Power Factor Correction Selection Guide
Power factor correction raises the power factor by supplying the reactive power drawn by inductive loads locally with capacitors. Panel components and calculation method.
What is power factor correction? Power factor correction is the process of raising the power factor (cos φ) by supplying, locally with capacitor banks, the reactive power that inductive loads in a facility, such as motors, transformers and ballasts, draw from the grid. This avoids reactive energy charges and reduces line and transformer current. In an automatic power factor correction panel, the reactive power relay switches the capacitor steps in and out through contactors.
This guide explains what to look at when choosing products in the Power Factor Correction category.
Components of a power factor correction panel
In an automatic power factor correction panel, a relay measures the power factor of the facility and switches the capacitor steps in and out through contactors. Each link in the chain is in a separate subcategory:
- Current transformers: transfer the line current to be measured by the relay and the analyzer to the secondary circuit.
- Reactive power control relays: manage the steps according to the measured reactive power.
- Power factor correction contactors: are built for the inrush current a capacitor draws when it is switched in. In facilities where the load changes very quickly, the steps are switched with thyristor static contactors from the contactor switching modules group.
- Power capacitors: are the steps that supply the inductive reactive power; they are selected by kVAr rating and rated voltage.
- Harmonic filters and harmonic filter reactors: are reactors connected in series with the capacitor in a facility with harmonics; active filter modules are also found under harmonic filters.
- Shunt reactors and SVC solutions: balance in the inductive direction when the facility remains on the capacitive side; the SVC group includes inductive load drivers and the shunt reactors used with them.
- Power network analyzers and power quality analyzers: are for measurement before selection and monitoring after commissioning.
A charge remains on a capacitor that has been disconnected; it must discharge before the step is switched in again. Capacitor discharge units (discharge coils) are used to shorten the discharge time.
How to calculate power factor correction
The required capacitor power is found from the facility's active power and the existing and target power factors: Qc = P × (tan φ1 − tan φ2). For example, raising a facility that draws 100 kW at a power factor of 0.80 to 0.95 requires 100 × (0.75 − 0.33) ≈ 42 kVAr.
The input to the calculation is measurement, not the nameplate value. The variation of active and reactive power over the day is recorded with an analyzer; the total power is divided into steps that can follow the smallest load change. The target power factor is set so as to stay within the inductive and capacitive reactive energy limits defined in the electricity tariff; confirm the current values of the limit ratios from the tariff text in force.
Power factor correction in facilities with harmonics
Nonlinear loads such as drives, rectifiers, UPS units and electronic ballasts produce harmonic currents in addition to the base frequency. Capacitors can resonate with the grid inductance and become overloaded by harmonic currents. In these facilities, each step is built with a series reactor connected to the capacitor; the reactor shifts the resonance frequency of the step below the dominant harmonics. Because a series reactor raises the voltage across the capacitor terminals, the rated voltage of the capacitor in a reactor step is chosen higher than the grid voltage. The reactor ratio and the capacitor pairing are decided after current and voltage harmonics have been measured.
Single-phase steps for unbalanced loads
A three-phase step supplies equal reactive power to each phase. In buildings where single-phase loads predominate, the reactive demand of the phases differs from one another; a three-phase step overcompensates one phase and undercompensates another. In this case, a relay that measures the phases separately is used, together with the products in the single-phase subgroups of the power capacitors and shunt reactors categories.
Difference between a power capacitor and a motor capacitor
A power capacitor is a power factor correction step and is referred to by its kVAr rating. Motor capacitors are used in the auxiliary winding circuit of single-phase motors and are selected by their microfarad (µF) value; there are permanent (run) and start types. The two products are not interchangeable.
Brands in this category
- Entes: current transformers, power capacitors, power network analyzers, harmonic filters, shunt reactors and reactive power control relays.
- Schneider Electric: current transformers, power capacitors, harmonic filter reactors, and network and power quality analyzers.
- ABB: CLMD series power capacitors, current transformers, power network analyzers and harmonic filter reactors.
- Tibcon: cylinder and box type power capacitors, harmonic filters, shunt reactors, power factor correction contactors and thyristor static contactor modules.
Tense is listed with current transformers, motor and power capacitors and reactive power control relays, and Hawcon with motor capacitors.
Order notes
Products are sold per piece. Some of the products are listed as made to order; for these products, the lead time is confirmed before ordering. Product names carry the manufacturer code (such as METSECT5GD050, TBC1504, STC-30); you can reach a product directly by typing the code into the search.
Power Factor Correction Panel Components and Selection Table
| PFC Component | Key Selection Criterion | Function and Protection | Typical Application | Category Link |
|---|---|---|---|---|
| Power Capacitors (Cylindrical / Box) | kVAr rating (0.5–50 kVAr) and voltage (400–525 V) | Compensating inductive reactive power demand | Standard and detuned harmonic-filter capacitor steps | Power Capacitors |
| Reactive Power Control Relay & SVC | Step count (12–24) and thyristor (SVC) trigger outputs | cosφ monitoring and automatic step / shunt reactor switching | Unbalanced and rapidly fluctuating industrial facility loads | Reactive Power Control Relays |
| Shunt & Harmonic Filter Reactors | kVAr rating and p=7% / 14% detuning factor | Absorbing capacitive reactive power and preventing resonance | Long cable/LED loads (shunt) and VFD-heavy plants (harmonic) | Inductive Load Drivers & Reactors |
| Capacitor Duty Contactor & NH Fuse | Pre-charge damping contacts · gG blade fuse protection | Limiting capacitor inrush current peaks and short-circuit clearing | Reliable switching and protection of each capacitor bank step | Capacitor Contactors |
Frequently asked questions
What components are needed for a power factor correction panel?
A basic panel consists of a reactive power control relay, a current transformer that gives current information to the relay, as many power capacitors as there are steps, and for each step a power factor correction contactor with a fuse or switch. In a facility with harmonics, a series reactor is added to each step; in a facility that remains on the capacitive side, a shunt reactor is added. A network analyzer is used for measurement and monitoring. Step ratings and the number of steps are determined by a measurement-based project.
How is power factor correction calculated?
The required capacitor power is found with the relation Qc = P × (tan φ1 − tan φ2). P is the active power of the facility, φ1 is the angle of the existing power factor and φ2 the angle of the target power factor. Raising the power factor from 0.80 to 0.95 at a 100 kW load requires about 42 kVAr. Because the load changes during the day, the calculation is made not with a single nameplate value but with the active and reactive power values recorded by the analyzer, and the total power is divided into steps.
How is a capacitor fuse selected?
The fuse is not selected equal to the rated current of the capacitor. A capacitor draws a high inrush current when it is switched in; if there are harmonics or voltage rise in the grid, the continuous current also exceeds the rated value. For this reason the fuse or switch is selected with the value the capacitor manufacturer gives for the step rating. The contactor and cable cross-section are likewise determined according to manufacturer data and project conditions.
Why does a facility move to the capacitive side, and how is it corrected?
Capacitive reactive power comes from loads where the current leads the voltage. Fixed capacitors left in circuit when the load is low, long cable runs and electronic loads with a capacitor in the input filter push the facility to the capacitive side. First, the relay settings and whether the steps are working correctly are checked. If the capacitive reactive power is persistent, balancing is done with shunt reactor steps; if the load changes continuously, with SVC solutions; the reactor power is selected according to the measured value.
What is the difference between a power factor correction contactor and a normal contactor?
At the moment a capacitor is switched in, especially when other steps are already connected, it draws a very high, short-duration inrush current. Power factor correction contactors are designed for this current, are mostly built with pre-insertion contacts and resistors that limit the current, and are referred to by their kVAr rating. If a contactor made for motors and general loads is to be used in the same step, the values the manufacturer gives for capacitor switching are the basis.