Tettex MIDAS 2881, Operating Instructions Manual

Page: 23 / 164

Theory 13
An increase of dissipation factor above a typical value may indicate conditions such as those showed
above: If the dissipation factor varies significantly with voltage down to some voltage below which it is
substantially constant, then ionization is indicated. If this extinction voltage is below the operating
level, then ionization ma progress in operation with consequent deterioration. Some increase of
capacitance (increase in charging current) may also be observed above the extinction voltage
because of the short-circuiting of numerous voids by the ionization process.
An increase of dissipation factor accompanied by a marked increase of the capacitance usually
indicates excessive moisture in the insulation. Increase of dissipation factor alone may be caused by
thermal deterioration or by contamination other than water.
Unless bushing and pothead surfaces, terminal boards, etc., are clean and dry, measured values not
necessarily apply to the insulation under test. Any leakage over terminal surfaces may add to the
losses of the insulation itself and may give a false indication of its condition.
Dissipation Factor of Typical Apparatus Insulation
Values of insulation dissipation factor for various apparatus are shown in this table. These values are
useful in roughly indicating the range to be found in practice; however, the upper limits are not reliable
service values.
Equipment Dissipation factor @ 20°C
Oil-filled transformer, New, HV ( > 115kV) 0.25% .. 1.0%
Oil-filled transformer, Age 15 years, HV ( > 115kV) 0.75% .. 1.5%
Oil-filled transformer, Age 15 years, LV, distribution 1.5% .. 5%
Circuit breakers, oil-filled 0.5% .. 2.0%
Oil-paper cables, "solid" (up to 27.6 kV) new 0.5% .. 1.5%
Oil-paper cables, HV, oil-filled or pressurized 0.2% .. 0.5%
Stator windings, 2.3 .. 13.8kV 2.0% .. 8.0%
Capacitors 0.2% .. 0.5%
Bushings, (solid or dry) 3.0% .. 10.0%
Bushings, compound-filled, up to 15kV 5.0% .. 10.0%
Bushings, compound-filled, 15 .. 46kV 2.0% .. 5.0%
Bushings, oil-filled, below 110 kV 1.5% .. 4.0%
Bushings, oil-filled, above 110 kV 0.3% .. 3.0%
Dissipation Factor and Dielectric Constant of Typical Insulation Materials
Typical values of 50/60Hz dissipation factor and permittivity (dielectric constant ε ) of some typically
used insulating materials.
Material Dissipation factor @ 20°C ε
Acetal resin (Delrin™) 0.5% 3.7
Air 0.0% 1.0
Askarels 0.4% 4.2
Kraft paper, dry 0.6% 2.2
Transformer oil 0.02% 2.2
Polyamide (Nomex™) 1.0% 2.5
Polyester film (Mylar™) 0.3% 3.0
Polyethylene 0.05% 2.3
Polyamide film (Kapton™) 0.3% 3.5
Polypropylene 0.05% 2.2
Porcelain 2.0% 7.0