Alternating and Direct Current

1. AC/DC,
TRANSFORMERS &
SUBSTATIONS
SUBMITTED BY: NISHI YADAV, YASH KHARBANDA, YASHIKA MADAN

2. Alternating Current (AC)
Alternating current describes the flow of charge that changes direction periodically. As a result, the
voltage level also reverses along with the current. AC is used to deliver power to houses, office
buildings, etc.
Generating AC
AC can be produced using a device called an
alternator. This device is a special type of electrical
generator designed to produce alternating current.
A loop of wire is spun inside of a magnetic field,
which induces a current along the wire. The
rotation of the wire can come from any number of
means: a wind turbine, a steam turbine, flowing
water, and so on. Because the wire spins and enters
a different magnetic polarity periodically, the
voltage and current alternates on the wire.

3. A.C. Distribution system
A.c. distribution system is the electrical system
between the step-down substation fed by the
transmission system and the consumers’ meters.
As we all know, electrical power is almost
exclusively generated, transmitted and
distributed in it's AC form. A distribution
system usually begins from a substation where
the power is delivered by a transmission
network. In some cases, the distribution system
may start from a generating station itself, such
as when consumers are located near the
generating station. For larger areas or
industrial areas, primary and secondary
distribution may also be used.

4. Primary distribution
• Voltages somewhat higher than general
utilisation and handles large blocks of electrical
energy than the average low-voltage consumer
uses.
• Commonly used primary distribution voltage
11KV, 6.6KV,3.3 KV.
• Electric power from the generating station is
transmitted at high voltage to the substation
located in or near the city.
• At this substation, voltage is stepped down to
11 kV with the help of step-down transformer.
• Power is supplied to various substations for
distribution or to big consumers at this voltage.
• This forms the high voltage distribution or
primary distribution.

5. Secondary distribution
• It is that part of a.c. distribution system which
includes the range of voltages at which the
ultimate consumer utilizes the electrical energy
delivered to him.
• The secondary distribution employs 400/230 V,
3 phase, 4-wire system.
• The primary distribution circuit delivers power
to various substations, called distribution
substations. The substations are situated near
the consumers localities and contain step-down
transformers.
• At each distribution substation, the voltage is
stepped down to 400 V and power is delivered
by 3-phase,4-wire a.c. system. The voltage
between any two phases is 400 V and between
any phase and neutral is 230 V.

6. Single phase, 2-wire system; Single phase, 3-wire system; Two phase, 3-wire system; Two phase, 4-
wire system; Three phase, 3-wire system; Three phase, 4-wire system
According to phases and wires involved, an AC distribution system can be classified as :-
Single Phase, 2-Wire Distribution
This system may be used for very short
distances. The following figure shows a
single phase two wire system with - fig (a)
one of the two wires earthed and fig. (b) mid-
point of the phase winding is earthed.
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Single Phase, 3-Wire System
This system is identical in principle with 3-wire dc
distribution system. The neutral wire is center-tapped
from the secondary winding of the transformer and
earthed. This system is also called as split-phase
electricity distribution system. It is commonly used in
North America for residential supply.

7. Two Phase, 3-Wire System
In this system, the neutral wire is taken from
the junction of two phase windings whose
voltages are in quadrature with each other.
The voltage between neutral wire and either
of the outer phase wires is V. Whereas, the
voltage between outer phase wires is √2V. As
compared to a two-phase 4-wire system, this
system suffers from voltage imbalance due
to unsymmetrical voltage in the neutral.
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Two Phase, 4-Wire System
In this system, 4 wires are taken from two phase
windings whose voltages are in quadrature with
each other. Mid-point of both phase windings are
connected together. If the voltage between the two
wires of a same phase is V, then the voltage
between two wires of different phase would be
0.707V.

8. Three Phase, 3-Wire Distribution System
Three phase systems are very widely used for AC
power distribution. The three phases may be
delta connected or star connected with star point
usually grounded. The voltage between two
phases or lines for delta connection is V, where V
is the voltage across a phase winding. For star
connection, the voltage between two phases is
√3V.
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Three Phase, 4-Wire Distribution System
This system uses star connected phase windings and the fourth
wire or neutral wire is taken from the star point. If the voltage
of each winding is V, then the line-to-line voltage (line voltage)
is √3V and the line-to-neutral voltage (phase voltage) is V. This
type of distribution system is widely used in India and many
other countries. In these countries, standard phase voltage is
230 volts and line voltage is √3x230 = 400 volts. Single phase
residential loads, single phase motors which run on 230 volts
etc. are connected between any one phase and the neutral.
Three phase loads like three-phase induction motors are put
across all the three phases and the neutral.

9. Home and office outlets are almost always AC. This is because generating and transporting AC
across long distances is relatively easy.
At high voltages (over 110kV), less energy is lost in electrical power transmission. Higher
voltages mean lower currents, and lower currents mean less heat generated in the power line
due to resistance. AC can be converted to and from high voltages easily using transformers.
AC is also capable of powering electric motors. Motors and generators are the exact same
device, but motors convert electrical energy into mechanical energy (if the shaft on a motor is
spun, a voltage is generated at the terminals!). This is useful for many large appliances like
dishwashers, refrigerators, and so on, which run on AC.
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Applications of AC

10. DC can be generated in a number of
ways:
An AC generator equipped with a device
called a "commutator" can produce direct
current.
Use of a device called a "rectifier" that
converts AC to DC.
Batteries provide DC, which is generated
from a chemical reaction inside of the
battery.
Generating DC
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Direct Current (DC)
Direct current is a bit easier to understand than alternating current. Rather than oscillating
back and forth, DC provides a constant voltage or current.

11. ------------------------------------------------------------------------------------------------------------------------------
D.C. Distribution system
It is a common knowledge that electric power is almost exclusively generated, transmitted
and distributed as a.c. However, for certain applications, d.c. supply is absolutely necessary.
For instance, d.c. supply is required for the operation of variable speed machinery (d.c.
motors), for electro-chemical work and for congested areas where storage battery reserves
are necessary. For this purpose, a.c. power is converted into d.c. power at the substation by
using converting machinery e.g., mercury arc rectifiers, rotary converters and motor-generator
sets. The d.c. supply from the substation may be obtained in the form of
( i) 2-wire
( ii) 3-wire for distribution.

12. ------------------------------------------------------------------------------------------------------------------------------
D.C. Distribution system
It is a common knowledge that electric power is almost exclusively generated, transmitted and
distributed as a.c. However, for certain applications, d.c. supply is absolutely necessary. For
instance, d.c. supply is required for the operation of variable speed machinery (d.c. motors),
for electro-chemical work and for congested areas where storage battery reserves are
necessary. For this purpose, a.c. power is converted into d.c. power at the substation by using
converting machinery e.g., mercury arc rectifiers, rotary converters and motor-generator sets.
Mercury
arc rectifier
Rotary
converters

13. ---------------------------------------------------------------------
D.C. Distribution system
The d.c. supply from the substation may be obtained in the form of:
( i) 2-wire d.c. system.
As the name implies, this system of distribution
consists of two wires. One is the outgoing or
positive wire and the other is the return or
negative wire. The loads such as lamps, motors
etc. are connected in parallel between the two
wires as shown below. This system is never used
for transmission purposes due to low efficiency
but may be employed for distribution of d.c.
power.
( ii) 3-wire d.c. system.
It consists of two outers and a middle or neutral wire
which is earthed at the substation. The voltage between
the outers is twice the voltage between either outer and
neutral wire as shown below. The principal advantage of
this system is that it makes available two voltages at the
consumer terminals viz., V between any outer and the
neutral and 2V between the outers. Loads requiring high
voltage ( e.g., motors) are connected across the outers,
whereas lamps and heating circuits requiring less voltage
are connected between either outer and the neutral.

14. This form of power is most commonly produced by sources such as solar cells, batteries, and
thermocouples. DC power is widely used in low voltage applications such as charging
batteries, automotive applications, aircraft applications and other low voltage, low current
applications. All solar panels nowadays produce DC power. Common applications with DC
power in the PV industry are portable solar systems and other off-grid appliances. Not
using a solar inverter to convert DC to AC will keep the costs down for such systems. The
application of DC can also be seen in: mobiles, air conditioners, fans, lights, sockets,
flashlights, flat-screen TVs, electric vehicles, computers, radios.
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Applications of DC

15. Electrical power transformer is a static device which transforms electrical energy from one
circuit to another without changing its frequency but may be in different voltage level.
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TRANSFORMER
Use of Power Transformer
• Electrical power is generated in low voltage level as
this is very much cost effective .
• To prevent the transmission loss, low level power
must be stepped up for efficient electrical power
transmission.
• This is done by step up transformer at the sending
side of the power system network.
• This high voltage power may not be distributed to the
consumers directly, this must be stepped down to the
desired level at the receiving end with the help of step
down transformer.

16. Basis of construction:
Core type transformer:
Its core has two limbs. The windings are
wounded on two limbs of the core material.
Shell type transformer:
Its core has three limbs and two windows. Both
the windings are wounded on the central limb.
(one over the other)
Spiral core transformer:
The core constructed is similar to wheels of
spokes. The windings are wounded these
spokes like structure.
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TRANSFORMER
Spiral core

17. Basis of Windings
Step up Transformer:
The no of windings on Primary side is less than
the no. of windings on the secondary side.
Np < Ns
Step down Transformer:
The no. of winding on Primary side are more
than the no. of windings on the secondary side.
Np > Ns
Isolation Transformer:
The no. of winding on Primary side are equal to
the no. of windings on the secondary side.
Np = Ns
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TRANSFORMER

18. Basis of coolant
Oil filled self cooling:
The assembled windings and core of such transformers
are mounted in a welded, oil-tight steel tanks provided
with a steel cover. The oil helps in transferring the
heat from the core and the windings to the case from
where it is radiated out to the surroundings.
Oil filled water cooled:
This type is used for much more economic construction
of large transformers. The cooling coil is mounted near
the surface of the oil, through which cold water keeps
circulating. This water carries the heat from the device.
Air Blast:
This type is used for transformers that use voltages
below 25,000 volts. The transformer is used at houses.
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TRANSFORMER

19. WHERE DOES ELECTRICITY IN OUR HOME COME FROM?
POWER PLANTS?
BUT ITS COMPLICATED.
Generation is only the first of many steps our power
takes on its nearly instantaneous journey from
production to consumption.
The behaviour of electricity doesn't always follow our
intuitions, which means the challenges associated
with constructing, operating, and maintaining the
power grid are often complicated and sometimes
unexpected.
Many of those challenges are overcome at facility
which, at first glance, often looks like a chaotic and
dangerous mess of wires and equipment, but which
actually serves a number of essential roles in our
electrical grid, the substation.

21. Electrical substations
• 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.

23. Classifications of Substations
The substations may be classified in numerous ways, such as by nature of duties,
service rendered operating voltage, importance, and design.
Nature of Duties
• Step-up or Primary Substations- It is located near the generating substation
• Step-down or Distribution Substations
Service Rendered
• Transformer substations
• Switching Substations
• Converting Substations
Operating Voltage
• High Voltage Substations - Involving voltages between
11 KV and 66 KV.
• Extra High Voltage Substations - voltages between 132
kV and 400 KV.
• Ultra High Voltage – Operating voltage above 400 KV.
Importance
• Grid Substations
• Town Substations
Design
• Indoor Type Substations
• Outdoor Substations
• Pole Mounted Substations
• Foundation Mounted Substations

24. Switching Substations
– The substations use for switching the power line without disturbing the voltage is known as the
switching substations. This type of substations is placed between the transmission line.
DISCONNECT SWITCH
These high voltage
conductors connect
to a series of some
or many pieces of
equipment before
heading out to their
next step
in the power grid.
Like all equipment, substations occasionally have malfunctions or things that simply
require regular maintenance. To avoid shutting down the entire substation, we need
switches that can isolate equipment, transfer load, and control the flow of
electricity along the bus.

25. As a junction point in the grid, a substation often serves as the termination of many
individual power lines. This creates redundancy, making sure that the substation stays
energized even if one transmission lines goes down. But, it also creates complexity.
The connections to these various devices are called buses, often rigid, overhead conductors.
The arrangement
of the bus is a
critical part of the
design of any
substation
because it can
have a major
impact on the
overall reliability.

26. OUT IN THE OPEN FOR ANYONE TO HAVE A LOOK
One of the most important parts
of the power grid is that different
segments flow at different
voltages

27. REGULATOR
At large power plants, electricity is produced at a somewhat
low voltage of around 10-30 kilovolts or kV.
From there, the voltage is increased much higher using
transformers so that it can travel along transmission
lines.Using a higher voltage reduces the losses along the way,
making them more efficient but also much more dangerous.
This is why overhead transmission lines are so tall - to keep
them out of the way of trees and human activities.
Transformer substations – In
such type of substation
transformers are installed for
transforming the power from
one voltage level to another
level as per need.

28. So, prior to distribution, the voltage of the grid needs to be brought back down, again
using transformers located within a substation.
In the event of a fault, the substation needs to be able to sink lots of current into the
ground to trip the breakers as quickly as possible.
But, when transmission lines reach the populated areas which they serve, it's not
feasible to keep them so high in the air.

29. A step-up transmission substation receives electric power from a nearby generating facility
and uses a large power transformer to increase the voltage for transmission to distant
locations. A transmission bus is used to distribute electric power to one or more transmission
lines. There can also be a tap on the incoming power feed from the generation plant to provide
electric power to operate equipment in the generation plant.
A substation can have circuit breakers that are used to switch generation and transmission
circuits in and out of service as needed or for emergencies requiring shut-down of power to a
circuit or redirection of power.
The specific voltages leaving a step-up transmission substation are determined by the
customer needs of the utility supplying power and to the requirements of any connections to
regional grids. Typical voltages are:
High voltage (HV) ac:69 kV, 115 kV, 138 kV, 161 kV, 230 kV
Extra-high voltage (EHV) ac:345 kV, 500 kV, 765 kV
Ultra-high voltage (UHV) ac:1100 kV, 1500 kV
Direct-current high voltage (dc HV):±250 kV, ±400 kV, ±500 kV

30. Step-down transmission substations are located at switching points in an electrical grid. They
connect different parts of a grid and are a source for subtransmission lines or distribution lines. The
step-down substation can change the transmission voltage to a subtransmission voltage, usually 69
kV. The subtransmission voltage lines can then serve as a source to distribution substations.
Sometimes, power is tapped from the subtransmission line for use in an industrial facility along the
way. Oterwise, the power goes to a distribution substation.

31. Distribution substations are located near to the end-users. Distribution substation transformers
change the transmission or subtransmission voltage to lower levels for use by end-users. Typical
distribution voltages vary from 34,500Y/19,920 volts to 4,160Y/2400 volts.
34,500Y/19,920 volts is interpreted as a three-phase circuit with a grounded neutral source. This
would have three high-voltage conductors or wires and one grounded neutral conductor, a total of
four wires. The voltage between the three phase conductors or wires would be 34,500 volts and the
voltage between one phase conductor and the neutral ground would be 19,920 volts.
From here the power is distributed to industrial, commercial, and residential customers.

32. Underground distribution substations are also located near to the end-users. Distribution
substation transformers change the subtransmission voltage to lower levels for use by end-users.
Typical distribution voltages vary from 34,500Y/19,920 volts to 4,160Y/2400 volts.
An underground system may consist of these parts:
Conduits
Duct Runs
Manholes
High-Voltage Underground Cables
Transformer Vault
Riser
Transformers
From here the power is distributed to industrial, commercial, and residential customers.

33. Substations are designed to accomplish the following functions, although not all substations have
all these functions:
Change voltage from one level to another
Regulate voltage to compensate for system voltage changes
Switch transmission and distribution circuits into and out of the grid system
Measure electric power qualities flowing in the circuits
Connect communication signals to the circuits
Eliminate lightning and other electrical surges from the system
Connect electric generation plants to the system
Make interconnections between the electric systems of more than one utility
Control reactive kilovolt-amperes supplied to and the flow of reactive kilovolt-amperes in the
circuits

34. Converting Substations
– In such types of substations, AC power converting into DC power or vice versa or it can convert
high frequency to lower frequency or vice versa.
Grid Substations
– This substation is used for transferring the bulk power from one point to another. If any fault
occurs on the substation, then the continuity of whole of the supply is affected by it.
Town Substations
– These substations step down the voltage at 33/11 kV for more distribution in the towns. If there is
any fault occurs in this substation, then the supply of the whole town is blocked.

35. REQUIREMENT AS PER THE CODES (NBC)
Location and Requirement of Substation
• It should be located in separate building and could be adjacent to the generator room,
if any.
• Location of substation in the basement floors should be avoided, as far as possible.
• The ideal location for an electrical substation for a group of buildings would be at the
electrical load centre on the ground floor.
• Generally the load centre would be somewhere between the geometrical centre
and the air conditioning plant room, as air conditioning plant room would
normally be the largest chunk of load, if the building is air conditioned,
• Substations with oil filled equipment will require great consideration for the fire
detection, protection and suppression. Oil cooled transformers require a suitable soak
pit with gravity flow to contain the oil in the event of the possibility of oil spillage from
the transformer on its failure.