High Voltage Circuit Breakers
A circuit breaker is defined as “a mechanical switching device capable of making, carrying, and breaking currents under normal circuit conditions and also making, carrying, and breaking for a specified time, and breaking currents under specified abnormal conditions such as a short circuit” (IEEE Standard C.37.100).
Circuit breakers are generally classified according to the interrupting medium used to cool and elongate the electrical arc permitting interruption.
The types of circuit breakers are:
- Air magnetic
- Air blast
- Oil (bulk oil and minimum oil)
- SF6 gas
Air magnetic circuit breakers are limited to older switchgear and have generally been replaced by vacuum or SF6 gas for switchgear applications. Vacuum is used for switchgear applications and for some outdoor breakers, generally 38 kV class and below.
Oil circuit breakers have been widely used in the utility industry in the past but have been replaced by other breaker technologies for newer installations.
Two designs exist: bulk oil (dead tank) designs dominant in the United States and minimum oil (live tank) designs prevalent in some other parts of the world. Bulk oil circuit breakers were designed as single-tank or three-tank devices, 69 kV and below ratings were available in either single-tank or three-tank configurations and 115 kV and above ratings in three-tank designs.
Bulk oil circuit breakers were large and required significant foundations to support the weight and impact loads occurring during operation.
Environmental concerns and regulations forced the necessity of oil containment and routine maintenance costs of the bulk oil circuit breakers coupled with the development and widespread use of the SF6 gas circuit breakers have led to the selection of the SF6 gas circuit breaker in lieu of the oil circuit breaker for new installations and the replacement of existing oil circuit breakers in favor of SF6 gas circuit breakers in many installations.
Oil circuit breaker development had been relatively static for many years. The design of the interrupter employs the arc caused when the contacts are parted and the breaker starts to operate. The electrical arc generates hydrogen gas due to the decomposition of the insulating mineral oil.
The interrupter is designed to use the gas as a cooling mechanism to cool the arc and also to use the pressure to elongate the arc through a grid (arc chutes) allowing extinguishing of the arc when the current passes through zero.
Vacuum circuit breakers use an interrupter that is a small cylinder enclosing the moving contacts under a hard vacuum. When the breaker operates, the contacts part and an arc is formed resulting in contact erosion. The arc products are immediately forced to be deposited on a metallic shield surrounding the contacts. Without a restrike voltage present to sustain the arc, it is quickly extinguished.
Vacuum circuit breakers are widely employed for metal clad switchgear up to 38 kV class.
The small size of the vacuum breaker allows vertically stacked installations of vacuum breakers in a two-high configuration within one vertical section of switchgear, permitting significant savings in space and material compared to earlier designs employing air magnetic technology. When used in outdoor circuit breaker designs, the vacuum cylinder is housed in a metal cabinet or oil-filled tank for dead tank construction
popular in the U.S. market.
Gas circuit breakers employ SF6 as an interrupting and insulating medium. In “single puffer” mechanisms, the interrupter is designed to compress the gas during the opening stroke and use the compressed gas as a transfer mechanism to cool the arc and also use the pressure to elongate the arc through a grid (arc chutes), allowing extinguishing of the arc when the current passes through zero.
In other designs, the arc heats the SF6 gas and the resulting pressure is used for elongating and interrupting the arc. Some older dual pressure SF6 breakers employed a pump to provide the high-pressure SF6 gas for arc interruption.
Recommended Reading: – Mastering switchgear control circuits (AC/DC circuits and circuit breaker closing circuit)
Gas circuit breakers typically operate at pressures between 6 and 7 atm. The dielectric strength and interrupting performance of SF6 gas reduce significantly at lower pressures, normally as a result of lower ambient temperatures.
For cold temperature applications (ambient temperatures as cold as −40°C), dead tank gas circuit breakers are commonly supplied with tank heaters to keep the gas in vapor form rather than allowing it to liquefy; liquefied SF6 significantly decreases the breaker’s interrupting capability.
For extreme cold temperature applications (ambient temperatures between −40°C and −50°C), the SF6 gas is typically mixed with another gas, either nitrogen (N2) or carbon tetra fluoride (CF4), to prevent liquefaction of the SF6 gas. The selection of which gas to mix with the SF6 is based upon a given site’s defining critical criteria, either dielectric strength or interrupting rating.
An SF6–N2 mixture decreases the interrupting capability of the breaker but maintains most of the dielectric strength of the device, whereas an SF6–CF4 mixture decreases the dielectric strength of the breaker but maintains most of the interrupting rating of the device.
For any temperature application, monitoring the density of the SF6 gas is critical to the proper and reliable performance of gas circuit breakers. Most dead tank SF6 gas circuit breakers have a density switch and a two-stage alarm system.
Stage one (commonly known as the alarm stage) sends a signal to a remote monitoring location that the gas circuit breaker is experiencing a gas leak, while stage two sends a signal that the gas leak has caused the breaker to reach a gas level that can no longer assure proper operation of the breaker in the event of a fault current condition that must be cleared. Once the breaker reaches stage two (commonly known as the lockout stage), the breaker either will trip open and block any reclosing signal until the low-pressure condition is resolved or will block trip in the closed position and remain closed, ignoring any signal to trip, until the low-pressure condition is resolved.
The selection of which of these two options, trip and block close or block trip, is desired is specified by the user and is preset by the breaker manufacturer.
|Title:||The basics of switching equipment in high voltage substations by David L. Harris and David Childress|
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