A CASE STUDY

The bolt for the feed from the transformer to the collector ring vibrated loose.

This case has before- and after-repair test results and is a great example of the DVtest graph indication of problematic connection in the OLTC schema. The tap changer in question is FPE model 546, reactance type OLTC.

The figure below shows three phase DVtest graphs recorded both directions UP and DOWN
Direction UP
Phase x1 Phase x2 Phase x3
Direction DOWN
Phase x1 Phase x2 Phase x3
     

 It is obvious that the X2 phase ripple is much larger than the other two phases, they are not consistent, some are shorter others longer, which is not normal. Any loose connection in the moving parts of the tap changer would produce longer than normal ripples.

Repair:

The maintenance crew opened the OLTC and found the bolt for the feed from the transformer to the collector ring had vibrated loose.
They cleaned and retightened the bolt, followed by a retest.

Graphs in the figure below show comparison of DVtest results before and after repair. The difference is huge and points to a problem with tap changer - in this case contacts of the collector ring.
Phase x2 before and after repair Phases x1 and x2 after repair

One can see that the ripples after repair compare between phases.

This was a simple repair and avoided a big problem if left unattended as loose contacts create overheating or even arcing and gasses in the transformer OLTC compartment.

Levico,  Incorporated 1995        Back         Home