Substation Current Transformer Failure

Introduction High Voltage Current Transformers, like any power electrical piece of equipment, are designed and manufactured to operate for at least 20 years. Extensive laboratory and factory tests are performed in new equipment to ensure this. These tests have been also applied in instrument transformers retired from service.

Problems & Findings:
The current transformer used in substation for measuring the current in substation. Usually Current transformer have a long life in substation; we don’t need to change it frequently. It can work continuously up to 10 years but somehow environment condition & successive transients affect its performance. In this case, we found that CT suddenly burst out and oil spurt with fire surrounding it and the porcelain petty coat broken into pieces & whole substation needed to shut down.

Check points:

Sr No

Check points



Checked the CT condition properly & insulation status.

Completely worst condition


Checked manufacturer recommended manual for CT

Every year Tan Delta Test recommended


Busbar protection operated

All other protection circuit checked for any other cause


Master relays of auto transformer operated

Master relays checked for any other abnormality


All other tripped feeder checked

Other tripped feeder found healthy


Preventive maintenance records checked

Tan Delta test annual basis record not found


1. ABB current transformer recommended in place of any other because of its long term services & reliability.

2. Tan Delta test recommended annually for getting its insulation status, so that before failure of insulation, it can be replaced on time.