Power Engineering Society Chicago Chapter

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Power Engineering SocietyChicago Chapter

• Reactive Power Sources and Solutions
• 12 February 2003
• David E. Mertz, PE
• Burns McDonnell Engineers, Inc

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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.

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Reactive Power

• Where does it come from?
• All conductors are an inductor.

• Multiple conductors are inductors with a mutual
  capacitance.

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Reactive Power

• Where does it come from?
• Placing conductors in a magnetic raceway
  increases their inductance.

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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.

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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.

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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.

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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

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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.

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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

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Reactive Power

• How do we control it?
• The single largest controllable source of lagging
  (inductive) reactive power is

LAZY MECHANICAL ENGINEERS
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Reactive Power

• Reduction by design
• Dont oversize motors

Large Motor, Large Load
Reactive Power (VAr)
Total Power (VA)
Real Power (Watts)
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Reactive Power

• Reduction by design
• Dont oversize motors

Large Motor, Small Load
Reactive Power (VAr)
Total Power (VA)
Real Power (Watts)
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Reactive Power

• Reduction by design
• Dont oversize motors

Small Motor, Small Load
Reactive Power (VAr)
Total Power (VA)
Real Power (Watts)
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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.

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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 wont 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.

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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).

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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.

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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.

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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-to-diagnose harmonic current flows can
  result if the resonant frequency or frequencies
  coincide with the fundament or system frequency
  or its harmonics.

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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.

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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.

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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.

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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.

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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.

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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

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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.