Site Map
Return to a Note on Generator Core Failure Mechanisms

Generator Core Fault Case History : Pacificorp Hunter 1 : 24/11/2000

[Information source : Public Service Commission of Utah  and summarised by Dr. Antony Anderson C.Eng FIEE ]

According to information laid before the Public Service Commission of Utah, by Mr Barry Cunningham, Vice President of Generation, Pacificorp, a  forced outage occurred on Hunter Unit 1 on November 24th 2000,  in which the generator stator core was severely damaged. The rebuilt stator was restored to service on May 8th 2001. Pacificorp's attorneys claim that the Company  incurred $270.1 million in net purchased power costs in relation to the outage.  Summarising the information in Mr  Cunningham's deposition :

Comments by AFA:  Unfortunately, Mr Barry Cunningham's deposition  is no longer available on the Utah State Government website. [See old references below]. The Hunter 1 failure is however briefly mentioned on the Exponent website at:   It appears that Exponent were asked by Pacificorp to examine the body of evidence after the unit had been repaired and was back on line. Exponent concluded that "the failure was an unprecedented and unpredictable event." 

Hunter 1 is by no means the only recorded case of a core failure on a machine with through-bolt core clamping. Therefore, in my view, the core failure cannot strictly be said to be unprecedented. Nor could the core failure be claimed  to be unpredictable, since, should a through bolt become loose, eventually a core fault would become a distinct possibility. Only the timing of the core fault would be unpredictable.  

"On line failure vs Preventive Maintenance" by Jim Taillon AGT Services [Iris Rotating Machines Conference 2003 , Santa Barbara CA]  describes core damage at through-bolt  positions on  Machine A , 10 years old, of  68.8 MVA and Machine B, 40 years old, of 64 MVA. Of particular note is Machine B on which, according to Taillon  page 3: " ...The thru bolt insulation also had core iron impressions down its length. The bolt was apparently loose enough to vibrate and hit the core hard enough to cut small grooves into the insulation..."  These observations of incipient core damage on Machine B, suggest strongly the mechanisms at work in  this type of  internal core fault to be: (1) a loose tie rod that allows  transverse  vibration of  the rod and  its insulation under the influence of  the  rotating radial/circumferential magnetic field in the  axial duct  (2) fretting  of insulation because of vibration and eventual thinning to the extent that the  through  bolt makes contact with lamination edges at one or more points along its axial length (3) given core to frame contacts that correspond with tie rod to lamination contacts, a circulating radial/axial eddy current with high  local losses at tie rod to lamination and lamination to keybar points will result (4) eventually a runaway core fault may be established.

Generally speaking core faults of this type in the body of the core are unlikely in machines where core clamping is by a cantilever arrangement involving the core end plate and the core fingers and where the through bolts are dispensed with.  The exception is when a conducting foreign body lodges in one of the axial vent ducts. See Image

Note: 12 January 2006 : The following Utah Government references listed below seem to have been removed from the Internet: -
Outage application:
Details of the core failure:
Photographs of the damage:

Return to a Note on Generator Core Failure Mechanisms

Summary prepared 1st December  2001 and updated Jan 12th 2006 by Antony Anderson