(rip) bushings - Doble Engineering

EVALUATION OF THE RELIABILITY OF
RESIN IMPREGNATED PAPER (RIP) BUSHINGS
Bernhard Heil
HSP Hochspannungsgeräte GmbH
ABSTRACT
Resin Impregnated Paper (RIP) Bushings are a reliable well established solution for many applications.
Although such bushings are maintenance-free, some parameters might be controlled on a regular basis
or prior to bringing into service. For any reasonable evaluation of the reliability a founded understanding
of the physical aspects is essential. Since the condenser core is a solid material the parameters for any
evaluation are different compared with oil impregnated paper (OIP) bushings. This paper presents the
physical background in regard of relevant parameters for any evaluation. Examples from the field are
given to demonstrate how a reasonable evaluation of the reliability can be carried out.
INTRODUCTION
All types of bushings can be realized by using a resin impregnated paper (RIP) condenser core. This
condenser core is a solid one thus it does not contain any oil in contrast to oil impregnated paper (OIP)
bushings. The basic construction of the condenser core is the same for both, RIP and OIP, however, in
regard of the evaluation of the operation reliability some differences have to be taken into consideration.
Different types of RIP bushings
Figure 1
A well-known evaluation measure for OIP bushing is the dissolved gas analysis (DGA). Since RIP
bushings do not contain any oil, no aging in relation to any oil insulation occurs. However, a partial
breakdown between some grading layers is difficult to be used as an evaluating parameter in case of OIP
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bushings, since a partial breakdown may lead to a complete breakdown very rapidly. Thus it may happen,
that with offline measurements no partial breakdown is detected prior to a complete breakdown, since the
complete failure mechanism takes place between two offline measurements.
In case of a RIP bushing there is no interaction between a local partial breakdown and the remaining
condenser core since due to the solid material each partial breakdown is limited to its particular location.
Thus the remaining life time of a RIP bushing after the first partial breakdown is usually much longer
compared to an OIP bushing. Therefore the detection of a partial breakdown during an offline
measurement might be an adequate parameter for evaluation of the operation reliability of a RIP bushing.
Beside the capacitance the power factor of a bushing is an adequate parameter for evaluation. Due to the
different design of OIP and RIP condenser cores, the influencing parameters for the power factor are
different.
For each evaluation the influence of the environmental conditions (e.g. stray capacitances or
temperature) must be taken into consideration. Some parameters are influenced in the same way for OIP
and RIP bushings, but some are different. The knowledge about these influencing parameters is
important for each evaluation.
RIP BUSHINGS – PRODUCTION AND CONSTRUCTION
RIP condenser cores are manufactured by using crepped paper which is wound continuously while
inserting aluminium foils to build up a capacitive grading of the electric field strength.
After the winding process the core is impregnated with epoxy resin under vacuum and heat. The solid
core is machined afterwards to complete the final size.
Impregnating (left) and machining (right) of a RIP condenser core
Figure 2
Typically a RIP bushing has an outer composite insulator and the gap in between the condenser core and
the outer composite insulator is filled with foam (see figure 3). However, applications with porcelain
insulators can be realized as well. Due to the hydrophobicity of silicone any cleaning prior to an electrical
measurement is usually not necessary.
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dry material foam
(polyurethane)
Cutaway of a RIP bushing with composite insulator
Figure 3
AGING OF BUSHINGS
Aging under normal operation conditions is caused by electrical stress in terms of nominal voltage and
overvoltage e.g. caused by lightning or switching operations. Accelerated aging is caused by unaccepted
overvoltages or by thermal stress and humidity ingress.
OIP insulation systems are sensitive to overheating. An increase of 6 K above maximum operating
temperature causes a bisection of the life time. Any humidity ingress will have a significant influence on
the life time as well. Especially in case of an already significantly aged insulation system it might happen
that the whole insulating system fails subsequent to a partial breakdown. A failure might result in a
catastrophic failure if the bushing catches fire and the transformer is involved in the fire as well.
Failure of an OIP bushing
Figure 4
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Figure 4 (middle picture) shows a breakdown of an aged OIP bushing taken out of service because of
increased power factor. The breakdown happened in the HV-lab during testing without any indication prior
to the breakdown. In the field a catastrophic failure like shown in the right picture would have been likely.
Figure 5 shows a partial breakdown happened with a RIP bushing during operation (right picture). Since
this is not only a single partial breakdown, but several grading layers are involved, it has to be concluded
that this failure took place over a longer time period. Indeed, this failure has been found during a
scheduled outage by indicating an increase of the capacitance. Thus, the bushing could be replaced
without having a catastrophic failure.
Failure of a RIP bushing
Figure 5
It is well known that the power factor and the DGA are adequate parameters for an evaluation of the
operation reliability of an OIP bushing. The relevant parameters for a RIP bushing are different. Therefore
it is important to know the relevant parameters and the potential influence on these parameters.
RELEVANT PARAMETERS FOR THE EVALUATION OF OIP AND RIP BUSHINGS
A partial breakdown results in an increase of the main capacitance C1. The increase is dependent on the
number of short circuited grading layers and the total number of grading layers. The higher the voltage
level of a bushing the lower the increase of C1 in case of a single partial breakdown will be. Therefore the
influencing parameters on the capacitance C1 must be taken into consideration for the evaluation,
especially for higher voltage levels.
Even if the relevance in regard of the evaluation of any increase of C1 is different for OIP and RIP
bushings, the influencing parameters are the same. The value of C1 is influenced by the stray
capacitance and by temperature.
The stray capacitance is a geometrical parameter. Thus, keeping the set up constant, the stray
capacitance will be constant as well. Therefore, a fingerprint measurement after installation of a bushing
in its final position is the best reference for any further measurement.
The temperature dependency of C1 for OIP and RIP is 0.025%/K and 0.04%/K, respectively. Usually this
is only relevant in case of big temperature differences. For off line measurements the resulting deviation
is mostly a minor one and can be neglected (see example 6 in the following).
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In figure 6 a measurement of an aged OIP bushing with different temperatures is shown. The bushing has
been heated up in an oven for these measurements. The temperature dependency of 0.025%/K results in
an increase of C1 of 1.75% when heating up the bushing from 20°C to 90°C, thus from 406pF to 413pF.
The temperature dependency for RIP is a little bit higher (0.04%/K), however, mostly it can be neglected
in case of off line measurements as well.
Temperature dependency of C1 of an OIP bushing
Figure 6
Stray capacitance on test taps has a stronger influence on C2 capacitance than C1 capacitance. This is
due to the fact that C2 is composed of both the condenser core and the “surrounding of the bushing”
which is determined by the geometry of transformer turret and other components. As a result, higher C2
deviations can be expected, up to +-50% from the routine test value.
In case of a potential tap for IEEE bushings, the influence of the stray capacitance is significantly
reduced, since the capacitance C2 of the potential tap is dominated by the last two grading layers.
Figure 7 illustrates the influence of the stray capacitance on C2 in case of a potential tap and test tap,
respectively.
Influence of the stray capacitance on C2 in case of a potential tap vs. test tap
Figure 7
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The power factor is an aging parameter for OIP bushings. In addition, it is influenced by temperature. For
RIP bushings an increased power factor is an indication for humidity ingress. But the value is temperature
dependent as well.
Table 1 summarizes the above mentioned relevant parameters for evaluation and the influencing
parameters for these values.
Table 1
Relevant parameters in regard of the evaluation of OIP and RIP bushings
The following examples will give an overview of typical situations found along with the evaluation of RIP
bushings.
EXAMPLE 1: INCREASE OF C1
PARTIAL BREAKDOWN
In 2008 HSP has been informed about a measurement during a scheduled outage of an RIP bushing
indicating an increase of the capacitance C1 of 53%. The bushing was in operation for 10 years and this
was the first measurement after installation. Based on the measurement it had to be concluded that
several partial breakdowns took place.
Table 2
On-site measurement after 10 years in operation
The bushing has been dissected at HSP after having confirmed the on-site measurement results with an
electrical measurement. A hole inside the condenser core has been found. The inner surface of this hole
was carbonized thus the involved grading layers are short circuited. The number of short circuited layers
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corresponds with the measured increase of the capacitance C1. Figure 5, right picture, shows the hole of
this investigated bushing. The root cause analysis indicated that obviously a minor defect had been
present in the beginning. This defect caused partial discharge and results in erosion of the resin in the
affected area. With the time the erosion increased and finally results in the hole.
Usually defects in the condenser core will be found during the routine test. However, in case of a very
small defect it might be possible, that it could not be detected since any partial breakdown is a statistical
event. Under continuous electrical stress it might happen that after a longer time (e.g. several years)
partial discharge started and the above mentioned failure mode begins.
EXAMPLE 2: DEVIATION OF C1
STRAY CAPACITANCE
If a small deviation is identified for the capacitance C1 it is helpful for the evaluation if not only one
bushing but several are measured in the same way. Table 3 shows the measurement results of 9 new
delivered transformer-GIS bushings. A deviation of approximately 5% has been found.
Table 3
Measurement of several bushings with comparable deviation of C1
After having received these measurement results from the customer, the set-up for the measurements
has been compared with the set-up for the routine test at HSP. Figure 8 illustrates the different set-ups.
The customer installed the bushings in a big oil vessel. The SF6 side remains in air since only a 10kV
measurement was carried out. For the routine test at HSP the bushings were installed in a smaller oil
vessel. Since the bushings have been tested up to test voltage the SF6 side has to be insulated, too. This
is realized by using an additional vessel filled with SF6. With these different set-ups the stray
capacitances are different. However, it is the same for all bushings for the routine test and the customer
measurement, respectively. Thus, it results in a systematic deviation which is constant for all bushings.
After having identified this situation, the deviation could be compensated. The average deviation for the 9
measurements has been calculated and added to each individual measurement. The result is shown in
table 3 on the right hand side. One can see that the deviation is now in a range from -1.4% to +1.6 %
only.
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Customer set-up (left) vs. set-up for routine test at HSP (right)
Figure 8
EXAMPLE 3: DEVIATION OF C1
STRAY CAPACITANCE
With increasing voltage level of the bushing the number of grading layers increases as well. Therefore the
deviation of the capacitance C1 in case of a single partial breakdown decreases with increasing voltage
level.
The evaluation becomes more difficult if this deviation is in a range where the deviation caused by stray
capacitance is as well.
The picture on the left hand side in figure 9 shows a typical set up for a control measurement of a
bushing. In this case it is an 800kV wall bushing. Due to high number of grading foils it is hardly possible
to detect any partial breakdown with this measurement set-up (the measurement was made to measure
the %PF). For comparison the set-up for the routine test is shown with the picture on the right hand side
in figure 9.
Typical set-up for control measurement (left) vs. set-up for routine test (right)
Figure 9
To demonstrate the influence of the set-up on the measurement result the measurements shown in figure
10 have been carried out. For the first measurement the bushing was supported 1 m above ground. For
the second measurement the bushing was lifted with a crane 0.1 m above ground.
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The measured C1 value changed from 756pF to 748pF. This is a deviation of 1%. Due to the high
number of grading layers this deviation might already indicate a partial breakdown. However, in this
situation it is clearly caused by the different stray capacitance. With the two measurement set-ups shown
in figure 9, typical set-up for control measurement vs. set-up for routine test, the deviation in the
measured capacitance C1 is even higher.
Test set-up: Bushing supported 1 m above ground and in crane 0.1 m above ground,
measurement of indoor side (left) and outdoor side (right)
Figure 10
For any further evaluation it is therefore necessary to have a fingerprint measurement with the bushing
mounted in its final position. This becomes more important with increasing voltage level and with
decreasing number of bushings which can be compared to each other.
EXAMPLE 4: DEVIATION OF C2
STRAY CAPACITANCE
The deviation of C2 is much stronger influenced by the stray capacitance than C1. Therefore a change to
larger deviation is acceptable. For transformer bushings this deviation is mainly caused by the different
size of the test vessel used for the routine test and the transformer turret. Usually with a new installation
multiple bushings are added. In this situation it becomes apparent if any deviation is caused by the stray
capacitance. Table 4 shows the measurement results of both capacitances, C1 and C2 for 3 transformer
bushings after installation in the transformer. The values are compared with the routine test results.
As expected, the deviation of C1 is very small but constant. The values can be corrected to eliminate this
deviation completely. The deviation of C2 is 16.8%, 27.6% and 31.2%, respectively. A correction can be
made as well, but due to the large difference in the individual deviation this is not useful. It is more
reasonable to define an expected range for the C2 measurement. According to HSP´s recommendation
this range is +-50%. For further measurements, the measured values from the fingerprint measurement
should be used as well for any comparison.
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Table 4
Commissioning test of transformer
EXAMPLE 5: INCREASE OF %PF OF C1
HUMIDITY INGRESS
Due to the fact that a RIP condenser core is a solid one, no additional housing is needed for the oil part
end or SF6 side. In operation these parts are protected by the oil or SF6 from the transformer and GIS,
respectively. For transportation and storage these parts need protection since the RIP is sensitive to
humidity ingress. Normally, a storage bag for the whole bushing with drying bags inside is sufficient if the
bushings are going to be installed on short notice. For long time storage a protection tank for the oil part
end is the recommended solution (see figure 11).
Storage bag for the whole bushing (left) and protection tank for the oil part end (right)
Figure 11
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If a RIP condenser core is exposed to humidity it will absorb a certain volume of the humidity. This will
affect the power factor. The influence of the humidity strongly depends on the volume and duration the
bushing is exposed to it.
In figure 12 the measurement result of several transformer bushings is shown. These bushings have
been exposed to normal weather conditions for several weeks but with partly damaged storage bags. The
comparison with the routine test results show a systematic deviation (increase) for all bushings caused by
different temperatures (see example 6) and an additional deviation for some bushings.
Humidity ingress during storage under unclear condition, increase of %PF of C1
Figure 12
The bushings with additional increased power factor have been selected for a drying procedure. They
have been dried in an oven with a temperature of 70°C for 3 days. After cooling down the power factor
has been measured again. As a result the power factor decreased to a normal value.
If necessary, the drying procedure could be continued for another few days if the decrease is not
sufficient.
EXAMPLE 6: INCREASE OF %PF OF C1
TEMPERATURE
Another influencing parameter for the power factor is the temperature. Since a deviation caused by
temperature is a reversible one, a correction might be done by calculating the corresponding power factor
for a temperature of 20°C.
Table 5 shows the measurement results of 6 wall bushings measured during the commissioning test on
site. Since wall bushings have a complete housing for the condenser core, any influence by humidity
ingress could be excluded. An increase from 11.8% to 21.2% compared to the routine test results has
been identified.
The ambient temperature during the measurement was 10°C, thus significant lower than during the
routine test which was approximately 20°C.
The typical power factor of a RIP condenser as a function of the temperature is shown in figure 13. By
taking the power factor at 20°C and 10°C (e.g. the middle value) a correction factor for 10°C can be
calculated. In table 5 the corrected power factor and the deviation of the corrected value compared with
the routine test result is shown as well. The deviation is now reduced from -7.2% to 0.6%.
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Table 5
Increase of power factor with low temperature
Temperature dependency of power factor and capacitance of RIP
Figure 13
In figure 13 the above mentioned temperature dependency of the capacitance (0.04%/K) is shown, too.
The measured values for C1 can be corrected to 20°C as well. In table 6 both deviations from the routine
test are shown, the measured values and the corrected values. Without correction the deviation of C1 is
from -3.3% to -1.5%. With correction the deviation is reduced, however it is still from -2.9% to -1.1%.
Obviously the stray capacitances have a bigger influence on C1 than the temperature. Therefore any
temperature correction for C1 can be usually neglected.
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Table 6
Decrease of capacitance C1 with low temperature
EXAMPLE 7: INCREASE OF %PF OF C2
HUMIDITY INGRESS
As discussed in example 5 humidity ingress does have an influence on the power factor. Both power
factors might be affected, the power factor of C1 and the power factor of C2.
For the power factor of C2 it should be taken into consideration that this value is not only composed of the
bushing itself, but also by e.g. the transformer oil inside the transformer turret.
If humidity ingress affects mainly the part of the condenser core close to the flange, it might happen that
the power factor of C2 increases significantly whereas the power factor of C1 only slightly increases.
Figure 14 shows the condenser core of bushings which have been returned to HSP for evaluation after
improper storage. The bushings have been stored outside, exposed to rain. The oil part end had been
covered by using a plastic covering. However, the plastic wrap was only up to the flange without any
sealing (see below picture right hand side in figure 14).
RIP condenser core after storage under rain without proper protection
Figure 14
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As a result the humidity was let into the condenser core close to the flange. In this area the ground layer
protects the inner part of the condenser core, thus the power factor of C1 was not significantly increased.
The power factor of C2 was increased significantly, up to nearly three times of the routine test value (see
table 6).
The same drying procedure mentioned in example 5 has been carried out. The result of the power factor
measurement of C2 is shown in table 6 as well. With almost all bushings the original values from the
routine test have been reached.
Table 7
Increase of capacitance C2 due to humidity ingress
CONCLUSION
The basic design of an OIP bushing and a RIP bushing is the same. However, there are some specific
differences resulting in different aging parameters.
For an OIP bushing an increase of the power factor is a well-known aging parameter. In addition a DGA
might give information about the operation reliability. Any increase of the capacitance C1 indicates a
partial breakdown. However, the first partial breakdown might be followed directly by a complete
breakdown of the insulation resulting in a catastrophic failure. Therefore C1 is not really an adequate
aging parameter.
For a RIP bushing any partial breakdown is limited to its position without any interaction with the
remaining condenser core. This is due to the fact, that the condenser core is a solid one, in contrast to an
OIP condenser core, where generated gases and contaminated oil can distribute to the whole condenser
core.
RIP bushings do not contain any oil, thus no aging of oil could occur. An increase of the power factor is
not caused by aging directly but by humidity ingress, which might weaken the insulation. This is usually
not relevant for installed bushings, but might occur with improper storage conditions. With a drying
procedure, humidity ingress into the condenser core might be removed.
For both bushings, OIP and RIP type, the influence of stray capacitances should be taken into
consideration if the measured values are compared with the routine test values. A fingerprint
measurement taken after installation of the bushing in its final position is the best reference for any further
measurement.
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Both, power factor and capacitance are temperature dependent. Any deviation of the power factor can be
compensated by using a correction factor. For the capacitance, a compensation can be made as well,
however, usually this is not necessary since the difference caused by temperature is only a minor one.
The best evaluation can be made if not only one bushing is measured but several. In this situation, the
measurement results can be compared to each other as well.
Examples are presented for each influencing parameter. The possibilities to identify systematic deviations
from individual deviation as well as the possibilities for compensation are discussed.
REFERENCES
[1] HSP Technical Recommendation, Evaluation guide on C2–measurement results, 2011, HSP
Hochspannungsgeräte GmbH
[2] HSP Technical Recommendation, Drying Procedure for RIP-Bushings, 2010, HSP Hochspannungsgeräte GmbH
[3] HSP Manual, Transfomer Bushings Type SETFta, 2014, HSP Hochspannungsgeräte GmbH
BIOGRAPHY
Bernhard Heil has been employed at HSP Hochspannungsgeräte GmbH since 2008, and currently works
as Head of the Technical Support. He received his diploma and his PhD at RWTH Aachen University in
2001 and 2006, respectively. From 2006 to 2008 he worked as Chief Engineer and Group Leader of the
research group Insulating Systems and Diagnostics at RWTH Aachen University.
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