Medium Voltage Power Circuit Breakers.

By medium voltage, we mean a voltage range of 1000 volts to 38 kV. (Some consider 72.5 kV as the upper end of medium voltage, but we will work with 38 kV for the purpose of this training module.)

Component.

 There are four main parts to a medium voltage power circuit breaker. These are:

1-     Fram

2-     Operating Mechanis

3-     Trip Unit

4-     Arc Extinguisher

Figure 1. 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.Although medium voltage power circuit breakers are electrically operated, the closing springs can be charged manually.For more on operating mechanism, see 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. 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.

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.

Arc Extinguisher

The Arc Extinguisher extinguishes the Arc produced when the contacts are pulled apart to interrupt current flow. The higher voltage make the  interrupt of the flow of current becomes harder.

There are a number of arc extinguishing technologies in use today. We will look at the most

arc extinguishing technologies

In a low voltage circuit breaker, using air or Arc Chutes is sufficient to extinguish an arc. In the medium voltage range, a different technology needs to be used. The main technology used today is the Vacuum Interrupter.

Vacuum Interrupter Technology

The vacuum interrupter (VI) was briefly discussed in Fundamentals of Circuit Breakers.

Figure 2. Schematic for a Typical Vacuum Interrupter

Basically, the vacuum interrupter is a pair of separable contacts (called "primary contacts") enclosed in a vacuum-tight envelope. The Envelope itself is a ceramic material, with a metal end plate brazed to each end. The metal plates seal the ends and provide support for the parts inside.

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