Substation
A substation is a facility which is used to step up or step down the voltage in the utility power lines, the voltage will be stepped up if the power is transmitted through long-distance transmission lines and it will be stepped down when the power is to enter the local distribution line. In general, an electrical substation is a place which transmits electricity from one part of the production system to others.
Classification of substations
There
are several ways of classifying substations. However, the two most important
ways of classifying they 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:
Ø Transformer
substations:
The largest transmission substations can cover a large area with multiple
voltage levels, many circuit breakers. Transmission substations can range from
simple to complex. A transmission substation connects two or more transmission
lines. The simplest case is where all transmission lines have the same voltage.
Ø Distribution
substation:
The input for a distribution substation is typically at least two transmission
or sub-transmission lines. A distribution substation transfers power from the
transmission system to the distribution system of an area.
Ø Collector
substations:
Collector substations also exist where multiple thermal or hydroelectric power
plants of comparable output power are in proximity. In distributed generation
projects such as a wind farm, a collector substation may be required.
Ø Switching
substations:
A switching station may also be known as a switchyard. The function of the
switching station is to isolate the faulted portion of the system in the
shortest possible time. Switching stations are sometimes used as collector and
distribution stations. A switching station is a substation without transformers
and operating only at a single voltage level.
Ø Power
factor correction substations: Those substations which improve the power factor of the
system are called power factor correction substations.
Ø Frequency
changer substations:
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.
Ø Converting
substations:
Converter substations may be associated with HVDC converter plants, traction
current, or interconnected non-synchronous networks. These stations contain
power electronic devices to change the frequency of current, or else convert
from alternating to direct current or the reverse.
Ø Industrial
substations:
Those sub-stations which supply power to individual industrial concerns are
known as industrial substations.
According to constructional features
A substation 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 substations are classified as:
Ø Indoor
substations:
A substation in which the apparatus is equipped inside the substation building
is called indoor substation. Such type of substation is mainly used for the
voltage up to 11 kV, but when the surrounding air is contaminated by impurities
such as metal corroding gases and fumes, conductive dust, etc., their voltage
can be raised up to 33 k V to 66 kV. The indoor substation is subdivided
into several compartments like control compartment, indicating and metering
instruments and protective device compartment main bus-bar compartment,
current transformer and cable sealing box compartment.
Ø Outdoor
substations:
A substation that is used for all voltage levels between 55 kV to 765 kV is
called outdoor substation. Such type of substation requires less time for
construction but uses more space. The outdoor substations are mainly
classified into two types, namely, pole-mounted substation and
foundation-mounted substations.
Ø Underground
substations:
In 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:
Ø The substation can be built under a
building and it will be an effective solution for saving space in the city area.
Ø City planning can be done freely and
there will be less impact on the environment.
Single line diagram of 132/33 kV substation
The electrical power distribution system
About 40% of power system investment is in the distribution system equipment (40% in the generation, 20% in transmission). The main function of an electrical power distribution system is to provide power to individual consumer premises. Distribution of electric power to different consumers is done with much low voltage level. The transmitted electric power is stepped down is substations, for primary distribution purposes. Now, these stepped down electric power is fed to the distribution transformer through primary distribution feeders. Overhead primary distribution feeders are supported by mainly supporting iron pole (preferably rail pole). The conductors are strand aluminum conductors and they are mounted on the arms of the pole by means of pin insulators. Distribution transformers are mainly 3 phase pole mounted type. The secondary of the transformer is connected to distributors. Different consumers are fed electric power by means of the service main. These service mains are tapped from different points of distributors. The distributors can also be re-categorized by distributors and sub-distributors. Distributors are directly connected to the secondary distribution transformers whereas sub-distributors are tapped from distributors. Service main of the consumers may be either connected to distributors or sub-distributors depending upon the position and agreement of consumers.
Types of distribution
The distribution system may also be
divided into two distinct subsystems.
Ø Primary
distribution system
Ø Secondary distribution system
Primary distribution system
The primary distribution system consists of the feeders emanating from the substation and supplying power to 1 or more secondary distribution systems. Such feeders are usually 3-phase circuits. As indicated, the standard primary distribution voltage levels include 66 kV, 33 kV or 11 kV. Gazipur districts utilities have been gradually increasing the percentage of distribution primaries at the higher voltage range, mainly because it allows for greater power carrying capability using less current.
Secondary distribution system
Branching from the main feeder is
laterals also referred to in the industry as taps or branches. The laterals
may be three-phase, two-phase (two phases of the three-phase feeder with a
neutral), or single-phase (one phase from the single-phase feeder and a
neutral). The laterals are usually protected with fuses so that faulted
laterals do not cause interruption at the feeder level.
Standard secondary voltage levels are:
Ø 120/240
single phase
Ø 120/208
3 phase
Ø 277/480 3 phase
Purpose of the distribution system
Distribution Systems start from transmission grid exit power at
the substations;
transformers reduce the voltage to a lower level for distribution to
commercial and residential users. This distribution is accomplished with a combination
of sub-transmission (33 kV to 132 kV) and distribution (3.3 to 25 kV).
Finally, at the point of use, the energy is transformed to low voltage (varying
by country and customer requirements—see Mains
electricity by country). With power
generating stations being remote to urban centers, it’s required that the
generated voltage be to step up to higher voltage levels for transmission in
order to reduce the electrical losses in the overhead transmission lines. By
transmitting the electricity at high voltage levels, this reduces the
transmission line losses and makes the transmissions line more efficient. Since
transmission voltage levels cannot be used by consumers it is required to
step-down the transmission voltage to more usable voltages. The transmission
power lines will enter a distribution substation where the voltage will be
stepped down to distribution levels where it will be distributed for use by
industrial, commercial, and residential customers. As the distribution network
will typically be rated at no greater than 11 kV, this voltage will be
distributed to geographical areas where connection to industrial and commercial
customers will be achieved. Residential consumers are typically supplied from
overhead feeders emanating from utility-owned substations. Pole mounted
transformers are typically used in older residential neighborhoods and rural
areas. Pole mounted transformers will be limited in the number of customers
they supply in order to minimize the interruption time should a fault occur. The connection from the pole-mounted transformer is made to each home. Distribution
transformers for the final service connection to each home or business.
Distribution network
Distribution of electric power is done by distribution
networks. Distribution networks consist of following main parts:
Ø Distribution
substation
Ø Primary
distribution feeder
Ø Distribution
line
Ø Distribution
transformer
Ø Construction
of Distribution System
Ø Distributors
Distribution substation
A distribution substation transfers power from the transmission system to the distribution system of an area. The input for a distribution substation is typically at least two transmission or sub-transmission lines.
Primary distribution feeder
Distribution feeder circuits are the connections between the output terminals of a distribution substation and the input terminals of primary circuits. The distribution system starts with 11 kV and 33 kV voltage levels, where these voltage levels are directly distributed to industries and too heavy loads. So, this voltage level distribution is called a primary distribution.
Distribution line
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