Disturbances & Interruptions

Power quality disturbances can be generated from both the utility and customer sides of the distribution network. Utility generated disturbances are typically caused by events related to the following:

Transformers - Tap-switching generates transients
Protective Apparatus - Re-closure of circuit breakers causes cycling of power (This includes: Fuses, circuit breakers, lightning arresters and suppressors, insulators, and switches)
Corrective Apparatus - Power factor correction capacitors and voltage regulating equipment generate spikes and transients

Disturbances to power quality generated from the customer-side are typically the result of:

Grounding mistakes - Multiple N-G bonds
Wiring errors and electrical code violations: Reversed polarity
Nonlinear loads - Switch power supplies and pulse rectification
System not designed properly - High impedance neutrals
Interference from other devices or loads - For example, the lights flicker when an air conditioner comes on may indicate that the air conditioner is on the same circuit as other electronic equipment
Some equipment is sensitive to utility operations - Capacitors switching
Operation of portable radio transmitters near electronic equipment
Electro-static discharge

Interruptions to power service resulting from a fault in the power system are not necessarily power quality issues. These faults are typically due to physical impacts to the distribution network by:

Trees

Lightning
Equipment Failure
Wind
Animals
Icing
Public Interference

Transients

Impulsive Transient
	A sudden and short duration disturbance 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
Causes	Lightning
Impacts	Transformer failures Arrester failures Customer equipment damage due to low-side surges

Oscillatory Transient
	A temporary rapid fluctuation in the steady-state condition of voltage, current, or both, that includes positive and negative polarity values.
Low Frequency Transients	<500 Hz, <30 cycles
Causes	Capacitor switching
Impacts	Tripping of ASDs and other sensitive equipment Voltage magnification at customer capacitors
Medium Frequency Transients	500 Hz - 2 kHz, < 3 cycles
Causes	Traveling waves from lightning impulses Capacitor and circuit switching transients
Impacts	Failure of customer equipment
High Frequency Transients	>2 kHz, < 0.5 cycle
Causes	Switching on secondary systems Lightning-induced ringing Local ferro-resonance
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 Sags (dips)
	Decrease between 0.1 and 0.9 per unit (pu) in root mean square (rms) voltage or current at the power frequency for duration from 0.5 cycles to 1 min
Causes	Local and remote faults
Impacts	Dropouts of sensitive customer equipment

Voltage Swells (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
Impacts	Equipment overvoltage Metal oxide varistors (MOV) are forced into conduction

Long Duration Voltage Variations

Overvoltage
	Increase in the rms voltage greater than 110 percent at the power frequency 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

Undervoltage
	Decrease in the rms voltage to less than 90 percent at the power frequency for a duration longer than 1 min.
Causes	Load switching on Capacitor switching off System voltage regulation
Impacts	Problems with equipment that require constant steady-state voltage

Interruptions

	Momentary Interruption: 1/2 - 3 secs
	Temporary Interruption: 3 - 60 secs
	Long-Term Interruption (outage): > 1 minute
Causes	Temporary Faults Lightning stroke Tree limb falling across two conductors then dropping clear Faults that do not clear
Impacts	Operation interruption, production losses, and revenue losses

Waveform Distortions

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, can reduce equipment operating reliability and service life (0-100th harmonic).
Causes	Nonlinear loads
Impacts	Malfunction of sensitive equipment 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	Malfunction of equipment

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	Disturbing electronic devices such as microcomputer and programmable controllers

Voltage Fluctuation

Voltage Fluctuation (flicker):
	The impact of the voltage fluctuation on lamps such that they are perceived to flicker by the human eye. This is due to loads which exhibit continuous, rapid variations in the load current, particularly the reactive component.
Causes	Arc furnaces Intermittent loads
Impacts	Lighting flicker Malfunction of sensitive loads

Frequency Variation

Frequency Variation:
	The deviation of the power system fundamental frequency from its specified nominal value (e.g., 50 or 60 Hz).
Causes	Poor speed regulations of local generation Faults on the bulk power system Large block of load being disconnected Disconnecting a large source of generation
Impacts	Equipment failure