Retrofitting of a 420 kV draw-lead type bushing with a draw-rod type – Part I
Challenges in replacement of the bushing Since the same type / make of bushing was not available and limitation of time of shutdown (outage period) due to grid constraints, replacement of bushing was carried out without draining top oil. There was no other option other than to insert new bushing with stress shield at a very accurate and precise angle without damaging bushing stress shield and turret CT (Current Transformer).
Keeping in view our previous experiences of the transformer tripping on harmonic restrain setting of 12 % after taking static winding resistances, a decision was taken to lower the 2nd harmonic restrain setting from 12 % to 10 % and the transformer was energized successfully. Such a decision was taken to avoid any confusion of maltripping, attributing to the retrofitting job of OIP bushing.
The bushings play an important role by providing insulation to the live leads from the power equipment. The OIP (oil-impregnated paper) bushings are one of the types of bushings for oil to air application for power transformers / shunt reactors in the air insulated substation.
The bushings are the very vital and sensitive accessory of any extra high voltage transformers from 66 kV to 765 kV. Bushing failure can lead to catastrophic failure of an EHV transformer resulting in a heavy fire causing colossal damage in the substation and system disturbance in the grid network.
The health of bushings is assessed through periodic measurement of tan δ and capacitance values. The limiting value for tan δ is defined by IEC 60137 to be 0.7 % or, in absolute terms, 0.007 (for new OIP bushings) . Keeping this in view, the Madhya Pradesh Power Transmission Co. Ltd has defined the periodicity of recording tan δ and the capacitance value of bushings of EHV transformers as 6 months.