ABB Transformer unit – outdoor live tank SF6 circuit breaker Outdoor Breaker

Description

ABB Transformer unit – outdoor live tank SF6 circuit breaker Outdoor Breaker

The live tank circuit breaker, OHB medium voltage circuit-breakers for outdoor installation use sulphur hexafluoride gas (SF6) to extinguish the electric arc and as the insulating medium. Breaking in SF6 gas takes place without current chopping or generation of over voltages. These characteristics guarantee a long electrical life for the circuit-breaker and limited dynamic, di-electric and thermal stresses on the installation. The stored energy and free release mechanical operating mechanism allows opening and closing operations through local and remote control. The operating mechanism, the activating kinematics of the moving contacts and the anti-condensation heater are located inside a tight metal enclosure which also acts as the support for the poles.

The above-mentioned structure is supported by a frame made of metal sections, to mount the circuit breaker on foundation. The circuit breaker is compact and ensures a high level of sturdiness and excellent mechanical reliability. The enclosure is made out of a steel plate with adequate surface protection by a special metallizing and painting process. The support structures are hot dip galvanized.

Fields of application

OHB circuit-breakers are used in power distribution for control and protection of lines and for control and protection of transformers, rectifier units, capacitor banks, etc. Due to the SF6 autopuffer breaking technique, the OHB circuit-breakers do not generate overvoltage during switching, and are therefore also highly suitable for retrofitting, upgrading and enlarging older installations where the insulating materials of the cables and apparatus may have already been stressed during service.

Standards and approvals

The OHB circuit-breakers comply with the IEC 62271-100 standard as well as of other major industrialised countries. They have undergone the tests indicated below and

ensure service safety and reliability of the apparatus in all installations.

Type tests:

– Temperature rise test

– Di-electric test

– Short circuit test duties

– Short time current test

– Mechanical endurance test

Routine tests:

– Power frequency voltage with stand test

– Insulation test on auxiliary and control circuits

– Measurement of main circuit resistance

– Mechanical and electrical operation test

Service safety The mechanical and electrical locking devices have been  provided to prevent incorrect operations and to carry out inspection of the installations while guaranteeing maximum

BREAKING TECHNIQUE

The breaking principle of OHB circuit-breakers is based on compression and self-blast technique to obtain top performances at all breaking current values, with minimum arc times, gradual arc extinction without chopping, and no restricting or switching overvoltage’s. This breaking technique already widely appreciated and used in high voltage circuit breakers, has been introduced into medium voltage with the

Main contact separation

No electric arc is struck because the current flows through thearcing contacts. During its run downwards, the moving apparatus compresses the gas in the lower chamber. The compressed gas  flows from the lower to the upper chamber, thereby taking both the chambers to the same pressure.

Arcing contact separation

The current flows due to the electric arc which has struck between the arcing contacts. The gas cannot escape through the nozzle because the hole is still closed by the fixed arcing contact and cannot even escape through the inside of the arcing contact, because of the clogging effect of the electric arc.– at low current values, the arc is extinguished when the current passes through its natural zero and the gas flows through the contacts. The low pressure level cannot chop the current and the modest amount of gas which “escapes” is sufficient to restore di-electric resistance between the two contacts, preventing restriking on the rising front of the recovery voltage. – at high short-circuit values, the pressure wave generated by

the electric arc closes the valve between the two chambers so that the circuit-breaker starts to operate as “pure self-blast” apparatus. The pressure increases in the upper volume due to the gas heating and molecular dissociation due to the high temperature. The pressure increase is proportional to the arc current and ensures arc extinction during the first passage through zero current.

Circuit-breaker open

The arc has been extinguished, the self-generated pressure in the upper volume decreases because the gas is flowing through the contacts. The valve re-opens and a new jet of fresh gas flows in and “washes” the chamber. This means the apparatus is immediately ready to close and trip again up to its maximum

• Terminal
• Insulating enclosure
• Blasting nozzle
• Moving arcing contact
• Moving contact
• Fixed arcing contact
• Fixed contact
• Insulating tie-rod

9* Cap with bursting valve (upon request)

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