Three-Phase Two-Winding 33kv off-Circuit Tap Changing Power Transformers

1. Low loss, low noise
The core laminations are composed of imported high quality grain-oriented cold-rolled silicon steel sheets with miter joints and
the joints are lapped with 5 steps, thus ensuring low loss and low noise. The inside of the tank is lined with shields. Measures
have also been taken for metallic structural parts located in high magnetic field, to reduce stray loss, thereby the total load loss,
and to relieve local overheating.
2. Safe and reliable service
Reliability of the products is the everlasting objective of Silverstone Electric and we ensure that the transformers can be
operated under long service voltage and also under short time overvoltage. Appropriate cooling arrangement and relatively
low current density ensure that the winding temperature rise is much lower, at least 5K, than specified in the national standard.
The mechanical stress in winding conductors during short circuit is also kept well below safe limit. Together with technological
procedures during manufacture and other means to elevate mechanical strength, 220kV~500kV transformers that we
supplies could withstand severe short circuit in all cases during service.
3. Easy erection and maintenance
To minimize erection work at site, Silverstone Electric transformers are so fabricated that lifting the active part out of the tank
for inspection after arrival at site is not mandatory. Besides, all components to be mounted on the transformer at site have
been pre-assembled before dispatch, and all secondary wirings have been laid down into conduits on the transformer and
connected to the terminals in the control cabinet or terminal box. Natural oil circulation and forced air cooling (ONAF) are
generally adapted in our transformers below 480MVA, and 70% nameplate rating can be achieved in case the fans are out of
service. Natural oil circulation is adapted in transformers below 240MVA. Little maintenance work is required for the cooling
devices.
4. Careful fabrication
Vertical winding machines with tension device are used for winding, so that the windings can be wound tightly and therefore
mechanically strong as a whole. All insulation parts subjected to mechanical stress during short circuit, such as spacers in
windings are made with T4 densified pressboard to reduce shrinkage. Individual windings are dried first, followed by a second
drying of the assembled phase block. After core and coil assembly, Vapor Phase Drying (VPD) is once again carried out to ensure
that all insulation parts are thoroughly dried and no further shrinkage is possible to occur during service. All the high voltage
terminals and nearby metallic structural parts are rounded up to prevent partial discharge. High voltage bushing terminals are
contained within turrets, and kept concentric with the turret. All the leads are properly crimped and metallized to be rounded
up. VPD process insures that the moisture content within the transformer is kept at a very low level, and the core and coil
assembly is once more cleaned by condensed vapor flowing down the surfaces. The fabricated tanks are relieved from welding
stress and shot-blasted integrally, so as to be free from oil leakage trouble and to give the tank a clean, pleasant looking
appearance.