Oil immersed electrical power transformers

1.The core

The iron core forms the central element of every transformer. High-grade, cold-rolled and laser-treated metal sheets with thicknesses of 0.3 mm or less are precision-cut with computer-controlled machines, to ensure compliance with even the smallest tolerances. The individual sheets are then assembled into cores manually using the “step-lap” technique. In this way we achieve an especially good flux distribution at the joints, resulting in exceptionally low losses and minimal no-load noise. When the transformer core is ready, a hydraulic platform moves it into the vertical installation position in which the windings are assembled.

2. The windings

Transformer windings are subject to continuously high electrical and mechanical loads. Disk and cylindrical coils with transposed conductors of copper wire guarantee high mechanical strength here and long operational reliability.

3. Voltage control

The grid and generator voltage can vary significantly from the rated value as a result of external influences. Transformers therefore provide facility for adapting the voltage to the grid conditions. It can either be changed in the deenergized state by means of a deenergized tap changer or adjusted in steps under load using on-load tap changers. On-load tap changers and optionally deenergized tap changers are fitted with a motorized drive and can be controlled either locally or remotely.

4.The tank

High-grade, quality-assured tanks define the appearance of our transformers. This protective shell accommodates the core-and-winding assembly and the insulating oil.Together these often weigh several hundred tons. The tank should therefore not add too much extra weight, but at the same time it has to be structurally sound and leakproof, and must withstand mechanical stresses. First-class corrosion protection is therefore a basic requirement for long tank life.

5. Cooling

The necessity of not exceeding the generally permissible maximum heating and of avoiding “hot spots” in the transformer require the cooling system to be dimensioned accordingly. Various methods of cooling can be used depending on the individual service conditions to guarantee reliable and problem-free operation over many years. The most noteworthy are the ONAN, ONAF, OFAF, and ODAF oil-air cooling and OFWF and ODWF oil-water cooling systems. The radiator banks and oil-air and oil-water coolers can be attached to the transformer or installed separately.

6. Final assembly

The core-and-coil assembly of the transformer is made up of core, windings, tap switches, and connecting cables. Particular emphasis is placed on the mechanical stability of the windings. Axial shear forces are minimized by a joint press ring for all windings on a core limb, since this fixes the geometrical position of the individual windings. The core-and-coil assembly is then dried in a vapor-phase system under vacuum at 130°C. After drying, all bolted connections on the core-and-coil assembly, which is still at a temperature of over 100°C are tested and secured, and the assembly is installed in the tank as quickly as possible. For increased life expectancy, the transformer is filled with insulating oil without delay so that the insulating parts absorb no moisture from the ambient air.

7. Final inspection and testing

Only after final inspection and testing is successfully completed do we dispatch a transformer to the customer. Each of our factories has a high-voltage test bay for this purpose in which a wide range of tests is carried out, from voltage tests – including lightning impulse voltage withstand tests and temperature-rise tests, to special tests for insulation resistance, harmonics, or noise level. Shortcircuit tests are performed in internationally recognized and approved institutes.

8. Transformer service

To complement its position as a market leader of transformer manufacturing and supply, ZHONGYI provides testing and commissioning services.

Our highly qualified and competent commissioning engineers undertake the complete electrical testing and commissioning of transformers and ancillary equipment.