Transformer Transportation Damage, A Case Presentation of a Low ...

columns. The outside wraps over the windings, limited room in the tank and the normal obstructions of a fully
assembled transformer prevented a comprehensive inspection of the active parts. However, the blocking items that
were inspected showed movement and in some cases it was significant.
The utility decided that they should receive a transformer in a “new” condition and based on these findings they
requested that the transformer be returned to the factory to make further inspections and perform the repairs with
proper access and tooling. The manufacturer decided that the repairs at the customer’s site were possible and
proceeded to make preparations for field repairs.
FIELD REPAIRS
The transformer utilized a fixed clamping frame design. A hydraulic jack was placed between the top core clamping
frame and the top clamping ring to press the winding package down and remove the top main blocks (the blocks
between the top frame and top clamping ring). Due to limited access only one hydraulic jack was used, so jacking
was performed at one location at a time. With the main blocks removed, the pressure was relieved from the coils in
that area allowing blocks that were shifted to be tapped back into place. Shifted coil blocks at the top of the outer
windings were realigned by tapping them back in place while the pressure was released. Blocks knocked off the
radial spacer columns on the outer coils were put back in place but this was not an easy task for the blocks over the
inner coils because of the limited access. The space available to access the inner coil blocks in the top cooling duct
was about 5 in. wide, 1.5 in. high and 15 in. deep. This proved to be very difficult to manipulate the displaced
blocks back onto their radial spacer columns, in many cases the blocks could not be realigned back onto the radial
spacer columns over the inner windings.
As the work proceeded it became more apparent that the repairs would not be adequate to return the transformer to a
new condition primarily due to the limited access in the tank. The blocks in the core window region on each phase
winding package were not accessible. The blocks over the inside coils were all but inaccessible and it was not
possible to place many of them back on their columns. Nearly 25 to 30 % of the inside winding blocks were off
their radial columns. After the tightening procedure was performed, the windings were inspected again. It was
recognized after several attempts to tighten the windings that some areas still remained loose. Loose blocks were
identified in the blocking over each coil of each winding phase assembly.
At the conclusion of the field repair work, the manufacturer agreed to return the transformer to the factory for
further inspection and repairs. The major inspection and repair work to be addressed at the factory included the
following:
Remove core and coils assembly from tank to allow adequate access to perform repairs.
Thorough inspection of core and coils and tank.
Remove top yoke and top clamping rings to allow inspection of the coils.
Removal of outer pressboard wraps to facilitate inspection of the main outside HV winding.
Realignment of blocking.
Retightening of coils.
UNTANKING AND INSPECTION
Due to the tight clearances between the top of the core and the cover, it was difficult to inspect the top core yoke.
Once the cover was removed, some problems were quickly identified. Overall, the top core yoke had mild but
noticeable waves throughout the laminations that became increasingly worse as they progressed to one end, where
they appeared to build up to a single large ripple that crossed nearly all steps of the yoke, see Figure 9. The miter
joints at each end of the top yoke did not show signs of opening up, except in one location involving about three
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