Huntly Power Station has 4 X 250 MW single reheat turbo-alternators which were commissioned between 1982 and 1985. These machines are at the end of their design life but can be kept in service by carrying out major maintenance such as rewinding the generator rotors as required. The Huntly generator rotors are 10 metres long and weigh 42 tonnes. Over the life of the generator the stop/ start and loading cycling causes thermal expansion and contraction which gradually degrades the quality of the insulation. If left, this can lead to one of two faults;
1. Inter turn fault. Where the insulation breaks down between adjacent winding layers. This causes a higher current in one side of the rotor and therefore the rotor bows from uneven thermal expansion. This is seen as an imbalance which is load related. To some extent this can be compensated for by adding balance weights but ultimately the machine will become un-runnable.
2. Rotor earth fault. This is where the insulation between the winding and the rotor body breaks down which results in severe localised heating and will instantly trip the machine.
The ideal is to plan for a rewind just before either of these two events occurs so that the repair can be managed around other outages and load requirements.In the case of the Huntly unit 4’s generator there were subtle indications from the changing vibration patterns and a known deterioration in the insulation quality that prompted the rotor rewind which was carried out at BWE in Geelong in 2012.In the case of unit 1, the generator was again known to have deteriorating insulation quality and we had de-commissioned unit 3 and so had the unit 3 rotor rewound at Siemens in the UK in 2014 to be fitted into unit 1 during an extended outage in 2015.This paper looks at these two balance facilities and the reasoning behind requiring the high speed balance.