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Reactive power flow
In power transmission and distribution, significant effort is made to control the reactive power flow. This is typically done automatically by switching inductors or capacitor banks in and out, by adjusting generator excitation, and by other means. Electricity retailers may use electricity meters which measure reactive power to financially penalise customers with low power factor loads. This is particularly relevant to customers operating highly inductive loads such as motors at water pumping stations.
Explanation of Reactive power
Engineers use several terms to describe energy flow in a
system:
Real power (P)
Reactive power (Q)
Complex power (S) ; |S|, the modulus of complex power, is referred to as
apparent power.
In the diagram, P is the real power, Q is the reactive power (in this case
negative), and the length of S is the apparent power.
The unit for all forms of power is the watt (symbol: W). However, this unit is
generally reserved for the real power component. Apparent power is
conventionally expressed in volt-amperes (VA) since it is the simple product of
rms voltage and current. The unit for reactive power is given the special name “VAR”,
which stands for volt-amperes reactive. Since reactive power flow transfers no
net energy to the load, it is sometimes called “wattless” power.
Understanding the relationship between these three quantities lies at the heart
of understanding power engineering. The mathematical relationship among them can
be represented by vectors or expressed using complex numbers,
(where j is the imaginary unit).
The complex value S is referred to as the complex power.
Consider an ideal alternating current (AC) circuit consisting of a source and a
generalized load, where both the current and voltage are sinusoidal. If the load
is purely resistive, the two quantities reverse their polarity at the same time,
the direction of energy flow does not reverse, and only real power flows. If the
load is purely reactive, then the voltage and current are 90 degrees out of
phase and there is no net power flow. This energy flowing backwards and forwards
is known as reactive power.
If a capacitor and an inductor are placed in parallel, then the currents flowing
through the inductor and the capacitor oppose and tend to cancel out rather than
adding. Conventionally, capacitors are considered to generate reactive power and
inductors to consume it. This is the fundamental mechanism for controlling the
power factor in electric power transmission; capacitors (or inductors) are
inserted in a circuit to partially cancel reactive power of the load. A practial
load will have resistive, inductive, and capacitive parts, and so both real and
reactive power will flow to the load.
The apparent power is the product of voltage and current. Apparent power is
handy for sizing of equipment or wiring. However, adding the apparent power for
two loads will not accurately give the total apparent power unless they have the
displacement between current and voltage.
from Wikipedia