Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
ISBN 978-0-620-44584-9
DESIGN AND TESTING OF UHV BUSHINGS
1
1
P. Cardano ,2 A.Pigini, 1G.Testin
Areva T&D SpA Italy(Unit RPV), 2Consultant
*Email: <paolo.cardano@areva-td.com>
Abstract: UHV systems are developing in the world, UHV bushings being one of the fundamental
components. Aspects related the design of UHV bushings for AC and DC application are
examined in the paper, paying special attention to the external insulation requirements. The
required external insulation length as a function of the system voltage is analysed, both under
service stress and under overvoltages. The alternative application of porcelain and composite
housing is discussed.
Examples of the UHV bushings developed and successfully tested are reported. At the light of the
experience critical testing aspects are pointed out and the need of implementations/modifications
of the Standards are discussed looking specifically to the UHV applications.
1.
However the reduction of the strength under rain can
be contained adopting suitable profiles and composite
housings [5], [6].
For the considerations in the following reference is
made to the conditions recommended by the Standard
(H=0, that means no turret).
INTRODUCTION
UHV applications imply extreme challenges for
bushings from the design testing and manufacturing
point of view [1], [2], some of them being examined in
the following, indicating, among others, the need of
implementation/modification of the recent Standard
[3].
10
9
2.
DESIGN ASPECTS
8
7
Evaluation of the required arching distances
L (m)
2.1.
Switching impulse (SI) requirements. The size of the
actual bushings may vary depending on the choices
made about field control, housing characteristics,
standardisation of the housings. A certain benefit on
the strength can derive from the capacitive field
control. However the testing experience indicate that
the influence of the active parts on the SI performance
in dry conditions is limited. Thus a preliminary
conservative evaluation of the strength in dry condition
can be made considering the configuration as a rod
structure one. The required arching distance versus the
required rated switching impulse withstand level VSI,,
evaluated according to [4] with reference to a rod
structure configuration, is given in Fig. 1, for different
heights H of the lower structure having a base width of
1.5 m. The longest arching distance L is required when
the base is at ground plane, situation which is
practically required to be simulated in the standard test
[3], which prescribes that the bushing shall be mounted
on an earthed plane, radially extended from the axis of
the bushing at least 0,4 L in every direction. The
arching distance dimensioned according to this
criterion is thus conservative with respect to actual
configurations (e.g., with the bushing mounted on
turrets) and may in particular penalise some UHV
applications. The required withstand can be reached in
dry conditions even with shorter values of L by
adopting suitable shielding electrodes, whose
efficiency is however lost under rain conditions and
other environmental perturbed conditions (e.g. ice and
snow). As a matter of fact higher clearances than those
in Fig. 1 can be required under rain.
6
5
4
H=0
H=1,5
H=3
3
2
1
0
500
1000
1500
2000
USI (kV)
Fig. 1 - SI, dry condition.
Simplified simulation of the bushing by a rod structure
configuration. Required arching distance as a function
of the rated switching impulse withstand value, for
different heights of the lower structure.
Lightning impulse (LI) requirements Also with LI
the strength depends on the configuration, even if at a
less extent than for SI [4]. For the evaluation in the
following reference is made to a withstand value (10%
flashover probability) of 500 kV/m
Pollution (P) requirements are reported in Table 1.
The specific creepage distances necessary for AC are
taken from [7]. Pending the new Standard provisional
values for DC are derived from [8], [9] on the basis of
test experience. The values appear much larger than
those required in AC and may be subjected to
optimization taking into account field experience. To
this purpose a questionnaire has been circulated to
collect field experience in terms of adopted creepage
distances, contamination level on the insulators in the
field and actual field performance.
Pg. 1
Paper E-51
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
ISBN 978-0-620-44584-9
standardised for higher system voltages in the UHV
range [11]. By chance, it is pointed out that one of the
more stringent standardisation need is the extension of
the standardisation of the voltage levels to UHV,
possibly to UHVDC also, to avoid the proliferation of
too many levels.
Table 1 Required specific creepage distances (phase to
ground)
AC
DC
mm/kV
mm/kV
22
22
Light (L)
27,8
35
Medium (M)
34,7
55
Heavy (H)
43,3
75
Very heavy (VH)
53,7
95
Very light (VL)
AC: data from [7]
DC: data derived from [8], [9]
Fig. 2- Creepage distance. Correction factor as a
function of average insulator diameter for porcelain
insulators AC [7]. Dotted line: expected test
performance. Continuous line: expected service
performance taking into account the influence of
diameter on contamination.
As far as composite insulators are concerned the
Standard [10], basically says that for AC the same
reference values can be assumed as for ceramic
insulators, even if other values can be taken under the
responsibility of the users. As a matter of fact in many
parts of the world there is the tendency to adopt lower
creepage distances for composites than for ceramics,
taking into account their better performance. This is
one of the matters deserving further investigation
especially in view of UHV applications. For the
evaluation in the following, taking also into account
the results in [6], it is assumed that the specific
creepage necessary for composites is 0.8 that for
porcelain insulators.
The above creepage distances refer to insulators of
small diameter. For insulators of large diameter the
correction in Fig. 2 is prescribed by the Standards for
ceramic insulators under AC voltage [7], while for
composite insulators under AC voltage the trend is not
clearly specified [10], saying that the influence can be
negligible for hydrophobic conditions and the same as
for ceramic for hydrophilic conditions (Fig. 3). As a
matter of fact recent tests have shown that the
influence of the diameter remains low even for
insulators which have lost part of their hydrorepellency
[6]. Also additional investigations on this aspect would
be very important for UHV applications.
In the following, based on the results in [6] a negligible
influence of the diameter on the required creepage
distance is assumed for composites both in AC and DC
systems.
Fig. 3 - Creepage distance. Correction factor as a
function of average insulator diameter for composite
insulators AC [10].
It is evident that SI predominates with respect to LI in
the upper range of EHV and in the UHV range.
The insulation levels are not specified for DC bushings
[12], however taking into account the values assumed
in the design practice, the same conclusion can be
extended to DC bushings also.
A comparison of the requirement dictated by SI and
pollution is made in Fig 5, 6 and 7. The pollution
requirements are evaluated considering typical bushing
diameters and a creepage factor of 4.
Furthermore it is assumed that for DC the required
creepage distance for composite is, as for AC, 80% of
that required for ceramic insulators.
2.2.
As shown in Fig. 4 SI generally predominates in design
for AC, even considering the heavy pollution
requirements for insulators of large diameter.
Predominant design stresses
A comparison of the requirements dictated by LI and
SI for AC systems is reported in Fig. 4, where the
required arching distances are evaluated according to
the criteria in the previous paragraph and on the basis
of the insulation levels specified in [3] for rated
voltages up to 800 kV and those going to be
The predominance of SI would be even greater in case
composite housings should be adopted, with a better
pollution performance than porcelain (with a creepage
distance of about 80% of that of Table 1 and
considering a negligible influence of the diameter).
Pg. 2
Paper E-51
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
ISBN 978-0-620-44584-9
12
An other important aspect to be pointed out is that the
determination of the site severity is much more
important in DC than in AC.
LI min
10
Limax
SI min
L (m)
8
25
SI
P VL
PL
PM
PH
P VH
SI max
20
6
L (m)
4
2
15
10
0
5
0
500
1000
1500
Um(kV)
0
0
Fig. 4 - AC bushings. Estimate of the required arching
distances as a function of the system voltage to
withstand LI and SI (minimum and maximum
insulation levels in [3], [11])
200
400
600
800
1000
U (kV)
Fig. 6 – DC bushings. Estimate of the required arching
distance as a function of system voltage to withstand SI
and service voltage under pollution. Porcelain
insulators with Creepage Factor=4.
14
SI min
12
SI max
10
P VL
16
8
14
PM
6
PH
12
P VH
10
L (m)
L (m)
PL
4
2
SI
P VL
PL
PM
PH
P VH
8
6
0
4
0
500
1000
1500
2
Um (kV)
0
Fig. 5 – AC bushings. Estimate of the required arching
distance as a function of system voltage to withstand SI
and service voltage under pollution. Porcelain
insulators with Creepage Factor=4.
0
200
400
600
800
1000
U (kV)
Fig.7 DC bushings. Estimate of the required arching
distance as a function of system voltage to withstand SI
and service voltage under pollution. Porcelain
insulators with Creepage Factor=4.
The same evaluations are made in Fig. 6 for DC,
assuming SI overvoltages of 2.1 p.u, being SI levels
not standardised [12]. It appears that for DC pollution
is always the dominating design stress. It is evident that
taking into account existing constraints in length (of
about 10 m based on manufacturing and transportation
limits), only light to medium contamination bushing
may be to day produced for UHVDC.
By adopting composite housing, requiring less specific
creepage distance and with a limited influence of the
housing diameter on the creepage distance, more
reasonable arching distances are required, making
possible to realize UHV bushings even for medium to
heavy pollution conditions, as shown in Fig.7.
The above considerations point out as the development
of the composite solution is essential toward UHVDC
development and confirm the need to further
investigate, agree and standardise the performance of
composite solutions.
Simplified pollution design approaches which make
reference to basic categories on the basis of the site
location as in AC, are not recommended in DC, at least
for the final design. Test campaigns with energised
insulators are strongly recommended before making
detailed design of insulators. The investigation could
prove that most of the sites are characterized by
pollution level lower than the so called medium one
and that in very few extreme cases the pollution
severity ranges to heavy and very heavy conditions.
2.3.
Influence of altitude on the insulation
requirements
An other aspect which has to be considered for UHV is
the influence of the atmospheric conditions and
Pg. 3
Paper E-51
ISBN 978-0-620-44584-9
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
simulation of a bushing mounted on a transformer
mock up or a GIS mock up).
particularly the influence of altitude on the sea level.
To take into account the influence of altitude (air
density) the Standard [3] assumes conservative values
for the coefficient m, and namely 1 for LI and 0,75 for
SI, for all the voltages levels (arching distance values)
and prescribes that for installations at an altitude higher
than 1000 m, the arcing distance under the standard
reference atmospheric conditions shall be designed to
withstand the voltages obtained by multiplying the
withstand voltages required at the service location by a
factor k in accordance in accordance with [3].
A comparison of the coefficient K from [3] and that
which should be more correctly applied on the basis of
[7] for a SI voltage of 1050 and 1800 kV respectively,
is reported in Fig. 8. It appears that the Standards
penalize to much the design especially for UHV
bushings.
Let now we make an example with reference to a
bushing for 1100 kV to be designed according to the
following insulation levels: LI: 2400 kV, SI: 1800 kV.
A bushing with about 9 m arching distance is adequate
for application at sea level (see Fig. 1).
If we have to design a bushing for 3000 m, according
to [3], we should design the bushing for the following
voltage values:
LI=2400*1,27=3048 kV,
SI=1800*1,2=2160 kV.
In spite of the large increase LI does not dominate the
design (just about a 6m arching distance is sufficient to
comply with the new LI requirements). Things are
different eith SI: the corresponding bushing length
should be about 12,5 m as dictated by SI. On the
contrary a correct design made according to [13] and
taking into account the predominance of SI on design
would imply the following voltage values:
SI=1800*1,03=1850 kV. The corresponding required
arching distance would be, according to Fig. 1, about
9,5 m.
This aspect of the Standard is extremely penalizing and
should be possibly modified soon, not only from the
point of view of UHV.
Particularly critical are also wet test that may need to
be made closer to the wet apparatus ( risk of
discharge) and for which the uniformity of the rain
distribution along the typical UHV arching distances is
difficult to be reached. A solution to limit the influence
of the laboratory walls is to make the tests outside,
however the criticality of the rain test remains.
1,5
K[3]
K[13] 1050 kV
k[13] 1800 kV
K (p.u.)
1,4
1,3
1,2
1,1
1
1000
2000
3000
altitude (m a.s.l.)
4000
Fig. 8 - Coefficient K for SI levels of 1050 and 1800 kV
as a function of the altitude above sea level (as derived
from [13]. Comparison with values from [3].
As far as DC is concerned, since Dc pollution is the
design stress , the altitude correction for DC needs to
be carefully assessed.
3.
3.1.
TESTING
Testing of the external insulation
As far as the external insulation is concerned, one of
the most critical tests to perform according to the
Standards for UHV bushing is the SI one. The data in
Fig. 9 [14] indicate that to make the influence of
laboratory size negligible at least a free space of 1.5
times the arching distance is necessary. This means
that laboratories with a size of at least 30*30*30 m are
necessary to perform correct tests in dry condition.
Tests in smaller laboratories can lead to too
conservative results and design: this is a critical aspect
considering the already very large arching distances
necessary. The matter is even more complicate if the
simulation of actual service conditions is required (e.g.
Fig. 9 - Influence of the laboratory size on the
flashover voltage [14]
One of the other challenges is the correct testing of the
lower part of the bushing. One particular aspect which
is to be considered in design and testing is the
interaction of the apparatus to which the bushing is to
be applied with the bushing design. The vessel should
be large enough such that the lower part of the bushing
is tested correctly, avoiding non representative
Pg. 4
Paper E-51
5000
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
ISBN 978-0-620-44584-9
discharges to the vessel walls and partial discharges
which could make difficult the measurement of the PD
on the bushing, as required by Standards. For AC the
design of the testing vessel has to take into account that
the specific strength of transformer oil greatly
decreases by increasing the spacing, as shown by
typical data in Fig. 10, taken from [15]. Furthermore
the performance of the lower part of the bushing under
test depends very much on the quality of the
transformer oil since the specific oil strength and the
ratio between the strength of the oil under LI and AC
depends very much on the quality of the oil: highly
degasified and filtered oil is essential for a successful
testing.
The matters are even more complicated in DC. In this
case DC stresses, overvoltage stresses and transients
during voltage polarity reversal dominate the design
such that the design of the bushing side immersed in oil
can not be made without considering the actual
application. The design of shielding electrodes and
barriers in the converter transformer to assure the
correct field distribution in DC and during transient is a
challenge especially for UHV.
Fig. 11 and 12 report the 1100 kV bushing under test at
Areva T&D Italy Spa premises and in China. The
bushing, have the following main characteristics:
Um
PFWV
LI
CW
SI
In
1100 kV
1200 kV
2400 kV
2760 kV
1950 kV
2500 A
Fig.11 - 1100 kV bushing under test at Areva T&d Italy
Spa premises
The bushing has satisfactorily passed the tests
prescribed and now several bushing of this type are in
service in China.
7
UF (KV/mm)
6
5
AC rms
LI
4
3
2
1
0
0
100
200
300
spacing (mm)
Fig. 10 - Typical strength trends of transformer oil as a
function of the spacing [15]
Fig. 12 - 1100 kV bushing under test In China
Pg. 5
Paper E-51
ISBN 978-0-620-44584-9
4.
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
for DC and the need to reproduce the actual
installation condition in DC.
CONCLUSION
o
Present standards prescribe the testing
voltages only for AC up to Um 800 kV. There
is urgent need to standardise the voltage
ratings for UHV AC and DC also.
o
SI testing with the simulation of an earth
plane at the base of the bushing may be too
severe especially for UHV, leading to
unnecessary overdesign.
o
The size of the testing laboratory should be
large enough to avoid too much influence of
the laboratory walls and ceiling on the
strength. Minimum laboratory sizes for UHV
testing should be 30*30*30, but larger
laboratories can be useful to make tests on
more
complex
and
representative
configurations.
o
SI rain tests remain any way critical due to the
difficulty of having suitable rain apparatus to
create uniform rain conditions at large
distances and on long insulator sets.
Alternative procedures may need to be found.
o
The correction for altitude in the present
bushing Standards is too conservative and is
to be corrected, at least for UHV, since leads
to unnecessary and costly overdesign.
o
SI dominates the design for UHV AC, while
pollution dominates the design for UHVDC.
The feasibility of UHVDC bushing for severe
contamination conditions may be facilitated
by the adoption of composite housings.
Additional investigations may be needed to
give more ground to the knowledge of the
performance of composite housings.
o
Due to the criticality of the pollution in DC a
proper assessment of the site pollution is
recommended. Specific investigations could
prove that probably most of the sites are
characterized by a pollution level not higher
than medium.
o
Even the design and testing of the lower part
of the bushing may deserve particular
attention for UHV. Design should be made in
tight cooperation between the bushing
manufacturer and the apparatus manufacturer.
The need of cooperation is essential for
UHVDC for which the design of the bushing
is very much related to that of the converter
transformer, with the consequent pressboard
barrier system.
o
As far as testing is concerned the test vessel
need to be designed taking into account the
rather poor performance of oil on large gaps
5.
REFERENCES
[1] P. Cardano , A. Pigini , R. Berti , M. de Nigris , E.
Moal , G Rocchetti.”Application of composite
housing to high voltage bushings” CIGRE 2008
paper A3 307
[2] A. Pigini “ External insulation for UHV: design
requirements and research needs” IEC CIGRE
UHV Symposium Bejing 2007
[3] IEC 60137 Ed. 6.0 “Insulated bushings for
alternating voltages above 1 000 V” 2008
[4] WG 33.07 “ Guidelines for the evaluation of the
dielectric strength of external insulation” Cigre
brochure 72 -1992
[5] CIGRE TF 33.03.03 “Switching Impulse
performance of post insulators” Electra n 109 1986
[6] P. Cardano, M. de Nigris, A. Pigini, G. Rocchetti
“Dielectric performance of composite housings”
ISH 2009
[7] IEC 60815-2008: Selection and dimensioning of
high-voltage insulators intended for use in polluted
conditions - Part 2: Ceramic and glass insulators
for a.c. systems
[8] A. Pigini, A.C. Britten, C. Engelbrecht
“Development of guidelines for the selection of
insulators with respect to pollution for EHV- UHV
DC: state of the art and research needs” CIGRE
2008
[9] A.Pigini, N. V. Ramkumar “Aspects related to
design and testing of uhv insulator strings with cap
and pin insulators” CIGRE IEC UHV Symposium
New Delhi 2009
[10] IEC 60815-2008: Selection and dimensioning of
high-voltage insulators intended for use in polluted
conditions - Part 3: Polymer insulators for a.c.
systems.
[11]E. Zaima, C Neumann “Insulation Coordination
for UHV AC Systems based on Surge Arrester
Application (CIGREC4.306)” Cigre IEC workshop
on UHV New Delhi 2009
[12]IEC 62199 “Bushings for DC application “ 2005
[13] IEC 60 IEC 60060-1 Ed. 3.0: High-voltage test
techniques Part 1: General definitions and test
requirements
[14]Cigre TF 33.03.03 “Evaluation of the SI strength
of external insulation” Electra N94 May 1984
[15]F. Clark “Insulating Materials for Design and
Engineering Practice” Book Wiley, New York,
1962
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