（a）NK
Influence of Copper on the By-products and Dielectric Properties of
Different Oil-paper Insulations
Jian Hao1, 2, Ruijin Liao1, George Chen2 and Chao Ma3
1University of Chongqing, Chongqing, China
2University of Southampton, Southampton, UK
3Gansu Electic Power Reaserch Institute, China
Introduction
Experiments
The pretreatment of samples and the flow chart of ageing test are shown in Figure 1. After
being pretreated, the moisture content of paper or paper-wrapped copper strip is 0.70%. The
oil samples have a moisture content of 9 mg/kg. 400 ml oil and the paper (7.5 g) or paperwrapped copper strip (paper: 7.5 g, copper strip: 215 cm2) were placed in the same glass vial
(500 ml), respectively, as presented in Figure 2.
7
6
5
4
3
2
0
0
10
20
3.0
immersion oil in vacuum for
24 hours(400C, 50Pa)
NK
PK
2.0
1.5
1.0
0
10
Figure 9 shows the dissipation factor (tanδ) and volume conductivity (dv) plots of oil and oil
impregnated paper in NK and NKC sample aged for 77 days, respectively. It can be seen that
the oil in NKC sample has higher tanδ and dv values than the oil in NK sample. In addition,
the oil impregnated paper in NKC sample also has higher tanδ and dv values than the oil
impregnated paper in NK sample.
-8
0.10
0.08
10
NK
NKC
0.012
0.010
0.06
tand
Figure 2. Photographs of samples
10
0.006
0.004
0.04
0.002
0.02
Experimental Results
10
0
10
1
10
2
10
3
10
4
NK
NKC
-9
0.008
paraffinic oil＋Kraft paper＋copper
Figure 7. Color of innermost paper in
NK, NKC, PK and PKC (from left to right)
The innermost paper next to the copper plate of NKC samples aged for 77 days were
measured by SEM. Figure 8 shows that there is copper element deposited on the surface of
paper.
paraffinic oil＋Kraft paper
PKC
80
Conductivity (S/cm)
PK
70
It’s noteworthy that the moisture content of paper aged in PKC is higher than that aged in
PK, and the moisture content of paper aged in NKC is higher than that aged in NK.
Table 1. Samples used for thermal ageing test
naphthenic oil＋Kraft paper＋copper
20 30 40 50 60
Ageing Time (day)
(a) NK
(b) NKC
Figure 8. SEM results of innermost paper in NK and NKC
Figure 1. Pretreatment of samples
NKC
NKC
PKC
bottling oil and paper-wrapped copper
plates samples samples(NKC, PKC)
samples ageing at 130 C
naphthenic oil＋Kraft paper
80
Figure 6. Water content in paper varying
with ageing time
0
NK
70
2.5
vacuumizing the bottles and
sealing them with nitrogen
Sample composition
60
Paper Property:
vacuum drying for 48
hours(900C, 50Pa)
Sample
30 40 50
Time (days)
Figure 5. Content of copper substance
dissolved in oil
Kraft paper or paper-wrapped
copper plates samples
bottling oil and paper samples without
copper plates(NK, PK)
The increase formation of insoluble sludge will
cause more heavy copper-organic compounds
sediment in the sludge. Consequently, a decreased
content of copper substance dissolved in oil is
observed after ageing 35 days, and the content of
copper substance dissolved in paraffinic oil is less
than that in naphthenic oil.
1
0.5
vacuum cooling
oil to 400C
The increased oil acidity will increase the
corrosion rate of copper. Therefore, the copper
corrosion in PKC sample is more serious than that
in NKC sample. The content of copper substance
dissolved in paraffinic oil is larger than that in
naphthenic oil up to ageing 35 days as shown in
Figure 5.
PKC
NKC
Water Content of paper(%)
naphthenic oil or
paraffinic oil
vacuum drying for 48
hours(900C, 50Pa)
8
Copper Products Content
of Oil(mg/Kg)
Transformer failure caused by the corrosion of copper material in transformer attracts great
attention of researchers and engineers. In this paper, 25# naphthenic mineral oil, 25#
paraffinic mineral oil, Kraft paper and copper were used to compose four combinations of oilpaper insulation samples. The ageing by-products and dielectric properties of the four
combinations of oil-paper insulation samples were compared after being thermally aged at
130°C for 77 days. The influence of copper on the by-products and dielectric properties of
different oil-paper insulations was obtained.
10
5
10
6
-10
10
-11
10
-12
10
-13
10
-14
10
-15
10
0.00
-16
-3
-2
10
-1
10
0
10
10
1.0
0.8
0.6
0.4
0.2
0.0
0
10
20
3
4
5
10 10 10 10
Frequency (Hz)
10
6
10
10
-3
10
30 40 50
Time (days)
60
70
80
60
55
0.06
NK
NKC
0.4
0.04
0.3
0.02
0.2
45
0.1
40
0.0
35
0.00 0
1
2
3
4
5
6
10 10 10 10 10 10 10
-3
10
30
-2
10
-1
10
0
10
1
2
3
4
10 10 10 10
Frequency (Hz)
5
10
(c) tanδ of oil impregnated paper
25
0
10
20
30 40 50
Time (days)
60
70
80
Figure 3. Changes of oil acidity during oil-paper Figure 4. Breakdown voltage of oils versus
ageing time
insulation samples ageing process
Figure 3 shows that NKC samples with copper have higher oil acidity than NK samples
without copper all the time. However, PKC samples with copper have lower oil acidity than
PK samples without copper initially, after aged 29 days the oil acidity of PKC samples
becomes higher than that in PK samples.
Figure 4 indicates that the existence of copper substance in oil-paper insulation during the
ageing process weakens oil AC breakdown property. Naphthenic oil performs better than
paraffinic oil under the same ageing condition.
-1
10
0
10
1
2
3
4
10 10 10 10
Frequency (Hz)
5
10
6
10
-6
0.5
50
-2
10
(b) dv of oil
6
10
Conductivity (S/cm)
PKC
NKC
PK
NK
PKC
PK
NK
NKC
65
tand
1.2
2
(a) tanδ of oil
Breakdown Voltage of Oil (kV)
Acid Value of Oil(mgKOH/g)
Oil Property:
1
10
NK
-7
10
NKC
-8
10
-9
10
-10
10
-11
10
-12
10
-13
10
-14
10
-15
10
-3
-2
-1
0
1
2
3
4
5
6
10 10 10 10 10 10 10 10 10 10
Frequency (Hz)
(d) dv of oil impregnated paper
Figure 9. Dielectric loss and volume conductivity of insulating oil and oil-immersed paper
of NK and NKC sample aged for 77 days
Conclusions
 Copper can accelerate the ageing rate of insulating oils and reduce their AC breakdown
voltage. The content of copper substance dissolved in insulating oil increases with ageing time
at first and then decreases.
Copper has great effect on the moisture content of oil impregnated paper in the ageing
process. SEM results show that there is copper product deposited on the insulating paper
surface , which will deteriorate the dielectric properties of oil-paper insulation.
Contact details :
cquhaojian@126.com
University of Southampton, Highfield, Southampton, SO17 1BJ, UK