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Depending upon the rate structure of your
electric utility, you may be able to save a substantial amount of money on
your electric bill. Pay-back period for an equipment purchase including
installation cost may be six months up to three years. Utility rate
structures that account for reactive power consumption, by either a KVA or
var demand usage, or a power factor penalty are the ones that can provide
this pay-back. Other ancillary benefits to be gained by optimizing power
factor are, lower energy losses, better voltage regulation and released
system capacity. This page explains the fundamentals of power factor and
how KVAR EC UNITs can benefit you.
All electric equipment requires "vars" - a term used by electric power
engineers to describe the reactive or magnetizing power required by the
inductive characteristics of electrical equipment. These inductive
characteristics are more pronounced in motors and transformers, and
therefore, can be quite significant in industrial facilities. The flow of
vars, or reactive power, through a watt-hour meter will not effect the
meter reading, but the flow of vars through the power system will result
in energy losses on both the utility and the industrial facility. Some
utilities charge for these vars in the form of a penalty, or KVA demand
charge, to justify the cost for lost energy and the additional conductor
and transformer capacity required to carry the vars. In addition to energy
losses, var flow can also cause excessive voltage drop, which may have to
be optimized by either the application of KVAR EC Units, or other more
expensive equipment, such as load-tap changing transformers, synchronous
motors, and synchronous condensers.
Figure 1 - Power Factor Triangle
The power triangle shown in figure 1, is the simplest way to understand
the effects of reactive power. The figure illustrates the relationship of
active (real) and reactive (imaginary or magnetizing) power. The active
power (represented by the horizontal leg) is the actual power, or watts
that produces real work. This component, is the energy transfer component,
which represents fuel burned at the power plant. The reactive power, or
magnetizing power, (represented by the vertical leg of the upper or lower
triangle) is the power required to produce the magnetic fields to enable
the real work to be done. Without magnetizing power, transformers,
conductors, motors, and even resistors and capacitors would not be able to
operate. Reactive power is normally supplied by generators, capacitors and
synchronous motors. The longest leg of the triangle (on the upper or lower
triangle), labeled total power, represents the vector sum of the reactive
power and real power components. Mathematically, this is equal to:
Electric power engineers often call total power, kVA, MVA, apparent power,
or complex power. Some utilities measure this total power, (usually
averaged over a 15 minute load period) and charge a monthly fee or tariff
for the highest fifteen minute average load reading in the month. This
tariff is usually added to the energy charge or kilowatt-hour charge. This
type of billing is often called kva demand billing and can be quite costly
to an industrial facility. KVAR EC UNITs can save your company money by
decreasing your reactive power component supplied by the utility to near
zero vars.
The power triangle and the equation above show, that as the reactive power
component is decreased by adding KVAR EC UNITs, the total power will also
decrease. This is shown by the decreased length of the dashed lines in the
power triangle as the reactive power component approaches zero. Therefore,
adding KVAR EC UNITs, which will supply reactive power locally, can reduce
your total power and monthly kva demand charge.
The angle "phi" in the power triangle is called the power factor angle and
is mathematically equal to:
The ratio of the real power to the total power in the equation above (or
the cos of phi) is called power factor. As the angle gets larger (caused
by increasing reactive power) the power factor gets smaller. In fact, the
power factor can vary from 0 to 1, and can be either inductive (lagging)
or capacitive (leading). Capacitive loads are drawn down, and inductive
loads are drawn up on the power triangle. Most industrials normally
operate on the upper triangle (inductive or lagging triangle). As an
industrial adds capacitors, the length of reactive (inductive) power leg
is shortened by the number of capacitive KVAR EC that were added. If the
number of capacitive KVAR EC added exceeds the industrials inductive KVAR
EC
load, operation occurs on the lower triangle. This is commonly referred to
as over compensation.
Utilities charge for reactive power in a countless number of ways. Some
utilities charge for KVAR EC demand, while others charge a strait fee for a
power factor less than their target. To fully understand the benefits of
the KVAR EC UNIT, you must acquire your electric billing rate structure. This
rate structure will describe how cost for poor power factor are added to
your monthly bills
You could put the KVAR EC UNIT anywhere on the system as shown (between the
transformer and load and not only at Points A, B, and C) and achieve unity
power factor for the system. The utility company will perceive this power
system as having a unity power factor no matter where it is located on the
distribution line as long as it's sized correctly to deliver the proper
amount of KVAR EC.
However, optimum efficiency and economics will be achieved if the KVAR EC
UNIT bank is located as close to the load as possible.
The reason for this is because when you optimize power factor, you can
reduce the total line current to the load and therefore you reduce the
total losses in the line conductor and decrease the voltage drop in the
line. This decrease in voltage drop will only occur if you locate the KVAR
EC
UNIT close to the load, as explained below.
Assume the load is a motor. A motor uses KW to perform work. It uses KVAR
EC
to magnetize its coil windings. (We will refer to the magnetic
requirements of the motor's windings as the motor's "inductance". It is
this inductance that utilizes the KVAR EC.)
The motor load draws a line current that has two components. The first
component is the amperage that supplies the KW to the load, so that the
motor can perform work such as lifting an object. The second part supplies
the amperage to provide the load with KVAR EC which in the case of the motor
is the power necessary to energize the magnetic fields in the motor's
windings. Together the two amounts of current supply the total KVA to the
load.
Normally the system generator or transformer supplies all this current.
But when a KVAR EC UNIT is used to optimize the power factor, the KVAR
EC UNIT
supplies the KVAR EC reactive current component to the load. The KVAR EC UNIT
is, in effect, a reactive power generator. (Remember, the KVAR EC UNIT stores
energy. The KVAR EC UNIT stores reactive energy in its electric field when it
charges up, and releases it when it discharges.)
The generator (or transformer) must still supply the load's KW
requirements.
The reactive current component is now supplied by the KVAR EC UNIT and not
the generator. By moving the KVAR EC UNIT closer to the load, the reactive
current does not have to travel as far through the line conductors to get
to the load.
If the KVAR EC UNIT is placed at the load, the reactive current only needs to
travel through a short distance (e.g. the lead length of connecting wire)
to get to the load. Since this reactive current component no longer
travels through the conductor line from the generator to the load, it does
not travel through the impedances in the line conductor.
Since this reactive current no longer flows through the line impedances,
there is less heating of the line, less losses (in the form of heat), and
less voltage drop across these in - line impedances (which reduces the
overall voltage drop from generator to load).
The KW current component is all that the generator has to supply to the
motor. Therefore the generator now runs at unity power factor and allows
the KVAR EC UNIT to supply the KVAR EC requirement of the motor's inductive
windings.
The energy "contained" in the KVAR EC current component is transferred back
and forth between the KVAR EC UNIT and the motor 2 times for every voltage
sine wave cycle (i.e. at 120 times a second).
This reactive energy is never consumed by either the KVAR EC UNIT or the
motor. (NOTE: The KW energy, on the other hand, performs real work and is
totally consumed.)
Rather, the reactive energy is only "BORROWED" half of the time by the
KVAR EC UNIT and half of the time by the motor. The energy is used to charge
the AC electric field of the KVAR EC UNIT and to energize and create the AC
magnetic fields contained in the motor's windings.
A KVAR EC UNIT absorbs this energy from the power system and stores this
energy in its electric field when it charges up (120 times a second). The
KVAR EC UNIT releases this energy back into the power when it discharges (120
times a second).
The motor's inductance absorbs the reactive energy from the power system
and stores this energy in its windings' magnetic fields when the fields
are expanding (120 times a second). The inductance releases this energy
back into the power system when the windings magnetic fields are
collapsing (120 times a second).
The secret is that when the motor's inductance requires reactive energy to
expand its magnetic field, the KVAR EC UNIT discharges to supply the energy.
And when the magnetic field in the motor's inductive windings is
collapsing and returning energy to the system, the KVAR EC UNIT uses this
energy to charge up.
So the capacitance in the KVAR EC UNIT and the inductance in the motor's
windings "slinky" this reactive energy back and forth 120 times a second,
each supplying the others needs. The reactive current of the KVAR EC UNIT is
180 degrees out of phase with the reactive current of the inductance. When
one is giving, the other is taking and vice versa.
Again, the reactive energy is never consumed (except for some small and
usually insignificant losses); it is only borrowed. The generator needs to
supply the original reactive KVAR EC energy only once when the system is
first energized. After that, this amount of energy is simply transferred
back and forth between inductance and capacitance.
Power Factor is a measurement of how much of the KVA is actually in the
form of KW. The advantage of a high power factor is that line currents can
be reduced which will in turn reduce voltage drop and decrease line
losses. This saves money. It also means that since equipment such as
transformers will supply only KW, the KVA rating of the equipment can be
reduced, or alternatively, more load can be added to the system without
purchasing larger equipment.
The KVA rating of a transformer is based on the transformers ability to
supply power either all in KW or all in KVAR EC or in a combination of both.
Drawing more than rated KVA from a transformer is easily done, but the
transformer's life will be reduced due to increasing heat which destroys
the transformer's winding insulation.
By increasing the power factor, all of a transformer's KVA can be utilized
to supply KW in order to perform useful work rather than to supply KVAR
EC
just to energize electric and magnetic fields.
Increasing the power factor seen by the transformer creates "room" on the
transformer for adding more load. Room can also be created on circuit
breakers. Since line current is reduced by increasing power factor, load
can be added to the system without upgrading the breaker to a larger size. |
