Measuring instrument control

Update time : 2024-11-01 08:49:57

6.1.5 Error measurement device

The ratio difference and phase difference indication resolution of the error measurement device shall not be less than 0.001% and 0.01'.

Under the calibration environment, the measurement error caused by the error measurement device shall not be greater than 1/10 of the basic error limit of the transformer under test.

The influence of the secondary load of the difference circuit on the error of the standard instrument and the transformer under test shall not be greater than 1/20 of their basic error limits.

[Article interpretation] The error measurement device of the transformer generally refers to the transformer calibrator.

The transformer calibrator with the largest usage is level 2, which can meet the requirements.

However, JIG 1021-2007 does not specify the requirements for its use accuracy level, but puts forward the requirements for measurement error.

In fact, the high accuracy level of the instrument does not mean that the measurement error can be guaranteed during use.

Because all instruments have a range problem, the accuracy is related to the range and the measurement indication point.

For the calibrator with level 2 accuracy, to ensure an error of 2%, the measurement point indication should be no less than 1/5 of the full scale value of the range.

Traditional transformer testers and error testers all use the working principle of AC bridge, that is, using impedance circuit to generate differential voltage source and differential current source, differential medium voltage source and power frequency galvanometer are connected in series with the measured voltage and standard voltage series circuit, differential current source and power frequency galvanometer are connected in parallel with the measured current and standard current circuit, and the differential voltage or differential current is adjusted to make the galvanometer point to zero.

At this time, the circuit is balanced, and the error between the measured voltage and current and the standard voltage and current can be read from the differential source indication.

JJG 1021-2007 defines the error of the transformer according to the amplitude difference and phase angle difference, which belongs to the polar coordinate method mathematically.

For the actual situation of δ<0.01rad under normal measurement conditions, there is no perceptible difference in the measurement results given by the calibrator with rectangular coordinate indication and the calibrator with polar coordinate indication.

The difference in their indications can generally only be found during polarity tests.

When the secondary current and voltage are connected in an additive polarity mode, the performance of the electrical tester is over-range, such as a ratio difference exceeding 10%; most electronic testers also show over-range, such as a ratio difference exceeding 20%; some digital testers can show a ratio difference close to 200%.

But the correct display should be that the ratio difference is close to the error limit, such as 0.2%, and the phase difference is close to 180°.

Therefore, all the indications of the above testers can be considered as abnormal measurement results, and the test personnel should change the polarity and test again.

It can be seen that only polar coordinate testers can measure transformers with different ratios, because the measurement result of the ratio difference is their amplitude difference.

When the ratio difference measurement value is f under the subtractive polarity connection, the ratio of the tested transformer is 1 (f+1) of the standard transformer.

For example, when f=-50%, the ratio multiple is 2; when f=100%, the ratio multiple is 1/2. Some digital calibrators (such as HEW20A) can accurately indicate the error according to the definition of transformer error through digital processing, and have a wider error measurement range and use.

The ratio difference indication of the transformer calibrator used for on-site verification of current transformers should be able to read 0.001%, and the phase difference indication should be able to read 0.01'; it should be able to measure the 1% point of the rated secondary current 1A, and the measurement error at this point should not be greater than 5%.

In order to facilitate polarity tests and measurement of transformation ratios, it is advisable to use a digital calibrator with a wide indication range.

In addition, in order to facilitate the work of the calibrators, the calibrator should have admittance and impedance measurement functions, and the indication range should meet the needs of on-site current and voltage load measurement.

The circuit for measuring admittance and impedance of the transformer calibrator shares the measurement channel with the circuit for measuring the transformer.

Under normal circumstances, the allowable input voltage of the differential current and differential pressure circuits does not exceed 10V, and the current does not exceed 0.5A.

It cannot measure a current load of 25 ohms and a voltage load of 60VA at a rated current of 1A.

However, since the load has certain linear characteristics, it is allowed to measure the impedance and admittance of the load box at a current and voltage lower than the rated value during on-site inspection.

In this way, only a transformer calibrator with a wide indication range is needed to meet the requirements of measuring the load box.

With the development of electronic technology and computer technology, a large number of transformer calibrators based on electronic measurement technology, digital measurement technology and even virtual instrument technology are now used.

This type of instrument can greatly reduce the labor intensity of the calibration personnel, but if the instrument fails and the calibration personnel cannot find it, it will result in the issuance of erroneous data.

Therefore, when using this type of calibrator, the calibration personnel must confirm that the instrument is working properly based on the circumstantial data.

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