Refit for a new climate

Following a successful diagnosis and refit proposal for one unit, industrial services specialist Wyko has won a contract to overhaul nine 30 MW 15 tonne turbo alternator rotors that had been relocated into an unsuitable environment.

The work is to include rebalancing and a new insulation system specially designed and developed by Wyko to improve rotor performance and extend wear life in an unusual context - where the rotors had been moved to a location unsuitable to their original engineering.

Wyko was initially contacted by an (unnamed) UK client to establish if effective repairs would be economically viable for the 30 MW rotor, part of a unit generating power from steam turbines. An essential requirement was to demonstrate a cost effective and reliable insulation system that would also take into account the history of similar rotors at the client’s facility sharing the same design.

Alternator rotors are built with different design characteristics specific to the intended application and its environment. One of the common reasons for reduced operational life is re-located to an environment quite different to the one initially intended.

Wyko’s engineers saw evidence of such a relocation in this case, and further investigation identified insufficient insulation for the application as one of the main reasons for failure. Wyko was then able to put forward design improvements to the insulation systems that would meet all the site operational and environmental requirements.

The proposed new design included improved slot insulation and a new inter-turn design to improve thermal, mechanical and insulation characteristics. Owing to physical wear in the original insulation, Wyko included the rebuilding and balancing of the giant rotors in the proposal to improve performance and promote more reliable operation. The resulting unit would be far less susceptible to this type of failure and the solution could also be economically applied to the eight units still in service.

Overhauling the rotors involved completely dismantling the units then annealing, cleaning, non-destructive testing and condition reporting on every element of the coils. To prepare the rotor body to accept new coils, it was cleaned and all insulation removed from within the slots. Electrical tests were carried out to ensure integrity of the rotor laminations and a full mechanical datum check of the rotor body, rotor slots, rotor shaft and coupling also conducted.

Wyko's Newcastle upon Tyne workshop manufactured the specifically designed rotor slot insulations for insertion into the rotor body prior to the fitting of coils. The coils had also been insulated and had met the designated class H temperature classification (180°C operating temperature). Following completion of the rotor rewind and reassembly, electrical tests were carried out prior to stoving and curing of the rotor winding. After the curing cycle, further electrical tests were then carried out before and during a 20% over-speed dynamic balancing operation.

As a result of the successful rewinding of this first turbo rotor, which met all the required design parameters and was in full operation, the client awarded Wyko the contract to rewind the remaining eight turbo rotors over a given period.