The electric power is made at the facility stations that square measure placed at favorable places, usually quite far from the customers. it’s delivered to the customers through an oversized network of transmission and distribution. At several places within the line of the facility system, it should be fascinating and necessary to alter some characteristic (e.g. voltage, a.c. to d.c., frequency, p.f. etc.) of electrical provide. this can be accomplished by appropriate equipment known as sub-station. for instance, generation voltage (11 kilovolts or 6·6 kilovolt) at the facility station is stepped up to high voltage (say 220 kV or 132 kV) for transmission of electrical power. The assembly of equipment (e.g. electrical device etc.) used for this purpose is that the sub-station.
Similarly, close to the consumer’s localities, the voltage might be got to be stepped all the way down to the activity level. This job is once more accomplished by acceptable equipment known as sub-station. however, at some places within the line of the facility system, it should be fascinating to convert massive quantities of a.c. power to d.c. power e.g.
It is clear that the sort of kit required in a very sub-station can depend on the service demand. though there are many sorts of sub-stations, we have a tendency to shall primarily confine our attention to solely those sub-stations wherever the incoming and outgoing provides square measure a.c. i.e. sub-stations that modification the voltage level of the electrical provide.
Classification of Sub-Stations:
There area unit many ways in which of classifying sub-stations. However, the 2 most significant ways in which of classifying them area unit in line with
(1) Service demand and (2) Constructional options.
A sub-station is also known as upon to alter voltage level or improve power issue or convert a.c. power into d.c. power etc. in line with the service demand, sub-stations are also classified into :
- Electrical device sub-stations. Those sub-stations that modification the voltage level of electrical provide area unit known as electrical device sub-stations. These sub-stations receive power at some voltage and deliver it at another voltage. Obviously, the electrical device is going to be the most element in such sub-stations. Most of the sub-stations within the facility area unit of this kind.
- Switch sub-stations. These sub-stations don’t modification the voltage level i.e. incoming and outgoing lines have identical voltage. However, they merely perform the switch operations of power lines.
- Power issue correction sub-stations. Those sub-stations that improve the facility issue of the system area unit known as power issue correction sub-stations. Such sub-stations area unit usually settled at the receiving finish of transmission lines. These sub-stations usually use synchronous condensers because of the power issue improvement instrumentation.
- Frequency changer sub-stations. Those sub-stations that modification the provision frequency area unit referred to as frequency changer sub-stations. Such a frequency modification is also needed for industrial activity.
- Changing sub-stations. Those sub-stations that modification a.c. power into d.c. power area unit is known as changing sub-stations. These sub-stations receive a.c. power and convert it into d.c. power with appropriate equipment (e.g. ignitron) to produce for such functions as traction, electroplating, electrical fastening, etc.
- Industrial sub-stations. Those sub-stations that provide power to individual industrial considerations area unit referred to as industrial sub-stations.
A sub-station has several elements (e.g. circuit breakers, switches, fuses, instruments, etc.) that should be housed properly to make sure continuous and reliable service. per constructional options, the sub-stations area unit classified as :
(i) Indoor sub-station, (ii) out of doors sub-station,
(iii) Underground sub-station, (iv) Pole-mounted sub-station,
- Indoor sub-stations. For voltages up to eleven potential units, the instrumentality of the sub-station is put in indoor thanks to economic issues. However, once the atmosphere is contaminated with impurities, these sub-stations is erected for voltages up to sixty-six potential unit.
- Out of doors sub-stations. For voltages on the far side sixty-six potential unit, instrumentality is invariably put in out of doors. it’s as a result of for such voltages, the clearances between conductors and also the area needed for switches, circuit breakers and alternative instrumentality becomes thus nice that it’s not economical to put in the instrumentality indoor.
- Underground sub-stations. In thickly inhabited areas, the area accessible for instrumentality and building is restricted and also the value of land is high. beneath such things, the sub-station is made underground. The reader might notice any discussion on underground sub-stations in Art.
- Pole-mounted sub-stations. this is often an out of doors sub-station with instrumentality put in overhead on H-pole or 4-pole structure. it’s the most affordable kind of sub-station for voltages, not olympian 11kV (or thirty-three potential unit in some cases). wattage is sort of distributed in localities through such sub-stations. For a complete discussion on pole-mounted sub-station, the reader might talk over with Art.
Difference between Outdoor and Indoor Sub-Stations:
From the higher than comparison, it’s clear that every sort has its own benefits and drawbacks. However, comparative social science (i.e. the annual value of operation) is that the most powerful issue influencing the selection between indoor and outside sub-stations. The bigger value of indoor sub-station prohibits its use. however typically non-economic factors (e.g. public safety) exert tidy influence in selecting indoor sub-station. In general, most of the sub-stations area unit of the outside sort and therefore the indoor sub-stations area unit erected solely wherever outside construction is infeasible or prohibited by the native laws.
The majority of the sub-stations within the facility area unit involved with the dynamical of the voltage level of electrical offer. This area unit referred to as electrical device sub-stations as a result of the electrical device is that the main part utilized to vary the voltage level. relying upon the aim served, electrical device sub-stations could also be classified into :
- Change of magnitude sub-station
- Primary grid sub-station
- Secondary sub-station
- Distribution sub-station
the diagram of a typical electrical offer system indicating the position of higher than forms of sub-stations. it’s going to be noted that it’s not necessary that all-electric offer schemes embrace all the stages shown within the figure.
- Change of magnitude sub-station: The generation voltage (11 kV during this case) is stepped up to high voltage (220 kV) to have an effect on the economy within the transmission of electrical power. The sub-stations that accomplish this job square measure referred to as increase sub-stations. These square measure usually placed within the powerhouses and square measure of the out of doors sort.
- Primary grid sub-station. From the increase station, electrical power at 220 kV is transmitted by 3-phase, 3-wire overhead system to the outskirts of town. Here, electrical power is received by the first grid sub-station that reduces the voltage level to sixty-six kV for secondary transmission. the first grid sub-station is mostly of the out of doors sort.
- Secondary sub-station. From the first grid sub-station, electrical power is transmitted at sixty-six kV by 3-phase, 3-wire system to varied secondary sub-stations placed at the strategic points within the town. At a secondary station, the voltage is more stepped all the way down to eleven kV.
- Distribution sub-station. the electrical power from eleven kV lines is delivered to distribution sub-stations. These sub-stations square measure placed close to the consumer’s localities and step down the voltage to four hundred V, 3-phase, 4-wire for supply to the customers. The voltage between any 2 sections is 400V and between any phase and neutral, it’s 230 V.
The eleven-kilovolt line is connected to the electrical device (11kV / four hundred V) through gang isolator and fuses. The lightning arresters are put in on the H.T. aspect to guard the station against lightning strokes. The electrical device steps down the voltage to 400V, 3-phase, 4-wire provide. The voltage between any 2 lines is 400V whereas the voltage between any line and neutral is 230 V. The oil electrical fuse (O.C.B.) put in on the L.T. aspect mechanically isolates the electrical device from the shoppers within the event of any fault.
In thickly inhabited cities, there’s a deficiency of land, yet because of the costs of land, ar terribly high. This has a diode to the event of underground sub-station. In such sub-stations, the instrumentality is placed underground. A typical underground sub-station. the planning of underground sub-station needs additional careful thought than different sorts of sub-stations. whereas birthing out associate underground sub-station, the subsequent points should be unbroken in view:
- The scale of the station ought to be as minimum as potential.
- There ought to be cheap access for each instrumentality and personnel.
- There ought to be provision for emergency lighting and protection against the hearth.
- There ought to be sensible ventilation.
- There ought to be provided for remote indication of excessive rise in temperature so H.V. provides will be disconnected.
- The transformers, switches, and fuses ought to be cool to avoid transferral oil into the premises.
Symbols for Equipment in Sub-Stations:
It is a usual follow to indicate the varied components (e.g. electrical device, fuse, isolator, instrument transformers, etc.) of a sub-station by their graphic symbols within the affiliation schemes. Symbols of the necessary instrumentation in sub-station square measure given below: [read more]