Circuit Breakers and Disconnects in Power Systems
Circuit breakers are the “final control elements” of the electric power industry, akin to control valves in the process industries. They are strictly on/off devices, used to make and break connections under load in power systems. Circuit breakers automatically open when dangerous circuit conditions are detected. Some low-voltage circuit breakers are strictly local-controlled devices, but larger circuit breakers (especially medium- and high-voltage units) may also be operated remotely by electrical signals.
Disconnects are switches designed to isolate sections of a power system in case of damage or to allow for routine maintenance. These may be manually-operated devices, or operated remotely by electric motor, and are typically not intended to make or break load current. Circuit breakers, by contrast, are designed to interrupt very high levels of electric current so they may safely cut off power in the event of a short-circuit fault.
This next photograph shows a set of three disconnect switches used on the line (input) side of a three-phase 500 kV power transformer bank in a large substation. As you can see, each disconnect is a simple “knife” switch design, bearing a striking similarity to its electrical schematic symbol. The long metal arm hinges on the left-hand side (at the top of the double insulator stands) and makes or breaks electrical contact on the right-hand side (where the arm is tipped with a sphere):
This particular high-voltage disconnect is motor-actuated, allowing all three disconnects to be operated in unison by remote control. When in the “open” state each metal arm points vertically toward the sky, clearly revealing its status to visual inspection. Lower-voltage disconnects are often built as manually-actuated devices, a crank handle or lever installed at ground level for a human lineman to actuate.
In series with these disconnect switches are a triad of 500 kV circuit breakers (with only two of them appearing in this next photograph). The circuit-breaking elements reside in the horizontal tube portion of each unit (called the “tank”), the tall ribbed structures being insulated conductors bringing the power down to the low-mounted breaker tank. Since disconnect switches are generally not rated for load current interruption, circuit breakers are necessary to “break” the current and safely extinguish the inevitable arc that forms when a live circuit is broken. In the background you can see a hinged disconnect switch in series with the furthest circuit breaker, a set of which serve to isolate power from the three circuit breakers (and every other component “downstream” of the disconnects) to permit maintenance on those components:
All three circuit breakers are remotely controlled by 125 VDC signals energizing solenoid coils within the breaker units. One solenoid coil called the “close” coil causes the breaker mechanism to move to the closed (conducting) position when momentarily energized. Another solenoid coil called the “trip” coil causes the breaker mechanism to move to the open (non-conducting) position when momentarily energized. The breakers also contain status contacts signaling the breaker’s change of state. It is these solenoid coils and status contacts which permit the circuit breaker to be a part of an automatic control system rather than function merely as a manual switching device.
Circuit breakers can be further classified into three categories based on voltage level:
- Low voltage circuit breakers
- Medium voltage circuit breakers
- High voltage circuit breakers
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