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Understanding Power Quality


  • Basic Utility System Operations
    • The Giant Electric Machine
    • Power Quality - Definitions
    • Voltage Sags
    • Ringing Transients
  • Trends in Utilization Equipment
    • Increased Sensitivity
    • Unconventional Demands
    • Harmonics
    • All Equipment Not Created Equal


  • Wiring and Grounding Practices for Sensitive Loads
    • Power Wiring
    • Preventing Equipment Interference
    • Control Wiring
  • Monitoring Equipment
  • Mitigation Equipment
  • Questions/Open Discussion


Voltage Unbalance

The correct way to calculate percent unbalance is to use phase to phase voltages in the equation below.

Equation 1

Phase to Phase Voltage Equation

For example, phase-to-phase voltages of 470, 479, and 482 average to 477. The maximum deviation from average is the phase with 470 volts. The deviation between 470 and 477 is 7 volts. The percent unbalance is

Equation 2

Unbalance = 7/477 * 100

Sensitive Equipment in today's facilities can increase production losses.  Click on the link to see an illustration.   This picture shows an area (in grey) where a fault can occur.  One piece of equipment can tolerate a 70% sag, while another device shuts down a process when it sees a >90% sag.