Insulation testing of generators for BP

Date: March 2012
Location: Ula field

Karsten Moholt was hired to run high voltage insulation testing on three generators and a WAG-motor at one of BP’s rigs. The test results showed that the generators were in a good condition, but there were a lot of rust on the cooling system.
Karsten Moholt recommended replacing the coolers. The WAG-motor got poor test result, showing a lot of weaknesses in the insulation.
The motor was inspected visually prior to the testing, and there were discovered missing woodruff keys. The motor had 105 100 hours of operation time when it was tested. The results showed an insulation resistance (IR) of 11.9 GΩ and polarization index (PI) of 5.8, both good values. Tanδ was measured to 0.7 % which also is a good value. The test of partial discharges (PD) was run stepwise from 40 % to 100 % of line voltage. At line voltage, the level of PD was over 30 nC. Compared with the result from the test with 80 % of line voltage, showing under 2nC, these were very high values. A nonlinear increase of PD as a function of voltage can indicate weak spots or damage in the insulation.
After analyzing the test results, Karsten Moholt recommended that the motor be taken out of operation and replaced with a spare motor. There was also presented a plan of actions should the customer decide to replace the motor. This plan explained what needed to be done regarding adjustments on the spare motor, and an estimate of how long the job would take.
If the customer should decide not to replace the current motor, Karsten Moholt offered to install online PD-surveillance, to track the condition of the windings. The customer didn’t wish to take the motor out of operation, but decided that the consequences of a possible breakdown would be greater than the loss of a few days of operation. Both motors were sent to Karsten Moholts workshop on Askøy for mechanical measuring. On a rig, space is often an issue, and it was therefore crucial that the motors were of identical dimensions. The workshop made sure that this was the case, and also tested the spare motor to make sure it was still in a good condition.
The spare motor was transported back to the rig, and the old stayed in the workshop for rewinding. The whole operation took two days less than estimated, and the customer was very satisfied with the service Karsten Moholt had provided.
This job is a good example that high voltage insulation testing of the stator is a good way to check the condition of the windings. Weaknesses in the insulation are often impossible to spot visually, and should therefore be checked regularly. The test can give an early indication of when the stator needs maintenance, so this can be done at a convenient time.