It is very much required to establish an electrical substation at load center. Since, establishing a substation at load center is quite economical and profitable in many aspects. As it reduces length of feeders and due to short length feeders, the quality of voltage regulation improves. But the main obstruction of establishing a substation at load center is space. Generally main load center of any place is situated at very congested place where, sufficient land for establishing conventional electrical substation is very hardly available.
This problem can be solved by using gas insulated switchgear technology. In this type of switchgear, all the necessary components of switchgear can be assembled in very limited space. GIS is a kind of metal enclosed switchgear. That means, all the equipments of the electrical switchgear are enclosed by gas tight metal enclosure and SF6 gas is used as insulation between live parts of the equipments and earthed metal enclosure. This type of switchgear, means, gas insulated switchgear is available from 12 KV system to 800 KV system. For establishing electrical substation in very limited place this type of SF6 insulated electrical switchgear plays the major role.
Gas insulated components of substation are generally,
- Electrical bus bars.
- Electrical isolators or disconnectors.
- Circuit breakers.
- Current transformers.
- Voltage transformers.
- Earth switches.
- Surge arrestors or lightning arresters.
The substation, assembled by gas insulated switchgear, is popularly known as gas insulated metal enclosed substation (GIMES). GIMES technology is not very recent invention, it is successfully running for over thirty years.
In gas insulated medium voltage switchgear, vacuum technology is used as interrupting purpose and SF6 gas is used as insulation material. Although for both interruption and insulation, SF6 gas is used in many medium voltage GIS system. But for such equipments rated SF6 gas pressures are different for interruption and insulation. SF6 gas pressure for insulating purpose is generally kept below 2.5 bar whereas SF6 gas pressure for interrupting purpose is ranged from 5 bar to 7 bar.
As vacuum technology is not available for high voltage, so for GIS or gas insulated switchgear system above 72.5 KV, only SF6 is used both for interruption medium and insulation.
There are different types of gas insulated metal enclosed switchgears available depending upon their constructional feature.
Isolated Phase GIS
In this configuration, each phase of the bay is assembled separately. That is, for each phase, one pole of circuit breaker, a single pole of electrical isolator, one phase assembly of current transformer are assembled together. This type of GIS requires larger bay width as compared to other gas insulated switchgear system.
Integrated 3 Phase GIS
In this configuration all three phase of circuit breaker, 3 phases of disconnectors and three phase current transformer are encapsulated in an individual metal enclosure. The arrangement forms a three phase module for the element. The size of this type of module is one third of the isolated phase GIS.
Hybrid GIS System
It is suitable combination of isolated phase and three phase common elements. Here three phase common bus bar system simplifies the connection from the bus bar. The isolated phase equipment prevents phase to phase faults. This is an optimum design considering, both facts in mind, i.e. space requirement and maintenance facility.
In this GIS or gas insulated switchgear system than one functional element are encapsulate in a single metal enclosure. For example, in some design, a three phase circuit breaker, current transformer, earth switches, even other feeder elements are covered together in a single metal capsule.
Highly Integrated System
This design was introduced in the year of 2000, where, total substation equipments are encapsulated together in single enclosure housing. This single unit gas insulated substation has gained user appreciation as it is a complete solution for a outdoor substation, in a single unit. As such, only equipment (HIS) is substitute of a total outdoor switch yard.