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POWER QUALITY TERMS

IMPULSE TRANSIENT, OSCILLATORY TRANSIENT, VOLTAGE DIPS, VOLTAGE SWELLS OR SURGES, OVER VOLTAGE, UNDER VOLTAGE, WAVEFORM DISTORTION OR HARMONICS, NOTCHING, NOISE

IMPULSE TRANSIENT

A sudden and short-duration disturbance caused by a very rapid change in the steady-state condition of voltage, current, or both, that is unidirectional in polarity.

Frequency range: >5 kHz (high frequency)
Duration: 30-200 uSec

LOW-FREQUENCY TRANSIENTS: <500 HZ

Duration: <30 cycles

Causes:
•Capacitor switching

Impacts:
•Tripping of ASDs and other sensitive equipment
•Voltage magnification at customer capacitors

OSCILLATORY TRANSIENT

A sudden and short-duration disturbance caused by a very rapid change in the steady-state condition of voltage, current, or both, that is bi-directional in polarity.

MEDIUM-FREQUENCY TRANSIENTS (500 HZ-2 KHZ)

Duration: < 3 cycles)

Causes:
•Traveling waves from lightning impulses
•Capacitor and circuit switching transients

Impacts:
•Failure of customer equipment

HIGH FREQUENCY TRANSIENTS >2 KHZ

Duration: < 0.5 cycle

Causes:
•Switching on secondary systems
•Lightning-induced ringing
•Local ferroresonance

Impacts:
•Radiated noise may disrupt sensitive electronic equipment
•High rate of rise oscillations may cause low-voltage power supplies to fail
•Short-duration voltage variations

VOLTAGE DIPS

Decrease between 0.1 and 0.9 pu in rms voltage or current at the power frequency for duration from 0.5 cycles to 1 minute

Causes:
•Local and remote faults

Impacts:
•Dropouts of sensitive customer equipment

VOLTAGE SWELLS OR SURGES

Increase to between 1.1 and 1.8 pu in the rms voltage or current at the power frequency for durations from 0.5 cycle to 1 min

Causes:
•Single-line-to-ground faults
•Equipment overvoltage

Impacts:
•Failure of MOVs forced into conduction
•Long-duration voltage variations

OVER VOLTAGE

Increase in the rms ac voltage greater than 110% at the power fre quency for a duration longer than 1 min.

Causes:
•Load switching off
•Capacitor switching on
•System voltage regulation

Impacts:
•Problems with equipment that require constant steady-state voltage

UNDER VOLTAGE

Decrease in the rms ac voltage to less than 90% at the power frequency for a duration longer than 1 minute

Causes:
•Load switching on
•Capacitor switching off

Impacts:
•Problems with equipment that require constant steady-state voltage, system voltage regulation

WAVEFORM DISTORTION OR HARMONICS

Nonlinear loads, such as power electronic equipment, produce non-sinusoidal current waveforms when energized with a sinusoidal voltage. They inject currents at harmonic (integer multiple of the fundamental frequency) frequencies into the system. Harmonic currents, and the voltage distortion they create as they flow through the system impedance.

Causes:
•Nonlinear loads

Impacts:
•Improper operation of sensitive equipment
•Reduction of equipment operating reliability and service life (0-100th harmonic)
•Capacitor failures or fuse blowing
•Telephone interference

NOTCHING

Periodic voltage disturbance caused by the normal operation of power electronics devices when current is commutated from one phase to another

Causes:
•Normal operation of electronic equipment

Impacts:
•Improper operation of equipment
•Equipment failure

Noise

Any unwanted electrical signals with broadband spectral content lower than 200 kHz superimposed upon the power system voltage or current in phase conductors, or found on neutral conductors or signal lines

Causes:
•Improper grounding
•Normal operation of electronic equipment
•Arcing devices
•Switching power supplies

Impacts:
•Interference with electronic devices such as microcomputer and programmable controllers

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