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11-05-2011, 08:09 PM
Post: #1
GAS INSULATED SWITCHGEAR SUBSTATION
GAS INSULATED SWITCHGEAR SUBSTATION
L A COMPLEX, TRIVANDRUM

IV report



2. GAS INSULATED SWITCHGEAR SUBSTATION

INTRODUCTION
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 .
A GIS has many advantages over a conventional substation, including a space saving and flexible design, less field construction work resulting in quicker installation time, reduced maintenance, higher reliability and safety, and excellent seismic withstand characteristics. Aesthetics of GIS are far superior to that of a conventional substation, due to its substantially smaller size. A GIS can even be installed in a building, if desired. When all of these advantages are taken into consideration, a GIS is a cost effective alternative to a conventional substation that offers many benefits to the end user.

COMPONENTS OF GIS SUBSTATION
• Bus bars
• Circuit breakers
• Disconnecting switches
• Earthing switches
• Current transformers
• Voltage transformers
• Densimeter
• Cable and boxes
• Gas supply and gas monitoring equipment
• Local control

CIRCUIT BREAKERS
Under short circuit conditions, the current in a circuit may reach tens of thousands of amperes at a power factor as low as 0.1. A circuit breaker is used to interrupt such currents as soon as possible to avoid equipment damage. High-voltage circuit-breakers have greatly changed since they were first introduced about 40 years ago, and several interrupting principles have been developed that have contributed successively to a large reduction of the operating energy. These breakers are available for indoor or outdoor applications, the latter being in the form of breaker poles housed in ceramic insulators mounted on a structure.
Current interruption in a high-voltage circuit-breaker is obtained by separating two contacts in a medium, such as SF6, having excellent dielectric and arc quenching properties. After contact separation, current is carried through an arc and is interrupted when this arc is cooled by a gas blast of sufficient intensity.
Gas blast applied on the arc must be able to cool it rapidly so that gas temperature between the contacts is reduced from 20,000 K to less than 2000 K in a few hundred microseconds, so that it is able to withstand the transient recovery voltage that is applied across the contacts after current interruption. Sulphur hexafluoride is generally used in present high-voltage circuit-breakers (of rated voltage higher than 52 kV).

DISCONNECTORS
Disconnectors or isolators are used for electrical isolation of circuit parts. 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.

LOCAL CONTROL CUBICLES (LCC)
LCC is the interface cubicles to all secondary systems of a substation which are represent a station control and protection. It includes control and alarm functions as well as the correct distribution of auxiliary power supply for the relevant GIS bay.

EARTHING SWITCH
Slow-operating earthing switch are used for protection purpose when work is being done in the substation, but are operated only when it is certain that the high-voltage system is not energized. The fast-closing earthing switch can close against full voltage and short circuit power. The high speed earthing switch is achieved by means of a spring-closing device.

VOLTAGE TRANSFORMER
The potential transformers are employed to step down the voltage and feed the potential coils of indicating and metering instruments and relays. It is identified by single incoming connection and a grounded outgoing connection.

CURRENT TRANSFORMERS
Current transformers are used to feed the current coils of indicating, metering instruments and protective relays. They are identified by the series connection. Normally they have a turns ratio of 1:2000. Tapings are taken in order to change the turns ratio as per requirement
The primary and secondary turns are immersed in SF6, which can be placed above or below the insulator. The advantage of providing the tank above the insulator over providing it below is the saving of copper. When the current in the primary exceeds the rated current, the relay coil is energized and this trips the circuit breaker. In order to avoid flashing, the secondary of the CT should never be left open and should be shorted.
PROPERTIES OF SF6
• SF6 does not harm to the ozone layer.
• Sf6 gas is chemically stable.
• Non poisonous & non inflammable.
• Colourless & heavier than air.
• Almost water insoluble.


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