Normal APFC vs Detuned APFC: 7% vs 14% Reactor

A normal APFC panel switches capacitor steps directly onto the bus. A detuned APFC panel puts a reactor in series with every step, which shifts the tuning frequency of the capacitor-reactor combination below the dominant harmonic order so the bank cannot resonate with the supply system at that frequency. Which of the two a plant needs is decided by the harmonic content of the network, not by the kVAr figure in the enquiry.

On a 50 Hz system, a 7% reactor tunes the step at approximately 189 Hz — tuning order 3.8, below the 250 Hz fifth harmonic. A 14% reactor tunes at approximately 135 Hz — order 2.7, below the 150 Hz third harmonic. Neither percentage is a grade of quality and neither is a percentage of harmonic reduction: they are reactance ratios that place the bank at two different points on the impedance curve, with different capacitor voltage, reactor current and heat as a consequence.

The Question Behind “200 kVAr APFC Panel”

A customer asks for:

“200 kVAr APFC Panel.”

One manufacturer quotes a normal capacitor bank.

Another quotes a 7% detuned APFC panel.

A third recommends a 14% detuned reactor.

All three quotations may have completely different prices.

So which one is correct?

The answer cannot be decided from kVAr alone.

The correct APFC design depends on the electrical network, especially:

Existing harmonic levels
VFD and UPS loading
Transformer size and impedance
Nonlinear loads
Capacitor-bank location
Dominant harmonic orders
Required power factor
Load variation
Existing capacitor banks
Future expansion

The most important principle is:

An APFC panel should not be selected only to improve power factor. It must also be compatible with the harmonic conditions of the electrical network.

In an electrical system containing significant nonlinear loads, installing ordinary capacitors without checking harmonics can create resonance and actually make the electrical system worse.

IEC 61642 provides guidance specifically for the application of shunt capacitors and passive harmonic filters in industrial AC networks affected by harmonics. IEC 61921 covers low-voltage AC shunt capacitor banks used for power-factor correction.

This guide explains:

What a normal APFC panel does
What a detuned APFC panel does
Why reactors are used
What 7% and 14% actually mean
Why 14% is not simply “better” than 7%
How VFDs affect APFC selection
How resonance occurs
Why capacitor voltage changes with a reactor
What engineers should specify in an APFC RFQ
What should be checked during FAT

What Is an APFC Panel?

APFC means:

Automatic Power Factor Correction

The panel automatically switches capacitor steps ON and OFF according to the reactive-power requirement of the electrical system.

A typical APFC system contains:

Main incomer
APFC controller
Current transformer input
Capacitor steps
Switching contactors or other switching devices
Protection
Busbars
Ventilation
Indication and metering
Detuned reactors where required

Its primary purpose is to supply part of the reactive power required by inductive loads locally instead of drawing all of that reactive power from the upstream network.

Industrial loads that can create reactive-power demand include:

Induction motors
Transformers
Pumps
Fans
Compressors
Conveyors
HVAC equipment
Other inductive loads

But improving power factor is only one part of APFC engineering.

In modern factories containing power-electronic loads, harmonics must also be considered.

Normal APFC vs Detuned APFC — Quick Comparison

Parameter	Normal APFC	Detuned APFC
Capacitor steps	Yes	Yes
Series reactor	No	Yes
Power factor correction	Yes	Yes
Suitable for clean/low-harmonic network	Often	Can be
Harmonic resonance protection	Limited	Designed specifically to reduce resonance/amplification risk
Suitable for significant nonlinear loading	Requires careful assessment	Often preferred after engineering assessment
Panel heat	Lower	Higher
Panel size	Smaller	Larger
Cost	Lower	Higher
Reactor thermal protection	N/A	Typically required
Capacitor voltage stress from detuning	Normal network-related duty	Must account for reactor-induced voltage rise
Engineering complexity	Lower	Higher

What Is a Normal APFC Panel?

A normal APFC panel typically connects capacitor steps directly to the electrical bus through:

Protection → Switching Device → Capacitor

There is no series detuning reactor.

For an electrical network with sufficiently low harmonic pollution and no problematic resonance condition, this can be a simple and economical solution.

Advantages include:

Lower initial cost
Smaller panel
Lower heat generation
Simpler maintenance
Lower losses
Simpler component arrangement

But there is an important condition:

The electrical network must be suitable for direct capacitor application.

A normal APFC panel should not be selected simply because:

“We have always used normal APFC.”

Electrical systems change.

A plant that previously contained mostly DOL and Star-Delta motors may later add:

VFDs
UPS systems
Rectifiers
Welding equipment
Chargers
Power electronics
LED loads

The harmonic environment can therefore change significantly over time.

Most of the detuned retrofits we are asked to quote follow exactly this history. A chemical or pharma plant installs a normal 250 kVAr bank behind a 1600 kVA transformer when the load is almost all DOL and star-delta motors, then adds 300 to 500 kW of drives over the next few years for pumps, blowers and dryers. The bank was correctly engineered on the day it was installed; the network it sits in is no longer the same network. Our recommendation: re-measure the spectrum whenever nonlinear load grows past roughly a quarter of the transformer rating, rather than waiting for capacitors to start failing.

What Is a Detuned APFC Panel?

In a detuned APFC panel, a reactor is connected in series with each capacitor step.

Typical arrangement:

Bus → Protection → Switching Device → Reactor → Capacitor

The reactor and capacitor together create an LC circuit.

The combination is deliberately tuned to a frequency below the harmonic order that the design intends to avoid interacting with.

The series reactor shifts the capacitor-bank tuning frequency below the dominant harmonic, so that the combination becomes inductive above its tuning frequency. That is what helps avoid harmful parallel resonance with the supply system.

The word detuned is important.

The bank is not normally designed to attract a particular dominant harmonic in the way a tuned harmonic filter would.

It is deliberately positioned away from the dangerous resonance region.

Why Can Capacitors Create Harmonic Problems?

A capacitor itself does not create the main nonlinear harmonic current in an industrial network.

However, adding capacitance changes the network impedance.

The capacitor bank and upstream system inductance—often dominated by transformer/source impedance—can create a resonant circuit.

If that resonance frequency lies near a harmonic already present in the network, the harmonic can be amplified. The consequences typically show up as:

Capacitor overloading
Transformer/cable/switchgear overloading
Increased voltage distortion
Higher losses
Nuisance operation of protection equipment

This is why a capacitor bank can fail even though:

The calculated power factor correction kVAr was correct.

The reactive-power calculation may have been correct.

The harmonic engineering may not have been.

Where Do Harmonics Come From?

Typical nonlinear industrial loads include:

Variable Frequency Drives
UPS systems
Rectifiers
Battery chargers
Switched-mode power supplies
Welding equipment
Some lighting systems
Other power-electronic converters

A conventional six-pulse VFD commonly produces characteristic lower-order harmonics dominated by the 5th and 7th orders on its input side.

But the actual harmonic spectrum at the APFC bus depends on the complete electrical installation, not merely one VFD.

Therefore, the correct question is not:

“Do we have VFDs?”

It is:

“What harmonic spectrum exists at the location where we intend to connect the capacitor bank?”

What Does “7% Reactor” Actually Mean?

This is one of the most misunderstood APFC terms.

A 7% reactor does NOT mean:

7% harmonic reduction
7% THD
7% filtering efficiency
Only 7% of harmonics are allowed

The percentage is the detuning factor, representing the relationship between the reactor's inductive reactance and the capacitor's capacitive reactance at the fundamental frequency.

p = XL / XC × 100%

where:

XL = reactor inductive reactance
XC = capacitor reactance
p = detuning factor

This relationship determines the tuning frequency of the reactor-capacitor combination.

7% Reactor at 50 Hz — What Does It Mean?

For a 50 Hz system, a 7% detuned combination has a tuning frequency of approximately:

189–190 Hz

or a tuning order of approximately:

3.8

The 5th harmonic in a 50 Hz system is:

5 × 50 = 250 Hz

Therefore the 7% combination is tuned below the 5th harmonic.

This is why 7% detuning is widely encountered in industrial networks where 5th-harmonic behaviour is a major consideration.

What Does “14% Reactor” Mean?

A 14% detuned reactor uses a larger reactance relative to the capacitor.

For a 50 Hz system, a 14% detuned reactor range typically works out at approximately:

135 Hz

with a tuning order of:

2.7.

The tuning frequency is therefore well below:

3rd harmonic = 150 Hz

as well as below the 5th harmonic.

This does not mean that a 14% reactor is automatically better.

It means it creates a different electrical impedance characteristic.

7% vs 14% Reactor — Practical Comparison

Parameter	7% Detuned	14% Detuned
50 Hz tuning frequency	~189–190 Hz	~134–135 Hz
Approx. tuning order	3.8	2.7
Position relative to 5th harmonic	Below 5th	Well below 5th
Position relative to 3rd harmonic	Above 3rd	Below 3rd
Reactor impedance	Lower	Higher
Fundamental capacitor-voltage rise	Lower	Higher
Physical reactor duty/impact	Lower	Generally greater
Selection basis	Harmonic spectrum + manufacturer/application design	Harmonic spectrum + manufacturer/application design

Therefore:

Never specify 14% simply because 14 is greater than 7.

A higher detuning percentage changes the tuning frequency, capacitor voltage, reactor current, losses and thermal design.

The correct value should come from the electrical network and the selected capacitor/reactor design.

Why 7% Is Common in VFD-Heavy Plants

Consider a plant with many conventional six-pulse VFDs.

If the dominant harmonic concern is around the 5th harmonic, a 7% reactor creates a resonant point at approximately 189 Hz in a 50 Hz system—below the 250 Hz fifth harmonic.

But this should still not become a blanket rule:

“VFD present = always 7% reactor.”

A proper assessment should also consider:

3rd harmonic
7th harmonic
11th/13th harmonics
Total nonlinear load
Source impedance
Existing capacitors
Transformer rating
System operating modes

When Should 14% Be Considered?

A 14% / 2.7-order reactor shifts the tuning frequency down to approximately 135 Hz on a 50 Hz network.

Because this lies below the 150 Hz third harmonic, it gives a significantly different response to lower-order harmonic content than a 7% system.

This is one reason lower tuning orders can be considered where the measured harmonic spectrum or system design requires the capacitor bank to be detuned below lower-order harmonics.

However, the presence of third harmonic alone does not justify blindly specifying 14%.

The engineer must examine:

Where the third harmonic is flowing
Network topology
Transformer connection
Neutral system
Capacitor connection
Actual voltage/current spectrum
Manufacturer's application limits

Published reactor application data assigns different allowable and design harmonic-current assumptions to 14%, 7% and 5.7% reactor systems, which reinforces that they are different engineered solutions rather than interchangeable options.

The Reactor Also Increases Voltage Across the Capacitor

This is another extremely important point.

When a reactor is connected in series with the capacitor, the capacitor terminal voltage at fundamental frequency becomes higher than the network voltage.

For an ideal series LC detuned arrangement, the fundamental-frequency relationship is approximately:

VC = VS / (1 − p)

where p is the detuning factor expressed as a decimal.

This gives an important theoretical comparison.

7% Detuning

For:

p = 0.07

capacitor voltage becomes approximately:

1 / 0.93 = 1.075 times network voltage

So on an ideal 400 V system:

VC ≈ 430 V

before considering supply tolerance and harmonic stresses.

14% Detuning

For:

p = 0.14

capacitor voltage becomes approximately:

1 / 0.86 = 1.163 times network voltage

So on an ideal 400 V system:

VC ≈ 465 V

before considering other operating stresses.

This does not mean you should simply buy a 430 V capacitor for 7% or a 465 V capacitor for 14%.

The final capacitor-rated voltage must be selected as part of the matched capacitor-reactor system considering:

Network voltage
Voltage tolerance
Detuning
Harmonic loading
Manufacturer design
Operating conditions

Capacitor selection therefore changes with the tuning order and the harmonic environment, rather than following the nominal network voltage alone.

This Is Why Capacitor Voltage Cannot Be Selected Independently

A common purchasing mistake is:

“Network is 415 V, so use 440 V capacitor.”

That may or may not be correct.

For a detuned bank, the engineer should first know:

Detuning factor
Effective required kVAr
Capacitor/reactor combination
Network voltage
Maximum voltage
Harmonic spectrum
Manufacturer's matched design

Only then should the capacitor voltage be finalized.

A 7% and 14% APFC bank delivering the same useful reactive-power output may require different capacitor and reactor selections.

Nameplate Capacitor kVAr vs Effective APFC Output

Another point frequently missed during quotation comparison:

The capacitor nameplate kVAr and the effective reactive-power output of the detuned step at the system voltage are not always identical.

The series reactor changes the electrical conditions across the capacitor.

Manufacturers therefore often specify detuned systems in terms of the effective filter/correction output required at the network bus, while selecting an appropriate matched capacitor internally. Reactor data is normally published as detuned-reactor data using effective filter output alongside the reactor and capacitor design parameters.

Therefore, when a customer asks for:

300 kVAr APFC

the important requirement is:

300 kVAr effective compensation at the specified network operating condition

not simply:

The arithmetic total of capacitor-can nameplates must equal 300 kVAr.

This becomes especially important when comparing quotations.

Normal APFC vs Detuned APFC With VFDs

Consider two factories.

Factory A

Mostly DOL motors
Few electronic loads
Low measured harmonic distortion
Stable transformer-fed network

A normal APFC bank may be perfectly reasonable after confirming the network condition.

Factory B

Large number of VFDs
UPS systems
Electronic rectifier loads
Significant measured 5th/7th harmonic content

Directly connecting a normal capacitor bank could create or worsen a resonance problem.

A detuned APFC arrangement should therefore be evaluated.

Does Every Plant With a VFD Need Detuned APFC?

No.

One 2.2 kW VFD in a plant with a 2000 kVA transformer does not create the same harmonic environment as:

100 × large VFDs operating simultaneously.

The correct decision depends on:

VFD total kW
VFD loading
Transformer kVA
Source impedance
Other nonlinear loads
Existing harmonic levels
Capacitor-bank size
Operating configuration

This is why the best answer comes from:

measurement or harmonic assessment

rather than counting the number of VFDs.

Detuned APFC Is Not the Same as a Harmonic Filter

This is critical.

A detuned capacitor bank is primarily intended to:

Provide reactive-power compensation while reducing the risk of resonance and harmful harmonic amplification.

It should not automatically be marketed as:

“This panel will remove all harmonics.”

If the objective is to substantially reduce harmonic currents or achieve a specific power-quality target, the project may require:

Passive tuned filters
Active harmonic filters
Low-harmonic VFDs
Active-front-end drives
Multi-pulse arrangements
Combination solutions

depending on the actual network.

Detuned Reactor vs Active Harmonic Filter

These are also different devices.

Detuned Reactor

Used in series with the PFC capacitor bank.

Primary objective:

Safe/reactively useful capacitor operation in a harmonic environment and avoidance of problematic resonance.

Active Harmonic Filter

Uses power electronics to measure and inject compensating harmonic currents.

Primary objective:

Actively reduce selected harmonic-current distortion in the electrical network.

It is entirely possible for a plant to require:

Detuned APFC + Active Harmonic Filter

because the two devices solve different problems.

APFC and True Power Factor

Another important concept is the difference between:

Displacement Power Factor

and

True Power Factor

Capacitors primarily compensate fundamental-frequency reactive power.

In a highly distorted network, simply improving the displacement angle does not necessarily eliminate distortion-related effects.

Therefore, if a plant has:

High harmonic current
High THDi
Significant distortion

the APFC controller displaying:

0.99 PF

does not automatically mean:

The overall power-quality problem has been solved.

Reactive-power compensation and harmonic mitigation should be treated as related but different engineering tasks.

How to Calculate Required APFC kVAr

For a conventional power-factor correction calculation:

Required kVAr = kW × (tan φ1 − tan φ2)

where:

φ1 corresponds to existing power factor
φ2 corresponds to target power factor

For example, suppose:

Actual load = 500 kW

Existing PF:

0.75

Target PF:

0.98

The calculation can determine the approximate fundamental reactive power required.

But this calculation alone still does not answer:

Normal or detuned?
7% or 14%?
Capacitor voltage?
Step configuration?
Harmonic filter required?

Those require additional electrical-system information.

Do Not Size APFC From Transformer kVA Alone

A common shortcut is:

“Transformer is 1000 kVA, therefore install 400 kVAr APFC.”

This may result in overcompensation or poor step utilization.

APFC should ideally be selected from actual or expected:

kW demand
Existing PF
Target PF
Load profile
Minimum load
Maximum load
Transformer reactive consumption if it is to be compensated
Future expansion

A 1000 kVA transformer may serve a plant operating at only 300 kW for much of the day.

A large fixed capacitor bank designed only from transformer nameplate data can therefore behave poorly.

APFC Step Selection Matters

Suppose the required APFC bank is:

300 kVAr

You could theoretically create:

6 × 50 kVAr

But is that the best step arrangement?

Not necessarily.

Step sizes should consider the actual load variation.

For example, a plant whose reactive demand changes gradually may benefit from smaller initial steps.

A plant with large discrete motors may need a different configuration.

The objective is to avoid:

Hunting
Frequent switching
Overcompensation
Poor PF at light load
Excessive wear of switching devices

This is the item we push back on at clarification stage more often than the reactor question. A plant whose reactive demand sits around 40 kVAr for most of the night shift and rises to 280 kVAr at peak production is badly served by six equal 50 kVAr steps — the controller either overcompensates into leading power factor or hunts between two steps and wears out the switching devices. A graded arrangement, with the smallest step matched to the lightest expected reactive demand, gives usable resolution across the whole load range. Ask the customer for the minimum load, not only the maximum.

APFC controller capability and switching logic should therefore be coordinated with the step design.

Capacitor Switching Contactors Are Different From Ordinary Motor Contactors

Capacitor energization can create high transient inrush current.

That is why capacitor-duty switching arrangements are commonly used for conventional contactor-switched APFC steps.

They are designed specifically for capacitor switching rather than being selected solely from steady-state current.

For rapidly fluctuating loads, a project may alternatively require:

Thyristor-switched capacitor steps
Static switching
Hybrid solutions

depending on switching speed and application.

Do not substitute an ordinary contactor merely because its continuous ampere rating appears sufficient.

Reactor Heat Must Be Considered in Panel Design

Adding reactors introduces:

Copper losses
Core losses
Heat
Additional panel volume

Detuned-reactor installation instructions consistently call for adequate ventilation and thermal protection on the reactor itself. Reactors run hot, and in a closed APFC panel that heat has nowhere to go.

Therefore, a detuned APFC panel may require:

Larger enclosure
Greater ventilation
Forced cooling
Temperature monitoring
Correct reactor spacing
Thermal trip integration

A normal 300 kVAr APFC enclosure should not simply be copied and reactors squeezed into the remaining space.

The thermal design changes.

To put a number on it: detuned-reactor losses are commonly in the region of 5 to 8 W per kVAr, so a 300 kVAr detuned bank can add roughly 1.5 to 2.5 kW of heat inside the enclosure that the same bank in normal configuration never had — before counting capacitor, contactor and busbar losses. In a closed panel standing in a 45 °C Gujarat plant room, that is the difference between a bank that runs for years and one that starts self-healing its way through the capacitor cans. We size APFC ventilation from the summed component losses declared by the selected capacitor and reactor, and treat the panel heat and cooling calculation as part of the APFC design rather than a separate exercise.

Reactor Thermal Protection Should Be Used Properly

Detuned reactors can operate at significant temperature.

Detuned reactors are commonly supplied with normally closed thermal-protection contacts, intended to disconnect that step if the reactor overheats. Wire them. We have opened APFC panels where these contacts were left unterminated because the wiring drawing did not show them.

If such protection is part of the selected reactor design, it should be correctly integrated into the control circuit.

A thermal switch that is physically present but not wired into the APFC protection philosophy provides little benefit.

Harmonic Study: What Data Should Be Collected?

Before upgrading a major industrial APFC system, useful data may include:

Bus voltage
Load kW
kVAr
Existing PF
True PF
THDv
THDi
Individual harmonic spectrum
Transformer kVA
Transformer impedance
Existing capacitor kVAr
VFD/UPS loading
Operating modes

Measurements should ideally represent relevant plant conditions such as:

Peak production
Light load
Major VFDs operating
DG operation if applicable

A single five-minute measurement when half the factory is stopped may not represent the real electrical environment.

Where Should Harmonics Be Measured?

Ideally at electrical points relevant to the design question.

For an APFC bank connected to the main LT bus, useful measurements often include the intended APFC connection point and upstream/downstream conditions as required by the study.

This helps determine:

What harmonic current reaches the bus
Voltage distortion
Existing resonance
Effect of capacitor switching
Dominant harmonic orders

The correct measurement point depends on the electrical network architecture.

Example 1 — Factory With Mostly Conventional Motors

Assume:

1600 kVA transformer
Majority DOL/Star-Delta motors
Few small VFDs
Low measured harmonic distortion
Required compensation 350 kVAr

A properly engineered normal APFC system may be sufficient.

There is no engineering benefit in automatically adding reactors simply to make the panel look more advanced.

Example 2 — Plant With Heavy VFD Loading

Assume:

1600 kVA transformer
900 kW of VFD-driven motors
Several rectifier loads
Dominant 5th and 7th harmonics
Required compensation 400 kVAr

A normal APFC bank should not be selected blindly.

A harmonic assessment may support a detuned solution.

Where 5th harmonic dominance is the main concern, a 7% / ~189 Hz detuned design is a common engineering approach, and it is the case most often used to illustrate reactor selection.

Example 3 — Significant Lower-Order Harmonic Content

Suppose measurements show significant harmonic behaviour around lower orders that makes a 3.8-order/7% design unsuitable according to the selected manufacturer's limits.

The engineer may need to evaluate a lower tuning order, such as a 2.7/14% system.

But the decision should be supported by:

Harmonic data
Network configuration
Manufacturer limits
Capacitor/reactor design

not by:

“14% is heavy duty, so use 14%.”

Example 4 — APFC Capacitors Are Failing Repeatedly

Suppose a plant reports:

Bulged capacitors
Frequent fuse operation
Hot APFC panel
Contactor failure
Reactor noise if reactors exist
Unstable PF
Nuisance tripping

Do not immediately replace capacitors with a higher voltage rating.

Investigate:

Harmonic current
Network voltage
Existing reactor selection
Resonance
Ventilation
Switching frequency
Step current
Capacitor age
Connection integrity
Temperature

In our experience, the single most common finding in a repeatedly failing detuned bank is a capacitor voltage class that was never coordinated with the reactor. Take a 7% bank on a 415 V system: the fundamental voltage across the capacitor is approximately 415 / 0.93 ≈ 446 V, and if the bus is running 4% high at 433 V the capacitor sees roughly 466 V. A 440 V can in that position is at or past its rating before a single harmonic is added. Our recommendation: confirm the capacitor voltage class against the detuning factor and the maximum bus voltage as a matched pair, and note that the actual class must come from the selected capacitor-reactor combination rather than from a generic table.

Replacing failed capacitors without identifying the cause may simply restart the failure cycle.

Normal APFC vs Detuned APFC — Cost Comparison

Normal APFC is generally less expensive because it avoids reactors and the associated:

Larger enclosure
Additional bus/wiring
Heat management
Thermal protection
Higher voltage-rated capacitor design where required

Detuned APFC costs more.

But comparing only initial cost can be misleading.

If a normal capacitor bank creates resonance or repeatedly fails in a harmonic-rich system, the cheaper initial panel may become much more expensive through:

Capacitor replacement
Production downtime
Contactor failures
Fuse failures
Transformer/cable heating
Maintenance cost

The correct comparison is:

Lifecycle cost + electrical compatibility

not just initial panel price.

7% vs 14% — Which Is Better?

Neither.

This is the wrong question.

The correct question is:

Which tuning frequency is suitable for the measured harmonic spectrum and electrical network?

7% may be appropriate when:

The selected design is intended to detune below the dominant 5th harmonic
Lower-order harmonics are within acceptable limits
Manufacturer application requirements are satisfied

14% may be considered when:

A lower tuning frequency is required
System harmonic characteristics justify a 2.7-order design
Selected equipment is specifically engineered for that application

Manufacturer-specific harmonic current limits and complete network behaviour must still be checked.

What Should Be Included in an APFC RFQ?

Instead of writing:

“Supply 300 kVAr APFC Panel with 7% reactor.”

provide:

Electrical System

System voltage
Frequency
Transformer kVA
Transformer impedance
Fault level
Earthing arrangement
DG operation

Load

Maximum kW
Minimum kW
Existing PF
Target PF
Major motor loads
VFD kW
UPS/nonlinear load kW

Harmonics

THDv
THDi
Harmonic spectrum
Measurement point
Measurement operating condition
Harmonic study if available

APFC Requirement

Required effective kVAr
Step arrangement
Normal/detuned requirement
Detuning factor if already engineered
Capacitor duty
Switching type
Approved makes
Ventilation requirement

Integration

APFC controller
Metering
Communication
Alarm contacts
Reactor thermal trips
BMS/SCADA interface if required

This allows different manufacturers to quote substantially the same technical solution.

Standards Relevant to APFC Panels

Several standards may be relevant depending on the exact equipment and project.

IEC 60831-1:2014

IEC 60831-1 covers self-healing shunt power capacitors for AC systems up to and including 1,000 V, including performance, testing, rating, safety and guidance for installation and operation.

IEC 61921:2017

IEC 61921 applies to low-voltage AC shunt capacitor banks used for power-factor correction and notes that, where applicable, LV PFC banks also comply with IEC 61439-1 and IEC 61439-2 requirements.

IEC 61642

IEC 61642 provides guidance for industrial AC networks affected by harmonics, including the application of shunt capacitors and passive AC harmonic filters.

Indian Standard for Power-Factor Correction Capacitors

For Indian projects, BIS currently lists:

IS 13340 (Part 1):2024

which adopts IEC 60831-1:2014 for self-healing AC shunt power capacitors.

Therefore, Indian consultant or tender specifications can reference the applicable IS/BIS requirements together with the relevant IEC requirements and project-specific standards.

As always, the exact contractually applicable edition should be confirmed during project engineering.

APFC Panel FAT — What Should Be Tested?

A proper APFC FAT should not be limited to:

“Controller ON, capacitor contactor ON — PASS.”

Depending on the project, inspect and verify:

Construction

GA
Enclosure
Ventilation
Component layout
Busbars
Cable connections
Earthing

Capacitors

Make
Type
Voltage rating
kVAr rating
Step configuration

Detuned Reactors

Detuning percentage
Tuning data
kVAr compatibility
Current rating
Thermal protection
Connections
Physical spacing
Cooling

Protection

MCCB/fuse
Step protection
Control MCB
Reactor thermal trip
Over-temperature alarm where provided

APFC Controller

CT ratio
CT polarity
Target PF
Step sequence
Switching delay
Alarm functions

Functional Testing

Manual step operation
Automatic step simulation
Contactors/switches
Indication
Interlocks
Alarm
Fan operation
Reactor thermal circuit

Electrical Testing

IR test
Applicable dielectric test
Continuity
Wiring verification

We perform applicable in-house panel FAT using calibrated testing equipment, and we support customer/consultant witnessing or TPI where required by the project.

How Wisdom Techno Solutions Approaches APFC Panel Engineering

At Wisdom Techno Solutions, an APFC panel should not begin with:

“How many capacitor cans fit in the enclosure?”

It should begin with the electrical system.

Useful project inputs include:

Transformer rating and impedance
Actual kW demand
Existing and target power factor
Required effective kVAr
Load profile
VFD/UPS loading
Harmonic measurements if available
Existing capacitor banks
DG operation
System voltage
Ambient conditions
Customer specification

Wisdom Techno Solutions is a licensed partner for three design-verified switchgear platforms: Rittal Ri4Power (Rittal Solutions Partner since 2021 — the Ri4Power system carries Intertek ASTA design verification to IEC 61439-1 and -2 Edition 3, with copper and aluminium busbar systems up to 6300 A), C&S CX (Licensed Partner since 2023, up to Form 4B Type II) and Siemens SIEPAN Elite/8PU (IEC 61439 Solutions Partner since 2024). We build to each system's verified configuration and routine-verify every assembly in-house at our Vadodara, Gujarat facility, for industrial, EPC and infrastructure projects across India. That places the harmonic-detuned capacitor stage construction inside a verified design envelope.

Based on the approved project requirement, we manufacture customized:

Conventional APFC panels
Detuned APFC panels
Multi-step APFC systems
APFC integrated with PCC/PMCC panels
PLC/monitoring-integrated solutions where required

For a detuned system, the capacitor and reactor should be treated as a matched electrical combination, not as two independently selected catalogue items.

The design must account for:

Detuning percentage
Reactor current
Capacitor voltage
Effective kVAr
Harmonic environment
Thermal performance
Ventilation
Protection
Control logic

The objective is not simply:

“Achieve 0.99 PF.”

The objective is:

Achieve the required reactive-power compensation without creating an unhealthy electrical condition elsewhere in the plant.

Common APFC Design Mistakes

Mistake 1: Installing Normal Capacitors in a Harmonic-Rich Network Without Assessment

This can create resonance and equipment stress.

Mistake 2: Assuming Every VFD Plant Needs the Same 7% Reactor

Harmonic spectrum and network impedance differ between plants.

Mistake 3: Assuming 14% Is Better Than 7%

It is a different tuning design, not a quality grade.

Mistake 4: Treating 7% as “7% Harmonic Reduction”

The percentage is the detuning/reactance ratio.

Mistake 5: Ignoring Capacitor Voltage Rise

A series reactor increases fundamental voltage across the capacitor.

Mistake 6: Selecting Capacitor and Reactor Independently

They should be designed as a matched combination.

Mistake 7: Using Capacitor Nameplate kVAr as Effective Bank kVAr Without Checking

Detuned-bank effective output should be properly established.

Mistake 8: Ignoring Reactor Heat

Detuned reactors require proper thermal and ventilation design.

Mistake 9: Ignoring Reactor Thermal Protection

Available thermal protection should be integrated correctly.

Mistake 10: Assuming APFC Will Remove Harmonics

Reactive-power compensation and harmonic filtering are different functions.

Mistake 11: Sizing APFC Only From Transformer kVA

Actual load and power factor determine useful compensation.

Mistake 12: Targeting PF = 1.00 Without Understanding Operating Conditions

Overcompensation can create leading power factor and unnecessary switching.

APFC Selection Checklist

Before selecting an APFC panel, ask:

Power Factor

✓ Current PF known? ✓ Target PF defined? ✓ Maximum kW known? ✓ Minimum load known? ✓ Required effective kVAr calculated?

Network

✓ Transformer kVA known? ✓ Transformer impedance known? ✓ DG operation considered? ✓ Existing capacitor banks known?

Harmonics

✓ VFD loading known? ✓ UPS/nonlinear load known? ✓ THDv measured? ✓ THDi measured? ✓ Harmonic spectrum available? ✓ Dominant harmonics identified?

APFC Design

✓ Normal or detuned justified? ✓ Detuning factor selected technically? ✓ Capacitor voltage coordinated? ✓ Effective kVAr confirmed? ✓ Reactor current checked? ✓ Thermal protection included? ✓ Ventilation designed?

Operation

✓ Step sizes appropriate? ✓ Switching frequency acceptable? ✓ Controller CT location correct? ✓ CT polarity correct? ✓ Alarm and monitoring requirements defined?

Only after these questions are answered should the final APFC design be frozen.

A Better Way to Compare APFC Quotations

Suppose two manufacturers quote:

Manufacturer A

300 kVAr APFC — ₹X

Manufacturer B

300 kVAr APFC — ₹Y

Do not compare price yet.

First create this table:

Parameter	Manufacturer A	Manufacturer B
Effective output at bus	?	?
Normal / detuned	?	?
Reactor %	?	?
Tuning frequency	?	?
Capacitor voltage	?	?
Capacitor duty	?	?
Reactor thermal protection	?	?
Switching device	?	?
Step arrangement	?	?
Ventilation	?	?
Harmonic design basis	?	?
Standards	?	?

Only after technical normalization should commercial comparison begin.

Two panels with:

“300 kVAr”

written on their datasheets can represent very different electrical designs.

Conclusion

A normal APFC panel and a detuned APFC panel both correct power factor.

But they are designed for different network conditions.

Use a normal APFC system when the electrical network is suitable for direct capacitor compensation and harmonic/resonance risks have been appropriately assessed.

Evaluate a detuned APFC system when nonlinear loads and network conditions create a meaningful harmonic or resonance concern.

And when selecting between:

and

14%

remember:

These numbers define the electrical tuning of the capacitor-reactor combination. They do not represent percentage harmonic reduction.

At 50 Hz:

7% → approximately 189–190 Hz / tuning order 3.8

14% → approximately 134–135 Hz / tuning order 2.7.

Do not choose the reactor because:

“7% is standard.”

or:

“14% is heavy duty.”

Study:

Harmonic spectrum
Transformer impedance
Nonlinear load
Capacitor size
Network voltage
Operating modes

Then engineer the capacitor and reactor together.

A good APFC panel should not simply make the power-factor number on a meter look better.

It should improve the electrical system without introducing a new resonance, thermal or harmonic problem.

Planning a normal or detuned APFC panel for an industrial project?

Share your SLD, transformer details, actual load, existing power factor, target power factor, VFD/UPS loading and available harmonic measurements with Wisdom Techno Solutions for project-specific APFC panel engineering and manufacturing.

Related Guides

Frequently Asked Questions

What is a detuned APFC panel?

A detuned APFC panel connects a reactor in series with each capacitor step, which shifts the tuning frequency of the capacitor-reactor combination below the dominant harmonic region. Above that tuning frequency the combination behaves inductively, so it cannot form a parallel resonance with the supply-system inductance at the harmonic orders of concern. It still performs the same power-factor correction duty as a normal bank; the reactor exists to keep that compensation from amplifying harmonics that are already present in the network.

What does a 7% reactor mean?

It refers to the reactor-to-capacitor reactance ratio used as the detuning factor. It does not mean 7% harmonic reduction. On a 50 Hz system, a 7% detuned bank typically has a tuning frequency around 189–190 Hz.

What does a 14% detuned reactor mean?

A 14% reactor uses a higher reactor-to-capacitor reactance ratio. On a 50 Hz system this gives a tuning frequency of approximately 135 Hz, a tuning order of 2.7.

Which is better: 7% or 14% reactor?

Neither is universally better. The correct selection depends on the measured harmonic spectrum, network configuration and the manufacturer's capacitor-reactor design.

Why is a 7% reactor commonly used with VFD loads?

A conventional six-pulse VFD produces characteristic input harmonics dominated by the 5th and 7th orders, so the 250 Hz fifth harmonic is usually the main concern on a 50 Hz network. A 7% detuned step tunes at approximately 189 Hz, which sits below 250 Hz, so the combination behaves inductively at the fifth harmonic instead of forming a parallel resonance with the source impedance. This is why 7% is the most commonly encountered detuning factor in VFD-heavy plants — but it should be confirmed against the measured spectrum rather than applied as a rule.

Does every VFD installation require a detuned APFC panel?

No. A single 2.2 kW VFD on a plant fed by a 2000 kVA transformer does not create the same harmonic environment as several hundred kW of drives running together. The decision depends on total nonlinear load as a proportion of the transformer rating, source impedance, the size of the capacitor bank relative to the supply, and the measured harmonic levels at the intended connection point. Counting VFDs is not an assessment; measurement or a harmonic study is.

Can a detuned APFC panel remove harmonics?

Its primary purpose is power-factor correction while reducing resonance/harmonic-amplification risk. It is not automatically a substitute for a dedicated active or passive harmonic filter.

Why does a capacitor need a higher voltage rating with a detuned reactor?

The reactor-capacitor series combination causes the fundamental voltage across the capacitor to be higher than the bus voltage. Capacitor rating must therefore be coordinated with the detuning factor, system voltage, harmonic duty and manufacturer design.

What is the tuning frequency of a 7% reactor on 50 Hz?

Approximately 189–190 Hz, which corresponds to a tuning order of about 3.8. That places the step below the 250 Hz fifth harmonic but above the 150 Hz third harmonic. On a 60 Hz system the same 7% detuning factor gives a proportionally higher frequency, so always state the system frequency alongside the detuning percentage.

What is the tuning frequency of a 14% reactor on 50 Hz?

Approximately 134–135 Hz, corresponding to a tuning order of about 2.7. This sits below both the 150 Hz third harmonic and the 250 Hz fifth harmonic, giving a different impedance characteristic to lower-order harmonic content than a 7% design. The trade-off is a higher fundamental voltage rise across the capacitor and a physically larger reactor duty.

Does 14% reactor provide twice the harmonic filtering of 7%?

No. The percentages represent different detuning factors and therefore different resonant frequencies. They are not filtering-efficiency percentages.

What happens if normal APFC capacitors are installed in a harmonic-rich system?

The added capacitance changes the network impedance and can form a resonant circuit with the upstream system inductance, which is usually dominated by the transformer. If that resonance lands near a harmonic order already present, that harmonic is amplified rather than absorbed. The visible symptoms are bulged capacitor cans, repeated fuse operation, a hot APFC panel, contactor failures and rising voltage distortion — often within months of commissioning, even though the kVAr calculation was correct.

What is the difference between a detuned reactor and an active harmonic filter?

A detuned reactor is part of a capacitor-reactor PFC step and primarily helps avoid problematic resonance. An active harmonic filter uses power electronics to actively compensate harmonic currents.

How much APFC kVAr is required?

The basic reactive-power requirement can be calculated from actual kW and the difference between the tangent values corresponding to existing and target power factor. The final bank size should also consider operating load range and system requirements.

Can APFC be sized only from transformer kVA?

It should not normally be selected from transformer size alone. Actual kW loading, existing PF, target PF, load variation and system requirements provide a better engineering basis.

Which IEC standard applies to LV capacitor banks?

IEC 61921:2017 applies to low-voltage AC shunt capacitor banks used for power-factor correction. It also notes that, where applicable, LV power-factor-correction banks comply with the assembly requirements of IEC 61439-1 and IEC 61439-2. In practice this means an APFC panel is both a capacitor bank and a low-voltage assembly, and both sets of requirements have to be satisfied.

Which IEC standard applies to self-healing PFC capacitors?

IEC 60831-1:2014 covers self-healing shunt power capacitors for AC systems up to and including 1,000 V. It addresses performance, testing, rating, safety requirements and guidance for installation and operation of the capacitor itself. It governs the component, not the complete APFC assembly, so a capacitor certificate to IEC 60831-1 does not by itself describe the panel.

Which Indian standard applies to self-healing power capacitors?

BIS currently lists IS 13340 (Part 1):2024, which adopts IEC 60831-1:2014 for self-healing AC shunt power capacitors. Indian tender specifications can therefore reference IS 13340 (Part 1) for the capacitors alongside the relevant IEC requirements for the assembly. As always, confirm the contractually applicable edition during project engineering rather than writing "as per latest IS".

What information should I provide for a detuned APFC quotation?

Provide the system voltage and frequency, transformer rating and impedance, actual maximum and minimum kW, existing and target power factor, and the required effective kVAr at the bus. Then add the harmonic picture: VFD and UPS kW, other nonlinear loads, THDv and THDi, the harmonic spectrum with its measurement point and operating condition, and any existing capacitor banks. Finally state DG operating modes, step arrangement, switching type and ambient conditions. With that set of data, competing manufacturers quote substantially the same technical solution — without it, "300 kVAr APFC" can be answered by three very different panels.

APFC vs Detuned APFC

Normal APFC vs Detuned APFC Panel: 7% vs 14% Reactor, Harmonics & Selection Guide