Instrument transformers are mainly divided into voltage transformers (VTs) and current transformers (CTs). They are widely used in power supply systems to supply power to the voltage coils or current coils of measuring instruments and relays. A voltage transformer converts the high voltage of a high-voltage circuit or low-voltage circuit into low voltage, which is supplied to instruments and relay protection devices for measurement, metering, protection and other purposes. A current transformer proportionally converts the large current on the primary side into a standard low current of safe level required for relay protection and secondary instrument measurement on the secondary side.

Main Testing Items of Instrument Transformers

Measuring the insulation resistance of windings

Measuring the dielectric loss tangent value tanδ of instrument transformers with a voltage class of 35kV and above

Partial discharge test

AC withstand voltage test

Insulating medium performance test

Measuring the DC resistance of windings

Checking the connection group and polarity

Error measurement

Measuring the excitation characteristic curve of current transformers

Measuring the excitation characteristics of electromagnetic voltage transformers

Testing of capacitive voltage transformers

Sealing performance inspection

Test items such as volt-ampere characteristics, ratio, angular error, 10% error test, DC resistance of primary and secondary windings, and power frequency withstand voltage are listed as required test items in GB50150-2016.

Precautions for CT Volt-ampere Characteristic Test

The comparison method is usually adopted for verification: that is, the volt-ampere characteristic curves obtained from the test of current transformers of the same model should be basically the same.

The volt-ampere characteristic test of current transformers is only performed on the secondary windings with requirements for relay protection.

Compare the measured volt-ampere characteristic curve with the volt-ampere characteristic curve from previous operation or factory inspection; there should be no significant voltage drop. If a significant voltage drop occurs, check whether there is an inter-turn short circuit in the secondary winding.

Instrument Transformer Test Methods

The CTP series transformer testers have been launched since 2008. After continuous innovation and upgrading, they have been widely used in many units such as State Grid Corporation of China, China Southern Power Grid, PowerChina, Energy China, and the five major power generation groups, and have a good reputation and market share.

The equipment adopts the technology combining the low-frequency frequency conversion method and the voltage method, which can perform CT tests at a frequency lower than the rated frequency, avoid the windings and secondary terminals from bearing intolerable voltage, and can accurately measure multiple important items including ratio, polarity, and volt-ampere characteristics. The CTP-120 and CTP-220 are applicable to protection class and metering class respectively, with accuracies reaching 0.1% and 0.05%. They are competent for testing high-precision CTs for power system metering, steady-state and transient CTs for protection, as well as instrument transformers with a ratio as high as 30000/1.

This series of equipment is equipped with a small printer. After the test, the test results are automatically displayed, including data such as CT/PT excitation curves, 10% and 5% error curves, ratio, polarity, angular error, and ratio error. The software supports U disk and computer synchronous network export of test reports, and can flexibly customize test report templates, avoiding repeated copying of test report data, and can automatically generate standard-format test reports and upload them to enterprise management software with one click.

 

1. Test Wiring

The panel of the CTP transformer tester is marked with the primary and secondary test terminal wiring diagrams for CT/PT ratio, volt-ampere characteristic test, etc. The schematic diagram is as follows:

 

2. Parameter Settings

Set parameters according to the nameplate of the instrument transformer, and click to start the test after the settings are completed.

 

3. Test Report

Print the test report after the test is completed, compare the test data of the tester with the factory data of the instrument transformer, and judge whether the CT/PT is qualified.