBC uses AC power to charge the power battery; when working in the reverse direction, the OBC is actually an AC inverter that converts the DC power from the power battery into AC power for the user. Depending on the standards of different users' countries or regions, the inverter output frequency is either 50Hz or 60Hz.

[0003] To ensure the safety of users' electricity use, relevant industry and national standards require that the insulation resistance of the power frequency AC bus be monitored online when the OBC is in inverter operation. When the insulation resistance of the power frequency AC output bus of the OBC inverter (hereinafter referred to as the inverter) to ground (PE) drops to the specified safety threshold due to the OBC itself or the inverter load, it should quickly enter the protection state, including shutting down the inverter and issuing a corresponding alarm.

[0004] The national standard GB / T-18384.3-2015, "Electric Vehicles - Safety Requirements - Part 3: Protection Against Electric Shock," stipulates that under maximum operating voltage, the minimum insulation resistance of a DC circuit should not be less than 100Ω / V, and that of an AC circuit should not be less than 500Ω / V. To adapt to different national or regional power grid voltages, the operating voltage range of an OBC (On-Board Circuit Breaker) is typically 90Vac-265Vac. Based on this voltage range and the aforementioned national standard's insulation resistance requirements, the minimum insulation resistance range of the OBC's AC busbar during reverse operation is 45kΩ-132.5kΩ. Automakers generally propose higher standards of 45kΩ-500kΩ based on this, which is also the detection range for the insulation resistance of the configured insulation monitoring circuit. Generally, the insulation resistance detection error is required to be no greater than ±10%.

[0005] The inverter's power frequency AC output bus has Y capacitors connected to ground (PE) on both the L (live) and N (neutral) lines. Their capacitance is the sum of the Y capacitors on the inverter's output terminals and the Y capacitors of the inverter load (which can be power electronic conversion devices, and these loads are generally designed with Y capacitors at their input terminals). However, the inverter load is uncertain, so the capacitance values ​​of the Y capacitors on the power frequency AC output bus are also uncertain, and may even be asymmetrical. That is, the capacitance values ​​of the Y capacitors on the L and N lines are uncertain and cannot be guaranteed to be symmetrical.

[0006] The existing technology in publication number CN109406977 has the following problems: 1) The technology described in this document uses the power frequency AC voltage output by the inverter as the test signal source. As long as the test signal is an AC signal, the influence of Y capacitor cannot be avoided. In this technology, the Y capacitor of the power frequency AC output bus to ground (PE) only considers the Y capacitor of the inverter output terminal of the OBC itself. The insulation resistance is calculated with the Y capacitor as a known parameter and the Y capacitors of the L line and N line being symmetrical as a premise. In actual operation, the load input terminal of the power frequency AC output may also have a Y capacitor, and the Y capacitor of the load cannot be guaranteed to be symmetrical between L and N. Under such conditions, the insulation resistance calculated by this technology will be incorrect; 2) When the insulation resistance of the L and N lines decreases by a similar order of magnitude at the same time, the calculation of the insulation resistance by this technology will also be incorrect; 3) It is necessary to detect two power frequency AC voltages, L to PE and N to PE, at the same time, and a compensation bias voltage is required to raise the signal voltage to above zero level to ensure that the detection circuit works normally. This increases the complexity, cost and size of the hardware circuit. At the same time, the compensation bias voltage also reduces the range of the AD conversion of the subsequent MCU, reducing the measurement accuracy; 4) This technology detects two power frequency AC signals, and the output result is also a power frequency AC signal, which is more susceptible to interference than the acquisition of DC signals, affecting the measurement accuracy.

[0007] The prior art in publication number CN114391105 has the following problems: 1) The technology described in this document also uses the AC voltage output by the inverter as the test signal source. As long as the test source is an AC signal, the Y capacitor is an unavoidable problem. The patent does not mention how to avoid the influence of the Y capacitor, or even mention the Y capacitor; 2) The prior art uses four controllable switches, and switches SW1 and SW2 are switched on and off once during each measurement. This requires high performance of the switching technology, and the high cost and large size of the solution are not conducive to the miniaturization of the whole machine; 3) It needs to measure two voltage signals, and the voltage measurement circuit is more complex, with higher cost and larger size; 4) Each measurement requires SW1 and SW2 to be switched on and off once, which inevitably affects the measurement speed, so the measurement speed is relatively slow. Summary of the Invention

[0008] The primary objective of this invention is to provide a high-precision insulation resistance detection circuit.

[0009] A second objective of this invention is to provide a monitoring method applicable to the above-mentioned insulation resistance detection circuit.

[0010] A third objective of this invention is to provide an on-board charger having the aforementioned insulation resistance detection circuit. To achieve the first objective of this invention, a high-precision insulation resistance detection circuit is provided. This circuit is applied to an AC power supply with a power conversion module, the output of which outputs AC power. The insulation resistance detection circuit includes a controllable switch, a bus impedance module, a live wire impedance module, a neutral wire impedance module, a voltage sampling module, and a processing module. The controllable switch and the bus impedance module are connected in series between the input terminal and the ground terminal of the power conversion module. The voltage sampling module is used to collect the voltage between the output terminal of the power conversion module and ground. The live wire impedance module is connected between the live wire output terminal and the ground terminal of the power conversion module. The neutral wire impedance module is connected between the neutral wire output terminal and the ground terminal of the power conversion module. The first input port of the processing module is used to receive the insulation resistance voltage sampling signal output by the voltage sampling module, and the second input port of the processing module is used to receive the bus voltage sampling signal.

[0011] As can be seen from the above scheme, by setting the controllable switch, the inverter output voltage Vac and the power frequency square wave voltage Vmid are connected in series to form a detection voltage signal source. This detection voltage signal source is injected into the detection circuit through the bus impedance module. Through the voltage division of the bus impedance module, the live wire impedance module, and the neutral wire impedance module, and through the parallel connection of the live wire impedance module and the neutral wire impedance module, an insulation voltage signal is generated, which is collected by the voltage sampling module and sent to the processing module. It can be seen that, through the above detection voltage signal source, the test signal can be equivalently converted from the original power frequency AC signal state to a smooth DC signal using the "average value equivalent smooth DC method" before the insulation resistance is measured and calculated. Therefore, the insulation resistance measurement completely avoids the influence of the power frequency AC output bus Y capacitor, and the test signal is equivalent to a smooth DC voltage. Therefore, the insulation resistance measurement result is accurate, stable, has small error, and strong anti-interference ability. It does not require the addition of a dedicated measurement signal source. The hardware is simple, reliable, low-cost, and small in size. The algorithm logic is simple, reliable, and fast in calculation, and it occupies few software resources of the processing module.

[0012] A further approach is to include a bus impedance module that includes bus resistors and / or bus capacitors.

[0013] A further approach is to include a live wire impedance module that includes live wire insulation resistance and / or live wire capacitance.

[0014] A further proposed solution is to include a live wire impedance module comprising a live wire insulation resistor and a live wire capacitor, with the live wire insulation resistor and the live wire capacitor connected in parallel.

[0015] A further approach is to include a neutral impedance module that includes neutral insulation resistance and / or neutral capacitance.

[0016] A further proposed solution is to include a neutral line impedance module comprising a neutral line insulation resistance and a neutral line capacitance, with the neutral line insulation resistance and neutral line capacitance connected in parallel.

[0017] As can be seen from the above, there are various ways to set the impedance. The impedance module can include at least one of resistors and capacitors, and the connection method is not specifically limited in this case. It can be adjusted according to the actual application requirements, but the internal component parameters of the impedance module are all known quantities.

[0018] A further approach is to connect the acquisition terminal of the voltage sampling module to the live wire output terminal of the power conversion module.

[0019] A further proposed solution is to connect the first terminal of the controllable switch to the positive input terminal of the power conversion module, connect the second terminal of the controllable switch to the first terminal of the bus impedance module, and ground the second terminal of the bus impedance module.

[0020] As can be seen above, by connecting the acquisition terminal to the live wire, and by connecting the controllable switch in series with the bus impedance module and then connecting it to the positive input terminal, the inverter output voltage Vac and the power frequency square wave voltage Vmid are connected in series to form a detection voltage signal source. This detection voltage signal source is injected into the detection circuit through the bus impedance module. With the arrangement of the controllable switch, when it is necessary to monitor the insulation resistance, the controllable switch is closed for monitoring. When monitoring is not required, the controllable switch is opened to isolate the bus impedance module from the power conversion module, thus not affecting the normal operation of the power conversion module.

[0021] A further proposed solution is that the voltage sampling module includes a voltage divider module, a dual-T notch filter module, and an operational amplifier signal amplification module. The first input terminal of the voltage divider module is connected to the live wire output terminal of the power conversion module, the second input terminal of the voltage divider module is grounded, the voltage divider output terminal of the voltage divider module is connected to the filter input terminal of the dual-T notch filter module, the filter output terminal of the dual-T notch filter module is connected to the operational amplifier input terminal of the operational amplifier signal amplification module, and the operational amplifier output terminal of the operational amplifier signal amplification module is connected to the first input port of the processing module.

[0022] As can be seen from the above, placing a power frequency dual-T notch filter between the resistor divider module and the operational amplifier signal amplifier in the voltage sampling module significantly attenuates the power frequency AC component in the insulation voltage signal output by the resistor divider module. This ensures that the input voltage signal of the operational amplifier signal amplifier is free of negative voltage components, thus eliminating the need to increase the compensation bias voltage to raise the operational amplifier input voltage signal above zero level. This allows for maximum utilization of the AD conversion range of the subsequent processing module, which is beneficial for improving the measurement accuracy of insulation resistance and meeting the requirement of a wide insulation resistance detection range. The entire detection circuit requires only one operational amplifier, making the hardware circuit more streamlined and cost-effective. In addition, the notch frequency of the power frequency dual-T notch filter can be selectively set to 55Hz, thereby accommodating inverters with output frequencies of 50Hz and 60Hz. This allows inverters with different output frequencies to share a set of hardware parameters for the insulation monitoring circuit, which is beneficial for manufacturing and after-sales service management.

[0023] To achieve the second objective of this invention, this invention provides a monitoring method for an insulation resistance detection circuit applied to the above-described scheme, the monitoring method comprising: Close the controllable switch; The voltage sampling module collects the voltage between the output terminal of the power conversion module and ground; The processing module acquires insulation resistance voltage sampling signals and bus voltage sampling signals; The insulation resistance impedance value of each phase of the AC power supply is obtained based on the impedance of the bus impedance module, the insulation resistance voltage sampling signal, and the bus voltage sampling signal.

[0024] As can be seen from the above, by setting the controllable switch, the inverter output voltage Vac and the power frequency square wave voltage Vmid are connected in series to form a detection voltage signal source. This detection voltage signal source is injected into the detection circuit through the bus impedance module. Through the voltage division of the bus impedance module, the live wire impedance module, and the neutral wire impedance module, and through the parallel connection of the live wire impedance module and the neutral wire impedance module, an insulation voltage signal is generated, which is collected by the voltage sampling module and sent to the processing module. It can be seen that, through the above detection voltage signal source, the test signal can be equivalently converted from the original power frequency AC signal state to a smooth DC signal using the "average value equivalent smooth DC method" before the insulation resistance is measured and calculated. Therefore, the insulation resistance measurement completely avoids the influence of the power frequency AC output bus Y capacitor, and the test signal is equivalent to a smooth DC voltage. Therefore, the insulation resistance measurement result is accurate, stable, has small error, and strong anti-interference ability. It does not require the addition of a dedicated measurement signal source. The hardware is simple, reliable, low-cost, and small in size. The algorithm logic is simple, reliable, and fast in calculation, and it occupies less software resources of the processing module.

[0025] A further approach is to include, after obtaining the insulation resistance value, the monitoring method also includes: Determine whether the insulation resistance impedance value is less than the preset insulation resistance alarm value; If so, a power-off signal will be output.

[0026] As can be seen above, if the resistance value is less than the preset insulation resistance alarm value, the power supply will be turned off to improve power supply safety.

[0027] A further solution is to determine whether the insulation resistance impedance value is less than the preset insulation resistance alarm value. If not, then determine whether the insulation monitoring needs to be terminated. If so, disconnect the controllable switch.

[0028] A further proposed solution is to determine whether insulation monitoring needs to be terminated. If not, return to the step of obtaining the insulation resistance impedance value of each phase of the AC power supply based on the impedance of the bus impedance module, the insulation resistance voltage sampling signal, and the bus voltage sampling signal.

[0029] As can be seen from the above, the insulation monitoring function can be selectively turned on or off by controlling the switch.

[0030] A further approach is to include the following monitoring methods when obtaining insulation resistance values: Based on the insulation resistance voltage sampling signal and the bus voltage sampling signal, the average insulation resistance voltage and the average bus voltage are calculated using a moving average filtering algorithm. The insulation resistance impedance value is calculated based on the average insulation resistance voltage, the average bus voltage, and the impedance of the bus impedance module.

[0031] A further approach involves, after calculating the average insulation resistance voltage and the average bus voltage, including the following monitoring methods: Determine whether the average value of the insulation resistance voltage and the average value of the bus voltage are stable within half a power frequency cycle; If so, the insulation resistance impedance value is calculated based on the average insulation resistance voltage, the average bus voltage, and the impedance of the bus impedance module.

[0032] As can be seen from the above, the synchronous moving average filtering algorithm is used to simultaneously measure and calculate the power frequency average value of the insulation resistance voltage sampling signal and the bus voltage sampling signal. The purpose is to avoid the possibility of incorrect insulation resistance impedance value results due to the different measurement time windows of the insulation resistance voltage average value and the bus voltage average value during dynamic processes. It can also improve the insulation resistance detection accuracy. In addition, it is used to judge whether the average value is stable within half a power frequency cycle. When it is stable, the calculation and measurement are performed to further improve the detection accuracy.

[0033] To achieve the third objective of this invention, this invention provides an on-board charger, including the insulation resistance detection circuit and power converter described above; the output terminal of the power converter outputs AC power; the insulation resistance detection circuit is used to detect the insulation resistance impedance value of each phase of the AC power output by the power converter. Attached Figure Description

[0034] Figure 1 This is a circuit diagram of an embodiment of the insulation resistance detection circuit of the present invention.

[0035] Figure 2 This is an equivalent circuit diagram of an embodiment of the insulation resistance detection circuit of the present invention.

[0036] Figure 3 This is a circuit diagram of the voltage sampling module in an embodiment of the insulation resistance detection circuit of the present invention.

[0037] Figure 4 This is a schematic diagram of the monitoring method of the insulation resistance detection circuit of the present invention.

[0038] The present invention will be further described below with reference to the accompanying drawings and embodiments. Detailed Implementation

[0039] To enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0040] Reference Figures 1 to 4 The insulation resistance detection circuit is applied to an AC power supply with a power conversion module. The output terminal of the power conversion module outputs AC power. In this embodiment, Vbus is the input DC voltage source of the power frequency inverter (hereinafter referred to as: inverter). The power conversion module is a power frequency inverter, which is an inverter H-bridge composed of switching transistors K1, K2, K3 and K4. K1 and K2 form one half of the H-bridge and switch at the power frequency, which is the power frequency commutation switch in the H-bridge. K3 and K4 form the other half of the H-bridge and switch at a high frequency, which is the high frequency switch of the H-bridge. The inductor L and the filter capacitor C form a power frequency filter. The input terminal of the power frequency filter is connected to the midpoint of the K3 and K4 half-bridge, and the output terminal is connected to the L line (live wire) output by the inverter. The N (neutral wire) output by the inverter is connected to the midpoint of the K1 and K2 half-bridge. The above is the main power circuit of the H-bridge inverter. The DC voltage source Vbus is converted into a power frequency AC voltage with an effective value of Vac by this power conversion module.

[0041] The insulation resistance detection circuit includes a controllable switch JK, a bus impedance module Rbus, a live wire impedance module, a neutral wire impedance module, a voltage sampling module, and a processing module. The first terminal of the controllable switch JK is connected to the positive input terminal of the power conversion module, that is, connected to the positive bus of the DC voltage source Vbus. The second terminal of the controllable switch JK is connected to the first terminal of the bus impedance module Rbus. The second terminal of the bus impedance module Rbus is grounded, that is, connected to the protective ground PE of the inverter system (PE is generally connected to the metal casing of the inverter).

[0042] The acquisition terminal of the voltage sampling module is connected to the live wire output terminal of the power conversion module. That is, one acquisition input terminal of the voltage sampling module is connected to the L line output by the inverter, and the other acquisition input terminal of the voltage sampling module is grounded to PE. The voltage sampling module is used to acquire the voltage of the live wire output terminal of the power conversion module to ground. The output signal of the voltage sampling module is VRi_AD, which is used to output to one AD (analog-to-digital converter) port of the processing module MCU. The processing module MCU also uses another AD port to receive the bus voltage sampling signal Vbus_AD from the DC voltage source Vbus sent by the control circuit of the inverter.

[0043] The live wire impedance module includes live wire insulation resistance and / or live wire capacitance, and the neutral wire impedance module includes neutral wire insulation resistance and / or neutral wire capacitance. In this embodiment, the live wire impedance module includes a resistor RLi, and the neutral wire impedance module includes a resistor RNi. Figure 1 CNy is the Y capacitor of the N line, CLy is the Y capacitor of the L line. This Y capacitor is the sum of the inverter's own Y capacitor and the Y capacitor of the inverter load. The resistor RNi is the insulation resistance of the N line to the PE. CNy and RNi are connected in parallel. The resistor RLi is the insulation resistance of the L line to the PE. CLy and RLi are connected in parallel. When the insulation is normal, the insulation resistance value is much greater than the safety threshold.

[0044] The detection principle of this invention is as follows: Figure 1 K1 and K2 in the inverter's H-bridge serve as power frequency commutation switches. During the positive half-cycle of the inverter's output voltage Vac, K2 closes and K1 opens; during the negative half-cycle, K1 closes and K2 opens. Thus, the voltage level between the positive bus Vbus+ of the DC voltage source Vbus and the midpoint (N line) of the K1 and K2 bridge arms can be equivalent to a power frequency pulsating square wave voltage source with an amplitude of Vbus and a duty cycle of 50%. This invention names this power frequency square wave voltage source Vmid.

[0045] When insulation testing is required, the MCU controls the controllable switch JK to close. At this time, the equivalent circuit of the insulation resistance detection circuit of this invention is as follows: Figure 2 In Figure A, Vmid and Vac are connected in series to form a detection voltage signal source, which is injected into the detection circuit through the bus impedance module Rbus.

[0046] The detection voltage signal source in Figure A is a power frequency AC voltage signal containing a DC component. Taking its average power frequency period and transforming it into a smoothed DC voltage signal, the equivalent circuit in Figure A becomes... Figure 2In the equivalent circuit of Figure B, the average power frequency period of the Vac part of the detection signal source in Figure A is 0, and the average power frequency period of the Vmid part is Vbus / 2. Therefore, after the average equivalent transformation, the detection signal source in Figure B is a smoothed DC voltage source with a level of Vbus / 2, which is also named Vbus / 2 in the figure. Since the capacitive reactance of the capacitor is infinite when smoothing DC, it can be ignored when connected in parallel with other impedances. Therefore, the Y capacitors in Figure B are omitted, and the influence of the Y capacitors on the measurement is completely avoided. Since the average power frequency period of the Vac part is equal to 0, N and L in Figure A are considered to be at the same potential and combined into one in Figure B. Therefore, the insulation resistances RNi and RLi are also connected in parallel to form Ri in Figure B, where Ri is the insulation resistance to be detected in this case.

[0047] In Figure B, the DC voltage signal source Vbus / 2 is divided by Rbus and Ri, generating an insulation resistance voltage sampling signal VRi on Ri. This signal is acquired by the voltage sampling module and output to the processing module MCU. The processing module MCU also receives the bus voltage sampling signal Vbus_AD. Through the calculation of the MCU, the average insulation resistance voltage VRi during the power frequency cycle can be measured as VRi_avg. When measuring the average value of VRi during the power frequency cycle, the MCU simultaneously measures the average value of the bus voltage Vbus during the power frequency cycle as Vbus_avg.

[0048] Rbus is a known constant. With VRi_avg and Vbus_avg, Ri can be calculated using Formula 1.

[0049] ; This invention uses two techniques—a power frequency dual-T notch filter pre-amplified by an operational amplifier signal amplifier and a synchronous sliding power frequency average filtering algorithm—to transform the power frequency AC signal in Figure A into a smooth DC signal in Figure B. The former first transforms the power frequency AC signal into a non-smooth DC signal that still contains power frequency periodic components, while the latter further transforms the non-smooth DC signal into a smooth DC signal.

[0050] Figure 3 This is a schematic diagram of the voltage sampling module of the present invention. The voltage sampling module includes a voltage divider module, F1-1, a dual-T notch filter module F1-2, and an operational amplifier signal amplification module F1-3.

[0051] The voltage divider module F1-1 includes resistors R1 and R2, which are connected in series and used to sample the voltage VRi. The first end of resistor R1 is the first input terminal of the voltage divider module, and the second end of resistor R2 is the second input terminal of the voltage divider module. By dividing the voltage with resistors, the insulation resistance voltage sampling signal VRi is converted from a higher level to a lower level acceptable to the subsequent circuit. The voltage signal output by F1-1 is an AC signal with a power frequency cycle. The total resistance value of R1+R2 should be much greater than Rimax, which is the upper limit of the insulation resistance measurement range specified by the OBC technical standard. As a preferred embodiment, R1+R2=200MΩ. Of course, other resistance values ​​can also be used outside of this embodiment.

[0052] The dual-T notch filter module F1-2 includes capacitors C1, C2, and C3, and resistors R3, R4, and R5. The first terminals of capacitor C1 and resistor R3 are connected to the midpoint of the series connection of resistors R1 and R2. The midpoint of the series connection of resistors R1 and R2 is the filter input terminal. The second terminals of capacitor C1, capacitor C2, and resistor R5 are connected to the first terminal of resistor R5. The second terminal of resistor R5 is grounded. The second terminals of resistor R3, capacitor C3, and resistor R4 are connected to the first terminal of resistor R4. The second terminal of capacitor C3 is grounded. The second terminals of capacitor C2 and resistor R4 are connected to the first input terminal of operational amplifier AP1. The second terminal of resistor R4 is the filter output terminal.

[0053] The function of the dual-T notch filter module F1-2 is to significantly attenuate the power frequency AC component in the insulation resistance voltage sampling signal after voltage division by the voltage divider module F1-1, and to completely filter out the negative voltage component. However, the insulation resistance voltage sampling signal after filtering by F1-2 is still a power frequency periodic DC signal containing power frequency components, rather than a smoothed DC voltage signal. As a preferred design, the notch frequency of the dual-T notch filter is set to 55Hz, so that inverters with output frequencies of 50Hz and 60Hz can share a set of hardware parameters, which is beneficial for manufacturing and after-sales service management. Of course, in other embodiments, the frequency can also be 50Hz or 60Hz.

[0054] The operational amplifier signal amplification module F1-3 includes operational amplifier AP1, resistor R6, resistor R7, and capacitor C4. The first end of resistor R6 is grounded. The second end of resistor R6, the second input terminal of operational amplifier AP1, the first end of resistor R7, and the first end of capacitor C4 are connected. The second end of resistor R7, the second end of capacitor C4, and the output terminal of operational amplifier AP1 are connected. The output terminal of operational amplifier AP1 is connected to the processing module MCU.

[0055] The function of the operational amplifier signal amplification module F1-3 is to condition and amplify the insulation resistance voltage sampling signal after it has been filtered by F1-2, so that its output signal VRi_AD matches the range of the AD conversion of the subsequent MCU, so as to make full use of the AD conversion range. Capacitor C4 is responsible for filtering out high-frequency components in the signal.

[0056] The insulation resistance voltage sampling signal VRi_AD and the bus voltage sampling signal Vbus_AD output from the operational amplifier signal amplification module F1-3 are input to the MCU. The following power frequency synchronous moving average digital filtering algorithm is used to quickly calculate the power frequency period average value of VRi and Vbus, the average value of insulation resistance voltage VRi_avg, and the average value of bus voltage Vbus_avg.

[0057] The principle of the moving average filtering algorithm for measuring and calculating the average value of a power frequency periodic signal is as follows.

[0058] First, a power frequency cycle is used as the sliding time window, and this time window is divided into N equal sampling cycles. The measured signal is sampled once in real time in each sampling cycle, thus forming a circular data queue composed of N sampled data.

[0059] Secondly, the data in the upper circular data queue are summed and then divided by N to obtain the average value of the measured signal within the time window.

[0060] Finally, for each sample, the first data in the circular data queue is replaced with the new data, and the average is calculated using the method described above to obtain a new average.

[0061] The above-mentioned measurement scheme not only has high measurement accuracy, but also the measurement accuracy is related to the value of N. The larger N is, the better the measurement accuracy. In this invention, the preferred value of N is 1000. It also has a fast dynamic response speed. When the detected power frequency periodic signal changes from one steady state to another, the average value of the new steady state can be obtained after the measured signal reaches the first power frequency period of the new steady state.

[0062] The bus voltage sampling signal Vbus also carries a power frequency ripple component. This invention uses a synchronous moving average filtering algorithm to synchronously measure and calculate the power frequency average values ​​of Vbus and VRi. The purpose is to avoid incorrect Ri results that may be caused by the different measurement time windows of VRi_avg and Vbus_avg during the dynamic process, which can improve the accuracy of insulation resistance detection.

[0063] Reference Figure 4 The monitoring method applied to the above insulation resistance detection circuit includes the following steps.

[0064] First, execute step S11. After the system activates the insulation monitoring function, close the controllable switch JK.

[0065] Then, step S12 is executed, where the MCU synchronously acquires VRi_AD and Vbus_AD, and synchronously calculates the power frequency average values ​​VRi_avg and Vbus_avg of VRi and Vbus using a moving average filtering algorithm.

[0066] Then, step S13 is executed to determine whether VRi_avg and Vbus_avg are stable within half a power frequency cycle. If the result is "yes", then proceed to S14; if the result is "no", then proceed to S12.

[0067] Execute step S14, substitute VRi_avg and Vbus_avg into the above formula 1, calculate the insulation resistance Ri through the formula, and then obtain the insulation resistance impedance value of each phase of the AC power output of the power converter.

[0068] Then, step S15 is executed to determine whether the insulation resistance Ri is less than the preset insulation resistance alarm value. If the result is "yes", then proceed to S16; if the result is "no", then proceed to S17.

[0069] Execute step S16, INV shutdown protection, and issue an alarm signal.

[0070] Then, proceed to step S17 to determine whether the insulation monitoring has ended. If the result is "yes", proceed to step S18; if the result is "no", proceed to step S12.

[0071] Finally, execute step S18 to disconnect the controllable switch JK and end the operation.

[0072] Of course, the above embodiments are only preferred embodiments of this case, and can be adjusted according to actual application. For example, there are various arrangements of the bus impedance module, live wire impedance module and neutral wire impedance module. For example, the live wire impedance module includes live wire insulation resistance and live wire capacitance, and the live wire insulation resistance and live wire capacitance are connected in parallel. Another example is the neutral wire impedance module, which includes neutral wire insulation resistance and neutral wire capacitance, and the neutral wire insulation resistance and neutral wire capacitance are connected in parallel. The live wire impedance module and the neutral wire impedance module can be modules arranged on the insulation resistance detection circuit, or modules integrated on the power converter. The series positions of the controllable switch JK and the bus impedance module Rbus can be interchanged. The acquisition terminal of the voltage sampling module is connected to the neutral wire output terminal of the power conversion module. By acquiring the voltage of the neutral wire output terminal to ground, the above changes can achieve the purpose of this invention.

[0073] The on-board charger includes the insulation resistance detection circuit and the power converter described above. The power converter outputs AC power, and the insulation resistance detection circuit is used to detect the insulation resistance impedance value of each phase of the AC power output by the power converter.

[0074] As can be seen from the above, this case uses the "average value equivalent smoothed DC method" to convert the test signal from the original power frequency AC signal state into a smoothed DC signal before measuring and calculating the insulation resistance. The insulation resistance measurement completely avoids the influence of the power frequency AC output bus Y capacitor, and the test signal is equivalent to a smoothed DC voltage. Therefore, the insulation resistance measurement results are accurate, stable, have small errors, and strong anti-interference ability. There is no need to add a special measurement signal source. The hardware is simple, reliable, low-cost, and small in size. The algorithm logic is simple, reliable, and fast in calculation, and it occupies less software resources of the processing module.

Claims

1. A high-precision insulation resistance detection circuit, wherein the insulation resistance detection circuit is applied to an AC power supply having a power conversion module, wherein the output terminal of the power conversion module outputs AC power; Its features are: The insulation resistance detection circuit includes a controllable switch, a bus impedance module, a live wire impedance module, a neutral wire impedance module, a voltage sampling module, and a processing module. The controllable switch is connected in series with the bus impedance module and between the input terminal and the ground terminal of the power conversion module; The voltage sampling module is used to collect the voltage of the output terminal of the power conversion module relative to ground; The live wire impedance module is connected between the live wire output terminal and the ground terminal of the power conversion module; The neutral impedance module is connected between the neutral output terminal and the ground terminal of the power conversion module; The first input port of the processing module is used to receive the insulation resistance voltage sampling signal output by the voltage sampling module, and the second input port of the processing module is used to receive the bus voltage sampling signal.

2. The insulation resistance detection circuit according to claim 1, characterized in that: The bus impedance module includes bus resistors and / or bus capacitors.

3. The insulation resistance detection circuit according to claim 1, characterized in that: The live wire impedance module includes live wire insulation resistance and / or live wire capacitance.

4. The insulation resistance detection circuit according to claim 3, characterized in that: The live wire impedance module includes the live wire insulation resistance and the live wire capacitor, with the live wire insulation resistance and the live wire capacitor connected in parallel.

5. The insulation resistance detection circuit according to claim 1, characterized in that: The neutral line impedance module includes neutral line insulation resistance and / or neutral line capacitance.

6. The insulation resistance detection circuit according to claim 5, characterized in that: The neutral line impedance module includes the neutral line insulation resistance and the neutral line capacitance, wherein the neutral line insulation resistance and the neutral line capacitance are connected in parallel.

7. The insulation resistance detection circuit according to any one of claims 1 to 6, characterized in that: The acquisition terminal of the voltage sampling module is connected to the live wire output terminal of the power conversion module.

8. The insulation resistance detection circuit according to claim 7, characterized in that: The first terminal of the controllable switch is connected to the positive input terminal of the power conversion module, the second terminal of the controllable switch is connected to the first terminal of the bus impedance module, and the second terminal of the bus impedance module is grounded.

9. The insulation resistance detection circuit according to claim 7, characterized in that: The voltage sampling module includes a voltage divider module, a dual-T notch filter module, and an operational amplifier signal amplification module. The first input terminal of the voltage divider module is connected to the live wire output terminal of the power conversion module, the second input terminal of the voltage divider module is grounded, the voltage divider output terminal of the voltage divider module is connected to the filter input terminal of the dual-T notch filter module, the filter output terminal of the dual-T notch filter module is connected to the operational amplifier input terminal of the operational amplifier signal amplification module, and the operational amplifier output terminal of the operational amplifier signal amplification module is connected to the first input port of the processing module.

10. A monitoring method applied to the insulation resistance detection circuit according to any one of claims 1 to 9, characterized in that, The monitoring method includes: Close the controllable switch; The voltage sampling module collects the voltage between the output terminal of the power conversion module and ground; The processing module acquires the insulation resistance voltage sampling signal and the bus voltage sampling signal; The insulation resistance impedance value of each phase of the AC power supply is obtained based on the impedance of the bus impedance module, the insulation resistance voltage sampling signal, and the bus voltage sampling signal.

11. The monitoring method according to claim 10, characterized in that: After obtaining the insulation resistance value, the monitoring method further includes: Determine whether the insulation resistance impedance value is less than the preset insulation resistance alarm value; If so, a power-off signal will be output.

12. The monitoring method according to claim 11, characterized in that: When determining whether the insulation resistance impedance value is less than the preset insulation resistance alarm value, if not, then determine whether the insulation monitoring needs to be terminated. If so, then disconnect the controllable switch.

13. The monitoring method according to claim 12, characterized in that: When determining whether insulation monitoring needs to be terminated, if not, return to the step of obtaining the insulation resistance impedance value of each phase of the AC power supply based on the impedance of the bus impedance module, the insulation resistance voltage sampling signal, and the bus voltage sampling signal.

14. The monitoring method according to claim 10, characterized in that: When obtaining the insulation resistance value, the monitoring method further includes: Based on the insulation resistance voltage sampling signal and the bus voltage sampling signal, the average insulation resistance voltage and the average bus voltage are calculated using a moving average filtering algorithm. The insulation resistance impedance value is calculated based on the average insulation resistance voltage, the average bus voltage, and the impedance of the bus impedance module.

15. The monitoring method according to claim 14, characterized in that: After calculating the average insulation resistance voltage and the average bus voltage, the monitoring method further includes: Determine whether the average value of the insulation resistance voltage and the average value of the bus voltage are stable within half a power frequency cycle; If so, the insulation resistance impedance value is calculated based on the average insulation resistance voltage, the average bus voltage, and the impedance of the bus impedance module.

16. An on-board charger, characterized in that, Includes the insulation resistance detection circuit and power converter as described in any one of claims 1 to 9; The power converter outputs alternating current. The insulation resistance detection circuit is used to detect the insulation resistance impedance value of each phase of the AC power output from the power converter.

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