EP-3:Transformer Insulation Materials

The insulation system of a power transformer is understood as the complete internal assembly of dielectric insulating materials. This includes parts and supporting structures that cover the winding wires, insulate the turns from each other in each winding, separate different winding bodies from each other and from the core and tank, see fig.1 for winding insulation . Such insulation systems may be divided to two main categories according to insulation material types as follows:

  1. Solid insulation materials 
  2. Fluid insulation materials 

A good insulation material must have the following properties:

  • High dielectric strength.
  • Good mechanical properties.
  • Long lifetime at operating temperature.
  • Easily workable.

Insulation materials must withstand the operating temperature that occurs in the transformer during the lifetime of the transformer.
The insulation materials to be used in liquid immersed transformers must be compatible to the liquid.
see fig.2 for winding production.


1- Solid insulation materials:
Solid insulation materials include the following types:

  • Cellulose materials 
  • Wood 
  • Porcelain 
  • Enamels 
  • Epoxy resins 
  • Polyesters 
  • Aramid fiber 

A- Cellulose materials:
Mainly used in oil immersed transformers with thermal class 105.
Cellulose insulation is made of slow growing types of wood, having long fibers that give long life time and high density gives high dielectric strength.
Cellulose products are compatible to mineral oil and are easy to oil impregnate.
Cellulose insulation is specified under IEC 60554-3 for paper and IEC 60641-3 for board.
pressboard production machine is shown in fig.3.


B- Wood:
Laminated wood is used for different support purposes in liquid immersed transformers and the applicable IEC standard is 61061-1/2/3.

C- Porcelain:
Porcelain is a mainly used for bushings in oil immersed transformers, in some case also used as supports or spacers in dry type transformers.

D- Solid synthetic insulation materials:
These materials are mainly used in dry type transformers or reactors having higher thermal classes 130, 155, 180, 220 these materials are more expensive than cellulose insulation

D-1- Enamels: Enamels are used as conductor insulation and normally double coated under reference made to IEC 60317.
D-2- Epoxy resins:Epoxy resins used in combinations with fillers (glass fiber and quartz powder ) is used for insulation barriers and complete vacuum cast windings.
D-3- Polyesters:
Polyesters can be used as insulation barriers, spacers and duct sticks reference is made to IEC 60893-3 and IEC 61212-3.
see fig.4 for polyester insulation.


D-4- Aramid fiber:Aramid fiber are used to manufacture insulation paper or board sheets in different thickness, the material has very good thermal properties, thermal class 220.
Aramid insulation is specified in IEC 60819-3 for paper and IEC 60629-1 for board.

2- Fluids:
The fluid in a transformer has several functions ; the two most important are insulation and cooling. Another function is to carry information about the condition of the active part inside the transformer.

Several requirements have to be fulfilled as follows:

A- Chemical:
  • Oxidation stability 
  • Oxidation inhibitor content 
  • Corrosive sulphur 
  • Water content 

B- Physical:
  • Viscosity 
  • Appearance 
  • Density 
  • Pour point 
  • Surface tension 
  • Flash point 

C- Electrical:
  • Breakdown voltage AC 
  • Breakdown voltage, impulse 
  • Dissipation factor 
  • Streaming charging 

D- Additional:
  • Low particle content 
  • Compatibility with other transformer materials 
  • Gassing properties 
  • Aromatic structure 
  • Solubility properties 

Insulation Fluids types:
  • Mineral oil 
  • Dimethyl silicone 
  • Synthetic ester 
  • Synthetic hydrocarbon 
  • Agricultural Ester 

A- Mineral oil :
Important properties of mineral oil are specified in IEC 60296.
Flash point 145 deg c, density 0.88 kg/cubic dm, relative permittivity 2.2.
Mineral oil is the most liquid used; mineral oil is normally the reference to which all other liquids are compared.

Mineral oil offers in the most cases the best compromise between cost and technical properties and compatibility with other transformer materials is also very good.

B- Other fluids:
These fluids are reserved for special applications, and are typically 5-6 times more expensive than mineral oil.
The main motivation for using these fluids is improved fire safety and environmental impact.
Further these fluids are applicable for operation at elevated temperatures, however have limited capabilities in extremely cold climates.

B-1-Dimethyl silicone:
Important properties of silicone fluid are specified in IEC 60836.
Flash point 310 deg c, density 0.96 kg/cubic dm, relative permittivity 2.7.
Silicone fluid has lower dielectric and cooling properties compared to mineral oil.
Silicone fluid is not used in on-load tap changers due to its poor lubrication properties.

B-2-Synthetic ester:
Important properties of Synthetic ester are specified in IEC 61099.
Flash point 275 deg c, density 0.97 kg/cubic dm, relative permittivity 3.2.

B-3- Synthetic hydrocarbon :
Important properties of Synthetic ester are specified in IEC 60867.
Flash point 230 deg c, density 0.83 kg/cubic dm, relative permittivity 2.1.

B-4- Agricultural Ester :
No applicable IEC specifications.
Flash point 330 deg c, density 0.91 kg/cubic dm, relative permittivity 3.2.

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