How to do CT Primary Injection Test

CT test & CT Secondary Injection Test should be completed before the CT Primary Injection Test. Now we are confident that all the CT secondary circuit is verified, therefore, we can inject high current on the primary side with any primary current injection kit. But before injection, we have to go for a mechanical check & visual inspection to make sure that:

Visual Inspection
I. No physical damage
II. Connections as per approved drawings
III. Ferrules Ok
IV. Tightness of all connections Ok
V. Isolate all high-impedance relays.
VI. Put all relays on 'out of service position' to prevent false trip

Three Steps Process
I. Core Identification
II. Current ratio & Confirmation for Short/Isolation facilities for Secondary Connection
III. Bus Bar Stability & Sensitivity

Primary Injection
During Primary Injection the above-mentioned three steps need to be verified at different input current magnitudes. For core identification inject 20% of the lowest Primary Rated current & short all protection & metering cores one by one. Make sure that the shorted core secondary current should be zero.
Similarly, for shorting and isolation along with CT ratio confirmation inject 40% & Inject in two steps.

1. Phase to Ground
2. Phase to phase

In step 1 inject in all phases one by one with reference to the ground and make sure the current should be available only in injected phase and neutral with proper magnitude. In step 2 inject in phase to phase with all possible combinations and make sure the neutral current is zero. Measure the current on all terminals, test switches, relays, BCU's, IED's & meters, then measure the current from the device display (from the first terminal to the device display and verify the CT adopted ratio.

However, for stability and sensitivity inject 10% of the highest primary rated current. During normal injection, the magnitude of the differential current on the busbar protection display should be zero but when we reverse or swap the secondary connection with a jumper, now we will get a differential current in the protection relay.

Note:
• Make sure that during primary injection, there is no noise from the CT under test.
• Normalize all the circuits
• In case, more than one CT is involved, then the primary injected current shall be limited to the minimum primary rated current.