Comparison between 33kV Air Insulated Switchgear and Gas Insulated Switchgear

Updated: Feb 2

Gas Insulated Switchgear – 33kV


The 33kV GIS has all the live components that are enclosed in a grounded metal enclosure, and the whole system is housed in a chamber filled with SF6 gas. Gas Insulated Switchgear (GIS) primarily uses Sulphur Hexafluoride gas as the primary insulation. SF6 is non-toxic; it maintains atomic and molecular properties even at high voltages, has high cooling properties, and has superior di-electric properties. Its superior dielectric properties compared to other gases; thereby provide favorable insulation for the phase to phase and phase to ground insulation.


The GIS contains the same compartments and components as the normal (AIS) configuration. The only difference is the insulation medium, and of course the size. The live components and parts are housed in metal enclosures filled with SF6 gas at moderate pressure. ‘O’ rings are installed at the equipment and enclosure flanges to ensure the gas tightness.


Another important tool is the gas monitoring system. This includes an integrated alarm system, automatic tripping with lockout facility in case of low pressure in the event of an unlikely gas leakage.



Air Insulated Switchgear – 33kV


33KV AIS switchgear unit consists of three power compartments viz. circuit-breaker compartment, bus-bar compartment and cable compartment. Each switchgear is fitted with a low voltage compartment, where all the auxiliary instruments are housed. The compartments are segregated from each other by metallic partitions. The live parts are exposed to air and have sufficient clearances. The AIS switchgear will have a withdrawable circuit breaker and will meet Service Continuity Class of LSC-2B, as per standards. LSC-2B stands for - Loss of Service Continuity category, wherein medium voltage parts are compartmentalized using metal partitions (PM class) which are connected to earth. It provides a guarantee that there is a high level of protection to people; when a compartment containing a main circuit is open, the other compartments and/or functional units may remain energized.


Utilities across the world have been widely using both Gas Insulated Switchgear as well as Air insulated Switchgear for their 33kV distribution networks. There are many differences between the 33kV AIS and GIS and each of them have their own pros & cons


Unlike in 11kV range, where almost all switchgear used are air insulated, and 66kV and above range, where all switchgear are gas Insulated, each utility chooses to use gas insulated or air insulated versions of 33kV switchgear depending on one that suits them better.


Following section highlights the key differences between and also lists the benefits and limitations of the 33kV AIS and GIS switchgear.


The Differences


  • Space

The GIS switchgear is very compact compared to AIS switchgear. SF6 has very good di-electric properties and allows the switchgear to be very compact. AIS has air as the insulation medium and requires larger space to meet the necessary clearances as per standards.


Size of Megawin GIS - (W x D x H): 600 x 1225 x 2300

Size of Megawin AIS - (W x D x H): 1200 x 2325 x 2100



  1. Installation

Installation of 33kV AIS and GIS switchgear are very different. Installation of GIS requires more skilled personnel. If gas handling is done on site, then the installation of GIS becomes very complicated. SF6 gas allows the switchgear to be much more compact and lighter. For both gas-insulated and air-insulated, the panels must be screw-fastened to one another. Air insulated switchgear also requires bus bar connections and boots on the joints of the switchgear. The circuit breaker must be installed and padlocked on air-insulated gear.


Setting up termination is simpler for Air Insulated Switchgear, which only requires a holed lug. The cable chamber is sufficiently large and sufficient clearances are available, while gas-insulated switchgear relies on an insulated cable connection on top of the lug. The compact size of the GIS also makes the access to the cable chamber difficult. Insulated cable termination is more expensive. The need for skilled personnel will make the cost of installing GIS more than a AIS.



  1. Construction

Main difference between Gas-Insulated Switchgear (GIS) and Air Insulated Switchgear (AIS) is the use of sulfur hexa-fluoride (SF6) gas for di-electrical insulation in the GIS. Natural Air with sufficient clearances is used as dielectric insulation in AIS. SF6 is a gas five times heavier than air and its dielectric strength is approximately 2.5 to three times that of air at 1 atmosphere pressure. It has excellent arc extinction behavior—about 100 times better than air. The gas is colorless, odorless, non- flammable, non-poisonous, physiologically harmless, and chemically stable.

AIS Switchgear features draw-out circuit breakers with three positions— “Service”, “test” and “isolated ”. GIS switchgear has a fixed type circuit breaker, mounted in the gas tank, usually made of Stainless Steel. The Gas pressure in the tank must be regularly monitored to ensure healthy operation of the GIS.

AIS Switchgear uses external discharge rods or external earth trucks to ground the cables or a separate earthing switch. GIS Switchgear uses an integrated three-position disconnector (with “on”, “off”, and “grounded” positions). When the “grounded” position is reached, the circuit breaker switches ON automatically and the cable side is grounded.

AIS switchgear enables the circuit breakers to be withdrawn in order to perform preventive maintenance. It is designed to allow regular maintenance on all components and thus optimize troubleshooting. By contrast, the GIS switchgear with fixed vacuum circuit breakers need not be removed during maintenance due to the “sealed for life” technology.



  • Cost

The initial capital expense of GIS switchgear is much more than similarly rated AIS switchgear. The price difference is normally expected to be about 30%, but in certain cases difference of more than 50% can also be expected.


  • Operation

All operations of the Gas Insulated Switchgear can be performed from the front, including access to the cable chamber. All operations of the Air insulated switchgear can be performed from the front, except that the power cable access from the rear.


All testing instruments are integrated within the GIS switchgear. This reduces human error.


Metal Clad Air Insulated Switchgear is prone to arc failure due to external environmental factors like entry of rodents, dust, pollutants in air, etc. Whereas in the GIS, all live parts are housed within the sealed Gas Tank, thereby protecting it from any external damage. GIS is better suited for installation in harsh and dusty environments.

The Metal Clad AIS switchgear has a fully withdrawable circuit breaker. The operator can visually ensure that the circuit breaker is physically removed before performing any activities on the downstream side. In the case of the GIS, the separation is internal and any indication is not available to visually confirm the isolation.



  • Maintenance

GIS switchgear offers the added benefit of requiring lesser maintenance. The metal-clad AIS switchgear should have inspections (torque, clean, draw-out, lubricate, and vacuum) performed on all compartments The GIS should be visually inspected and the mechanism of the circuit breaker, earthing mechanism, isolators etc. must be test run as done in AIS. For instance, a typical AIS withdrawable circuit breaker requires four hours of maintenance every two years.


Megawin can offer AIS switchgear with Magnetic actuator mechanism. This mechanism is proven for 100,000 operations. This is 10 times the life of a conventional circuit breaker mechanism. This mechanism greatly reduces the overhead of switchgear maintenance.



  1. Breakdown Maintanance

There are almost no options for any trouble shooting at site for GIS switchgear. Any event causing a gas leakage or a mal-function of the switchgear, a full switchgear replacement will have to be sought from the same manufacturer.


AIS switchgear lends itself to complete trouble shooting. The cause of the trouble can be easily identified and only the affected components need to be replaced.

Thus downtime in case of the AIS can be much lesser compared to GIS, depending on the nature of the trouble encountered.


Some GIS designs have the instrumentation transformers (CTs and PTs) placed within the gas tanks. Considering that the CTs and PTs are vulnerable to failure, troubleshooting/replacing any faulted instrumentation transformers will be very difficult.



  1. Environment

SF6 is a greenhouse gas. Any leak of SF6 into the atmosphere will increase the green-house effect and shall lead to global rise in temperature. SF6 gas is colorless, odorless, non- flammable, non-poisonous, physiologically harmless, and chemically stable. During the course of operation, the gas can potentially decompose into toxic gases like thyonyl fluoride (SOF2) and the byproducts of SOF2 hydrolysis like Sulphur dioxide (SO2) and Hydrogen Fluoride (HF).


IEC recommends very stringent measures to reduce any SF6 emissions to the atmosphere.



Summary

Both Gas Insulated Switchgear and Air Insulated Switchgear have different strengths and weaknesses. SEWA substations are built in standard dimensions and the sizes of the 33kV switchgear rooms is constrained by the room requirement for 11kV switchgear which can easily accommodate AIS panels. The substations are also climate controlled ensuring that there is no impact of external factors on performance of the switchgear. The Air Insulated Switchgear offers the benefits of much lesser initial costs and easier installation. Using Megawin’s technologies of Magnetic actuator, Current and voltage sensor, the AIS can be made much more reliable and maintenance can be reduced.