Gas Insulated Substations (GIS) Tutorial: Applications & Reliability

1
Introduction and Applications of
Gas Insulated Substation (GIS)
Phil Bolin,
Hermann Koch,
IEEE Substations Committee
IEEE Substations Committee
A. Overview
Gas-Insulated Substations (GIS) are in use world-wide
since more than 30 years with in general very good experiences. GIS are most common in use in Japan, the largest single
GIS market in the world, in Europe, the Middle East, and also
the Far East and South East Asia.
In the USA, where this technology was founded, the success of GIS is a limited one. There are two major reasons , one
in that available space did not give high pressure to use GIS,
as it was the case in Japan, Europe and places in Asia, and as a
second reason some design difficulties at the beginning 30
years ago. Some of this first days equipment is still in service
due to the long living type of technology.
This tutorial is meant to present and teach about all the improvements done in the design of GIS technology and to
report about the excellent experiences with GIS in other
countries.
Restricted availability of space for new substations and the
replacement of old equipment in cities and metropolitan areas
indicate that in the near future also in the USA a larger
number of GIS will be installed and operated.
This tutorial is dedicated to all engineers who are or might
be in charge of GIS technology today or in the future.
The tutorial was set up by professionals all round the world
of leading equipment manufacturer, turnkey project executer,
industry consultant, operator and experiences user of GIS.
B. Introduction to GIS
1) GIS Reliability IEEE GIS Tutorial
This chapter addresses the reliability of GIS and provides a
brief comparison to an AIS substation. This information is
mostly based on CIGRE publications, in particular the latest
one from 1998. The details can be found in the Bibliography.
The first GIS’s were put in operation in 1967 in Switzerland and Germany. The GIS in Germany is still in operation,
whereas the GIS in Switzerland was recently decommissioned
after 35 years of operation without major fault or gas leak. The
utility made an assessment of the gas leak over the lifetime of
this first GIS and concluded that overall leakage rate was
about 0.4% per year.
US users were among the early adopters of the emerging
GIS technology in the 1970’s, however, bad experience with
immature designs resulted in a totally different view of GIS
reliability between North America (in particular the United
States of America) and the rest of the world.
Reliability, the economic advantages of reduced life cycle
cost and the physical compactness have resulted in the widespread use of GIS over the last 35 years.
Even though today’s GIS technology can be considered
mature many users’ approach to its application is still rather
unique. GIS is used in specific types of applications only.
The reliability of GIS has markedly improved since its introduction 35 years ago. The overall trend shows a reduced
failure rate for GIS commissioned after 1985. Consequently,
CIGRE distinguishes between GIS commissioned before 1985
and after 1985. Table II of the CIGRE report gives an overview over this data [I-1].
The table below gives the voltage class and corresponding
voltage levels.
TABLE I
CIGRE SURVEY 2000
VOLTAGE CLASSES
Voltage
class kV
1: 60 ≤ Un < 100
2: 100 ≤ Un < 200
3: 200 ≤ Un < 300
4: 300 ≤ Un < 500
5: 500 ≤ Un <700
6: Un > 700
The outdoor GIS population represents about 43% of the
total CB-Bay-Years.
Tables III and IV of the CIGRE report provide the major
failure characteristics of the GIS.
One increasing trend is the involvement of circuit breaker
failure for the newer GIS observed at all voltage levels.
TABLE II
CIGRE SURVEY 2000
MAJOR FAILURE FREQUENCY (FF) - 2ND GIS SURVEY TOTAL POPULATION
AND COMPARISON BETWEEN THE 1ST AND THE 2ND SURVEYS RESULTS
Voltage
Class
1
2
3
4
5
6
1 to 5
TOTAL
Voltage
Class
1
2
3
4
5
1 to 5
GIS in total
2nd GIS survey
1st GIS survey
No. of CB-bayFF
CB-bay- FF
failures years
years
27
56884
0.05
38471
0.13
465
32048
1.45
23845
1.1
138
16040
0.86
12955
1.1
179
6371
2.81
4735
4.3
49
4525
1.08
3453
4.2
12
200
6.00
80
14.0
855
115868
0.74
83459
0.96
867
116068
0.75
83539
0.97
GIS commissioned before 1.1.1985
2nd GIS survey
1st GIS survey
No. of CB-bayFF
CB-bay- FF
failures years
years
16
28669
0.06
21304
0.17
351
19504
1.80
16035
1.3
100
10362
0.97
8596
1.5
110
3694
2.98
3287
4.4
32
3252
0.98
2532
3.7
609
65481
0.93
51754
1.18
GIS commissioned after 1.1.1985
2nd GIS survey
1st GIS survey
No. of CB-bayFF
CB-bay- FF
failures years
years
11
28215
0.04
9792
0.06
114
12544
0.91
4605
0.6
38
5678
0.67
2636
0.4
69
2677
2.58
970
4.0
17
1273
1.34
654
1.8
246
50387
0.49
18657
0.51
Voltage
Class
1
2
3
4
5
1 to 5
Notes :
Failure frequency (FF) = No. of Failures per 100CB-bay-years
2
TABLE III
CIGRE SURVEY 2000
IDENTIFICATION OF MAIN COMPONENT INVOLVED IN THE FAILURE FROM GIS
VOLTAGE CLASS POINT OF VIEW
Main component involved
in the failure
(whole period)
Total number of answers
(reported failures)
Circuit Breaker or Switch
Disconnector
Grounding Switch
Current Transformer
Voltage Transformer
Bus bars
Bus ducts and Interconnecting Parts
SF6/Air Bushing
Cable Box/Cable Sealing
Power Transf. Interface
Chamber/Bushing
Surge arrester
Other
GIS in total
Class 2
%
801
=100%
43.4 (30.1)
17.9 (19.2)
4.4
0.9
5.6 (7.7)
5.5 (7.3)
11.9 (17.2)
%
435
=100%
54.7
17.2
5.3
0.7
6.2
3.7
4.1
Class
3+4+5
%
335
=100%
29.9
18.2
3.6
1.2
4.8
6.9
22.4
3.6
3.5
0.9
0.9
4.4
0.2
6.9
1.8
1.8
0.7
1.7
0.5
2.1
1.2
1.5
Based on published failure rates for AIS and GIS substation
by international organizations such as CIGRE it can be shown
that the failure rate of a six breaker ring bus in GIS is lower
than that of a nine breaker AIS in a breaker-and-a-half arrangement. There are several commercially available software
programs available to make these calculations. Below is a
comparison of the failure rates of a six-feeder 230 kV GIS
substation versus a nine-breaker AIS.
6 Breaker GIS 230kV
TABLE IV
CIGRE SURVEY 2000
IDENTIFICATION OF MAIN COMPONENT INVOLVED IN THE FAILURE FROM GIS
AGE POINT OF VIEW (5 MOST INVOLVED COMPONENTS)
Main component involved
in the failure
(whole period)
Total number of answers
(reported failures)
Circuit Breaker or Switch
Disconnector
Voltage Transformer
Bus bars
Bus ducts, Intercon. Parts
GIS in total
%
801
=100%
43.4
17.9
5.6
5.5
11.9
GIS before
1.1.1985
%
562
=100%
42.2
18.5
4.4
5.7
14.4
GIS after
1.1.1985
%
239
=100%
46.2
16.3
8.4
5.0
5.9
The worldwide experience with GIS as documented by several organizations including CIGRE, IEEE etc. indicates that
GIS has a lower failure rate than a comparable AIS substation.
Users in North America might dispute this fact. There are still
many first generation GIS in operation, which never lived up
to their promise. As a consequence GIS is still viewed with
skepticism and often not considered as a default choice. It
should be noted that also in North America GIS commissioned
after 1985 shows similar failure rates as observed by CIGRE
during the last worldwide GIS survey.
There is no question that a GIS compared with the same
configuration in AIS will always be more expensive if looked
at first cost only, unless special conditions exist such as land
availability and/or cost, soil conditions, environment etc. But
as a substation builder one should not compare a GIS and AIS
in the same configuration, but define the required availability
of the substation first and then look for the substation configuration that meets those requirements. As an alternative, the
availability of the AIS substation in a particular configuration
can be used. Then the substation builder should evaluate a GIS
configuration, which meets this base availability.
9 Breaker AIS 230kV
Failure outages
OF (1/yr) OD [h/yr]
Line 1
0.0214
0.117
Line 2
0.0214
0.117
T1
0.0214
0.117
AIS 1 1/2
T2
0.0214
0.117
cb
T3
0.0214
0.117
T4
0.0214
0.117
0.702
Line 1
Line 2
T1
GIS ring T2
T3
T4
0.0081
0.0081
0.0081
0.0081
0.0081
0.0081
0.072
0.072
0.072
0.072
0.072
0.072
0.432
The expected outage frequency of the GIS feeders is 2.5 times
less than in AIS
AIS feeder: 47 yrs! GIS feeder: 123 yrs.
The expected outage duration of the GIS feeders is 1.6 times
less than in AIS.
The result of this reliability study is shown in the table be-
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low. Based on the failure rate the 6 breaker GIS ring bus substation is superior to the 9 breaker AIS in a breaker-and-a- half
scheme and should be the preferred choice due to all the
advantages of GIS.
2) Design Features
The use of SF6 within the power delivery system is mainly
driven by gas-insulated switchgear. There are single phase and
three phase encapsulated designs. For the distribution voltage
level up to 145/170 kV mainly three phase enclosures are
used. For higher voltage levels single-phase encapsulation is a
standard.
In the last years the development of SF6 insulated switchgear was mainly aimed at reducing the use of material and
costs while maintaining the extremely high reliability.
Main steps of the development were as follows:
• progress of circuit-breaker technology, which allowed to
reduce the number of interrupter units despite increasing
breaking capability
• progress of casting and machining technology of aluminum casted parts, which allowed the use of minimized
shapes and volumes
• use of computerized production and testing equipment
with high quality standards
• design of integrated components with several functions
such as combined disconnect and grounding switches
within one gas compartment
• use of intelligent monitoring and diagnostic tools to postpone maintenance activities and avoid unnecessary tasks
As a result very compact GIS substations designs are available
on the market with the following changes in comparison to
older equipment:
• up to 98% of space reduction in comparison to air insulated switchgear
• up to 75% reduction of SF6 volume
• delivery of completely sealed and tested bays up to
245 kV
• leakage rates down to less than 0.5% per compartment
and year
110
100
[% ]
90
80
70
60
50
40
30
Size of building
Space requirement
Packing volume
20
10
0
68
70
72
74
76
78
80
82
84
86
88
90
92
94
96
98
Year
Fig. I-14. Progress of GIS Development (145 kV)
The mean time between failures according to international
statistics (IEC; CIGRE) has reached levels of 400 to 1000
years depending on the kind of switchgear and its voltage
level.
The importance of quality and reliability of all kind of
switchgear equipment has become an ever increasing topic
over recent years. Quality and reliability are the result of a
complex process which includes design, manufacturing, delivery and erection and after sales service.
Quality and reliability have to be already an integral part of
development activities. Reliability starts with the right design,
followed by the choice of the suitable materials, using the
relevant testing procedures and the appropriate manufacturing
techniques and all that accompanied by a stringent quality
control. Using the very latest procedures for computer aided
design, optimization of parts and components and failure
mode effective analysis (FMEA), dynamic calculations of arc
extinction and drive behavior accompanied by quality checks
are well known methods to meet the users’ expectations.
After the design stage, materials and components are subjected to thorough development tests. In this respect particular
significance is given to long-term strength even at extremely
high numbers of operations, as well as resistance to all kinds
of environmental influences. Due to the dominance of mechanical failures, major components such as drive mechanisms
are tested on a hydraulic test rig independently from the
switchgear but using the previously measured loads given as
stress level. Thousands of operating cycles will be performed
in just a few hours. In this way the components can be
subjected intentionally to a higher load than encountered in the
switchgear itself during normal operation in order to
determine their safety reserves. This method of testing is
proven for contact systems, operating rods, drive components
etc. and has enabled considerable improvement in their
reliability.
These development tests will be followed by prototype
testing, comprising mechanical, power, dielectric, heat run and
environmental tests even such as seismic testing, up to the
limits, on several test objects in parallel.
Although specified very clearly, type tests were run at different levels. For some manufacturers it is already common
practice today to conduct up to 10000 operations or even more
on one or several test objects for mechanical type testing.
Furthermore, it is also specified by several utilities to extend
the number of successful short circuit interruptions beyond the
requirements of the applicable standards for power type testing.
The testing during development and type tests are performed in the manufacturer’s own certified test fields or, if
requested, in independent laboratories.
Special attention will be given to the life time behavior of
insulating material. Especially for the GIS insulators an extensive test program has been in place ever since the first delivery
of GIS.
As a result of tests and more than 35 years of service experience it can be concluded that the life time of GIS insulators
will reach more than 50 years without a failure, and that they
are nearly insensitive to aging.
Many years of experience in switchgear design and usage
have allowed to optimize test and measurement techniques to
the appropriate severity of the duty of the components with
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respect to their material and functional characteristics.
All these activities were accompanied by parallel theoretical calculations and ever more exact definitions of requirements.
Manufacturing has also changed. A customer’s order will
be entered into an electronic data processing system, which
details the substation down to the components and finally into
the single parts. The steps of the manufacturing processes are
defined by a process map and accompanied by quality
assurance milestones to make sure that the same level of
reliability will be reached with every single piece of
equipment. The interdependence of manufacturing processes
in terms of technical specifications, used materials, available
machinery, logistics and personnel is highly complex. The
optimal point has been reached when the module meets all the
technological demands while being produced in few and
simple manufacturing steps.
The long time service experience and extensive tests have
shown that there is no difference between three-pole and single-pole bus arrangements. In general the bus conductors are
arranged symmetrically with insulating and supporting elements like spacers made of cast resin.
The enclosures nowadays are no longer made of steel but of
aluminum alloy, which offers several advantages such as reduced weight, excellent gas tightness due to excellent sealing
area surfaces, high corrosion resistance and negligible
resistive and eddy current losses.
The enclosure design is adequate to withstand the electrical
arc. By extensive research and using the new technologies of
3D-CAD and finite element method (FEM) the enclosure has
been designed and optimized including testing the worst case
scenarios. The result is a level of safety far greater than that
required by the IEC and IEEE standards.
The design of a modern GIS substation looks similar to the
figures given above without significant differences between
the individual manufacturers.
However, today’s activities hint at future changes. The following paragraphs provide some highlights of the future
trends of GIS development.
New techniques such as 3D CAD as a computerized design
tool, finite-element method for the mechanical safety of enclosures and housings, stereo-lithography for producing test models, and field distribution calculation for the predetermination
of the dielectric stress allow optimizing the components of the
HV equipment very precisely.
With the use of 3D CAD-systems 3-dimensional modeling
is simple, and the data can be used basically for mechanical
and electrical optimization. The same data is also the basis for
the computerized machining process and measurement system
of quality control.
The finite element method is an ideal tool for complex
components or shapes in the design and for the calculation of
critical areas of component stressing either on internal loads
such as pressure or on external loads such as seismic activities.
Stereo-lithography is a method for the manufacturing of
testing models based on 3D CAD design. It is made from
photopolymer and semi-cured with a UV laser, which pro-
duces the contours. This experimental model can be used for
mechanical and dielectric tests and eventually for the manufacturing of casting pattern. For the dielectric tests the models
are coated with silver.
The effective dielectric design receives considerable support from basically two field distribution analyzing programs:
the equivalent charge method and the finite element method.
Values of field strength along given contours are one result
obtained, minimum-maximum distribution the other. The field
distribution analysis programs are linked to the CAD system
and allow iteration procedures for the optimization of the dielectric design.
The development of ever smaller and more compact substations seems to be limited by the needs of the utilities for
convenient service and maintenance access. However, such
small equipment offers the advantage that it can be shipped as
complete, factory assembled and pre-filled double-bays
(245 kV) and triple-bays (145 kV) with shortest erection time
on site.
A further possibility offered by the space saving design is
the installation within a container for mobile use.
The circuit diagram in following figure describes the feed
back loop to development and production and is the basis for
an efficient cost benefit optimization between user and manufacturer of high voltage switchgear.
Stresses
Basic
Design
Realisation
and
Verification
Application
in
Service
Field
Experience
Maintenance
Monitoring
Failure
Rates/
Reliability
Fig. I-15. Field Experiences - Feed Back for the Development
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C. Applications
1) Jacksonville, Florida, USA
Ratings
Ur
Ir
UBIL
Is
138 kV
2500 A
650 kV
40 kA
Fig. I-20 Taylor Street
3) Puget Sound, Bow Lake Substation,
USA
Fig. I-19. Jacksonville, USA
The single line shows a ring bus with 4 bus coupling bays
and 16 feeder bays.
The bus coupling bays allow to separate the ring bus in up
to 4 segments.
Each feeder bay has a circuit breaker, grounding switches
before and after the circuit breaker, towards the ring bus and
towards the in- and outgoing line.
Disconnectors are placed at each feeder bay towards the
ring bus, towards the in- and outgoing line, and on each side
of the circuit breaker of the coupling bays.
The use of GIS to solve the single line requirements allows
a very compact substation as this bird‘s eye view shows.
Easy to see the 4 feeder bay sections and the 4 couple bays,
two at the end and two in the middle.
The local control cubicles are placed at the center
walkway.
The GIS substation is placed inside buildings which fits
nicely into the neighborhood. Beside the overhead lines
nothing reminds the public that there is a 138 kV switchyard.
The compactness of GIS and its modular structure allows
the use of small buildings and offer a space saving solution.
2) Taylor Street, Chicago, USA, ComEd
Ratings
Ur
Ir
UBIL
Is
362 kV
4000 A
950 kV
63 kA
Ratings
Ur
Ir
UBIL
Is
145/115 kV
2000 A
650 kV
40 kA
Fig. I-21. Puget Sound, USA
The Puget Sound Project at Bow Lake Substation in
Oregon USA.
This outdoor GIS solution is placed on a small spot in the
residential and office neighborhood in an often used 4
breakers, two overhead line feeders and two cable feeders.
The design voltage is 145 kV and the operation voltage
today is 115 kV with a current rating of 2000 A.
The Plan View shows the physical arrangement of the GIS
ring bus with 6 circuit breakers, two overhead line connections
and two cable connections.
The size of the installation is about 6 m by 10 m. The
outdoor GIS is three phase insulated.
In this side view it can be seen how the GIS is positioned
on the steel structures and how the overhead line is connected
to the three phase insulated GIS.
The small size and the metallic encapsulation offer great
advantages of GIS in locations like Puget Sound. Safety to the
public is given by GIS because of its metallic enclosure. Even
6
if someone (children searching for their play ball) is jumping
over the fence he is not in immediate danger.
The small size of the GIS equipment also improves the
esthetics of this outdoor installation.
4) Sargans Substation - Switzerland
Ratings
Ur
Ir
UBIL
IS
110 kV
2500 A
650 kV
40 kA
Fig. I-23. Termobahia
Modules have been selected by customer as optimized
solution for retrofitting the existing AIS substation.
There is no advanced condition monitoring installed.
Potential Transformers have been installed on the module
itself.
6) Braintree
Ratings
Ur
Ir
UBIL
Is
115/13.8 kV
2000 A
550 kV
31,5 kA
Fig. I-22. Sargans, Switzerland
Mainly due to very small footprint and also for aesthetic
purpose.
There is no advanced condition monitoring installed.
Relative cost is high for 110 kV substation.
Transformers are located into the building and connection
between GIS and transformer is through indoor SF6/Air
bushing.
That is an interesting possibility for 115 kV stations in the
US as it enables to have the surge arresters mounted
conventionally.
5) Termobahia - Rio de Janeiro, Brazil
Ratings
Ur
Ir
UBIL
IS
245 kV
2000 A
1050 kV
40 kA
Fig. I-24. Braintree
7) Barbana
Ratings
Ur
Ir
UBIL
Is
145 kV
2000 A
650 kV
40 kA
7
Fig. I-25. Barbana
The GIS and Transformers are located under an public
park. The substation supplies the inner city with energy