How to do primary injection test for current transformer

Primary injection tests are essential to validate the integrity of current transformers. These tests help determine if the current transformers are accurately measuring the current and are functioning correctly. In this article, we will discuss how to perform the primary injection test for current transformers.

This test is to ensure the CT circuits are properly connected with respected cores and there is no mix up in the circuit (phase identification).The circuit

connections shall be made. Single point grounding shall be verified for CT circuits, before starting this test. Inject 25% of rated primary current between one phase and earth with all connected burden. Measure secondary current at all points of CT circuits. It shall be done for other phases.

The primary injection test measures the current transformer's performance, including saturation, ratio, and phase shifts. The test requires specialized equipment, which is costly and requires a high level of experience to use accurately.

The following steps are essential to ensure a successful primary injection test:

Step 1: Set up the test equipment

The test equipment consists of the primary current injector and a suitable measuring instrument to record the secondary winding output. Ensure that all connections are tight and adequately insulated to avoid any accidents. Interlock the test equipment to prevent accidental activation during the test.

Step 2: Prepare the CT for the test

Disconnect the CT's secondary cable from the system and short the secondary winding to a ground. Set the CT to the nominal secondary current rating. Ensure that the primary winding is appropriately connected to the current injector. Verify that the primary winding polarity is correct.

Step 3: Perform the test

Activate the primary current injector and verify that the current waveform is close to the sinusoidal waveform. Gradually increase the primary current to the rated full-load current. Record the actual primary current and the measured secondary current output.

Step 4: Analyze the test results

Compare the actual primary current and the measured secondary current output. Calculate the current transformer ratio and determine the percentage error between them. Check for any phase shifts and ensure they are within the manufacturer's specified limits.