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 up on the following:
i. 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.
ii. 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.
 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.

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.

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:

i. 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 1:2:2:…) 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.
ii. ii. 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

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.

Power Factor Improving:

 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.

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
= 1000 kVA = 8% of KVA