Power factor is an important concept in electrical engineering and power distribution. It is a measure of how efficiently electrical power is being used in a system. A power factor of 80% indicates that the system is using power fairly efficiently, but with some room for improvement.
What is Power Factor?
Power factor is defined as the ratio of real power (measured in watts) to apparent power (measured in volt-amperes). Real power performs the actual work and apparent power is the product of voltage and current in the circuit.
Mathematically, power factor is calculated as:
Power Factor = Real Power (Watts) / Apparent Power (Volt-Amperes)
Power factor ranges from 0 to 1.0. A perfectly efficient system with no reactive power would have a power factor of 1.0. Systems with power factors less than 1.0 have some amount of reactive power flowing back and forth that is not doing useful work.
Why is Power Factor Important?
There are several reasons why power factor matters in electrical systems:
- Higher power factor means less current is required to deliver the same amount of real power. This reduces losses in the distribution system.
- Low power factor requires larger conductor sizes and bigger transformers to handle the extra current.
- Utility companies often charge commercial customers extra fees if their power factor is below a certain threshold, usually around 0.9.
- Generators and motors operate less efficiently at low power factors, wasting energy.
By improving power factor, utilities and customers can reduce costs and use energy more efficiently.
What Causes Low Power Factor?
The main cause of low power factor is having a large amount of reactive power flow. Reactive power occurs when the current and voltage are out of phase with each other. Devices like inductors and capacitors cause reactive power to circulate in the system.
Common causes of low power factor include:
- Motors – The large inductors in motors produce reactive power and lower power factor.
- Transformers – Leakage reactance in transformers consumes reactive power.
- Fluorescent lighting – The ballasts in fluorescent lights are inductive and require reactive power.
- Welding equipment – Arc welders use inductors that require substantial reactive power.
- Inductive loads – Any device with coils like solenoids, actuators, induction furnaces, etc. will draw reactive power.
By adding power factor correction equipment like capacitors, the reactive power can be reduced, bringing the power factor closer to unity.
What Does 80% Power Factor Mean?
A power factor of 80% means that the real power being used is 80% of the total apparent power. The remaining 20% of apparent power is reactive power circulating in the system. This is considered reasonably good for most electrical systems.
For example, say a facility has the following loads:
- Real power used = 500 kW
- Apparent power = 625 kVA
The power factor would be:
Power Factor = Real Power / Apparent Power
= 500 kW / 625 kVA
= 0.80 (80%)
While 80% is decent, there are still opportunities to improve the power factor closer to unity by adding power factor correction equipment. This would reduce the reactive power and current, saving energy.
Is 80% Power Factor Good or Bad?
Generally, a power factor of 80% is considered reasonable for most commercial and industrial facilities. While not ideal, a PF of 80% is typically acceptable and usually does not incur utility penalties.
Here are some guidelines for evaluating power factor:
- 90-100% – Excellent power factor
- 80-90% – Reasonable, but could be improved
- 70-80% – Often the threshold for utility charges
- Below 70% – Poor power factor, penalties likely
Facilities aiming for maximum efficiency will want to achieve a power factor over 90%. However, getting above 80% is a good interim goal for older systems and requires less extensive equipment upgrades.
It is rarely cost-effective to try improving power factor above 95-96%. The amount of correction equipment required reaches a point of diminishing returns. Keeping power factor above 80% provides meaningful efficiency improvements without major expense.
How to Improve Power Factor
There are several ways facilities can improve their power factor closer to unity:
- Capacitors – Installing capacitor banks counters inductive loads and provides reactive power.
- Synchronous condensers – These specialized motors supply reactive power.
- Phase shifting transformers – Specialty transformers correct poor power factor.
- Harmonic filters – Filters out harmonics that distort power factor.
- Replacing equipment – Upgrading to higher efficiency motors and lights improves power factor.
- Power factor correction devices – Automatic controllers can connect capacitors when needed.
Adding shunt capacitors is one of the most common and cost-effective ways to correct poor power factor. The capacitors supply reactive power to cancel out inductive loads. This balances the real and reactive components for a power factor closer to unity.
Why Utilities Penalize Low Power Factor
Utility companies often penalize commercial and industrial customers for having a power factor below a certain limit. Typically the threshold is around 0.9 to 0.95. The penalties are in place because low power factor causes greater line losses and requires oversized wiring and transformers.
With a low power factor, the utility has to generate extra apparent power (kVA) beyond what is actually consumed (kW). This strains the capacity of generators, transformers and conductors. It also leads to higher losses in the transmission and distribution system. The excess kVARs increase costs for the utility company.
By penalizing customers with poor power factor, the utility motivates them to install correction equipment. This reduces the burden on the electrical grid and allows the system to run more efficiently.
Typical Power Factor Requirements
While requirements vary between utilities, some typical power factor thresholds are:
- Residential customers – No requirements
- Commercial customers – 0.85 to 0.95 lagging
- Industrial customers – 0.90 to 0.95 lagging
- Data centers/servers – 0.95+ lagging
Customers with a leading power factor (capacitive load) may also be penalized. Utilities want to discourage leading power factor since it can cause voltage instability.
Within these ranges, penalties or surcharges are applied to the customer’s bill. Typical power factor penalties can range from 0.5% to 3% of the total bill. Correcting the power factor avoids these extra costs.
Power Factor Correction Methods
Here are some common ways to correct low power factor in electrical systems:
Capacitors
Capacitors supply reactive power that counters inductive loads. They are a low cost, reliable way to improve lagging power factor. Shunt capacitors are connected in parallel to correct the overall system power factor.
Synchronous Condensers
These specialized synchronous motors have oversized excitation systems to generate reactive power. Their rotating mass provides inertia to the system. Often used when large capacitor banks are impractical.
Static VAR Compensators (SVC)
An SVC uses thyristors and capacitors to provide fast reactive power compensation. It dynamically adjusts to both inductive and capacitive loads.
Static Synchronous Compensators (STATCOM)
Similar to an SVC but uses voltage source inverters instead of capacitors and reactors. STATCOMs provide very rapid power factor correction.
Phase Shifting Transformers
Specialty transformers can adjust the phase angle of the voltage relative to the current. This directly controls the power factor at the secondary side.
Harmonic Filters
Filters out harmonic currents caused by non-linear loads. This reduces distortion and improves true power factor.
Equipment Upgrades
New energy efficient motors, lights and other devices will have a higher power factor. Replacing outdated equipment improves overall facility power factor.
Power Factor Controllers
Automatically switch capacitor banks to maintain a target power factor. Often programmed with a utility’s power factor requirements.
Advantages of High Power Factor
Here are some of the benefits of improving power factor in electrical systems:
- Reduces utility penalties and surcharges
- Decreases losses in lines, transformers and generators
- Allows same capacity loads to be served with less current
- Minimizes voltage drops across circuits
- Increases capacity of existing electrical equipment
- Improves voltage regulation and stability
- Enhances generator and motor efficiency
- Decreases energy consumption and cost
Disadvantages of Low Power Factor
Drawbacks of operating at a low power factor include:
- Higher electricity bills due to utility fees and penalties
- Increased power generation and transmission costs
- Oversized cables, transformers and conductors required
- Greater line losses and voltage drops
- Equipment operates less efficiently
- Capacities limited for existing electrical equipment
- Power quality issues like voltage flickering
- Higher reactive power causes greater system losses
Power Factor Correction Economics
The costs of power factor correction equipment must be weighed against the savings from reduced utility penalties and energy consumption. Steps for evaluating the economics include:
- Measure existing power factor and load profiles
- Estimate utility penalties for poor power factor
- Audit reactive power sources like motors and fluorescent lights
- Model power factor improvement scenarios and costs
- Analyze lifecycle savings from reduced utility charges
- Consider ancillary benefits like improved voltage regulation
- Determine optimal power factor correction plan
Since the costs are relatively fixed but utility rates rise over time, power factor correction investments often payback over 5-10 years. Maintenance of capacitors and controllers should also be included in cost projections.
Power Factor Correction Examples
Here are two examples demonstrating how power factor correction capacitors can improve a low power factor:
Example 1
- Load: 500 kW 120 VAC
- Power Factor: 0.7 lagging
- Apparent Power = kW / PF = 500 kW / 0.7 = 714 kVA
- Reactive Power = kVAR = Sqrt(kVA2 – kW2) = 536 kVAR
- Capacitor Size = kVAR / (V x 1.73) = 536 kVAR / (120 x 1.73) = 387 μF
By adding a 387 μF capacitor, the power factor is improved to 0.95 lagging.
Example 2
- Load: 350 kW 480 VAC
- Power Factor: 0.8 lagging
- Apparent Power = kW / PF = 350 kW / 0.8 = 438 kVA
- Reactive Power = kVAR = Sqrt(kVA2 – kW2) = 262 kVAR
- Capacitor Size = kVAR / (V x 1.73) = 262 kVAR / (480 x 1.73) = 87 μF
The 87 μF capacitor brings the power factor up to 0.95 lagging.
Key Takeaways about Power Factor
- Power factor measures how efficiently electrical power is being utilized in a system.
- Lagging power factor is caused by inductive loads like motors and transformers.
- Leading power factor results from capacitive loads.
- Utility companies penalize customers for low lagging power factor.
- Capacitors are commonly used to correct lagging power factor.
- Power factor correction reduces losses, increases capacity, and improves voltage.
- The economics depend on utility costs versus equipment costs.
- Maintenance of capacitors and control systems should be included.
- Aim for >0.9 PF, but careful evaluation above 0.95 PF.
- Power factor requirements vary by utility and customer type.
Conclusion
A power factor of 80% means that 80% of the apparent power in a system is being usefully consumed as real power. The other 20% is reactive power being circulated. While an 80% power factor is reasonable, most facilities can benefit by improving it to 90-95% through capacitors and other methods.
Higher power factors reduce electrical losses, increase capacity, improve voltage regulation, and avoid utility penalties. Power factor correction makes the electrical system more efficient and lowers costs. Implementing capacitors, condensers, filters, new equipment, and control systems can provide meaningful power factor improvements for commercial and industrial customers.