Selection of SF6 or VCB

How to you select SF6 or VCB for different applications . What are the technical comparision between the two ? Why usage of vacuum contactors for lower currents are still not widely used for MV systems ?

 

I have chosen not to use SF6 in our 25 kV circuit breakers. I did some web research and found that SF6 is an extremely harmful greenhouse gas - much worse than CFC's. At this point it has not been regulated but like PCB's it may be a matter of time. The U.S. EPA has some info on their site. The utility industry is taking a pro-active approach to avoiding regulation. A salesman had said to me that it is sealed so none on this should be a concern. Yet in the brochure he gave me they advertised re-fill cannisters!

 

[cuky2000]

QUESTION 1: How to you select SF6 or VCB for different applications.
ANSWER 1: In General, the selection of vacuum or SF6 interrupting devices is dictated based on the following factors: Pricing, individual preferences, national traditions and switching requirements.
Typical application of vacuum and SF6 circuit breakers are as follow:

STANDARD APPLICATIONS:
-a) Motor starter and circuit breaker starter.
-b) Transformer with lactch contactor or circuit breaker.
-c) CircuIt breaker for furnace.
NOTE: Overvoltage assessment it is recommended to determine the requirement of surge protection (arrester and/or surge capacitor) especially if vacuum device is selected for starting large motors and dry type transformers.

ESPECIAL APPLICATIONS:
- a) Back-to-Back Capacitor Bank with reactors: SF6 breaker or Vacuum breaker with synchronous control.
- b) For low frequency application such as railway systems, vacuum breaker is better choice than SF6.
- c) Shunt Reactor application the target maximum overvoltage is limited to 2.5 pu.

QUESTION 2: What is the technical comparison between the two?
ANSWER 2:
Vacuum or SF6 interrupting devices technologies have similar electrical and mechanical performances on standard circuit applications.
Production of overvoltage is the most remarkable difference in performance between vacuum and SF6 interrupting technology. Vacuum devices have tendency to produce significant level of chopping waves and multiple reignitions that could be amplified by the distribution system forcing to exceed the insulation of rotating machines, dry-type transformers and reactors.

QUESTION 3: Why usage of vacuum contactors for lower currents are still not widely used for MV systems?

ANSWER 3:
The smaller standard MV vacuum contactor is rated for 400A(360A enclosed) for a range of voltage between 2200 to 7200 Volts. The require investment associate with starter, MCC, etc. became difficult to justify for small motor capacity.

On the other hand, for large motors the largest standard contactor in the voltage range described above is 720 A (enclosed). The use of the circuit breaker is justifiable since provides larger continuous rating (1200 A minimum), more operation flexibility and better protection features.

 

[Elecme]

Also as the Production of overvoltage is very significant with vacume circuit breakers they normally use surge suppressors with MV vacume circuit breaks and not with SF6 circuit breakers.

Elecme