Apparent Power
The practical significance of apparent power is as a rating
unit. For example, an ac generator supplies apparent power at essentially constant
voltage and frequency. Its output capacity is then described in Mega Volt Amps
(MVA).
Transformer and motor capacities are also rated in MVA or
KVA for similar reasons. Although the utility must provide apparent power, it
receives direct compensation only for active power utilised by its customers.
The ratio of active to apparent power is therefore, an
important quantity and is defined as the power factor. This number can range
from zero to one but good economics requires it to be as close to unity as
possible.
It can be thought of as a measure of the system’s
effectiveness in using apparent power to do useful work. The terminology can be
in any one of the following forms:
Power Factor = PF
= Real Power/Apparent Power
= Watts/ Volt Amps
Note: There is a time lag between the apparent power and
real power. This corresponds to the time lag between voltage and current for
reactive loads.
For capacitive loads, we have a leading power factor.
For inductive loads, it is lagging. Power Factor Angle is a
measurement that describes how close the apparent power is to being totally
real or supplying a pure resistive load. The Power Factor rating can be
summarized in two main areas:
• Efficiency: The lower the power factor, demanded by the
load which requires a given amount of active power, the greater the size of
line current that has to be supplied by the generator and sent through the
transmission system. This means higher winding and line losses and reduced
efficiency;
• Voltage Regulation: The lower the power factor and the
greater the generator current, the greater the reactance voltage drops along
the line. This means a lower voltage at the load and, consequently, poorer
system voltage regulation.
The relationship between apparent, active and reactive power
is that of a right-angled triangle.
The following relationships exist between voltage, current,
apparent power (U), active power (P) and reactive power (Q).
U^2 = P^2+Q^2
U = VI
P = VI cosθ
Q = VI sinθ
The power factor is equal to cosθ
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