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GIS substation

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GAS INSULATED SUBSTATION & SWITCHGEAR
Asif Eqbal: Electrical & Electronics
Engineer
Contents
1. Introduction and governing standard
2. Conventional substation (AIS)
3. Limitation of AIS
4. Need for GIS
5. Introduction to GIS
6. Service condition of GIS as per IEEE
7. Services offered by GIS user & GIS supplier
8. Technical requirements & design features
9. Maintenance philosophy of GIS switchgear
10. Comparison of GIS and AIS substation
11. Drawbacks of GIS
12. Growth of GIS substation
13. Future trends in GIS
14. Conclusion
Introduction and governing standard
This presentation covers the technical requirements and design aspects of a gas-insulated
substation (GIS) as per governing standards of GIS substations.
Governing standard of GIS substation are:
1.
IEEE Standard C37.123-1996 (Rev-2002)- IEEE Guide to Specifications for Gas-Insulated,
Electric Power Substation Equipment.
2.
IEEE Standard 1300–1996, Guide for Cable Connections for Gas-Insulated Substations.
3.
Components inside GIS switchgears (CB, CT, PT and Disconnecters) are governed by
their respective IEC standard. Prominent among them is IEC 62271-203, 200, 100
4. ASTM D2472–1992, Specification for Sulfur Hexafluoride.
5.
NEMA CC 1–1993, Electric Power Connectors for Substations.
Conventional substations (AIS)
A:Primary power lines' side B:Secondary power lines' side
1.Primary power lines 2.Ground wire 3.Overhead lines 4.Transformer for measurement of electric voltage 5.Disconnect
switch 6.Circuit breaker 7.Current transformer 8.Lightning arrester 9.Main transformer 10.Control building 11.Security
fence 12.Secondary power lines
Conventional substations
(AIS)……continued




Power generating stations may be hydro, thermal or nuclear. Depending upon the availability of
resources these stations are constructed at different places. These places may not be nearer to load
centers where the actual consumption of power takes place. So it is necessary to transmit these
huge power blocks from generating station to their load centers.
Long and high voltage transmission networks are needed for this purpose. Power is generated at
medium voltage level. It is economical to transmit power at high voltage level. Distribution of
electrical power is done at lower voltage levels or as specified by consumer requirement.
For maintaining proper voltage levels at transmission and distribution level and for providing greater
stability a number of electrical transformation and switching setups have to be created in between
generating station and consumer ends. These transformation and switching setups are generally
known as electrical substations.
There are various ways to classify a substation. Based on purpose substation may be transmission
substation, distribution substation etc….Based on nature of dielectric medium a substation is of two
types AIS and GIS
Conventional substations
(AIS)……continued
 Air used as a dielectric.
 Normally used for outdoor substations.
 All the equipment like circuit breaker, CT, VT and
disconnecters are outdoor
 Equipment are mounted on lattice or pipe structure
and connected with rigid tubes or flexible conductors
 Easy to expand (in case space is not an issue)
 Excellent overview, simple handling and easy access.
Limitations of
AIS………….continued
1. Site selection of AIS substation is a complex process and requires consideration
of lots of factor like:
 Technical factor. Technical factor consist of mainly area requirement and
pollution.
 For deciding the area requirement of a substation its voltage level(s), number of
feeders, requirements of step-up/ down transformers & reactors, infrastructural
facilities like housing etc., for present and future expansion on a 10-15 year
scenario are to be planned. After such an assessment, estimate of required area
is made.
 Substation location should be away from the polluted area as far as possible. As
pollution levels increase, the insulator creepage distance of equipment will also
increase, which may increase the cost of the equipment. In extreme cases, in
heavily polluted area cleaning facilities or the use of protective products may be
necessary. The risk of failure of equipment increases with the pollution level.
Limitations of AIS
 Physical factor. Physical factor mainly
consist of Topographical, Geological and
Geographical fcators.
 Topographical uniformity is maintained
through standardization of supporting
structures and uniform equipment
terminal height, AIS land needs to be
levelled. Wherever, high cost of leveling
and retaining walls is anticipated,
various terraces needs to be formed.
AIS substation needs to be located on
the level around 0.5 m higher than
highest recorded flood level. It is
preferable to select the site as even as
possible to save not only time and cost
of leveling but also cost of civil
foundation .

Limitations of
AIS…………..continued
 Physical factor. Physical factor mainly consist of Topographical, Geological and
Geographical fcators.
 Most important geological factor is the type of soil & its bearing capacity, which
needs investigation. Soil should be suitable for construction of roads and
foundation. A high water level may require special kind of treatment for
foundation. Hence cost of substation may vary depending upon nature of soil.
Similarly, if the natural soil has a high resistivity, the earth mat becomes costly
accordingly.
 Geographically a AIS substation should be selected away from the hazardous
area like mines, land slides, flood prone areas. They should be away from
airports and aeronautic al installation as there are usually restrictions on the
maximum height of structures and due to possible disturbance on navigation
equipment. The orientation of substations should be selected keeping in view
the outgoing line orientations.
Limitations of AIS…………….continued
 Infrastructural factor. Easy access, amenities availability, reliable power & water supply constitute the
infrastructural factors. Other amenities such as medical, communication facilities etc. should be
reasonably available. The availability of reliable power supplies for substation auxiliaries, and
construction purpose should be examined. Water should be available be construction of substation as
well as for drinking purpose.
 Social & Environmental factor. Land accusation and forest encroachment constitute the social and
environmental factors.
 With strict laws on land acquisition, resettlement of PAPs (Project Affected Persons), greater attention
is required for acquiring an AIS site. It is preferable to go for a govt. land rather than private land as the
process may take less time.
 Out of various options available for locating an AIS substation, the choice which involves minimum
forest encroachment by lines needs to be preferred. Low noise transformers and reactors should be
installed wherever station is near residential areas.
 Commercial factor. While selecting substation site, cost of substation considering land cost & its
development needs to be examined with respect to the cost of transmission lines.
Limitations of
AIS……………….continued
2.
Large dimensions due to statutory clearances and poor dielectric strength of air. For a typical
400/220 kV Substation, the area requirement for switchyard is around 30 to 35 acres with I-type
layout for 400 kV system and double main and transfer bus arrangement for 220 kV system.
3.
Life of steel structures degrades. Saline and other types of industrial pollution cause corrosion in
supporting structures and protective coating may be required.
4.
Seismic instability. All the AIS equipment needs to be type tested individually for respective seismic
zone under which AIS site falls.
5.
Large planning & execution time. For above mentioned site selection process large planning and
thereafter execution time is required.
6.
Regular maintenance of the substation and its equipment is required.
Need for GIS
1.
Non availability of sufficient space. It is very much required to establish a substation at load
center. Establishing a substation at load center is quite economical and profitable in following
ways:
 Reduction in length of feeders
 Improvement of the quality of voltage regulation due to short length feeders
Generally main load center of any place is situated at very congested place where, sufficient land for
establishing conventional AIS is very hardly available. This problem can be solved by using gas
insulated switchgear technology. Total space required for a GIS is 10% of that needed for a
conventional substation.
2. Difficult climatic and seismic conditions at site, like high altitude and atmospheric pollution
3. Aesthetically “superior” to air insulated substations
4. The higher the voltage, the more favorable gas insulated technology becomes. The footprint of
765kV conventional substation is enormous, and GIS technology allows a significant size
reduction. GIS technology can be used for installations in areas where the cost of real estate is
appreciable.
5. Overcomes or decreases the magnitude of limitations of AIS site selection.
Introduction to GIS
 GIS was first developed in various
countries between 1968 and 1972.
After about 5 years of experience, the
use rate increased to about 20% of
new substations in countries where
space is limited. In other countries
with space easily available, the higher
cost of GIS relative to AIS has limited
use to special cases.
 For example, in the U.S., only about
2% of new substations are GIS.
International experience with GIS is
described in a series of CIGRE papers
(CIGRE, 1992; 1994; 1982).
Introduction to
GIS…………continued
The atmospheric air insulation used in a conventional, AIS requires meters of air insulation to do what
SF6 can do in centimeters.
 Compact, multi-component assembly.
 Enclosed in a ground metallic housing.
 Sulphur Hexafluoride (SF6) gas – the primary insulating medium.
 (SF6) gas- superior dielectric properties used at moderate pressure for
phase to phase and phase to ground insulation
 Preferred for voltage ratings of 72.5 kV, 145 kV, 220 kV and 420 kV and
above.
 Various equipment like Circuit Breakers, Bus-Bars, Isolators, Load Break
Switches, Current Transformers, Voltage Transformers, Earthing
Switches, etc. housed in metal enclosed modules filled with SF6 gas.
 Phase conductor is almost always of Aluminium. The outer enclosure is
also of mild steel, although Aluminium is also available.
Introduction to
GIS…………continued
Properties of SF6 Gas
 Sulfur hexafluoride is an inert, nontoxic, colorless, odorless, tasteless, and
nonflammable gas consisting of a sulfur atom surrounded by and tightly bonded
to six fluorine atoms.
 It is about five times as dense as air. SF6 is used in GIS at pressures from 400 to
600 kPa absolute. The pressure is chosen so that the SF6 will not condense into
a liquid at the lowest temperatures the equipment experiences.
 SF6 decomposes in the high temperature of an electric arc, but the decomposed
gas recombines back into SF6 so well that it is not necessary to replenish the SF6
in GIS. Lifetime – Very long (800 to 3200 years!).
 Even if some reactive decomposition byproducts formed because of the trace
presence of moisture, air, and other contaminants, the quantities formed are
very small. Molecular sieve absorbents inside the GIS enclosure eliminate these
reactive byproducts.
Service condition of GIS
As per IEEE 122, IEEE 123 and IEC62271
 Indoor or outdoor
: As per customer requirement
 Elevation above sea level
: As per site location
 Design ambient temperature
: 40°C As per IEC 62271
 Maximum ambient temperature
: 105°C As per IEC 62271 or lesser than
temperature at which self sustaining reaction
may start between Sf6 and desiccants
Service condition of GIS
 Minimum ambient temperature
 Enclosure design pressure
 Seismic
 As per manufacturer recommendation as SF6
liquefies at a sufficiently low temperature for a
wide range of power engineering applications.
 Enclosures shall be designed to withstand an
external pressure of one atmosphere (with a
1.5 safety factor) with zero internal pressure
absolute and shall be designed and tested to
withstand an internal test pressure of 1.5 times
the design pressure.
Service offered by GIS user and GIS supplier
User
Unless otherwise specified or agreed upon, the user should receive, install, and field test
the apparatus specified herein. Installation and field testing of the GIS should be
completed under the supervision of the supplier to ensure correct installation. Unless
otherwise specified, the following equipment and services should also be furnished by
the user:
 Concrete foundations
 Power transformers or reactors, including bushings
 Surge arresters outside the GIS
 The station ground grid, below ground, and vertical connection risers of adequate
length
 Ac and dc auxiliary power to furnished equipment
 Hoist or crane
 Supervisory control and data acquisition (SCADA) equipment
 Conduit or troughs and wiring to supplier-furnished equipment
 Gas handling equipment
 Batteries and battery chargers
User
Service offered by GIS user and GIS
supplier……….continued
 Protective relaying systems
 Transmission-line dead-end terminations
 Control and power cable trenches
 Erection labor, with the supervision and assistance of the GIS supplier
 Control house and related equipment or building in case of indoor
installation, including crane, if applicable
 Job-site unloading and storage
 Field welding, if required, with the supervision and assistance of the GIS
supplier
 Test equipment, coordinated with the GIS supplier
 Specific tertiary bus and other related equipment
 High-voltage power cables and terminations
 Anchor bolts and/or embedded steel, if part of foundation pouring
Service offered by GIS user and GIS
supplier……….continued
Supplier
GIS supplier should design, manufacture, test, deliver, and guarantee the following services as
recommended by IEEE:
 The complete gas-insulated switchgear, including connections to power transformers and line with
associated circuit breakers, disconnect and grounding switches, voltage transformers, and surge
arresters.
 All metal-enclosed gas-insulated buses for interconnecting various switchgear assemblies, including
flexible joints to ensure service continuity during thermal cycling and vibration.
 All cable connections, including SF6 enclosures, terminator support structures, and mounting insulators
should be as specified in IEC859 : 1986 and IEEE Standard 1300–1996 .
 All transformer and reactor bushing sulfur hexafluoride (SF6) enclosures.
 All coordination with power transformer, reactor, cable, and cable termination suppliers to assure
proper electrical and mechanical interface, in accordance with IEC859 : 1986 and IEC1639 : 1996.
 All auxiliary equipment, for emergency control and local supervision, including interlocks; operating
mechanisms; and control, monitoring and protective devices, installed in suitable cabinets.
 Ground buses and ground connection pads for connection to the ground grid.
 Gas density monitors, pressure relief devices, and gas-filling connections.
 New gaskets, sealant, and desiccant for permanent sealing of all field assembled joints
Design features & Technical
requirement
COMPONENTS OF GIS:
1. Bus bar
2. Circuit Breaker
3. Disconnecter (line or bus)
4. Earthing switch (line or bus)
5. Current transformer (feeder / bus)
6. Voltage transformer (feeder/ bus)
7. Feeder Disconnecter
8. Feeder Earthing switch
9. Lightning / Surge Arrester
10. Cable termination
11. Control Panel.
Design features & Technical requirement
Design features & Technical requirement
Design features & Technical
requirement
Circuit Breaker:
 Under short circuit conditions, however, the current may reach tens of thousands
of amperes at a power factor as low as 0.1. It is duty of a circuit breaker to
interrupt such currents as soon as possible to avoid equipment damage.
 Duty cycle O-0.3s-CO-3 min-CO. In case of HV circuit breakers duty cycle (ANSI) or
operating sequence (IEC) refers to how rapidly breaker operations can be
sequentially performed. Breaker has a 0.3 second delay between the first
opening and the next close-open sequence, and then it must wait 3 minutes until
the next close-open operation. It "shows" that you have a breaker meant for
rapid reclosing duty. The first delay gives the spring operating mechanism time to
recharge, the second delay gives the contacts time to cool down.
Design features & Technical
requirement
Disconnecter:
 Disconnectors or isolators are used for electrical isolation of circuit parts
 Disconnect switches can be three-pole, group-operated, or single-pole
operated,
 They are slow acting and operating at off load
 Disconnectors must be carefully designed and tested to be able to break small
charging current without generating too-high over voltage.
Design features & Technical
requirement
Voltage transformer:
 Variable location on feeder and busbar.
 Integrated disconnecting facility for GIS and power cable testing without
dismantling and gas handling.
 Flexible gas compartment allocation for optimal service oriented gas
supervision.
 The following should be specified and considered in selection of the voltage
transformer.
I.
II.
III.
IV.
V.
VI.
Number of secondary windings
Number of taps in each secondary winding
Ratio of primary voltage to each secondary winding voltage
Thermal rating of each secondary winding
Thermal rating of primary winding
Accuracy class
Design features & Technical
requirement
Local control cubicle:
 LCC is the interface cubicles to all secondary systems of a substation which are represent a
station control and protection.
 LCC includes control and alarm functions as well as the correct distribution of auxiliary
power supply for the relevant GIS bay.
 In general, each cabinet should contain the following equipment for control, indication and
protection of switches, circuit breakers, and associated components:
1.
2.
3.
4.
5.
6.
One control switch for each three-phase circuit breaker.
One remote-local switch for each three-phase circuit breaker.
One open-close control switch for each motor-operated grounding switch.
One open-close control switch or push-button for each motor-operated grounding switch.
One or two red light-emitting diodes or mechanical semaphore and one green for each circuit
breaker, each disconnect and grounding switch, or contact position indication on the mimic
diagram.
Control switches for ac and dc supply to each compartment.
Design features & Technical
requirement
Local control cubicle
7.
A mimic diagram showing connections of all furnished equipment and showing location
of all gas zones.
8.
An annunciator panel with retransmit contacts.
9.
Terminal blocks and terminations for each gas density relay.
10. Terminal blocks and terminations for electrical interlock contacts.
11.
Terminal blocks and terminations for alarm and miscellaneous remote-control circuits.
12. Terminal blocks and terminations for all instrument transformer leads. A lead should be
installed and terminated for each tap of a multi-ratio current transformer and all
secondary winding taps for voltage transformers.
13. Terminal blocks and terminations for all required spare contacts.
Design features & Technical
requirement
Current transformer:
 In the single phase enclosed Core of CT is located outside the enclosure & inside
for three phase gas compartment to reduce access of moisture and to suppress
gas-tight bushings for secondary connections.
 CT current rating is 120%of rated primary current
 Number of cores is decided based on metering and protection requirement
Design features & Technical requirement
Cable connection:
 The following should be specified and
considered in design of the cable
connection, and values should be chosen
from IEEE Standard 48-1990.
I.
Voltage—kV rms phase-to-phase
II.
Basic insulation level—kV peak
III.
Continuous current—A rms
IV.
Short-time current—kA rms for 1 s or
3s
V.
Peak short circuit current—kA peak
VI.
Maximum dielectric fluid pressure for
cable system—kPa
VII.
Power frequency factory withstand
voltage—kV rms phase-to-ground
Design features & Technical requirement
Dielectric medium SF6
 The leakage rate of SF6 gas from an
individual gas compartment shall not
exceed 1% per year. The total leakage rate
from the GIS system shall not exceed 0.5%
per year.
 Each gas zone should be furnished with a
gas monitoring device capable of signaling
two adjustable, independent alarms. The
user should specify the signaling
requirement as it pertains to the user's
protection and control system.
 Typically, the signaling is done by two sets
of adjustable, electrically-independent
contacts that operate at the alarm levels as
follows:
I.
First alarm—refill gas density normally
5–10% below the nominal fill density
II.
Second alarm—minimum gas density to
achieve equipment ratings
Maintenance philosophy
Time based Maintenance and condition based Maintenance
 The criteria for time-based maintenance are aging of material based on environmental conditions of
installation
 To maintain the correct equipment at the right time after lapse of certain condition is condition based
maintenance. With this type of maintenance one can exchange just the components or parts that have
been subjected to the most wear over their many years of reliable operation
Comparison of AIS & GIS
Drawbacks of GIS substation
 Excessive damage in case of internal fault.
 Diagnosis of internal fault and rectifying
takes very long time (high outage time).
 SF6 gas pressure must be monitored in
each compartment.
 Reduction in the pressure of the SF6 gas in
any module results in flash over and faults.
 SF6 causes ozone depletion and global
warming. SF6 is a gas specifically
mentioned in Kyoto protocol. Search is on
for a replacement gas or gas mixture.
 Currently, 80% used by Electrical
Power
industry.
I.
II.
7000 metric tons/year in 1993.
Reached 10,000 metric tons/year by 2010.
Drawbacks of GIS substation
Transient enclosure voltage TEV or Transient ground potential rise TGPR can be a very
serious EMC and personnel safety problem. Voltage rise on grounded shields of several kV
at distances up to several km have been observed in early days.
Such transient voltages on the “grounded” enclosure arise from an internal collapse of
voltage in the SF6 gas, internal re-strikes across circuit breaker or disconnect switch
contacts, or flashover of external insulation close to GIS, e.g., and air-SF6 bushing.
Internal voltage collapse produces travelling waves, in both directions, from the point of
breakdown. Such transients are often called VFTO (very fast transient over voltages).
At the points of discontinuity (changes in surge impedance) these VFTO waves get
reflected and refracted. Such points are junctions of transmission lines, air/SF6 bushing,
GIS/cable connections, ground leads connecting the enclosure to the earthing
grid/mat/plate, or a ZnO arrester.
Growth of GIS substation
Future trends in GIS
Small quantities of SF6 in N2 can improve dielectric strength drastically.
All of the dielectric strength of SF6, nearly, can be achieved by adding less than
20% SF6 into N2.
SF6/N2 mixtures less susceptible to effects of field non uniformity than pure
SF6.
Compact design of switch gear by using three phase encapsulated design for
higher voltages.
Development of DC GIS for incorporating into expanding national/international
HVDC systems.
Conclusion
 GIS generate no noise & have no radio interference, being located closure to
load centers, easy solution for mountain areas where ice & snow are major
problems and due to many other advantages described in this presentation GIS
is necessary for Extra HV & Ultra HV substations.
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