TERMINAL FAULT, SHORT LINE FAULT AND OUT OF PHASE SWITCHING INTERRUPTING CONDITIONS OF POWER CIRCUIT BREAKERS BASIC INFORMATION AND TUTORIALS

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.