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Birla vishvakarma mahavidhyalaya
Electrical Power System
Supply Systems
Prepared by:
Savdas Ambaliya(130070109001)
Amritkumar ojha(130070109002)
Payal Bhadresa(130070109003)
Parth Bhalodiya(130070109004)
Guided by:
Pro.Ajay Patel
Electrical Supply system
 The conveyance of electric power from a power station to
consumers’ premises is known as electric supply system.
 Electric supply system consist of three principle components:
1)Power station
2)Transmission lines
3)Distribution system
 The electric supply system can be broadly classified into:
1)DC or AC system
2)Overhead or underground system
Typical AC power supply scheme
Different blocks of typical ac power supply
scheme are as per following::
1)Generating Station:
 In generating station power is produced by
three phase alternators operating in parallel.
 the usual generation voltage is 11 kV.
 For economy in the transmission of electric
power, the generation voltage is stepped up to
132 kV or more at generating station with the
help of three phase transformer.
2)Primary transmission:
 The electric power at 132 kV is transmitted by
3-phase,3 wire overhead system to the out skirts of the city.
3)Secondary transmission:
 At the receiving station the voltage is reduced to 33kV by step
down transformer.
4)Primary distribution:
 At the substation voltage is reduced 33kV to 11kV. The 11kV
line run along important road sides to city. This forms the
primary distribution.
5)Secondary distribution:
 The electric power form primary
distribution line is delivered to distribution
substation. The substation is located near
the consumers location and step down the
voltage to 400V, 3-phase ,4-wire for
secondary distribution.
 The voltage between any two phases is
400V and between any phase and neutral
is 230V.
Comparison of DC and AC transmission
DC transmission
Requires only two conductors.
There is no inductance, capacitance, phase difference and surge problem.
Better voltage regulation.
No skin effect.
DC line requires less insulation.
Less corona loss and reduced interference with communication circuit.
The high voltage DC transmission is free from dielectric losses.
No stability problems and synchronising difficulties.
• Electric power can not be generated at high DC voltage due to commutation
• DC voltage can not be stepped up.
• The switches and circuit breakers have their own limitations.
AC transmission
• The power can be generated at high voltages.
• The maintenance of AC substation is easy and cheaper.
• The AC voltage can be steeped up or stepped down by
• Requires more copper than DC.
• Due to skin effect in the AC system the effective resistance of
line is increased.
• AC line has capacitance therefore there is continuous loss of
power due to charging current even when the line is open.
Advantages of high transmission voltage
1) Reduces volume of conductor material
P= power transmitted in watt
V= line voltage in volt
COSØ= Power factor of the load
l= Length of the line in meters
R= Resistance per conductor in ohm
ρ= Resistivity of conductor material
a= Area of cross-section of conductor
Volume of the conductor material
 From the equation it is concluded that the greater the transmission voltage, lesser is
the conductor material required
2) Increases transmission efficiency
Transmission efficiency
 AS J,ρ and l are constant , therefore, transmission efficiency increases
when the line voltage is increased.
3) Decreases percentage line drop
 AS J,ρ and l are constant therefore; percentage line drop decreases
when the transmission voltage increases.
Various system of power transmission
1) DC system
 DC two-wire
• In the 2-wire dc system one is the outgoing or positive
wire and the other is the return or negative wire.
 DC two-wire with mid-point earthed
• In the 2-wire dc system with mid point earthed ; the
maximum voltage between any conductor and earth is
Vm so that maximum voltage between conductors is
 DC three-wire
• In a 3 wire dc system there are two outer and a
middle or neutral wire which is earthed at the
generator end.
• If load is balanced ; the current in neutral wire is
2) Single-phase two-wire
 Single-phase two-wire
 Single-phase two-wire with mid-point earthed
 Single-phase three-wire
3) Two-phase AC system
 Two-phase four-wire
• The four wires are taken from the ends of the
two phase windings and the midpoints of the
two windings are connected together .
• This system can be considered as two
independent single phase systems.
 Two-phase three-wire
4) Three-phase AC system
 Three-phase three-wire
 Three-phase four-wire
• In this case forth for neutral wire is taken from
the neutral point . The area of X-section of the
neutral wire is generally one half that of the
line conductor.