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Comparison of GIS and AIS systems for urban supply networks
20 kV, so that four different HV/MV networks are evaluated. The comparison is based on the life-cycle costs of the different supply concepts.
Design of the AIS/GIS network variants
High-voltage network The best locations for the HV injections into the distribution network depend to a
Studies carried out by ABB show that for urban supply networks the com-
large extent on the technology (AIS or GIS)
bination of HV gas-insulated switchgear (GIS) and HV cable has important
chosen for the HV system. Sites which are
advantages over systems with air-insulated switchgear (AIS) and overhead
large enough for AIS substations are sel-
lines. Due to their compactness and flexibility, GIS substations can be lo-
dom available, and when they are their
cated close to load centers, allowing a much more efficient configuration
cost is extremely high. But it is not just the
for both the HV system and the MV distribution network. The saving in in-
smaller size of the site that makes GIS the
vestment and operating costs more than compensates for the higher cost
lower-cost option: GIS is also the more
of the GIS and cables. Benefits include higher reliability and the option of
economic alternative when expanding or
integrating a complete GIS substation in an existing building, eg when
replacing existing substations. An inner-
extra site area is not available.
city site that has been used previously for an AIS installation can be sold or rented
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esides the vital role they play in mod-
parison does not take into account the
the new substation. The compactness of
ern market economies, electricity supply
fact that by locating a GIS transformer
GIS enables an HV transformer substation
systems also have a large impact on the
substation close to the load centers a far
to be fully integrated in an existing build-
environment, making them subject to
more efficient network structure is ob-
ing, which may only have to be made
changes in social paradigms. It is in part
tained at both the HV and the MV distribu-
higher or have a basement added.
due to this that the medium-voltage sys-
tion level. As a result, the investment and
tems supplying power to urban areas are
operating costs are reduced.
out and the income used to help finance
The issue of space extends beyond the transformer substations to the high-volt-
designed today almost entirely as cable
To quantify the respective impact of AIS
age connections. Overhead lines are
networks with indoor switching stations.
and GIS technology at the HV and the MV
today practically ‘no-go’ as an option for
Besides its reduced visual ‘intrusion’ on
levels of an urban supply system, a look is
inner-city areas. And even where rights-of-
everyday life, gas-insulated switchgear
taken in the following at an existing net-
way are available, these can often be util-
(GIS) also offers operators of high-voltage
work with a maximum load of 120 MW.
ized more economically in other ways.
supply systems reliable and flexible so-
The MV network is optimized for each of
Typical complaints about overhead lines
lutions in areas where load densities are
the HV variants (GIS and AIS), in each case
are that they are unsightly and generate
high and substation sites have to be kept
for distribution voltages of 10 kV and
electromagnetic fields, the effects of which
small. Urban supply systems combining
are subject to constant and intensive pub-
GIS technology and HV cable are safe, re-
lic debate. Modern-day HV cable not only offers a
liable and environmentally benign 1 . A direct comparison of the component investment for identical switchgear con-
Werner Zimmermann André Osterholt
high level of reliability but also some technical benefits for overhead line connec-
figurations suggests that the GIS variant is
Dr. Jürgen Backes
tions. Given these advantages, there is
more costly than the air-insulated switch-
ABB Calor Emag Schaltanlagen AG
today no realistic alternative to HV cables
gear (AIS) solution. However, such a com-
for electric supplies in urban areas.
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Use of ABB gas-insulated switchgear allowed the 132-kV transformer substation Barbaña in the center of Orense, Spain, to be constructed underground and a park built over it which harmonizes with the surroundings. The sound of the waterfall, which acts as a heat-exchanger, hides the noise made by the fans.
1
GIS The GIS variant 2 considered in the study 4.5 km
consists of three HV transformer sub2.7 km
stations located in the center of a city. The connection to the surrounding 110-kV network is provided by three cables run from
TS1
the nearest HV substation to the main substation of the urban HV network. This main substation is configured as a double
63 MVA
busbar system, allowing maintenance to be carried out on one busbar without having to de-energize the complete station.
3.7 km
The remaining substations in the HV cable
TS2
TS3
ring, which are also located in the city center, are H-type transformer substations with a bus-tie. They also allow maintenance and repairs within the gas compart33.1 MVA 4.1 km
27.8 MVA GIS variant of the 110-kV network TS1, 2, 3 Transformer substations 1, 2, 3
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ment on one half of the busbar while the 3 km
other half remains in operation.
4.2 km AIS In the AIS variant 3 there is an overhead line loop around the supply region, the
TS1
right half of which consists of double lines (due to the load flows). As in the GIS vari-
30.3 MVA
ant, the main HV substation consists of a double busbar system, the remaining HV transformer substations having H-type configurations. Transformer substation
5.6 km 1.6 km TS2
TS2 has a double-T connection to the SS1
double overhead line, substation TS3
SS2
1 km
2 km
being looped into the single line on the TS3
left-hand side. Due to the larger space required, the transformer substations are
1 km
located in the less densely populated outer regions of the city. 33.2 MVA
Double lines connect the outer ring to 28.1 MVA
the transformer substations, ie there are
TS4
two circuits on the same poles. This solution allows efficient utilization of the available space. However, it also reduces the reliability of the HV supply as both circuits can trip for the same reason (‘common
35 MVA
8.6 km
11.6 km
mode failure’, eg due to back-flashover from the earth wire to both circuits caused by lightning striking the earth wire or con-
3
AIS variant of the 110-kV network
tact with trees). TS1, 2, 3, 4 SS1 ,2
Transformer substations 1, 2, 3 Satellite stations 1, 2
Medium-voltage network A comparison of the GIS and the AIS vari-
•
ants based only on the differences in the
siderably influence operation of the net-
HV network is too limited, since the lo-
work. Each utility therefore has its own
Use of standard components: – XLPE MV cables with an Al cross-
cations of the HV transformer substations
planning rules, according to which the net-
section of 150 mm2 (distribution
are of decisive importance and exert a
work configuration is adapted to custom-
cables) and 240 mm2 (transmission
large influence on the structure of the MV
ers’ requirements and to the geographic
cables)
network. The study therefore takes the
locations of the MV loads.
– 110-kV/MV transformers with a rated power of 31.5/40 MVA
load situation in an actual urban network,
•
including the load values and the geo-
Planning rules
graphical location of the MV transformer
The technical constraints for each network
network. The distribution cables start
substations, as its starting point 4 .
variant are the allowed voltage band and
from the MV busbar of the HV/MV
Radial operation of an MV network
the limits for the short-circuit power, which
transformer substations, run between
results in a large number of possible net-
are dictated by the rating of the switch-
customers’ substations and are looped
work concepts. Besides differing in terms
gear. There are also some design rules to
back to the MV busbar of the same HV
of their initial capital costs, they also con-
be formulated:
transformer substation. During normal
Open-loop topology for the distribution
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operation one of the cables in this ring
substation) does not load any cable be-
required power transmission capability of
remains open to provide simple pro-
yond 120 % of its capacity. The maxi-
the MV network, allowing an additional
tection.
transformer
mum load of 5.2 MVA per cable at 10
saving due to the smaller MV cable cross-
substation has its own back-up trans-
kV (10.2 MVA at 20 kV) at the beginning
sections required. And thirdly, trans-
former, allowing maintenance to be
of the planning period takes into ac-
mission losses are avoided. The reduction
carried out on a transformer without
count the reduced ampacity for cables
in operating costs due to this third advan-
having to operate switches in the MV
bundled in the same cable trench close
tage is especially evident at low distribu-
network.
to the transformer as well as a margin
tion voltages.
Maximum of 14 customer substations
for load growth during the planning
in a loop. This limits the number of cus-
period.
Each
HV/MV
in the results obtained with the described network solutions. The MV network receiv-
tomers whose power supply would be
•
These general effects are also reflected
interrupted in the event of an MV net-
Planning results
ing power from the GIS substations con-
work failure, as only the feeder con-
The inherent flexibility of GIS allows
sists of radially operated (open) loops, all
nected to the HV/MV station busbar is
planners to place the injection points
of which are fed from the MV busbars of
equipped with overcurrent protection
close to the load centers 5 . The first
the HV substations.
and a circuit-breaker.
effect of this is on the number of loads
The peripheral locations of the trans-
Normal loading of the feeder cables, so
considered an optimum for each HV sub-
former substations in the AIS variant make
that a worst-case cable failure (feeder
station, and thus on its installed trans-
additional satellite stations necessary 6 .
failure close to the HV/MV transformer
former capacity. Secondly, it reduces the
These have remote MV busbars, fed by the HV transformer substations via several parallel and selectively protected trans-
4
Distribution and load rating of MV transformer substations taken as a starting point for the GIS/AIS system comparison
mission cables. Their reliability is comparable with the reliability of the MV busbar of the HV transformer substations, but
6.2 MW
1.9 MW
0.4 MW
they call for extra capital investment and
0.2 MW
cause additional losses. The AIS variant with a voltage of 10 kV requires 6 parallel 1.4 MW
2.7 MW
10 MW
11.3 MW
7.4 MW
3.1 MW
0.9 MW
cables from the HV injection to the satellite station, the 20-kV variant requiring 4 cables. These satellite busbars, like the
5.5 MW
4.9 MW
11.3 MW
19.7 MW
7.1 MW
6.7 MW
4.5 MW
MV busbars of the HV substations, supply power to the MV substations via open loops.
1.3 MW
3.6 MW
4.2 MW
7.4 MW
3.9 MW
8.4 MW
1.5 MW
GIS/AIS cost comparison 1.7 MW
7.7 MW
6.1 MW
2.3 MW
1.1 MW
7 and 8 compare the respective cost of
the AIS and the GIS variant (the figures for the different components and external ser1.5 MW
2.9 MW
1.8 MW
3.5 MW
0.3 MW
vices apply to the German market). An interest rate of 8 % and an inflation of 3 % were assumed for the calculation of the
0.5 MW
0.9 MW
0.8 MW
0.5 MW
cash values. A load growth of 1.5 % per year (linear) was set and the cash value calculated for a planning horizon of 10
0
1.0 km
years. It should be noted that planning times of 20 years and more, which have
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TS1
TS2
TS1 TS2
TS4 TS3 TS3
5
Topology of the GIS variant with injection points close to the load centers (distribution voltage 20 kV) TS1, 2, 3
6
Topology of the AIS variant with peripheral transformer substations and additional satellite stations (distribution voltage 20 kV)
Transformer substations 1, 2, 3 TS1, 2, 3, 4
Transformer substations 1, 2, 3, 4
often been used in the past to evaluate dif-
transformers results from the different
operation, so that only a single additional
ferent variants, are not realistic in today’s
number of transformers in the two vari-
unit is necessary for back-up. The AIS
rapidly changing market environment.
ants. In the case of GIS, two transformers
variant requires a dedicated back-up unit
are required at substation TS1 for normal
for the transformer in each HV substation.
The first cost to be considered in the comparison is that of the HV switchgear installations. This is significantly higher for the GIS than for the AIS solution. However, the difference in system costs is smaller than the difference in component (eg,
7
Comparison of the cost of the GIS and AIS variants (cash value over a period of 10 years) CVrel
Relative cash value
switchbay) costs. This is because the inherent flexibility of the GIS/cable topology
45
allows the HV network to be configured
%
AIS ( 20kV )
more efficiently, leading to a smaller
35
AIS ( 10kV )
number of HV substations (3 for GIS, 4 for
30
GIS ( 20kV )
AIS) and also switchbays per HV trans-
25
GIS ( 10kV )
former substation.
20 15
Interruption costs
Maintenance / repairs
the case of GIS.
Losses
equipment and civil works are higher in
Land / construction
lines with AIS, the capital cost of the
I&C
0 Cables MV
5
shorter than the length of the overhead
Switchgear MV
the cable length in the GIS variant is much
Transformers 110 kV
10
Lines / cables 110 kV
nections shows a similar picture. Although
CV rel
Switchgear 110 kV
A look at the cost of the 110-kV con-
The difference in the cost of the 110-kV
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GIS. It is costs such as these that under100 % 90
score how much the results depend on the basic assumptions of the comparison, eg the interest rate and the planning horizon.
80
As regards the losses, the short planning 70
horizon chosen for this comparison results
60
in the saving due to GIS being underesti-
50
mated. When state-of-the-art AIS and GIS
CV rel 40
components are used, the cost of their
30
maintenance and inspection will be a mini-
20
mum. As the comparison shows, the maintenance costs have no impact on the
10
variant ranking. The GIS variant exhibits
0 AIS ( 20kV )
AIS ( 10kV )
GIS ( 20kV )
the lower costs as it needs fewer inspec-
GIS ( 10kV )
tions per switchbay (GIS once every 8
Comparison of life-cycle costs with GIS and AIS (cash value over a period of 10 years)
8 years, AIS once every 5 years), of which it also has fewer. The final factor in the comparison is the
CVrel Relative cash value
cost of supply interruptions, ie the costs at
110-kV switchgear
MV switchgear
Land/construction
110-kV lines/cables
MV cables
Losses
110-kV transformers
I&C
Maintenance/repairs
the load end resulting from interrupted service. Although it is the customer who foots the bill, these costs enable the nonreliability of the network to be defined in monetary terms. Significant differences also exist be-
Another factor that speaks for the GIS variant is the cost of the MV switchgear in-
tion, control and instrumentation) for the
tween the 10-kV and the 20-kV MV supply.
GIS variant.
A 20-kV network can, roughly speaking,
stallations. The difference here is due to
In addition, the cost of the land, foun-
transfer twice as much power as a 10-kV
the additional switchgear panels needed
dations and buildings is lower for the GIS
network over the same cable cross-
for the satellite stations in the AIS con-
variant. This is due to the smaller space
section. This either reduces the invest-
figuration.
required, which can even compensate for
ment in primary equipment (due to the
The cost of the MV cables is extraordi-
the higher cost of land in the inner city.
smaller number of MV cables) or leads to
narily high for both variants (about 40 % of
Also, the GIS substation can be integrated
lower losses for the same cable cross-
the life-cycle costs/cash value). Their
in the basement of an existing building,
section.
absolute value as well as the difference
thus providing the required space without
This effect is relativized in actual
between AIS/GIS shows how important it
having to add to the footprint. Since this
planning. The maximum number of MV
is to compare complete supply concepts
solution is still unusual in Germany, it has
substations supplied by the same cable in
rather than just the HV substations if
not been considered in the comparison. In
high load density areas is limited not only
results are to be reliable and realistic: HV
South-East Asia and other regions, util-
by the ampacity of the feeder cables but
injection close to the load centers reduces
ities often take advantage of this option in
also by the need to keep the number of
the MV network costs significantly.
order to install HV stations in highly popu-
customers affected by a single failure as
lated load centers.
low as possible. Another constraint is due
The smaller number of HV substations, the absence of remotely controlled MV
The difference in the cash value of the
to the open-loop topology: the MV sub-
satellite stations and the reduced number
losses quantifies the additional trans-
stations must be assigned to two half-
of HV connections also reduce the cost of
mission loads in the MV network. A saving
loops, each of which starts at the HV
the secondary equipment (ie, for protec-
of approximately 25 % is possible with
substation busbar and is linked to the
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other via a disconnect point in the net0.035
work. The cost comparison of the GIS and
GIS
0.030
the AIS variant in the case of the 20-kV network shows reduced differences in
AIS
0.025
losses and in the investment in MV cable. 0.020
However, the costs are still significantly (approx 4 %) higher for the AIS variant.
0.015
The direct comparison of the 20-kV and 10-kV variants for similar HV tech-
0.010 IF
nologies shows an advantage for the
0.005
20-kV solution. This result is plausible and in line with actual planning experience.
TS1
SS1
TS2
SS2
TS3
TS4
Interruption frequencies (IF) at the MV busbars of the HV substation
9
Reliability of the AIS/GIS variants
TS1, 2, 3, 4 SS1, 2
Transformer substations 1, 2, 3, 4 Satellite stations 1, 2
A reliable electric supply is especially important in urban networks where load densities are high and users are sensitive to
of cables. Overhead lines exhibit high
cables. The high value for the outage rate
interruptions. The outage and reclosure
outage rates but short outage times. To
is a pessimistic estimate in which the ma-
behaviour is an area in which there is
keep overhead line systems compact,
jority of the faults are assumed to be
a fundamental difference between the
parallel circuits are run on the same
caused by external cable damage. A relia-
GIS/cable and the AIS/overhead line sol-
pylons. As mentioned before, a single
bility calculation was performed to quantify
utions, at least as far as the main power
event can therefore lead to common mode
the differences between the 10-kV vari-
users are concerned.
failures in which several circuits are
ants of the AIS/GIS concepts. The calcu-
110-kV cable faults are rare, being due
tripped. This aspect is important and has
lation simulates relevant component failure
in most cases to accidental damage dur-
to be considered in every reliability com-
modes and evaluates and quantifies the
ing construction work. Since HV cables
parison.
consequences for the customers. The
are buried deeper than MV cables, the
Table 1 shows the values used in the
latter, which are laid above them, give
reliability calculations for the HV lines/
component behaviour is derived from statistics about failures in the past.
them a certain degree of ‘protection’. If the HV cable itself is not sufficiently robust (eg, protected by a surrounding steel tube, as in the case of pipeline compression cable)
Table 1: Reliability data for cables and overhead lines (110 kV)
concrete ducts can be used to provide the Outage1) rate per year and 100 km
necessary shielding. This minimizes the
Repair duration (h)
failure rate even in supply regions in which large-scale construction work is carried out. Besides their cost, the main drawback of HV cables is the long time needed for their repair, as HV cables are typically
Overhead HV line (1 circuit)
short
0.21
1
long
0.04
20.5
Overhead HV line (common mode)
0.15
2.7
HV cables
0.35 0.35
3.4 298
spliced by the supplier’s staff.
short long
The characteristic behaviour of over1)
head lines is somewhat different to that
Independent, stochastic outages
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Conclusions 140
Although the GIS solution appears initially GIS
120
to be the more costly, its flexibility allows
AIS
the HV transformer substations to be sited
100
in optimum locations. The number of injections from the HV system can be opti-
80
mized and the transmission load of the MV network reduced. This leads to a signifi-
60
cant saving in investment as well as oper40
ating costs which more than compensates
n
for the additional cost of the GIS and HV
20
cables.
0
Another advantage of the GIS/cable 0.02 0.04 0.06 0.08
0.1
0.12 0.14 0.16 0.18
0.2
0.22 0.24
combination is that it offers better reliabil-
IF
ity than the AIS/overhead line solution.
Interruption frequency distribution, as experienced by customers
10
Major customers linked to the HV substation via parallel MV cables also
n Number of MV nodes IF Range of interruption frequencies (per year, upper value)
experience this in everyday operation. And being inherent advantages of the GIS/cable variant, these benefits cost the
The results of the calculation are shown
multiple cable failure will affect the satellite
in 9 . It is evident that the interruption
supply as long as the protection devices
rates are much lower for the GIS/cable
work correctly.
network operator nothing. GIS technology also offers further benefits which are harder to quantify but
variant, although high failure rates for the
As shown in 10 , there are no significant
which can be decisive for the realization of
HV cables have been assumed. The effect
differences between the AIS and the GIS
a project. One example is the option of
on the reliability is felt at the MV busbars
variant within the MV network.
complete integration of a GIS substation in
of the HV transformer.
The reason lies in the radial operation of
an existing building when no extra site
The values for GIS are of the order of
the MV loops. Every fault leads to the at-
area is available. Summing up, GIS is a
0.01 per year, ie one interruption every 100
tached feeder tripping and interrupting the
cost-efficient, flexible and reliable solution
years. The interruption frequencies for the
supply to all customers in the same half-
for supply systems in regions with high
AIS substations TS1 and TS4 are of the
loop. This interruption of service is usually
load densities.
same magnitude, but TS3 and especially
ended by (manual or remotely controlled)
TS2 are more frequently affected (by a fac-
switching. The large number of events in
tor of 3 to 4). The reason is the loop-in via
the MV network compared with those in
a double overhead line, which has been
the 110-kV system dictates the inter-
assumed in both cases, and in addition for
ruption behaviour and ensures com-
Authors
TS2 the double-T connection to the sur-
parative reliability at all customer nodes.
Werner Zimmermann
rounding ring (also a double overhead
This effect is underscored by the com-
André Osterholt
line).
parable interruption costs for AIS and GIS
Dr. Jürgen Backes
The values for the satellite stations re-
7.
ABB Calor Emag Schaltanlagen AG
ceiving power from TS1 and TS2 are the
In cases where a special-tariff custom-
same as for the MV busbars of the HV
er is connected to the HV transformer sub-
D-68167 Mannheim
transformer substations. This is plausible
station via parallel cables, the reliability of
Germany
because of the selective protection pro-
the HV substation is passed on to the cus-
E-mail:
vided for the parallel transmission cables
tomer, who also benefits in this way from
[email protected]
between the transformer substations and
the GIS solution.
[email protected]
the satellite stations; neither a single nor
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