• Frame
• Operating Mechanism
• Trip Unit
• Arc Extinguisher
Typical Medium Voltage Vacuum Circuit Breaker
with Front Cover Removed, Exposing Operating Mechanism
Frame
A medium voltage power circuit breaker is essentially an assembly of parts on a rugged metal Frame. Depending upon factors such as ratings and interrupting method, they come in a variety of shapes, sizes and configurations.
Operating Mechanism
The medium voltage power circuit breaker uses a stored-energy Operating Mechanism to open the circuit breaker. It has a motor-charged, spring-type, stored-energy closing mechanism. Closing the breaker charges the accelerating springs. Protective relays on the control switch energize a shunt trip coil to release the accelerating springs and open the breaker.
This is a trip-free design - truly mechanically and electrically trip free. Breaker contacts will not touch or close onto a fault, even when a mechanical or electrical "close" command is issued.
The manual controls are usually accessed from the front of the circuit breaker. Although medium voltage power circuit breakers are electrically operated, the closing springs can be charged manually.
For more on operating mechanisms, see Module 5, Fundamentals of Circuit Breakers.
Trip Unit
A Trip Unit is typically integral to a circuit breaker. But, the medium voltage power circuit breaker uses externally mounted trip units to provide the operational intelligence. These devices are called Protective Relays.
The protective relays are normally wired to the circuit breaker and Current Transformers. They are mounted on a panel or door of the switchgear assembly. They function to detect a defective line or apparatus, as well as dangerous or undesirable system conditions. The relay energizes the trip coil of the circuit breaker to clear a fault.
Typical Medium Voltage Switchgear Assembly with Protective Relays and
Trip Intelligence
The protective relay intelligence devices fall into two broad categories. These are:
Electromagnetic-The electromagnetic protective relay has been used widely in the industry for many years, with a high degree of success. It functions to tell the circuit breaker when to operate, based on the specific relay type selected. Many different types of electromagnetic protective relays are available, and each type performs rather specific functions.
Common relay types include: instantaneous overcurrent, time overcurrent and overvoltage. Because these devices are limited in scope, a number of different relays (each with different protective capabilities) must be used to provide a comprehensive protective package.
Microprocessor-Based-The microprocessor-based protective relay is a multi-function device. It can provide all the features of several electromagnetic relays in one box. It is also easier to mount and wire. Overall, it is a smaller investment than an electromagnetic protective relay solution.
In most cases, only one device is required for each three-phase circuit, not one device for each phase. One of the more sophisticated microprocessor-based protective relays is capable of replacing the normal complement of three or four electromagnetic relays, as well as a number of associated meters and switches.
Because these are individual devices, apart from the medium voltage circuit breaker itself, there will be no further discussion of the tripping intelligence in this module.
Arc Extinguisher
The Arc Extinguisher extinguishes the Arc produced when the contacts are pulled apart to interrupt current flow. The higher the voltage, the harder it is to interrupt the flow of current.
There are a number of arc extinguishing technologies in use today. We will look at the most prominent types in the next section. Labels: SF6 Circuit Breaker
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