Medium Voltage Circuit Breakers

In Electrical Power Distribution industry the term medium voltage refers to voltage range extending from 2.4 kV to 35 kV.

Circuit breaker design and construction becomes more complicated at higher voltages, such as the voltage range extending from 2.4 kV to 35 kV commonly classified as “medium-voltage” in the electrical power distribution industry. Aside from being physically larger than low-voltage circuit breakers, medium-voltage circuit breakers are generally not self-tripping as low-voltage circuit breakers are. Rather, medium-voltage circuit breakers are electrically commanded to trip (and to close) by external devices called protective relays monitoring dangerous electrical conditions. Internally, these circuit breakers are equipped with “trip” and “close” electromagnet solenoids allowing the mechanism to be triggered by remote electrical signals. Medium-voltage circuit breakers are designed to be unplugged from the breaker panel for maintenance and replacement. This is referred to in the electrical power industry as racking out a circuit breaker. Some circuit breakers “rack” by moving horizontally, sliding in and out of the panel on guide rails. Other circuit breakers “rack” by moving vertically, sliding up into and down out of the stationary panel terminals, as shown in the following illustration:

The primary purpose of being able to “rack” a medium voltage breaker into and out of its place in a breaker panel is to facilitate regular maintenance on the circuit breaker mechanism. Unlike the circuit breakers you find in your home, these units may be frequently cycled and will suffer definite wear with each actuation. After a certain number of closing/tripping cycles, the breaker must be removed from service for inspection and testing.

Racking out a circuit breaker also provides another advantage, and that is an extra measure of safety when securing a power circuit in a zero-energy state. When a circuit breaker has been locked into its “racked out” position, the load conductors serviced by this breaker absolutely cannot become energized even if the circuit breaker contacts were made to close. This is analogous to unplugging an electrical appliance from a wall receptacle: it cannot be powered up even if the switch is turned on!

An example of a vertically-racking circuit breaker is the General Electric “Magneblast” unit shown below, designed for use in power systems operating up to 15 kV. The particular unit shown rests on a wooden pallet in a storage area. Normally, it would be installed in a metal-clad breaker panel, its components hidden from direct view:

The six “stabs” seen on the top of this breaker unit engage with six sockets connected to the six bus-bar conductors inside the breaker panel (three for the three phases of the line supply, plus three more for the three phases of the load conductors). When this circuit breaker is “racked out,” it is dropped down so that these six stabs disengage from the bus-bar connections, making it impossible to energize the load conductors even if the breaker contacts close.

A detail not seen in this photograph is the hoisting mechanism necessary to lift this breaker into its “racked-in” position. Medium-voltage circuit breakers such as the General Electric Magneblast are quite heavy, requiring special “lift truck” frames to hoist into and out of their engaged positions in the circuit breaker panel.

Not only does “racking out” a circuit breaker add an extra measure of safety for personnel working on the load circuit, but it also allows the breaker to be tested in-place without energizing the load. The electrical connections commanding the breaker to open (trip) and close may still be connected to the control circuitry even in the racked-out state, permitting such tests. The following illustration shows how such a test may be performed in the “racked-out” condition:

At medium-voltage and greater levels of potential, a significant design problem is how to rapidly extinguish the arc formed when contacts separate under load. Low-voltage circuit breakers simply rely on a wide and rapid enough separation of contact points to ensure the electric arc formed when the breaker trips cannot continue more than a fraction of a second. In medium-voltage circuits, both the heat output and the potential length of the electric arc formed by separating contacts is huge and therefore the arc must be extinguished as quickly as possible, both for personnel safety and for extending the operating life of the circuit breaker.

The original General Electric Magneblast circuit breaker design used a series of arc chutes, electromagnet coils, and pneumatic jets to direct the arc away from the separating contacts and thereby extinguish it rapidly. Other medium-voltage circuit breaker designs submerge the electrical contacts in an oil bath to keep them completely isolated from air so that an arc could never form. This oil has a high dielectric value (i.e. it is an excellent electrical insulator with a high breakdown rating), but needs to be tested on a regular basis to ensure good integrity.

A modern approach to the problem of extinguishing the arc drawn by opening circuit breaker contacts is to encapsulate the contacts inside of an air-tight vacuum chamber. This rear view of this GE Magneblast circuit breaker shows it retrofitted with vacuum contacts (the three white-colored components seen inside the breaker frame), replacing the old open-air contacts and arc chutes:

By removing all air from the vicinity of the contacts, there are no gas molecules to ionize when the contacts separate. Not only does this completely eliminate the problem of contact arcing, but it also permits the circuit breaker mechanism to perform its job with a shorter “throw” (less contact motion), since less gap distance is necessary to prevent current in a vacuum than in air. The only real challenge now is ensuring the integrity of the vacuum inside these chambers. This requires periodic testing of the contacts’ dielectric rating by maintenance personnel using high-voltage testing equipment.

An interesting feature of the GE Magneblast and other medium-voltage circuit breakers is the mechanism for actuation. These circuit breaker contacts must be moved swiftly and with significant force in order to ensure quick and repeatable make/break times. In order to achieve this rapidity of motion, the breaker is designed to actuate by the stored energy of large mechanical springs. A side-view of a Magneblast circuit breaker shows a pair of large coil springs used to trip and close the circuit breaker contacts:

Much like the spring on the hammer of a firearm, the springs inside this Magneblast circuit breaker provide the mechanical driving force for opening and closing the breaker’s three electrical power contacts. The act of opening or closing this circuit breaker is analogous to pulling the trigger of a firearm: a small mechanical movement unleashes the stored energy of these springs to do the actual work of rapidly opening and closing the contacts.

These springs are tensed (“charged”) by an electric motor in the times following an actuation cycle, so they will be ready for the next actuation. Typically these charging motors are powered by125 VDC supplied by the substation’s “station power” battery bank, so they may operate even in the event of a total black-out condition where the substation loses AC line power from its incoming transmission lines. Indicator flags on the front of the circuit breaker reveal the breaker’s contact status as well as its spring charge status:

Green-colored flags seen on the front panel of this breaker show the contact status as “open” and the spring status as “discharged”. This circuit breaker is incapable of any action until its spring is charged. Once the spring has been charged, pushing the button labeled “Manual trip” will cause the breaker contacts to open, and pushing the button labeled “Manual close” will cause the breaker contacts to close.

A photograph of the front panel of a Westinghouse vacuum circuit breaker reveals the same basic indicators and manual controls seen on the (older) General Electric circuit breaker:

In this particular example, the actuating spring is charged, which means the breaker is in a state of readiness to switch from its present status (open, or tripped) to its opposite status (closed).

Both of these medium-voltage circuit breakers share another feature of interest: a mechanical counter tracking the number of close/trip cycles the breaker has experienced. The act of making and breaking high-power electric circuits takes a toll on the components of a circuit breaker – especially the contacts – and therefore this count value is a useful parameter for maintenance purposes. The breaker should be serviced at manufacturer-specified intervals of close/trip cycles, just like an automobile should be serviced at manufacturer-specified intervals of distance traveled.

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

Leave a Reply