APFC Panel Step Size Selection Procedure

The selection of step size and the number of capacitor steps is one of the most important aspects in the design of Automatic Power Factor Correction (APFC) panels. 

Proper selection directly impacts 

• Power factor correction accuracy, 
• System stability, 
• Equipment life and 
• Overall panel cost.

An optimized design ensures an efficient reactive power compensation while minimizing switching stress & investment cost.

APFC Panel Step Size Selection

Stage-1: Maximum Step Size Selection

The maximum capacitor step size depends on 

• Load variation, 
• Current transients and 
• Voltage transients.

Large load variations need larger capacitor steps.

Switching large capacitors (>100 kVAr) produces high inrush current up to 75 kA.

Large switching may cause voltage transients affecting sensitive equipment.

Recommendation

Maximum step size must be 100 kVAr.

Use capacitor duty contactors (or) thyristor switching modules.

Practically, utilize 2 x 50 kVAr banks in parallel for a 100 kVAr step.

Stage-2: Minimum Step Size Selection

Minimum step size determines accuracy of power factor (PF) correction.

APFC relay sensitivity ~2.5%.

Electricity boards consider power factor (PF)  up to 0.99.

Ideal power factor (PF) range: 0.96 to 0.99.

Thumb Rule

Minimum step size = 5% to 10% of total kVAr.

Panel Size	Minimum Step
Up to 100 kVAr	5 kVAr
100–500 kVAr	10–25 kVAr
500–1000 kVAr	25 kVAr

Stage-3: Number of Steps in APFC Panels

Number of steps in APFC panels depends on controller technology and cost.

Conventional controllers need more steps.

Intelligent controllers allow for mixed step sizes and fewer steps.

Design Objectives

• Maximum electrical combinations.

• Minimum physical steps.

The number of steps must balance:

• Accuracy
• Cost
• Panel size

Controller Technology Impact

Conventional Controllers

• Need more steps
• Follow linear (or) circular switching

Advanced Controllers (Intelligent)

• Use optimized switching logic.
• Allow mixed step sizes.
• Reduce number of steps.

Stage-4: Design Objectives

An ideal APFC panel should have:

Maximum Electrical Steps

• Improves flexibility
• Enhances PF correction accuracy

Minimum Physical Steps

Reduces:

• Panel size
• Cost
• Maintenance

Stage-5: Step Configuration Examples

For 100 kVAr Panel

Case A:

• 10 steps of 10 kVAr
• 10 physical steps
• 10 electrical steps

Case B:

• 50 + 25 + 15 + 5 + 5
• 5 physical steps
• 20 electrical steps

Here, Case B is more efficient.

For 300 kVAr Panel

Case A:

• 25 kVAr x 12
• 12 physical steps

Case B:

• 100 + 3 x 50 + 2 x 25
• 6 physical steps

Case B reduces the hardware while maintaining performance.

For 600 kVAr Panel

Case A:

• 50 kVAr x 12
• 12 steps

Case B:

• 3 x 100 + 5 x 50 + 2 x 25
• 10 physical steps
• 24 electrical combinations

Case B provides higher resolution with fewer steps.

Stage-6: Step Combination Strategy

Large Steps (Up to 100 kVAr)

• Used for the base load compensation

Medium Steps

• Used for a variable load correction

Small Steps (5–10% or 25 kVAr)

• Used for the fine tuning

General Recommendations Table

Panel Rating	Steps	Suggested Combination
100 kVAr	5	50 + 25 + 15 + 5 + 5
300 kVAr	6	100 + 3 x 50 + 2 x 25
600 kVAr	10	3 x 100 + 5 x 50 + 2 x 25
1000 kVAr	12	Mixed (100, 50, 25 kVAr)

APFC Panel Step Size Selection Table

This table is primarily suitable for most of the industrial applications.

Final step selection should always be based on:

• Load profile study,
• Harmonic conditions and
• Switching technology.

Panel Rating (kVAr)	Physical Step Size (kVAr)	No. of Physical Steps	Electrical / Logical Steps (kVAr)	No. of Electrical Steps
35	2×12.5 + 2×5	4	5, 10, 12.5, 17.5, 22.5, 25, 30, 35	8
50	2×12.5 + 2×10 + 1×5	5	5, 10, 12.5, 15, 17.5, 20, 22.5, 25, 27.5, 30, 35, 37.5, 40, 45, 50	15
75	2×25 + 2×10 + 1×5	5	5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 70, 75	13
100	2×25 + 2×15 + 1×5	5	5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 95, 100	13
125	2×12.5 + 2×25 + 50	5	12.5, 25, 37.5, 50, 62.5, 75, 87.5, 100, 112.5, 125	10
150	2×12.5 + 2×25 + 50	6	12.5, 25, 37.5, 50, 62.5, 75, 87.5, 137.5, 150	12
175	2×12.5 + 2×25 + 50	6	12.5, 25, 37.5, 50, 62.5, 75, 87.5, 162.5, 175	14
200	2×12.5 + 2×25 + 50	6	12.5, 25, 37.5, 50, 62.5, 75, 87.5, 187.5, 200	16
225	2×12.5 + 2×25 + 50	6	12.5, 25, 37.5, 50, 62.5, 75, 87.5, 212.5, 225	18
250	4×50 + 2×25	6	25, 50, 75, 100, 125, 150, 175, 200, 225, 250	10
275	1×50 + 1×25	6	25, 50, 75, 100, 125, 150, 175, 200, 225, 250, 275	11
300	1×100 + 3×50 + 2×25	6	25, 50, 75, 100, 125, 150, 175, 200, 275, 300	12
350	1×100 + 3×50 + 4×25	8	25, 50, 75, 100, 125, 150, 175, 200, 325, 350	14
400	2×100 + 2×50 + 4×25	8	25, 50, 75, 100, 125, 150, 175, 200, 375, 400	16
450	2×100 + 3×50 + 2×25	8	25, 50, 75, 100, 125, 150, 175, 200, 425, 450	18
500	2×100 + 4×50 + 2×25	8	25, 50, 75, 100, 125, 150, 175, 200, 475, 500	20
550	3×100 + 3×50 + 2×25	10	25, 50, 75, 100, 125, 150, 175, 200, 525, 550	22
600	3×100 + 5×50 + 2×25	10	25, 50, 75, 100, 125, 150, 175, 200, 575, 600	24
650	4×100 + 4×50 + 2×25	10	25, 50, 75, 100, 125, 150, 175, 200, 625, 650	26
700	4×100 + 5×50 + 2×25	12	25, 50, 75, 100, 125, 150, 175, 200, 675, 700	28
750	5×100 + 3×50 + 2×25	12	25, 50, 75, 100, 125, 150, 175, 200, 725, 750	30
800	5×100 + 5×50 + 2×25	12	25, 50, 75, 100, 125, 150, 175, 200, 775, 800	32
850	6×100 + 4×50 + 2×25	12	25, 50, 75, 100, 125, 150, 175, 200, 825, 850	34
900	6×100 + 5×50 + 2×25	12	25, 50, 75, 100, 125, 150, 175, 200, 875, 900	36
950	7×100 + 4×50 + 2×25	12	25, 50, 75, 100, 125, 150, 175, 200, 925, 950	38
1000	8×100 + 2×50 + 2×25	12	25, 50, 75, 100, 125, 150, 175, 200, 975, 1000	40

Important Design Guidelines

• More steps do not always have a mean better performance
• Intelligent controllers reduce the required steps
• Always mix:
  • Large
  • Medium
  • Small capacitors
• Avoid the overcompensation (leading PF)

• Also consider harmonic conditions
• Maximum step size must not exceed 100 kVAr
• Minimum step size must be 5–10% of total capacity
• Use a mixed step configuration
• Prefer intelligent APFC controllers
• Always analyze load profile before design

Conclusion

The optimum selection of capacitor step size and number of steps is essential for achieving an efficient and economical APFC panel performance.

A balanced combination of different step sizes that ensures an accurate

• Power factor correction, 
• Reduced switching stress and 
• Optimized system cost. 

For better results detailed load analysis and modern controller selection are highly recommended.