- Power factor correction (PFC) aims to make the power factor as close to 1 as possible.
- Apparent power, true power, and power factor must be calculated to determine PFC.
- Capacitors are commonly used for PFC to provide reactive power and offset inductive loads.
- The PFC capacitance required depends on the existing reactive power and desired power factor.
- PFC calculations differ for single-phase vs three-phase systems and by equipment.
What is power factor correction and why is it important?
Power factor correction (PFC) is the process of compensating for a lagging power factor in an electrical system. The power factor measures how efficiently power is being utilized. It is the ratio between the real power consumed (kW) and apparent power drawn (kVA).
A low power factor is undesirable and can lead to increased electricity costs, greater line losses, and overheating of transformers or conductors. By implementing PFC, the power factor can be improved closer to a maximum value of 1, which allows electrical systems to function at optimum efficiency.
Some key reasons PFC is important include:
- Avoiding utility penalties – Many utilities charge higher rates when the power factor goes below a specified level. PFC can help maintain a high power factor to reduce these penalties.
- Increased system capacity – Improving power factor allows more usable power to be drawn for the same current. This effectively increases the supply capacity.
- Reduced losses – At lower power factors, higher currents are needed to transfer usable power, increasing I2R heating losses in lines and equipment. PFC reduces wasted energy.
- Improved voltage regulation – Large inductive loads can cause voltage drops. PFC helps support voltage.
- Longer equipment life – Lower losses and less heat dissipation with PFC extend the life of transformers, generators, motors, and conductors.
- Avoiding oversizing – Without PFC, larger cables, switches, and transformers may be needed to account for the higher apparent power.
How is power factor calculated?
To determine the amount of PFC needed, the power factor must first be calculated. This requires finding the apparent power and true power.
Apparent power (S or VA) is the product of root mean squared (RMS) voltage and RMS current:
S = V x I
Where S is in volt-amps (VA), V is in volts, and I is in amps.
True power (P or W) is the real power consumed and converted to useful work. It is voltage multiplied by current multiplied by the power factor:
P = V x I x PF
Where P is in watts (W).
With the apparent power and true power known, the power factor (PF) is simply:
PF = P / S
PF = W / VA
A properly loaded electrical system will have a power factor between 0.85 and 0.95. Values below 0.85 are considered poor and indicate the need for PFC.
Why is power factor low and how can it be corrected?
The main cause of a low power factor is inductive loading from equipment like transformers, electric motors, inductive ballasts, and welding sets. These devices require reactive power to produce a magnetic field for operation. This reactive power causes the current to lead the voltage, unlike resistive loads where current and voltage are in phase. The phase difference lowers the power factor.
To correct the power factor, capacitors are added. These provide reactive power by generating a leading current to offset the inductive loads. With the reactive currents cancelling out, the resulting power factor is improved. Capacitors supply the reactive power so the utility only needs to provide the real working power.
Other PFC methods include synchronous motors and variable speed drives. But capacitors are simple, reliable, and commonly used for PFC. The capacitors can be installed at individual loads or grouped at a centralized panel.
How do you calculate power factor correction capacitance?
Once the existing power factor is found, the specific PFC capacitance required can be determined using the following steps:
- Calculate the reactive power (QL):QL = S x sin(cos^-1 PF)Where S is the apparent power and PF is the measured power factor.
- Choose the desired or target power factor (PFtarget). A value of 0.99 is common.
- Calculate the capacitive reactive power needed:Qc = QL x (PFtarget / PFcurrent)
- Determine the PFC capacitance:C = Qc / (2πfV2)Where f is the system frequency in Hz and V is voltage in volts.
The result, C, is the capacitance in farads needed for PFC. When sizing the capacitors, a safety margin of 15-25% above the calculated capacitance is usually added. Over-correction should also be avoided.
What are the key considerations for power factor correction?
Some important factors to keep in mind when implementing PFC:
- Measure power factor during both light and heavy loading conditions to size PFC correctly for the full range.
- Capacitors can create harmonics so harmonic filtering may be needed.
- Use capacitors with a voltage rating 20% higher than the operating voltage as temporary over-voltages can occur.
- PFC capacitors should have fuse or circuit breaker protection. Disconnect switches allow safe isolation.
- Staggered capacitor banks prevent large steps in PFC.
- Detuned reactors may be used to isolate harmonics created by PFC capacitors.
- Placement of capacitors should consider inductive load locations to maximize reactive power cancellation.
- Multi-stage PFC can be used for better regulation across a wide load range.
- KVAR generators help provide dynamic compensation for fluctuating loads.
How does power factor correction differ for single vs three phase systems?
The same basic PFC principles apply to single and three phase circuits. However, there are some key differences:
- Single-phase PFC – Only one capacitor bank is required and it is connected across the supply. The current leads the voltage by 90°.
- Three-phase PFC – Requires a three-phase capacitor bank with one on each phase or in a delta formation. The capacitive current leads voltage by 120°.
- Measuring power factors in three-phase systems can use either the average PF or fundamental PF.
- Balanced three-phase loads normally have higher power factors so require less correction than single-phase.
- With three-phase, the compensation kVAR can be calculated per phase or for the total load.
- Overcorrection can cause leading power factors so should be avoided in three-phase systems.
The steps for calculating PFC capacitance and sizing remain the same. But appropriate PF measurements and calculations must be used for the specific single or three-phase system involved.
What standards and guidelines apply to power factor correction?
There are some standards and recommended practices that provide guidance on PFC:
- IEEE 18-2002 – Standard for Shunt Power Capacitors
- IEEE 141-1993 – Recommended Practice for Electric Power Distribution for Industrial Plants
- IEEE 519-2014 – Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems
- NEMA MG 10-2017 – Energy Management Guide for Selection and Use of Fixed Capacitors
- IEC 60831-1 & 2 – Shunt Power Capacitors of the Self-Healing Type for AC Systems
These documents address topics like PFC equipment application, design, harmonics, protection, safety, testing, and capacitance calculations. Adhering to the standards ensures proper implementation of PFC.
Utilities also specify requirements like the acceptable power factor range, limits on harmonics introduced by PFC, and necessary permitting. These requirements should be consulted.
Optimizing power factor through PFC capacitors and other methods provides significant benefits for electrical efficiency, capacity, regulation, and cost. Correctly calculating the existing power factor and required PFC involves measuring key parameters and using the proper formulas. PFC design must account for the type of system, load variations, harmonics, and other specific factors. Standards and utility requirements help guide proper PFC implementation. Overall, power factor correction is a valuable means of maximizing electrical system performance