High Voltage Circuit Breakers

At voltages 46 kV and above (classified as “high voltage” in the electrical power industry), the challenge of extinguishing the electric arc formed by separating breaker contacts becomes severe. Two popular strategies for mitigating contact arc in modern high voltage circuit breakers are oil immersion and gas quenching.

A set of three oil-bath circuit breakers (OCB’s) rated for 230 kV service is shown here, retired from service:

Each of the three circuit breakers (one for each line of the three-phase circuit) is mechanically linked by a common shaft at the top of the breaker tanks, so they all trip and close as one unit.

The fast and reliable actuation of such a bulky mechanism requires a large amount of stored energy, and in the case of the oil circuit breaker shown above the energy storage medium is compressed air. An on-board electric air compressor powered by “station power” maintains air pressure inside a pressure vessel, and this compressed air is directed to a piston actuator through solenoid valves to provide the actuation force necessary to move the breaker contact assemblies open and closed.

A view inside the enclosure on the far side of this oil circuit breaker reveals the air compressor (upper-right), compressed air storage tank (right) and actuation cylinder (middle):

The man shown in this photograph is pointing to a solenoid valve designed to pass compressed air to and from the piston actuator. A large-diameter black hose runs from this solenoid through the bottom of the enclosure, allowing compressed air from the cylinder to vent to atmosphere.

A more modern breaker design for 230 kV service is this gas-quenched circuit breaker unit, a mere fraction of the physical size of the oil circuit breaker shown previously:

The ribbed porcelain structures are the high-voltage terminals for this circuit breaker: three for the three-phase lines coming in, and three for the three-phase load terminals exiting. The actual contact assemblies reside in the gas-filled horizontal metal tubes (“tanks”). It is striking to note that the same current interruption and isolation functions performed by the gigantic oil-filled tanks of the retired circuit breaker previously shown are performed by this relatively tiny gas-quenched breaker.

The gas inside the breaker’s tanks is sulfur hexafluoride, a very dense gas (about 5 times denser than air) with excellent electrical insulating and arc-extinguishing properties. SF6 gas is contained in these breaker contact chambers under pressure to maximize its dielectric breakdown strength (its ability to withstand high voltage without ionizing and passing current across the gap between the circuit breaker’s open contacts). SF6 gas is non-toxic and safe to handle.

Like the large oil-filled circuit breakers seen previously, this SF6 circuit breaker has an enclosure on one side where the actuation and control components are located. Inside this enclosure we see a large stack of Belleville spring washers (the dark-colored discs located in the center of the enclosure), which are used as the mechanical energy-storage medium instead of compressed air. This stack of spring-steel washers is compressed by an electric motor and gear mechanism, then the spring tension is released through another mechanism to close and trip the breaker’s contacts on demand. As usual this charging motor receives its power from the uninterruptible “station power” supply of the substation, allowing the breaker to actuate even in the event of a total “blackout” condition:

Inside this enclosure we also see a small pair of pushbuttons (one red, one green) just below and to the right of the Belleville washer stack for manually closing and tripping the breaker, respectively. The common color coding used in the United States for electric power switchgear is red for energized, green for de-energized. This may seem backward to most people familiar with red and green traffic lights, where red means “stop” and green means “go,” but the concept here is one of safety: red means “dangerous” (power on) while green means “safe” (power off ).

It should be noted that most of the time these high-voltage circuit breakers are triggered remotely, rather than manually by someone standing near them. These command signals may come from a manual switch located in the control room, or from some automatic circuit such as a protective relay instructing the breaker to open due to an abnormal system condition.

The nameplate photographed on a similar SF6 circuit breaker reveals some interesting features:

According to this nameplate, the normal operating pressure for the SF6 gas is 98.6 PSI. A low-pressure alarm triggers if the SF6 gas pressure happens to fall below 85 PSI. When opening (tripping), the circuit breaker only takes 3 cycles’ worth of time at 60 Hz to completely interrupt the current. One solenoid coil closes the breaker, and that coil requires a signal of 125 volts DC at just over 3 amps of current. The breaker’s contacts may be tripped by energizing one or more

redundant solenoid coils at 125 VDC and 1.8 amps (each). In either direction, the breaker’s actuation is powered by a pre-charged spring, much like the 230 kV breaker seen previously. This particular breaker is rated for 123 kV at 2000 amps full-load.

Sulfur hexafluoride gas circuit breaker technology is popular for higher voltage applications as well, such as these 500 kV circuit breakers seen here:

In this application, where three separate circuit breaker units independently interrupt current for the three-phase power lines, there is no mechanical link to synchronize the motion of the three contact sets. Instead, each single-phase circuit breaker actuates independently.

So far all of the high-voltage circuit breakers shown in previous photographs are of the dead tank type, where the structure housing the interrupting contact(s) is maintained at ground potential (i.e. the outside surface of the circuit breaker mechanism is electrically “dead”). Some high-voltage circuit breakers are built such that their interrupting assemblies are at line potential, the entire breaker suspended above ground from insulators. This type of circuit breaker is called a live tank, because the “tank” containing the contact(s) operates at a high voltage with respect to earth ground. A photograph of a 500 kV, single-pole, SF6 -quenched, live tank breaker appears next:

The actuating mechanism for this live-tank breaker is housed in the “can” assembly seen at the base, where the vertical insulator meets the steel supporting tower.

Another 500 kV, single-pole, live-tank circuit breaker assembly appears in the next photograph, this particular breaker being an older unit using compressed air as the interrupting medium rather than sulfur hexafluoride gas:

Air nozzles powered by hundreds of PSI of compressed air are used to “blow out” the arc formed when the circuit breaker’s contacts separate. These air nozzles are not visible in the photograph, being internal to the circuit breaker’s construction. An interesting feature of this style of circuit breaker is the loud report generated when it trips: the sound of the compressed air jets extinguishing the arc across the separating contact poles within the breaker is not unlike that of a firearm discharging.

Multiple series-connected contact assemblies comprise this circuit breaker, distributing the energy of the arc across multiple points in the breaker assembly rather than across a single contact. This is evident in the photograph as multiple clusters of “tanks” on the top of the left-hand assembly, as well as the second live-tank assembly connected in series to the right. Such arrangements are necessary because air is a less effective medium for extinguishing an electric arc than either oil or SF6 gas.

Article from Lessons In Industrial Instrumentation by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License

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