Calibration device for calibrating voltages of a fault monitoring device, fault monitoring device and method of calibrating voltages of a fault monitoring device

Abstract

Calibration device for calibrating voltages of a fault monitoring device, fault monitoring device and method of calibrating voltages of a fault monitoring device A calibration device for calibrating voltages of a fault monitoring device for a power system (1) The calibration device comprises a fault monitoring device with first and second voltage dividers (21, 22) and first and second voltage measurement devices (81, 82), first and second calibrated voltage sensors (LABH, LABL), and first and second variable resistors (R5, R6). A controller (2) is configured to, in a first step, determine first and second linear regression curves based on voltages (UH, UL) determined by means of the first and second voltage measurement devices (81, 82) and a based on voltages measured by the first and second calibrated voltage sensor (LABH, LABL), and first and second constant parameters are deducted from the first and second linear regression curves. In a second step, asymmetrical voltage values for the high side voltage (UH) and the low side voltage (UL) are provided in steps by adjusting the resistance of the first and second variable resistors (R5, R6), and in each step several voltage values are determined. Third and fourth constant parameters (GAINERRHm, GAINERRHb) are determined from the determined voltage values using linear regression. The invention further provides for a fault monitoring device and a method of calibrating voltages of a fault monitoring device.

Description

[0001] CALIBRATION DEVICE FOR CALIBRATING VOLTAGES OF A FAULT MONITORING DEVICE, FAULT MONITORING DEVICE AND METHOD OF CALIBRATING VOLTAGES OF A FAULT MONITORING DEVICE

[0002] Field of the disclosure

[0003] The present disclosure relates to a calibration device for calibrating voltages of a fault monitoring device system for a power system, a fault monitoring device and a method of calibrating voltages of a fault monitoring device.

[0004] Background

[0005] United States patent applications US 2022 / 0413034 A1 and US 2022 / 0413035 A1 disclose an insulation monitoring device comprising a primary resistance circuit and a secondary resistance circuit arranged in parallel to a high side insulation resistance and a low side insulation resistance, wherein the secondary resistance circuit comprises additional resistors which are selectively connectable to a high voltage bus via a switching circuit. Different states can be implemented by the switches. Voltage values associated with the different states are used to calculate the resistance values for the high side insulation resistance and the low side insulation resistance.

[0006] The relevant voltage values are measured by means of voltage measurement devices such as Analog-to-digital converters (ADCs). The ADCs or any other voltage measurement devices have to be calibrated to provide satisfactory accuracy.

[0007] There is thus a desire to provide for an accurate calibration of voltage measurements made in insulation monitoring devices.

[0008] There is a need to provide a calibration device and a method of calibrating voltages of a fault monitoring device which calibrate voltage measurements in an accurate manner.

[0009] Summary of the disclosure

[0010] According to an aspect of the invention, a calibration device for calibrating voltages of a fault monitoring device for a power system is provided. The power system for which the fault monitoring device is provided for is of the kind that comprises a positive voltage rail, a negative voltage rail, and a chassis. The positive voltage rail and the negative voltage rail form part of a high voltage bus connected to a DC power source. A high side voltage is present between the positive voltage rail and the chassis and a low side voltage is present between the chassis and the negative voltage rail. The fault monitoring device comprises a first voltage divider arranged between the positive voltage rail and the chassis, wherein the first voltage divider comprises a first resistor and a second resistor, and wherein a first voltage measurement device is arranged in parallel to the second resistor and configured to measure the voltage of the second resistor. The fault monitoring device further comprises a second voltage divider arranged between the chassis and the negative voltage rail, wherein the second voltage divider comprises a third resistor and a fourth resistor, and wherein a second voltage measurement device is arranged in parallel to the third resistor and configured to measure the voltage of the third resistor. The calibration device comprises the fault monitoring device and further comprises a first calibrated voltage sensor connected between the positive voltage rail and the chassis, a second calibrated voltage sensor connected between the chassis and the negative voltage rail, a first variable resistor connectable between the positive voltage rail and the chassis, and a second variable resistor connectable between the chassis and the negative voltage rail.

[0011] A controller is configured to determine constant parameters (which are calibration parameters) that allow to calculate a double calibrated high side voltage and a double calibrated low side voltage from voltages measured by the first and second voltage measurement devices. The calibration that provides for such parameters is in two steps, wherein in the first step the first variable resistor and the second variable resistor are disconnected and thus do not play any role. In the second step, on the other hand, the first variable resistor is connected between the positive voltage rail and the chassis in parallel to the first voltage divider and the second variable resistor is connected between the chassis and the negative voltage rail in parallel to the second voltage divider.

[0012] In the first step, a first linear regression curve is determined. The first linear regression curve is determined on the basis of values of high side voltages that are determined differently. More particularly, a first high side voltage is determined by means of the first voltage measurement device. A second high side voltage is measured by the first calibrated voltage sensor. The first linear regression curve comprises first constant parameters that are determined and allow to provide a first calibrated high side voltage dependent on the first high side voltage. Similarly, a second linear regression curve is determined on the basis of values of the low side voltages that are determined differently. More particularly, on a first low side voltage is determined by means of the second voltage measurement device. A second low side voltage is measured by the second calibrated voltage sensor. The second linear regression curve comprises second constant parameters that are determined and allow to provide a first calibrated low side voltage dependent on the first low side voltage. Accordingly, in the first step, a first calibrated high side voltage and a first calibrated low side voltage are provided for by means of linear regression. In the second step, asymmetrical voltage values are provided in steps for the high side voltage and the low side voltage by adjusting the resistance of the first and second variable resistors. In each step, the values for the first calibrated high side voltage, the first calibrated low side voltage, a third high side voltage measured by the first calibrated voltage sensor, and a third low side voltage measured by the second calibrated voltage sensor are determined. Subsequently, third constant parameters are determined from the determined voltage values using linear regression, wherein the third linear parameters allow to correct a first calibrated high side voltage determined by the first constant parameters to arrive at a double calibrated high side voltage. Further, fourth constant parameters are determined from the determined voltage values using linear regression, wherein the fourth constant parameters allow to correct a first calibrated low side voltage determined by the second constant parameters to arrive at a double calibrated low side voltage.

[0013] Aspects of the invention are thus based on the idea to provide for a two-step calibration, wherein in the first step first and second constant parameters that allow to determine a first calibrated high side voltage and a first calibrated low side voltage are provided for by means of linear regression. For the first step calibration, the high side voltage and the low side voltage measured through the first and second voltage measurement device is compared to the high side voltage and the low side voltage measured by the first and second calibrated voltage sensor, wherein the calibrated voltage sensors provide for a highly accurate value of the voltage. It is pointed out that, in the first calibration step, the high side voltage and the low side voltage are the same or in a fixed ratio (as the resistance of the first voltage divider and the resistance of the second voltage divider have a fixed relationship (they may be equal)) such that the first calibrated high side voltage and the first calibrated low side voltage are measured in a symmetrical system (wherein during calibration the DC voltage which is applied to the positive voltage rail and the negative voltage rail is increased in predetermined steps).

[0014] In the second step, asymmetrical voltages are provided, wherein the high side voltage is different from the low side voltage, and calibration takes place on the basis of the asymmetrical voltages, wherein for all asymmetrical high side / low side voltage pairs the first calibrated high side voltage and the first calibrated low side voltage as well as the respective high side voltage and low side voltage measured by the first and second calibrated voltage sensors are measured. From these measurements, third constant parameters and fourth constant parameters are deducted by linear regression.

[0015] The result of the calibration are the first, second, third and fourth constant parameters that may be stored in hardware / software of a fault monitoring device and subsequently (subsequent to the calibration) be used to correct voltage values measured by the first and second voltage measurement devices of the fault monitoring device. The first and second constant parameters achieved in the first calibration step allow to calculate a first calibrated high side voltage and a first calibrated low side voltage. The third and fourth constant parameters achieved in the second calibration step allow to correct the first calibrated high side voltage to arrive at a double calibrated high side voltage and further allow to correct the first calibrated low side voltage to arrive at a double calibrated low side voltage.

[0016] Implementation of the second step is based on the realization that calibration based on asymmetrical voltages allows to compensate for voltage measurement errors which are caused by the internal resistance of the voltage measurement device. In this respect, it is to be noted that a voltage measurement device such as an ADC has a high internal resistance which negatively influences the accuracy of the measurement. By the second step calibration it is possible to take this internal resistance into consideration: the internal resistance of the voltage measurement devices particularly increases a measurement error if the asymmetry between the high side voltage and the low side voltage is high. This is addressed by the second step calibration.

[0017] The term “controller” identifies any processing capacity which is able to perform the method to determine the double calibrated high side voltage and the double calibrated low side voltage. The controller may comprise one or several processors, and memory storing instructions that when executed perform the method. The controller may also include output lines to control the first and second variable resistors, and may include input lines to receive measurement values from the first and second voltage measurement devices and from the first and second calibrated voltage sensors.

[0018] In some embodiments, the controller is configured such that first step comprises: for a plurality of voltages in a voltage range, determine the first high side voltage from the first resistor, the second resistor, and the voltage measured by the first voltage measurement device, and further determine the second high side voltage by measuring the voltage between the positive voltage rail and the chassis by the first calibrated voltage sensor, for a plurality of voltages in a voltage range, determine the first low side voltage from the third resistor, the fourth resistor, and the voltage measured by the second voltage measurement device, and further determine the second low side voltage by measuring the voltage between the chassis and the negative voltage rail by the second calibrated voltage sensor, determine the first linear regression curve on the basis of the first high side voltages and the second high side voltages, the first linear regression curve providing a first line defined by the parameters slope and y-intercept as first constant parameters, the first line providing the first calibrated high side voltage dependent on the first high side voltage, determine the second linear regression curve on the basis of the first low side voltages and the second low side voltages, the second linear regression curve providing a second line defined by the parameters slope and y-intercept as second constant parameters, the second line providing the first calibrated low side voltage dependent on the first low side voltage.

[0019] Accordingly, linear regression curves and their slope and y-intercept parameters are determined for the high side voltage and the low side voltage, thereby providing the first calibrated high side voltage and the first calibrated low side voltage as a function of the first high side voltage and the first low side voltage and thus as a function of the first and second voltage measurement devices.

[0020] The mentioned voltage range may be defined by the DC power source. For example, if the DC power source has a maximum voltage difference of 1000 V (between -500 V at the negative terminal and +500 V at the positive terminal), such voltage difference represents the voltage range in which the plurality of voltages is determined. A test voltage may be applied between 0 V and 1000 V in predefined steps. As discussed before, due to the symmetry of the system in the first step, the same voltage or voltage ratio is present over the first voltage divider and the second voltage divider.

[0021] In some embodiments, the first high side voltage is determined from the first resistor, the second resistor, and the voltage measured by the first voltage measurement device by means of the formula wherein UH is the first high side voltage, R1 is the first resistor, R2 is the second resistor and UADCH is the voltage measured by the first voltage measurement device. The first low side voltage may be determined in a similar manner. The formula follows from the voltage present at a voltage divider.

[0022] In some embodiments, the controller is configured in the second step to provide in steps asymmetrical voltage values in that it is configured to adjust the resistance of the first and second variable resistors such that the voltage over the first variable resistor is changed in steps between a minimum and a maximum and such that the voltage over the second variable resistor is changed in steps between a maximum and a minimum. Further, in each step the values for the first calibrated high side voltage, the first calibrated low side voltage, the measured third high side voltage measured by the first calibrated voltage sensor, and the measured third low side voltage measured by the second calibrated voltage sensor are recorded.

[0023] In some embodiments, the controller is configured in the second step to correct the first calibrated high side voltage and correct the first calibrated low side voltage in that it is configured to: calculate in each step a high side gain error for the first calibrated high side voltage based on the third high side voltage, calculate in each step a low side gain error for the first calibrated low side voltage based on the third low side voltage, determine a third linear regression curve on the basis of the quotient of the first calibrated high side voltage and the first calibrated low side voltage on the x-axis and this quotient times the high side gain error on the y-axis, the third linear regression curve providing a third line defined by parameters slope and y-intercept which represent the third constant parameters, and determine a fourth linear regression curve on the basis of the quotient of the first calibrated low side voltage and the first calibrated high side voltage on the x-axis and this quotient times the low side gain error on the y-axis, the fourth first linear regression curve providing a fourth line defined by parameters slope and y-intercept which represent the fourth constant parameters.

[0024] Accordingly, linear regression curves are provided for the quotient of the first calibrated high side voltage and the first calibrated low side voltage and for the quotient of the first calibrated low side voltage and the first calibrated high side voltage. Thereby, two linear equations are provided for the quotient of the double calibrated high side voltage and the double calibrated low side voltage which allows it to calculate these values.

[0025] The third constant parameters of the third line allow - subsequent to the calibration - to correct a first calibrated high side voltage to arrive at a double calibrated high side voltage, and the fourth constant parameters of the fourth line allow - subsequent to the calibration - to correct a first calibrated low side voltage to arrive at a double calibrated low side voltage.

[0026] In some embodiments, it may be provided that in each step the high side gain error for the first calibrated high side voltage is calculated as the quotient of the measured high side voltage and the first calibrated high side voltage. Similarly, it may be provided that the low side gain error is calculated as the quotient of the measured low side voltage and the first calibrated low side voltage. The high side gain error and the low side gain error are used in the linear calibration. In some embodiments, the controller is configured to adjust the resistances of the first and second variable resistors in steps such that the respective high side voltage is varied in steps in a voltage range defined by the voltage difference between a positive terminal and a negative terminal of a DC power source to which the positive voltage rail and the negative voltage rail are attached, respectively, and that the respective low side voltage is varied in steps over the same voltage range, wherein the sum of the high side voltage and the low side voltage is constant.

[0027] To give an example, if the voltage range is 1000 V (when assuming a DC power source with a +500 V terminal and a -500 V terminal), this range may be passed in steps of 50 V or 100 V by adjusting the first and second variable resistors in steps accordingly. More particularly, both the high side voltage between the chassis at the positive voltage rail is varied over most of this voltage range and the low side voltage between the negative voltage rail and the chassis is varied over most of this voltage range, such that both the high side voltage and the low side voltage are calibrated over a range of nearly 1000 V. For example, the resistance of the first adjustable resistor may be 5000 kQ and the resistance of the second adjustable resistor may be 50 kQ at the beginning of a stepwise measurement (such that the high side voltage is close to 1000 V and the low side voltage is close to 0 V) and these values are reversed at the end of the stepwise measurement (such that the high side voltage is close to 0V and the high side voltage is closer to 1000 V).

[0028] In some embodiments, the fault monitoring device further comprises a third voltage measurement device, wherein the third voltage measurement device is arranged to measure the voltage over the second resistor of the first voltage divider and the third resistor of the second voltage divider. The controller is configured to, in the first stage, determine a fifth linear regression curve based on a first battery voltage determined by means of the third voltage measurement device and a second battery voltage measured by the first and second calibrated voltage sensors, and determine fifth constant parameters from the fifth linear regression curve that allow to provide a first calibrated battery voltage dependent on the first battery voltage. Such third voltage measurement device is used as a validity check I for redundancy.

[0029] The fault monitoring device may generally be configured to define two different states of the fault monitoring device by selectively switching on an off resistors arranged in parallel to the first and second voltage dividers, wherein the controller determines from the voltage changes associated with the two different states the resistance values for a high side insulation resistance and a low side insulation resistance, wherein the chassis is insulated by the high side insulation resistance from the positive voltage rail and insulated by the low side insulation resistance from the negative voltage rail. For the respective calculations, the high side voltage and the low side voltage need to be exactly calibrated. The first and second voltage measurement device may, in principle, be any voltage measurement devices. In some embodiments, the first and second voltage measurement devices are Analog-to-Digital Converters (ADCs). The use of ADCs as measurement devices is associated with the advantage that the voltage measurement provides as output a digital signal which can be used in software calculations without further transformation. ADCs, as all voltage measurement devices, have a high internal resistance to limit the influence on the measurement, wherein a remaining influence is addressed in accordance with the present invention.

[0030] In a further aspect of the invention, a fault monitoring device for a power system is provided for. The fault monitoring device comprises a first voltage divider arranged between the positive voltage rail and the chassis, the first voltage divider comprising a first resistor and a second resistor, wherein a first voltage measurement device is arranged in parallel to the second resistor and configured to measure the voltage of the second resistor. The fault monitoring device further comprises a second voltage divider arranged between the chassis and the negative voltage rail, the second voltage divider comprising a third resistor and a fourth resistor, wherein a second voltage measurement device is arranged in parallel to the third resistor and configured to measure the voltage of the third resistor.

[0031] The fault monitoring device further comprises a controller which has stored (in a memory or as a software) the first, second, third and fourth constant parameters determined by the calibration device of any of claims 1 to 6 (or by the method of claim 12), wherein the controller is configured to: determine a first calibrated high side voltage from a voltage measured by the first voltage measurement device and the first constant parameters, determine a first calibrated low side voltage from a voltage measured by the second voltage measurement device and the second constant parameters, correct the first calibrated high side voltage using the third constant parameters to determine a double calibrated high side voltage, correct the first calibrated low side voltage using the fourth constant parameters to determine a double calibrated low side voltage.

[0032] This aspect of the invention is based on a previous determination of the first, second, third and fourth constant parameters which constitute the calibration of the fault monitoring device. Based on these stored parameters, the fault monitoring device can calculate for every voltage sample received from the voltage measurement devices the double calibrated voltage and the double calibrated low side voltage. In some embodiments, the controller is configured to correct the first calibrated high side voltage and the first calibrated low side voltage by applying the formulas wherein:

[0033] Uncaiibi is the first calibrated high side voltage;

[0034] UHcaiib2 is the double calibrated high side voltage;

[0035] Ui_caiibi is the first calibrated low side voltage;

[0036] GAINERRHm is the slope of the third linear regression curve;

[0037] GAINERRHb is the y-intercept of the third linear regression curve;

[0038] Ui_caiibi is the first calibrated low side voltage;

[0039] Ui_caiib2 is the double calibrated low side voltage;

[0040] Uncaiibi is the first calibrated high side voltage;

[0041] GAINERRL-m is the slope of the fourth linear regression curve; and GAINERRL-b is the y-intercept of the fourth linear regression curve.

[0042] In a further aspect of the invention, a power system is provided. The power system comprises a DC power source having a positive terminal, a negative terminal and a voltage, a positive voltage rail connected to the positive terminal, a negative voltage rail connected to the negative terminal, a chassis, and a fault monitoring device in accordance with the invention.

[0043] In a still further aspect of the invention, a method of calibrating voltages of a fault monitoring device is provided for. The calibration method provides for first, second, third and fourth constant parameters that are determined in the first and second steps as defined in claim 12. The first, second, third and fourth constant parameters allow to determine a double calibrated high side voltage and a double calibrated low side voltage from voltage values determined by the first and second voltage measurement devices.

[0044] Embodiments of the inventive method correspond to the embodiments of the calibration device discussed above.

[0045] In some embodiments of the inventive method, the method further comprises determine a first calibrated high side voltage from a voltage measured by the first voltage measurement device and the first constant parameters, determine a first calibrated low side voltage from a voltage measured by the second voltage measurement device and the second constant parameters, correct the first calibrated high side voltage using the third constant parameters to determine a double calibrated high side voltage, correct the first calibrated low side voltage using the fourth constant parameters to determine a double calibrated low side voltage.

[0046] These steps take place after calibration and allow to determine a double calibrated high side voltage and a double calibrated low side voltage based on the constant parameters that have been determined by the calibration.

[0047] The skilled person will appreciate that except where mutually exclusive, a feature or parameter described in relation to any one of the above aspects may be applied to any other aspect. Furthermore, except where mutually exclusive, any feature or parameter described herein may be applied to any aspect and / or combined with any other feature or parameter described herein.

[0048] Brief description of the drawings

[0049] The invention will be explained in more detail on the basis of exemplary embodiments with reference to the accompanying drawings in which:

[0050] FIG. 1 is an embodiment of a calibration device for a fault monitoring device for power system in accordance with the present invention, the calibration device comprising the fault monitoring device, first and second calibrated voltage sensors, first and second variable resistors, and a controller;

[0051] FIGS. 2-4 show linear regression curves for a high side voltage, a low side voltage and a DC power source voltage in the calibration device of FIG. 1 , wherein the linear regression curves are provided for during a first step of calibration;

[0052] FIG. 5a indicates asymmetrical resistance values for a first variable resistor and a second variable resistor provided for during a second step of calibration, and further indicates corresponding high side voltages determined in different ways;

[0053] FIG. 5b indicates asymmetrical resistance values for a first variable resistor and a second variable resistor provided for during a second step of calibration, and further indicates corresponding low side voltages determined in different ways; FIG. 6 indicates for the asymmetric resistance values of FIG. 5a a plurality of values used in the second step of calibration;

[0054] FIG. 7 is a flow chart of a method for determining a double calibrated high side voltage and a double calibrated low side voltage; and

[0055] FIG. 8 is a fault monitoring device for a power system.

[0056] Detailed description

[0057] FIG. 8 shows a fault monitoring device for a power system. The fault monitoring device is configured to measure a high side insulation resistance RISOH and a low side insulation resistance RISOL of a power system 1.

[0058] The power system 1 is shown on the left-hand side of FIG. 8. It comprises a DC power source 7 such as a battery that has a positive terminal 71 and a negative terminal 72. Between the positive terminal 71 and the negative terminal 72 a DC voltage UB is present. A positive voltage rail 3 is connected to the positive terminal 71 and a negative voltage rail 4 is connected to the negative terminal 72. The positive voltage rail 3 and the negative voltage rail 4 form a high-voltage bus.

[0059] The power system 1 further comprises a chassis 5. The chassis 5 is insulated from the positive voltage rail 3 by the high side insulation resistance RISOH. The chassis 5 is further insulated from the negative voltage rail 4 by the low side insulation resistance RISOL.

[0060] The voltage between the positive voltage rail 3 and the chassis 5 is the high side voltage UH. The voltage between the chassis 5 and the negative voltage rail 4 is the low side voltage UL. The sum of the high side voltage and the low side voltage is equal to the DC power source / battery voltage: UB = UH + UL.

[0061] In parallel to the high side insulation resistance RISOH a capacitance CISOH is arranged between the chassis 5 and the positive voltage rail 3. Similarly, in parallel to the low side insulation resistance RISOL a capacitance CISOL is arranged between the chassis 5 and the negative voltage rail 4. The capacitances CISOH and CISOL represent capacitive loads of the system.

[0062] In a power system such as the power system 1 of FIG. 8, it is required that the insulation of the positive voltage rail 3 and of the negative voltage rail 4 from the chassis 5 is monitored. This can be done by monitoring the values of the resistances RISOH and RISOL. The fault monitoring device of FIG. 8 serves to provide for such monitoring. The fault monitoring device comprises a first parallel circuit of resistors, the first parallel circuit comprising a first branch 21 and a second branch 22 which are both connected at one end thereof to the positive voltage rail 3 of the power system 1. The first branch 21 comprises a first resistor R1 and a second resistor R2 which are arranged in series and form a first voltage divider. The resistance of the first resistor R1 is typically substantially larger than the resistance of the second resistor R2. For example, the first resistor R1 may have a resistance in the range between 10 MQ and 50 MQ, while the second resistor R2 may have a resistance in the range between 10 and 100 kQ.

[0063] A voltage UADCH is measured by a first voltage measurement device 81 between a point between the first and second resistors R1, R2 and the chassis 5. The voltage measurement device 81 may be implemented as an analog-to-digital converter (ADC) which provides the measured voltage as a digital signal which is convenient for further processing of the signal.

[0064] By measuring the voltage UADCH, the high side voltage UH can be calculated using the formula:

[0065] R1+R2 ...

[0066] UH ~ UADCHR?(1)

[0067] The second branch 22 of the first parallel circuit comprises three electrical resistors R7, R8, R9 arranged in series, wherein two of the electrical resistors R8, R9 can be short- circuited by a first switch S1 and a second switch S2.

[0068] The fault monitoring device further comprises a second parallel circuit of resistors, the second parallel circuit comprising a third branch 23 and a fourth branch 24 which are both connected at one end thereof to the negative voltage rail 4 of the power system 1. The third branch 21 comprises a third resistor R3 and a fourth resistor R4 which are arranged in series and form a second voltage divider. The resistance of the fourth resistor R4 is typically substantially larger than the resistance of the third resistor R3. For example, the fourth resistor R4 may have a resistance in the range between 10 MQ and 50 MQ, while the third resistor R3 may have a resistance in the range between 10 and 100 kQ.

[0069] A voltage UADCL is measured by a second voltage measurement device 82 between a point between the third and fourth resistors R3, R4 and the chassis 5. the second voltage measurement device 82 may also be implemented by an ADC. By measuring the voltage UADCL, the low voltage UL can be calculated using the formula:

[0070] The voltage dividers of branches 21, 23 thus serve to measure the high voltage UH and the low voltage UL. The fourth branch 24 of the second parallel circuit comprises three electrical resistors R10, R11 , R12 arranged in series, wherein two of the electrical resistors R10, R11 can be short-circuited by a second switch S3 and a fourth switch S4.

[0071] The first branch 21 , the second branch 22, the third branch 23 and the fourth branch 24 are each connected at the other end thereof to the chassis 5 of the power system 1.

[0072] When applying the fault monitoring device to the power system 1 , the first parallel circuit

[0073] 21 , 22 with branches 21 , 22 is connected in parallel to the high side insulation resistance RISOH and the second parallel circuit 23, 24 with branches 23, 24 is connected in parallel to the low side insulation resistance RISOL. By switching the switches S1 to S4 a common mode voltage UCM different from zero can be injected.

[0074] The fault monitoring device further comprises a controller 6 schematically depicted in FIG. 8. The controller 6 may be implemented in software and / or hardware. For example, the controller 6 may comprise software stored in a memory and executed by a processor. The controller 6 is operatively coupled to the switches S1 , S2, S3, S4 and configured to selectively switch the switches S1 , S2, S3, S4 of the second branch 22 and of the fourth branch 24, thereby providing for different states of the first and second parallel circuits 21 ,

[0075] 22, 23, 24. The controller 6 is further configured to determine from voltage changes associated with the different states the resistance values for the high side insulation resistance RISOH and for the low side insulation resistance RISOL of the power system 1. In this respect, the controller 6 may control and / or read values of other elements of the fault monitoring device 2 as well such as of voltage measurement devices 81 , 82.

[0076] Further examples to determine the high side insulation resistance RISOH and the low side insulation resistance RISOL are discussed in US 2022 / 0413034 A1 and US 2022 / 0413035 A1.

[0077] As a common thread, it is essential to measure exact values of the high side voltage UH and of the low side voltage HL to determine if there is an insulation problem. According to formulas (2) and (3), to this end, it is necessary to calibrate the voltage measurement devices 81 , 82. One problem associated therewith lies in the fact that voltage measurement devices have a large internal resistance that influences the measurement. This needs be taken into account to determine exact, calibrated values for the high side voltage UH and of the low side voltage UL.

[0078] In the following, methods and systems are described to determine calibrated values of the high side voltage UH and of the low side voltage UL.

[0079] FIG. 1 shows a calibration device in accordance with the present invention. The calibration device comprises a fault monitoring device similar to the fault monitoring device discussed with respect to FIG. 8. Regarding the features of a DC power source 7, a positive voltage rail 3, a negative voltage rail 4, a chassis 5, a high side voltage UH, a low side voltage Ui_, a first voltage divider 21 and a second voltage divider 23, wherein a voltage measurement device 81 measures the voltage over resistor R2 of first voltage divider 21 and a voltage measurement device 82 measures the resistance over resistor R3 of the second voltage divider 23, reference is made to the description of FIG. 8.

[0080] There is further provided a first calibrated voltage sensor LABH connected between the positive voltage rail 3 and the chassis 5 and a second calibrated voltage sensor LABL connected between the chassis 5 and the negative voltage rail 4. The sensors LABH, LABL represent calibrated laboratory voltage sensors which are able to measure the high side voltage and the low side voltage independent of the voltage measurement devices 81, 82 with high precision. The voltage they measure is indicated as ULABH and ULABL in the following. First and second calibrated voltage sensors LABH and LABL are part of the calibration device but not part of the fault monitoring device. They are needed for calibration only.

[0081] The fault monitoring device of FIG. 1 further comprises a first variable resistor R5 connectable between the positive voltage rail 3 and the chassis 5 and a second variable resistor R6 connectable between the chassis 5 and the negative voltage rail 4. The variable resistors R5, R6 are part of the calibration device but not part of the fault monitoring device. As will be discussed further below, the variable resistors R5, R6 are connected to the positive voltage rail 3, the chassis 5 and the negative voltage rail 4, respectively, in a second step of calibration only.

[0082] In some embodiments, the first and second calibrated voltage sensors LABH and LABL and the first and second variable resistors R5, R6 are added to the fault monitoring device at a factory or calibration site. After the fault monitoring device is calibrated, they are not needed any more and may be removed. Accordingly, the calibration is typically done after manufacturing of the fault monitoring device. The fault monitoring device is connected to the other calibration equipment and the calibration process is done. The result of the calibration is a set of constant parameters which, subsequently to calibration, can be used to correct actual voltage samples from the voltage measurement devices.

[0083] The first voltage measurement device 81 may be implemented as an ADC and is in the following referred to as ADC 81. The voltage it measures is indicated as voltage UADCH. The second voltage measurement device 82 may also be implemented as an ADC and is in the following referred to as ADC 82. The voltage it measures is indicated as voltage UADCL. There is further provided a third voltage measurement device 83 which may also be implemented as an ADC and is in the following referred to as ADC 83. ADC 83 measures the voltage between a point between the first and second resistors R1, R2 and a point between the third and fourth resistors R3, R4. The voltage it measures is indicated as voltage UADCB. From UADCB, the voltage UB of the DC power source 7 (which is the sum of UH and UL) can be measured as follows:

[0084] ADC 83 provides for redundancy and a validity check in the measurements.

[0085] The calibration device further comprises a controller 2 which is depicted schematically. The controller 2 may be implemented in software and / or hardware. For example, the controller 2 may comprise software stored in a memory and executed by a processor. The controller 2 is operatively coupled with the other elements of the circuit which may receive a control signal from the controller to and / or provide an input signal to the controller 2. FIG. 1 depicts schematically control lines 21 for controlling elements of the calibration device and input signal lines 22 for receiving input signals from elements of the calibration device and / or from other sensors and / or from other controllers.

[0086] For example, controller 2 is configured to set the resistance of the first and second variable resistors R5, R6. It may also connect the resistors R5, R6 automatically to the positive voltage rail 3, chassis 5 and negative voltage rail 4 by closing respective switches (not shown). Controller 2 is further configured to read the voltage values of voltage sensors LABH, LABL and of ADCs 81-83.

[0087] Controller 2 further comprises a memory that allows to store voltage values as well as constant parameters determined during the calibration process and / or is configured to provide such constant parameters to another entity such as another controller. It is further pointed out that controller 2 of FIG. 1 may be identical to controller 6 of FIG. 8, while this is not necessarily the case.

[0088] The controller 2 also contains software to implement a method for calibrating the high side voltage UH and the low side voltage UL. However, the method may alternatively be carried out by another entity or several other entities.

[0089] The method comprises two steps, a first calibration step which yields as result first and second constant parameters that allow to provide a first calibrated high side voltage Uncaiibi and a first calibrated low side voltage Uicaiibi , and a second calibration step which yields as result third and fourth constant parameters which allow to correct the first calibrated high side voltage Uncaiibi and a first calibrated low side voltage Uicaiibi to provide a double calibrated high side voltage UHcaiib2 and a double calibrated low side voltage Ubcalib2.

[0090] In the first step, the variable resistors R5, R6 are disconnected from the positive voltage rail 3, the chassis 5 and the negative voltage rail 4 and thus do not play any role. In the first step, a defined voltage is applied between the positive voltage rail 3 and negative voltage rail 4. This voltage is changed in predetermined steps over a voltage range, such as between a minimum such as 0 V and a maximum such as 1000 V. As the only resistances which are present are the resistances of the first voltage divider 21 and the second voltage divider 23, there is a fixed ratio between the high side voltage UH and the low side voltage UL. If the resistances of the voltage dividers 21 , 23 are equal, the high side voltage UH and the low side voltage UL are equal. This situation is considered in the following is an example implementation.

[0091] The first step, the first calibrated high side voltage Uncaiibi and the first calibrated low side voltage Uicaiibi are determined by a linear regression. To this end, for a plurality of voltages in the voltage range (e.g., 0 V to 1000 V), the first high side voltage UH is determined from the first resistor R1 , the second resistor R2, and the voltage UADCH measured by ADC 81 by means of formula (1). At the same time, the high side voltage is determined independently by measuring the voltage ULABH between the positive voltage rail 3 and the chassis 5 by means of the first calibrated voltage sensor LABH.

[0092] Accordingly, the high side voltage is measured independently through ADC 81 and voltage sensor LABH. The respective value pairs (UH; ULABH) 94 are inserted into a diagram as shown in FIG. 2. Subsequently, a line is drawn through these values by linear regression. The result is a line 91 which is defined as follows:

[0093] UHcallbl= UHm■ UH+ UHb(4) wherein Uncaiibi is the first calibrated high side voltage, Unm is the slope of line 91 , UH is the high side voltage as measured through formula (1) (and thus by means of ADC 81), and UHb is the y-intercept of the line 91 . Accordingly, Unm and UHb are the parameters that define line 91. These parameters Unm and UHb represent first constant parameters determined by the calibration.

[0094] In a similar manner, in the first step, for a plurality of voltages in the voltage range (e.g., 0 V to 1000 V), the first low side voltage UL is determined from the third resistor R3, the fourth resistor R4, and the voltage UADCL measured by ADC 82 by means of formula (2). At the same time, the low side voltage is determined independently by measuring the voltage ULABL between the chassis 5 and the negative voltage rail 4 through the second calibrated voltage sensor LABL. Accordingly, the low side voltage is measured independently through ADC 82 and voltage sensor LABL. The respective value pairs (UL; ULABL) 95 are inserted into a diagram as shown in FIG. 3. Subsequently, a line is drawn through these values by linear regression. The result is a line 92 which is defined as follows:

[0095] ULcallbl=ULm ■UL + ULb(5) wherein Ui_caiibi is the first calibrated low side voltage, Ui_m is the slope of line 92, UL is the low side voltage as measured through formula (2) (and thus by means of ADC 82), and Ui_b is the y-intercept of the line 92. Accordingly, Ui_m and Ui_b are the parameters that define line 92. These parameters Ui_m and Ui_b represent second constant parameters determined by the calibration.

[0096] Optionally, in addition, in a similar manner, a first calibrated DC power source voltage Uscaiibi is measured in the first step. For a plurality of voltages in the voltage range (e.g., 0

[0097] V to 1000 V), the DC power source voltage UB is determined from resistors R1 to R4 and the voltage UADCB measured by ADC 83 by means of formula (3). At the same time, the DC power source voltage is determined independently by adding the voltages measured by calibrated voltage sensors LABH and LABL. The respective value pairs (UB; ULABH+LABL) 96 are inserted into a diagram as shown in FIG. 4. Subsequently, a line is drawn through these values by linear regression. The result is a line 93 which is defined as follows:

[0098] UBcalib. =UBm■ UB+ UBb(6) wherein Uscaiibi is the first calibrated DC power source voltage, Usm is the slope of line 93, UB is the DC power source voltage as measured through formula (3) (and thus by means of ADC 83), and UBb is the y-intercept of the line 93. Accordingly, Usm and UBb are the parameters that define line 93. These parameters Usm and UBb represent fifth constant parameters determined by the calibration.

[0099] This first calibration step already provides for a helpful calibration of the voltages Uncaiibi , Ui_caiibi and Uscaiibi , addressing errors / tolerances in R1 , R2, R3, R4, and general errors of the ADCs 81 , 82, 83. However, two problems remain. First, the internal resistance of the ADCs 81 , 82, 83 in the arrangement of FIG. 1 which negatively influences the accuracy of the voltage measurements is not addressed. In particular, the internal resistance of ADC 83 is a main course for asymmetry errors. Second, the calibration for the high side voltage and for the low side voltage is over a lower range than the calibration for the DC power source voltage, the reason being that the voltage dividers 21 , 23 have a fixed resistance which naturally limits the voltage range. For example, if the voltage range is 1000 V and if the resistance of the first and second voltage dividers 21 , 23 is the same, the high side voltage and the low side voltage can both be calibrated over a range of 500

[0100] V only, wherein the DC battery voltage can be calibrated over a range of 1000 V (see FIGS. 2 to 4). However, it is preferable to provide for a calibration over a high range.

[0101] These problems are addressed in the second calibration step. The second calibration step comprises a plurality of substeps. Initially, the first and second variable resistors R5, R6 are connected between the positive voltage rail 3 and the chassis 5 and the chassis 5 and the negative voltage rail 4, respectively. This may be done manually or by control signals provided by controller 2.

[0102] Subsequently, a voltage range is defined. The voltage range may be the same as in the first calibration step. For example, the voltage range is 1000 V (wherein, e.g., the DC power supply positive terminal is +500 V and the DC power supply negative terminal is - 500 V). The variable resistors R5, R6 are then adjusted in steps in a way to achieve values between 0 V - 1000 V between the positive voltage rail 3 and the chassis 5 and respectively values between 1000 V - 0 V between the chassis 5 and the negative voltage rail 4. FIGS. 5a, 5b are illustrative examples showing, among others, values for R5 and R6.

[0103] With R5 and R6 changing, there is also a change in the high side voltage and the low side voltage. For example, with R5 being parallel to R1 and R2, the total resistance of the two parallel branches (one branch with R5, the other branch with R1 and R2) is changed. Therefore, the voltages UADCH and consequently UH change, as is shown in the third column of FIG. 5a. The same is true for the voltages UADCL and UL, as shown in the third column of FIG. 5b. For each pair of values of R5 and R6 (corresponding to a row of the table of FIGS. 5a, 5b), UH is determined through formula (1), and from UH the first calibrated high side voltage Uncaiibi is determined in accordance with formula (4), see fourth column of FIG. 5a. Similarly, for each pair of values of R5 and R6 UL is determined through formula (2), and from UL the first calibrated low side voltage ULcaiibi is determined in accordance with formula (5), see fourth column of FIG. 5b.

[0104] At the same time and independently, ULABH and ULABL are measured for each pair of values of R5 and R6 through a direct measurement by voltage sensors LABH and LABL.

[0105] Accordingly, by changing the resistance of the first and second variable resistors R5, R6, the voltage over the first variable resistor R5 is changed in steps between a minimum (0 V) and a maximum (1000 V) and the voltage over the second variable resistor R6 is changed in steps between a maximum (1000 V) and a minimum (0 V). At the same time, in each step the values for the first calibrated high side voltage Uncaiibi, for the first calibrated low side voltage ULcaiibi, for the (third) high side voltage ULABH measured by the first calibrated voltage sensor LABH, and for the (third) low side voltage ULABL measured by the second calibrated voltage sensor LABL are recorded.

[0106] Subsequently, for each step a high side gain error GAI NERRH for the first calibrated high side voltage Uncaiibi is calculated based on the high side voltage ULABH, wherein the high side gain error GAI NERRH is calculated as the quotient of the measured high side voltage ULABH and the first calibrated high side voltage Uncaiibi. The gain error GAI NERRH for the first calibrated high side voltage Uncaiibi is shown in column 4 of FIG. 6, wherein the first three columns of FIG. 6 are identical to the third to fifth columns of FIG. 5a.

[0107] Further, for each step a low side gain error GAI NERRL for the first calibrated low side voltage Ui_caiibi is calculated based on the low side voltage ULABL, wherein the low side gain error GAI NERRL is calculated as the quotient of the measured low side voltage ULABL and the first calibrated low side voltage ULcaiibi .

[0108] Next, two linear regressions are performed with the following inputs and outputs.

[0109] A first linear regression (the third linear regression when also counting the linear regressions of the first stage) is carried out on the basis of the quotient of the first calibrated high side voltage Uncaiibi and the first calibrated low side voltage ULcaiibi on the x-axis and this quotient times the high side gain error GAI NERRH on the y-axis:

[0110] X-axis:UHcaliblin each step u licaLibl

[0111] Y-axis: — ■ GAINERRHin each step

[0112] ^Lcalibl

[0113] The values for the x-axis are shown in the fifth column of FIG. 6 and the values for the y- axis are shown in the sixth column of FIG. 6.

[0114] The third linear regression curve approximated through the respective value pairs provides a third line defined by parameters slope GAINERRHm and y-intercept GAI NERRH, wherein the linear regression curve is generally defined as (y = m ■ x + b), and wherein m = GAINERRHm and b = GAI NERRH and x = UHcaiibi I ULcaiibi. These parameters GAINERRHm and GAINERRHb represent third constant parameters determined by the calibration.

[0115] In a similar manner, a second linear regression (the fourth linear regression when also counting the linear regressions of the first stage) is carried out on the basis of the quotient of the first calibrated low side voltage ULcaiibi and the first calibrated high side voltage Uncaiibi on the x-axis and this quotient times the low side gain error GAI NERRL on the y- axis:

[0116] X-axis:ULcallbin each step

[0117] Uncaiibi

[0118] Y-axis: —C-S^L■ GAINERRLin each step

[0119] ^^callbl

[0120] The fourth linear regression curve approximated through the respective value pairs provides a fourth line defined by parameters slope GAINERRLm and y-intercept GAI NERRLL, wherein the linear regression curve is generally defined as (y = m ■ x + b), and wherein m = GAINERRLm and b = GAI NERRU, and x = ULcaiibi I Uncaiibi. These parameters GAINERRLm and GAINERRLb represent fourth constant parameters determined by the calibration. The calibration thus yields first, second, third and fourth constant parameters Unm, Unb, Ui_m, U i_b, GAINERRHm, GAINERRHb, GAINERRLm, GAINERRLb. These parameters are stored in controller 2 and / or are provided to another processing entity or a memory.

[0121] After calibration, these parameters can be used to calculate double corrected values of the voltages measured by ADCs 81-83.

[0122] After calibration, the first and second calibrated voltage sensors LABH, LABL and the first and second variable resistors R5, R6 can be removed from the circuit of FIG. 1 , thereby reducing the circuit of FIG. 1 to a fault management device similar to the fault management device of FIG. 8 (with additional ADC 83 though), wherein the determined parameters remain stored in controller 2.

[0123] The controller 2 is then further configured to determine a first calibrated high side voltage Uncaiibi from a voltage UADCH measured by the first voltage measurement device 81 and the first constant parameters Unm, Unb in accordance with formula (4) and formula (1).

[0124] The controller 2 is further configured to determine a first calibrated low side voltage Ui_caiibi from a voltage UADCL measured by the second voltage measurement device 82 and the second constant parameters Ui_m, Ui_b in accordance with formula (5) and formula (2).

[0125] The controller 2 is further configured to determine a first calibrated battery voltage U scaiibi from a voltage UADCB measured by the third voltage measurement device 83 and the fifth constant parameters Usm, UEb in accordance with formula (6) and formula (3).

[0126] The controller 2 is further configured to correct the first calibrated high side voltage U ncaiibi using the third constant parameters GAINERRHm, GAINERRHb to determine a double calibrated high side voltage Uncaiib2, and to correct the first calibrated low side voltage Ui_caiibi using the fourth constant parameters GAINERRLm, GAINERRLb to determine a double calibrated low side voltage Ui_caiib2- To this end, the formulas are applied: wherein:

[0127] Uncaiibi is the first calibrated high side voltage;

[0128] Uncaiib2 is the double calibrated high side voltage;

[0129] Ui_caiibi is the first calibrated low side voltage; GAINERRHm is the slope of the third linear regression curve;

[0130] GAINERRHb is the y-intercept of the third linear regression curve;

[0131] Ui_caiibi is the first calibrated low side voltage;

[0132] Ui_caiib2 is the double calibrated low side voltage;

[0133] Uncaiibi is the first calibrated high side voltage;

[0134] GAINERRL-m is the slope of the fourth linear regression curve; and GAINERRL-b is the y-intercept of the fourth linear regression curve.

[0135] In the above formulas, the term g1 : represents a first gain error that allows to correct the first calibrated high side voltage UHcaiibi , wherein g1 is multiplied with the first calibrated high side voltage Uncaiibi . The gain error g1 can be determined from the third linear regression curve discussed above. It represents an improved gain error.

[0136] The values for the first gain error g1 are shown in the seventh column of FIG. 6 and the values for the double calibrated high side voltage lineal are shown in the eighth column of FIG. 6 by example.

[0137] Similarly, the term g2: represents a second gain error that allows to correct the first calibrated low side voltage Ui_caiibi , wherein g2 is multiplied with the first calibrated low side voltage Ui_caiibi - The gain error g2 can be determined from the fourth linear regression curve discussed above. It represents an improved gain error.

[0138] Last not least, it is pointed out that a double calibrated DC power source voltage UBcaiib2 is equal to the first calibrated DC power source voltage UBcaiibi , wherein UBcaiibi is determined in accordance with equation (6): (11)

[0139] The reason for this lies in that ADC 83 is not affected by the asymmetry error. It is rather a main cause of the asymmetry error.

[0140] FIG. 7 summarizes main steps of the calibration method. In a first step 71 , first and second constant parameters are determined by means of linear regression that allow to provide for a first calibrated high side voltage and a first calibrated low side voltage, such as discussed with respect to FIGS. 2 to 4. In a second step 72 (more particularly, a first substep in the second step), asymmetrical voltages for the high side voltage and the low side voltage are provided, and values for the first calibrated high side voltage, the first calibrated low side voltage, a third high side voltage measured by a first calibrated voltage sensor, and a third low side voltage measured by a second calibrated voltage sensor are determined in steps over a voltage range, as discussed with respect to FIGS. 5a, 5b. More particularly, the first calibrated high side voltage Uncaiibi, the first calibrated low side voltage Ui_caiibi, the third high side voltage Ui_ABH and the third low side voltage ULABL are determined.

[0141] Subsequently, in step 73, third constant parameters are determined from the determined voltage values of Uncaiibi , Ui_caiibi , UuxBH and ULABL using linear regression, wherein the third constant parameters allow to correct a first calibrated high side voltage determined by the first constant parameters to arrive at a double calibrated high side voltage. In particular, the parameters GAINERRHm and GAI NERRHL are determined.

[0142] In step 74, fourth constant parameters are determined from the determined voltage values of Uncaiibi , ULcaiibi , ULABH and ULABL using linear regression, wherein the fourth constant parameters allow to correct a first calibrated low side voltage determined by the second constant parameters to arrive at a double calibrated low side voltage. In particular, the parameters GAINERRLm and GAINERRLb are determined.

[0143] It should be understood that the above description is intended for illustrative purposes only, and is not intended to limit the scope of the present disclosure in any way. Also, those skilled in the art will appreciate that other aspects of the disclosure can be obtained from a study of the drawings, the disclosure and the appended claims. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. Various features of the various embodiments disclosed herein can be combined in different combinations to create new embodiments within the scope of the present disclosure. In particular, the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein. Any ranges given herein include any and all specific values within the range and any and all sub-ranges within the given range.

Claims

CLAIMS1. A calibration device for calibrating voltages of a fault monitoring device for a power system (1), the power system (1) comprising a positive voltage rail (3), a negative voltage rail (4) and a chassis (5), wherein a high side voltage is present between the positive voltage rail (3) and the chassis (5) and a low side voltage is present between the chassis (5) and the negative voltage rail (4); wherein the fault monitoring device comprises: a first voltage divider (21) arranged between the positive voltage rail (3) and the chassis (5), the first voltage divider (21) comprising a first resistor (R1) and a second resistor (R2), wherein a first voltage measurement device (81) is arranged in parallel to the second resistor (R2) and configured to measure the voltage (UADCH) of the second resistor (R2); and a second voltage divider (23) arranged between the chassis (5) and the negative voltage rail, the second voltage divider (23) comprising a third resistor (R3) and a fourth resistor (R4), wherein a second voltage measurement device (82) is arranged in parallel to the third resistor (R3) and configured to measure the voltage (UADCH) of the third resistor (R3); wherein the fault monitoring device is part of the calibration device, and wherein the calibration device further comprises: a first calibrated voltage sensor (LABH) connected between the positive voltage rail (3) and the chassis (5) and a second calibrated voltage sensor (LABL) connected between the chassis (5) and the negative voltage rail (4); a first variable resistor (R5) connectable between the positive voltage rail (3) and the chassis (5) and a second variable resistor (R6) connectable between the chassis (5) and the negative voltage rail (4); and a controller (2) configured to: o in a first step, when the first variable resistor (R5) and the second variable resistor (R6) are disconnected:■ determine a first linear regression curve (91) based on a first high side voltage (UH) determined by means of the first voltage measurement device (81) and a second high side voltage (ULABH) measured by the first calibrated voltage sensor (LABH), and determine first constant parameters (Unm, UH from the first linear regression curve (91) thatallow to provide a first calibrated high side voltage (Uncaiibi) dependent on the first high side voltage (UH); and■ determine a second linear regression curve (92) based on a first low side voltage (UL) determined by means of the second voltage measurement device (82) and a second low side voltage (ULABL) measured by the second calibrated voltage sensor (LABL), and determine second constant parameters (Ui_m, Ui_b) from the second linear regression curve (92) that allow to provide a first calibrated low side voltage (Ui_caiibi) dependent on the first low side voltage (UL); o in a second step, when the first variable resistor (R5) is connected between the positive voltage rail (3) and the chassis (5) in parallel to the first voltage divider (21) and the second variable resistor (R6) is connected between the chassis and the negative voltage rail in parallel to the second voltage divider (23):■ provide in steps asymmetrical voltage values for the high side voltage (UH) and the low side voltage (UL) by adjusting the resistance of the first and second variable resistors (R5, R6), and determine in each step the values for the first calibrated high side voltage (Uncaiibi), the first calibrated low side voltage (ULcaiibi), a third high side voltage (ULABH) measured by the first calibrated voltage sensor (LABH), and a third low side voltage (ULABL) measured by the second calibrated voltage sensor (LABL);■ determine third constant parameters (GAI NERRHm, GAI NERRH from the determined voltage values (Uncaiibi , ULcaiibi , ULABH, ULABL) using linear regression, wherein the third linear parameters allow to correct a first calibrated high side voltage (Uncaiibi) determined by the first constant parameters (Unm, Unb) to arrive at a double calibrated high side voltage (Uncaiib2); and■ determine fourth constant parameters (GAI NERRLm, GAI NERRL from the determined voltage values (Uncaiibi , ULcaiibi , ULABH, ULABL) using linear regression, wherein the fourth linear parameters allow to correct a first calibrated low side voltage (ULcaiibi) determined by the second constant parameters (ULm, UL to arrive at a double calibrated low side voltage (ULcalib2).

2. The calibration device of claim 1 , wherein the controller (2) is configured such that first step comprises:for a plurality of voltages in a voltage range, determine the first high side voltage (UH) from the first resistor (R1), the second resistor (R2), and the voltage (UADCH) measured by the first voltage measurement device (81), and further determine the second high side voltage (ULABH) by measuring the voltage between the positive voltage rail (3) and the chassis (5) by the first calibrated voltage sensor (LABH); for a plurality of voltages in a voltage range, determine the first low side voltage (UL) from the third resistor (R3), the fourth resistor (R4), and the voltage (UADCL) measured by the second voltage measurement device (82), and further determine the second low side voltage (ULABL) by measuring the voltage between the chassis (5) and the negative voltage rail (4) by the second calibrated voltage sensor (LABL); determine the first linear regression curve on the basis of the first high side voltages (UH) and the second high side voltages (ULABH), the first linear regression curve providing a first line defined by the parameters slope and y-intercept (Unm, UH as first constant parameters (Unm, UHL), the first line providing the first calibrated high side voltage (Uncaiibi) dependent on the first high side voltage (UH); and determine the second linear regression curve on the basis of the first low side voltages (UL) and the second low side voltages (ULABL), the second linear regression curve providing a second line defined by the parameters slope and y-intercept (ULm, UL as second constant parameters (Unm, UHL), the second line providing the first calibrated low side voltage (ULcaiibi) dependent on the first low side voltage (UL).

3. The calibration device of claim 2, wherein the first high side voltage (UH) is determined from the first resistor (R1), the second resistor (R2), and the voltage measured by the first voltage measurement device (81) by means of the formulaR1 + R2UH=UADCH wherein UH is the first high side voltage, R1 is the first resistor, R2 is the second resistor and UADCH is the voltage measured by the first voltage measurement device, and wherein the first low side voltage (UL) is determined from the third resistor (R1), the fourth resistor (R2), and the voltage measured by the second voltage measurement device (82) by means of the formulawherein UL is the first low side voltage, R3 is the third resistor, R4 is the fourth resistor and UADCL is the voltage measured by the second voltage measurement device.

4. The calibration device of any preceding claim, wherein the controller (2) is configured to provide in steps asymmetrical voltage values in that it is configured to: adjust the resistance of the first and second variable resistors (R5, R6) such that the voltage over the first variable resistor (R5) is changed in steps between a minimum and a maximum and such that the voltage over the second variable resistor (R6) is changed in steps between a maximum and a minimum, and record in each step the values for the first calibrated high side voltage (Uncaiibi), the first calibrated low side voltage (Ui_caiibi), the measured third high side voltage (ULABH) measured by the first calibrated voltage sensor (LABH), and the measured third low side voltage (ULABL) measured by the second calibrated voltage sensor (LABL).

5. The calibration device of any preceding claim, wherein the controller (2) is configured to determine the third and fourth constant parameters (GAI NERRHm, GAI NERRHL, GAI NERRLm, GAI NERRL in that it is configured to: calculate in each step a high side gain error (GAI NERRH) for the first calibrated high side voltage (Uncaiibi) based on the third high side voltage (ULABH); calculate in each step a low side gain error (GAI NERRL) for the first calibrated low side voltage (ULcaiibi) based on the third low side voltage (ULABL); determine a third linear regression curve on the basis of the quotient of the first calibrated high side voltage (Uncaiibi) and the first calibrated low side voltage (ULcaiibi) on the x-axis and this quotient times the high side gain error (GAI NERRH) on the y-axis, the third linear regression curve providing a third line defined by parameters slope and y-intercept (GAI NERRHm, GAI NERRHL) which represent the third constant parameters; and determine a fourth linear regression curve on the basis of the quotient of the first calibrated low side voltage (ULcaiibi) and the first calibrated high side voltage (Uncaiibi) on the x-axis and this quotient times the low side gain error (GAI NERRL) on the y-axis, the fourth first linear regression curve providing a fourth line defined by parameters slope and y-intercept (GAINERRL-m, GAI NERRL-L) which represent the fourth constant parameters; wherein the third constant parameters (GAI NERRHm, GAI NERRHL) of the third line allow to correct a first calibrated high side voltage (Uncaiibi) to arrive at a double calibrated high side voltage (Uncaiib2), and wherein the fourth constant parameters (GAI NERRL-m, GAI NERRL-L) of the fourth line allow to correct a first calibrated low side voltage (ULcaiibi) to arrive at a double calibrated low side voltage (ULcaiib2).

6. The calibration device of claim 5, wherein the controller (2) is configured to: calculate in each step the high side gain error (GAI NERRH) for the first calibrated high side voltage (Uncaiibi) in that the high side gain error (GAI NERRH) is calculated as the quotient of the measured high side voltage (ULABH) and the first calibrated high side voltage (UHcaiibi); and calculate in each step the low side gain error (GAI NERRL) for the first calibrated low side voltage (Ui_caiibi) in that the low side gain error (GAI NERRL) is calculated as the quotient of the measured low side voltage (ULABL) and the first calibrated low side voltage (ULcaiibi).

7. The calibration device of any preceding claim, wherein the controller (2) is configured to adjust the resistances of the first and second variable resistors (R5, R6) in steps such that the respective high side voltage (UH) is varied in steps in a voltage range defined by the voltage difference between a positive terminal and a negative terminal of a DC power source to which the positive voltage rail (3) and the negative voltage rail (4) are attached, respectively, and that the respective low side voltage (UL) is varied in steps over the same voltage range, wherein the sum of the high side voltage (UH) and the low side voltage (UL) is constant.

8. The calibration device of any preceding claim, wherein the fault monitoring device further comprises a third voltage measurement device (83), wherein the third voltage measurement device (83) is arranged to measure the voltage over the second resistor (R2) of the first voltage divider (21) and the third resistor (R3) of the second voltage divider (23), wherein the controller (2) is configured to, in the first stage, determine a fifth linear regression curve (93) based on a first battery voltage (UB) determined by means of the third voltage measurement device (83) and a second battery voltage (ULABH+ULABL) measured by the first and second calibrated voltage sensors (LABH, LABL), and determine fifth constant parameters (Usm, UBL) from the fifth linear regression curve (93) that allow to provide a first calibrated battery voltage (Uscaiibi) dependent on the first battery voltage (UB).

9. The calibration device of any preceding claim, wherein the first voltage measurement device (81) is an Analog-to-Digital Converter and that the second voltage measurement device (82) is an Analog-to-Digital Converter.

10. A fault monitoring device for a power system (1) that comprises a positive voltage rail (3), a negative voltage rail (4) and a chassis (5), wherein a high side voltage is present between the positive voltage rail (3) and the chassis (5) a low side voltage is present between the chassis (5) and the negative voltage rail (4), wherein the fault monitoring device (2) comprises:a first voltage divider (21) arranged between the positive voltage rail (3) and the chassis (5), the first voltage divider (21) comprising a first resistor (R1) and a second resistor (R2), wherein a first voltage measurement device (81) is arranged in parallel to the second resistor (R2) and configured to measure the voltage (UADCH) of the second resistor (R2); a second voltage divider (23) arranged between the chassis (5) and the negative voltage rail, the second voltage divider (23) comprising a third resistor (R3) and a fourth resistor (R4), wherein a second voltage measurement device (82) is arranged in parallel to the third resistor (R3) and configured to measure the voltage (UADCL) of the third resistor (R3); and a controller (2) having stored the first, second, third and fourth constant parameters (Unm, Unb, U|_m, U|_b, GAI NERRHm, GAI NERRHb, GAI NERRLm, GAI NERRLb) determined by the calibration device of any of claims 1 to 6, wherein the controller (6) is configured to: o determine a first calibrated high side voltage (Uncaiibi) from a voltage (UADCH) measured by the first voltage measurement device (81) and the first constant parameters (Unm, UHb); o determine a first calibrated low side voltage (Ui_caiibi) from a voltage (UADCL) measured by the second voltage measurement device (82) and the second constant parameters (Uim, UL ; o correct the first calibrated high side voltage (Uncaiibi) using the third constant parameters (GAI NERRHm, GAI NERRHb) to determine a double calibrated high side voltage (UHcaiib2); and o correct the first calibrated low side voltage (Uicaiibi) using the fourth constant parameters (GAI NERRLm, GAI NERRLL) to determine a double calibrated low side voltage (ULcaiib2).

11. The fault monitoring device of claim 10, as far as the third and fourth constant parameters (GAI NERRHm, GAI NERRHb, GAI NERRLm, GAI NERRLL) were determined by the calibration device of claim 5, characterized in that the controller (2) is configured to correct the first calibrated high side voltage (Uncaiibi) and the first calibrated low side voltage (ULcaiibi) by applying the formulaswherein:Uncaiibi is the first calibrated high side voltage;UHcaiib2 is the double calibrated high side voltage;Ui_caiibi is the first calibrated low side voltage;GAINERRHm is the slope of the third linear regression curve;GAINERRHb is the y-intercept of the third linear regression curve;Ui_caiibi is the first calibrated low side voltage;Ui_caiib2 is the double calibrated low side voltage;Uncaiibi is the first calibrated high side voltage;GAINERRL-m is the slope of the fourth linear regression curve; and GAINERRL-b is the y-intercept of the fourth linear regression curve.

12. A method of calibrating voltages of a fault monitoring device for a power system (1) that comprises a positive voltage rail (3), a negative voltage rail (4) and a chassis (5), wherein a high side voltage is present between the positive voltage rail (3) and the chassis (5) a low side voltage is present between the chassis (5) and the negative voltage rail (4);- wherein the fault monitoring device comprises: o a first voltage divider (21) arranged between the positive voltage rail (3) and the chassis (5), the first voltage divider (21) comprising a first resistor (R1) and a second resistor (R2), wherein a first voltage measurement device (81) is arranged in parallel to the second resistor (R2) and configured to measure the voltage of the second resistor (R2); and o a second voltage divider (23) arranged between the chassis (5) and the negative voltage rail, the second voltage divider (23) comprising a third resistor (R3) and a fourth resistor (R4), wherein a second voltage measurement device (82) is arranged in parallel to the third resistor (R3) and configured to measure the voltage of the third resistor (R3);- wherein the method further uses o a first calibrated voltage sensor (LABH) connected between the positive voltage rail (3) and the chassis (5) and a second calibrated voltage sensor (LABL) connected between the chassis (5) and the negative voltage rail (4),o a first variable resistor (R5) connectable between the positive voltage rail (3) and the chassis (5) and a second variable resistor (R6) connectable between the chassis (5) and the negative voltage rail (4);- wherein the method comprises: o in a first step, when the first variable resistor (R5) and the second variable resistor (R6) are disconnected;■ determine a first linear regression curve (91) based on a first high side voltage (UH) determined by means of the first voltage measurement device (81) and a second high side voltage (ULABH) measured by the first calibrated voltage sensor (LABH), and determine first constant parameters (Unm, Unb) from the first linear regression curve (91) that allow to provide a first calibrated high side voltage (Uncaiibi) dependent on the first high side voltage (UH); and■ determine a second linear regression curve (92) based on a first low side voltage (UL) determined by means of the second voltage measurement device (82) and a second low side voltage (ULABL) measured by the second calibrated voltage sensor (LABL), and determine second constant parameters (Ui_m, Ui_b) from the second linear regression curve (92) that allow to provide a first calibrated low side voltage (Ui_caiibi) dependent on the first low side voltage (UL); o in a second step, when the first variable resistor (R5) is connected between the positive voltage rail (3) and the chassis (5) in parallel to the first voltage divider (21) and the second variable resistor (R6) is connected between the chassis and the negative voltage rail in parallel to the second voltage divider (23);■ provide in steps asymmetrical voltage values for the high side voltage (UH) and the low side voltage (UL) by adjusting the resistance of the first and second variable resistors (R5, R6), and determine in each step the values for the first calibrated high side voltage (Uncaiibi), the first calibrated low side voltage (ULcaiibi), a third high side voltage (ULABH) measured by the first calibrated voltage sensor (LABH), and a third low side voltage (ULABL) measured by the second calibrated voltage sensor (LABL);■ determine third constant parameters (GAI NERRHm, GAI NERRH from the determined voltage values (Uncaiibi , ULcaiibi , ULABH, ULABL) using linear regression, wherein the third linear parameters allow to correct a firstcalibrated high side voltage (Uncaiibi) determined by the first constant parameters (Unm, Unb) to arrive at a double calibrated high side voltage (Uncaiib2); and■ determine fourth constant parameters (GAI NERRLm, GAI NERRL from the determined voltage values (Uncaiibi , Ui_caiibi , ULABH, ULABL) using linear regression, wherein the fourth linear parameters allow to correct a first calibrated low side voltage (Ui_caiibi) determined by the second constant parameters (Ui_m, Ui_b) to arrive at a double calibrated low side voltage (U caiib2); and o store the first, second, third and fourth constant parameters (Unm, Unb, Ui_m, Ui_b, GAI NERRHm, GAI NERRHb, GAI NERRLm, GAI NERRLL) determined in the first and second steps.

13. The method of claim 12, wherein the first step comprises: for a plurality of voltages in a voltage range, determine the first high side voltage (UH) from the first resistor (R1), the second resistor (R2), and the voltage (UADCH) measured by the first voltage measurement device (81), and further determine the second high side voltage (ULABH) by measuring the voltage between the positive voltage rail (3) and the chassis (5) by the first calibrated voltage sensor (LABH); for a plurality of voltages in a voltage range, determine the first low side voltage (UL) from the third resistor (R3), the fourth resistor (R4), and the voltage (UADCL) measured by the second voltage measurement device (82), and further determine the second low side voltage (ULABL) by measuring the voltage between the chassis (5) and the negative voltage rail (4) by the second calibrated voltage sensor (LABL); determine the first linear regression curve on the basis of the first high side voltages (UH) and the second high side voltages (ULABH), the first linear regression curve providing a first line defined by the parameters slope and y-intercept (Unm, Unb) as first constant parameters (Unm, Unb), the first line providing the first calibrated high side voltage (Uncaiibi) dependent on the first high side voltage (UH); and determine the second linear regression curve on the basis of the first low side voltages (UL) and the second low side voltages (ULABL), the second linear regression curve providing a second line defined by the parameters slope and y-intercept (UL-m, UL-L) as second constant parameters (Unm, Unb), the second line providing the first calibrated low side voltage (ULcaiibi) dependent on the first low side voltage (UL).

14. The method of claim 13, wherein the first high side voltage (UH) is determined from the first resistor (R1), the second resistor (R2), and the voltage measured by the first voltage measurement device (81) by means of the formulawherein UH is the first high side voltage, R1 is the first resistor, R2 is the second resistor and UADCH is the voltage measured by the first voltage measurement device; and wherein the first low side voltage (UL) is determined from the third resistor (R1), the fourth resistor (R2), and the voltage measured by the second voltage measurement device (82) by means of the formulawherein UL is the first low side voltage, R3 is the third resistor, R4 is the fourth resistor and UADCL is the voltage measured by the second voltage measurement device.

15. The method of any one of claims 12 to 14, wherein providing in steps asymmetrical voltage values comprises: adjust the resistance of the first and second variable resistors (R5, R6) such that the voltage over the first variable resistor (R5) is changed in steps between a minimum and a maximum and such that the voltage over the second variable resistor (R6) is changed in steps between a maximum and a minimum, and record in each step the values for the first calibrated high side voltage (Uncaiibi), the first calibrated low side voltage (ULcaiibi), the measured third high side voltage (ULABH) measured by the first calibrated voltage sensor (LABH), and the measured third low side voltage (ULABL) measured by the second calibrated voltage sensor (LABL).

16. The method of any one of claims 12 to 15, wherein determining the third and fourth constant parameters (GAINERRHm, GAI NERRHL, GAINERRLm, GAINERRLb) comprises: calculate in each step a high side gain error (GAI NERRH) for the first calibrated high side voltage (Uncaiibi) based on the third high side voltage (ULABH); calculate in each step a low side gain error (GAI NERRL) for the first calibrated low side voltage (ULcaiibi) based on the third low side voltage (ULABL); determine a third linear regression curve on the basis of the quotient of the first calibrated high side voltage (Uncaiibi) and the first calibrated low side voltage (ULcaiibi) on the x-axis and this quotient times the high side gain error (GAI NERRH) on the y-axis, the third linear regression curve providing a third line defined by parameters slopeand y-intercept (GAI NERRHm, GAI NERRHb) which represent the third constant parameters; and determine a fourth linear regression curve on the basis of the quotient of the first calibrated low side voltage (Ui_caiibi) and the first calibrated high side voltage (Uncaiibi) on the x-axis and this quotient times the low side gain error (GAI NERRL) on the y-axis, the fourth first linear regression curve providing a fourth line defined by parameters slope and y-intercept (GAINERRL-m, GAI NERRL) which represent the fourth constant parameters.

17. The method of claim 16, wherein the method further comprises: calculate in each step the high side gain error (GAI NERRH) for the first calibrated high side voltage (Uncaiibi) based on the third high side voltage (ULABH) in that the high side gain error (GAI NERRH) is calculated as the quotient of the measured high side voltage (ULABH) and the first calibrated high side voltage (Uncaiibi); and calculate in each step the low side gain error (GAI NERRL) for the first calibrated low side voltage (ULcaiibi) based on the third low side voltage (ULABL) in that the low side gain error (GAI NERRL) is calculated as the quotient of the measured low side voltage (ULABL) and the first calibrated low side voltage (ULcaiibi).

18. The method of any one of claims 12 to 17, wherein the resistances of the first and second variable resistors (R5, R6) are adjusted in steps such that the respective high side voltage (UH) is varied in steps in a voltage range defined by the voltage difference between a positive terminal and a negative terminal of a DC power source to which the positive voltage rail (3) and the negative voltage rail (4) are attached, respectively, and that the respective low side voltage (UL) is varied in steps over the same voltage range, wherein the sum of the high side voltage (UH) and the low side voltage (UL) is constant.

19. The method of any one of claims 12 to 18, wherein the fault monitoring device further comprises a third voltage measurement device (83), wherein the third voltage measurement device is arranged to measure the voltage over the second resistor (R2) of the first voltage divider and the third resistor (R3) of the second voltage divider, wherein the method further comprises that, in the first stage, a fifth linear regression curve (93) is determined based on a first battery voltage (UB) determined by means of the third voltage measurement device (81) and a second battery side voltage (ULABH+ULABL) measured by the first and second calibrated voltage sensors (LABH, LABL), and fifth constant parameters (Usm, UB are determined from the fifth linear regression curve (93) that allow to provide a first calibrated battery voltage (Uscaiibi) dependent on the first battery voltage (UB).

20. The method of any one of claims 12 to 19, wherein the method further comprises determine a first calibrated high side voltage (Uncaiibi) from a voltage (UADCH) measured by the first voltage measurement device (81) and the first constant parameters (Unm, Unb); determine a first calibrated low side voltage (Ui_caiibi) from a voltage (UADCL) measured by the second voltage measurement device (82) and the second constant parameters (Ui_m, Ui_b); correct the first calibrated high side voltage (Uncaiibi) using the third constant parameters (GAI NERRHm, GAI NERRHb) to determine a double calibrated high side voltage (UHcaiib2); and correct the first calibrated low side voltage (Ui_caiibi) using the fourth constant parameters (GAI NERRLm, GAI NERR^) to determine a double calibrated low side voltage (U|_calib2).

21. The method of claim 20, wherein the first calibrated high side voltage (Uncaiibi) and the first calibrated low side voltage (Ui_caiibi) are corrected by applying the formulaswherein:Uncaiibi is the first calibrated high side voltage;UHcaiib2 is the double calibrated high side voltage;Ui_caiibi is the first calibrated low side voltage;GAINERRHm is the slope of the third linear regression curve;GAINERRHb is the y-intercept of the third linear regression curve;Ui_caiibi is the first calibrated low side voltage;Ui_caiib2 is the double calibrated low side voltage;Uncaiibi is the first calibrated high side voltage;GAINERRL-m is the slope of the fourth linear regression curve; and GAINERRL-b is the y-intercept of the fourth linear regression curve

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