Live-tank SF6 gas puffer-type
interrupters are utilized by most circuit switchers today. In the
closed position, the contacts are surrounded by a flow guide and
piston assembly which is ready to mechanically generate a “puff”
of SF6 to cool and deionize the arc that is established prior to
circuit interruption.
The moving cylinder attached to the
contact assembly is driven by the main opening spring, causing the
gas to be pressurized by the stationary piston. The stationary
contact “follows” the moving contact as the piston assembly
achieves the prepressurized gas condition.
When the contacts (which are hollow
tubes) part, an arc is established and the gas flow divides into two
parts and flows down the stationary and moving contact tubes. The
alternating nature of the arc current waveform results in two current
zeros every cycle. As long as the arc is sufficiently “hot” or
conductive through the SF6 dielectric medium, the current will
reestablish.
At the first current zero where the SF6
density is sufficient to stop the arc from reestablishing itself and
to provide necessary dielectric strength, the arc is interrupted.
This entire process from trip signal initiation to current
interruption requires from 3 to 8 cycles or 50 to 133 ms in modern
circuit switchers.
Figure above illustrates a typical
“blade-disconnect model” circuit switcher with the interrupter
and blade connected in series. For opening, the trip device, called a
“shunt trip,” receives a trip signal when the relay system
detects an abnormal condition within the specified range or when the
operator desires a high-speed circuit opening. By discharging its
operating spring, the shunt trip rotates the insulator above it at
high speed, thus tripping and discharging the opening spring in the
driver mechanism.
This actuates the interrupter to open
the circuit. If the insulator above the shunt trip continues to
rotate, by motor or manual actuation of the drive train controls, the
blade opens to achieve visible isolation. The blade-hinge mechanism
is actuated directly by the rotating insulator through the driver
mechanism.
Continued rotation of the insulator
after the blade is open will “toggle” the drive train controls to
lock the blade in its open position. For closing, the reverse
rotation of the insulator first releases drive train toggle and
allows the blade to begin closing.
The shunt-trip units have already
recharged during the opening operation. As the blade closes, the
closing springs are charged in the driver. The last few degrees of
closing rotation lock the blade in position and release the closing
springs in the driver, thus closing the interrupter.
The opening springs are charged as the
closing springs discharge. If the unit has closed into a circuit
condition that provides a trip signal to the shunt trip units, the
opening process may immediately proceed since all springs are charged
and all controls are ready.
The closing operation may be achieved
in other designs by closing the interrupter during the opening stroke
of the blade. When a close operation is called for, all that is
necessary is to close the blade, because the interrupter is already
closed. Because of the arc established in air for this type of
closing, high-speed operation of the blade is necessary to minimize
damage to contacts and prevent flashovers.
Both methods of closing are proven over
many years of field use. Bladeless circuit switchers operate exactly
the same as blade models, except that on opening, the insulator
rotation is used only for driver and interrupter actuation. Models
that depend on high-speed blade operation for closing are available
in bladeless nondisconnect configuration, but circuit closing must be
accomplished by other means.
For models without shunt trip, opening
is accomplished by rotating the insulator to the point where the
driver opening spring would normally be tripped by the shunt trip’s
rotation. This configuration is used where protection duty is not a
function of the circuit switcher.
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