ABSTRACT A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions. Between the generating station and consumer, electric power may flow through several substations at different voltage levels. A substation may include transformers to change voltage levels between high transmission voltages and lower distribution voltages, or at the interconnection of two different transmission voltages. Generally substations are unattended, relying on scada for remote supervision and control. Substations are needed but in the case of urban area, we have to search for the area than the need and ordinary substation need a lot of space. Here we can think of the underground substation or indoor substation. we can commonly call them as invisible substation. Which is not visible for a man passing by or cannot see openly and call them as invisible substation Underground substation is the substation which is situated under the ground in the urban area we can built a underground substation which will use the minimum space and in the top the substation we can built building or shopping mall etc indoor substation is the substation which is situated in a building a specially designed house. CHAPTER 1 INTRODUCTION A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions. Between the generating station and consumer, electric power may flow through several substations at different voltage levels. A substation may include transformers to change voltage levels between high transmission voltages and lower distribution voltages, or at the interconnection of two different transmission voltages. Generally substations are unattended, relying on scada for remote supervision and control. Substations are needed but in the case of urban area, we have to search for the area than the need and ordinary substation need a lot of space. Here we can think of the unerground substation or indoor substation . we can commonly call them as invisible substation . which is not visible for a man passing by or cannot see openly.and call them as invisible substation Underground substation is the substation which is situated under the ground.in the urban area we can built a underground substation which will use the minimum space and in the top the substation we can built building or shopping mall etc. Indoor substation is the substation which is situated in a building a specially designed house. In this two type of substation the main part is the gas insulated transformer and gas insulated controls which reduces the space. CHAPTER 2 ORDINARY SUBSTATION (AIS) Substations with air-insulated switch gear (AIS) are the most commonly used high-voltage substations for rated voltages of up to 800 kV. They are popular wherever space restrictions and environmental circumstances are not an issue. The AIS uses air as the primary dielectric from phase to phase, and phase to ground insulation. They have been in use for years before the introduction of GIS. Actually, most substations across all regions are AIS. They are in extensive use in areas where space, weather conditions, seismic occurrences, and environmental concerns are not an issue such as rural areas, and favourable off site terrain. The indoor AIS version is only used in highly polluted areas, and saline conditions, as the air quality is compromised. The fig.2.1 shows the ordinary substation with air insulated switch gear Fig. 2.1 Substations with air-insulated switch gear (AIS) 2.1 MERITS OF AIS: Low cost comparing to other substation Easy implementation of virtually any application and substation layout thanks to modular design Outstanding degree of availability, reliability, and operational safety 2.2 LIMITATIONS Large dimensions due to statutory clearances and poor Dielectric strength of air. Insulation deterioration with ambient conditions and susceptibility to pollutants. Wastageof space. Life of steel structures degrade Seismic instability. Large planning & execution time. Regular maintenance of the substation required CHAPTER 3 INVISIBLE SUBSTATION It has been divided by two type mainly underground substation and indoor substation 3.1 UNDERGROUND SUBSTATION Fig 3.1 Schematic Layout of Underground Substation A substation which is situated underground is called underground substation. The underground substation usually will be installed under the densely populated area for distributing the electricity to the city. Benefits of underground substations are The substation can be built under a building and it will be an effective solution for saving space in city area. City planning can be done freely and there will be less impact on the environment 3.2. INDOOR SUBSTATION Indoor substations are usually found in urban areas to reduce the noise from the transformers, for reasons of appearance, or to protect switch gear from extreme climate or pollution conditions. A grounding (Earthing) system must be designed. 3.3 THE NEEDS OF INVISIBLE SUBSTATION Non availability of sufficient space. Difficult climatic and seismic conditions at site. Urban site (high rise building). High altitudes. Rising population Need of electricity CHAPTER 4 WHAT IS GIS? A gas-insulated substation (GIS) uses a superior dielectric gas, SF6, at moderate pressure for phase-to phase and phase-to-ground insulation. The high voltage conductors, circuit breaker interrupters, switches, current transformers, and voltage transformers are in SF6 gas inside grounded metal enclosures. The atmospheric air insulation used in a conventional, air-insulated substation (AIS) requires meters of air insulation to do what SF6 can do in centimeters. GIS can therefore be smaller than AIS by up to a factor of 10. A GIS is mostly used where space is expensive or not available. In a GIS the active parts are protected from the deterioration from exposure to atmospheric air, moisture, contamination, etc. As a result, GIS is more reliable and requires less maintenance than AIS. GIS was first developed in various countries between 1968 and 1972. After about 5 years of experience, the use rate increased to about 20% of new substations in countries where space is limited. In other countries with space easily available, the higher cost of GIS relative to AIS has limited use to special cases. For example, in the U.S., only about 2% of new substations are GIS. International experience with GIS is described in a series of CIGRE papers which is an association joining many countries to improve power electrical system for today and tomorrow (CIGRE, 1992; 1994; 1982). 4.1 SF6 Sulfurhexaflouride is an inert, nontoxic, colorless, odorless, tasteless, and nonflammable gas consisting of a sulfur atom surrounded by and tightly bonded to six fluorine atoms. It is about five times as dense as air. SF6 is used in GIS at pressures from 400 to 600 kPa absolute. The pressure is chosen so that the SF6 will not condense into a liquid at the lowest temperatures the equipment experiences. SF6 has two to three times the insulating ability of air at the same pressure. SF6 is about 100 times better than air for interrupting arcs. It is the universally used interrupting medium for high voltage circuit breakers, replacing the older mediums of oil and air. SF6 decomposes in the high temperature of an electric arc, but the decomposed gas recombines back into SF6 so well that it is not necessary to replenish the SF6 in GIS. There are some reactive decomposition byproducts formed because of the trace presence of moisture, air, and other contaminants. The quantities formed are very small. Molecular sieve absorb ants inside the GIS enclosure eliminate these reactive byproducts. SF6 is supplied in 50-kg gas cylinders in a liquid state at a pressure of about 6000 kPa for convenient storage and transport. Gas handling systems with filters, compressors, and vacuum pumps are commercially available. Best practices and the personnel safety aspects of SF6 gas handling are covered in international standards (IEC, 1995). 4.2 ADVANTAGE OF GIS The earthed metal enclosure makes for a safe working environment for the attending personnel Compartmentalizedenclosure of the live parts makes for a very reliable system due to reduced disruption of the insulation system. By reducing the distance between active and non active switch gear parts, less space is required than in the normal AIS system: this comes in handy in densely populated areas and unfavourable terrain (minimum requirements for an AIS is about 47,000m2, while GIS with the same power properties will require approx 523m2). For the AIS, the highest element is approximately 28m, whereas for GIS you have 11m at the highest point for a 400kV substation. Low maintenance requirements due to expedient design and protection against external elements. Under scheduled maintenance, SF6 neither ages nor depletes. There is no need to top up the gas levels throughout the equipment lifetime (approx 40 years). Quick assembly due to extensive pre -assembly 4.3 DISADVANTAGE High installation costs compared to AIS systems Procurement and supply of SF6 gas can be a problems especially in rough terrain and off site locations. This further increases the costs High level of maintenance is required. This requires highly skilled personnel Internal faults tend to be very costly and severe when they occur. They often lead to long outage periods. For example, the use of impure gas, as well as leakage due to ‘O’ ring failure, as well as presence of dust can lead to flash overs and explosions. Though the gas is quite inert, flash problems can break it down into harmful by-products such as metal fluoride powders. This poses a health hazard such as physical asphyxiation and other respiratory problems. The below figures ( fig.4.1, fig 4.2, fig.4.3 ) shows the photographs of the Gas Insulated Substation Fig.4.1 Fig.4.2 Fig.4.3 CHAPTER 5 GIS ASSEMBLY The essential parts of GIS is list below 5.1. Bus Bar The below figure (fig 5.1 ) shows the design view of the bus bar Fig 5.1 the design view of the bus bar 5.2 Circuit Breaker The operation and working of the circuit breaker is shown in fig 5.2 Fig 5.2 The operation and working of the circuit breaker 5.3.Dis-Connector (line or bus) The photograph of a dis-connector is shown in fig 5.3 Fig 5.3 The photograph of a dis-connector 5.4 Earth Switch The schematic diagram of a earth switch is shown in fig 5.4 Fig 5.4 The schematic diagram of a earth switch 5.5 Current Transformer (feeder / bus) The below figure ( fig 5.5 ) shows the photograph of a current transformer Fig 5.5 photograph of a current transformer 5.6 Voltage Transformer (feeder/ bus) Fig 5.6 Voltage transformer (feeder/ bus) 5.7 Lightning / Surge Arrester Fig 5.7 Lightning / Surge Arrester 5.8.Cable Termination Fig 5.8 Cable Termination CHAPTER 6 CONCLUSION The GIS Substation is Necessary for Extra HV & Ultra HV Some important areas to be studied include: More conservative design. Improved gas handling. Decomposition product management techniques. Achieving & maintaining high levels of availability require . more integrated approach to quality control by both users and manufactures. REFERENCES [1] H.M. Xu, T.S. Bi, , Q.X. Yang, "Study on Wide Area Backup Protection to Prevent Cascading Trips Caused by Flow Transferring", IEEE Trans. 2006 [2] M. Vaiman, P. Hines, J. Jiang, G. Zweigle, “Mitigation and Prevention of Cascading Outages: Methodologies and Practical Applications", IEEE Trans. 2013. [3] https://arxive.org/abs/1011.4098 [4] https://www.scientificamerican.com/article/preventing blackout-power-grid/ [5] https://en.m.wikipedia.org/wiki/cascading failure [6] http://www.ee.co.za/article/reducing-blackouts-power-stability-control.html