This case study was written by Tim Golden at British Sugar. Tim is the CHP manager at the plant in Wissington, King's Lynn, and wrote this piece prior to purchasing a number of HAWK Inspection Windows.

"These photos are of a failed generator stator in Australia at Origin Energy, where the stator end windings had overheated and failed catastrophically, requiring a complete stator re-wind. No one was injured in this particular event, but the unit was down for repair for around six months in total."

"This particular gas turbine generation package was delivered and commissioned by European Gas Turbines in 1999, which was just after they had completed the Bury St Edmunds and Wissington packages for British Sugar. Origin Energy took delivery of two LM6000 PC gas turbine units, both of which have water injection (air inlet fogging) systems to boost power output to around 49 MW during the hot summer months."

"The British Sugar units have a slightly different LM6000 gas turbine configuration, which is designated 'PD' to classify it as a Dry Low Emissions (DLE) design. The British Sugar gas turbines do not presently have water injection for power boosting in hot weather, and are therefore limited to around 33 MW on a hot summer's day, but increase to around 44 MW in the winter."

"It has been noted that the British Sugar generators are maximum rated at 58 MVA, whereas the Origin Energy generators are only rated at 49 MVA, which means that they are running at 100% of design capability. The electrical generators at both companies are 'air cooled' by design, which means that the generator cooling is less effective on a hot day than on a cold day (i.e. the Origin Energy generator is loaded the highest on a hot day, when the air cooling is least efficient)."

"As part of Origin Energy's design review, they had noted that the embedded stator temperature thermocouples were reading a steady 70°C, which is the same as ours. By using an infrared camera, they recorded the stator end windings surface temperature running in excess of 110°C, which is very close to the maximum permissible, and makes any manufacturing defects likely to show themselves sooner rather than later."

"The excessive stator end winding temperatures are thought to be due to poor design of the cooling air through the stator, where the central core is adequately cooled in the main cooling airflow. The stator end windings are in a static air flow region, which doesn't receive sufficient cooling."

"British Sugar will now be fitting HAWK Inspection Windows to each side of the generator, to enable a direct line of sight to the stator end windings when using an infrared camera. We intend to use the Electrical Research Association (ERA) for this study, and they'll issue a report on their findings."

"The ERA will also fit temperature sensitive 'strips' on these end windings, which can be reviewed during a future shutdown to show the maximum temperature that the end windings have reached. These will be used as supporting evidence should the ERA conclude that a design change is required."

"The downside of these 'strips' is that they can only be reviewed when the unit is shut down, whereas infrared can be safely carried out while the equipment is online, and at a range of gas turbine power outputs and ambient air temperature conditions. In addition, the ERA will fit partial discharge sensors to the generator, and record readings for two weeks. Further readings can then be taken if a fault is suspected."