Terminal Fault.
After interruption of short-circuit
current, the recovery voltage oscillates toward the service frequency
driving voltage via an initial peak. The natural frequency is
determined by the inductance and capacitance of the driving system.
The dc component of the short-circuit
current depends on the time constants of the network components like
generators, transformers, cables, and high-voltage lines and their
reactances of the zero-sequence and the positive sequence networks.
The recovery voltage will accordingly
vary depending on the location of the circuit breaker within the
network.
Short-Line Fault.
In the case of a short-line fault, a
section of line lies between the breaker and the fault location.
After the short-circuit current has been interrupted, the oscillation
at the line side (L) of the breaker assumes a superimposed
“saw-tooth” shape.
The rate of rise of this line
oscillation is directly proportional to the effective surge impedance
and the time rate of change of current\ (di/dt) at current zero. The
component on the supply side (S) basically exhibits the same waveform
as a terminal fault.
The circuit breaker is stressed by the
difference between these two voltages. Because of the high frequency
of the line oscillation, the transient recovery voltage has a very
steep initial rate of rise.
Since the initial rate of rise
increases with increasing rate of current change, the limiting
interrupting capability of many breaker designs is determined by the
short-line fault.
Out-of-Phase Switching.
Two network systems with driving
voltages E1 and E2 are connected via a high-voltage transmission
line. Since the circuit is closed via the closed circuit breaker, the
resulting driving voltage is equal to the sum of the two system
voltages.
Driving voltage E2 may, for example,
exceed voltage E1 by the voltage drop across the transmission line.
After opening the breaker, the transient recovery voltages of the
disconnected networks oscillate independently.
The circuit breaker is stressed by the
difference of these two voltages. In the case of disconnection of
long lines, the recovery voltage across the breaker could be
increased because of the Ferranti effect, where the voltage of the
receiving end can be up to 15% higher than the sending end if the
line is lightly loaded.
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