ISBN 978-0-620-44584-9
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
NEW COMPOSITE INSULATOR STRING DESIGNED FOR 400 KV
OVERHEAD LINE USING STATISTICAL INSULATION
COORDINATION PROCEDURE
S. Vižintin1*, I. Uglešić2, B. Vertačnik3, H. Kifle3, B. Barl3, S. Lesjak3
Elektroinštitut Milan Vidmar, Hajdrihova 2, SI-1000 Ljubljana, Slovenia
2
University of Zagreb, Unska 3, 10000 Zagreb, Croatia
1
Elektro Slovenija, Hajdrihova 2, SI-1000 Ljubljana, Slovenia
*Email: stane.vizintin@eimv.si
1
Abstract: The progress in the field of overhead line insulation, good experiences with composite
insulators on the 110 kV level and appearance of some new insulation coordination computersupported techniques led us to the conclusion that for the next 400 kV overhead line project in
Slovenia composite insulators should be used for the first time. In order to define the insulation
level, a thorough overvoltage analysis was accomplished. Calculations and computer simulations
were used to assess the temporary and transient (lightning and switching) overvoltages regarding
their frequency and amplitudes. The lightning activity was represented by the actual highresolution lightning density map. With a bunch of data collected it was possible to perform a
statistical insulation coordination procedure to determine the required insulation level. Some other
system aspects were considered. The tests in the high-voltage laboratory showed appropriate
dielectric strength for the insulator string that was designed at the beginning. Unfortunately the
corona voltage inception level was too low due to unfavourable electric field distribution for the
contemporary environmental requirements. Another solution is needed, so the insulator string with
new metal fittings (potential rings, arching horns) is being designed. The paper shows results of
the developed insulation coordination procedure in detail and dielectric tests.
1.
INTRODUCTION
2.1. Electric power network
The simulation scheme mapped the existing Slovenian
400 kV and 220 kV power transmission systems. The
Beričevo-Krško OL is of a double-system with one
ground wire. Its basic data are as follows:
The insulation level for Slovenian 400 kV overhead
lines was defined by using the deterministic insulation
coordination procedure more than 40 years ago. The
overvoltage data was established by means of
calculations and more rarely by measurements. The
standard insulator string was designed with glass
cap/pin insulators. In the meantime a considerable
progress in the field of insulation has been made
especially by introducing composite insulators and
some new insulation coordination computer-supported
techniques.
OL length:
Number of towers:
Medium span length:
In our calculations of the switching overvoltages we
took into account also the contribution of other 400 kV
OLs. Five of them are those that connect power
distribution transformer substations (TS) inside
Slovenia (the length of individual OLs is between
31.1 km and 76.2 km), and four of them allow for
connection with the Croatian, Austrian and Italian
system (the length of individual OLs is between
32.2 km and 66.5 km). The two basic insulation levels
(BIL) used in Slovenia are given in Table 1:
In the last decade at the 110 kV level in Slovenia
almost exclusively composite insulators were installed.
Many discussions about their possible insulation
properties degradation were raised in the past, but no
problems have been experienced in service up to now.
A thorough investigation [1] has revealed that aging
has not left any serious deterioration on insulators. It is
for this reason that a decision has been taken to design
for the planned 2 x 400 kV Beričevo-Krško overhead
line (OL) a new string with composite insulators with
their dielectric parameters determined in the insulation
coordination procedure.
2.
80.4 km
227
354 m
Table 1: Standard insulation levels in Slovenia.
MODELLING
Um
Insulation
U250/2500
U1,2/50
420 kV
BIL 1300
BIL 1425
950 kV
1050 kV
1300 kV
1425 kV
On the 400 kV OLs there is solely single-pole
automatic reclosure (AR) used. It is not likely that in
the near future a three-pole AR will be used. The shortcircuit disconnection time does never exceed 0.5 s.
Modelling of the Beričevo-Krško OL to allow for the
calculation of the switching and lightning overvolatges
was made by using the input data given bellow.
Pg. 1
Paper E-58
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
ISBN 978-0-620-44584-9
2.2. Beričevo-Krško OL
2.3. Equipments
Geometrical circumstances (distances) for a 52.3 m
high tower are shown in Figure 1:
At both OL ends there are transformers in TS. Their
basic data are given in Table 3:
Table 3: Data of transformers in TS.
Station
Number (parallel)
Power [MVA]
Voltage ratio[kV]
uk [%]
Beričevo
Krško
2 (autotransformer)
400
400/231
11,9
2
300
400/110
12,9
In our simulations we considered the magnetizing
curve with a hysteresis for a typical 300 MVA
transformer.
In TSs, there are SF6 gas-filled circuit-breakers (CB)
installed. As to their operational times, we considered
the typical values:
Closing time:
Opening time:
Breaking time:
Most of the values obtained in our measurements of
simultaneous closures of individual poles of modern
(new) CBs (110 kV, 220 kV or 400 kV) were around
1 ms (between the first and the last pole). They never
exceeded 2 ms. The warranty (manufacturer's) value
was maximally 5 ms. The pessimistic values used in
our simulations ranged from 2 ms to 5 ms.
Figure 1: Spatial distribution of conductors (distances
in [m]) for the Beričevo-Krško OL.
While the distances between conductors do not change,
tower heights Ht may vary. The actual values for Ht
were taken into account in the analysis of lightning
overvoltages. Dimension data are given in Table 2:
In each of the TSs, the ZnO surge arresters (SA) are
sited by transformers and there is none of them in the
OL bays. We took into consideration the U-t
characteristic for a typical SA of the rated voltage
Ur = 336 kV (Ir = 10 kA, line discharge class 3).
Table 2: Data for the medium heights of conductors
and ground wires on the Beričevo-Krško OL.
Conductor
Phase
x [m]
y [m]
1
2
3
4
5
6
7
A
B
C
A
B
C
z.v.
-6,2
-10,2
-7,0
6,2
10,2
7,0
0
30,0
21,5
14,3
30,0
21,5
14,3
43,6
Number of
conductors
D [m]
2
(horizontal)
0,4
1
-
60 … 90 ms
22 ms (± 3 ms)
40 ms
3.
OVERVOLTAGES
In our investigation we calculated all the types of
overvoltages which are in accordance with the rules
imposed on insulation coordination (IEC 60071)
assessed to be of a decisive importance for the 400 kV
OLs. The overvoltages were dealt with in the light of
their cause.
Value x in Table 2 means horizontal deflection of the
conductor from the tower symmetral and y the average
distance above the ground in which the sag is already
taken regard of. The phase conductor represents a
bundle of two Al/Fe 490/65 wires of the diameter
dc = 30.6 mm (R20°C = 0,059 Ω/km) in the horizontal
design and with the distance among themselves D. The
ground wire is of the Optoflex ASB 4.1.3s type
(Ay/ACS
131/25-13.4)
with
the
diameter
dgw = 18.0 mm (R20°C = 0.233 Ω/km).
3.1. Single-phase earth-fault overvoltages
The usual causes for the occurrence of temporary
overvoltages are: single-phase earth-faults, load
rejections, ferro-resonance etc. The overvoltages
important for the needs of insulation coordination in
our circumstances are those taking place at singlephase earth-faults. Their value is determined with
earth-fault factor k. Besides the characteristics of the
power system (mostly the applied network grounding
system), it is also the fault location that affects k.
The measured specific ground resistance along the
right-of-the way usually varies in the range from
100 Ωm < ρ < 1000 Ωm. The value used in our
simulations was 500 Ωm.
Our analysis was made with the PSS/E25 software
package. Earth-fault factor k can be estimated by using
complex impedance Z1 (direct system), Z2 (inverse
system) and Z0 (zero system) with simultaneous
Pg. 2
Paper E-58
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
ISBN 978-0-620-44584-9
consideration of failure resistance R. We determined it
for all the points of the Slovenian 400 kV electric
power system (Table 4):
700
Table 4: Impedances of the 400 kV network.
100
Z
[Ω]
TS
Cirkovce
Šoštanj
Beričevo
Divača
Krško
Maribor
Okroglo
Podlog
R1
1,18
1,31
0,91
1,02
0,70
0,94
1,34
1,06
X1
16,0
18,4
11,7
12,2
10,6
13,6
16,0
15,1
R0
3,58
1,39
1,97
4,18
0,90
2,77
3,26
1,63
[kV]
400
-200
Ratios Z
X0
26,3
16,6
14,9
18,9
12,7
22,5
28,1
15,3
R0/X1
0,224
0,076
0,168
0,342
0,084
0,203
0,204
0,108
X0/X1
1,639
0,902
1,273
1,543
1,196
1,653
1,757
1,014
-500
R1/X1
0,074
0,071
0,078
0,084
0,066
0,069
0,084
0,070
-800
0,00
0,02
0,04
(f ile DV400kV.pl4; x-v ar t) v :X0009A
v :X0009B
0,06
0,08
[s]
0,10
v :X0009C
Figure 3: Phase voltages UA, UB and UC at the end of
the open OL of the Krško TS at connection from the
Beričevo TS.
The other OL system was at this time loaded. As seen
from Figure 3, at the beginning (10 % of the overall
time), the induced voltage (not equal to zero) is in each
of the three phases. The highest inter-phase voltages
(close to 1200 kV) are also at the end of the open OL
for the same case as above. The difference between the
first and last CB pole closure time is approximately
Δt ≈ 5 ms. Figure 4 shows another case - disconnection
of OL at the Beričevo TS 5:
Since the neutral point of the 400 kV network is
directly grounded, k is determined by using the family
of curves given in the IEC 60071-2 standard for R1 = 0.
The family is shown in Figure 2:
400
[kV]
250
100
-50
-200
-350
-500
0
Figure 2: Short-circuit factor k for various values of
R0 X 1 in X 0 X 1 at (R1 = 0).
10
20
(f ile DV400kV.pl4; x-v ar t) v :X0009A
v :X0009B
30
40
[ms]
50
v :X0009C
Figure 4: Phase voltages UA, UB and UC at the
beginning of OL at the Beričevo TS at disconnection.
As seen from values in Table 3, the earth-fault factor
does not exceed the value k = 1.2 in none of the points
of the power system. This means that low temporary
overvoltages may be expected.
Further we analysed the overvoltage phenomena by
using a statistic CB with which closures take place at
arbitrary selected times as foreseen by the Monte Carlo
method; this is of course within the selected limits
(depending on pole dissipation  = 1,45 ms). The
applied distribution was even with the pole set at some
3.2. Switching overvoltages
It is known that the highest overvoltage amplitudes
take place at CB closings at the end of an open OL.
They depend on the moment of the CB pole closure so
that the highest overvoltages occur at closure in the
moment when the driving voltage is at its maximum.
The amplitude of the supply phase (1 p.u = 343 kV) or
inter-phase (1 p.u = 594 kV) voltage is used as a basis
for calculation of the overvoltage factor [p.u.].
Connection of one system of a loadless OL is first
analysed in a deterministic and then in a statistic way.
The EMTP software was used.
t = 10 ms  3   . We simulated 100 of such arbitrary
selected closure times and statistically processed
overvoltage values. Results for CB closings at the end
of the open OL are given in Tables 5 and 6:
Table 5: OL CB closing from the Beričevo TS.
When connecting a loadless OL, the wave deflects and
the voltage gets doubled at the OL open end. Figure 3
shows the voltage time development at the Krško TS
for such case if connected from the Beričevo TS:
Pg. 3
Line
Beginning (CB)
Open end
Voltage [p.u.]
Phase A
Phase B
Phase C
A-B
B-C
A-C
Umean
1,242
1,172
1,202
−
−
−
Umean
1,694
1,546
1,541
1,430
1,386
1,535
St.d.
0,105
0,065
0,098
−
−
−
U2%
1,44
1,34
1,43
−
−
−
St.d.
0,153
0,206
0,204
0,140
0,188
0,120
U2%
1,96
2,10
1,95
1,74
1,80
1,75
Paper E-58
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
ISBN 978-0-620-44584-9
Table 6: OL CB closing from the Krško TS.
Line
Beginning (CB)
Open end
Voltage [p.u.]
Phase A
Phase B
Phase C
A-B
B-C
A-C
Umean
1,216
1,174
1,198
−
−
−
Umean
1,748
1,587
1,570
1,428
1,393
1,579
St.d.
0,090
0,066
0,092
−
−
−
U2%
1,39
1,33
1,40
−
−
−
St.d.
0,181
0,197
0,214
0,136
0,149
0,141
Based on our analysis, we determined the values for
the lightning current maximal amplitude Imax, median
value Mp and standard deviation σp. The values are
given in Table 7:
U2%
2,10
2,95
1,98
1,73
1,63
1,85
Figure 5 shows a cumulative distribution
overvoltages for one of the observed cases:
Table 7: Properties of the lightning stroke currents for
the Beričevo-Krško OL.
of
Lightning
strokes
Period
[years]
I max
[kA]
Mp
[kA]
σp
[kA]
1587
8
142,9
12,5
12,5
The data were obtained by Slovenian lightning location
system SCALAR. The average lightning stroke density
on the OL right-of-the way is:
100
90
80
ng = 2.4 str./km2/year
70
%
60
Our simulations of lightning overvoltages, in which the
lightning stroke parameters were changed according to
the Monte Carlo method (current, shape, angle, and
place of stroke), were made with the SigmaSlp
software package. For each OL configuration we made
1000 simulations. The place of stroke was determined
with an electro-geometrical model.
50
40
30
20
10
2,1
2,15
2
2,05
1,9
1,95
1,8
1,85
1,7
1,75
1,6
1,65
1,5
1,55
1,4
1,45
1,3
1,35
1,2
1,25
0
Napon [p.u.]
p.u.
Voltage
Following our analysis of simulation results, the below
major characteristics can be drawn:
Figure 5: Cumulative overvoltage probability at CB
closing at the end of the open OL (KrškoTS) phase B.
-
We also made overvoltage simulations at connection of
OL terminated with a transformer at its no-load
operation, connected additional OLs and during singlephase automatic fast reclosure. In each of the observed
states, overvoltages were lower.
-
3.3. Lightning overvoltages
In our calculation of lightning overvoltages we took
into account the actual values for the lightning stroke
density ng and the probability distribution of currents in
accordance with the log-normal distribution. Figure 6
shows a section of the right-of-the way of the
Beričevo-Krško OL where colours represent the
lightning stroke density from ng = 0.5 str./km2/year
(yellow) to ng = 3.0 str./km2/year (blue).
-
-
-
-
The number of strokes into the phase conductor
(shielding failures) is substantially lower than the
number of strokes into the ground wire. This share
is approximately from 12 % to 15 % of the total
number of strokes into OL.
The majority of strokes into the phase conductor
triggers a flashover (for any of the possible
circumstances and insulation level). Their share is
from 75 % to 100 %. The impact of the tower
height or footing resistance Roz is rather irrelevant.
When lightning strikes into the ground wire, it is
only its small share that gives rise to a backflashover. For the footing resistance of up
Roz = 30 Ω and insulation level BIL 1300 kV this
share is less than 7 %. By using the strengthened
insulation BIL 1425 kV, these values are further
decreased and are below 6 %.
As expected, the role of Roz at lightning strokes into
the shielding wire when using the BIL 1300 kV and
BIL 1425 kV insulation is important. The maximal
values of the flashover shares are between 2.7 %
(Roz = 20 Ω) to over 12 % (Roz = 40 Ω).
When values of Roz are low, the majority of the BIL
1300 kV and BIL 1425 kV insulation flashovers are
due to strokes into the phase conductor; when Roz
values are higher, the share of back-flashovers may
increase by over 50 % of their total number.
The high trees along both sides improve OL
performance very much. The flashover frequency
decreases by over 75 % for BIL 1300 kV.
The final criterion is a comparison of the 400 kV OL
fault occurrence rate when using either BIL 1300 kV or
BIL 1425 kV.
Figure 6: Lightning stroke density ng between
0.5 str./km2/year (yellow) and 3.0 str./km2/year (blue).
Pg. 4
Paper E-58
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
ISBN 978-0-620-44584-9
4.
Value of Urw-50Hz must be recalculated so as to obtain
switching withstand impulse overvoltage Urw-250/2500 :
INSULATION COORDINATION
Based on our calculations and statistical processing of
overvoltages, we determined the insulation level by
combining the deterministic (temporary overvoltages)
and statistic (transient overvoltages) method.
U rw250/ 2500  1,7  U rw50Hz  1,7  361,7  614,9 kV
since it covers Urw-50Hz.
4.1. Statistical method
4.3. Slow-front overvoltages
The point of reference in the statistical method is the
OL acceptable flashover risk or OL failure occurrence
rate. Flashover probability Rfo for a certain overvoltage
of the same type can be calculated on the basis of the
overvoltage occurrence distribution f(U) and flashover
probability on the insulator P(U) with the below
expression:
Judging from our computer simulations, the maximal
voltage at OL connections on its open end is equal to
U2% = 2.10 p.u., thus making the coordination
insulation withstand voltage to be:
420
 2,10  2
 720,1kV
3
3
By taking into account the correction factors, it is
increased up to the value of:
U cw  2,10  2

Rfo 
 f U   PU   dU
0
This means that besides the overvoltage there should
also be a model provided to demonstrate flashover on
the insulator string. The procedure is graphically
illustrated in Figure 7:
Um
U rw250/ 2500  ks kaUcw  1,11,11 720,1  879,2 kV
which determines the required withstand voltage.
P(U)
4.4. Fast-front overvoltages
1
Our approach to fast-front overvoltages was to some
extent different than the one given above. Our intention
was to determine the increase in the flashover rate ∆N
for the Beričevo-Krško OL if instead of BIL 1425 kV
we use BIL 1300 kV. Our calculations were made for
the values of OL tower footing resistances Rgr = 20, 25
and 30 Ω and OL tower heights Ht = 52, 57 and 62 m.
The calculated OL flashover frequency for the two
observed voltage levels is:
0,8
f(U)
0,6
0,4
0,2
Rfo
Figure 7: Determination of flashover probability Rfo .
N1425kV = 1.740 flashovers/year
N1300kV = 1.831 flashovers/year
According to IEC 60071-1, insulation of the 400 kV
system belongs to range II. It is then dielectrically
defined with the following withstand voltages:
This means that by decreasing the insulation from BIL
1425 kV to BIL 1300 kV, the flashover frequency
increases by 5.2 %. By taking into account the OL
length lOL = 80.4 km and the average lightning stroke
density ng = 2.4 str./km2/year we obtain (norming at
lOL = 100 km and ng = 1 str./km2/year) the value
N*1425kV = 0.901 flashovers/100km/year .
U250/2500 standard switching impulse (250/2500 μs)
U1,2/50 standard lightning impulse (1,2/50 μs)
As foreseen by the insulation coordination procedure,
we had to define an appropriate insulation level. To
determine the required withstand voltages Urw, we used
the appropriate insulation coordination factors for
safety ks and above-sea-level altitude ka (maximally
1000 m).
4.5. Insulation level
By considering results of our analysis of temporary and
transient overvoltages, the required standard withstand
overvoltages can be determined. For the switching
withstand voltage Urw-250/2500 the following applies:
4.2. Temporary overvoltages
For the maximum earth-fault factor that may occur in
the Slovenian 400 kV network, k = 1.2 is assumed.
With consideration of Um = 420 kV we determine the
representative overvoltage that is the same as the
insulation coordination withstand voltage, e.g.
U rp  1,2  420
Temporary overvoltages:
Slow-front overvoltages:
614,9 kV < 950 kV
879,2 kV < 950 kV
As to the lightning overvoltages, when determining the
lightning withstand voltage Urw-1,2/50, we find it
reasonable to adopt a 5.2 % increase in the flashover
frequency at N1300kV. This makes the standard
insulation level to be:
3  291,0 kV . By taking into account
insulation coordination factors, we get:
U rw50Hz  ks kaUcw  1,11,13  291,0  361,7 kV
Pg. 5
Paper E-58
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
ISBN 978-0-620-44584-9
On the potential ring, the corona (Figure 9) appeared at
a relatively low voltage of 195 kV and extinct at
193 kV.
U250/2500 = 950 kV
U1,2/50 = 1300 kV
In compliance with specifications of the Slovenian
legislation, there shall be electrically strengthened
insulation applied on certain locations. This in practice
means that instead of the insulators rated BIL 1300 kV
those rated BIL 1425 kV should be used.
5.
INSULATOR STRING
The adopted standard withstand voltages have to be
considered in construction of insulator strings. The first
to be defined are the basic demands for selection of
composite insulators and amongst them the use of:
- silicone rubber (HTV) for housing, and
- ECR glass fibers for rod.
Figure 9: Corona on the bottom part of the insulator at
the voltage of 292 kV.
The pollution state being moderate we opted for the
minimum specific creepage distance of 20 mm/kV
(pollution level II). As a result of a number of positive
effects [2], the selected construction involves inbuilding of potential gradient control rings on both
insulator ends. There were two types of the insulator
ring constructed with an adjustable distance lad between
arcing horns. The insulator length is lins = 2940 (BIL
1300 kV) or lins = 3280 (BIL 1425 kV). Our results of
the dielectric tests are given in Table 8:
Judging from the measurement results, with this
construction we are quite far away from the limit value
laid down for the corona evoking voltage which is set
at > 291 kV and is being used as a criterion in several
European countries. In re-designing the metal armours,
the calculated maximal value of the single-phase short
circuit of 37.5 kA (< 0.5 s) will be taken into account.
6.
Following our experience, calculations and field
measurements regarding the use of new 400 kV
insulator strings, the conclusions to be drawn are:
Table 8: Insulator strings withstand voltages.
Impulse shape
Polarity
Single (suspension)
BIL 1300
Double (tension)
BIL 1425
U50% [kV]
U10% [kV]
U50% [kV]
U10% [kV]
1,2/50 μs
+
−
1493
1647
1435
1583
1717
1844
1650
1772
CONCLUSION
250/2500 μs
+
−
1123
1124
1036
1037
1176
1330
1084
1226
- The insulators of our preference are composite
insulators (silicone rubber, ECR glass).
- Potential rings will be fitted on both insulator ends.
- In the insulation coordination procedure, our
analysis of overvoltages showed the appropriate
insulation level to be U250/2500 = 950 kV, U1,2/50 =
1300 kV.
- In the used design of the potential rings, the corona
takes part at a considerably lower voltage than the
adopted minimal limit value set at 291 kV, for
which reason certain alterations shall have to be
effected (larger diameter, double ring, etc.).
The above withstand voltages were obtained at the
flashover distance between the arcing horns
lad ≈ 2655 mm (BIL 1300 kV) or lad ≈ 3000 mm (BIL
1425 kV). The acceptable withstand voltages for
insulators (with no armours) are by 5 % to 10 %
higher.
We also made a radio interference (IEC 60437) and a
corona voltage test (IEC 61284) on a single suspension
string. The results of the first measurement at the
atmospheric conditions (T = 17.0 °C, b = 96.4 kPa,
hr = 37.5 %) are shown in Figure 8:
The tower construction being old, the inter-phase
insulation corresponds to the BIL 1425 level and was
therefore not addressed in our analysis.
7.
REFERENCES
[1] S. Vižintin, A. Varl, S. Jamšek, M. Hrast, B. Barl,
“State Analysis of 110 kV Composite Insulators
after Being in Service in the Electric Power
System of Slovenia for Several Years”, 14 th
International Conference on High Voltage
Engineering, Beijing, China, Aug. 2005.
[2] WG 22.03, “Use of Stress Control Rings on
Composite Insulators”, ELECTRA 143, pp. 69–71,
Aug. 1992.
Figure 8: Results of radio interference (RIV) test.
Pg. 6
Paper E-58