Gas Insulated Substation (GIS)
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Gas Insulated Substation (GIS)
Gas Insulated Substation (GIS) is a compact, multi component assembly enclosed in a ground metallic housing which the primary insulating medium is compressed sulphurhexaflouride(SF6) gas. It generally consists components of
a) Bus bars
b) Circuit Breakers
c) Disconnecting switches
d) Earthing switches
e) Current transformers
f) Voltage transformers
g) Cable and boxes
h) Gas supply and gas monitoring equipment
j) Local control
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Conventional substations requires, small installation size, protection against atmospheric pollution and moisture, noiseless operation, nonexplosive and flame resistant, reduced maintenance, minimal radio interference, but totally enclosed substations using SF6 gas as insulation that are also known as GIS is now in widespread use in the electrical power industry.
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 were decommissioned after 35 years of operation without major fault or gas leak.
GIS – GAS INSULATED SUBSTATION
Gas Insulated Substations (GIS) is a compact, multicomponent assembly enclosed in a ground metallic housing in which the primary insulating medium is compressed Sulphur hexafluoride (SF6) gas. SF6 acts as an insulation between live parts & the earthed metal closure. The introduction of SF6 gas has revolutionized not only the technology of circuit breakers but also the layout of substations.
The dielectric strength of SF6 gas at atmospheric pressure is approximately three times that of air. It is incombustible, non toxic, colorless and chemically inert. It has arc-quenching properties 3 to 4 times better than air at equal pressure. Space requirement is only 10 to 25 percent of what is required is a conventional substation.
WHY WE NEED GIS?
GIS has small ground space requirements.
Gas insulated Substations have easy maintenance (nearly zero Maintenance)
Less field erection time & less erection cost.
For underground power house of Hydroelectric power project and implimentation where space constraint is a major issue.
For Fast Growing Major Cities where land availability is costlier.
Non-Flammability & Non-Explosive , Oil-free & Less Pollution.
DESIGN OF GIS
SPECIFICATION OF GIS
The range of application of SF6 gas-insulated switchgear extends from voltage ratings of 72.5 up to 800 kV with breaking currents of up to 63 kA, and in special cases up to 80 kA. Both small transformer substations and large load-centre substations can be designed with GIS technology.
GIS equipment is usually of modular construction. All components such as busbars, disconnectors, circuit-breakers, instrument transformers, cable terminations and joints are contained in earthed enclosures filled with sulphur hexafluoride gas (SF6).
Up to ratings of 170 kV, the three phases of GIS are generally in a common enclosure, at higher voltages the phases are segregated. The encapsulation consists of non- magnetic and corrosion-resistant cast aluminium or welded aluminium sheet.
The weight and size of the GIS equipment do not change appreciably with the voltage class as the bulk of the current – carrying components and enclosures have identical dimensions for similar thermal and short time current. The additional insulation required for the next voltage class is achieved by increased gas density. Owing to these flexibilities, a few manufacturers offer the same equipment for two voltage classes (like 170/145 kV).Even when the GIS equipment is designed for an individual voltage class, the dimensions and weights of the equipment differ marginally.
All enclosed compartments are filled with gas once at the time of commissioning. This includes allowance for any leakage during operation (less than 1 % per year). All the gas compartments have vacuum couplings, making gas maintenance very easy, most of which can be done while the station remains in operation. The gas is monitored by density relays mounted directly on the components.
Electrical protection system
A reliable protection system and electrical or mechanical interlocks provide protection for service staff when carrying out inspections and maintenance or during station extension, and safeguard the equipment against failure and serious damage.
The fast-response busbar protection system is recommended for protecting the equipment internally.
BASIC MODULE OF GIS STATION
TYPES OF MODULES IN A GIS
Bus Bar Module: This module is equivalent to the Bus Bar arrangements in normal AIS station, and tapping's are taken at different points.
Line Module: In this module as shown in the second figure it has one Isolator, CB, One CT and One LA all built inside the closed chamber.
Transformer Module : In this module as shown in the third figure it has one Isolator, Circuit Breaker and One CT all built inside the closed chamber.
PT Module: For the sake of measuring the Bus Bar voltage at any time, the PT is connected to the Bus Bar through PT Module .
Bus Coupler Module : In case if it is a two Bus Bar arrangement, for interconnecting those two buses Bus Coupler module is used.
Bus Bar Module Line Module Transformer Module PT Module
The busbars are segregated by barrier insulators at each bay and form a unit with the busbar disconnectors and the maintenance earthing switches.
The circuit-breaker operates on the self-blast principle. Conventional puffer-type breakers use the mechanical energy of the actuator to generate the breaker gas stream while the self-blast breaker uses the thermal energy of the short-circuit arc for this purpose. This saves up to 80% of the actuation energy. Depending on their size, the breakers have one to four breaker gaps per pole. They have single- or triple-pole actuation with hydraulic spring mechanisms.
The current transformers for measuring and protection purposes are of the toroidal- core type and can be arranged before or after the circuit-breaker, depending on the protection concept. Primary insulation is provided by SF6 gas, so it is resistant to ageing. Iron-free current transformers using the Rogowski coil principle are used with SMART-GIS. They allow quantized evaluation of short-circuit currents and so make it possible to create a contact erosion image of the circuit-breaker.
Voltage transformers for measurement and protection can be equipped on the secondary side with two measuring windings and an open delta winding for detecting earth faults. Inductive voltage transformers are contained in a housing filled with SF6 gas. Foil- insulated voltage transformers are used, with SF6 as the main insulation.
Maintenance earthing switches, which may be required during servicing, are usually placed before and after the circuit-breaker. Normally mounted on or integrated in the isolator housing, they are operated by hand or motor only when the high-voltage part is dead. The maintenance earthing switch after the circuit-breaker may be omitted if there is a high-speed earthing switch on the line side.
Each bay has a control cubicle containing all the equipment needed for control, signalling, supervision and auxiliary power supply.
The gastight enclosure of high-grade aluminium is of low weight so that only light foundations are required. The enclosure surrounds all the live parts, which are supported on moulded-resin insulators and insulated from the enclosure by SF6gas at a pressure of 350 to 450 kPa.