The continuing growth in global population and economy is leading to the rising requirement of electrical power and diminishing availability of space (land).
For this purpose, the substation equipment have to be made more efficient and reliable to cope up with increase in demand – specially adapting current power distribution and transmission system – and space saving design may help enhancement of utility power distribution system in this scenario – which may be fulfilled by GIS.
The phenomenon is quite simple, a gas Insulated substation is an electric power station, in which all live equipment and bus-bars are housed in grounded metal – that is sealed and placed in a chamber filled with SF6 gas.
A large space is required for the sub-station with normal Air Insulation Switchgears (AISs), because the required phase-to-phase and phase-to-ground clearances are very large. But as the dielectric strength of SF6 gas is higher than the air, which is necessary for phase-to-phase and phase-to-ground clearances, the required spaces are much lower for all equipment. Therefore, the overall size of each bay – and consequently the whole substation is reduced to about 10% of the conventional air insulation substation.
For example:- phase-to-ground clearance for 132 kV systems is around 1200 mm in air, compared to 80 mm in SF6 gas at 4.0 bar(g).This gives a direct reduction in dimensions of the high-voltage equipment by 15 times. Consequently, the size of SF6 insulated equipment is around 6% that of air insulated equipment for this voltage class (132kV).
The compactness of Gas Insulated Switchgears (GIS) has a direct influence on land requirement, and land cost. Compact modular designs offer many possibilities in layout design, & allow tailor made applications.
The GIS is preferred in:
- Major cities and towns
- Underground stations
- Heavily contaminated saline environment
- Substations and power plants located
- off shore
- Mountains and valley regions
Limitations of Air Insulated Switchgears (AISs)
- Large dimensions due to statutory clearances & poor dielectric strength of air
- Insulation deterioration with ambient conditions and susceptibility to pollutants
- Wastage of space above
- Life of steel structures
- Seismic instability
- Large planning and execution time
- Grounding-mat is essential for containing touch and step potentials
- Hot line washing and regular maintenance of the substation is essential, requires more spares inventory and man-power
Advantages of GISs over AISs
- Compact space-saving design
- Minimal operating cost
- Minimal weight by lightweight construction
- Safe encapsulation
- Environmental compatibility
- Economical transport
- Smooth and efficient installation and commissioning
Fig. 1: A typical single phase GIS assembly…
Gas Insulated Switchgears (GISs)
Main components of gas insulated substations are:
- Bus bar
- Disconnector (line or bus)
- Earthing switch
- Voltage transformer
- Current transformer
- Feeder disconnector
- Feeder earthing switch
- Lightening or surge arresters
- Cable termination
- Control panel (control cube)
Different types of GIS
There are different types of gas insulated, metal enclosed switchgears available – depending upon their constructional features.
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 simple pole of electrical isolator, one phase assembly of current transformer are assembled together. This type of GIS requires larger bay as compared to other gas insulated switch gear systems.
Integrated three-phase GIS
In this configuration all 3 phases of the circuit breaker, 3 phases of disconnectors and 3 phases of current transformers are encapsulated in an individual metal enclosure. The arrangement forms a 3 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 a 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 space requirement and maintenance facility.
In this GIS system, more than one functional elements are encapsulated in a single metal enclosure. For example, in some designs, a three phase circuit breaker, current transformer, earth switches, even other feeder elements are covered together in a single metal capsule.
Highly Integrated System (HIS): 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 of outdoor substation, in a single unit. As such, only the equipment HIS substitutes a total outdoor switch yard.
Economics and environment
In order to ensure the future supply of electricity, we require sustainable concepts that preserve resources – and do not harm our environment. Humankind needs a reliable and secure power supply – that is also environmentally friendly and economic – today and in the future.
Synthetically produced sulphur hexafluoride (SF6 gas) makes an important contribution to this task. As an insulation and arc-extinguishing medium in high and medium-voltage switchgears, SF6 has enabled a continuous development of advanced solutions in the transmission and distribution of electrical energy since 1960.
Fig. 2: The three essentials (Es) of the past, present and future…
An analysis of the contribution made by the distribution grids to the Global Warming Potential (GWP) shows that the greatest share is attributable to ohmic losses in cables, transmission lines and transformers. At present, SF6 emissions from medium-voltage switchgear contribute less than 0.005% to the global warming potential.
The GIS substation has a lower environmental impact than the AIS one – on the majority of environmental indicators. This is due to electricity losses that are four times lower with GIS than with AIS.
Furthermore, the following conclusions can be drawn from the results of the life cycle assessment. The life cycle assessment study demonstrates the advantages of the SF6-insulated (GISs) switchgears compared to the Air Insulated Switchgears (AISs) at a switchgear level. The switchgear themselves, however, only make a very minor contribution to the global warming potential. The design and capacity utilisation of the power distribution grids have a much greater influence, regardless of whether AIS or GIS technology is used. Therefore, application restrictions on the use of SF6-insulated medium-voltage switchgear cannot be justified from an ecological point of view.
Thus, regulations aimed at switchgear and components will not make any significant contribution towards climate protection.
Rather, grid operators – for whom also other criteria such as economic efficiency and personal safety are relevant – should be given freedom of choice when selecting a new switchgear technology.
New requirements in substation design or remodeling of existing ones are putting pressure on the utilities to develop more feasible, economical and easier solutions – based on compaction and integration of each of the elements. The three essentials (Engineering, Economics and Environment) are considered as the deciding criteria for implementation of a project that have inter-changed, as follows, their priority held in the past, present and future.
Issues and challenges of use of GIS in India
- Need for manufacturing units for GIS sub-station up to 765 kV in India
- Skilled manpower availability
- Ready availability of spares
- Provision of services
- Availability of equipment & diagnostic tool
- Increase in agencies for erection and commissioning of GIS sub-stations
- Transportation of GIS equipment.
Although the GIS initially appears to be more costly, its compact modular designs offer many possibilities in layout design – and allow tailor made applications. This leads to a significant saving in investment as well as operating costs that more than compensates for the additional cost of the GIS.
Another advantage of the GIS is that it offers better reliability than the AIS.
GIS technology also offers further benefits, that are harder to quantify but can be decisive for the realisation of a project. One example is the option of complete integration of a GIS substation in an existing building – when no extra site area is available. Summing up, GIS is a cost-efficient, flexible, environment friendly and reliable solution for supply systems in regions with high load densities.