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Chapter: Transmission and Distribution : Mechanical Design of Lines and Grounding

Substation: Classification and Layout

1.Classification of Sub-Stations 2. Substation Layout

Substation - introduction


The assembly of apparatus used to change some characteristic ( e.g. voltage, a.c. to d.c., frequency, p.f. etc. ) of electric supply is called a sub-station. Sub-stations are important part of power system. The continuity of supply depends to a considerable extent upon the successful operation of sub-stations. It is, therefore, essential to exercise utmost care while designing and building a sub-station. The following are the important points which must be kept in view while laying out a sub-station :


( i ) It should be located at a proper site. As far as possible, it should be located at the centre of gravity of load.


( ii ) It should provide safe and reliable arrangement. For safety, consideration must be given to the maintenance of regulation clearances, facilities for carrying out repairs and maintenance, abnormal occurrences such as possibility of explosion or fire etc. For reliability, consideration must be given for good design and construction, the provision of suitable protective gear etc.


(iii) It should be easily operated and maintained.


(iv)It should involve minimum capital cost.


1.Classification of Sub-Stations


There are several ways of classifying sub-stations. However, the two most important ways of classifying them are according to (1) service requirement and (2) constructional features.

1.According to service requirement


A sub-station may be called upon to change voltage level or improve power factor or convert a.c. power into d.c. power etc. According to the service requirement, sub-stations may be classified into :


i)Transformer sub-stations.


Those sub-stations which change the voltage level of electric supply are called transformer sub-stations. These sub-stations receive power at some voltage and deliver it at some other voltage. Obviously, transformer will be the main component in such sub- stations. Most of the sub-stations in the power system are of this type.


( ii ) Switching sub-stations


These sub-stations do not change the voltage level i.e. incoming and outgoing lines have the same voltage. However, they simply perform the switching operations of power lines.


(iii) Power factor correction sub-stations.


Those sub-stations which improve the power factor of the system are called power factor correction sub-stations. Such sub-stations are generally located at the receiving end of transmission lines. These sub-stations generally use synchronous condensers as the power factor improvement equipment.


( iv ) Frequency changer sub-stations


Those sub-stations which change the supply frequency are known as frequency changer sub-stations. Such a frequency change may be required for industrial utilisation.


( v ) Converting sub-stations


Those sub-stations which change a.c. power into d.c. power are called converting sub-stations. These sub-stations receive a.c. power and convert it into d.c power with suitable apparatus to supply for such purposes as traction, electroplating, electric welding etc.


( vi ) Industrial sub-stations


Those sub-stations which supply power to individual industrial concerns are known as industrial sub-stations.


2. According to constructional features


A sub-station has many components ( e.g. circuit breakers, switches, fuses, instruments etc.) which must be housed properly to ensure continuous and reliable service. According to constructional features, the sub-stations are classified as :


( i ) Indoor sub-station

 ( ii Outdoor sub-station

( iii ) Underground sub-station

( iv ) Pole-mounted sub-station


( i ) Indoor sub-stations


For voltages upto 11 kV, the equipment of the sub-station is installed indoor because of economic considerations. However, when the atmosphere is contaminated with impurities, these sub-stations can be erected for voltages upto 66 kV.


( ii ) Outdoor sub-stations


For voltages beyond 66 kV, equipment is invariably installed out- door. It is because for such voltages, the clearances between conductors and the space required for switches, circuit breakers and other equipment becomes so great that it is not economical to install the equipment indoor.


(iii) Underground sub-stations


In thickly populated areas, the space available for equipment and building is limited and the cost of land is high. Under such situations, the sub-station is created underground.


(iv) Pole-mounted sub-stations


This is an outdoor sub-station with equipment installed over- head on H-pole or 4-pole structure. It is the cheapest form of sub-station for voltages not exceeding 11kV (or 33 kV in some cases). Electric power is almost distributed in localities through such sub- stations. For complete discussion on pole-mounted sub-station,



2. Substation Layout


1.Substation Layout AIS


AIS Substation Description


An Air Insulated Switchgear substation (AIS substation) uses atmospheric air as the phase to ground insulation for the switchgear of an electrical substation. The main advantage of the AIS substation is the scope of the substation for future offloading, for this reason AIS substations tend to be the most popular 400kV substation type. The equipment of an AIS substation is easily sourced and has a short lead-time; this means that the required future offloading does not need to be built immediately, unlike GIS where it must be considered. The main disadvantage to the AIS substation is its overall size. At 400kV level these substations can have a significant footprint and require sensitive locating in any rural environment. AIS are usually installed outdoor.


AIS Substation Size


Based on the single line diagrams given in Appendix B the minimum size of an AIS substation for this project would be as follows:


1.Overall substation Compound Size 46,864.5m2(235.5m x 199m or approximately 11.6 acres) 2.Height of highest element of substation ~ 28m (lightning protection structures situated in the substation compound)


Note: The switchgear in an AIS substation is outdoors therefore no building sizes are considered.


AIS Maintenance Requirements


1.Ongoing maintenance requirements, all equipment exposed to weather conditions 2.Disconnect contacts must be cleaned regularly, operating mechanisms must be checked and maintained


2.Substation Layout GIS


A gas insulated substation (GIS) is a high voltage substation in which the major structures are contained in a sealed environment with sulfur hexafluoride gas as the insulating medium. GIS technology originated in Japan, where there was a substantial need to develop technology to make substations as compact as possible. The clearance required for phase to phase and phase to ground for all equipment is much lower than that required in an air insulated substation; the total space required for a GIS is 10% of that needed for a conventional substation.


Gas insulated substations offer other advantages in addition to the reduced space requirements. Because the substation is enclosed in a building, a GIS is less sensitive to pollution, as well as salt, sand or large amounts of snow. Although the initial cost of building a GIS is higher than building anair insulated substation, the operation and maintenance costs of a GIS are less.


The primary applications for gas insulated substations include:


High voltage installations


The higher the voltage, the more favorable gas insulated technology becomes. The footprint of 765kV conventional substation is enormous, and GIS technology allows a significant size reduction.


Urban Installations


GIS technology can be used for installations in areas where the cost of real estate or aesthetic appeal is a significant consideration.


Indoor Installations


Building an air insulated substation indoors is usually impractical, but a GIS can easily go inside buildings.


Environmentally Sensitive Installations


GIS technology is popular in desert and arctic areas because it can be enclosed in a building with environmental control. Gas insulated substations also contain the electrical components within a Faraday cage and are therefore totally shielded from lightning.

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