Marthinusen & Coutts’s solution to an irregularity that occurred in the stator of a large 36 MW compressor motor deployed at Sasol’s Secunda plant, has yielded the best ever test results.
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During a routine inspection in January 2018, a Sasol maintenance team discovered that the flux shield mountings on the stator were faulty. Sasol awarded M&C a contract to identify the cause of the irregularity and offer a remedy.
“We tested the stator winding, which we found to be fine, but confirmed that there was a defect in the flux shield,” said Rob Melaia, M&C’s engineering and technical executive.
“We recommended that it be repaired because of the risk of it damaging the winding if left to continue operating in its existing condition. In order to repair the flux shield, we also had to remove the winding and perform a rewind on the stator.”
Sasol accepted M&C’s recommendation and in August last year assigned the company to perform the required repairs. “In addition to replacing the old bars with new bars purchased from a reputable coil manufacturer in the US, we did a very specific modification to repair the flux shield to prevent a recurrence of the defect in future,” Melaia stated.
“On investigating the defect we found that the electrical current, instead of flowing only in the flux shield as it ought to have done to prevent the core from overheating, had started flowing in the mounting bolts, and consequently, causing wear by electrical arcing in the mounting holes and the mounting studs,” he explained.
The solution provided by M&C’s repair team was to fit copper braid straps from several points on the flux shield to the stator body to reroute the current in such a way as to prevent a repeat of the damage as witnessed. To confirm the effectiveness of the solution M&C arranged to have the refurbished stator tested by local independent test authority, H.V. Test Field Services.
Being the recognised tests for determining the integrity and efficiency of medium voltage windings, partial discharge and tan delta tests were conducted.
The results were as follows:
- A maximum partial discharge of below 250 picocoulombs (pC) at 120% of phase voltage.
- In the tan delta tests the dielectric dissipation factor was found to be 65 x 10-4 at 20% of phase voltage and 105 x 10-4 at 100% of phase voltage.
“The results of final tests, conducted by H.V. Test Field Services on the stator after M&C had completed all the necessary repairs on it, were found to be the best among many such tests conducted on similar equipment over a period of several decades,” said Melaia.
“We have every reason to be proud of this outcome as it says volumes about M&C’s expertise in this field, both in terms of correctly diagnosing and repairing faults in a wide range of large rotating equipment, as well as providing the appropriate and most effective solutions for them,” he concluded.
