17-04-2012, 04:07 PM
SWITCHING TRANSIENTS
Introduction
A transient is created whenever a sudden change occurs in a power circuit, especially during power switching - either closing or opening a circuit.
1. Simple Switching Transients
2. Abnormal Switching Transients
Simple Switching Transients
Simple switching transients include circuit closing transients, transients initiated by clearing a short-circuit.
In circuits having inductance and capacitance with little damping, these simple switching transients are inherently limited to twice the peak amplitude of the steady-state sinusoidal voltage.
current chopping
Current chopping is the name given to the rapid current reduction, prior to the natural current zero of the power system, which fuses or circuit breakers can force when clearing a circuit. When there is inductance in the circuit, this rapid current change can produce high overvoltage - some 10 times the normal circuit voltage
Capacitor switching can be troublesome if the switch restrikes after current interruption:
the capacitor voltage remains nearly constant at maximum system voltage, since the interruption occurred at zero current, which is 900 apart from the voltage zero, while the system voltage follows the normal sine wave .
At 180 degrees after interruption, the switch has to support twice the system voltage, a stress it might not be able to support with its contacts incompletely separated
CONCLUSIONS
The two major causes of transient overvoltage & lightning or switching surges, have been identified with greater precision in low-voltage ac circuits as well as in communication systems. While standardizing the definition of the environment will not change the environment itself, the emergence of realistic standards will enable designers to increase the reliability of their products. Likewise, users will be able to protect their equipment more effectively.