Method and apparatus for monitoring insulation between a DC bus and protective earth - Patents.com

Abstract

A method and apparatus for monitoring insulation between a DC bus and protective earth, the bus being connected to a DC power source. Terminals of the power source are connected to protective earth by a first electrical network for a first time period, and a transient first electrical value associated with the first electrical network is measured at a plurality of time steps during the first time period. A first steady-state value is calculated using a plurality of first measurement points during the first time steps. Further, terminals of the power source are connected to protective earth by a second electrical network for a second time period, and a transient second electrical value associated with the second electrical network is measured at a plurality of second time steps during the second time period. A second steady-state value is calculated using a plurality of second measurement points during the second time steps. An indicator of insulation resistance between the terminals of the power source and protective earth is determined based on the calculated steady-state values ​​of the first and second electrical values.

Description

[Technical field]

[0001] The present invention relates to a method and a device for monitoring the insulation between a DC bus and a protective earth. The present invention further relates to the use of the device for insulation fault monitoring. Furthermore, the present invention relates to a system comprising a DC bus possibly capacitively coupled to an AC grid, in which the device is arranged to monitor the insulation between the DC bus and the protective earth. [Background technology]

[0002] Direct current (DC) power networks are used in a variety of applications, including direct current voltage distribution systems, electric vehicles, charging stations for electric vehicles, and solar power generation.

[0003] In a floating DC grid, insulation monitoring is necessary as a safety measure to ensure proper electrical insulation between the DC grid and protective earth. Such insulation resistance monitoring devices can be employed to detect insulation failures or insulation faults before a fault current can flow, for example to a human body. The device can automatically interrupt and / or disconnect the DC supply voltage when an insulation failure or insulation fault is detected. Thus, active protection can be obtained so that electric shock of users and other personnel can be effectively avoided. Insulation monitoring devices are compact and can be easily integrated with other devices. Contrary to DC grids, AC grids can be protected by residual current devices. These devices need to pass a fault current before they can operate.

[0004] Insulation monitoring in DC networks can be performed in several ways. The device can be configured to measure the insulation resistance between the power lines and protective earth (e.g., enclosure) in a floating system. Typically, a small electric excitation (small DC or AC voltage or current) is applied between the DC power lines and protective earth, and the response is observed. This tends to give adequate results for small DC island grids (e.g., vehicles) with relatively few connected (known) equipment.

[0005] However, in various applications and DC networks, many more external devices are connected to the DC power source. These devices may not be known or defined at the time of design. Furthermore, these devices may contain power electronics that provide some coupling between the protective earth and the power line for filtering purposes. For example, in larger grids, the filters included in the devices may significantly interfere with the operation of the insulation monitoring device. If the DC grid is coupled to the AC grid, for example through power electronics, the insulation monitoring device may not operate correctly. For example, an isolation transformer may be used to provide isolation when the DC grid is connected or coupled to the AC grid. However, since the protective earth is common to the DC power source and the AC grid, faults in the DC grid may not be accurately detected by the residual current device.

[0006] There is a need for insulation monitoring that can accommodate such situations and / or provide improved insulation monitoring for a wide variety of DC networks. Summary of the Invention

[0007] It is an object of the present invention to provide a method and system which obviates at least one of the above mentioned disadvantages.

[0008] Additionally or alternatively, it is an object of the present invention to provide improved insulation monitoring.

[0009] Additionally or alternatively, it is an object of the present invention to provide an insulation monitoring method and apparatus having improved safety.

[0010] Additionally or alternatively, it is an object of the present invention to provide improved insulation monitoring between the DC bus and the protective earth of the AC grid.

[0011] Therefore, the present invention provides a method for monitoring insulation between a DC bus and a protective earth, the bus being connected to a DC power source, the method comprising: connecting a terminal of the power source to protective earth by a first electrical network during a first time period; measuring a transient first electrical value associated with the first electrical network at a plurality of time steps within the first time period, a first steady-state value being calculated using a plurality of first measurement points at the plurality of first time steps; connecting a terminal of the power source to protective earth by a second electrical network during a second time period; measuring a transient second electrical value associated with the second electrical network at a plurality of second time steps within the second time period, a second steady-state value being calculated using a plurality of second measurement points at the plurality of second time steps; and determining an indication of insulation resistance between the terminal of the power source and protective earth based on the calculated steady-state values ​​of the first electrical value and the second electrical value.

[0012] Advantageously, insulation monitoring can be used in larger DC networks. For example, in the case of a DC grid that is coupled to a larger DC grid and / or an AC grid, insulation monitoring can provide an accurate indication of the insulation resistance. The insulation resistance value can be determined accurately and quickly even when the response of the electrical excitation contains a resistor-capacitor (RC) component, thereby improving the overall safety of insulation monitoring. This can reduce the risk of electric shock.

[0013] The protective earth can be excited at two voltage levels, one after the other. In many cases and applications, it is desirable for the insulation monitoring to react within 300ms (depending on the magnitude of the fault) after an insulation fault occurs, to guarantee safety and protect personnel from electric shock. The RC time constant of the parasitic capacitance can easily exceed 50ms, when combined with the high impedance of the resistor used for excitation. Reducing the RC time constant by reducing the value of the resistor in the excitation circuit would increase the excitation current to values ​​that are dangerous to personnel. It can also be difficult to influence the capacitance C, since it may depend on other equipment that is coupled to the electrical network.

[0014] Waiting 3-5 times the RC time constant (which can be 200 ms or more) before taking a measurement may not be desirable or acceptable, as the total insulation monitoring reaction time would exceed 800 ms. According to the invention, the RC response can be monitored for a limited time (see time period) and the steady-state final value can be estimated or predicted and used in further calculations to find the resistive insulation value faster. Various types of models can be used to predict / estimate the steady-state final value.

[0015] The insulation resistance value between the terminals to be determined may be calculated (e.g., by a microcontroller) based on voltage measurements obtained when the terminals of the DC power supply are connected to protective earth by a first electrical network and when the terminals of the DC power supply are connected to protective earth by a second electrical network.

[0016] An indication of the insulation resistance can be determined, from which the insulation resistance value can be derived directly or indirectly. The calculated resistance value and / or the calculated parallel resistance value (or an indication thereof) can be used to trigger an alarm when an alarm level for insulation failure is reached.

[0017] Optionally, the first and second steady-state values ​​are calculated before the measured first and second electrical values ​​reach a steady state in the first and second time periods, respectively. Before the stable or steady-state values ​​are reached, a model can be used (e.g., fitting) to calculate or estimate a predicted steady-state value, i.e., a steady-state value that would have been obtained after a period long enough for stabilization.

[0018] In this way, instead of waiting for the first and second electrical values ​​to settle to a constant and / or stable value (e.g., for a time longer than 5 times the RC component: 5*RC), multiple different measurement points (sampled data points) in a limited first and second time period can be used to calculate / estimate the final value, respectively. For example, fitting can be performed to identify an overall trend, so that the final value can be accurately determined without requiring undesirable or unacceptable waiting times. As a result, safety can be significantly improved.

[0019] Optionally, the first period and / or the second period is less than three times a resistor-capacitor (RC) time constant.

[0020] In order to improve the speed of detecting insulation faults, the slow response as a result of resistor-capacitor (RC) delay can be avoided. The detrimental RC delay plays an important role, since there is often not enough time to wait until the stable steady-state final values ​​of the measured first and second electrical values ​​are obtained.

[0021] The total time to obtain the steady-state final value is 5 times the time constant RC. However, according to the present invention, only a limited portion of the charge / discharge curve is used to determine the steady-state final value. The first and second electrical values ​​may be voltages. As a result of the electrical circuitry, the voltage can increase at a faster rate during the first time constant and at a slower rate during the last time constant, or vice versa. Multiple measurement points in the charge curve can be used to determine the steady-state final value.

[0022] The RC time constant may vary depending on the resistance obtained when the terminals of the power supply are connected to the protective earth by the first or second electrical network. Furthermore, the RC time constant may vary depending on the capacitance between the protective earth and the positive and negative terminals of the DC power supply. The capacitance may be, for example, a parasitic capacitance obtained when a DC power supply is connected to an AC electrical network.

[0023] Optionally, the first and second periods are selected taking into account an estimate of the RC time constant, which may be set in advance.

[0024] Optionally, the RC time constant is determined based on the measurement points.

[0025] Optionally, the first period and / or the second period is less than 400 ms, more preferably less than 200 ms, even more preferably less than 100 ms. The sum of the first period and the second period may be less than 800 ms, more preferably less than 400 ms, even more preferably less than 120 ms.

[0026] The first and second steady-state values ​​can be predicted in various ways. For example, a computational model configured to predict the final steady-state value based on a limited number of measurement points can be employed. For example, model fitting, estimator algorithms, machine learning algorithms, etc. can be used.

[0027] In some examples, a 400 ms duration for the first and / or second period may be too long, since it may not be possible to perform a reaction (e.g., alarm) within the 300 ms period. Optionally, the first and second periods are selected such that the time required to determine the insulation resistance value and perform an alarm action is less than 300 ms, more preferably less than 200 ms.

[0028] Optionally, the first steady-state value is calculated by a first curve fitting using a plurality of measurement points at a plurality of first time steps, and the second steady-state value is calculated by a second curve fitting using a plurality of measurement points at a plurality of second time steps.

[0029] The time required to obtain an estimate of the steady-state value of the measured electrical quantity can be significantly reduced by performing curve fitting. Curve fitting can provide an accurate prediction of the steady-state value. Thus, it is not necessary to wait (for a full cycle) until the first / second electrical value (e.g. voltage) reaches the steady-state value. Based on the predicted final value, an index of the insulation resistance can be determined. It is also possible to directly and / or indirectly calculate the value of the insulation resistance instead of the index.

[0030] Optionally, the time interval between successive time steps is selected based on a power cycle.

[0031] Because the protective earth can be capacitively coupled to the AC line, the voltage on the protective earth can follow the grid frequency or its harmonics. By measuring during any number of power periods or power cycles, the effects of this destructive coupling can be effectively averaged out.

[0032] Optionally, the time interval is selected as a multiple of a power supply period, preferably equal to one power supply period.

[0033] The measurement points used to predict the first and second steady-state values ​​can be selected at a distance of n times the duration of the power cycle of the AC grid, which is 20 ms (cf. 50 Hz) in Europe and 16.7 ms (cf. 60 Hz) in the United States.

[0034] Optionally, three measurement points are used for each of the first and second curve fits.

[0035] The curve fitting may be an exponential fit utilizing at least three measurement points. By employing three measurement points, three equations with three unknowns can be obtained. Other curve fitting methods can also be performed, including, for example, least squares or Levenberg-Marquart. It will be understood that more than three measurement points can be used to perform the curve fitting.

[0036] The processing method (e.g. in a microcontroller) of the measured electrical values ​​may be selected in such a way that a better accuracy is obtained. By choosing an asymmetric ratio like 1 / 3 and 2 / 3, the calculations (formulas) and the processing method (e.g. in a microcontroller) of the measured electrical values ​​can be significantly simplified. In this way, the accuracy can be improved. Furthermore, symmetrical faults can be detected.

[0037] Optionally, the first electrical value and the second electrical value are measured at the same measurement location relative to the same reference, and the excitation voltage is normalized to a DC power supply voltage provided by the DC power supply.

[0038] When used to calculate the isolation voltage, the measured response to the excitation voltage can be normalized to the DC supply voltage. The excitation voltage can be scaled with respect to the DC voltage. The scaling can be implemented in a microcontroller. Optionally, the excitation voltage is linearly proportional to the DC voltage. By dividing the DC supply voltage by the same factor as the first and second voltage values ​​and using the first one as the reference for the A / D converter, multiple A / D channels are no longer necessary. Furthermore, the calculation time can be reduced since the ratio is measured directly by the A / D converter.

[0039] A DC voltage (Vdc) can be used as a reference. Advantageously, it is no longer necessary to divide by the full voltage. Higher accuracy can be obtained and the full scale of the measurement range can be used. Also, the DC voltage may vary to some extent. By measuring in relative values ​​(e.g. 1 / 3, 2 / 3), inaccuracies due to DC voltage variations can be eliminated. The arrangement of resistors in the electrical network may result in asymmetric ratios of 1 / 3 and 2 / 3. It will be understood that other ratios may be used.

[0040] Optionally, the reference voltage Vref is generated internally to the microcontroller using a DC power supply voltage or a direct supply voltage of the microcontroller.

[0041] Optionally, during a first period, the protective earth is excited with a blocking voltage of a first fraction of the DC supply voltage, and during a second period, the protective earth is excited with a blocking voltage of a second fraction of the DC supply voltage, the first fraction being different from the second fraction. Advantageously, this monitoring also works for symmetrical insulation faults occurring at the positive and negative terminals of the DC supply. Symmetrical insulation faults can be detected, where there is a similar insulation fault at the positive and negative voltage terminals. In DC systems, this type of fault is not infrequently likely to occur.

[0042] As an example, symmetrical failures can occur at the intermediate voltage level of a stacked DC supply if the DC supply itself fails. In addition, symmetrical failures can occur if the DC bus is equipped with a motor inverter or if an insulation failure occurs after the inverter (e.g. in an electric vehicle). Degradation of insulation materials can also cause symmetrical failures.

[0043] Optionally, the sum of the first rate and the second rate is substantially equal to 1. Optionally, the first rate is 1 / 3 and the second rate is 2 / 3.

[0044] By adopting dimensionless values ​​(e.g. 1 / 3 and 2 / 3), the accuracy and calculation time can be improved. In order to be able to measure volts so that the ratio of 1 / 3, 2 / 3 can be measured directly, a reference value (Vref) can be used for the A / D converter. The reference value Vref can be defined as the DC supply voltage divided by a number to obtain a voltage low enough to be usable by the microcontroller. In this case, Vref can vary depending on the DC supply voltage. The ratio of V1 (i.e. the first electrical value) to the DC supply voltage can be measured directly. Vref can be the reference voltage for the A / D conversion.

[0045] A first electrical value (e.g., voltage V1) and a second electrical value (e.g., voltage V2) may be measured at different times as the DC voltage changes. If the DC voltage is unknown (e.g., not measured), then both V1 and V2 will be divided by the same number. This can be overcome by working with dimensionless fractions (instead of absolute voltages) when making measurements.

[0046] Optionally, one or more bias resistors are used to increase the first rate and decrease the second rate, the one or more bias resistors configured to provide a bias of between 0.1% and 2%, more preferably between 0.1% and 1%.

[0047] Bias resistors can allow for compensation of component tolerances (e.g., use of cheaper components). To further increase noise immunity, bias resistors can be introduced such that the first excitation voltage (Vx1) is slightly greater than a first fraction (e.g., 1 / 3 Vdc) and the second excitation voltage (Vx2) is slightly less than a second fraction (e.g., 2 / 3 Vdc). As a result, Vx2-Vx1 is less than Vdc / 3 in all circumstances, taking into account injected noise and component tolerances.

[0048] Advantageously, the bias resistor can provide a lower insulation resistance estimate (i.e., safe) when resistance values ​​are high, and can provide a negligible error in the estimate in the event of an insulation failure. If the insulation value is significant, errors may occur in the insulation measurement. However, even if the insulation value becomes small, i.e., an insulation failure occurs, the measurement can still be made accurately.

[0049] Optionally, a filter is employed to measure the first electrical value and the second electrical value, the filter being a low pass filter.

[0050] Filters can be used to filter noise and crosstalk. However, in some cases, such filters (e.g., filters configured to filter 50 Hz or 60 Hz components and optionally their harmonics) can introduce undesirable (additional) delays, e.g., of the order of milliseconds, in the electrical measurement. Furthermore, detrimental phase shifts can be introduced into the measurement results. However, by employing high frequency filters, the delays and phase shifts can be suppressed while an improved signal is obtained. The calculation of the steady-state final values ​​of the transient first and second electrical values ​​can be performed in various ways.

[0051] Optionally, the steady state electrical values ​​are calculated using curve fitting only when the DC power source is coupled to the AC network.

[0052] The method may further include determining whether the DC power source is coupled to an AC network. If there is a coupling to an AC network, the first and second electrical values ​​may be calculated / predicted. Waiting until a steady state value is reached may take too long as a result of an RC time constant (e.g., capacitive coupling). If there is no coupling to an AC network, the first and second electrical values ​​in steady state may be measured directly. This can be performed quickly enough since there is no RC delay. Optionally, the operating mode may also be selected manually.

[0053] Optionally, a switch is provided which is switchable between a first state and a second state, in the first state the switch connects the terminals of the power supply to a first network and in the second state the switch connects the terminals of the power supply to a second network, said first period and second period are equal, the sequence of said first period and second period is repeated with a fixed frequency, and the measuring, calculating and comparing are repeated with the same frequency.

[0054] Optionally, the determined value of the insulation resistance (R1, R2) is compared with a predefined alarm level and an alarm action is performed if the value of the insulation resistance exceeds the predefined level.

[0055] Optionally, the device of the invention comprises means for sending a signal, in case of an insulation fault, to a load connected to the DC power supply in order to communicate said fault to a remote control station.

[0056] Optionally, the transient first electrical value and the transient second electrical value are measured with a ratiometric A / D conversion. Advantageously, as a result no fixed reference is required.

[0057] Optionally, the DC voltage of the DC power supply is measured. Additionally or alternatively, an indication of the DC voltage is provided to the microcontroller.

[0058] According to one aspect, the invention provides an apparatus for monitoring insulation between a DC bus and a protective earth, the bus being connected to a DC power source, the apparatus being configured to: connect a terminal of the power source to protective earth by a first electrical network during a first time period, measure a transient first electrical value associated with the first electrical network at a plurality of time steps within the first time period, a first steady-state value being calculated using a plurality of first measurement points at the plurality of first time steps, connect a terminal of the power source to protective earth by a second electrical network during a second time period, measure a transient second electrical value associated with the second electrical network at a plurality of second time steps within the second time period, a second steady-state value being calculated using a plurality of second measurement points at the plurality of second time steps, and determine a measure of insulation resistance between the terminal of the power source and protective earth based on the calculated steady-state values ​​of the first electrical value and the second electrical value.

[0059] Advantageously, the device can operate correctly even when the electrical network to be monitored is a DC network coupled to an AC grid via capacitive coupling (e.g. a charging station for electric vehicles). The resulting capacitance obtained by capacitive coupling can significantly affect the measurement of electrical quantities (e.g. voltage). The RC time constant can create a significant delay until the steady-state final value is reached. When a blocking voltage is applied, a delayed square wave may be obtained. The device can respond fast enough to detect insulation faults, even in the presence of said RC time constant due to capacitive coupling.

[0060] In addition, the device can be used in DC networks that are not connected to an AC grid. In both cases, an accurate indicator of the insulation resistance can be determined.

[0061] The device may include a microcontroller configured to control a switch between an "on" state and an "off" state, where in the "on" state the terminals of the power supply are connected to protective earth by a first electrical circuitry and in the "off" state the terminals of the power supply are connected to protective earth by a second electrical circuitry. Depending on the state of the switch, a first electrical value and a second electrical value (e.g., two voltages) may be measured. An additional voltage measurement may be performed to measure the DC voltage generated by the DC power supply.

[0062] Optionally, the device is configured to perform other monitoring functions.

[0063] It will be appreciated that the insulation resistors R1 and R2 are physically present, but not in the form of resistive components. The fault resistors behave electrically as resistors and can therefore be considered as resistors.

[0064] It will be understood that any of the aspects, features and options described in terms of the method apply equally to the apparatus and / or vice versa. It will also be apparent that any one or more of the above aspects, features and options may be combined. [Brief description of the drawings]

[0065] The invention will be further explained on the basis of exemplary embodiments shown in the drawings, which are given by way of non-limiting example, and it should be noted that the drawings are only schematic representations of embodiments of the invention, given by way of non-limiting example.

[0066] [Figure 1] FIG. 1 shows a schematic diagram of the method. [Diagram 2] FIG. 2 shows a schematic diagram of an embodiment of the DC bus and protective earth. [Figure 3a] FIG. 3a shows a schematic diagram of the measured transient electrical values ​​as a function of time. [Figure 3b] FIG. 3b shows a schematic diagram of the measured transient electrical values ​​as a function of time. [Figure 4a] FIG. 4a shows a schematic diagram of the measured transient electrical values ​​as a function of time. [Figure 4b] FIG. 4b shows a schematic diagram of the measured transient electrical values ​​as a function of time. [Diagram 5] FIG. 5 shows a schematic diagram of the measured transient electrical values ​​as a function of time. [Figure 6] FIG. 6 shows a schematic diagram of an insulation monitoring device according to an embodiment of the present invention. [Figure 7] FIG. 7 shows a schematic diagram of an insulation monitor. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0067] FIG. 1 shows a schematic diagram of a method 100 for monitoring insulation between a DC bus and a protective earth, the bus being connected to a DC power source. In a first step 101, a terminal of the power source is connected to the protective earth by a first electrical network during a first time period. In a second step 102, a transient first electrical value associated with the first network is measured at a number of time steps during the first time period and a first steady-state value is calculated using a number of first measurement points at a number of first time steps. In a third step 103, a terminal of the power source is connected to the protective earth by a second electrical network during a second time period. In a fourth step 104, a transient second electrical value associated with the second network is measured at a number of second time steps during a second time period and a second steady-state value is calculated using a number of second measurement points at a number of second time steps. In a fifth step 105, a measure of the insulation resistance between the terminals of the power supply and the protective earth is determined based on the calculated steady-state value of the first electrical value and the steady-state value of the second electrical value.

[0068] When a DC power source is connected to an AC electrical network, a parasitic capacitance may exist between the protective earth and the positive and negative terminals of the DC. This capacitance is unknown and may change over time. This capacitance may depend, for example, on the equipment connected to the AC grid. As a result of capacitive coupling, the time required to reach the steady-state final value may be too long (unacceptable for safety reasons).

[0069] There may be a significant delay before the first and second electrical values ​​reach a stable final value. This delay depends on a time constant equal to R*C. The R value (resistance) is relatively large to obtain a limited leakage current. The C value (capacitance) may be relatively large as a result of the device being connected to an electrical network. If the RC value is relatively large, it may take too long to obtain a steady-state final value. For example, a time constant RC of 3 to 5 times may be unacceptable for safety and / or for a particular application. By predicting the first and second steady-state final values, for example by performing curve fitting of the RC curve, a faster and more accurate response may be obtained.

[0070] Figure 2 shows a schematic diagram of one embodiment of a DC bus and a protective earth. The DC bus is connected to a DC power source 1. The terminals of the power source 1 are connected to a protective earth 4. The insulation resistances are represented by R1 and R2. A first electrical excitation can be applied by connecting the power source 1 to the protective earth 4 via a first network (not shown in this figure). A second electrical excitation can then be applied by connecting the power source 1 to the protective earth 4 via a second network (not shown in this figure).

[0071] The electrical excitation 50 can be performed by applying two different DC voltages V1, V2 determined by the configuration of the first and second networks. The electrical values ​​52 associated with the first and second networks are measured over a limited time period and the steady-state (stable) values ​​are predicted based on the measurement points at multiple time steps to limit the values ​​needed to calculate the insulation resistances R1 and R2.

[0072] In this example, a square wave voltage is applied (excited), but the measured voltage is not a square wave due to capacitance effects. If the applied voltage is maintained for a long time, a steady-state final value can be reached, but for safety reasons, it may be too late to activate an alarm at this point. Therefore, the steady-state value is predicted for a limited period of time that is shorter than the time required to reach steady state (e.g., shorter than 5*RC).

[0073] 3a and 3b show schematic diagrams of measured transient electrical values ​​with amplitude A as a function of time t. FIG. 3a shows an alternating sequence of a transient second electrical value 54 associated with the second network and a transient first electrical value 56 associated with the first network. As mentioned above, there may be coupling between the protective earth and the AC grid, which may result in capacitance. When the terminals of the DC power source are connected to the protective earth by the second network, the measured voltage may gradually increase (as a result of the RC delay) until it reaches a steady-state value 58.

[0074] The resulting effect of the RC circuit is that the voltage rises as a function of time towards the applied (excitation) voltage during the charge phase. A decrease in the excitation voltage results in a discharge phase where the voltage decreases. The voltage discharges towards the new excitation voltage. During charging, the voltage increase is fastest during the first time constant and slowest during the last time constant. Similarly, during discharging, the voltage decrease is fastest during the first time constant and slowest during the last time constant. The charge / discharge curve can reach five time constants, i.e., the point at which a substantially stable or steady-state final value is reached.

[0075] FIG. 3b shows an expanded view where the steady state value is calculated using three measurement points 60. These measurement points 60 are used to predict / estimate the steady state final value 58. This may be done, for example, by performing an exponential fitting. Other models and algorithms may also be used. To determine the steady state final value 58, it is necessary to use a time t rather than up to 5*RC. predTherefore, the time period during which the transient electrical values ​​associated with the first and second networks are measured can be significantly smaller than 5*RC. As shown in FIG. 3b, the second time period (t pred -t0) will be significantly less than (5*RC-t0). This is also valid for the first time period during which transient electrical values ​​associated with the first network are collected. In this example, charging phase 80a begins at time step t pred followed by a discharge phase 80b.

[0076] 4a and 4b show schematic diagrams of measured transient electrical values ​​as a function of time. For example, the switching between the connection of the power supply terminal to protective earth via a first electrical network and the connection of the power supply terminal to protective earth via a second electrical network, which is realized by a first state ("on") and a second state ("off") of a switch, should be performed fast enough to detect insulation errors in a timely manner while ensuring safety. As a result, there may not be enough time to wait for the steady-state values.

[0077] Asymmetric excitation may be employed. By making the ratio between V1 and V2 asymmetric, symmetrical insulation errors can also be detected. For example, insulation errors occurring as a result of insulation degradation (natural aging) may be symmetrical. The protective earth may for example first be excited with 1 / 3 of the DC line voltage for a first period and with 2 / 3 of the line voltage for a second period. At least two measurement cycles are required to determine and confirm an insulation fault.

[0078] In one example, several measurements can be made during two power line periods excited at 1 / 3 and two power line periods excited at 2 / 3. Each set of measurements can form an RC curve from which a final value can be calculated. Thus, the final value can be calculated without waiting for it to stabilize. Two power line periods may be sufficient to perform a measurement. In this way, the measurement time can be significantly improved. For example, a reaction time of 300 ms can be achieved.

[0079] In this example, the insulation of a 50V DC system is monitored. The excitation voltages are therefore Vx1=16.67V and Vx2=33.33V. The excitation voltages are exemplarily selected as 1 / 3 of the DC grid voltage and 2 / 3 of the DC grid voltage. Due to the capacitive load between the DC bus and the PE, Vx1, shown by line 61 in FIG. 4a, is not reached within 40 ms of the excitation time, and Vx2, shown by line 62 in FIG. 4b, is not reached during 60 ms of the excitation time. However, these final values ​​can be derived by extrapolating the exponential RC curve. This final value is used to calculate the insulation resistance. In FIG. 4b, the value to which the voltage would converge if one waited long enough, i.e. the steady value 62, is never reached. The invention is not limited in this context.

[0080] A first block voltage of 1 / 3 of the DC power supply voltage may be applied for a first time duration, and a second block voltage of 2 / 3 of the DC power supply voltage may be applied for a second time duration. This results in a protective earth potential difference of 1 / 3 of the potential difference between the positive and negative terminals of the DC power supply. The applied voltage ratios of 1 / 3 and 2 / 3 simplify the calculation of the insulation resistances R1, R2. It will be appreciated that other asymmetric ratios may also be used, e.g. (1 / 4 and 3 / 4), (2 / 5 and 3 / 5), (1 / 5 and 4 / 5), etc. Many ratio pair combinations may be implemented.

[0081] The first and second periods can be considered as the durations during which the protective earth is connected to the first and second (resistive) networks, respectively. In one example, the protective earth may be connected to one network for 60 ms and to the other network for 40 ms. It may be desirable to measure over multiple periods before an alarming insulation resistance value is reported. In the example of Figures 4a and 4b, a complete measurement to provide an indication of the insulation resistance between the power supply terminals and the protective earth can be made within 100 ms.

[0082] Figure 5 shows a schematic diagram of measured transient electrical values ​​as a function of time. The protective earth can be capacitively coupled to AC and DC lines, so that the voltage on the protective earth follows the grid frequency or its harmonics. By measuring over any whole number of power cycles Δt, the effect of the coupled grid frequency can be averaged out.

[0083] As shown in FIG. 5, a vibration signal 70 can be superimposed on an exponential function 72 (only a portion is depicted). The amplitude of the vibration signal may be limited (e.g. oscillating at 50 Hz). However, the vibration signal may adversely affect the estimation of the first and second steady-state values ​​(e.g. using curve fitting). Relatively small oscillations may already have a large effect on the calculated value of the insulation error. For example, a vibration signal causing oscillations of ±0.1 V on a signal of about 600 V may already cause large deviations between the calculated / estimated value and the actual steady-state value.

[0084] This effect may be even greater as the insulation resistance becomes smaller (e.g., approaching the limit of the minimum insulation value). When the insulation resistance is small, the difference between V1 and V2 may become smaller. For example, when using 1 / 3 and 2 / 3 excitation, the difference V2-V1 is no longer 1 / 3 of the DC supply voltage, but may become smaller.

[0085] The time distance ΔT between the two measurement points can be selected based on the power supply period of the AC grid to which the DC power supply is coupled. Errors due to poor sampling can be prevented. In an advantageous embodiment, the distance between the two points in time is an integer multiple of the power supply period. For example, the distance between the two points in time may be one power supply period in order to reduce the required processing time.

[0086] At least three consecutive measurement points may be determined to perform an estimation or prediction of the final value. In case of curve fitting, three measurement points may be used. More than two measurement points may also be used, for example, when employing the Lenberg-Marquart algorithm. In this example, three measurement points sampled at three time steps, namely T1, T2, T3, are used. The correct trend for determining the stationary value can be obtained if the fitting curve passes through the three points T1, T2, T3.

[0087] This time interval may be adjustable depending on the associated power line cycle. In one example, the power line cycle is determined, for example during initialization. The device may be configured to determine or receive an indication of the power line cycle. It is also envisioned that the power line cycle may be pre-set and / or user selectable (e.g., Europe: 50Hz, USA: 60Hz).

[0088] An exponential curve fitting may be performed to calculate the steady state voltage value 62. The steady state electrical value may be calculated based on the following equation:

[0089]

number

[0090] It will be appreciated that curve fitting with a larger number of measurement points is also contemplated. Many fitting algorithms may be implemented.

[0091] Figure 6 shows a schematic diagram of an embodiment of an insulation monitoring device according to the invention. There is a DC power source 1 which can be connected to one or more loads (not shown) via a DC bus including conductors 12 and 13. The insulation of the DC bus connected to the DC power source 1 is represented diagrammatically as a pair of resistors R1 and R2 between the terminals of the DC power source 1 and protective earth 4. The insulation between the DC bus and protective earth 4 is influenced by the insulation of the conductors 12 and 13 and all conductors leading to the loads, which in turn affect the insulation of the DC system consisting of these loads. The resistance values ​​of R1 and R2 should be determined accurately and fast enough for safety purposes.

[0092] The insulation monitoring device 5 can be electrically connected to the power supply 1. The insulation monitoring device 5 can include an electrical circuit 8, which can include a network of resistors, capacitors and / or inductors, a switching device 6 and a microcontroller 7. The switching device 6 can be, for example, a bipolar transistor or a MOSFET, and can be controlled by the microcontroller 7, which commands the "on" or "off" state of the switch 6 via connection 9. The state of the switch 6 defines the actual circuit that is connected to the power supply at any point in time. This means that by controlling the switch 6, two different electrical networks can be formed.

[0093] The microcontroller 7 is provided with means for measuring a value, preferably a voltage, at a particular point of the circuit. This measurement is taken via connection 10. The circuit is devised so that during each state of the switch 6 (on or off) two sufficiently different values ​​are measured at the same point. If necessary, more than two values ​​may be measured, for example one additional measurement independent of the state of the switch 6, or two additional measurements dependent on the state of the switch 6, or both.

[0094] The resistance values ​​of R1 and R2 to be determined are calculated based on these at least two measurements and combined with network laws applied to two different networks corresponding to the switches being on or off, respectively.

[0095] The calculation of R1 and R2 may be performed by the microcontroller 7 according to an appropriate formula. The microcontroller may be configured to check whether the values ​​of R1 and R2 are high enough. If not, an insulation fault is detected and the terminals of the power supply 1 may be disconnected from any load connected to these terminals. Optionally, a pre-alarm warning signal may be activated when R1 and R2 are close to a predefined alarm level but have not yet fallen below said alarm level.

[0096] For the calculation of R1 and R2, two successive measurements are taken during each one state (on or off) of switch 6. Continuous monitoring can be performed by sequentially changing the switch state at a given frequency.

[0097] Optionally, a main switch may be arranged to connect or disconnect the power source 1 to the load. For example, the main switch may be arranged with two synchronized switches, one on each of the conductors of the DC bus and operated by relays. The device may also operate an additional relay to prevent excessive inrush currents to the DC bus capacitors. The additional relay may also be operated to discharge any residual voltage on the DC bus capacitors after the main switch is turned off.

[0098] The illustrated apparatus may be configured to connect a terminal of a power source 1 to protective earth 4 by a first electrical network for a first time period, subsequently measuring a transient first electrical value associated with the first network at a number of time steps in the first time period, and calculating a first steady-state value using a number of first measurement points at a number of first time steps.

[0099] The apparatus may further be configured to connect the terminals of the power supply to the protective earth 4 by a second electrical network during a second time period, subsequently measuring a transient second electrical value associated with the second network at a plurality of second time steps during the second time period, and calculating a second steady-state value using a plurality of second measurement points at the plurality of second time steps. These steps may be repeated one or more times, and based on the calculated steady-state value of the first electrical value and the steady-state value of the second electrical value, a measure of the insulation resistance between the terminals of the power supply and the protective earth 4 may be determined.

[0100] 7 is a schematic diagram of an insulation monitoring device according to the invention. Two insulation resistances (Rf1, Rf2) representing the insulation between the DC bus terminals (positive +, negative -) of the DC power source 1 and the protective earth 4 need to be monitored. The switch S1 can be a MOSFET transistor. Other implementations are also possible for the switching between the two electrical networks. In this embodiment, an electrical network is provided comprising a resistor R3 between the positive power supply terminal and the protective earth 4, two resistors R4 and R8 arranged in series between the negative power supply terminal and the protective earth 4, a resistor R7 connected in series with S1, where R7 and S1 are arranged in parallel with the resistors R4 and R8, and two resistors R5 and R6 arranged in series and connected to the terminals of the power source 1.

[0101] When switch S1 is off, the resistor divider network includes only resistors R3, R4 and R8. When S1 is activated, the parallel branch including R7 is activated. In both states, the microcontroller measures the voltage with respect to protective earth 4 at point 16, which is the point between series resistors R4 and R8. This configuration as a resistor divider network allows the measured voltage to be reduced to a value low enough that it can be measured by the microcontroller's circuitry. Two successive voltage measurements give two values. These values ​​are used to calculate R1 and R2, as explained below.

[0102] Capacitive coupling between DC+ or DC- and protective earth 4 can arise, for example, due to parasitic capacitance of isolation transformers and / or electromagnetic interference decoupling capacitors in equipment connected to the DC bus. Devices known in the prior art usually employ high impedance resistors and require two measurements to be taken. Add to this the capacitive effects, and either measurement failures or long settling times occur, slowing down the operation of the device. It is generally accepted that insulation monitoring devices must react within 40ms to 300ms (depending on the magnitude of the fault) after an insulation fault occurs in order to protect personnel.

[0103] The time constant of the parasitic capacitance and the high impedance measuring resistor is of the order of 10 ms. The settling time is about 50 ms. However, these parameters can vary widely, so in practice a settling time of 100 ms per measurement has been found to be a good compromise in most cases. Two measurements are required to calculate one insulation resistance, and in order to create immunity to electromagnetic interference and other disturbances, the insulation resistance should be calculated at least two to three times before reacting.

[0104] It is clear that due to the parasitic capacitances present in the grid, the proposed insulation monitoring device cannot react fast enough. Furthermore, the grid's signals may crosstalk to the protective earth. The protective earth may carry traces of the mains voltage in the form of voltage ripples at the main frequency and / or one or more of its harmonics. Filtering these signals may slow down the acquisition of the signal applied to the protective earth. The method according to the invention is able to overcome these technical problems and can be implemented in various different types of insulation monitoring devices.

[0105] The protective earth is excited successively with two voltage levels. The first measurement of the protective earth voltage is taken immediately at the beginning of the procedure, i.e. immediately after opening the switch. The second and third measurements are taken after one grid cycle. These measurements are Vpe1.1+Vg1.1, Vpe1.2+Vg1.2 and Vpe1.3+Vg1.3. Vpe1 is the voltage on the protective earth due to the RC circuit with the switch open and Vg is the superimposed sinusoidal voltage due to crosstalk from the mains.

[0106] Next, with the switch closed and measurements taken in the same way, we get Vpe2.1+Vg2.1, Vpe2.2+Vg2.2, Vpe2.3+Vg2.3.

[0107] All measurements are taken at the same phase angle of the grid voltage, so Vg1.1=Vg1.2=Vg1, and Vg2.1=Vg2.2=Vg2. Furthermore, the time to close the solid-state switches is negligible compared to the grid period, so Vg1=Vg2=Vg. Vpe1.1 and Vpe1.2 are part of an exponential curve that starts at Vpe1.1 and ends at the settled value for the first measurement, Vpe1.

[0108]

number

[0109] Rf1, Rf2 are calculated insulation resistance values. It is also contemplated that metrics for Rf1 and Rf2 (e.g., values ​​based on equivalent electrical quantities and / or ratios of Rf1 and Rf2) may be determined.

[0110] The microcontroller 7 may be configured to open switch S1 for a predetermined period of time (eg 60 ms) and then close switch S1 (eg 40 ms), which may be repeated continuously.

[0111] The resistors in the network can be configured as follows:

[0112]

Number

[0113] By selecting the resistance values of R3, R4, R7, and R8, the protective ground can be excited at a blocking voltage of 2 / 3 of the DC power supply voltage (when S1 is open) and a blocking voltage of 1 / 3 of the DC power supply voltage (when S1 is closed). When S1 is open, Vx2 can be measured at pin 15 of the microcontroller, and then S1 can be closed. With S1 closed, Vx1 can be measured at pin 15. The voltages Vx1 and Vx2 can be measured at pin 15.

[0114] In this example, bias resistors are introduced (+0.5% of the resistance values of R3 and R7) so that Vx2 is slightly less than 2 / 3Vdc and Vx1 is slightly greater than 1 / 3Vdc to further enhance noise tolerance. As a result, in all situations considering the injected noise and component tolerance, Vx2 - Vx1 < Vdc / 3. The calculation formula processed by the microcontroller may only be valid in that case. When the bias resistor has a high resistance value, the estimated insulation resistance (on the safe side) is low, and it can be ignored in the case of poor insulation. The result of the resistor voltage division R5 / R6 forms a voltage reference (18) for AD conversion and can be made lower than the Vcc (power supply) of the microcontroller.

[0115] When a fixed voltage reference (e.g., 5V) is used as Vref for AD conversion, the 12 bits (4096 are referenced) of the AD converter correspond to a fixed voltage reference of 5V. The output of the AD converter needs to be converted to voltage units. Voltage unit (millivolt) = 4096 * ADresult / 5000.

[0116] When using the resistive divider R5 / R6 as a voltage reference, the ADC measurement is 2 / 3 or 1 / 3 of Vdc and this measurement is considered as Vdc. There is no need to convert to voltage units. The resistive divider R5 / R6 as a voltage reference has the additional advantage that the ADC resolution is reduced. For example, Vref=2V: 12 bits (4096) of the ADC corresponds to 2V: 1 bit: resolution=2000 / 4096=0.488mV / bit (resolution=5000 / 4096=1.22mV / bit for a fixed voltage of 5V).

[0117] Vref can be determined based on the DC voltage. Optionally, a voltage divider is used to determine Vref. Vref can be proportional to the DC power supply voltage. It will be understood that the voltage supply network of the microcontroller is not depicted. Vref can be a separate input that can be used to measure the DC power supply voltage.

[0118] For the Vx2 and Vx1 measurements a filter (low pass filter) can be added. The cut-off frequency of this filter must be at least 10 times the measurement frequency (60ms and 40ms). Low frequency filtering is not a solution as it increases the measurement time, which is unacceptable.

[0119] In one example, the device switches between states S1 "on" and S1 "off" at regular intervals, for example with a frequency of 10 Hz. Since a steady-state value is calculated (to provide a prediction), it is no longer necessary to perform a measurement after a certain delay time (for example 50 ms) after switching S1 in order to generate a stable voltage at the measurement location 16.

[0120] Optionally, an alarm is triggered if an insulation error is detected in one or more cycles. In one example, multiple measurements are taken and an alarm is triggered if an insulation error is detected in successive cycles. At each cycle, a measurement of the insulation resistance can be obtained. Although a single cycle is sufficient, repeated cycling can be used for increased reliability and the change in insulation resistance over time can also be monitored.

[0121] It will be appreciated that the protective earth may relate to a connection to a protective enclosure, such as a vehicle chassis, or to an actual earth, depending on the application.

[0122] It will be appreciated that in some applications knowledge of the DC power supply voltage is desired. In such cases, electrical measurements can be made using the reference voltage Vref as described above, and the DC power supply voltage can be determined with a fixed reference so that the DC power supply voltage, expressed in V (volts), can be determined. The present embodiment is not limited in this context.

[0123] It will be appreciated that the device can also perform other safety functions useful for dedicated applications, such as under- and over-voltage protection, temperature measurement, self-diagnostics, warning level indications, etc. This can be done by reprogramming the microcontroller. It can also be done by using a microcontroller designed to activate additional command signals for additional measurements, calculations, and / or relays, etc., so that values ​​other than the insulation resistances R1 and R2 are calculated. Multiple values ​​to be checked may be calculated by the microcontroller 7, thereby activating multiple safety measures, i.e. relays to interrupt the current, alarm signals, etc.

[0124] It will be understood that the method may include computer-implemented steps. All the steps above may be computer-implemented steps. The embodiment may include a computer device, and the process is executed on a computer device. The invention also extends to a computer program adapted to implement the invention, in particular a computer program on or in a carrier. The program may be in the form of source code or object code, or in any other form suitable for use in implementing the process according to the invention. The carrier may be any entity or device capable of carrying a program. For example, the carrier may include a storage medium such as a ROM, for example a semiconductor ROM or a hard disk. Furthermore, the carrier may be a transmissible carrier, such as an electrical or optical signal, which may be conveyed via electrical or optical cable, or by radio or other means, for example via the Internet or the cloud.

[0125] Some embodiments may be implemented, for example, using a machine or tangible computer-readable medium or article that may store instructions or sets of instructions that, when executed by the machine, cause the machine to perform methods and / or operations in accordance with the embodiments.

[0126] Various embodiments may be implemented using hardware elements, software elements, or a combination of both. Examples of hardware elements may include a processor, a microprocessor, a circuit, an application specific integrated circuit (ASIC), a programmable logic device (PLD), a digital signal processor (DSP), a field programmable gate array (FPGA), a logic gate, a register, a semiconductor device, a microchip, a chipset, etc. Examples of software include a software component, a program, an application program, a computer program, a system program, a machine program, an operating system software, a mobile app, a middleware, a firmware, a software module, a routine, a subroutine, a function, a computer implemented method, a procedure, a software interface, an application program interface (API), a method, an instruction set, a computing code, a computer code, etc.

[0127] Additionally, when a feature or element is referred to as being "connected," "attached," or "coupled" to another feature or element, it will be understood that it may be directly connected, attached, or coupled to the other feature or element, or there may be intervening features or elements present.

[0128] Terms such as "first," "second," and the like, as used herein, may distinguish one element from another by indicating different instances of similar elements. It should be understood that terms such as "first," "second," and the like, do not indicate any order, quantity, importance, sequence, whether temporally, spatially, ranking, or in any other manner.

[0129] Additionally, the terms "network," "circuit," "circuitry," and "controller" may include either a single component or multiple components, either active and / or passive, that are connected (e.g., as one or more integrated circuit chips) or otherwise coupled to provide the described functionality.

[0130] The present invention is described herein with reference to specific examples of embodiments of the present invention. However, it will be apparent that various modifications, variations, substitutions, and changes can be made without departing from the essence of the present invention. Features are described herein as part of the same or separate embodiments for clarity and conciseness. However, it is envisioned and understood that other embodiments having all or a partial combination of the features described in these separate embodiments also fall within the scope of the present invention as outlined by the claims. Thus, the specification, drawings, and examples should be regarded in an illustrative, rather than a restrictive, sense. The present invention is intended to embrace all such alternatives, modifications, and variations that are within the spirit and scope of the appended claims. Moreover, many of the described elements are functional entities that can be implemented as separate or distributed components, or in combination with other components, in any suitable combination and location.

[0131] In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of other features or steps than those listed in a claim. Moreover, the words "a" and "an" are not limited to "only one" but are instead used to mean "at least one" and do not exclude a plurality. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage. The following is a summary of the claims as originally filed: [1] A method for monitoring insulation between a DC bus connected to a DC power source and protective earth, comprising: connecting a terminal of the power supply to protective earth by a first electrical network during a first time period; measuring a transient first electrical value associated with the first electrical network at a plurality of time steps within the first time period, wherein a first steady-state value is calculated using a plurality of first measurement points at the plurality of first time steps; connecting a terminal of the power supply to protective earth by a second electrical network during a second time period; measuring a transient second electrical value associated with the second electrical network at a plurality of second time steps within the second time period, where a second steady-state value is calculated using a plurality of second measurement points at the plurality of second time steps; and determining a measure of insulation resistance between the terminals of the power source and protective earth based on the calculated steady-state values ​​of the first electrical value and the second electrical value; The method includes: [2] the first steady-state value and the second steady-state value are calculated before the first electrical value and the second electrical value to be measured reach a steady state during the first time period and the second time period, respectively; The method described in [1]. [3] The first period and / or the second period are less than three times a resistor-capacitor (RC) time constant. The method according to [1] or [2]. [4] The first period and / or the second period are less than 100 ms. A method according to any one of claims 1 to 3. [5] The first steady-state value is calculated by a first curve fitting using the plurality of measurement points at the plurality of first time steps; the second steady-state value is calculated by a second curve fitting using the plurality of measurement points at the plurality of second time steps; A method according to any one of claims 1 to 4. [6] The time interval between successive time steps is selected based on the power supply period. A method according to any one of claims 1 to 5. [7] The time interval is selected as a multiple of the power supply period, and is preferably equal to one power supply period. The method described in [6]. [8] Three measurement points are used for each of the first curve fitting and the second curve fitting. 1. A method according to any one of claims 1 to 7. [9] The first electrical value and the second electrical value are measured at the same measurement location relative to the same reference; the excitation voltage is normalized to a DC power supply voltage provided by the DC power supply; 1. A method according to any one of claims 1 to 8.

[10] During the first time period, the protective earth is excited with a block voltage that is a first percentage of the DC power supply voltage; during the second time period, the protective earth is energized with a blocking voltage that is a second percentage of the DC power supply voltage; the first ratio is different from the second ratio; 1. A method according to any one of claims 1 to 9.

[11] One or more bias resistors are used to increase the first rate and decrease the second rate; the one or more bias resistors are configured to provide a bias of 0.1-2%, more preferably 0.5-1%; The method described in

[10] .

[12] A filter is employed to measure the first electrical value and the second electrical value, the filter being a low pass filter. 1. A method according to any one of claims 1 to 11.

[13] The steady-state value is calculated using curve fitting only when the DC power source is coupled to an AC network. 1. A method according to any one of claims 1 to 12.

[14] A switch is provided that is switchable between a first state and a second state; In the first state, the switch connects the terminal of the power source to the first electrical network; In the second state, the switch connects the terminal of the power source to the second electrical network; the first period and the second period are equal; the sequence of the first period and the second period is repeated at a fixed frequency; the measuring, calculating, and comparing are repeated at the same said frequency; 1. A method according to any one of claims 1 to 13.

[15] A device for monitoring insulation between a DC bus connected to a DC power source and protective earth, comprising: connecting a terminal of the power supply to a protective earth by a first electrical network during a first time period; measuring a transient first electrical value associated with the first electrical network at a plurality of time steps within the first time period, and a first steady-state value is calculated using a plurality of first measurement points at the plurality of first time steps; connecting a terminal of the power supply to protective earth by a second electrical network during a second time period; measuring a transient second electrical value associated with the second electrical network at a plurality of second time steps within the second time period, and a second steady-state value is calculated using a plurality of second measurement points at the plurality of second time steps; determining a measure of insulation resistance between the terminals of the power source and protective earth based on the calculated steady-state values ​​of the first electrical value and the second electrical value. An apparatus configured as follows.

Claims

1. 1. A method for monitoring insulation between a DC bus connected to a DC power source and a protective earth, the method comprising the steps of: connecting a terminal of the power supply to a protective earth by a first electrical network for a first time period; measuring a transient first electrical value associated with the first electrical network at a plurality of time steps within the first time period, where a first steady-state value is calculated using a plurality of first measurement points at a plurality of time steps within the first time period; connecting a terminal of the power supply to protective earth by a second electrical network during a second time period; measuring a transient second electrical value associated with the second electrical network at a plurality of second time steps within the second time period, where a second steady-state value is calculated using a plurality of second measurement points at a plurality of time steps within the second time period; determining a measure of insulation resistance between the terminals of the power source and protective earth based on the calculated first and second steady-state values ​​for the first and second electrical values; Equipped with 11. A method according to claim 1, wherein connecting the terminal of the power supply to protective earth by the first electrical circuitry during the first time period and connecting the terminal of the power supply to protective earth by the second electrical circuitry during the second time period are performed by switching a single switch between a first state and a second state.

2. 2. The method of claim 1 , wherein the first steady-state value and the second steady-state value are calculated before the first and second measured electrical values ​​reach a steady state during the first and second time periods, respectively.

3. 3. The method of claim 1, wherein the first period and / or the second period is less than three times a resistor-capacitor (RC) time constant of the first electrical circuitry and / or the second electrical circuitry.

4. The method of claim 3 , wherein the first period and / or the second period are selected taking into account an estimate of the RC time constant.

5. The method of claim 3 or 4, wherein the RC time constant is determined based on measurement points.

6. The method according to claim 1 , wherein the first period and / or the second period is less than 100 ms.

7. the first steady-state value is calculated by a first curve fitting using the first measurement points at a plurality of time steps within the first time period; 7. The method of claim 1, wherein the second steady-state value is calculated by a second curve fitting using the second measurement points at multiple time steps within the second time period.

8. The method of claim 1 , wherein the time interval between successive time steps is selected based on a power supply period.

9. The method of claim 8 , wherein the time interval is selected as a multiple of the power supply period.

10. The method of claim 7 , wherein three measurement points are used for each of the first curve fitting and the second curve fitting.

11. the first electrical value and the second electrical value are measured at the same measurement location relative to the same reference; 11. The method of claim 1, wherein the excitation voltage is normalized to a DC power supply voltage provided by the DC power supply.

12. during the first time period, the protective earth is energized with a blocking voltage that is a first percentage of the DC power supply voltage; during the second time period, the protective earth is energized with a blocking voltage that is a second percentage of the DC power supply voltage; The method of claim 11 , wherein the first rate is different from the second rate.

13. The method of claim 12 , wherein the sum of the first rate and the second rate is substantially equal to one.

14. 14. The method of claim 12 or 13, wherein the first ratio is 1 / 3 and the second ratio is 2 / 3.

15. one or more bias resistors are used to increase the first rate and decrease the second rate; The method of any one of claims 12 to 14, wherein the one or more bias resistors are configured to provide a bias of 0.1 to 2%.

16. 16. The method of claim 1, wherein a filter is employed to measure the first electrical value and the second electrical value, the filter being a low pass filter.

17. 17. The method of claim 1, wherein the first and second steady-state values ​​are calculated using curve fitting only when the DC power source is coupled to an AC network.

18. a switch is provided that is switchable between a first state and a second state; In the first state, the switch connects a terminal of the power source to the first electrical network; In the second state, the switch connects a terminal of the power source to the second electrical network; the first period and the second period are equal; the sequence of the first period and the second period is repeated at a fixed frequency; 18. The method of any one of claims 1 to 17, wherein the measuring, calculating, and comparing are repeated at the same frequency.

19. The determined insulation resistance value is compared to a predetermined alarm level; 19. The method of any one of claims 1 to 18, wherein an alarm action is performed if the value of the insulation resistance exceeds a predetermined level.

20. 20. The method according to any one of claims 1 to 19, comprising means for transmitting a signal to a load connected to said DC power source in the event of an insulation fault, and communicating said insulation fault to a remote control station.

21. 21. The method of claim 1, wherein the transient first electrical value and the transient second electrical value are measured using ratiometric A / D conversion.

22. 22. The method of any one of claims 1 to 21, wherein the DC voltage of the DC power source is measured.

23. 1. An apparatus for monitoring insulation between a DC bus connected to a DC power source and protective earth, comprising: The apparatus comprises: connecting a terminal of the power supply to a protective earth by a first electrical network for a first time period; measuring a transient first electrical value associated with the first electrical network at a plurality of time steps within the first time period, where a first steady-state value is calculated using a plurality of first measurement points at a plurality of time steps within the first time period; connecting a terminal of the power supply to protective earth by a second electrical network during a second time period; measuring a transient second electrical value associated with the second electrical network at a plurality of second time steps within the second time period, where a second steady-state value is calculated using a plurality of second measurement points at a plurality of time steps within the second time period; determining an indication of insulation resistance between the terminals of the power source and protective earth based on the calculated first and second steady-state values ​​for the first and second electrical values; It is configured as follows: wherein connecting the terminal of the power supply to protective earth by the first electrical circuitry during the first time period and connecting the terminal of the power supply to protective earth by the second electrical circuitry during the second time period are performed by switching a single switch between a first state and a second state.

24. 24. The apparatus of claim 23, configured to perform other monitoring functions.

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