Power Factor Correction Capacitors Sizing Calculations – Part One

Today, we will continue explaining the glossary of all terms and expressions that we will deal with through our explanation of Power Factor Correction Capacitors (PFCC) Sizing Calculations.

This glossary will help you to understand the technical meaning for any term or expression which will facilitate your understanding for the Power Factor Correction Capacitors (PFCC) Sizing Calculations.

Please read this glossary carefully because you will need to come back to it while our explanation for Power Factor Correction Capacitors (PFCC) Sizing Calculations.


Glossary of Power Factor Correction Capacitors (PFCC) 

AC capacitor

 A capacitor designed for operation with alternating voltage.
NOTE: AC capacitors may be used with DC voltages only if authorized by the manufacturer.

Active power P

Power in an electrical installation that is converted to mechanical work or thermal energy.

Ambient temperature of capacitors

The temperature of the air measured at a distance of 10 cm from the capacitor and at 67% of its height.

APFC or Centralized PFC

APFC is the abbreviation for Automatic Power Factor Correction.
The capacitor branches / steps are controlled by a microprocessor-based PFC controller. This controller continuously monitors the relevant grid parameters and switches capacitors in/out to maintain the specified cos phi value.

Apparent power S

The power drawn by an electrical installation during operation. It consists of the active power P and the reactive power Q.


Audio frequency in PFC

See Mains signaling voltage

Base or fundamental frequency

The "frequency" or "rated frequency" of an electrical network or component.

Breaking capacity of a fuse

The current range or current value that can cause a fuse to disconnect at a specific voltage under specified conditions of use and behavior.


A passive electronic component capable of storing electric charges and thus electrical energy. It basically consists of two conductive surfaces a short distance apart separated by a dielectric. The properties of capacitors vary strongly depending on their conductive parts, dielectrics, contacts and geometrical configurations. In PFC LV capacitors, metalized polypropylene or paper is widely used as the conductive surface and polypropylene as the dielectric. Long strips of these materials are wound to form a cylindrical capacitor element (winding). The winding is finally contacted by metal wires and enclosed in a metal can. The relevant design and control standard is IEC 60831.

Capacitor bank (or PFC bank)

A number of capacitors and other components, such as fuses, contactors and controllers, connected together to function as a single system.

Capacitor contactor

Special contactors designed for connecting capacitors. They protect capacitors against high inrush currents that can reach values of up to 200 times the rated capacitor current. Without the use of such contactors, these inrush currents could damage the capacitors sufficiently to cause failures. This harmful effect also shortens the capacitor's operating life.

Capacitor element (or winding)

See Capacitor.

Capacitor losses

The power dissipated in the capacitor. The upper limit is stated in the capacitor specifications (e.g. 0.5 W/kvar). The capacitor losses consist of dielectric losses that depend on the applied voltage frequency and ohmic losses that depend on the applied current and serial capacitor resistance.

Centralized PFC

See APFC (Automatic PFC).


An electromechanical switch designed to connect or disconnect devices to a power network. For LV applications, it is usually implemented with two metal contacts per phase that are contacted or separated by a magnetic field. See also Capacitor contactors.

Controller (PFC)

Any device that controls a number of switches (in PFC usually contactors or thyristors) in order to connect and disconnect the steps of a capacitor bank to the power network. As a rule, a processor-controlled electronic device that measures the current and voltage in the grid, calculates the power factor / cos phi and switches in an optimum number of capacitors to reach the programmed cos phi. State-of-the-art devices offer additional features such as THD-I and THD-V measurement, automatic set-up on connection, zero voltage release, error messages etc.

Cos phi (cos f) PFC

Cosine of the angular difference between the current and voltage phases at a given time in an electrical network. In the case of an ideal sinusoidal waveform, it is equal to the power factor.

DC capacitor

A capacitor designed for operation with direct voltage.
NOTE: DC capacitors may be used with AC voltages only if authorized by the manufacturer.

Detuned PFC

A PFC application in which a reactor/capacitor combination is used to reduce harmonics by intentionally creating minima in the impedance spectrum at specified frequencies and to protect the capacitors. In contrast to tuned PFC, the LC filter frequency is set at a comfortable distance to the resonant frequencies of the harmonics to avoid overload due to resonance. The filter effect achieved is thus smaller than for tuned PFC, but still sufficient for most applications. Note that the 3rd harmonic and its multiples cannot be filtered this way. In PFC, the filter frequency is usually expressed in percent as a detuning factor p. Common standard factors are 7% (189 Hz) and 14% (134 Hz). 7% detuning is effective for the 5th, 7th and 11th harmonics. 14% detuning, although often called filtering, actually increases the impedance for the 5th harmonic and upward. The capacitors are protected from harmonic currents in this way, and while no big filter effect is obtained for the power network, the capacitors are not overloaded and can therefore correct the power factor.

Detuning factor p

Ratio of the impedances of a reactor/capacitor combination in detuned PFC.

Discharge reactor

An inductor that discharges a capacitor to 75 V or less within 3 minutes. It usually has a considerably shorter discharge time than a discharge resistor. Nevertheless, its main advantage is its substantially lower losses during capacitor operation, due to its high AC resistance.

Discharge resistor

An ohmic discharge device for a capacitor. It is usually pre-mounted or supplied together with an EPCOS PFC capacitor. IEC 60831 stipulates that after disconnection, a capacitor must be discharged to 75 V or less within ≤ 3 minutes by this device.

Dynamic PFC

A PFC method designed especially to handle fast-changing reactive power requirements. EPCOS dynamic PFC uses only thyristor switches with a reaction time of 5 to 20 ms. As thyristors keep the capacitors at the maximum network voltage (e.g. 566 V in a 400 Vrms AC network) when disconnected, the capacitor discharge time is also short (only overvoltages generated by switching need to be discharged) and a capacitor step can be switched in again after approximately 200 ms. Load changes in the millisecond range, such as transients or motor-start effects can thus be compensated, which is not possible for contactor-switched PFC with a switching time in the seconds range and capacitor discharge times of 60 to 75 s.


Any device that passes on only a selected part of its input as output. In power quality applications, it is usually used as a filter for specific voltage/current frequencies in a power network. For more information, refer to tuned and detuned PFC.

Fixed PFC

Power factor correction using fixed capacitors with no switching options.


The subjective impression of an unsteady visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time. Voltage fluctuations can cause changes in lamp luminance which can create the visual phenomenon known as flicker. Above a certain threshold, it becomes annoying. The annoyance grows very rapidly with the amplitude of the fluctuation. At certain repetition rates, even very small amplitudes can be annoying.

Frequency of the supply voltage

The repetition rate of the fundamental wave of the supply voltage measured over a given interval of time. Usually 50 or 60 Hz.

Fuse (electric)

A device that disconnects one part of an electrical installation from the rest when a given value of a parameter (usually a current) is reached for a defined time period.

Grid voltage

See Supply voltage.

Group compensation

This term refers to connecting fixed or automatically controlled capacitors to a group of simultaneously operating loads, such as motors.

Harmonics (current / voltage)

Harmonics are integral multiples of a fundamental frequency that, when added together, distort the normally sinusoidal waveform. They are usually derived by a Fourier transform of the measured current and/or voltage curves (over time) at multiples of the network's base frequency. They are usually specified as a percentage known as the total harmonic distortion referred to the RMS value of the base current or voltage. For example:


The abbreviations THD-U and THD-V are commonly used for the voltage terms, which are calculated in the same way as THD-I. Harmonics cause numerous problems in power networks and are increasing because they represent a growing problem for modern electrical and electronic equipment. The 3rd, 5th and 7th harmonics, and odd ones up to the 50th harmonic, are most common.

Harmonic order h

The ratio of the frequency of a harmonic to the fundamental (rated) network frequency.

Hot spot temperature

The temperature of the hottest spot inside a capacitor. It is caused by its power losses during operation. Its temperature and location depend not only on these losses, but also on the capacitor's heat dissipation properties and the ambient temperature. The hot spot is usually located close to the core of the capacitor winding at about two-thirds of its height from the base.

Hybrid or mixed compensation

A PFC system combining electromechanical and thyristor-switched steps to respond simultaneously to fast and slow-changing demands for reactive power.

Individual PFC

See Fixed PFC.

Inrush current

A transient current surge of high amplitude and frequency that may occur when a capacitor is connected to the power network. Its amplitude and frequency are determined by factors such as the short-circuit impedance of the power supply, the capacitance connected in parallel, the residual capacitor charge, and the grid conditions at the moment of switching.

IPxx degree of protection

A measure of the degree of protection: the first digit refers to electric shock hazards and the second one to the intrusion of unwanted substances.

Key components

All important parts required to set up a PFC/PQS system, such as capacitors, controllers, reactors, contactors or thyristor modules.

Low voltage (LV)

In PFC voltages up to 1 kV RMS.

Mains signaling voltage

A signal superimposed on the supply voltage for the purpose of transmission of information (e.g. signals to automatically switch in and out special loads).

Mean life expectancy

The average time up to which useful service of a device is expected under the conditions specified by the manufacturer and the relevant (international) standards.

Medium voltage (MV)

In PFC, voltages with RMS values above 1 kV and below 35 kV.

Metal-foil capacitor (non self-healing)

A capacitor whose electrodes consist of metal foils separated by a non-metallized dielectric of different material. In the event of dielectric breakdown, the capacitor cannot perform self-healing.

Mixed compensation

See Hybrid compensation.

Network voltage

See Supply voltage.


See Rated.

Optimized capacitor safety terminal

This terminal is used for PFC-capacitors of the new generation (PhaseCap C-series, PhaseCap Compact, PhaseCap HD and PhiCap E-series) for fast and easy cable connection to the capacitor. It offers two contacts per phase as well as electric shock protection to IP20. For further information,


Any kind of current load applied to a device above its rated value.


Any kind of voltage load applied to a device above its rated value.


Abbreviation for Power Factor Correction. Any method to achieve a power factor as close to unity as desired.

Power factor

The power factor is the ratio of active power P to apparent power S under sinusoidal conditions.

For further information, refer to the technical section of the EPCOS product profile.


Term used to designate the characteristic values of certain parameters that together define the operating conditions on which the tests for a device are based. The device is designed for the relevant parameter set of these values, usually given in the respective data sheet. The term "Nominal" is often used instead. The indices R and N are commonly used (e.g. IN or IR or the rated voltage) to indicate the rated value of a parameter.

Rated voltage of a capacitor (UN or VN)

In PFC, this is usually the rated AC RMS voltage. NOTE: for other capacitor applications, the peak voltage is often used as the rated voltage (e.g. 400 V L-L ACrms = 566 V L-L ACpeak).

Reactive power Q

The power drawn by an electrical installation for operation without being consumed. It is mainly due to the power needed to build up magnetic fields in inductive loads (e.g. in motors).

Reactor PFC

An inductor (usually with a metal core) used with a capacitor to form an LC filter circuit. It protects the capacitor by blocking harmonics (by increasing the impedance at certain frequencies) and damping inrush currents. It also reduces harmonics from the power network by creating impedance minima at certain frequencies.

Residual voltage

The voltage remaining at the terminals of a capacitor for a certain time after disconnection.

Resonance (frequency)

Resonance occurs when a combination of electrical components produces an impedance minimum at a frequency equal to a voltage frequency present in a power network (usually the base frequency and its harmonics). In such a case, the current in the power network can rise significantly depending on the value of the impedance minimum. If the latter is close to zero, the current can in theory become infinite (short circuit!). Operation at or close to a resonant frequency thus implies the danger of current surges and damage to components in the system. Discrete PFC capacitors usually have resonant frequencies in the kHz range. Refer to detuned PFC and/or reactors for additional information.

Ripple control signal

See Mains signaling voltage.

Ripple voltage

The peak-to-peak alternating component of a unidirectional voltage. This small regular variation in a DC voltage remains after rectification and filtering of an AC voltage.


Root mean square.


A safety feature of capacitors with a polypropylene dielectric. If a local discharge (caused by an over-voltage, for instance) penetrates the dielectric, the polypropylene evaporates at this location. The vapor condenses around the hole in the dielectric and isolates it electrically. The capacitor then only suffers a negligible capacitance loss and can safely continue operation.

Standard PFC

A PFC installation without reactors. Usually found in applications with no or very low harmonic content. It uses all key components such as controllers, contactors and thyristors, but is fast losing market share. In Europe more than 95% of all PFC installations currently employ reactors, and the global trend is likewise toward detuned PFC. In standard PFC, thyristors or capacitor contactors are mandatory to avoid high inrush currents. See also capacitor contactors.

Step of a capacitor bank

Combination of one or more capacitors and switches (and if applicable also reactors) that can be switched in/out by a single switching signal.

Supply voltage

The RMS value of the voltage supplied to an electrical component or installation.

Supply voltage dip

A sudden reduction of the supply voltage to an RMS value between 90 and 100% of its standard value rapidly followed by a voltage recovery. Usually, such a voltage dip lasts between 10 ms and 1 minute.

Tangent delta (tan δ) of a capacitor

The ratio between the equivalent series resistance ESR and the capacitive reactance Xc of the capacitor at a specified sinusoidal AC voltage and frequency.
tan δ = ESR / Xc

Tear-off fuse

An internal safety device designed to disconnect the capacitor elements from the power supply. Aging, overload and the consequent multiple self-healing processes lead to the production of gas inside PFC capacitors, resulting in considerable overpressure. The addition of a suitable bead in the capacitor casing allows the capacitor to expand in a specific direction at a designated overpressure. The wires leading internally from the terminal block to the capacitor elements then break at predetermined points.

Terminal (block)

Any kind of interface for a component.

Temperature class of a capacitor

Defines the permissible ambient temperature for a capacitor to IEC 60831:



Mean value over any period of



24 h

1 year


45 °C

35 °C

25 °C


50 °C

40 °C

30 °C


55 °C

45 °C

35 °C

Note that the three conditions for a temperature class are all simultaneously valid.

Thyristor (module)

An electronic switch for capacitor steps in dynamic PFC. Unlike electromechanical contactors, thyristors operate fully electronically using power semiconductors. This reduces the switching time to less than 5...20 ms compared to seconds in the case of contactors.


See Harmonics.

Tuned PFC

An efficient way of reducing harmonics. The series resonance of these filters is close to the order of the harmonics to be reduced. It is mandatory to perform an accurate power quality analysis and such filters must be designed by specialists because large harmonic reductions can also lead to large currents flowing through the capacitors. Inappropriate designs involve a high risk of damage.


A term used for an electric or power company or energy authority, and thus the (local) company that supplies electrical energy.

Voltage fluctuation

A series of voltage changes or a cyclic variation of the voltage.

Voltage unbalance

In a three-phase system, a condition in which the RMS values of the voltage for the three phases or the phase angles between consecutive phases are not equal.

Voltage variation

An increase or decrease of the supply voltage, usually due to a variation in the total load of a distribution system.

Wavy cut

A special method of cutting the metallized film used to manufacture a capacitor element. Cutting the edge of the film in a wavy rather than a straight pattern optimizes the contact area of the capacitor element. This creates a lower contact resistance and thus leads to lower contact losses and a higher inrush current capability compared to a straight cut.

Zero voltage release

A feature of a controller, allowing it to recognize an interruption in the power supply and disconnect charged capacitors from the power network after a specified time (e.g. 20 to 50 ms).

In the next article, we will continue explaining the Power Factor Correction Capacitors (PFCC) sizing calculations. So, please keep following.


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