I 12 13 Power Transformers Core and Coil Assembly Core Form Coils are sized and clamped after assembly with the core by being pressed with a special hydraulic fixture, then clamped to the end and lock plates. In the case of core-form transformer, core and coil are supported by vertically connecting leg core to lower yoke and assembling each winding to leg core where the upper tempering iron is assembled on top of the winding. Shell Form The coil assemblies are lowered into the form-fit tank bottom and wedged securely in place. Hipersil laminations, stacked on fill blocks around the coils and on a T-beam through the coil openings, are applied with overlapped joints to increase mechanical strength and keep exciting current at a minimum. The core section in the coil openings are wedged at the top to compress the core and to hold the coil assembly rigid. Wedges are driven between phases, top fill blocks are positioned and vertical wood pieces, with pressboard channels that are connected between the top and bottom fill blocks. Tanking Core Form The transformer is provided with a suitable steel tank of substantial construction which is oil-tight and gas-tight. Also, all gaskets are made of materials which do not deteriorate under service conditions. The tank has sufficient strength to withstand an internal pressure of full vacuum and vacuum drying without damage or permanent deformation. In the case of core-form transformer the joint between the tank and cover is provided with suitable flange and gaskets at top of the tank so that the assembly service is oil-tight and gas-tight. Shell Form The form-fit shell form tank designed and built into a unit assembly of high inherent strength withstands the effects of vibration, handling, application of vacuum during the oil filling and prevents oil leakage with suitable gaskets. The form-fit top section, with end frames and side channels welded inside, makes the core into a rigid body when it is lowered over the wood pieces, pulled down and welded to the tank bottom.