2006 Mobile Substation Repair

This picture shows the untanked core and coils assembly from the rear side. You can see part of the C-phase winding on the left edge of the photo. The two paper-enclosed boxes on the front of the assembly are the A and B high-side no-load tapchangers, and the leads at the bottom of the assembly go to the series/parallel tapchanger. The upper tapchanger here is the B tapchanger and the lower unit is the A tapchanger. The A tapchanger getting stuck on tap 7 was the reason for untanking the transformer.

This picture shows the untanked core and coils assembly from the front side. At the top of the photo, you can see the rod and gears for the C high-side no-load tapchanger. The tapchanger itself is in three assemblies which you can see as the three vertically-situated paper-enclosed boxes in the front of the photo. The large collection of leads at the left comprises the connections to the lower-voltage load tapchanger (LTC), and the smaller bundle of leads at the right comprise the connections to the reactor, or preventative autotransformer, which works in conjunction with the LTC. Also, on the right you have a bit better view of part of the C phase windings, and you can see the B phase and A phase windings in the background, behind the LTC leads and the C tapchanger.

Here is simply a close-up of the LTC and reactor leads to give you an idea of how many connections must be made and unmade to untank the core assembly.

Here is a close-up of the top of the core and coils assembly from the A phase end of the core. You can see the steel "C" channel support members at the top of the photo with the crane hooks attached. Between these members you can see part of the core of the transformer, which carries the magnetic current. It is made of many thin laminated layers put together to reduce eddy-current losses in the core (this occurs when the magnetic field in the core induces an electric current in the core itself rather than in the windings...making the core out of laminated layers greatly reduces this). Below this, you can see part of the A phase winding.

This is a photo of the empty transformer tank from the rear. Note that two covers, one on top of the unit and one on the rear side at the bottom of the unit, had to be cut off to untank the unit. You can also just see the series/parallel tapchanger through the opening at the bottom of the tank. We discovered that this tapchanger was bolted to the bottom of the transformer tank rather than to the core when attempting to lift the core assembly out of the unit the first time. At the right of the photo you can see the bushing wells for the high voltage bushings. The low voltage bushings are situated on the front side of the tank. You can also see the operating handle for the C tapchanger at the upper-left corner of the photo. This is a 7500 kVA FOA Westinghouse Mobile Substation with dual high-side voltages of 115 kV delta and 69 kV delta and a 12.5/7.2 kV wye secondary. This unit also has an LTC, high and low side arrestors, 7.2 kV metering, 7.2 kV transformers to run the oil pumps, and an electrostatic-controlled 7.2 kV oil circuit recloser.

These two pictures show two contacts on the A tapchanger. The top portion of the photo shows the C phase contact, and the bottom portion of the photo shows the B phase contact. Notice how the squared-off cylindrical part of the C phase contact is too close to the rounded horizontal peg, where the B phase contact is properly spaced so that it can move past the peg. Also, upon further examination, we found the C phase contact to be so loose on the tapchanger shaft that it can rotate in a direction it isn't supposed to rotate in. This extra rotation combined with the poor adjustment is the cause of the hangup on the A tapchanger.