wat is the difference between real power and reactive
power..which is useful..?and how it s useful..wat is the
effect of these on a power system...

Answer Posted / vasanth kumar s m

Alternating current
In alternating current circuits, energy storage elements
such as inductance and capacitance may result in periodic
reversals of the direction of energy flow. The portion of
power flow that, averaged over a complete cycle of the AC
waveform, results in net transfer of energy in one
direction is known as real power (also referred to as
active power). That portion of power flow due to stored
energy, that returns to the source in each cycle, is known
as reactive power.


Power triangle The components of AC powerThe relationship
between real power, reactive power and apparent power can
be expressed by representing the quantities as vectors.
Real power is represented as a horizontal vector and
reactive power is represented as a vertical vector. The
apparent power vector is the hypotenuse of a right triangle
formed by connecting the real and reactive power vectors.
This representation is often called the power triangle.
Using the Pythagorean Theorem, the relationship among real,
reactive and apparent power is:

(apparent power)2 = (real power)2 + (reactive power)2
Real and reactive powers can also be calculated directly
from the apparent power, when the current and voltage are
both sinusoids with a known phase angle between them:

(real power) = (apparent power) * cos(theta)
(reactive power) = (apparent power) * sin(theta)
The ratio of real power to apparent power is called power
factor and is a number always between 0 and 1.

To know the Power Triangle & the above concept here is the
hyperlink for this sub:
http://en.wikipedia.org/wiki/Power_factor#Importance_of_powe
r_factor_in_distribution_systems

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