How power circuit breaker works?
Circuit Breaker Performance During
Interruption; Basic Intervals
The Circuit breaker has two basic
positions: closed and opened. In the closed position the circuit breaker
conducts full current with negligible voltage drop across its contacts. In the
open position it conducts negligible current but with full voltage across the
contacts.
This defines two main stresses, the current
stress and the voltage stress, that are separated in time. However, the main
function of the circuit breaker is neither to conduct nor to isolate. It
performs its main function in changing from one condition to the other, that
is, the switching operation.
If closer attention is paid to the voltage
and current stresses during the interrupting test (figure below), three main
intervals can be recognized.
1. High-Current Interval
The high-current interval is the time from
contact separation to the significant change in arc voltage preceding the
interaction and high-voltage intervals.
2. Interaction Interval
The interaction interval is the time from
the significant change in arc voltage prior to current zero to the time when
the current including the post arc current, if any, ceases to flow through the
test breaker.
3. High-Voltage Interval
The high-voltage interval is the time from
the moment when the current including the post arc current, if any, ceases to
flow through the test breaker to the end of the test.
State of Interrupting Process
The three intervals described in 1 to 3
follow each other immediately, that is, they cover the whole interrupting
process without any discontinuities, even though it might be difficult to
establish precisely the moment when one interval ends and the other begins.
However, this accuracy may not be required.
State of Interrupting Process During Three
Basic Intervals
The quantities determining the physics of
the interrupting process change considerably during the circuit breaking
operation. In fact, the prevailing physical conditions have different
importance during the three time intervals.
High-Current Interval
During the high-current interval,
short-circuit current is flowing through the circuit breaker with a relatively
small voltage drop across the contacts. A large amount of energy is supplied to
the arc establishing the state of ionization, temperature, dynamic pressure,
etc, important for the switching function.
Interaction Interval
During the interaction interval, the
short-circuit current stress changes into high-voltage stress and the breaker
performance can significantly influence the currents and voltages in the
circuit. As the current decreases to zero, the arc voltage may rise to charge
parallel capacitance and distort current passing through the arc.
After the current zero the post arc
conductivity may result in additional damping of the transient recovery voltage
and thus influence the voltage across the breaker and the energy supplied to
the ionized contact gap.
The mutual interaction between the circuit
and the circuit breaker immediately before and after current zero (that is,
during the interaction interval) is of extreme importance to the switching
process.
High-Voltage Interval
During
the high-voltage interval, the gap of the breaker is stressed by recovery
voltage. The circuit breaker is now a passive element in the circuit.
No comments:
Post a Comment