An insulation system is an assembly of fabricated, processed and in-place combinations of component insulating materials with related structural parts as used in liquid-filled transformers, in this case power units as referenced in ANSI C57.12.00-1987.
Coordination of Insulation
Solid insulation is required in a transformer whenever a difference in potential exists between two points. The selection of insulation is generally made in proportion to the anticipated overvoltages and with a safety margin to compensate for decreases due to normal service aging. Various components are designed to work best together and achieve what is called "coordination of insulation" within the insulation system.
A conclusion that should have been reached by now is that insulation is one of the most important, if not the most important, component in a transformer. The internal insulation of the transformer are a number of critical areas that must be adequately insulated to assure that the transformer will operate properly and provide a long service life (Figures 24 and 25). These areas are:
• Turn-to-turn insulation
• High-voltage to low-voltage insulation • Low-voltage to core insulation
• Phase-to-phase insulation • Core-to-ground insulation
These insulation areas must have proper types and combinations of insulation selected and in place to have a transformer that will operate during normal and abnormal conditions and provide a long service life.
Engineering Encyclopedia Electrical Power Transformers
High Density Organic Winding Sticks (B) High Density Cellulose Spacers (B) High Density Kraft Paper Tube Betwween Primary & Secondary Windings (C) High Density Organic Winding Sticks (C) High Voltage Windig Paper Insulation (A) Laminated Magnetic Steel Core Paper Insulation (A) Low Voltage Winding Copper High Density Kraft Paper Tube (B)
Heavy Cellulose Phase-to-Phase Insulation (D)
Rule of Thumb:
0.3 x KVA Rating = Weight of cellulose paper in pounds Example: 0.3 x 1500 KVA = 450 pounds cellulose insulation Phase A Phase B Phase C Phase Insulation
Figure 25. Oil-Filled Cellulose System
[Basic insulation system of a core type power transformer where, (A) is insulation on wire; (B) is insulation to ground; (C) is insulation between windings; and (D) is
Engineering Encyclopedia Electrical Power Transformers
Types of Insulating Materials
Transformers use various types of insulating materials, which together, form an insulation system.
• Pressboard (1/8"-1/2" thick) • Kraft paper (5-20 mils thick) • Manila and hemp paper • High-density particle-board
• Pressboard collars and end insulation • Laminated (plywood-type particle-board)
• Enamels
• Inorganic and organic core • Lamination coatings
• Porcelain
• Epoxy power coatings • Maple wood structural forms • Vulcanized fiber
• Cotton
• Plastics and cements, adhesive tapes, glass-fiber bands, etc. • Liquid dielectric fluid
The transformer insulation system materials isolate the windings from each other and from ground to "insulate" the current-carrying parts of the transformer from the magnetic-iron and structural-steel parts. The insulation is more than just a mechanical means for keeping the wires or turns apart.
Coil Assembly Insulation
Winding Insulation – Insulation material provides barriers to voltage to prevent short
circuits. It must always be placed between individual turns and layers of the windings. Kraft paper and pressboard paper are the two principal materials employed. Paper or synthetic enamel having sufficient dielectric strength is suitable for strip on wire. For low voltage coils, two insulation enamels on the wires are sufficient, but paper is still needed for larger insulation at higher voltages. In some cases, enamel and paper are both used.
Regardless of the type of winding, paper spacers or wood are also provided to form an oil duct to allow circulation for cooling and insulation. The ducts have to be constructed to permit circulation of the liquid or air up through the coils.
Insulation Between Windings – Insulation between windings is also required or from coils to ground. Selected pressboard or synthetic resin-bonded cylinders and end insulation such as paper washers, collars, or spacing blocks are the major items.
Functions of Solid Insulation
The solid insulation in a oil-filled transformer "insulates" because it possesses two distinct properties:
1. Ability to withstand both electrical and mechanical stresses due to the voltage used
2. A poor conductor that allows only negligible to small current to flow through it This means a functional insulation system for an oil-filled transformer must contain material that performs four major functions.
1. Ability to withstand normal service high voltage (impulse and transient surges - dielectric strength)
2. Ability to withstand mechanical and thermal stresses (short circuit) 3. Ability to prevent excessive heat accumulations (heat transfer)
4. Ability to maintain desired characteristics for an acceptable service life period given proper maintenance
Any weakness of insulation may result in the failure of a transformer. Insulation is deteriorated when it has lost a significant portion of its original dielectric, mechanical, or impulse strength. The continuation of the deterioration process leads to inevitable mechanical
Engineering Encyclopedia Electrical Power Transformers
Classification of Solid Insulation
The effect of temperature upon insulation materials is so significant that the thermal characteristics has become the basis for the classification of electrical solid insulation (Table 1).
This temperature should not be confused with the actual temperature at which the insulating materials may be used in particular environments (air, oil or gas) or with temperatures on which specified temperature rise in equipment standards is based. These temperature classifications refer only to the thermal evaluation of insulating materials themselves or simple combinations. An example is that some materials which are suitable for operation at one temperature in air may be suitable for a higher temperature when used in a system operated in an inert gas atmosphere or in oil. Also, some materials when operated in dielectric liquids, may have lower or higher thermal endurance than when operated in air. Even though the thermal characteristics are recognized as the most important, other factors such as mechanical strength and moisture resistance are required in varying degrees for the successful use of insulating materials.
TABLE 1
THERMAL CLASSIFICATION