How to measure instrument transformer

A, voltage transformer (PT) measurement method

The main function of voltage transformer is to reduce the voltage of high-voltage system in proportion for use by measuring instruments. Measuring voltage transformer usually includes the following aspects:

1. Variable ratio measurement

Transformation ratio refers to the ratio of primary side voltage to secondary side voltage. When measuring, you can apply a known voltage to the primary side, measure the output voltage of the secondary side, and calculate whether the transformation ratio meets the rated value through the formula.

2. Polarity check

Polarity is related to the accuracy of instrument wiring. Usually, DC method or AC comparison method is used for inspection to ensure that the primary side and the secondary side with the same name are correctly connected.
　　. Error testing

Errors include ratio difference and angular difference. The ratio difference refers to the deviation between the actual ratio and the theoretical ratio, and the angular difference is the phase difference between the secondary voltage and the primary voltage. Error testing usually needs to use a special transformer calibrator to complete.
　　. Insulation resistance test

Use megohmmeter to measure the insulation resistance of primary winding to ground, secondary winding to ground and between primary winding and secondary winding to judge whether the insulation state is good or not.

Second, the current transformer (CT) measurement method
　　The function of current transformer is to convert large current into standard small current (usually 1A or 5A) for measurement and protection. The measurement method is as follows:

1. Variable ratio measurement

Introduce a known current into the primary winding, measure the output current of the secondary side at the same time, and calculate whether the actual transformation ratio meets the nominal value of the nameplate.

2. Polarity test

Commonly used methods are DC method and AC method. The correct polarity ensures the correct action of relay protection devices and measuring instruments.

3. Excitation characteristic test

By measuring the relationship between excitation current and voltage on the secondary side, whether the iron core is saturated or not can be judged, and the performance of CT can be evaluated.

4. Error testing

Similar to voltage transformer, it is also necessary to measure ratio difference and angle difference, which is often done by using transformer comprehensive tester.

5. Insulation resistance test

Measure the insulation resistance between windings and the insulation between windings and ground to ensure the safe operation of equipment.

Third, the measurement matters needing attention

During the measurement of instrument transformer, the safety operation procedures shall be strictly observed:

-Before measurement, power should be cut off and electricity should be checked to prevent electric shock accidents;

-The secondary side shall not be open (CT) or short (PT), otherwise the equipment may be damaged;

-Use qualified test instruments and calibrate them regularly to ensure the measurement accuracy;

-Record each measurement data, so as to facilitate the subsequent analysis of the running state of the equipment.

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As a key measuring element in power system, the accuracy and reliability of instrument transformer directly affect the operation safety and measurement accuracy of the system. Through the regular measurement and maintenance of voltage transformer and current transformer, the hidden trouble of equipment can be found in time and the stable operation of power system can be guaranteed. Therefore, in practical work, we should pay attention to the measurement of instrument transformers and operate in strict accordance with the regulations to ensure the safe and efficient operation of the power system.