Automatic Power Factor Correction Panel

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• Vision Tekniks

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Automatic Power Factor Correction Panel

• Optimum step size selection guidelines in APFC
  panels
• Major part in the design of APFC panels is the
  selection of step size of capacitor banks and
  number of steps.
• The right selection of step size and number of
  steps plays a significant role in the performance
  as well as cost of the APFC panel.
• This article focuses on the need and ways of good
  step size (maximum and minimum sizes) selection
  and number of steps.

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1. Maximum step size selection

• The maximum capacitor rating in an APFC panel
  depends up on the following
• Maximum amount of load variation that happens in
  the industry at a time Large load variations
  demand bigger capacitor steps so that target
  power factor is achieved in short time, by less
  number of switching operations.
• Current and voltage transient withstand
  capabilities of the system
• Current transients Switching of big capacitors
  (usually above 100 kVAr) introduces large
  magnitude of inrush current (current transients)
  for a small duration. This results in high
  thermal and electrical stress on capacitors,
  short circuit protection devices and mainly, the
  switching device. This may lead to their nuisance
  tripping and premature failures.

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1. Maximum step size selection -- Continued.

• Voltage transients Switching of big capacitors
  may cause transient over voltages, which might
  result in failure of sensitive electronic
  devices.
• Thus, the maximum size of the capacitor step is a
  trade-off between the points i and ii.
  Practically acceptable maximum possible rating in
  any APFC panel shall be 100 kVAr. If this 100
  kVAr rating capacitor is switched using power
  contactor, the peak inrush current may reach as
  high as 75 kA. Hence, the capacitors should be
  switched using either capacitor duty contactors
  or thyristor switching module.
• However, thyristor switching module or capacitor
  duty contactor for 100 kVAr rating is not readily
  available. Hence, the best way to switch a 100
  kVAr bank is by connecting two 50 kVAr TSM /
  contactor in parallel, each with a separate
  physical 50 kVAr capacitor banks. The control
  supply to both the 50 kVAr TSM/contactors shall
  be common (one relay output of the controller),
  so that both of the devices are triggered at the
  same time. To achieve this, the common output of
  the APFC relay should be programmed as 100 kVAr.
  Hence, physically they are two separate 50 kVAr
  banks, whereas electrically they behave as a
  single 100 kVAr bank.

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2. Minimum step size selection

• The minimum capacitor rating depends on how
  precise the power factor needs to be maintained.
  This minimum kVAr rating depends upon the minimum
  current sensitivity (typically around 2.5) of
  the APFC controller. However the change in PF due
  to these minimum selected kVAr rating would be
  usually in the 3rd or 4th decimal places
  (depending on panel size or kVAr requirement),
  whereas the electricity board is concerned only
  with the first two digits of PF (like 0.99).
• Typically many industries want the desired target
  PF to be unity (exactly 1.00), in order to get
  incentives from some EBs. But practically, the
  optimum target PF has to be 0.96 to 0.99. These
  are healthy levels of power factor as it will
  have safety margin that avoids the leading power
  factor as well as any dangerous harmonic
  amplification (due to more capacitance in the
  system).
• Hence, as a rule of thumb, the minimum kVAr
  rating in an APFC panel can be chosen to be 5-10
  of overall rating of APFC panel. For smaller
  rating APFC panels (up to 100 kVAr), the smallest
  step can be 5 kVAr and for bigger rating APFC
  panels (above 600 kVAr and up to 1000 kVAr) the
  minimum rating can be 25 kVAr.

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3. Number of steps / branches in APFC panels

• Once the minimum and maximum kVAr rating of the
  APFC panel is selected, the number of branches
  can be decided upon based of the following
• Technology of APFC controllers Latest APFC
  controllers like etaCON L Series employ self
  optimized intelligent switching where the
  controller calculates the exact kVAr requirement
  and switches ON / OFF the appropriate capacitors
  irrespective of the capacitors already in
  circuit. Traditional controllers employing linear
  or circular switching require more number of
  smaller steps (like 122) for effective power
  factor correction. Whereas the latest controllers
  like etaCON can have a mix of large and small
  steps reducing the number of steps in capacitor
  bank as well as the cost of associated
  switchgear.
• Size cost of APFC panel More the number of
  steps more will be the cost of APFC panel, due to
  more number of switchgear, bigger size of panel
  and others.

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An APFC panel should have

• a) Maximum number of electrical steps
  (combination of physical steps) to ensure more
  accurate and flexible power factor correction
• b) Minimum number of physical steps to reduce the
  size and cost of the panel

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Summary

• APFC panels with more number of steps (more than
  12) does not always mean better step resolution.
• With the latest APFC controller technology, fewer
  steps are sufficient to achieve better step
  resolution and control of power factor
• Hence, usually a combination of large, small and
  very small ratings is used in steps of capacitor
  banks, as given below
• 1. Large rating capacitors (100 kVAr maximum)
  are required to PF compensation of base load and
  coarse power factor compensation
• 2. Medium rating capacitors are meant for
  variable part of the reactive power
  compensation.
• 3. Small rating capacitors (usually 5 to 10 of
  total kVAr or 25 kVAr for panels above 500 kVAr)
  are meant for fine tuning of the power factor.

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• Please check if required kVAr of capacitors are
  installed.
• Check the type of capacitor installed is suitable
  for application or the capacitors are de rated.
• Check if the capacitors are permanently ON. The
  Capacitor are not switched off
• When the load is not working, under such
  condition the average power factor is found to be
  lower side.
• Check whether all the capacitors are operated in
  APFC depending upon the load operation.
• Check whether the APFC installed in the
  installation is working or not. Check the CT
  connection is taken from the main incomer side of
  transformer, after the fix compensation of
  transformer.
• Check if the load demand in the system is
  increased.
• Check if power transformer compensation is
  provided.

Power Factor Improving
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• Thumb Rule if HP is known.
• The compensation for motor should be calculated
  taking the details from the rating plate of motor
  Or
• the capacitor should be rated for 1/3 of HP
• Kvar Required For Transformer Compensation
• Transformer                                      
      Required Kva
• lt 315 kVA  T.C                           
      5 of  KVA
• 315kVA To 1000 kVA                        6
  of  KVA
• gt 1000 kVA                                 
     8 of  KVA

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• Thumb Rule if HP is known.
• The compensation for motor should be calculated
  taking the details from the rating plate of motor
  Or
• the capacitor should be rated for 1/3 of HP
• Kvar Required For Transformer Compensation
• Transformer                                      
      Required Kva
• lt 315 kVA  T.C                           
      5 of  KVA
• 315kVA To 1000 kVA                        6
  of  KVA
• gt 1000 kVA                                 
     8 of  KVA