In assessing dielectric strength of power transformers, three subsystems should be identified: energized components, grounding and insulation. The latter is intended to separate the former two, as well as separate energized parts.
Energized parts include all components that carry electrical current, as well as all the elements that are electrically connected thereto. Examples include windings, taps, buses, filters, capacitive rings, metal caps of wall bushings, etc.
Grounding system includes a magnetic system with all metal parts, tank and cooling system with all metal parts and metal fittings in oil-filled transformers or protective cover in dry transformers.
Insulation system is made of solid dielectric materials like porcelain, electrical insulation cardboard, cable paper, etc. Insulation spaces which are not filled with solid dielectrics are filled with transformer oil. In dry transformer the function of oil is performed by the air.
The insulation of windings is divided into: primary insulation (from earthed parts and other windings), and longitudinal insulation (between different points of each specific winding). The classification of switch and tap insulation has the same principle.
Insulation system of a transformer must withstand mechanical, thermal, electrical, physical and chemical loads that may arise during the operation of the transformer. Considering its importance, it is not surprising that most of the transformer cost is the cost of its insulation. For example, the cost of insulation of a transformer with voltage 220-500 kV (oil included) reaches 15-20% of the total cost of such transformer.
In order to ensure electrical and mechanical strength, as well as thermal resistance of transformer insulation, it is necessary to:
- consider all the factors that affect the insulation during operation;
- choose the right methods and values that affect the insulation during commissioning and tests;
- choose the right size, materials, technology of insulating structures manufacturing and processing.
If all the calculations regarding the transformer and its operation are correct, then service life of its insulation system can reach 20-25 years or more.
Insulation servicing technology is of great importance in ensuring its dielectric strength; an important role in this process is played by post-assembly vacuum drying before filling the transformer with oil. This operation is required to remove gases and moisture from insulation in order to increase its dielectric strength and reduce its dielectric losses.
GlobeCore “Iney”-type plants are used to perform this operation in practice. They are intended for vacuuming and drying of solid insulation. Due to its construction, “Iney” is capable of developing temperatures up to -70ºC, which causes the condensation of water located in windings. Also, these plants allow you to see how well the windings and the transformer tank are dried.