Concepts of transients- Power Quality

Concepts of transients:

^ü   Transient over voltages in electrical transmission and distribution networks result from the unavoidable effects of lightning strike and network switching operations.\

^ü   Response of an electrical network to a sudden change in network conditions.

ü     Oscillation is an effect caused by a transient response of a circuit or system. It is a momentary event preceding the steady state (electronics) during a sudden change of a circuit.

^ü   An example of transient oscillation can be found in digital (pulse) signals in computer networks. Each pulse produces two transients, an oscillation resulting from the sudden rise in voltage and another oscillation from the sudden drop in voltage. This is generally considered an undesirable effect as it introduces variations in the high and low voltages of a signal, causing instability.

^ü   Types of transient:

o  Impulsive transient

o   Oscillatory transient

1. Impulse transient:

A sudden, non power frequency change in the steady state condition of voltage or current that is unidirectional in polarity.

2. Oscillatory transient:

A sudden, non power frequency change in the steady state condition of voltage or current that is bidirectional in polarity.

3. Short duration variations – Interruption

The complete loss of voltage on one or more phase conductors for a time less than 1 min.

3.1 Types of Short Duration interruption:

^ü   Momentary Interruption < 1 min , <0.1 pu

^ü   Temporary Interruption < 1 min , <0.1 pu

4. Long duration variations – Sustained interruption

The complete loss of voltage on one or more phase conductors for a time greater than 1 min.