20-12-2012, 04:48 PM
SF6 gas Circuit breakers
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Why SF6?
Sulfur Hexafluoride (SF6) is an excellent gaseous dielectric for high voltage power applications. It has been used
extensively in high voltage circuit breakers and other switchgears employed by the power industry. Applications
for SF6 include gas insulated transmission lines and'gas insulated power distributions. The combined electrical,
physical, chemical and thermal properties offer many advantages when used in power switchgears. Some of the
outstanding properties of SF6 making it desirable to use in power applications are:
High dielectric strength
Unique arc-quenching ability
Excellent thermal stability
Good thermal conductivity
General Information
Elimsan SF6 circuit breaker is equipped with separated poles each having its own gas. In all types of the circuit
breakers, gas pressure is 2 bars (absolute 3 bars). Even if the pressure drops to I bar, there will not be any change
in the breaking properties of the circuit breaker due to the superior features of SF6 and Elimsan's high safety factor
for the poles. During arcing, the circuit breaker maintains a relatively low pressure (max 5-6 bars) inside the chamber
and there will be no danger of explosion and spilling of the gas around. Any leakage from the
chamber will not create a problem since SF6 can undergo considerable decomposition, in which some of toxic
products may stay inside the chamber in the form of white dust. If the poles are dismantled for maintenance, it needs
special attention during removal of the parts of the pole. This type of maintenance should be carried out only by the experts
of the manufacturer. (According to ELIMSAN Arcing Products and Safety Instruction for Working on SF6 Circuit Breakers)
Operation of Circuit Breaker
In general, the circuit breakers consist of two main parts, the poles and the mechanism. The poles consist of contact
and arc-extinguishing devices. The mechanism is the part to open or close the contacts in the poles at the same
time instantaneously (with max. 5 milisec. Tolerance). The closing and opening procedures are performed through
springs which are charged by a servomotor and a driving lever. In the system, the closing springs are first charged.
If "close" button is pressed the opening springs get charged while the contacts get closed. Thus, circuit breaker will
be ready for opening. The mechanical operating cycle of the circuit breaker is (OPEN-3 Min CLOSE/OPEN-3 Min-
CLOSE/OPEN) or (OPEN-0.3 sec-CLOSE/OPEN-3 Min CLOSE/OPEN). The second cycle is valid when the circuit
breaker is used with re-closing relay. In that case, after the closing operation, the closing springs are charged by
the driving lever or by driving motor (if equipped).
Auxiliary Switch
The auxiliary switch mounted on the circuit breaker has 12 contacts. One of them is for antipumping circuit, four
of them are allocated for opening and closing coils. The remaining 7 contacts are spare. Three of them are normally
opened and four are normally closed. When it is necessary, the number of the contacts can be increased.
Rapid Automatic Reclosing
The circuit breaker which opens due to a short circuit failure, can be re-closed automatically after a pre selected
time by arc closing relay, assuming the fault is temporary. Thus, we avoid long time power loss in case of temporary
short circuits. But, if the fault lasts after re-closure, the protection relay will trip to open the circuit breaker again.
What to Specify on The Order
1- Rated voltage of the circuit breaker
2- Rated current of the circuit breaker
3- Rated short circuit breaking current
4- Voltages of opening and closing coils
5- Motor supply voltage (if equipped)
Commissioning
The outer surfaces of epoxy insulating tubes of the poles are to be wiped out with a clean and dry cloth. The wiring
and connections of the auxiliary circuit are to be carefully examined. DC voltage should be checked to see whether
it is suitable for coil and motor or not (if equipped). The opening-closing coils are to be operated 15-20 times and
the accuracy of the relay circuit is to be checked before energizing the circuit breaker. The circuit breaker is to
be mounted with two MI2 bolts through its anchoring shoes. It should not move during operation. No excessive
load should be exerted to the poles and if possible flexible cables should be used. The incoming and outgoing contacts
must have clean surfaces and their contact resistance should be as low as possible. When connecting the circuit
breaker to protection system and auxiliary supply, the cable cross sections should be according to the table given.
The circuit breaker must be grounded through at least 16 mm steel tape (by cable shoe). After all, the following
procedure must be performed:
1. Open the isolator of circuit breaker,
2. Prepare the circuit breaker for closing operation by driving mechanism,
3. Close the isolator of circuit breaker firmly,
4. Send the closing signal to the circuit breaker,
The Maintenance Of Circuit Breaker During Operation
Normally, at least once a year or after every 500 operations, the circuit breaker must be maintained. During
maintenance, the moving parts of the mechanism must be lubricated carefully. The insulating parts are to be wiped
out by a clean and dry cloth. When maintaining, the circuit breaker should be open and high voltage sides must be
grounded. Auxiliary power supply should also be disconnected. On saline areas near seaside, the insulating parts
of the circuit breaker must be carefully cleaned, at least once every two months. If not, the microscopic salt particles
drawn by wind from the sea will create conductive layers on the insulating surfaces and may cause surface flashover.
Before maintenance, first circuit breaker, then isolator should be opened and grounded carefully. The maintenance
of circuit breaker must be done after checking the open position of isolator contacts by eye.