Failure of 220 kV Current Transformer at 220/132/33 kV 


The transformers are a highly expensive equipment in a switchyard/substation and is expected to serve the entire life of substation i.e. 35 years. It has become a matter of high concern that many transformers are failing before their useful life. In order to investigate the failure of 220KV CT, recommended measures should be proposed to avert recurrence.

Current transformer failed due to bursting of pressure release diaphragm. This current transformer was supplied for metering purpose and it was live tank CT. Current transformer was installed in the place of failed CT. After detailed internal inspection of transformer and thorough inspection of transformer, it was found that the transformer failed due to bursting of pressure release diaphragm and it was supplied for metering purposes. The transformer was further declared faulty and was immediately shifted to repair bay for detailed investigation.

Name of Substation: 220/132/33 kV substation

Faulty Equipment: CT

Rating: 220 kV

Make: SCT

Year of manufacturing: 2008

Date and time of occurrence/discovery of fault: 08.09.2013 @ 03:14 hrs

Fault discovered during: Operation

Present condition of equipment: Replaced

Details of previous maintenance: Last maintenance on 07.06.2013

Details of previous failure: Nil

Details of Tests done after failure:

Transformer testing was imperative to ensure that the distribution transformers are built with an adequate strength to withstand over voltage and develop the mechanical strength to withstand testing during short circuits. We conducted several checks on the following criteria:

  1. The CT measuring terminals connections were checked

  2. Shorting link status checked 

  3. Other damaged part properly checked

  4. Last maintenance status checked & its tan delta value checked. 

  5. Power Connection Tightness

  6. Cleaning Insulators

  7. Checking Earthing of Testing before CT

  8. Checking proper earthing of body connection

Specific Instruction 

Since a little negligence on the part of the worker may have to pay a heavy cost resulting in fatal accidents, the instructions for safe working and prevention of hazards should be strictly adhered. The supervisors or engineers should put maximum efforts to overcome any causality.


Since Current transformer had served for less than 6 months after commissioning, the matter should be investigated in consultation with OEM. We recommended ABB CT in place of current manufacturer for trial purpose so that further investigation regarding the manufacturer could be clarified.