Problem With 33kV Resin Cast Current Transformer

There could be several causes; both internal and external and a thorough failure analysis is necessary to determine the failure mechanism and mode. Age, design and condition of the PTs and CTs could be factors. Surge arresters on the breakers do not necessarily guarantee adequate overvoltage protection for the transformers. The type of arrester (gapped silicon-carbide or gapless metal-oxide), discharge voltage and lead lengths are also factors. If lightning or switching surges are suspected, insulation coordination and arrester protective margins should be reviewed.

Remember, while an overvoltage surge is discharged to ground through the arrester, a discharge voltage appears across the terminals of the arrester (and across any parallel insulation the arrester is protecting). That voltage is a product of the discharge current x the arrester internal resistance (IxR). To that must be added the combined voltage drop of the arrester line and ground leads in series with the arrester. Although a function of the lead inductance and the rate of voltage rise (Ldi/dt), a realistic estimate for the lead wire voltage drop is 6 kV per foot (which is the value commonly used for calculating protective margins under fast-front surge conditions.

Unfortunately, there are no shortcuts or universal answers and your analysis should probably include the load side to make sure nothing strange is happening there.

The Preventive maintenance of CRT ( Cast resin Transformers ) and OFT is different. The reasons for failure of HT and EHT Power Transformers are mainly due to the flash over caused due to Surge voltage which can be prevented with surge arresters and lightening protectors.

Insulation Failure due to over heating of winding is another cause for failure. Cause for over heating is either due to continued overloading of the Transformer or leakage current due to insulation failure during monsoon due to wet weather conditions. In OYT (Oil Filled Transformer) warning and tripping is done with suitable relays.The failure in OYT is due to contamination of Insulating oil and poor maintenance

In CRT temperature sensors are located at various points inside winding and are connected to the tripping controls of the breakers. These electronic sensors often fails but it cannot be timely noticed and protection become in effective. When I noticed those failure the manufacturers were requested to increase number of sensors and to place them uniformly.This made the protective tripping prompt and avoided transformer failures. Though Manufactures say that CRT are maintenance free it is always advisable to carry out routine cleaning of dust specially when the transformers are installed outdoor.Dust formation along with moisture causes insulation failure and flash over. Poor manufacturing of CRT is another cause . Air bubbles in casting can cause failure. Your substation earthing must be proper to ensure proper functioning of protective relays. routine ER & IR test should be conducted to ensure safety.

I have been using CRT for the past ten years and never had a failure where as there is saving in maintenance cost .

Check that they're not in a high temp. area of the equipment.