Insulation resistance tester is also called megohmmeter, suitable for the maintenance, repair, test and verification of electrical equipment for insulation testing. The following Wuhan Xinshengneng insulation resistance tester on the common problems and q&A summary, for your reference.
Q1: When measuring capacitive load resistance, what is the relationship between the output short-circuit current of insulation resistance tester and the measured data? What is the reason?
A1: insulation resistance tester output short circuit current can reflect the size of the instrument's internal output high voltage source resistance. When the tested product has capacitance, at the beginning of the test process, the high voltage source in the insulation resistance tester should charge the capacitor through its internal resistance, and gradually charge the voltage to the rated output high voltage value of the insulation resistance tester. Obviously, if the capacitance value of the tested product is very large, or the internal resistance of the high voltage source is very large, the time of this charging process will be longer. Its length can be determined by the product of loads in R and C (in seconds). Note that the current charged to the capacitor flows into the insulation resistance tester together with the current flowing through the insulation resistance of the tested product during the test. The current measured by the insulation resistance tester not only has the component of the insulation resistance, but also adds the capacitance charging current component, and then the measured resistance value will be small.
For example, the rated voltage is 5000V insulation resistance tester, if its short-circuit output current is 80μA, its internal resistance is 5000V/80μA = 62M ω
For example, the capacitance is 0.15μF, the time constant τ=62M ω ×0.15μF≈9(s), that is, at 18 seconds, the charging current is still 11.3μA.
Thus, the equivalent resistance formed by the charging current alone is 5000V/11.3μA = 442M ω. If the normal insulation is 1000M ω, the measured insulation value displayed is only 306M ω. This test value can not reflect the true condition of the insulation value, and the test value is mainly changed with the change of capacitive load capacity, that is, the capacity is small, the test resistance is large; Large capacity, small test resistance.
Therefore, in order to ensure the accurate measurement of R15s and R60s test values, a high-capacity insulation resistance tester with fast charging speed should be selected. China's relevant regulations require that the output short-circuit current of insulation resistance tester should be greater than 0.5mA, 1 mA, 2 mA, 5mA, the occasion of high requirements should try to choose the insulation resistance tester with larger output short-circuit current.
Q2: When measuring insulation, it is required not only to measure the pure resistance value, but also to measure the absorption ratio and polarization index. Why?
A2: In the insulation test, the insulation resistance value at a certain moment cannot fully reflect the advantages and disadvantages of the insulation performance of the test product. This is due to the following two reasons. On the one hand, the insulation resistance of the same performance of the insulating material is small when the volume is large, and large when the volume is small.
On the other hand, there are charge absorption ratio and polarization processes in insulating materials with high voltage. Therefore, in the insulation test of the main transformer, cable, motor and many other occasions, the absorption ratio, namely the ratio of R60s to R15s, and the polarization index, namely the ratio of R10min to R1min, should be measured in the power system, and the insulation condition can be judged by the data.
Q3: In the test environment of high pressure and high resistance, the instrument is required to be connected to the "G" end. Why?
A3: on both ends of tested products with higher rated voltage, and insulation resistance is high, the tested product surface moisture, pollution caused by leakage is larger, value error is big, the instrument will be "G" end test bypass surface leakage current, the leakage current does not pass the test circuit, eliminate the error caused by leakage current
Q4: When measuring the rated output DC high voltage at both ends of some insulation meters "L" and "E" at school, the pointer multimeter DCV file is used to measure the voltage at both ends of L and E, why the voltage drops a lot, but the digital multimeter does not?
A4: Use a common pointer multimeter to directly measure the output rated DC voltage at both ends of the insulation resistance tester "L" and "E", the measurement result and the nominal rated voltage value is much smaller (beyond the error range), and the digital multimeter will not. This is because the internal resistance of pointer multimeters is low, while the internal resistance of digital multimeters is relatively high. Pointer multimeter internal resistance is small, insulation resistance tester L-E end output voltage reduced a lot, is not the normal operation of the output voltage. However, it is wrong to use a multimeter directly to measure the output voltage of the insulation resistance tester. It should be measured by a high voltage meter with large internal resistance or a voltage divider and other load resistance ways that are large enough.
Q5: Can we use a megohm meter to directly measure the charged test product? What is the impact on the result? Why?
A5: For the sake of personal safety and normal test, tested in principle are not allowed to measure charged, to measure charged under test, won't cause damage to the instrument (short time), but the test result is not accurate, because when charged, the product is tested with other try link together, so the results can't reflect the real data, real It is the parallel or series resistance with other samples.
Q6: why several battery power supply can produce higher DC high voltage?
A6: This is based on the PRINCIPLE of DC transformation. The lower supply voltage is raised to the higher output DC voltage through the boost circuit. Although the high voltage is higher, the output power is smaller. (For example, a few batteries can produce tens of thousands of volts of high voltage.)
Q7: What are the factors that may cause inaccurate measurement data when measuring insulation resistance with insulation resistance tester? Why is that?
A7:
The battery voltage is insufficient. The undervoltage of the battery is too low, causing the circuit to not work properly, so the measured reading is inaccurate.
② The test line connection method is incorrect. The "L", "G", "E" three-terminal wiring is wrongly connected, or the "G", "L" wire "G", "E" wire is connected at both ends of the tested product.
③ The "G" end is not connected. The error caused by the current leakage of the tested product due to pollution and humidity and other factors causes the test to be inaccurate. In this case, the "G" end connection must be properly connected to prevent the error caused by the leakage current.
④ Too much interference. If the tested product is subjected to excessive environmental electromagnetic interference, resulting in meter reading jump. Or the pointer wobbles. Causing inaccurate readings.
⑤ Artificial reading error. When measuring with pointer insulation resistance tester, the error of visual Angle or scale scale results in inaccurate indication value.
⑥ Instrument error. The error of the instrument itself is too large, so it needs to be checked again.
Q8: High resistance insulation performance field capacitive load (such as the main transformer), the pointer shows the resistance value suddenly dropped in a certain range (not the normal test range of slow small swing), fast swing back and forth, why?
A8: This phenomenon is mainly caused by discharge ignition in a part of the test system. When the insulation meter is charged to the capacitive test product, when the capacitive test product is charged to a certain voltage, if the instrument internal test line or any part of the test product has a breakdown discharge ignition, the above phenomenon will occur. Discriminant method:
(1) The instrument test seat is not connected to the test line, turn on the power supply and high voltage, and see whether there is a spark phenomenon in the instrument (if there is a spark, you can hear the spark sound).
(2) Connect the L, G, E test line, do not connect the tested product, L test line end clip hanging, open the high voltage, see whether the test wire ignition phenomenon occurs. If there is ignition phenomenon, check: a) L and G test wire core (L end) is too close to the exposed line (G end), resulting in arc ignition. B) L end core wire plug and test seat shielding ring or test clip contact with the tested product, resulting in ignition. C) Virtual welding breaks between the test line and plug and clip, resulting in clearance discharge.
(3) Access to the tested product, check whether there is discharge ignition near the contact point between the end clamp and the test product.
(4) eliminate the above reasons, connect the tested product, open the high voltage, if the instrument still has the above phenomenon, it shows that the test product insulation breakdown caused by partial discharge or arc.
Q9: Why are there differences in the values measured by different insulation resistance testers?
A9: Due to the non-ideal voltage source of high voltage insulation resistance tester test power supply, different internal resistance Ri measurement loop series resistance Rm, different dynamic measurement accuracy, as well as the unreasonable or error of field measurement operation, there will be differences in the measurement results of different types of insulation resistance tester for the same tested product. During the actual measurement, it should be combined with the particularity of insulation test conditions of insulation resistance tester to minimize possible measurement errors:
① When different types of insulation meters measure the same sample, they should adopt the same voltage level and wiring method. For example, in measuring the insulation of the high voltage winding of the power transformer, when the winding leads out of the end is always connected with the insulation resistance tester L end button, there are: E end button connected with the low voltage winding and shell, and G end button hanging direct method; The E end button is connected to the low-voltage winding, and the G end button is connected to the shell shielding method (low potential shielding); The G end button is connected to the surface of the high voltage winding bushing, and the E end button is first connected to the low voltage winding, and then respectively connected with the shell or not connected to the two bushing shielding method (high potential shielding). E end button connected shell, and G end button connected low-voltage winding wiring method. Different structure, system insulation resistance tester, G end button potential is different, G end button on the surface of the casing should be changed.
(2) different types of insulation resistance tester range and show the value of the calibration method is different, the scale resolution is different, the measurement accuracy level is different, will cause the difference between the value. In order to ensure the accurate measurement of power equipment, low accuracy and inconvenient shaking should be avoided.
(3) Most of the samples contain capacitive components, and there is media polarization phenomenon. Even under the same test conditions, it is difficult to obtain ideal data repeatability.
④ When measuring, the temperature of the insulating medium and the oil temperature should be consistent with the ambient temperature, generally allowing a difference of ±5%.
⑤ The measured value should be read as soon as possible within the allowable time difference of a specific time period. To ensure that the measurement error is not greater than ±5%, the reading time of R60S is allowed to be ±3S, while the reading time of R15S should not differ by ±1S.
⑥ High voltage test power supply is not the ideal voltage source, heavy load (the insulation resistance of the tested product is small), the output voltage is lower than its rated value, which will lead to a single branch direct reading measurement method insulation resistance tester measurement accuracy due to the change of conversion coefficient and reduce. This change varies with the load characteristics of power supply tested by insulation resistance tester.
Different dynamic test capacity insulation resistance tester, test voltage on the test (and sampling resistance) establishment process and charging capacity of the test are different, the measurement results will be different, the use of insulation resistance tester is lower than the dynamic test capacity threshold value, Because the instrument memory in inertial network (including the pointer instrument mechanical inertia) result in slower response value, no time to correct the sample (s) reflect the insulation resistance changing with the time, especially in the initial stage of the test capacitor charging electric current was not fully attenuation is zero, more can make R15S and absorption measurements have larger error than the reader (small).
⑧ The dielectric polarization of the sample is related to the size of the applied test voltage. Due to the test voltage can not quickly reach the rated value, or because of the insulation resistance tester test power load characteristics of different lead to the difference in the test voltage applied to the test, the test initial polarization state is different, resulting in different absorption current, so that the edge resistance measurement values are different.
⑨ The test high voltage of some foreign insulation resistance tester can be adjusted continuously, from zero to the rated value after starting. The uncertainty of the initial reading time of the insulation resistance tester, as well as the uncertainty of the time when the high voltage reaches the rated value, makes the initial polarization of the sample different, and will also cause the difference between the indicated values.
⑩ Different insulation resistance testers have different sensitivity and resistance to field interference, so there will be differences in reading values for the same sample.
The difference between the indicated values is caused by the error of conventional measurement and the error of insulation resistance tester method.
The insufficiency of dielectric discharge is one of the important reasons for the difference of repeated measurement results. The sample (s) according to the absorption current charging and reverse discharging current corresponding and the characteristics of reversible, if the sample (s) should be the same for the second repeat the measurement, the sample (s) after the first measurement of short circuit discharge intermittent time should be longer than the general measurement time, to put all the absorption of accumulated charge amount, try to make product insulating medium fully restored to the original state of polarization, Otherwise, the accuracy of the second measurement data will be affected. In order to make no residual charge on the tested product, before each test, the measuring end should also be short-circuit discharge to the ground, sometimes even take nearly 1 hour, and should be removed from the connection between irrelevant equipment. In short, insulation measurement of the same sample in different periods should adopt the same test voltage level and wiring method, and use the same type of insulation resistance meter or similar performance as far as possible to ensure the comparability of measured data.
⑬ special emphasis on selection of dynamic measurement accuracy with low and high pressure low test power capacity instrument, because the capacitance charging current is not fully decay to zero, and the meter says value cannot be accurately in real time the sample (s) to follow the change of the apparent insulation resistance value, read R15S measuring low resistance, appear bigger difference, lead to try to absorb higher than the test value is false, Should cause testing personnel to pay special attention to. This may also be the main reason why there are differences in the absorption ratio of various types of hV insulation resistance tester when measuring the same sample. This shows that the absorption ratio is not as scientific and objective as the polarization index.
Understand the insulation resistance tester common questions and answers, do you have a deeper understanding of the insulation resistance tester? Xinshengneng is a professional electric power testing equipment manufacturers, a wide range of products, welcome to buy!
