Salient design features of 188 MVA, 220/96/28 KV biggest rating regulating transformer

Aluminium refinery uses prodigious amount of electricity for electrolysis process. It requires power to be controlled over large voltage range. Regulating transformer is connected in series with Rectifier transformer and finally to rectifiers. Rectifier gives DC power output to Aluminium smelter which is controlled by Regulating Transformer.

188 MVA Regulating Transformer with 304 T overall weight is produced for Hindalco projects in India. This Transformer is having 106 taps which can control the output voltage from 96 kV to 1 kV in step of 1 kV. The required voltage is achieved with multi coarse design which is having 5 coarse and 1 fine winding. Selection of tap in 107 positions is done with special on load tap changer.

Fibre optics sensors are installed in various windings to get the local hot spot temperature. Pt 100 sensor is provided for top oil temperature measurement. Online gas monitoring device is provided for online diagnostics against internal fault. Special 107 tap OLTC is provided with oil filter unit. Regulating transformer and rectifier transformer are connected with integrated oil filled cable box. Water base paint is used considering environment aspect.

Reliability of such large rating transformer is important to deliver continuous power to smelter due to high shutdown cost. Voltage, thermal and short circuit stresses are determined and controlled for each winding, core and connections. Design, manufacturing and processing are done with high level of engineering skill.

All tests are done inline with IEC 60076, Harmonic losses are determined as per IEC 61378-1. DGA and SFRA test are done which is finally compared with site results.

This paper describes about the specification requirement, salient design features, manufacturing and testing aspects of 188 MVA, 220/96/28 kV Regulating Transformer.