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Power Engineering Society
Chicago Chapter
Reactive Power: Sources and Solutions
12 February 2003
David E. Mertz, PE
Burns & McDonnell Engineers, Inc
Reactive Power
• What is it?
– Current that is 90 degrees out of phase with
the voltage in an alternating current system.
– Inherent in all alternating current systems
– Caused by capacitive (leading) and inductive
(lagging) loads.
– The complement of “real” power.
Reactive Power
• Where does it come from?
– All conductors are an inductor.
– Multiple conductors are inductors with a
mutual capacitance.
Reactive Power
• Where does it come from?
– Placing conductors in a magnetic raceway
increases their inductance.
Reactive Power
• Where does it come from?
– Magnetic devices are the largest source of lagging
(inductive) reactive power
– Transformer impedance contributes reactive
power, but also limits downstream short-circuit
currents.
Reactive Power
• Where does it come from?
– Magnetic devices are the largest source of lagging
(inductive) reactive power
– Magnetic lamp ballasts also produce lagging
reactive power.
Reactive Power
• Where does it come from?
– Magnetic devices are the largest source of
lagging (inductive) reactive power
– Synchronous electric machines (generators
and synchronous motors) can produce either
lagging or leading reactive power.
– Inductive electric machines (garden-variety
motors) produce only lagging reactive power.
Reactive Power
• What good is it?
– In small amounts, it helps transmission line
operators control the flow of electric power.
– The transmission of high-frequency or step
signals in power systems is greatly attenuated
by the properties that also give us reactive
power.
– The same electrical phenomena are used to
tune circuits for transmitting and receiving
signal broadcast at selected frequencies
Reactive Power
• Why is it not desirable?
– Transmitting reactive together with real power
power reduces the conductor ampacity,
transformer capacity, and generator capacity
available for the real power.
– It can lead to the overheating of electrical
transmission and distribution equipment.
– The same electrical phenomenon attenuates
signals on wire-based systems.
Reactive Power
• How do we control it?
– Limit the amount of lagging reactive power
required from the electrical power system.
– The single largest controllable source of
lagging (inductive) reactive power is:
Reactive Power
• How do we control it?
– The single largest controllable source of
lagging (inductive) reactive power is:
LAZY
MECHANICAL
ENGINEERS
Reactive Power
• Reduction by design:
Reactive Power (VAr)
– Don’t oversize motors
Large Motor,
Large Load
Total Power (VA)
Real Power (Watts)
Reactive Power
• Reduction by design:
Reactive Power (VAr)
– Don’t oversize motors
Large Motor,
Small Load
Total Power (VA)
Real Power (Watts)
Reactive Power
• Reduction by design:
Reactive Power (VAr)
– Don’t oversize motors
Small Motor,
Small Load
Total Power (VA)
Real Power (Watts)
Reactive Power
• Reduction by design:
– Select high power factor motors
• These are often high efficiency motors
• Be aware that high power factor motors often have
higher starting current requirements.
• Ensure that the high efficiency or high power factor
motors have the right mechanical characteristics,
such as starting torque, for the load.
Reactive Power
• Reduction by design:
– Use variable frequency drives (VFDs) where
applicable
• Power factor on line side of VFD is usually 0.95 or
greater.
• Reactive power reduction alone won’t justify cost
of the drive, but can be part of the total return on
investment.
• VFDs have rectifier front ends, which will add
harmonic currents to the system.
Reactive Power
• Reduction by design:
– Use synchronous motors for large, constant
speed and load applications
• Synchronous motors can be run as a source of
leading as well as lagging power factor
• A large, constant load is necessary to be able to
recover the added cost of the synchronous motor
and its field controller.
• Typically applied at higher voltages (4160, 13 800).
Reactive Power
• Reduction by design:
– Carefully select lighting ballasts
• Where possible, use electronic ballasts
• Otherwise, select high power factor ballasts.
• When using electronic ballasts, be aware of third
harmonic consideration.
Reactive Power
• Lagging power factor countermeasures:
– Reduce the demand for reactive power
through the measures previously mentioned.
• Reducing the amount of lagging reactive power on
a system has less potential for creating
undesirable conditions than trying to correct it
through adding sources of leading reactive power.
• Reducing reactive power demand will almost
always reduce the real power demand also.
Reactive Power
• Lagging power factor countermeasures:
– Add sources of leading reactive power once
opportunities to reduce lagging power
demand have been addressed.
• Once leading reactive sources have been added to
a power system, tuned “LC” circuits have been
created. Potentially severe and difficult-todiagnose harmonic current flows can result if the
resonant frequency or frequencies coincide with
the fundament or system frequency or its
harmonics.
Reactive Power
• Lagging power factor countermeasures:
– Distribution-level Fixed Capacitors:
• Most economical on a dollars-per-farad basis.
• No control system required
• Least flexible in response to changing system
conditions.
– Load-level Fixed Capacitors
• Very economical, easy to install, no control system,
switches automatically with the load.
Reactive Power
• Lagging power factor countermeasures:
– Distribution-level Switched Capacitors:
• More expensive than fixed banks, but responds to
changes in reactive power demand.
• Control system required, with added cost,
configuration, and maintenance required.
• Electromechanical type is less expensive than
semiconductor switched banks, but it responds
more slowly to load changes, which may be a
concern if avoiding utility penalties is a concern.
Reactive Power
• Lagging power factor countermeasures:
– Active Harmonic Compensation Systems:
• Most expensive on a dollars-per-farad basis.
• Works by “injecting” compensating current into the
power system.
• Highly responsive control system compensates on
a sub-cycle basis for both harmonic and reactive
power demands.
• Under normal configuration, reactive power takes a
back seat to harmonic cancellation.
Reactive Power
• Benefits of reactive power control:
– Better voltage stability
– More efficient use of existing power system.
• May be able to add load without increasing system
ampacities.
– Less heating of electrical equipment
• Extends useful life of equipment.
• Less real power needed to generate that heat.
– Potential reduction in utility charges.
Reactive Power
• Summary:
– Reactive power is inherent in AC systems and
serves some useful purposes.
– Reduce demand for reactive power before
adding capacitors to compensate.
– Select leading reactive power sources by
balancing cost with need for flexibility and
responsiveness.
Reactive Power
• For further reading:
– IEEE Std. 141-1993, Electric Power
Distribution for Industrial Plants, Chapter 8.
• This is a good resource for sizing power factor
correction capacitors.
• Questions and discussion
Reactive Power
For further reading:
IEEE Std. 141-1993, Electric Power
Distribution for Industrial Plants, Chapter
8. This is a good resource for sizing
power factor correction capacitors.
Questions and discussion.