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What is Power Factor?
Visualize a horse pulling a railroad car down a track. Because the railroad ties are uneven, the horse must
pull the car from the side of the track at an angle. The power required to move the car down the track is
the working (real) power. The effort of the horse is the total (apparent) power. Because of the angle that
the horse is pulling from not all of the horse's effort is used to move the car down the track. The railroad
car will not move sideways therefore, the sideways pull of the horse is wasted effort or non-working (reactive) power.
The angle of the horse's pull can be related to power factor, which is defined as the ratio of real (working) power
to apparent (total) power. When the horse is led closer to the center of the track, the angle of side pull decreases
and the real power approaches the value of the apparent power. Therefore, the ratio of real power to apparent power
(the power factor) approaches 1. As the power factor approaches 1, the reactive (non-working) power approaches 0.
Power Factor = Real Power/Apparent Power
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For example, if:
Real power = 100 kW
and
Apparent power = 142 kVA
then
Power Factor = 100/142 = 0.70 or 70% |
This indicates that the system is 70% efficient, or that only 70% of the current is being used to produce work.
Why is power factor important to me?
If a building has a low power factor, more current must flow through power distribution lines. Energy and money is
wasted due to additional energy being required from RPU for non-productive purposes.
Low power factor is caused by inductive loads such as transformers, electric motors and high intensity discharge lighting.
Unlike resistive loads that create work by consuming watts or kilowatts, inductive loads require the current to
create a magnetic field, and the magnetic field produces the desired work. The total or apparent power required
by an inductive device is a composite of the following:
* Real power (measured in kilowatts, kW)
* Reactive power, the non-working power caused by the magnetizing current, required to operate the device (measured in kilovars,
kVAR)
Reactive power required by inductive loads increases the amount of apparent power (measured in kilovolt amps, kVA) in our
distribution system. The increase in reactive and apparent power is reflected by the amplification of the angle between
the two, causing the power factor to decrease.
Power factor can be improved in several ways:
Reduce the amounts of reactive energy by avoiding unloaded motors and transformers and keeping within rated voltages of the loads.
Use synchronous motors, usually available only in very large sizes, instead of induction motors. Synchronous motors
have the same electrical effect on the distribution network as capacitors but are seldom used outside of heavy industrial
applications.
Compensate for the consumption of reactive energy by installing power factor capacitors.
What is Demand?
The cost of supplying electric service not only depends on the amount of energy (kWh or kilowatt-hours) consumed, but also on
the power requirement of a facility, known as electric demand. Electric demand is the maximum amount of electricity used
at any single point in time by a facility. Demand is measured in kilowatts (kWh) by an electric demand meter. The highest
level of electric demand during a billing month is called the peak demand.
RPU must be able to meet the peak coincident demand of all our customers with our existing generation, transmission and
distribution facilities. The higher the overall peak demand on an electric system, the more generating capacity and
equipment is required. Figure 1 shows a typical system demand profile for a peak day in summer. Electric demand is
usually greatest during the period from 10am to 10pm.
Demand Charges recover a portion of the cost of providing the generating capacity and equipment needed to serve our customer's
peak electrical demand. Your monthly bill will include a charge for kWh of energy and kW of demand. The demand portion of
your bill is based on the peak demand recorded in kW during the billing month. To calculate the demand portion of your bill,
multiple the demand charge by the peak demand (in kW) indicated on your bill.
How do I read my demand meter?
Most demand customers served by us have an indicating demand meter (Fig. 2) known as a thermal demand meter. Thermal demand
meters respond very slowly to fluctuations in the electric load. Thermal demand meters require 15 minutes to register 90%
of the electricity load.
There are two indicators or pointers on the demand meter. The red pointer indicates the present kW demand. The black pointer
indicates the peak kW demand for the billing period and is pushed to the right by the end red pointer. The demand pointers
lag behind and smooth out fluctuations of the actual load. Every month, the meter reader records the peak demand
(black pointer) and resets both pointers back to zero to allow a new peak to be set for the next billing period.
What Are Capacitors?
Install capacitors in your AC circuit to decrease the magnitude of reactive power. Reactive power caused by inductance always
acts at a 90 degree angle to real power.
Capacitors store kVARs and release energy opposing the reactive energy caused by the inductor. This implies that inductance and
capacitance react 180 degrees to each other. The presence of both in the same circuit results in the continuous alternating
transfer of energy between the capacitor and the inductor, thereby reducing the current flow from the generator to the circuit.
When the circuit is balanced, all the energy released by the inductor is absorbed by the capacitor.
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