
Raviteja Chivukula



If millions of users
start consuming
considerable amount
of reactive power,
the power utility
company will
collapse in losses. 
We keep hearing regularly that the low power factor is bad, and that
power factor should be kept greater than 0.95, low power factor is
inefficient, and so on. Many of us usually accept it as a thumb rule.
Rather than accepting it, let us go a step further in rigorously understanding
why low power factor is bad? Whom does it affect the most? What happens if
you don't worry about low power factor? How do you correct low power factor?
etc. To answer these questions, let us understand some commonly used terms
like instantaneous power, real power, reactive power, apparent power and power
factor.
Simply put, instantaneous power is given by P(t) = V(t) x I(t) . Instantaneous power is a
function of time. So if I measure the voltage level given to the load at this instant
and measure the current drawn by the load at the same instant and multiply
them, I get instantaneous power. Though it is easy to measure, instantaneous
power may not be all that useful. In a typical AC power system, the
instantaneous power may not always be positive. In a time of 20ms
(corresponding to 50Hz), the instantaneous power may change multiple times
between positive and negative as seen in Figure 1. How much time does it
remain positive, how much time does it remain negative depends on the kind of
load connected.
For purely resistive loads (like bulbs, electric iron etc), the voltage and
current are perfectly in phase with each other. This means that the
instantaneous power is always positive. The flow of electrical power is always
from the grid to the load and not in the other direction.


Figure 1: Voltage, Current and Instantaneous Power Waveforms 
On the other extreme, if the load is purely reactive, the voltage and current are 90
degrees out of phase.
If V(t) x I(t) is computed over time, it can be seen that the instantaneous
power changes direction every quarter cycle (5ms for 50 Hz AC). This means, the
average power transferred over time, from the supply to the load is zero.
Typical loads however are not purely reactive. In fact, if a load is purely
reactive, on an average, it cannot do any useful work for us. Typical loads are
a combination of resistive and reactive loads. The resistive
load component gives us useful work, while the reactive load
component is just an overhead. The Pythagorean sum of real
power and reactive power is termed as the apparent power
and the ratio of real power to apparent power is termed as
power factor.
Now, one may ask, if I am consuming power for 5ms and
then pumping back power for 5ms, what is the problem? In
order to answer this question, let us go back to the concept of
reactive power. If a load is purely reactive, it consumes energy
from grid for 5ms and then pumps energy back into the grid for
5ms. This means, I am not getting any useful work from this
load. But still, current has to flow back and forth into the load
from the grid. And this current travels all the way from the
generating power station to my load. Due to this flow of
current, all the way from the generating station to the load,
heating losses occur in the transmission lines. So the bottom
line is: if the utility company just charges for the 'real power'
that you consumed, it will not be able to bear the losses
occurred due to reactive power. And if millions of users start
consuming considerable amount of reactive power, the utility
company will collapse in losses. Hence the utility companies
need to penalize low power factor.




Figure 2: NI CompactRIO is a generic embedded platform that can programmed for various functionalities like Power Factor Correction, PMU,
Power Quality Analyzer, Smart Recloser etc. 
One thing to note here is that, power factor is entirely a
consequence of the load connected. The generation side does
not govern the power factor. So, the onus is upon the load to
correct it to make sure that the reactive power is very small as
compared to the apparent power. i.e. the power factor should
be close to 1.
The way to correct power factor is to reduce the reactive
power in the circuit. Most of the industrial loads have
extensive inductive load giving rise to a lagging current,
reducing the power factor. Inductive current and capacitive
current have an inverse relationship as capacitors discharge
current when inductors need it and vice versa. So the best way
to correct the lagging current due to inductive load is to add
capacitor banks to the circuit. So, instead of borrowing the
reactive current all the way from the generation site through
the transmission lines, we are borrowing it from the locally present capacitors. For nonlinear loads passive PFC systems
alone doesn't solve the problem. Active PFC systems which can
reshape the current waveform through a load provide better
power factor correction capabilities for circuits with nonlinear
loads. They also work in multiple stages providing
automatic power factor correction and continuously targeting
an optimum PF value. They perform high end online data
processing to provide realtime PF correction.
Advanced embedded systems like National Instruments
CompactRIO platform are used to make Smart Active
Power Factor Correction systems. The versatility,
modularity and flexibility of NI CompactRIO allow it to
directly acquire voltage and current signals from multiple
power lines simultaneously. The availability of FPGA on
these systems allow it to perform highend computation to
accurately perform power quality analysis and provide
control output signals to take corrective actions on a realtime
basis for e.g. controlling a thyristorswitched
capacitor banks on PF correction on fast changing dynamic
load circuits. The rigidity and reliability of this hardware
allows it to be used in hazardous condition continuously for
year's together. National Instruments LabVIEW electrical
power suite software provides ready to use functions which
helps to compute various power quality measurements like
frequency variation, flicker, voltage dips, harmonics etc. as
per IEC standards.
The CompactRIO system is programmed using NI LabVIEW
Graphical System Design platform, and it can be customized
as well as upgraded in the field. With minimal/no addition to
the hardware, it can be programmed to behave as a number of
different "personalities", such as Power Factor Correction
System, Phasor Measurement Unit, Power Quality Analyzer,
Smart Switch, Recloser, etc. Due to the modular nature of the
hardware & remotely upgradeable software, NI CompactRIO
is an ideal approach for smart grid applications that demand
evolving functionality and requirements.
( Raviteja Chivukula is Technical Marketing Engineer,
National Instruments )
