The influence of polypropylene laminated paper on the breakdown strength of insulation for high temperature superconducting cable

trength of PPLP insulation was reported by other researchers 
17. However, the difference between negative and positive breakdown strength in this study is 
much larger than that observed in 17. This is because the sphere-plane electrode system was 
used in this study while the more homogeneous electrode system, i.e. the coaxial system, was 
used in 17. From this result, it is inferred that the polarity of the applied voltage has a strong 
effect on the breakdown strength voltage of PPLP. In addition, the decline of the impulse 
breakdown strength is larger than that of the AC breakdown strength, and the slope of the 
breakdown strength line for the without butt gap samples is much larger than that of the with 
butt gap sample. From these results, it is clearly seen that the impulse breakdown strength is 
more thickness dependent than the AC breakdown voltage, and the without butt gap samples 
also have a stronger thickness dependence than that of the with butt gap samples. 
3.2. The breakdown strength characteristic of Kraft 
Figure 5 shows the breakdown strength of Kraft as a function of thickness. The breakdown 
strength decreases linearly as the thickness increases, and the value of the breakdown strength in 
the without butt gap samples is always higher than that of the with butt gap samples. This result 
is in line with that observed by other researchers 4. Moreover, the impulse breakdown strength 
is higher than the AC breakdown strength, and the standard deviation in the impulse case is also 
larger than in the AC case. The superior value of the impulse breakdown strength in comparison 
with AC breakdown strength of Kraft paper was reported in previous studies 10, 11. However, 
there is a small difference in the breakdown strength between positive and negative impulses for 
both with and without butt gap samples as increasing the thickness. A similar result was 
observed with the testing of one sheet of Kraft paper 18. This is possibly because positive and 
negative charges spread easily into Kraft paper 12. It is clear that the polarity of the applied 
voltage has a little effect on the breakdown strength of Kraft paper. The slope of the impulse 
breakdown strength line seems to be parallel to that of the AC breakdown strength line in both 
Nguyen Van Dung, Hoang Dang Khoa 
50 
with butt gap and without butt gap samples. However, the breakdown strength line in the without 
butt gap samples is much steeper than for the with butt gap sample. From these results, it is 
concluded that the effect of thickness on the breakdown strength of Kraft is only slightly 
dependent on the kind of applied voltage but strongly dependent on the state of the sample, i.e., 
whether or not it has a butt gap. 
 Figure 4. Breakdown strength versus thickness of PPLP. Full symbols for without butt gap; open 
symbols for with butt gap. 
Figure 5. Breakdown strength versus thickness of Kraft. Full symbols for without butt gap; 
open symbols for with butt gap samples. 
40
80
120
160
200
0 0.2 0.4 0.6 0.8 1
Thickness (mm)
B
re
ak
d
o
w
n
 s
tr
en
g
th
 (
k
V
/m
m
)
AC
Positive impulse
Negative impulse
Circular butt gap 
of 6 mm
With butt gap 
specimen
Without butt gap 
specimen
PPLP 
layers
40
80
120
160
0.0 0.2 0.4 0.6 0.8
Thickness (mm)
B
re
ak
d
o
w
n
 s
tr
en
g
th
 (
k
V
/m
m
)
AC
Positive impulse
Negative impulse
Circular butt gap 
of 6 mm
With butt gap specimen
Without butt gap specimen
Kraft 
paper 
layers
The influence of polypropylene laminated paper on the Breakdown strength of insulation  
51 
3.3. The breakdown strength characteristic of PPLP/Kraft 
The comparison of breakdown strength between PPLP and Kraft paper with butt gap 
sample as a function of thickness is shown in Fig. 6. The breakdown strength and its standard 
deviation of PPLP are higher than those of Kraft. The similar result was previously reported in 
references 4, 5, 10. This could be due to the partial discharges occurring inside porous 
structure of Kraft paper, but this is not the case of PP layer 18. However, the difference in the 
value of the breakdown strength between PPLP and Kraft becomes smaller as the thickness 
increases for positive impulse case. In addition, the slope of the breakdown strength line of 
PPLP tends to be steeper than that of Kraft, so the thickness of PPLP has a more significant 
effect on the breakdown strength characteristics as compared to Kraft paper. 
The breakdown strength of PPLP/Kraft for the without butt gap sample is shown in Fig. 7. 
It shows that the value for the impulse case is higher than that of the AC case and the value for 
the negative impulse case is highest. The breakdown strength strongly increases as the number 
of PPLP layers increases in the PPLP/Kraft samples for the negative impulse case, and just 
slightly increases for the positive impulse and AC cases. This result is in agreement with that 
observed under AC and positive impulse 10. Moreover, the AC breakdown strength varies only 
slightly for both the upper and lower cases, but the impulse breakdown strength shows 
somewhat more clear difference between the upper and lower cases. However, the difference in 
the values between the upper and lower cases is larger for the negative case. From these results, 
it shows that the number of PPLP in PPLP/Kraft has a strong effect on the negative impulse 
breakdown strength and a slight effect on the AC and positive impulse breakdown strength. 
However, the inserting place of PPLP in PPLP/Kraft has a strong effect only on impulse 
breakdown strength for the negative impulse. 
Figure 6. Breakdown strength comparison. Full symbols for PPLP; open symbols for Kraft samples. 
40
80
120
160
200
0.0 0.2 0.4 0.6 0.8 1.0
Thickness (mm)
B
re
a
k
d
o
w
n
 s
tr
e
n
g
th
 (
k
V
/m
m
)
AC
Positive impulse
Negative impulse
Circular butt 
gap of 6 mm
Specimen of PPLP
Circular butt 
gap of 6 mm
Specimen of Kraft 
paper
Nguyen Van Dung, Hoang Dang Khoa 
52 
Figure 7. Breakdown strength of PPLP/Kraft for the without butt gap samples. 
The breakdown strength of PPLP/Kraft with butt gap samples is shown in Fig. 8. It shows 
that the AC breakdown strength increases as the number of PPLP layers in PPLP/Kraft 
increases, and the value of specimen 9 (PPLP only) has the highest value while specimen 1 
(Kraft only) has the lowest value. However, the breakdown strength varies only slightly as the 
inserting place of PPLP (see 3, 4, 5) changes. These results was observed by other 
researchers 19. The results show that the AC breakdown strength of the PPLP/Kraft specimens 
is only slightly affected by the relative position of PPLP in the specimen but rather strongly 
affected by the component of PPLP and Kraft paper in PPLP/Kraft sample. The reason could be 
considered that PPLP has a higher partial discharges proof compared to Kraft paper 19. The 
partial discharges may occur both in the butt-gap and interfaces between layers. The positive 
impulse breakdown strength of PPLP (9) is a little higher than that of Kraft paper (1) due to 
the effect of the layer thickness of the latter. The breakdown strength varies greatly as the 
component ratio of PPLP and Kraft in PPLP/Kraft varies, and the value of specimen 5 has the 
highest value in comparison with those of specimens 3 and 4, which indicates that the 
breakdown strength becomes larger as the distance of the PPLP from positive electrode, i.e. the 
spherical electrode, becomes larger. This is due to the effect of trapping positive charges, which 
are formed by positive streamers in the butt gap, by PP film, leading to increase the local electric 
field intensity as seen in Fig. 9 [12]. When the distance from the PPLP to the positive electrode 
increases, therefore, positive charges have a change to spread into Kraft paper and hence the 
accumulation of positive charges on the surface of PP film reduces. This results in a lower local 
electric field and higher breakdown strength. The breakdown strength varied depending on the 
number and the position of PPLP in PPLP/Kraft as observed in specimen 5 which is in line 
with that observed in references 10, 12. This explains why the impulse breakdown strength of 
PPLP/Kraft is higher than that of Kraft or PPLP alone. 
Without buttgap
0
40
80
120
160
0 1 2 3 4 5 6 7
Number of PPLP
B
re
ak
d
o
w
n
 S
tr
en
g
th
 (
k
V
/m
m
)
Negative impulse
Positive impulse
AC
Upper case
Lower case
layers
Lower case specimen
Upper case specimen
PPLP layer
Kraft paper
layer
The influence of polypropylene laminated paper on the Breakdown strength of insulation  
53 
Figure 8. Breakdown strength of PPLP/Kraft for the with butt gap samples. 
Figure 9. Charge accumulation on PPLP 
Figure 10. Images of positive impulse breakdown spots. 
0
40
80
120
B
re
ak
d
o
w
n
 s
tr
en
g
th
 (
k
V
/m
m
)
Specimens
1 98765432       
AC
Positive impulse
With butgap
PPLP 
Kraft 
+
+
++++
+ +
+ -
-
-
-
+
+
+
+
+
+
+
+
+ +
+ +
+
+
+
+
+
+
+
+
++ + +
+
++++
+ +
+ -
-
-
-
+
+
+
+++ +++++ ++ + +
+ + +
Positive streamers
High electric field 
region
PPLP
Kraft 
paper
Butt gap
 Breakdown hole 
10 mm 
 Breakdown hole 
10 mm 
a. Positive impulse breakdown spot on PPLP 
layer
b. Positive impulse breakdown spot on Kraft 
layer
Nguyen Van Dung, Hoang Dang Khoa 
54 
Figure 10 shows the breakdown-spot images of the PPLP layer in specimen 3 and Kraft 
layer in specimen 5 after positive impulse breakdown. The breakdown spot of the PPLP layer is 
obvious, and the breakdown hole is big. On the other hand, the breakdown spot of the Kraft 
layer looks complicated, and breakdown hole is small. This result is due to the fact that positive 
charges are trapped on the surface of the PPLP, while they can spread easily into Kraft paper. 
Similar images were captured for PPLP and Kraft paper in a previous study 12. 
4. CONCLUSIONS 
The breakdown characteristics of PPLP/Kraft insulation was performed, and the 
experimental results show that PPLP has a significant effect on the breakdown strength of 
PPLP/Kraft insulation. In the AC case, the breakdown strength is greatly dependent on the 
component of PPLP in the PPLP/Kraft sample but only slightly dependent on the relative 
position of the PPLP due to high partial discharges proof of PP film. In the impulse case, the 
breakdown strength of PPLP/Kraft has the highest value in comparison with PPLP or Kraft. The 
impulse breakdown strength of PPLP/Kraft is strongly dependent on the component of PPLP and 
the position of the PPLP. This is due to the accumulation of positive charges on PPLP layer 
which increases the local electric field intensity. On the breakdown strength aspect, PPLP/Kraft 
composite insulation exhibits the best insulation in comparison with PPLP and Kraft. 
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