46.Power Delivery by John G. Webster (Editor)

By John G. Webster (Editor)

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69 kV and higher) transmission lines which can be hundreds of miles long. These lines are protected by circuit breakers close to the voltage source. Consider when short circuits occur between overhead lines, which are protected by a high-voltage circuit breaker. If the fault is line-to-line, the upstream breaker interrupts the fault current. Immediately following the current interruption, a TRV consisting of a sawtooth wave superimposed on the (1 Ϫ cos) voltage wave begins to appear across the breaker contacts.

The maximum operating voltage is the maximum safe circuit voltage at which the circuit breaker is designed to operate reliably. , source-side and load-side voltages are 180Њ out of phase) for a period of time across the separated electrodes. The transient recovery voltage (TRV) is the peak of the recovery voltage across the circuit breaker contacts immediately following current interruption. This voltage is a highfrequency voltage superimposed on the power-frequency source. The time to crest for the TRV depends on the inductance, capacitance, and cable effects of the breaker circuit.

As in all breakers, the current electric arc burns until a natural current zero is reached, and the arc is extinguished. Compressed air and SF6 gas both have excellent heat capacity to deionize the arc plasma and excellent dielectric resistance, so that the breaker can withstand the high transient recovery voltage. The compressed gas is supplied from a compressor or by a piston connected to the movable electrode. The former breaker is called a ‘‘two-pressure’’ breaker. The latter normally uses SF6 and is called a ‘‘puffer’’ breaker which uses a piston and electrode arrangement.

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