VTdrive Reactive Power Compensation0 pages

Reactive Power can best be described as the quantity of “unused” power
that is developed by reactive components, such as inductors or capacitors
in an AC circuit or system. In a DC circuit, the product of “volts x amps”
gives the power consumed in watts by the circuit. However, while this
formula is also true for purely resistive AC circuits, the situation is slightly
more complex in an AC circuits containing reactive components as this
volt-amp product can change with frequency.
In an AC circuit, the product of voltage and current is expressed as volt-amperes
(VA) or kilo volt-amperes (kVA) and is known as Apparent power, symbol S. In a
non-inductive purely resistive circuit such as heaters, irons, kettles and filament
bulbs, etc. their reactance is practically zero, and the impedance of the circuit is
composed almost entirely of just resistance.
For an AC resistive circuit, the current and voltage are in-phase and the power at
any instant can be found by multiplying the voltage by the current at that instant,
and because of this “in-phase” relationship, the rms values can be used to find the
equivalent DC power or heating effect.
However, if the circuit contains reactive components, the voltage and current
waveforms will be “out-of-phase” by some amount determined by the circuits phase
angle. If the phase angle between the voltage and the current is at its maximum of
90o, the volt-amp product will have equal positive and negative values.
In other words, the reactive circuit returns as much power to the supply as it
consumes resulting in the average power consumed by the circuit being zero, as the
same amount of energy keeps flowing alternately from source to the load and back
from load to source.
Since we have a voltage and a current but no power dissipated, the expression of P
= IV (rms) is no longer valid and it therefore follows that the volt-amp product in an
AC circuit does not necessarily give the power consumed. Then in order to
determine the “real power”, also called Active power, symbol P consumed by an AC
circuit, we need to account for not only the volt-amp product but also the phase
angle difference between the voltage and the current waveforms given by the
equation:VI.cosΦ.
Then we can write the relationship between the apparent power and active or real
power as: