At 132/33 kV Substation, R-ph CT was failed due to any some reason.
The ratio of the CT was 200/1 and replaced by the similar CT of 200/1 after checking of the ratio by the injection kit.
While loading the feeder, the tripping is not resulting immediately but after attaining certain amount, tripping is occurring.
Action taken
For proper of the current on each phase, purposefully the feeder was allowed to deliver a small load. The currents on each phase and on E/F element was measured and found with higher on the secondary of the replaced CT i.e R Phase CT and unbalance current on the E/F element.
Shut down on the feeder was availed for detail checking of the connection on the secondary side of the CT.
On checking it was observed with connection of the secondary circuit from S1-S2. But by mistake the electrician had shorted the other secondary tapings (S3-S4), thinking of unused secondary terminals to be shorted to avoid open of secondary.
The shorting on S3-S4 for R Phase Ct secondary terminal was removed and the feeder was charged and loaded successfully.
Analysis
In this feeder, the CTs that used are of secondary control type with having facility of ratio 200-400-800/ 1-1-1-1. For the selection of CTR as 200/ 1-1-1-1, the terminals on the secondary have to be used from S1-S2.
Similarly, for 400/1, it is to be selected from S1-S3 and accordingly for 800/1, it is to ken from S1-S4. In this condition, 100/1 was the choice and accordingly selection must be taken from S1-S2. This selection and as per the flux movement due to primary and secondary current, the ratio becomes 100/1. But due to the concept mistake of the electrician, as S3-S4 was shorted, the extra shorting and current movement for this mistake resulted extra flux in the core. So the ratio of the CT did not remain 100/1. The new ratio became less than 100/1, for which the corresponding secondary current of this CT became more as measured on the next day morning.
This un-balance current was also available in the night, but due to allowable tolerance setting on the E/F relay, the relay did not actuate for the minimum load that available in the night. But in the next morning, when load was raised, the un-balance current crossed the setting value and started tripping.