Megger DELTA 4000, Reference Manual

Page: 11 / 48

ZM-AH02E DELTA 4000 11
2 INTERPRETATION OF MEASUREMENTS
Dissipation factor (Power
factor) of typical apparatus
insulation
Values of insulation dissipation factor for various appara-
tus are shown in Table 2.1. These values may be useful in
roughly indicating the range to be found in practice. Please
note that the higher values are not to be regarded as “OK”
but instead examples of “to be investigated/at risk” data.
Table 2.1
DF (PF) of typical apparatus insulation
Type apparatus % DF (PF) at 20°C
Oil-filled transformer: New, high-
voltage (115 kV and up) 0.25 to 1.0
15 years old, high-voltage 0.25--
Low-voltage, distribution type 0.30--
Oil circuit breakers 0.5 to 2.0
Oil-paper cables, “solid” (up to 27.6
kV) new condition 0.5 to 1.5
Oil-paper cables, high-voltage oil-filled
or pressurized 0.2 to 0.5
Rotating machine stator windings, 2.3
to 13.8 kV 2.0 to 8.0
Capacitors (discharge resistor out of
circuit) 0.2 to 0.5
Bushings: Solid or dry 3.0 to 10.0
Compound-filled, up to 15 kV 5.0 to 10.0
Compound-filled, 15 to 46 kV 2.0 to 5.0
Oil-filled, below 110 kV 1.5 to 4.0
Oil-filled, above 110 kV and con-
denser type 0.25--
In IEEE 62-1995, typical values for dissipation/power fac-
tor are given as in Table 2.2.
Table 2.2
IEEE 62-1995 power factor values
Typical power factor values @ 20°C
“New” “Old” Warning/alert limit
Power trans-
formers, oil
insulated
0.2-0.4% 0.3-0.5% > 0.5%
Bushings
0.2-0.3% 0.3-0.5% > 0.5%
IEEE 62-1995 states; “The power factors recorded for
routine overall tests on older apparatus provide information
regarding the general condition of the ground and inter-
winding insulation of transformers and reactors. While the
power factors for older transformers will also be <0.5%
(20°C), power factors between 0.5% and 1.0% (20°C) may
be acceptable; however, power factors >1.0% (20°C) should
be investigated.”
Permittivity and % DF of
typical insulating materials
Typical values of permittivity (dielectric constant) k and
50/60 Hz dissipation factor of a few kinds of insulating
materials (also water and ice) are given in Table 2.3.
Table 2.3
Permittivity and dissipation factor of typical
insulating materials
Material k % DF (PF) at 20°C
Acetal resin (Delrin*) 3.7 0.5
Air 1.0 0.0
Askarels 4.2 0.4
Kraft paper, dry 2.2 0.6
Oil, transformer 2.2 0.02
Polyamide (Nomex*) 2.5 1.0
Polyester film (Mylar*) 3.0 0.3
Polyethylene 2.3 0.02-0.05
Polyamide film (Kapton*) 3.5 0.3
Polypropylene 2.2 0.05
Porcelain 7.0 2.0
Rubber 3.6 4.0
Silicone liquid 2.7 0.01
Varnished cambric, dry 4.4 1.0
Water** 80 100
Ice** 88 1.0 (0°C)
* Dupont registered trademark.
** Tests for moisture should not be made at freezing tempera-
tures because of the 100 to 1 ratio difference of % dissipation
factor between water and ice.