How to calibrate instrument transformer

In power system, instrument transformers (including current transformers and voltage transformers) play a vital role. They are used to convert high voltage or large current into standardized low voltage or small current in proportion, so as to facilitate the safe and accurate work of measuring instruments, relay protection devices and automation systems. Therefore, the accuracy of instrument transformer is directly related to the reliability of electric energy metering, system monitoring and protection action. In order to ensure its measurement accuracy, it must be calibrated regularly.

First, the basic concept of instrument transformer

Instrument transformers mainly include current transformers (CT) and voltage transformers (PT). A current transformer converts a large current into a standard current signal of 5A or 1A according to a certain proportion, while a voltage transformer converts a high voltage into a low voltage signal of 100V or 110V. These standard signals are convenient for instruments and protection devices.

Second, the significance of instrument transformer calibration

In the long-term operation of instrument transformer, the error may increase due to mechanical wear, thermal effect, insulation aging or external electromagnetic interference. Periodic calibration can ensure that the ratio error and angle error are within the allowable range, thus ensuring the accuracy of power system measurement and the reliability of protection action.

Third, the basic principle of calibration

The core of calibrating instrument transformer is to measure its ratio error and phase angle error. Usually, the standard transformer is used as a reference, and the error is calculated by comparing the outputs of the calibrated transformer and the standard transformer. Errors mainly include:

-Ratio error: the deviation between the actual ratio and the rated ratio;
-Angle error: the phase difference between primary current and secondary current.

Fourth, the calibration method

1. Comparative method
The standard transformer and the calibrated transformer are operated under the same primary current or voltage, and the difference of their outputs is compared by the error measuring device. This method has high accuracy and is suitable for laboratory and field detection.

2. Direct measurement method
Using a high-precision current or voltage source applied to the primary side, the actual output value of the secondary side is measured, compared with the theoretical value, and the error is calculated. This method is suitable for on-site rapid detection.

3. Automatic calibration device
Modern transformer calibration mostly adopts automatic calibration device, which can automatically apply current/voltage, collect data and analyze errors, and has the advantages of high efficiency and accuracy.

V. Calibration steps

1. Preparation stage: check the equipment connection and confirm that the calibration device and standard transformer are in good condition;
2. Wiring: Connect the calibrated transformer into the calibration loop;
3. Pressurization or current addition: gradually apply test voltage or current according to regulations;
4. Data acquisition: recording parameters such as ratio and angle error;
5. Data analysis: judge whether it is within the allowable error range;
6. Issue a report: fill in the verification data and make a conclusion on whether it is qualified or not.

VI. Matters needing attention

-During calibration, ensure that the primary side current/voltage is within the rated range of the transformer;
-The secondary side shall not be open (especially CT) to avoid high voltage danger;
-The verification environment should avoid strong electromagnetic interference;
-Periodic maintenance and calibration intervals are recommended to be 1 to 2 years, or according to the requirements of regulations.

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Accurate calibration of instrument transformer is very important for the operation safety and energy metering of power system. Through the scientific and standardized calibration process, the reliability of equipment operation can be effectively improved, its service life can be prolonged, and a strong guarantee can be provided for the intelligence and automation of power system. With the continuous development of detection technology, future transformer calibration will be more intelligent and convenient, further improving work efficiency and measurement accuracy.