Truth is in the Details ™

The Case of the Crappy Crimp

Synopsis:
Going off half-cocked is sometimes aggravated when shooting from the hip. Pre-emptive replacement assuming insurance coverage sometimes runs afoul of the facts. Here again, Truth is in the Details.

Background:
A municipal substation transformer rated at 30 MVA (like 30 megawatts) was tested annually for gas content in the insulating and cooling oil. Based on elevated test results an order for a replacement transformer was placed without determining the cause of the elevated readings. The $1 million cost was submitted for insurance coverage.

Assignment:
TI was requested to determine the cause of the elevated test results.
 

 

 
Analysis:
TI made arrangements with a major rebuilder to have the 100,000-pound transformer shipped to its shop for disassembly and testing. The gas of major concern in the oil was acetylene which is created at a temperature near 2,000°F and is a good indication that arcing has occurred within the transformer. The transformer input is 67,000 volts, output is 12,470 volts and thus any electrical anomaly is of major concern. Arcing, even at the ‘low’ voltage level creates significant physical damage but when the coils were removed from the transformer and carefully inspected no evidence of arcing was found. Yet, it was immediately noted that the physical assembly of the transformer coils to the transformer core was very sloppy and included deep hammer marks where misplaced components had been only been partially beat into proper position. Precision resistance tests were performed on the coils and it was found that one of the three low voltage coils had resistance 9% higher than the other two. Additional tests showed that the number of turns of wire in the three coils was identical and the physical size of the wire was identical to within .001”. It was concluded that the cause of high resistance was a poor connection between the basic rectangular-shaped coil wire and the fine, flexible lead wires that take power out of the coil. The connection between coil and lead wire is achieved by a hydraulically crimped copper sleeve. TI requested that the couplings for the high resistance coil be sent to its lab for analysis. The rectangular wires of the coil were found to have been sloppily bundled into the connector sleeve and heavily distorted when the sleeve was crimped.

The high resistance of the involved coil assembly was indeed solely the result of defectively performed crimp connections to the coil lead wires. Inspection of the coil and lead wire under microscope showed that some lead wires had been welded to the ends of coil wire that ended within the connector sleeve rather than passing completely through, and other lead wires had arced open.

 

 

 

 

 
No other defect was found in the transformer. The transformer could have been entirely rewound for 25% of the $1 million replacement cost and simply correcting the faulty lead wire connection would have cost a fraction of the rewind cost.

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