POWER QUALITY BASICS: TRANSIENTS

Saturday, May 7, 2011

Transients are power quality disturbances that involve destructive high magnitudes of current and voltage or even both. It may reach thousands of volts and amps even in low voltage systems. However, such phenomena only exist in a very short duration from less than 50 nanoseconds to as long as 50 milliseconds. This is the shortest among PQ problems, hence, its name. Transients usually include abnormal frequencies, which could reach to as high as 5 MHz.
Transients
Transients
In addition, transients are also known as surge. According to IEEE 100, surge is a transient wave of voltage, current or power in an electric circuit. Other IEEE definitions suggest that it is the part of the change in a variable that disappears during transition from one steady-state operating condition to another. Such description is too vague, which could be used to describe just about any unusual events occurring in the electrical system. Moreover, most electrical engineers would refer to the damped oscillatory transient phenomena in a RLC circuit when hearing such term.

Sources of Transients 

Lightning Strikes
Switching activities
·         Opening and closing of disconnects on energized lines
·         Capacitor bank switching
·         Reclosing operations
·         Tap changing on transformers
Loose connections in the distribution system that results to arcing
Accidents, human error, animals and bad weather conditions
Neighboring facilities

Effects of Transients

Electronic Equipment
Ø  Equipment will malfunction and produces corrupted results
Ø  Improper specification and installation of TVSS can aggravate the failures
Ø  Efficiency of electronic devices will be reduced

Motors
Ø  Transients will make motors run at higher temperatures
Ø  Result in micro-jogging leading to motor vibration, excessive heat and noise
Ø  Degrades the insulation of the motor winding resulting to equipment failure.
Ø  Increases the motor’s losses (hysteresis) and its operating temperature

Lights
Ø  Fluorescent bulb and ballast failure
Ø  Appearance of black rings at the fluorescent tube ends (indicator of transients)
Ø  Premature filament damage leading to failure of the incandescent light.

Electrical Equipment
Ø  Transients degrade the contacting surfaces of circuit breakers and switches
Ø  Nuisance tripping of breakers due to false activation to a non-existent current demand
Ø  Reduce transformer efficiency because of increased hysteresis losses

Damages due to such PQ problems are uncommon as compared to voltage sags and interruptions, but when it does occur it is more destructive. To protect against transients, end-users may use Transient Voltage Surge Suppressors (TVSS), while utilities install surge arresters.

Furthermore, transients are classified as:

1.    Impulsive
2.    Oscillatory

Synopsis:

Magnitude: Several thousands of volts and amps
Duration: <50 ns to 50 ms
Source: Lightning and switching activities
Symptoms: Electronic component damage
Occurrence: Low
Mitigating Devices: TVSS and Surge Arresters

References:
Dugan, R., McGranaghan, M., Santoso, S., and Beaty, H.W. (2004). Electrical Power Systems Quality (2nd ed.). New York: McGraw-Hill.
IEEE 1159-1995. Recommended Practice For Monitoring Electric Power Quality.

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I am an Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality, Diagnostic Testing and Protective Relaying. I have been working in an electric distribution utility for more than a decade. I handle PQ studies, power system analysis, diagnostic testing, protective relaying and capital budgeting for company projects.