An electric
current in which the periodic reversal of direction of flow of charge
particles takes place is called alternating current, in contrast to DC
where the flow of electric
charge is only in one direction. The waveform of alternating current is
usually sinusoidal wave and below figure shows its graphical representation of
single phase alternating current.
 |
| alternating current |
Where T=is
the time period
This is the
form of current
used throughout the world in electric power system. The main advantage of an
alternating current transmission is the AC voltage can be increased or
decreased easily and efficiently with the help of a device called transformer.
With
increase in voltage
of transmission the power loss reduces whereas the consumer loads at the
distribution end are of different voltage
levels and also such a high voltage is not safe. This necessitates the changes
in the voltage
levels at different parts of power system. This can be easily achieved
alternating current system with the help of a transformer. Whereas it not
possible in system.
Three
phase system:
Instead of a
single phase system three phase AC system is used has more power transfer
capability. In three phase system each single phase is placed at 120 electrical
degrees with each other.
Frequency:
Frequency of
inverse of time period is depends on the poles in the machine and the speed of
the rotation of machine. In a power system all the generators
are interconnected and hence their frequencies should be same and they should
be run in synchronism with each other throughout the system. The frequency of power
system varies from country to country usually 50 Hz or 60 Hz are used. In USA
the frequency is 60 Hz, and in India it is 50 Hz. In some countries like Japan
mixture of 50 Hz and 60 Hz also used.
Sources
of Generation of AC:
Alternator: alternator is a synchronous machine
which is the most widely used to generate alternating current and hence its
name alternator.
Induction
generator: Though
induction generators can generate alternating voltages and current,
it not used generally as the performance of induction generator is poor. But it
is used widely used in wind energy conversion.
Inverter: inverter converts direct
current DC
to AC. The DC
from various sources such as batteries and
solar cells can be converted to AC with the help of inverter.
Advantages
and disadvantages of AC:
Advantages
of AC:
1. The
principle advantage of an AC system is that the level of voltage can
easily altered with the help of simple transformer which offers wide range of voltages.
2. The cost of generation of AC is cheaper
than that of an equivalent DC.
3. The initial cost and maintenance cost
of AC machines are cheaper.
4. AC machines are robust and can almost
work in any kind of environment.
5. Ease of conversion to DC
with a rectifier to supply DC
loads.
Disadvantages
of AC:
1. Skin
effect, in which the charge
concentration is more near the surface of the conductor
and less at the center of the conductor.
This non uniformity in the charge
concentration in conductor
decreases the effective area of cross section of the conductor
thereby the increase in effective resistance.
2. Analysis
and design of AC systems involve complex numbers and is harder when compared
with DC analysis which requires simple mathematics.
