Power Substation
Sub-Station: 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:
- It should be located at a proper site. As far as possible, it should be located at the centre of gravity of load.
- 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.
- It should be easily operated and maintained.
- It should involve minimum capital cost.
In a large power system large number of Generating stations, Electrical Power Substations and
load centers are interconnected. This large internet-work is controlled from load dispatch
center. Digital and voice signals are transmitted over the transmission lines via the Power
substations. The substations are interlinked with the load control centers via Power Line
Carrier Systems (PLCC). Modern Power System is controlled with the help of several
automatic, semi - automatic equipment. Digital Computers and microprocessors are installed in
the control rooms of large substations, generating stations and load control centers for data
collection, data monitoring, automatic protection and control.
Functions of Electrical Power Substations are:
- Supply electric power to the consumers continuously
- Supply of electric power within specified voltage limits and frequency limits
- Shortest possible fault duration.
- Optimum efficiency of plants and the network
- Supply of electrical energy to the consumers at lowest cost
Classification of Sub-Stations
There are several ways of classifying sub-stations. However, the two most important ways of
classifying them are according to
- service requirement
- constructional features.
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 utilization.
(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 (e.g. ignitron) 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.
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:
- Indoor sub-station
- Outdoor sub-station
- Underground sub-station
- Pole-mounted sub-station
(i) Indoor sub-stations: For voltages up to 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 up to 66 kV.
(ii) Outdoor sub-stations. For voltages beyond 66 kV, equipment is invariably installed outdoor. 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 cities, there is scarcity of land as well as
the prices of land are very high. This has led to the development of underground sub-station. In
such sub-stations, the equipment is placed underground.
The design of underground sub-station requires more careful consideration than other
types of sub-stations.
While laying out an underground sub-station, the following points
must be kept in view:
(i) The size of the station should be as minimum as possible.
(ii) There should be reasonable access for both equipment and personnel.
(iii) There should be provision for emergency lighting and protection against fire.
(iv) There should be good ventilation.
(v) There should be provision for remote indication of excessive rise in temperature so that
H.V. supply can be disconnected.
(vi)The transformers, switches and fuses should be air cooled to avoid bringing oil into the
premises.
(iv) Pole-mounted sub-stations. This is an outdoor sub-station with equipment installed overhead 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.
Comparison between Outdoor and Indoor Sub-Stations
| S.No. | Particular | Outdoor Sub-station | Indoor Sub-station |
|---|---|---|---|
| 1 | Space required | More | Less |
| 2 | Time required for erection | Less | More |
| 3 | Future extension | Easy | Difficult |
| 4 | Fault location | Easier because the equipment is in full view | Difficult because the equipment is enclosed |
| 5 | Capital cost | Low | High |
| 6 | Operation | Difficult | Easier |
| 7 | Possibility of fault escalation | Less because greater clearances can be provided | More |
From the above comparison, it is clear that each type has its own advantages and disadvantages.
However, comparative economics (i.e. annual cost of operation) is the most powerful factor
influencing the choice between indoor and outdoor sub-stations. The greater cost of indoor substation prohibits its use. But sometimes non-economic factors (e.g. public safety) exert
considerable influence in choosing indoor sub-station. In general, most of the sub-stations are of
outdoor type and the indoor sub-stations are erected only where outdoor construction is
impracticable or prohibited by the local laws.
Transformer Sub-Stations
The majority of the sub-stations in the power system are concerned with the changing of
voltage level of electric supply. These are known as transformer sub-stations because
transformer is the main component employed to change the voltage level.
Depending upon the
purpose served, transformer sub-stations may be classified into:
(i) Step-up sub-station
(ii) Primary grid sub-station
(iii) Secondary sub-station
(iv) Distribution sub-station
(i) Step-up sub-station. The generation voltage (11 kV in this case) is stepped up to high
voltage (220 kV) to affect economy in transmission of electric power. The sub-stations which
accomplish this job are called step-up sub-stations. These are generally located in the power
houses and are of outdoor type.
(ii) Primary grid sub-station. From the step-up sub-station, electric power at 220 kV is transmitted by 3-phase, 3-wire overhead system to the outskirts of the city. Here, electric power is received by the primary grid sub-station which reduces the voltage level to 66 kV for secondary
trans-mission. The primary grid sub-station is generally of outdoor type.
(iii) Secondary sub-station. From the primary grid sub-station, electric power is transmitted at
66kV by 3-phase, 3-wire system to various secondary sub-stations located at the strategic points
in the city. At a secondary sub-station, the voltage is further stepped down to 11 kV. The 11 kV
lines run along the important road sides of the city. It may be noted that big consumers (having
demand more than 50 kW) are generally supplied power at 11 kV for further handling with their
own sub-stations. The secondary sub-stations are also generally of outdoor type.
(iv) Distribution sub-station. The electric power from 11 kV lines is delivered to distribution
sub-stations. These sub-stations are located near the consumers localities and step down the
voltage to 400 V, 3-phase, 4-wire for supplying to the consumers. The voltage between any two
phases is 400V and between any phase and neutral it is 230 V. The single phase residential
lighting load is connected between any one phase and neutral whereas 3-phase, 400V motor load
is connected across 3-phase lines directly. It may be worthwhile to mention here that majority of
the distribution sub-stations are of pole-mounted type.
Image source:https://totalshield.com/wp-content/uploads/2022/12/blog-cover-protect-power-substation.png