SF6 Gas Technology

00:47 / Posted by tech data / comments (3)

SF6 Gas Technology
Another arc extinguishing technology is SF6 (sulfur hexafluoride) technology. It is popular outside the United States. It is specifically associated with European manufacturers of medium and higher voltage circuit breakers.
SF6 technology was briefly discussed in Module 5, Fundamentals of Circuit Breakers. The main contacts are enclosed in a chamber of SF6 gas, which happens to be a very good dielectric


In short, this arc interruption technology results the in arc energy being used and absorbed, while the arc is simultaneously cooled.
There are several SF6 interrupter designs, but only two types will be covered here:
• Puffer
• Rotary Arc
Puffer-Type SF6 Gas Technology
The puffer-type of SF6 interrupter is the older of the two SF6 technologies. It is more capable but more complicated than the rotary arc-type.
The interrupter is shown below in the closed position, and in the opening sequence.

 Typical SF6 Puffer Interrupter in Closed Position


Typical SF6 Puffer Interrupter During Opening Sequence
High Current Arc on Left and Near Current Zero on Right


During current interruption, a piston compresses the SF6 gas in a cylinder, all of which is enclosed in an epoxy-type enclosure. After the main current-carrying contacts part, the current transfers to the arcing contacts. Once the arcing contacts part, the SF6 gas in the compression chamber blasts the arc through the nozzle. The heat created by the arc breaks the SF6 molecules into fluorine and sulfur. Arc energy is absorbed and the arc is cooled.
As current zero is approached, the heat energy subsides as more SF6 gas enters the system. At current zero, the high-pressure SF6 gas flows through the nozzle and extinguishes the arc.
Compressing the SF6 gas requires significant mechanical energy. A circuit breaker equipped with this type of technology requires a higher-energy operating mechanism than is required by an equivalent vacuum circuit breaker. In addition, this interrupter type has a large number of parts.
Rotary Arc-Type SF6 Gas Technology
Rotary arc SF6 technology is less complicated in design than the puffer type. It has fewer parts and does not require such a high-energy operating mechanism. However, it is not effective over as wide a range of short circuit currents and voltages as the SF6 puffer or vacuum. Suitable applications are somewhat limited.
Figure shows a typical SF6 rotary arc interrupter.



As the contacts part, the arc transfers from the main contacts to an annular contact. This causes the current to switch into the coil behind it. The coil's magnetic field, produced by the load current itself, causes the arc to rotate rapidly. The arc is cooled by moving through the SF6 gas.
The SF6 gas is normally at rest inside this interrupter. The arc's movement acts like a mixer, mixing hotter and cooler gas. This helps cool the arc. Contact erosion is also reduced due to this rapid arc movement.
As current zero is approached, the dwindling arc must sustain enough speed to be lost in the SF6 environment in order to withstand the transient recovery voltage. (This is mentioned because the magnetic field produced by the load current causes the all important arc rotation. As the current decreases, the magnetic field decreases, and the arc rotation slows.)
At current zero, the arc is cooled and extinguished.

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Arc Extinguishing Technologies

00:36 / Posted by tech data / comments (0)

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 Module 5, Fundamentals of Circuit Breakers.



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.
Of the two contacts (also called "electrodes") inside, one is fixed. The other is movable, through a bellows-type connection. Various shields inside the envelope provide different types of protection to interrupter parts.
 depicts the important arcing and interruption phenomena within a vacuum.


When the circuit breaker is closed, the contacts within the interrupter touch, allowing current to flow.
When a fault occurs and interruption is required, the contacts are quickly separated and an arc forms. An arc is formed because the voltage tries to keep the current moving.

Enclosing Contacts in a Vacuum

The arc burns in the metal vapor evaporated from hot spots on the contact surfaces. This metal vapor continuously leaves the contact region and recondenses on the contact surfaces and surrounding metal shield, which protects the ceramic envelope.
At Current Zero, the arc extinguishes, contact vapor production stops, and the original vacuum condition is restored. Current zero is a point in the AC current sine wave where the value is zero.



The vacuum in the envelope is considered a Dielectric. The Dielectric Strength is the maximum voltage the dielectric can withstand without breaking down. The Transient Recovery Voltage (TRV) is the most severe waveform the interrupter will have to withstand. This is why the speed of the dielectric recovery and the strength of the dielectric inside the interrupter are critical issues for successful circuit interruption. If the dielectric does not reach sufficient strength fast enough, the arc will re-ignite.
Vacuum interrupters for circuit breaker duty must be capable of interrupting currents of 12 to 50 kA (and up), at voltages up to 38 kV

Rear View of Circuit Breaker with
Vacuum Interrupters Installed (One Per Phase)

Spiral Contacts
A newer technology is now being used in the vacuum interrupter. It involves using spiral-shaped copper-chrome contacts inside the vacuum tube. They provide a self-induced magnetic effect that moves the arc root around the contact periphery. This very efficient arc control method prevents hot spots, minimizing contact erosion.

Spiral Contacts

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Components

00:27 / Posted by tech data / comments (0)


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

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Module 6:Medium Voltage Power Circuit Breakers.

00:21 / Posted by tech data / comments (0)

Module 6,which is about 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.)

Like the other modules in this series, this one presents small, manageable sections of new material followed by a series of questions about that material. Study the material carefully, then answer the questions without referring back to what you've just read. You are the best judge of how well you grasp the material. Review the material as often as you think necessary. The most important thing is establishing a solid foundation to build on as you move from topic to topic and module to module.

Introduction
To understand where the medium voltage power circuit breaker fits into the scheme of things, you need to understand the basics of power distribution in an industrial environment.
An industrial distribution system consists of:

• metering devices to measure power consumption
• main and branch disconnects
• protective devices
• switching devices to start and stop power flow
• conductors
• transformers

Power may be distributed through various Switchgear and Switchboards, transformers and Panelboards. The medium voltage power circuit breaker is found in switchgear assembly. A switchgear assembly controls electric power circuits.


The Circuit Breaker is the main device - the heart - of the switchgear. It provides centralized control and protection of medium voltage power equipment and circuits. Its operation covers load switching, control and fault protection for generators, motors, transformers, capacitors and all types of feeder circuits.

This type of power equipment is normally found in industrial, commercial and electric utility installations. Typical applications include electric utility systems, industrial distribution systems, commercial buildings, municipal pumping stations, and transportation systems.

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