1、 Materials and Classification of Leads
The wires that connect the external terminals of the transformer windings are called leads. Through the leads, external power is input into the transformer, and the electric energy transmitted through the leads is also output from the transformer to the outside.
Leads are mainly classified into the following types:
(1) Outlet leads connecting winding terminals to bushings;
(2) Connecting leads between winding ends;
(3) Tap leads connecting winding taps to switches.
The materials of leads generally include:
(1) Bare copper rods, applicable scope: 10kV class transformers with 6300kVA and below;
(2) Paper-insulated round copper rods, applicable scope: small-capacity transformers of 10～35kV;
(3) Bare copper bars, applicable scope: low-voltage winding leads of 10kV and below;
(4) Copper stranded wires, applicable scope: leads of various voltage levels, especially 110kV and above;
(5) Copper tubes, applicable scope: leads of 220kV and above transformers.
To ensure sufficient insulation distance, leads are insulated by laminated wood and paperboard parts, which must meet the requirements in three aspects: electrical performance, mechanical strength, and temperature rise. The selection of leads is also based on electric field strength, mechanical strength, temperature rise during short circuits, and temperature rise under long-term loads.
2、Connection of Leads
The connection forms of transformer leads include: copper welding, gas welding, cold pressure welding, and bolt connection.
Phosphorus-copper alloy should be used as the welding rod for copper welding, which is used for the connection between winding outlets and leads as well as between leads. Gas welding is used for the welding of copper bar leads and the welding of through-cable bushing joints. Cold pressure welding involves inserting the two terminals of the lead connection into a metal tube, then using a mold to squeeze the metal tube, pressing the two terminals tightly together. Cold pressure welding does not require heating, so the welding is relatively safe, with no false welding, no damage to the insulation of the leads and other parts due to burning, and it has good extrusion quality and tensile strength. Therefore, cold pressure welding is currently the main lead connection method for large transformers.
Bolt connection is mainly used for outlet leads connected to rod-type bushings. The outlet leads can be disassembled and can compensate for the deviation in lead length. Usually, a bendable arc-shaped lead structure that can expand and contract freely is adopted, which is also called a flexible connection.
3、Fastening of Leads
To ensure the insulation distance of the leads and prevent displacement and deformation under the vibration and impact of electric force during operation and short circuits, clamping parts must be used to fasten the leads.
Lead clamping parts should have sufficient mechanical strength and electrical strength. For this reason, the structure of lead clamping parts generally adopts a wooden bracket structure. When the clamping parts are fixed to the metal parts of the transformer body, metal bolts can be used to improve mechanical strength. However, when fixing between clamping parts, epoxy bolts must be used, and there should be anti-loosening devices. Insulating paperboard should be added as additional insulation at the position where the leads are clamped to prevent damage to the leads.
