Asset Development Group
Comparison of the Reliability of a 400 kV
Underground Cable with an Overhead
Line for a 200 km Circuit
TRANSPOWER
REPORT
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Last saved: 23/03/05 09:58
© 2005 - Transpower New Zealand Limited
EXCUTIVE SUMMARY
This report forms part of the preliminary investigations associated
Transpower’s North Island 400 kV Backbone investigation project.
with
International Consultants, PB Power have carried out some preliminary
investigations into the cost and feasibility of using underground cables for the
proposed 400 kV, 1000 MW 200 km long double circuit transmission line between
Auckland and Whakamaru.
The results of these investigations are reported
separately.i
This report investigates and reports on the expected performance of a 400 kV
underground cable with an overhead line for a 200 km long circuit. It compares
expected performance in terms of failure rates, outage times and availability.
If the present grid availability is to be maintained then the failure rates and outage
times for 400 kV lines would have to be to be equal to or better than those for the
existing 220 kV lines.
For 400 kV 200 km long overhead lines, there is little doubt that their availability
due to forced outages is likely to be similar or considerably better than the existing
220 kV lines.
There is a very high level of uncertainty in the failure rates for 400 kV cables
because of the small number of circuit kilometres installed and recent changes in
technology with the introduction of XLPE type cables at this voltage.
Repair times for faults on cables, joints and terminations are much longer than for
overhead lines and at best will take between 10 and 19 days. This assumes that
the cable jointers would be immediately available from overseas, that spares were
immediately available in New Zealand and the site is accessible and the fault easily
located.
The conclusion of the study is that even with optimistic assumptions on failure rates
and outage durations the availability of a 400 kV 200 Km long cable circuit will be
significantly worse than for an overhead line.
The requirement for shunt
compensation equipment every 50 km with a cable circuit will reduce the circuit
availability even further compared to an overhead line.
Also, the effect of the long cable repair times on grid security requires consideration
to determine if additional circuit redundancy is required.
The long cable repair times can be mitigated by provision of a spare cable per
circuit or a spare circuit. While both these options significantly reduce the risk of
complete loss of transmission capacity neither will increase the reliability to that of
a double circuit overhead line.
© 2005 - Transpower New Zealand Limited
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CONTENTS
EXCUTIVE SUMMARY.......................................................................................................... 3
1
2
INTRODUCTION. ..................................................................................................... 5
FAILURE RATES..................................................................................................... 5
2.1
2.2
3
REPAIR TIMES........................................................................................................ 8
3.1
3.2
4
Overhead Line Failure Rates. ........................................................................................ 5
Cable Failure Rates........................................................................................................ 6
Overhead Lines Repair Time. ........................................................................................ 8
Cable Repair Time. ........................................................................................................ 9
AVAILABILITY....................................................................................................... 10
4.1
4.2
Existing and Proposed Circuits. ................................................................................... 10
Availability Enhancement. ............................................................................................ 11
5
CONCLUSIONS..................................................................................................... 13
A
REFERENCES....................................................................................................... 14
© 2005 - Transpower New Zealand Limited
4
1 INTRODUCTION
This report forms part of the preliminary investigations associated
Transpower’s North Island 400 kV Backbone investigation project.
with
International Consultants, PB Power have carried out some preliminary
investigations into the cost and feasibility of using underground cables for the
proposed 400 kV, 1000 MW 200 km long double circuit transmission line between
Auckland and Whakamaru.
The results of these investigations are reported
separatelyii.
This report investigates and reports on the expected performance of a 400 kV
underground cable with an overhead line for a 200 km long circuit. It compares
expected performance in terms of failure rates, outage times and availability.
2
FAILURE RATES
A measure of overhead transmission line and underground cable reliability is the
rate at which they fail or are taken out of service (outages). Outages can be
categorised by the following types:
•
Automatic transient outage and reinstatement, due to operation of autoreclose circuit protection.
•
Automatic forced outage due to operation of circuit protection.
•
Forced outage due to manual operation of switchgear.
•
Planned outage for routine maintenance.
•
Planned outage for construction maintenance.
The outage rates are conventionally measured by considering the number of
outages per 100 circuit-km per annum, as they are proportional to circuit length.
Auto-reclose operation on an overhead line is not considered as a forced outage, as
the circuit is typically disconnected for less than a second or so, to clear the fault.
For the purposes of this report a forced outage will be considered as an outage
failure that cannot be deferrediii, hence will incorporate outages caused by
protection equipment and manual operation of switchgear.
Furthermore, outages for construction maintenance will not be considered for
comparison purposes, as this is a special case. Construction maintenance for a
transmission line would include tower rebuilding and conductor replacement, while
construction maintenance for a cable could include cable section replacement.
Hence the term ‘planned outage’ will refer to outages for routine maintenance only.
2.1
Overhead Line Failure Rates
Most faults on Transpower 110 and 220 kV overhead transmission lines are
transient in nature, caused by insulator flash-overs which are self healing and do
not result in the line being permanently out of service. The transient faults are
© 2005 - Transpower New Zealand Limited
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typically caused by lightning activity, are of very short duration, and are
automatically cleared by auto-reclose operation of the line circuit breakers. The
complete automatic clearing operation typically takes less than a second and does
not requiring human intervention. It is assumed that the 400 kV overhead lines will
behave in a similar manner. The failure rates for steel tower overhead lines
reported by the Canadian Electricity Association, for the period between 1998 and
2003, and excluding auto–reclose operationiv are:
Rated Voltage
Line
kilometre
(km.yr)
Years
Sustained
Line
forced
outage frequency (per
100 km.yr)
110 -149 kV
69,493
0.8922
150 – 199 kV
3,953
0.1518
200 – 299 kV
130,295
0.5656
300 – 399 kV
38,825
0.1056
500 – 599 kV
19,156
0.5481
Actual sustained recorded forced outage rates for all Transpower 220 transmission
line circuitsv measured over the last ten years, excluding auto-reclose operation;
indicate a forced outage rate of 0.34 per 100 km per annum and a planned outage
rate of 0.92 per 100 km per annum.
New Zealand 220 kV forced transmission line failure rates are similar to those
experienced in Canada, and it is assumed that the planned outage frequency would
also be similar.
Consequently it would be reasonable to assume that if the proposed New Zealand
400 kV overhead line is designed using similar principles to those used for the
existing 220 kV lines, then the expected forced outage rate would not be worse
than for the 220 kV system and not better than for the Canadian 300 – 399 kV
system i.e between 0.11 and 0.34 sustained forced outages per 100 km years.
Also the maximum expected planned outage rate for the 400 kV overhead line
could be expected to be not worse than for the 220 kV system. For a 200 km long
400 kV double circuit line it is reasonable to assume that each circuit would be
taken out of service once per year for maintenance. This gives gives a planned
outage rate of between 0.5 and 0.92 planned outages per 100 km years.
2.2
Cable Failure Rates
Cable insulation faults can not be cleared by auto-reclose operation as a break
down in cable insulation is never self healing as in air insulated transmission lines.
Hence a cable fault always results in the cable being out of service until repairs can
be completed.
The cable failure rates reported by the Canadian Electricity Association, for the
period between 1998 and 2002 are:
© 2005 - Transpower New Zealand Limited
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Rated Voltage
Cable kilometre
(km.yr)
Years
Cable
forced
outage
frequency
(per
100
km.yr)
110 – 149 kV
2,346
1.7903
200 – 299 kV
749
1.6238
300 – 399 kV
60
15.000
500 – 599 kV
698
1.1461
Unfortunately the cable kilometre years are not sufficient to give a statistically
meaningful result for voltages above 200 kV.
A recent submission to the Connecticut Siting Councilvi gave the results of
investigations into 400 and 230 kV cable fault rates, for XLPE type cables installed
in Europe, Asia and the USA. It estimated future failure rates of 1 (optimistic), 1.4
(realistic) and 4.1 (pessimistic) per 100 km.yr, based on data available up to the
year 2001. However this was based on less than 700 cable circuit km and included
faults occurring during commissioning and multiple faults from the same cause.
Most of these faults resulted from design, material and manufacturing problems
that occurred in the developing technology of XLPE cables and are likely to be
successfully remedied and consequently will not result in failures in new circuits.
Also cable circuits have sources of faults such as cable terminations and surge
arrestors that are not directly proportional to circuit length and so should be
accounted for separately.
A Cigre papervii gives information from a major cable manufacturer and two major
utilities, one Japanese and one Swedish based on 13000 cable circuit kilometre
years show a failure rate decreasing from 0.128/100 km.yr (1973-1989) to
0.0153/100 km.yr (1990-1993) for 110-170 kV cables. The paper chose an
intermediate fault rate of 0.07/100 km.yr to reflect this decreasing trend. This was
for the cable only. The paper also gave separate fault rates for joints, terminations
and surge arrestors. The PB Power reportii gives a maximum cable section length
of 50 km between points of shunt compensation on a 200 km long circuit. This
results in an circuit fault rate of 0.28/100 km.yr.
A Commission of the European Communities background paperviii quotes an average
failure rate of 0.72/100 km.yr and states this figure is also confirmed by a study
carried out by DISCAB Group over the last 12 years as presented in the ICF
Congress in Barcelona in 1995.
The background paper also mentioned that the fault rate of cable circuits should be
less than that of overhead lines because cables are not susceptible to storm
damage. However a critique of the background paperix said: “Security of supply in
the HV and EHV grid is not necessarily enhanced by cables even though cables are
not subject to transitory phenomena. Adverse weather conditions may not be a
problem for overhead lines if designed and maintained properly including right of
way management. Experience of some European countries (Austria) is best with
OHL even with heavy storm, flood, ice and snow.
Flood, earthquakes and
landslides may influence cable connections”.
© 2005 - Transpower New Zealand Limited
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Obtaining an accurate value of the failure rate for cables at voltages above 200 kV
is very difficult because of the statistically small population of cable km.yr for the
number of failures occurring. There are no recent international studies and most
Utilities are unwilling to share this information in the public domain.
In consequence of the above it is necessary at this time to assume a wide range of
expected failure rate.
The cable failure rates given in the PB Power reportii are taken from a submission
made to the Connecticut Siting Council and are considered to be unduly pessimistic
for the reasons given above. Therefore it is considered appropriate to use a
pessimistic failure rate equal to the report’s optimistic rate i.e 1 per 100 km.yr.
The optimistic failure rate of 0.28 per 100 km.yr is based on the Cigre papervii.
Note: failure rates of Transpower EHV cables, installed in New Zealand have not
been considered in this report due to the very small amount of cable installed.
3 REPAIR TIMES
Repair durations for faulted EHV overhead lines and cables can be measured in
hours, days or in extreme circumstances even months. The duration time is related
to the degree of technology complexity used, availability of skilled workforce at
short notice, ready availability of spare components, and ability to quickly identify
the position and type of fault.
3.1
Overhead Lines Repair Time
The number of recorded average forced outages and durations for Transpower’s
220 kV transmission lines, for the last 10 year period, indicate:
•
247 forced outages due to protection initiation, each taking an average of 1.2
hours for restoration of supply.
•
35 forced outages due to manual switching, each taking an average of 6.1 hours
for restoration of supply.
An average forced outage, for Transpower’s existing 220 kV OHL system,
considering protection and manual switching, is therefore 1.808 hours.
The recorded average planned maintenance outage duration for Transpower’s 220
kV transmission lines, for the last 5 year period, indicate an average of 17.45 hours
per planned outage. In exceptional circumstances this could conceivably extent to
four days duration.
It is assumed that the range of forced and planned repair times for the proposed
400 kV transmission line will be similar to the existing 220 kV transmission lines,
providing similar design and construction technology is used, similar stocks of
spares held locally and similar maintenance contracts organised with local
companies.
Therefore for the proposed 400 kV overhead line the expected average forced and
planned outage durations are 1.8 hours and 17.5 hours respectively
© 2005 - Transpower New Zealand Limited
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3.2
Cable Repair Time
A recent Consultant’s reportx, investigating repair times for proposed XLPE type 220
kV cables has the following conclusions:
220 kV Cables
Repair
(Days)
Termination fault at transition structure
10
Cable Joint Fault
18
Cable internal fault in duct
19
Cable internal fault in direct lay section
17
Serving fault on cable
6
Serving fault on bonding level
6
Sheath link box fault
1.5
Duration
The above forced outage repair times were estimated based on all cable repairs
being performed by specialist cable jointers brought into New Zealand from
Australia for each event. It also assumed that the contracting cable jointers would
be immediately available from overseas, that spares were immediately available in
New Zealand and the site is accessible and fault easily located.
It is reasonable to assume that repairs on 400 kV XLPE cables will be of similar
duration to the 220 kV XLPE cables, due to their similar technology complexity.
An average forced outage time of between 10 and 18 days (240 and 432 hours) is
assumed for the proposed 400 kV cables.
For XLPE cable technology, the associated routine maintenance should include
periodic sheath insulation testing, and possibly sheath voltage limiter testing,
maintaining cable sheath link bolt tightness, general inspection of link box
internals, possible cleaning of air insulated cable terminations or monitoring of oil
levels of GIS connected cable terminations. These activities would normally be
carried out on cable sections optimistically every four years, or pessimistically
every two years; requiring an associated planned outage for each cable circuit. It is
estimated that 8 hours would be required for each cable circuit outage and that a
10 km circuit section could be tested by one maintenance team. This equates to an
optimistic planned outage rate of 2.5 outages per 100-km per year and a
pessimistic planned outage rate of 5 outages per 100-km per year.
© 2005 - Transpower New Zealand Limited
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4 AVAILABILITY
4.1
Existing and Proposed Circuits
Availability is a measure of circuits being available for service and is hence
influenced by circuit failure rates and repair times. Or conversely, Unavailability is a
measure of a circuit being out of service. Conventionally both Availability and
Unavailability are measured in per-unit. A circuit that has 1.0 p.u. Availability has 0
p.u. Unavailability and is always available for service. A circuit that has 1.0 p.u.
Unavailability has 0.0 p.u. Availability and is never available for service.
To ensure overall grid availability is not reduced by the addition of 400 kV circuits it
is necessary for the 400 kV circuits to have availabilities not less than for the
existing 220 kV lines. If it is assumed over the lifetime of the circuit that the
unavailability due to planned outages will be the same as for existing 220 kV lines
then the unavailability due to forced outages will have to be the same or smaller.
Considering historical Transpower outage data, measured over the last ten yearsv,
the reliability of 400 kV overhead lines and underground cables for optimistic and
pessimistic failure rates and average outage times as discussed, are tabled below
and have been calculated using the following formulae:
Unavailability = outage rate / (outage rate + 8760/outage duration)
Where failure rate = failures per year & outage duration is hours per outage
Availability = (1 – Unavailability).
Circuit
Planned Forced
Average Average Planned
Forced
Total
200 km
Long
Outage
Rate
Planned Forced
Outage Outage
Time
Time
Outage
Unavailability
Outage
Unavailability
Unavail- Availabiability
lity
(p.u.)
(p.u.)
(p.u.)
Outage
Rate
(100 cct-(100 cct-(hrs.)
km/yr.) km/yr.)
220 kV
O/H Line
Historic.
400 kV
O/H Line
optimistic
pessimistic
400 kV
Cable
optimistic
pessimistic
(hrs.)
Total
(p.u.)
0.92
0.34
17.5
1.8
0.00366
0.00014
0.00380
0.99620
0.5
0.92
0.11
0.34
17.5
17.5
1.8
1.8
0.00200
0.00366
0.00005
0.00014
0.00204
0.00380
0.99796
0.99620
2.5
5
0.28
1
8.0
8.0
240.0
432.0
0.00456
0.00909
0.01523
0.09399
0.01978
0.10309
0.98022
0.89691
A 400 kV overhead line would easily meet the availability criteria on optimistic
values and be the same on pessimistic values.
© 2005 - Transpower New Zealand Limited
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A 400 kV cable would not meet the availability criteria on either the optimistic or
pessimistic values. Note this availability is based on a 200 km long circuit with 4 x
50 km cable sections. In practice cable availability would be reduced further by
outages of the shunt compensation equipment required between cable sections.
Also, the effect of the long cable repair times on grid security requires consideration
to determine if additional circuit redundancy is required to provide availability
enhancement.
4.2
Availability Enhancement
It is reasonable to assume that a 400 kV 200 km long cable circuit should have an
availability equal to or greater than 0.99620, which is the availability of an existing
Transpower 220 kV overhead line of the same length. The proposed 400 kV cable
circuit discussed above, consisting of one cable per phase for each circuit, will not
meet the required availability criteria.
A single cable circuit availability may theoretically be increased by: providing a
spare single phase cable; or providing a complete spare circuit, consisting of three
individual cables.
However if there are two cable circuits installed side by side, the availability of the
circuits could be theoretically increased by: providing one spare cable shared
between both circuits, or providing one spare cable for each circuit, or providing
one complete spare circuit consisting of three cables.
When considering possible cable circuit availability enhancement that may result as
a consequence of these additions, the following has been assumed:
4.2.1
•
The effect of associated substation or switchgear cable circuit equipment
availability has not been considered.
•
Cable circuits will utilize single phase cables, with cable sheath cross bonding in
link boxes situated at each cable joint. (Typically positioned approximately
every 800 metres or cable drum length).
•
A cable can be repaired with adjacent cables alive.
Cable circuit with spare cable
The spare cable would be installed in the same trench as the in-service cables so
that it could be connected in place of any one of them. This could be achieved by
disconnectors or bolted connections at the circuit’s terminations. It would also
require sheath bonding connections to be changed at every joint with cross bonded
sheath connections (i.e at two out of every three joints).
This would reduce the forced outage duration to the time required to confirm the
protection indication of which was the faulty cable, isolate and earth the circuit and
then connect the spare cable in place of the faulty one. For a 50 km cable section
involving changes to the sheath bonding connections in 67 link boxes this is
expected to take about 24 hours.
Although this increases the availability of a 200 km long cable circuit to between
0.994 and 0.985 pu. it is still below the target value of 0.996 pu
© 2005 - Transpower New Zealand Limited
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4.2.2
Two cable circuits with one spare cable
If two cable circuits share the same route then it is theoretically possible for a spare
cable to be installed so it can be used for either circuit. At 400 kV this is unlikely to
be practical as the circuits would normally be at least 3 m apart to reduce mutual
heating between circuits to an acceptable level. The spare cable would then have
to be installed at a significant distance from one or both circuits. Connection of the
spare cable in place of an existing cable would then lead to significant unbalance in
the sheath cross bonding arrangements. It would also require the joints in both
cable circuits to be at almost exactly the same positions along the cable route to
keep sheath bonding connections down to an acceptable length.
Cable circuit availability would be slightly less than for one spare cable per circuit
because of the small probability of a fault occurring on the second circuit before
repairs had been completed on the first circuit.
4.2.3
Spare cable circuit
Installing three cable circuits instead of two on the same route along public roads is
unlikely to be practical because of the space required. If a second route is available
then some additional security would result from the use of geographically separate
routes. The addition of a third circuit would increase costs by 50 %, which would
be slightly mitigated by the reduced cable losses with all 3 circuits were in service.
The probability of the loss of more than one circuit is given below:
Average Unavailability of Unavailability of Unavailability of 3 of
1 of 2 circuits
2 of 2 circuits
3 circuits
Forced
Outage
Time
(p.u.)
(p.u.)
(p.u.)
(100 cct- (hrs.)
km/yr.)
Circuit 200 Forced
km Long
Outage
Rate
220 kV
O/H Line
Historic.
400 kV
O/H Line
optimistic
pessimistic
400 kV
Cable
optimistic
pessimistic
0.34
1.8
0.00028
0.000000079
0.0000000000442
0.11
0.34
1.8
1.8
0.00009
0.00028
0.000000008
0.000000079
0.00000000000150
0.0000000000442
0.28
1
240.0
432.0
0.03045
0.18799
0.000927284
0.035340073
0.00002823702207
0.0066435646777
The table shows that there is a higher probability of losing all three cable circuits
than both circuits of a double circuit overhead line. The unavailability of all
transmission capacity in one year varies from 2.5 s for the existing 220 kV
overhead lines to between 8 hrs and 13 days for 2 cable circuits and 15 mins and 2
days for 3 cable circuits.
© 2005 - Transpower New Zealand Limited
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5 CONCLUSIONS
If the present grid availability is to be maintained then the failure rates and outage
times for 400 kV lines would have to be to be equal to or better than those for the
existing 220 kV lines.
For a 400 kV 200 km long overhead line, there is little doubt that their availability
due to forced outages is likely to be similar or considerably better than the existing
220 kV lines.
There is a very high level of uncertainty in the failure rates for 400 kV cables
because of the small number of circuit kilometres installed and recent changes in
technology with the introduction of XLPE type cables at this voltage.
Repair times for faults on cables, joints and terminations are much longer than for
overhead lines and at best will take between 10 and 19 days. This assumes that
the cable jointers would be immediately available from overseas, that spares were
immediately available in New Zealand and the site is accessible and the fault easily
located.
Even with optimistic assumptions on failure rates and outage durations the
availability of a 400 kV 200 km long cable circuit will be significantly worse than for
an overhead 400 kV line. The requirement for shunt compensation equipment
every 50 km with a cable circuit will reduce the circuit availability even further
compared to an overhead line.
Also, the effect of the long cable repair times on grid security requires consideration
to determine if additional circuit redundancy is required.
The long cable repair times can be mitigated by provision of a spare cable per
circuit or a spare circuit. While both these options significantly reduce the risk of
complete loss of transmission capacity neither will increase the reliability to that of
a double circuit overhead line.
© 2005 - Transpower New Zealand Limited
13
APPENDIX A
A
REFERENCES
i
PB Power Report, “Factors Affecting the Viability of a 200 km 400 kV Power Cable from
Auckland to Whakamaru”, 18 February 2005.
ii
Ibid
iii
IEEE Std 493-1990, “IEEE Recommended Practice for the Design of Reliable Industrial
and Commercial Power Systems.”
iv
Canadian Electricity Association Report, Pub. 2004, “Forced Outage Performance of
Transmission Equipment, For the Period January 1, 1998 to December 31, 2002”.
v
Transpower Reliability Data, from Trevor Weaver, for period July 1992 to June 2004,
dated 25 February 2005.
vi
Updated Cable fault Rate Information, provided by Brian Gregory for State of
Connecticut Siting Council, docket 272, 18 June 2004
vii
D Karlsson et al, “Comparison of 130 kV XLPE cable systems and OH lines – loading
capability, reliability and planning criteria”, Cigre paper 37-104, 2002.
viii
Commission of the European Communities, “Background Paper – Undergrounding of
electricity lines in Europe”, Brussels, 10 December 2003
ix
“First UCTE Comments on the Background Paper Undergrounding of Electricity Lines in
Europe” The "Union for the Co-ordination of Transmission of Electricity" (UCTE) ,2004
x
Meritec Report, “Repair Scenarios for 220 kV and 110 kV Cable Installations”,
November 2001
© 2005 - Transpower New Zealand Limited
(last page)
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