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Your location: Home > Related Articles > QINSUN interprets that there are differences in the values ​​measured by different megohmmeters-

QINSUN interprets that there are differences in the values ​​measured by different megohmmeters-

Author:QINSUN Released in:2023-05 Click:22

Due to the non-ideal voltage source of the high-voltage megohmmeter test power supply, the different internal resistance Ri of the measurement circuit, the series connection resistance Rm is different, the dynamic measurement accuracy is different and the on-site measurement operation is unreasonable or wrong, etc. Different types of megohmmeters are different

There may be differences in the measurement results of the same tested product. In the actual measurement, it should be combined with the particularity of the insulation test conditions of the megohmmeter to minimize the various measurement errors that may occur:

1. When measuring the same test product with different types of insulation meters, the same voltage level and wiring must be used. For example, to measure the insulation of high voltage windings of power transformers, when the winding leads are still connected to the L-te buttonterminal of the megohmmeter, there is a direct method: the E-terminal button is connected to the low-voltage winding and the shell, and the G-terminal button is suspended; the E-terminal button is connected to the low voltage.

The winding and the button of the G terminal are connected to the shielding method of the low-potential shielding shell; the G terminal button is connected to the surface of the high voltage winding socket, and the E terminal button is first connected to the low voltage winding, then connected to the shell or not connected respectively. Housing shielding method high potential shielding,. The E-terminal button is connected to the shell, and the G-terminal button is connected to the low-voltage winding and other wiring methods. Megohmmeters with different structures and standards have different potentials of the G terminal button, and the placement position of the G terminal button on the case surface must also change accordingly.in this.

2. Different types of megohmmeters have different scaling methods for measuring the range and indication value, different scaling resolutions and different levels of measurement accuracy, which will cause the indication values ​​to differ. In order to ensure accurate measurement of electrical equipment, poor precision and inconvenient shakers should be avoided.

3. Most of the test samples contain capacitive components and there are dielectric polarization phenomena. Even if the test conditions are the same, it is difficult to obtain ideal repeatability of the data.

4. When measuring, the temperature of the insulating medium and the temperature of the oil should be consistent with the ambient temperature, and the difference is generally allowed within ±5%.

5. The measured value should be read as soon as possible within the allowable time difference range of a specific period of time. In order tothat the measurement error does not exceed ±5%, the reading time of R60S allows an error of ±3S, and the reading time of 15S should not differ by ±1S.

6. The high voltage test power supply is not an ideal voltage source. When the insulation resistance value of the heavy-duty tested product is low, the output voltage is lower than its rated value, which will lead to the measurement accuracy of the single-limb direct-reading measurement method. Decrease due to change in conversion factor. This change will vary depending on the load characteristics of the megohmmeter test supply.

7. For megohmmeters with different indicators of dynamic test capability, there are differences in the process of establishing the test voltage on the product under test and the sampling resistor, and the load capacity of the product under test, and the results measurement will also be different. When measuring the valeur dynamic test capability index threshold with a megohmmeter, due to the inertial network of the instrument including the mechanical inertia of the needle instrument, the indication value response speed is slow, and it is too late to correctly reflect the change in insulation resistance value of the teer produced over time. Especially in the initial phase of the test, the capacitor charging current is not completely reduced to zero, which will cause large errors and small errors in the reading value of R15S and the absorption rate.

8. The state of polarization of the insulating medium of the product under test is related to the applied test voltage. Because the test voltage cannot reach the rated value quickly, or due to the different load characteristics of the European meter test power supply lead to the difference of the test voltage applied to the object under test, this which makes the state of polarizationinitial voltage of the object under test different, which causes the difference of the absorption current and makes the indication value of the insulation resistance measurement different.

9. The high test voltage of some foreign megohmmeters is continuously adjustable, and is adjusted from zero to the nominal value after start-up. The uncertainty of the start time of the megohmmeter reading and the uncertainty of when the high voltage reaches the nominal value make the initial bias of the product under test different, which will also cause the difference between the indications.

10. Different megohmmeters have different sensitivities and resistance to field interference, and there will be differences in reading values ​​for the same product being tested.

11. The difference between the conventional measurement error of random data fluctuation and the error of the megohmmeter method causes the difference between the indication values.

12. Insufficient dielectric discharge is one of the main reasons for differences in repeated measurement results. According to the corresponding and reversible characteristics of the charging and absorption current of the test product and its reverse discharge current, if it is necessary to repeat the second measurement of the same test product, the discharge interval time short-circuit of the product under test behind the first measurement should generally be longer than the measurement time, so as to discharge The accumulated absorbed charge can fully restore the insulating medium of the object under test to the state original unbiased, otherwise it will affect the accuracy of the second measurement data. In order to ensure that there is no residual charge on the product under test, the measuring terminal must be short-circuited and discharged to the ground before each test, sometimes it takes nearly an hour, and the connection with irrelevant equipment doit be deleted. In short, the same test voltage level and the same wiring method should be used for the insulation measurement of the same test product at different periods, and the insulation resistance meter of the same model or similar performance should be used whenever possible to ensure comparability. measurement data.

13. Particular emphasis should also be placed on the selection of instruments with low dynamic measurement accuracy andlow voltage test power supply capability, since the capacitor charging current has not yet completely fallen to zero and the indicated value of the instrument cannot accurately track the apparent value of the DUT in real time. The insulation resistance value change, the reading and measurement of the resistance value of R15S are on the low side, and there is a large error, which leads to the false high value of the absorption rate of the tested product, whichshould be given special attention by testers. This can also be the main reason for the difference in reading value of absorption ratio when different types of high voltage megohmmeters measure the same product under test. It also shows that the absorption ratio judgment index is not as scientific and objective as the polarization index.