Three-Phase 33kv off-Circuit Tap Changing Power Transformers with Low-Loss and Low-Noise

1. Low loss, therefore significantly energy-saving. The average no-load loss is 35% lower, and the load loss 10~20% lower, than
prescribed by the present National standard GB/T 6451.
2. The core laminations are made up from imported high-quality cold-rolled grain-oriented silicon steel sheets with miter joints
and without punched holes, and are stacked without the upper yoke sheets in place. After stacking, the core is pressed
integrally and finally glued together by epoxy resin on the cut edges, so that the three limbs and the upper and lower yokes are
bonded together into a rigid, vertically straightened entity, together with the plate-type upper and lower frame.
3. Zigzag directed flow of oil are provided within the windings, so that all parts of the windings can be cooled more evenly, thus
reducing the temperature rise of the windings as a whole as well as the hot-spot temperature rise, prolonging the service life
of transformers.
4. The regulating winding is a separate winding, apart from the high voltage main winding. In this way, total ampere-turn balance
between windings can be obtained irrespective of the tapping position; and the axial electrodynamic force during short circuit
can be much reduced, thus upgrading the short circuit withstand capability of the transformer as a whole. All the windings are
perfectly dried before assembly. The radial clearances between windings and between low voltage winding and core are filled
with pre-dried vertical spacers and insulating cylinders in such a way that the windings are tightly together radially with
practically ‘zero’ allowance between them.
5. The windings of one phase are assembled into an integral phase block before placing onto the core. The low voltage winding is
wound directly on strong thick paper cylinder. Vertical spacer strips are provided in spaces between windings and outside the
outermost winding. The radial dimensions of all windings are kept within strict tolerance, so that a rigid winding phase block
are obtained, capable of withstanding all the radial short-circuit forces encountered in service, and the possibility of buckling
of the inner winding is obviated. Axially, the height of all windings is kept within close tolerance. Horizontal spacers between
discs of a winding are pre-densified. The assembled winding phase black are pressed up and down by single support and
pressure plates after careful adjustment of the heights of individual windings. Axial pressures are co-ordinated with the
calculated short circuit force, so that the capability of the transformer to withstand axial short-circuit force is much increased.
6. The winding manufacture, winding assembly, and core and winding assembly work are all proceeded in totally closed clean
rooms inside workshops. The cleanliness and temperature stability requirement of the area are the same as that required for
the manufacture of 500kV class transformers. The final vapor phase drying of the core and coil assembly is practically a further
cleaning process.
7. Core frames in the form of a flat plate are used for clamping the core, and are joined together by side beams. The core and coil
assembly is fixed in the tank, being positioned against the tank at six sides rigidly, such that there can be no displacement of
the assembly due to impacts during transport. Three-dimensional impact recorders are installed on transformers during
transport to detect whether or not unacceptable impact force has been experienced and displacement of the core and coil
assembly occurred during transportation. If not, lifting of the core and coil assembly out of the tank is not necessary. Flat cover
permits the bushings to be mounted in a vertical position, thus eliminating oil leakage because of uneven pressure on the
gasket.
8. Transformer tanks are fabricated with steel plates wide enough so that there are no weld joints on each surface. The steel
plates for tank walls are pressed into corrugated shape to increase mechanical strength and to reduce noise due to vibration of
the tank wall. Secondary wirings are housed within stainless steel conduits and ended at terminals in a stainless steel control
cabinet, resulting in higher reliability and giving a neat appearance.
9. Diaphragm membrance is provided within the conservator, preventing direct contact of oil with air, protecting the oil from
aging, thereby prolonging the service life of oil. Magnetic oil-level gauge reflects correctly the oil level in conservator. Flat
plate-type radiators effectively cool down the transformer.
10. Quality assurance rules are observed from the very beginning of acquisition of raw materials and components. No rusting,
colliding, scratching or hammering is allowed for steel plate. Special attention is paid to those parts susceptible to oil leakage.
All the gasket groove surfaces are carefully machined, and limits are provided to prevent the gasket material from being
compressed over its elastic limit. The transformer tank is integrally shot-blasted, and burrs and welding slag are thoroughly
removed. Radiators and other component parts are tested in accordance with their respective service condition. Usually the
flanges of upper and lower parts of the tank are bolted together with double gasket grooves in their jointing surface and kept
tight with high quality gasket material, effectively preventing oil leakage from the jointing surface. If required by the customer,
the upper and lower part of the tank can also be welded oil tight.
11. High quality paint is applied as surface finish of the tank and exposed steel parts, after shot-blasting and pretreatment, thus
ensuring a durable life and glossy surface.
12. Quality assurance rules are also observed for all insulating parts, from raw material to final assembly. Strict regulations are
observed for drying, wrapping and storage, preventing the absorption of moisture, swelling, and deformation. Finished spacers
and pressure rings are all stored in practically isothermal oven. Finished windings, after pressing and adjustment are conserved
in specially made isothermal cylinders. For the core and oil assembly, the time interval between lifting out from oven and
tanking is strictly controlled, so that no significant absorption of moisture can occur due to exposure.