Measurement of insulation resistance of electrical insulator

Abstract

The measuring device (1) performs measurement of the insulation resistance of the electrical insulator (2) by applying a voltage to the electrical insulator (2), increasing the voltage over time, and measuring the resulting current through the electrical insulator (2). The output from a circuit (5) for measuring the current through the electrical insulator (2) is sampled by iteratively performing a step of generating a sample representative of the current measured when the voltage applied to the electrical insulator (2) increases in continuous increments. For each iteration, a threshold value for the current measured in the iteration is determined by processing at least one sample measured for the respective iteration and samples measured for a plurality of previous iterations. In response to the current measured in the iteration exceeding a determined threshold, the increasing of the voltage applied to the electrical insulator is stopped.

Description

Technical Field

[0001] The present invention generally relates to an improved operating method for a measuring device and to an improved measuring device for measuring the insulation resistance of an electrical insulator. Background Art

[0002] Measurements of the resistance of electrical insulators, such as the insulation of windings of an electric motor, are often performed periodically to check the condition of the insulator, which may degrade over time. The measurement may be performed by applying an increasing voltage (usually as a step function or as a ramp increase) and measuring the resulting current flowing in the insulator. The test may be performed in accordance with IEEE 95-2002 Recommended Practice of Insulation Testing of AC Electric Machinery and as described in the static article "Pruebas Eléctricas Estáticas Recommendendadas en Sistemas de Aislamiento de Máquinas Eléctricas Rotativas paraun Mantenimiento Predictivo Eficaz" by Luis Beltran PDF | PDF (scribd.com) (Recommended Static Electrical Tests for Effective Predictive Maintenance in Rotating Electric Machine Insulation Systems). If the applied voltage is increased to the point where the insulator breaks down, the insulator may be damaged, so it is desirable to stop the test before the breakdown voltage is reached. An operator can detect that the applied voltage is approaching the breakdown voltage by observing that the characteristic of the current to voltage deviates from a smooth curve. However, this requires judgment on the part of the operator, and the shape and absolute value of the current to voltage characteristic vary greatly depending on the type of insulator and its condition. In addition, especially for insulators with high resistance, or if the measurement is performed in a noisy environment, the current value to be measured may be very small, and the measurement may be noisy, resulting in irregular characteristics, making it difficult to judge when the current to voltage characteristic deviates from a smooth curve. Limits can be set on the ratio at which the current can increase from one value of the applied voltage to the next value. For example, if the current increases by twice when the voltage increases, the test can be stopped. However, due to the high variation between typical characteristics for different insulators, this method is unreliable. It is desirable to have a measurement device that uses a measurement technique that is automatically adapted to measure different types of insulators with widely different characteristics and operates using potentially noisy measurements. Summary of the invention

[0003] According to a first aspect, there is provided a method of operating a measuring device for performing a measurement of the insulation resistance of an electrical insulator by applying a voltage to the electrical insulator, increasing the voltage over time, and measuring a resulting current through the electrical insulator for a plurality of applied voltages, the method comprising:

[0004] sampling an output from a circuit for measuring current through an electrical insulator to produce at least one sample, the at least one sample representing the current measured when a first voltage is applied to the electrical insulator;

[0005] iteratively performing the step of sampling the output from the circuit to produce one or more additional samples, the one or more additional samples representing corresponding currents measured when the voltage applied to the electrical insulator is increased in successive increments;

[0006] for each iteration, determining a threshold for a first category for at least one sample representing current measured in the iteration by processing at least one sample measured for the respective iteration and samples measured for a plurality of previous iterations; and

[0007] In response to at least one sample of measurements representing that the current exceeds the determined threshold value for the first category for the respective iteration, increasing the voltage applied to the electrical insulator is stopped.

[0008] The method allows the measuring device to be operated automatically to measure different types of insulators having widely varying characteristics, while reducing the risk of causing insulator breakdown.

[0009] In an example, the method includes determining a threshold for a first category for at least one sample representing current measured in each iteration by fitting, for each iteration, a function relating current to voltage to at least one sample measured for the respective iteration and samples measured for a plurality of previous iterations.

[0010] This method allows the threshold for stopping the measurement to be automatically adjusted according to the characteristics of the insulation being measured.

[0011] In an example, fitting includes calculating an average gradient of the function for the current iteration and the previous iteration, wherein, depending on the corresponding gradient being negative, the corresponding gradient is set to zero for the purpose of calculating the average gradient.

[0012] The method allows to reliably determine the threshold for stopping the measurement, for example for insulators with high resistance, for which the current value to be measured may be very small and the measurement may be noisy, resulting in irregular characteristics of voltage versus current.

[0013] In an example, fitting includes calculating an average intercept of the function for the current iteration and the previous iteration, wherein, depending on the corresponding intercept being negative, the corresponding intercept is set to zero for the purpose of calculating the average intercept.

[0014] This method allows to reliably determine the threshold for stopping the measurement in situations where the measurement may be noisy, resulting in irregular characteristics of voltage versus current.

[0015] In an example, the method comprises calculating a threshold value for a first category for at least one sample representing current measured in an iteration by determining a value of a function for voltage values ​​applied for the iteration in question and multiplying the value of the determined function by a first predetermined factor.

[0016] This allows the threshold to be determined reliably and automatically using predetermined factors applicable to a variety of different insulation types.

[0017] In an example, the method includes calculating a threshold for a first category for at least one sample representing a current measured in an iteration by multiplying a value of the determined function by a predetermined factor and adding a factor related to a noise level in the measurement data.

[0018] In an example, a factor related to the noise level in the measurement data is generated by taking the standard deviation or variance of the difference between the measurement data and a function in which newer data is de-weighted compared to older data.

[0019] This approach allows a reliable estimate of the noise that is not unduly influenced by the effects of increased current due to approaching the breakdown voltage.

[0020] In an example, the method includes generating an electronic display showing measurement samples of current as a function of applied voltage and a function representing a threshold value of the first category as a function of applied voltage.

[0021] This approach allows an intuitive display to be provided to the user showing the progress of the test and indicating whether the test is approaching the breakdown voltage.

[0022] In an example, the method includes determining a threshold for the second category for each iteration by processing at least one sample measured for the corresponding iteration and samples measured for a plurality of previous iterations; and

[0023] In response to at least one sample of the measurement representing that the current exceeds the determined threshold value for the second category for the respective iteration, a second electronic signal is generated indicating that a risk of an insulation fault exists.

[0024] This method allows automatic warning of the risk of insulation failure before stopping the measurement.

[0025] In an example, the method includes generating an electronic display showing measurement samples of the current as a function of the applied voltage and the threshold value of the second category as a function of the applied voltage.

[0026] This approach allows an intuitive display to be provided to the user showing the progress of the test and indicating how close the test is to the breakdown voltage.

[0027] In an example, the method includes:

[0028] determining a threshold for a third category for each iteration by processing at least one sample measured for the corresponding iteration and samples measured for a plurality of previous iterations; and

[0029] Responsive to at least one sample of the measurement representing a current exceeding a determined threshold for a third category of a respective iteration, a third electronic signal is generated indicating a risk of insulation failure is present, wherein the third electronic signal indicates a higher risk of insulation failure than the second electronic signal.

[0030] This approach allows a more finely graded indication to the user of the risk level of insulation breakdown.

[0031] In an example, the method includes ceasing to apply voltage to the electrical insulator in response to at least one sample of measurements representing that the current exceeds a determined threshold value for the first category for the respective iteration.

[0032] This method allows reducing the risk of insulation breakdown.

[0033] In an example, the method includes, in response to at least one sample of the measurement representing that the current exceeds a determined threshold for the first category for the respective iteration, generating an electronic signal indicating that the measurement has ceased.

[0034] The method provides an indication of the status of the measurement to the user.

[0035] According to a second aspect of the invention, there is provided a measuring device for performing a measurement of the insulation resistance of an electrical insulator by applying a voltage to the electrical insulator, increasing the voltage over time, and measuring the resulting current through the electrical insulator for a plurality of applied voltages, the device comprising:

[0036] A circuit for applying a voltage to an electrical insulator;

[0037] A circuit for measuring current through an electrical insulator; and

[0038] At least one processor is configured to cause the measuring device to perform the following steps:

[0039] sampling an output from a circuit for measuring current through an electrical insulator to produce at least one sample, the at least one sample representing the current measured when a first voltage is applied to the electrical insulator;

[0040] iteratively performing the step of sampling the output from the circuit to produce one or more additional samples, the one or more additional samples representing corresponding currents measured when the voltage applied to the electrical insulator is increased in successive increments;

[0041] for each iteration, determining a threshold for a first category for at least one sample representing current measured in the iteration by processing at least one sample measured for the respective iteration and samples measured for a plurality of previous iterations; and

[0042] In response to at least one sample of measurements representing that the current exceeds the determined threshold value for the first category for the respective iteration, increasing the voltage applied to the electrical insulator is stopped.

[0043] In an example, the at least one processor is configured to cause the measuring device to perform an example of the method as already mentioned.

[0044] In an example, the measurement apparatus includes a meter and a device external to the meter, the device including a processor and a display, wherein the meter includes circuitry for applying a voltage to an electrical insulator, circuitry for measuring a current through the electrical insulator, and circuitry for communicating with the device.

[0045] This provides a convenient display of measurement results and indications.

[0046] According to a third aspect of the present invention, there is provided a computer-readable storage medium storing instructions for causing one or more processors to cause a measuring device for performing a measurement of an insulation resistance of an electrical insulator to perform the following steps:

[0047] receiving at least one sample, the at least one sample representing a current measured in the presence of a first voltage applied to the electrical insulator;

[0048] iteratively performing the step of receiving one or more additional samples, the one or more additional samples representing corresponding currents measured when the voltage applied to the electrical insulator is increased in successive increments;

[0049] for each iteration, determining a threshold for a first category for at least one sample representing current measured in the iteration by processing at least one sample measured for the respective iteration and samples measured for a plurality of previous iterations; and

[0050] In response to at least one sample of the measurement representing that the current exceeds the determined threshold value of the first category for the respective iteration, the measuring device is caused to cease increasing the voltage applied to the electrical insulator.

[0051] Further features and advantages of the invention will become apparent from the following description of exemplary embodiments given by way of example only. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 is a schematic diagram showing a measuring device for performing a measurement of the insulation resistance of an electrical insulator;

[0053] Figure 2 is a graph showing a step increase in voltage applied to an insulator by a measuring device as a function of time, the resulting current as a function of time, and an end-of-step current measurement sampled by the measuring device;

[0054] Figure 3 is a graph showing samples of end-of-step current measurements as a function of applied voltage;

[0055] Figure 4 is a graph showing samples of measured current as a function of applied voltage and thresholds for the first, second, and third categories for a first set of measurements;

[0056] Figure 5 is a graph showing samples of measured current as a function of applied voltage and thresholds for the first, second, and third categories for a second set of measurements;

[0057] Figure 6 is a graph showing samples of measured current as a function of applied voltage and thresholds for the first, second, and third categories for a third set of measurements;

[0058] Figure 7 is a graph showing samples of measured current as a function of applied voltage and thresholds for the first, second, and third categories for a fourth set of measurements, wherein the samples exceed the threshold for the first category;

[0059] Figure 8 is a graph showing samples of measured current as a function of applied voltage and thresholds for the first, second, and third categories for a fifth set of measurements, the graph showing the fit of the function to the measured samples;

[0060] Fig. 9 A time series of measurements showing how thresholds for the first, second, and third categories are generated is shown;

[0061] Fig.10 shows the generation of noise estimates by a process involving sample weighting; and

[0062] Fig.11is a flow chart showing a method of operating the measuring device. DETAILED DESCRIPTION

[0063] By way of example, embodiments will now be described in the context of a measuring apparatus capable of performing measurements of the insulation resistance of an electrical insulator by applying a voltage to the electrical insulator, increasing the voltage over time, and measuring the resulting current through the electrical insulator for a plurality of applied voltages, wherein the test apparatus takes the form of a dedicated test set designed for this purpose, but it will be understood that the measuring apparatus may take other forms, such as a multifunction meter or other test equipment.

[0064] Figure 1 An example of a measuring device 1 is shown for performing a measurement of the insulation resistance of an electrical insulator by applying a voltage to the electrical insulator 2, increasing the voltage over time, and measuring the resulting current through the electrical insulator 2 for a number of applied voltages. Figure 1 As can be seen, the insulator under test 2 is connected to the measuring device 1 via at least two connection terminals 3, 4. The measuring device includes a circuit 6 for generating and applying a voltage to the electrical insulator 2, and a circuit for measuring the current through the electrical insulator. The insulation resistance of the insulator under test 2 is measured by applying a high DC (direct current) voltage, typically 1kV or higher, and measuring the resulting current, which is typically very small, typically in the order of microamperes (uA). The insulation resistance is typically measured in MΩ, calculated based on the applied voltage and the measured current. In a step voltage (SV) test, the voltage is applied in increasing steps, where each step is effectively a shorter DC insulation resistance test. As an alternative, a ramp voltage (RV) test is an SV test in which the steps of the voltage are very small, so that the voltage is effectively increased as a continuous ramp. During the test, the applied voltage is typically increased to a value greater than the nominal operating voltage of the insulator under test, that is, an overvoltage is applied, such as twice the nominal voltage +1kV. If the insulation has deteriorated, overvoltage testing may result in insulation breakdown, such as a so-called "spark through", and therefore failure of the entire insulation system of the insulator under test (e.g. a motor). This is undesirable because a return to base for repairs may be required before the system can be energized again. The test equipment is configured to automatically detect the risk of insulation breakdown and stop increasing the voltage, and / or stop applying the voltage and stop the test, generating an electronic signal indicating that the test has stopped. Figure 1 Two terminals are shown in the , but some examples may be provided with an additional terminal, which may be referred to as a guard terminal, provided to improve measurement accuracy. The guard terminal may be maintained at the same potential as one of the terminals, but bypasses the meter's internal ammeter. The guard terminal may be connected to the surface of the insulator being tested so that unwanted surface leakage current does not affect the test results.

[0065] Figure 1 The measuring device comprises at least one processor 8, which comprises the functionality of a controller 9. The processor may be implemented by hardware, software and / or firmware. Typically, one or more processors have a memory in which machine executable code may be stored, which causes the one or more processors to cause the measuring device to perform insulation resistance measurements and automatically detect the risk of insulation breakdown and stop increasing the voltage. The one or more processors may be located in a dedicated test set, such as Figure 1 As shown in , and / or one or more processors may be located in a device remote from the dedicated test set, such as a user device (such as a smartphone). The user device may have a data connection to the processor in the dedicated test set, which uses a radio link such as Bluetooth or WiFi. One or more processors may be implemented by a remote data processing facility (such as cloud processing).

[0066] At least one processor 8 is configured to cause the measuring device to cause the sampler 7 (usually including one or more analog-to-digital converters) to sample the output from the circuit 5 for measuring the current through the electrical insulator to generate at least one sample. The at least one sample represents the current measured when there is a first voltage applied to the electrical insulator. The first voltage can be a step voltage in a step voltage (SV) test. The processor causes the measuring device to iteratively perform the steps of sampling the output from the circuit 5 to generate one or more additional samples, and the one or more additional samples represent the corresponding current measured when the voltage applied to the electrical insulator 2 is increased in continuous increments. For each iteration, a threshold value for the first category of the current measured in the iteration is determined by processing at least one sample measured for the corresponding iteration and samples measured for a plurality of previous iterations. One or more processors, in response to at least one sample of the measurement representing that the current exceeds the threshold value determined for the first category of the corresponding iteration, cause the measuring device 1 to stop increasing the voltage applied to the electrical insulator. This automatically protects the insulator from breakdown without requiring user intervention judgment. Thresholds are generated by processing at least one sample for each iteration and also for samples measured for previous iterations, allowing the thresholds to be adaptively adjusted to the characteristics of the insulator being tested as the test progresses. Different types of insulators can vary greatly in the range of measured currents and the slope and shape of the current vs. voltage characteristics.

[0067] The first category of thresholds is a threshold for stopping the increase in applied voltage and / or stopping the test, and crossing or reaching the threshold can cause one or more processors to generate an electronic signal that causes a display 10 in the measuring device 1 to indicate that the test has stopped, for example indicating "failure". The display can be located in a dedicated test device and / or a remote device such as a user device.

[0068] Other thresholds indicating the degree of risk of insulation breakdown may also be determined for each iteration, the other thresholds also being generated by processing at least one sample measured for the respective iteration and samples measured for a plurality of previous iterations. For example, the threshold may be set to generate an electronic signal causing the display 10 in the measuring device 1 to indicate "attention", "warning", or "failure".

[0069] In the described examples, the samples representing the current directly represent the measured current, for example in microamperes as the unit of measurement. However, it should be understood that the samples representing the current can be represented by the insulation resistance directly related to the current. Therefore, throughout the description, references to the current and the measured current can be replaced by the measurement of the resistance derived from the measurement of the current. Therefore, each threshold value can be expressed as a threshold value for measuring the resistance, which is equivalent to the threshold value for the current.

[0070] Figure 2 is a graph showing a step increase in voltage applied to an insulator by a measuring device 1 as a function of time. The voltage is shown as line 11 and the voltage values ​​are indicated by the scale on the left axis. The resulting current 12 is shown as a function of time and the current values ​​are indicated by the scale on the right axis. In this example, it can be seen that the form of the current with respect to time has an approximate sawtooth shape, where the current increases rapidly at the beginning of each step of increasing voltage and then decays during the step. A current sample is taken for each step, in this example towards the end of the step. Samples of the current 13a to 13g sampled by the measuring device for each step are shown. The characteristics of the current with respect to time for the samples are shown as follows Figure 2 As shown in line 14 in FIG.

[0071] like Figure 2 As can be seen, the applied voltage is increased stepwise. Each step is maintained for about 60 seconds, and then the voltage is quickly ramped up to the next step. After the voltage step is applied, the current increases rapidly immediately due to the capacitance and absorption current, and then decays. If the step is of infinite length, the current will stabilize at a fixed leakage value for that step. In practice, the current is measured as a current sample just before a given step is completed.

[0072] Figure 3 shows the voltage as a function of the applied voltage Figure 2 Samples of currents 13a to 13g are shown, and additional samples of currents 13h to 13k are also shown.

[0073] Figures 4 to 7 is a graph showing a sample of measured current as a function of applied voltage for a corresponding set of measurements on different insulators.

[0074] exist Figure 4, thresholds for the first category 17 (failure), the second category 19 (caution), and the third category 18 (warning) are shown for the last sample shown. The thresholds are determined by processing the last sample 15 as well as the previous sample 16. Each threshold may be determined by fitting, for each iteration, a function relating current to voltage to at least one sample measured for the respective iteration and to samples measured for the previous iteration. Figure 4 In the data in , the last sample acquired did not pass any thresholds, so the measurement can be indicated as a "pass" for that voltage step.

[0075] exist Figure 5 , for the last sample shown, the thresholds for the first category 25 (failure), the second category 27 (caution), and the third category 26 (warning) are shown. The thresholds are determined by processing the last sample 23 as well as the previous sample 24. Figure 5 , the last sample collected passed the threshold of the second category 27, so the display 10 can indicate a "caution" for this voltage step.

[0076] exist Figure 6 , for the last sample shown, the thresholds for the first category 33 (failure), the second category 35 (caution), and the third category 34 (warning) are shown. The thresholds are determined by processing the last sample 32 as well as the previous sample 31. Figure 6 , the last sample collected passed the threshold of the third category 34, so the display 10 can indicate a "warning" for this voltage step.

[0077] exist Figure 7 , for the third last sample 39 shown, the thresholds for the first category 41 (failure), the second category 43 (caution), and the third category 42 (warning) are shown. The thresholds are determined by processing the sample 39 as well as the previous sample 40. Figure 7 , the third to last sample 39 collected passes the threshold of the first category 41, so the display 10 can indicate a "failure" for this voltage step. Figure 7 In the example of , the voltage is shown to continue to increase after the third to last sample 39 that exceeds the "failure" threshold. For measurement purposes for this graph, the voltage continues to increase to show the effect of increasing voltage on current. However, in an actual implementation of the measurement device 1, the increase in voltage will stop when the current sample exceeds the threshold of the first category ("failure").

[0078] exist Figures 4 to 7In each of the Figures, a line of a function associating each threshold class with the voltage for the last iteration of the threshold calculation is shown. A function associating the thresholds of the first class with voltage 20, voltage 28, voltage 36, voltage 44, a function associating the thresholds of the second class with voltage 22, voltage 30, voltage 38, voltage 46, and a function associating the thresholds of the third class with voltage 21, voltage 29, voltage 37, voltage 45 are shown. If any previous sample exceeds the threshold for that voltage indicated by the shown function (recalculated for each iteration), then the corresponding threshold can be considered to have been crossed.

[0079] In an example, processing the samples to generate the corresponding thresholds includes: curve fitting by linear regression, statistical evaluation of noise by calculating the standard deviation of the samples, generating a running average for each iteration of the slope and intercept of the fitted curve (which may be a straight line), suppression of negative slopes and intercepts, and analysis of each data point (i.e., sample) relative to the calculated limits. In an example, the curve fitting may be performed by nonlinear regression.

[0080] There are several coefficients in the algorithm that determine the corresponding thresholds, which, once predetermined, can be kept constant to operate the measurement device on widely different motors under test. The method is intended to be applied to testing rotating machines, but can also be applied to any system requiring SV or RV testing.

[0081] The algorithm performs calculations after each step (or measurement point) or as data becomes available. This is an on-the-fly process where the last point of the data is evaluated relative to all the points acquired so far, and the algorithm will therefore adapt itself to any data collected. The evaluation is performed according to the constraints calculated on the real data. The constraints are re-evaluated at each step so that changing features of the curve, such as the pre-polarization effects mentioned above, can be taken into account.

[0082] More than one limit can be generated, so instead of just having a pass / fail assessment, a more graduated assessment can be made, such as: pass, caution, warning, fail, and any other number of such limits, as well as numbers or percentages, e.g., 90% probability of failure.

[0083] Figures 4 to 7 An example of a graphical display is shown in . The example shows 3 limits, which result in 4 ranges. This allows a more refined diagnosis of the insulation condition. "Pass" is for all points below the threshold of the second category.

[0084] A motor diagnosed with a “Caution” will most likely continue to operate for some time, but more regular diagnostics may need to be scheduled to detect possible further degradation. A motor with a “Warning” can continue to operate, but an immediate overhaul should be planned. A “Fail” requires immediate disassembly due to the risk of catastrophic failure during operation. A “Pass” allows further uninterrupted operation. The method gives a defined way of how to prevent insulation damage, quantified by the test data itself, without the need for previous iterations from the user to set the limits, thus avoiding operator error.

[0085] Figure 8 is a graph showing samples of measured current as a function of applied voltage and thresholds for the first, second, and third categories for a fifth set of measurements, the graph showing the fit of the function to the measured samples. Fig. 9 A time series of measurements a) to e) showing how the thresholds for the first, second and third categories are generated is shown.

[0086] Figure 8 A series of samples of current 48 plotted as a function of applied voltage is shown. The last sample 47 acquired for the iteration shown is shown. Thresholds for the first category 50, the second category 52, and the third category 51 are shown for the sample 47 acquired in the iteration shown. In addition, lines 55, 57, and 56 for the corresponding thresholds as a function of voltage are shown. A function 49 is also shown, which relates the current to the voltage to at least one sample measured for the corresponding iteration and to samples measured for a plurality of previous iterations. Function 49 may be referred to as a baseline or baseline function. In an example, the baseline function may be a nonlinear curve, which may be derived by nonlinear regression.

[0087] The corresponding threshold value for the current measured in each iteration is determined by the following process: for each iteration, a baseline function 49 relating current to voltage is fitted to at least one sample measured for the corresponding iteration and to samples measured for a plurality of previous iterations. The fitting includes calculating the average gradient of the baseline function for the current iteration and the previous iterations, wherein, depending on the corresponding gradient being negative, the corresponding gradient is set to zero for the purpose of calculating the average gradient. The fitting also includes calculating the average intercept of the function for the current iteration and the previous iterations, wherein, depending on the corresponding intercept being negative, the corresponding intercept is set to zero for the purpose of calculating the average intercept. As previously described, the samples representing the current can be represented by insulation resistance, which is directly related to the current, i.e., the applied voltage divided by the current. If the baseline function is represented in terms of resistance values ​​as a function of the applied voltage rather than current as a function of the applied voltage, the calculation of the baseline can be performed by applying an equivalence rule that will produce a threshold value in terms of resistance that is equivalent to the threshold value produced in terms of current.

[0088] The respective threshold value for the current measured in the iteration is calculated by determining the value of a function for the voltage value applied for the iteration in question and multiplying the value of the determined function by a first predetermined factor. The respective threshold value for the current measured in the iteration is calculated by multiplying the value of the determined function by a predetermined factor and adding a factor related to the noise level in the measured data. The factor related to the noise level in the measured data is generated by taking the standard deviation of the difference between the measured data and the function, wherein newer data is de-weighted compared to older data. Line 53 shows the fit of the sample 48 by linear regression and line 54 shows the corrected regression line with the negative intercept removed. A running average of the corrected regression line for each iteration is used to calculate the baseline function 49, from which the respective threshold value for the iteration is calculated. This is combined with the value calculated by Fig. 9 a) to Fig. 9 e) to explain the time series shown.

[0089] Fig. 9 a), Fig. 9 b) Fig. 9 c), Fig. 9 d) and Fig. 9 e) shows a time series of measurements showing an example of steps of how to generate thresholds for the first, second and third categories.

[0090] There are several coefficient factors to be determined in advance, which can be called as follows:

[0091] Noise-Noise Multiplier

[0092] Badness - multiplier for badness limit

[0093] Suspicious - multiplier for suspicious limit

[0094] Good - multiplier for the good limit

[0095] like Fig. 9 As shown in a), when the first sample 58a is acquired, a baseline 59a is set from the origin (V=0, I=0) to this point. The slope and intercept for this line (y=slope×x+intercept) are derived so that the intercept=0V and the slope=I / V at the measurement point. In this step, the noise is assumed to be zero. For each iteration, that is, for each step in the time series, for example, Fig. 9 a), Fig. 9 b) Fig. 9 c) and Fig. 9 As shown in e), three thresholds are generated by multiplying the equation for the baseline represented by the slope of the baseline ("slope_mean") and the intercept of the baseline ("intercept_mean") by the factors listed above, namely:

[0096] Good[] = (voltage[] × slope_average + intercept_average) × good + standard deviation × noise Suspicious[] = (voltage[] × slope_average + intercept_average) × suspicious + standard deviation × noise Bad[] = (voltage[] × slope_average + intercept_average) × bad + standard deviation × noise

[0097] like Fig. 9 As shown in b), for the samples, a linear regression 60b can be calculated for the first sample and the second sample 58b. If the slope or intercept is negative, it must be "corrected" to suppress negative values. This is because negative values ​​indicate non-physical behavior. Fig. 9 In the case shown in b), the intercept is positive and the slope is negative. Therefore, the slope is forced to zero and the intercept is calculated as the average of the two points taken so far. This produces the solid grey horizontal line 61b.

[0098] The new baseline 59b is calculated so that there is a running average of the slope values ​​and the intercept values ​​from this 59b and from all previous iterations 59a. Thus, the new baseline has a shallower slope than the previous iterations.

[0099] Fig.10 FIG. 4 shows how the noise measurement used to generate the threshold is calculated in an example. Fig.10 As shown above, the baseline is subtracted from the sample to produce the difference (diff). Fig.10 In addition, the corresponding weight set is generated so that in the example, the weight set starts at 1 for the first point and ends at near zero or zero for the last available point. Fig.10 The difference is multiplied by a weight, effectively suppressing the large difference for the last point. This ensures that the true measurement noise, which may be similar throughout the test for the initial point, is taken into account, but if the current starts to accelerate to a high value, typically for the last point, it will not be affected by the large difference. The weighted difference data is Fig.10 64. A standard deviation (stdev value) is calculated for the weighted difference data 64. The standard deviation is a measure of the noise level. In an alternative example, the variance may be used as a measure of the noise.

[0100] Back to Fig. 9 a) to Fig. 9 e) sequence, in this example, using a combination of Fig.10 The measure of standard deviation stdev calculated as described yields three limits for each iteration, "good", "bad" and "questionable", ie the thresholds for the second, first and third categories respectively.

[0101] like Fig. 9As shown in b), since stdev is significant in this case, the limit of "good" (which would indicate a passed "caution") is better than for Fig. 9 The first point shown in a) is further away from the baseline.

[0102] For each iteration, once the limits, i.e. thresholds, have been defined, the last point, i.e. the sample for the current iteration, and in the example any previous sample, can now be checked against those limits in order to produce a diagnosis of the insulation state. If the current sample is below the threshold for the second category, this indicates a pass, between the thresholds for the second and third categories indicates a caution, between the thresholds for the third category and the first category indicates a warning, and above the threshold for the first category indicates a fail.

[0103] If the result is pass / caution / warning, the test will proceed to the next point, where the processor will request another step of the test from the hardware, namely increasing the voltage. Once a "failure" is detected, the processor can stop the hardware from applying the increased voltage and risk permanent damage to the insulation. In an example, the automatic stop function can be enabled or disabled by the operator.

[0104] For the 3rd sample and all following samples, Fig. 9 a) to Fig. 9 As shown in e), the same algorithm is repeated: fit a curve to all previous data by linear regression, correct slope and intercept if necessary, calculate running averages of slope and intercept to produce a base, calculate the difference between the new base and the true data, apply weights to the difference data, calculate stdev of the difference between the base and the true data, calculate limits based on running averages and stdev of slope and intercept, checkpoint against limits to give a diagnostic. This sequence is repeated for each step.

[0105] Fig.11 is a flow chart showing a method of operation of the measuring device according to steps S10.1 to S10.5.

[0106] As already mentioned, the measuring device may include one or more processors for causing the device to perform the method as described. The one or more processors may include, or communicate with, a computer-readable storage medium, such as a memory chip or other data storage device. At least one of the one or more processors may be located within a user device (such as a smartphone) connected to a meter that includes a circuit for measuring current. The computer-readable storage medium may store instructions for causing the one or more processors to cause the measuring device to perform the following steps: receiving at least one sample, the at least one sample representing a current measured when there is a first voltage applied to the electrical insulator; and iteratively performing the step of receiving one or more additional samples, the one or more additional samples representing a corresponding current measured when the voltage applied to the electrical insulator is increased in successive increments. The samples may be received by the user device from the meter. For each iteration, the instructions may cause the measuring device to perform the following steps: determining a threshold for a first category of at least one sample representing a current measured in the iteration by processing at least one sample measured for the respective iteration and samples measured for a plurality of previous iterations; and in response to at least one sample of the measurement representing a current exceeding the determined threshold for the first category for the respective iteration, causing the measuring device to stop increasing the voltage applied to the electrical insulator. The instructions may cause one or more processors to cause the measuring device to perform the following steps: sampling the output from the circuit for measuring the electrical parameter for the electrical insulator to produce at least one sample and send the sample to at least one of the one or more processors; and sampling the output from the circuit to produce one or more additional samples, the one or more additional samples representing the corresponding current measured when the voltage applied to the electrical insulator is increased in successive increments, and sending the one or more additional samples to at least one of the one or more processors. The samples may be sent via a wired connection, or via a radio link such as Bluetooth or WiFi, or by any other method. Part of the instructions may be executed by a processor at the meter, and part of the instructions may be executed by a processor at the user device.

[0107] The above embodiments are to be understood as illustrative examples of the present invention. It should be understood that any feature described in connection with any embodiment may be used alone or in combination with other features described, and may also be used in combination with one or more features of any other embodiment, or in any combination of any other embodiment. In addition, equivalents and modifications not described above may also be employed without departing from the scope of the present invention as defined by the appended claims.

Claims

1. A method of operating a measuring device for performing a measurement of the insulation resistance of an electrical insulator by applying a voltage to the electrical insulator, increasing the voltage over time, and measuring the resulting current through the electrical insulator for a plurality of applied voltages, the method comprising: sampling an output from a circuit for measuring current through the electrical insulator to produce at least one sample representing the current measured in the presence of a first voltage applied to the electrical insulator; iteratively performing the step of sampling an output from the circuit to produce one or more further samples, the one or more further samples representing respective currents measured when the voltage applied to the electrical insulator is increased in successive increments; for each iteration, determining a threshold for a first category for the at least one sample representing current measured in the iteration by processing at least one sample measured for the respective iteration and samples measured for a plurality of previous iterations; and In response to the at least one sample indicating that the current measured in the iteration exceeds a determined threshold of the first category for the respective iteration, increasing the voltage applied to the electrical insulator is stopped.

2. The method of claim 1 , comprising determining the threshold for the first category for the at least one sample representing the current measured in each iteration by fitting, for each iteration, a function relating current to voltage to the at least one sample measured for the respective iteration and to samples measured for a plurality of previous iterations.

3. The method according to claim 2, wherein: The fitting comprises calculating an average gradient of the function for the current iteration and the previous iteration, wherein, in dependence on the respective gradient being negative, the respective gradient is set to zero for the purpose of calculating the average gradient.

4. The method according to claim 3, wherein: The fitting comprises calculating an average intercept of the function for the current iteration and the previous iteration, wherein, in dependence on a respective intercept being negative, the respective intercept is set to zero for the purpose of calculating the average intercept.

5. A method according to any one of claims 2 to 4, comprising calculating the threshold value of the first category for the at least one sample representing the current measured in an iteration by determining the value of a function of the voltage value applied for the iteration in question and multiplying the value of the determined function by a first predetermined factor.

6. The method of claim 5, comprising calculating the threshold for the first category for the at least one sample representing the current measured in the iteration by multiplying the value of the determined function by the predetermined factor and adding a factor related to the noise level in the measurement data.

7. The method according to claim 6, wherein: The factor related to the noise level in the measurement data is generated by taking the standard deviation or variance of the difference between the measurement data and the function, wherein newer data is de-weighted compared to older data.

8. A method according to any preceding claim, comprising generating an electronic display showing measured samples of current as a function of applied voltage and a function representing the threshold value of the first category as a function of applied voltage.

9. A method according to any one of the preceding claims, comprising: determining a threshold for a second category for each iteration by processing at least one sample measured for the respective iteration and samples measured for a plurality of previous iterations; and In response to at least one sample of measurements representing a current exceeding a determined threshold for the second category for the respective iteration, a second electronic signal is generated indicating that a risk of insulation failure exists.

10. The method of claim 9, comprising generating an electronic display showing measured samples of current as a function of applied voltage and the threshold value for the second category as a function of applied voltage.

11. The method according to claim 9 or claim 10, comprising: determining a threshold for a third category for each iteration by processing at least one sample measured for the respective iteration and samples measured for a plurality of previous iterations; and Responsive to at least one sample of measurements representing current exceeding a determined threshold for the third category for the respective iteration, generating a third electronic signal indicating the presence of a risk of insulation failure, wherein the third electronic signal indicates a higher risk of insulation failure than the second electronic signal.

12. A method according to any preceding claim, comprising ceasing to apply voltage to the electrical insulator in response to at least one sample of measurements representing a current exceeding a determined threshold of the first category for the respective iteration.

13. The method of claim 12, comprising, in response to at least one sample of measurement representing a current exceeding a determined threshold for the first category for the respective iteration, generating an electronic signal indicating that the measurement has ceased.

14. A measuring device for performing a measurement of the insulation resistance of an electrical insulator by applying a voltage to the electrical insulator, increasing the voltage over time, and measuring the resulting current through the electrical insulator for a plurality of applied voltages, the measuring device comprising: a circuit for applying a voltage to the electrical insulator; a circuit for measuring current through the electrical insulator; as well as At least one processor is configured to cause the measuring device to perform the following steps: sampling an output from a circuit for measuring current through the electrical insulator to produce at least one sample representing the current measured in the presence of a first voltage applied to the electrical insulator; iteratively performing the steps of sampling the output from the circuit to produce one or more additional samples representing respective currents measured when the voltage applied to the electrical insulator is increased in successive increments; for each iteration, determining a threshold value for a first category for the at least one sample representing current measured in the iteration by processing at least one sample measured for the respective iteration and samples measured for a plurality of previous iterations; and In response to at least one sample of measurements representing a current exceeding a determined threshold for the first category for the respective iteration, increasing the voltage applied to the electrical insulator is stopped.

15. The measuring device according to claim 14, wherein: The at least one processor is configured to cause the measuring device to perform the method of any one of claims 2 to 13 .

16. The measuring device according to claim 15, wherein: The measuring device includes a meter and a device external to the meter, the device including a processor and a display, wherein the meter includes the circuit for applying a voltage to the electrical insulator, the circuit for measuring a current through the electrical insulator, and the circuit for communicating with the device.

17. A computer-readable storage medium storing instructions for causing one or more processors to cause a measurement device for performing a measurement of an insulation resistance of an electrical insulator to perform the following steps: receiving at least one sample, the at least one sample representing a current measured in the presence of a first voltage applied to the electrical insulator; iteratively performing the step of receiving one or more additional samples, the one or more additional samples representing corresponding currents measured when the voltage applied to the electrical insulator is increased in successive increments; for each iteration, determining a threshold for a first category for the at least one sample representing current measured in the iteration by processing at least one sample measured for the respective iteration and samples measured for a plurality of previous iterations; and In response to at least one sample of measurements representing a current exceeding a determined threshold of the first category for the respective iteration, the measuring device is caused to cease increasing the voltage applied to the electrical insulator.

18. The computer-readable storage medium of claim 17, wherein the computer-readable storage medium stores instructions for causing the one or more processors to cause the measurement device to perform the following steps: sampling an output from a circuit for measuring an electrical parameter of the electrical insulator to produce at least one sample and sending the sample to at least one of the one or more processors; and The output from the circuit is sampled to produce one or more additional samples representing corresponding currents measured when the voltage applied to the electrical insulator is increased in successive increments, and the one or more additional samples are sent to at least one of the one or more processors.

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