10.4 Power Factor and Power Factor Correction

Power factor is a crucial concept in AC power systems, measuring how efficiently electrical power is used. It's the ratio of real power to apparent power, ranging from 0 to 1. A high power factor means better energy utilization, while a low one leads to inefficiencies and increased costs.

Understanding power factor is key to optimizing electrical systems. It affects equipment performance, system capacity, and energy costs. Improving power factor through various correction methods can lead to significant energy savings, reduced utility penalties, and better overall system efficiency.

Power factor in AC systems

Definition and importance

Top images from around the web for Definition and importance

• 

  Power factor - Wikipedia View original

  Is this image relevant?

• 

  Power factor - Wikipedia View original

  Is this image relevant?

• 

  Apparent power meaning - Electrical Engineering Stack Exchange View original

  Is this image relevant?

• 

  Power factor - Wikipedia View original

  Is this image relevant?

• 

  Power factor - Wikipedia View original

  Is this image relevant?

1 of 3

Top images from around the web for Definition and importance

• 

  Power factor - Wikipedia View original

  Is this image relevant?

• 

  Power factor - Wikipedia View original

  Is this image relevant?

• 

  Apparent power meaning - Electrical Engineering Stack Exchange View original

  Is this image relevant?

• 

  Power factor - Wikipedia View original

  Is this image relevant?

• 

  Power factor - Wikipedia View original

  Is this image relevant?

1 of 3

• Power factor represents efficiency of power utilization in AC electrical systems
• Ratio of real power (P) to apparent power (S)
• Expressed as decimal value between 0 and 1, or percentage (100% ideal)
• Mathematically defined as cosine of phase angle between voltage and current waveforms
• High power factor indicates efficient power usage
• Low power factor suggests inefficient power consumption and increased system losses
• Crucial for utilities and industrial facilities affects equipment performance, system capacity, and energy costs

Key components and concepts

• Reactive power measured in volt-amperes reactive (VAR) key component in understanding power factor
• Real power (P) measured in watts (W) represents actual power consumed by load for useful work
• Apparent power (S) measured in volt-amperes (VA) vector sum of real power and reactive power
• Power triangle graphically represents relationship between real power, reactive power, and apparent power
• Balanced three-phase system power factor calculated using line-to-line voltage (VL-L) and line current (IL)
  • Formula: $Power Factor = P / (\sqrt{3} * V_{L-L} * I_L)$
• Power analyzers and power quality meters directly measure power factor in electrical systems

Calculating power factor

Basic calculations

• Power Factor formula: $Power Factor = Real Power (P) / Apparent Power (S)$
• Real power (P) measured in watts (W)
• Apparent power (S) measured in volt-amperes (VA)
• Example: If real power 800 W and apparent power 1000 VA, power factor 0.8 or 80%
• Power factor always between 0 and 1 (or 0% to 100%)

Advanced calculations

• Three-phase balanced system power factor calculation: $Power Factor = P / (\sqrt{3} * V_{L-L} * I_L)$
  • VL-L line-to-line voltage
  • IL line current
• Power triangle method uses trigonometry to find power factor
  • Example: If real power 1000 W and reactive power 750 VAR, apparent power $\sqrt{1000^2 + 750^2} = 1250 VA$
  • Power factor $1000 / 1250 = 0.8$ or 80%
• Complex power method uses complex numbers to represent power components
  • S = P + jQ, where j imaginary unit
  • Power factor magnitude of real part divided by magnitude of complex power

Effects of low power factor

System performance impacts

• Increases current draw in electrical systems leads to higher I²R losses in conductors and transformers
• Causes voltage drops and reduced system efficiency due to increased current flow
• Reduces overall capacity of electrical distribution systems limits ability to add new loads
• Results in poor voltage regulation potentially causing equipment malfunction or failure
• Example: System with 1000 kVA capacity at 0.8 power factor can only deliver 800 kW of real power

Economic and equipment implications

• Utilities often impose penalties or higher rates for customers with low power factor increases operating costs
  • Example: 5% rate increase for power factor below 0.9
• Equipment (motors, transformers) may experience increased heating and reduced lifespan
  • Example: Motor rated for 100 hp at 0.8 power factor may only deliver 80 hp at 0.64 power factor
• Increased energy losses lead to higher electricity bills
• Requires larger capacity equipment (transformers, cables) to handle higher currents increases capital costs

Power factor correction methods

Passive correction techniques

• Capacitor banks widely used provide leading reactive power to offset lagging loads
  • Fixed capacitor banks for constant loads
  • Switched capacitor banks for varying loads
• Phase advancers improve power factor of induction motors by modifying rotor circuit
• Harmonic filters combine capacitors and inductors to correct power factor and reduce harmonics
• Example: 100 kVAR capacitor bank can improve power factor from 0.8 to 0.95 in a 500 kW system

Active correction techniques

• Static VAR compensators (SVCs) use thyristor-controlled reactors and capacitors for dynamic power factor correction
• Synchronous condensers (synchronous capacitors) rotating machines provide or absorb reactive power
• Active power factor correction (PFC) circuits use switching power supplies to improve power factor in electronic devices
  • Example: Computer power supplies with active PFC can achieve power factor >0.95
• Automatic power factor correction systems use controllers to switch capacitor banks based on real-time measurements
  • Example: System with multiple 50 kVAR steps can fine-tune power factor to target value (0.98)

Power factor correction design

Calculation and sizing

• Determine required capacitive reactance (Xc) to achieve desired power factor improvement using power triangle method
• Calculate capacitance value needed for correction: $C = 1 / (2\pi fX_c)$, where f system frequency
• Consider harmonics present in system when selecting correction capacitors to avoid resonance issues
• Example: To improve power factor from 0.8 to 0.95 in 1000 kW system at 480V, 60Hz:
  • Required reactive power $Q = P(\tan(\cos^{-1}(0.8)) - \tan(\cos^{-1}(0.95))) = 438 kVAR$
  • Capacitance $C = Q / (2\pi f V^2) = 3978 \mu F$

Implementation and control

• Implement step-wise capacitor bank control system provides flexible and accurate correction under varying loads
• Design protection systems for capacitor banks includes fuses, circuit breakers, and discharge resistors
• Conduct cost-benefit analysis determine optimal level of correction, considering energy savings, reduced penalties, and equipment costs
• Integrate correction equipment with existing power monitoring and control systems for efficient operation
• Example: Automated system with power factor target of 0.98:
  • Measures power factor every 15 seconds
  • Switches capacitor banks in 50 kVAR steps
  • Includes 5-minute delay to prevent rapid switching