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Optimum step size selection guidelines in APFC panels

Optimum step size selection guidelines in APFC panels
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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:


1. Maximum step size selection:


The maximum capacitor rating in an APFC panel depends upon the following:

a. 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.


b. 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 result 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.
  • Voltage transients: Switching of big capacitors may cause transient over voltage, which might result in failure of sensitive electronic devices.


Thus, the maximum size of the capacitor step is a trade-off between the points a & b. Practically acceptable maximum possible rating in any APFC panel shall be 100 kVAr. If this100 kVAr rating capacitor is switched using power contactor, the peak inrush current may reach as high as 75 kA. Hence, the capacitor should be switched using either capacitor duty contactors or thyristor switching module.


However, 100 kVAR Capacitor duty contactor can be used for 100 kVAR bank but in the case of a thyristor switched panel, thyristor switching module for 100 kVAR rating is not readily available. Hence the best way to switch a 100 kVAr bank is by connecting two 50kVAr TSM in parallel, each with a separate physical 50 kVAr capacitor banks. The control supply to both the 50 kVAr TSM 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.


A sample connection diagram for switching a step of 100 kVAr capacitor using two 50 kVAr TSM is shown as under:

Connection Diagram

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, minimum step size can be decided based on the following:

  1. For the continuously loaded transformers, throughout the month, the minimum kVAr rating step can be around 5 to 10% of the APFC panel kVAr rating. For example, 5 kVAr step for 100 kVAr APFC panel and 25 kVAr step for 300 kVAr APFC panel.
  2. If the transformer is lightly loaded during any time period of the day (like office buildings / industries), smaller kVAr ratings (1 kVAr to 5 kVAr) may be chosen depending up on the minimum load current. In addition to this, MV / HT sensing CTs can help to achieve close to Unity power factor at light-load conditions.

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:


  1. Technology of AFFC controllers: Latest APFC controllers like etaULTRA 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 1:2:2:...) for effective power factor correction. Whereas the latest controllers like etaULTRA can have a mix of large and small steps reducing the number of steps in capacitor bank as well as the cost of associated witchgear.
  2. 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.


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


Let us consider a few examples of step size selection in APFC panels.

I. 100 kVAr APFC panel

Case A Cases B
Step configuration 10 + 10 + 10+ 10+ ...10 times 50 + 25 + 15 + 5 + 5
Step resolution 10 kVAr 5 kVAr
Electrical Steps 10, 20, 30, 40, 50, ..., 100 10, 20, 30, 40, 50, ..., 100
Physical Steps 10 5
No. of Electrical Steps 10 20

In Case B, 20 electrical steps are possible with only 5 physical steps; whereas in Case A, 10 physical steps are required to achieve 10 electrical steps.


II. 300 kVAr APFC panel

Case A Cases B
Configuration 25 + 25 + 25 + 25 +, ..., 12 times 1x100 + 2x50 + 4x25
Step resolution 25 kVAr 25 kVAr
Electrical Steps 25, 50, 75, 100, 125, ..., 275, 300 25, 50, 75, 100, 125, ..., 275, 300
Physical Steps 12 7
No. of Electrical Steps 12 12

Case B is better as same electrical steps are achieved with just 7 physical steps.


III. 600 kVAr APFC panel

Case A Cases B
Configuration 50 + 50 + 50 + 50 + ... + 12 times 3x100 + 5x50 + 2x25
Step resolution 50 kVAr 25 kVAr
Electrical Steps 50, 100, 150, 200, 250, ..., 600 50, 100, 150, 200, 250, ..., 600
Physical Steps 12 10
No. of Electrical Steps 12 24

In Case B, 24 electrical steps are possible with only 10 physical steps; whereas in Case A, 12 physical steps are required to achieve 12 electrical steps.

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 for 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.

Considering the above points, following table suggests optimum number of steps and step sizes from 35 kVAr to 1000 kVAr. Beyond 1000 kVAr, it is always better to split the APFC panels and install them at different PCC / MCC levels.

Panel Rating (kVAr)Physical Step size (kVAr)No. of Physical StepsElectrical / Logical StepsNo. of Electrical Steps
35 15 + 10 + 2x5 4 5, 10, 15, 20, 25, 30, 35 7
50 25 + 2x10 + 5 4 5, 10, 15, 20, 25, 30, 35, 40, 45, 50 10
75 2x25 + 2x10 + 5 5 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, …., 70, 75 15
100 50 + 25 + 15 + 2x5 5 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, …., 95, 100 20
125 50 + 2x25 + 2x10 + 5 6 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, …., 120, 125 25
150 50 + 3x25 + 15 + 10 6 10, 15, 25, 35, 40, 50, 60, 65, 75, ….., 140, 150 18
175 2x50 + 2x25 + 15 + 10 6 10, 15, 25, 35, 40, 50, 60, 65, 75, ….., 165, 175 21
200 3x50 + 25 + 15 + 10 6 10, 15, 25, 35, 40, 50, 60, 65, 75, ….., 190, 200 24
225 3x50 + 2x25 + 15 + 10 7 10, 15, 25, 35, 40, 50, 60, 65, 75, ….., 215, 225 27
250 3x50 + 4x25 7 25, 50, 75, 100, 125, 150, 175, 200, 225, 250 10
275 4x50 + 3x25 7 25, 50, 75, 100, 125, 150, 175, …., 250, 275 11
300 100 + 2x50 + 4x25 7 25, 50, 75, 100, 125, 150, 175, …., 275, 300 12
350 100 + 3x50 + 4x25 8 25, 50, 75, 100, 125, 150, 175, …., 325, 350 14
400 2x100 + 2x50 + 4x25 8 25, 50, 75, 100, 125, 150, 175, …., 375, 400 16
450 2x100 + 3x50 + 4x25 9 25, 50, 75, 100, 125, 150, 175, …., 425, 450 18
500 2x100 + 5x50 + 2x25 9 25, 50, 75, 100, 125, 150, 175, …., 475, 500 20
550 3x100 + 3x50 + 4x25 10 25, 50, 75, 100, 125, 150, 175, …., 500, 550 22
600 3x100 + 5x50 + 2x25 10 25, 50, 75, 100, 125, 150, 175, …., 575, 600 24
650 4x100 + 3x50 + 4x25 11 25, 50, 75, 100, 125, 150, 175, …., 625, 650 26
700 4x100 + 5x50 + 2x25 11 25, 50, 75, 100, 125, 150, 175, …., 675, 700 28
750 5x100 + 3x50 + 4x25 12 25, 50, 75, 100, 125, 150, 175, …., 725, 750 30
800 5x100 + 5x50 + 2x25 12 25, 50, 75, 100, 125, 150, 175, …., 775, 800 32
850 6x100 + 3x50 + 4x25 13 25, 50, 75, 100, 125, 150, 175, …., 825, 850 34
900 6x100 + 5x50 + 2x25 13 25, 50, 75, 100, 125, 150, 175, …., 875, 900 36
950 7x100 + 3x50 + 4x25 14 25, 50, 75, 100, 125, 150, 175, …., 925, 950 38
1000 7x100 + 5x50 + 2x25 14 25, 50, 75, 100, 125, 150, 175, …., 975, 1000 40

The above table is for illustration only, which may be applicable for majority of industries. For accurate compensation, step ratings shall be decided after studying the Load Profile of the Industry.

Disclaimer

For accurate compensation, the load profile of the industry needs to be studied. This gives an indication of the size of peak loads, base loads, possible harmonics, power factor and thus, helps determine step sizes based on load variations. In some cases, the load requirements can also be established from equipment lists.

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About the Author

author

Abhinav L Purkar,

Manager Product Management & Marketing (PMM)

Abhinav Purkar is a Product Management, Marketing, and Application Engineering professional with over 10 years of experience in the Power Quality domain. He has extensive expertise in power quality analysis, harmonic mitigation, reactive power compensation, energy efficiency, and electrical system optimization, helping customers improve system reliability and operational performance. With proven experience in product lifecycle management, market development, portfolio growth, and go-to-market strategy, Abhinav has successfully driven initiatives that enhance customer value and strengthen market presence. His strong technical foundation, combined with strategic business acumen, enables him to bridge the gap between customer requirements and innovative product offerings.

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