Vacuum Circuit Breakers

Vacuum Circuit Breakers (VCB)

                

 In this breaker, vacuum is being used as the arc quenching medium. Vacuum offers highest insulating strength, it has far superior arc quenching properties than any other medium. When contacts of a breaker are opened in vacuum, the interruption occurs at first current zero with dielectric strength between the contacts building up at a rate thousands of times that obtained with other circuit breakers.

   

Principle: When the contacts of the breaker are opened in vacuum (10 -7 to 10 -5 torr), an arc is produced between the contacts by the ionization of metal vapours of contacts. The arc is quickly extinguished because the metallic vapours, electrons, and ions produced during arc condense quickly on the surfaces of the circuit breaker contacts, resulting in quick recovery of dielectric strength. As soon as the arc is produced in vacuum, it is quickly extinguished due to the fast rate of recovery of dielectric strength in vacuum.

 

Construction: It consists of fixed contact, moving contact and arc shield mounted inside a vacuum chamber. The movable member is connected to the control mechanism by stainless steel bellows .This enables the permanent sealing of the vacuum chamber so as to eliminate the possibility of leak .A glass vessel or ceramic vessel is used as the outer insulating body. The arc shield prevents the deterioration of the internal dielectric strength by preventing metallic vapours falling on the inside surface of the outer insulating cover.

 Electrical Maintenance of VCB:

DO’s: Along with PM schedule of Motors

1. Apply & take clearance from concerned Substation Engineer
2. Ensure breaker is in Tripped condition, insert the handle in to hole in interlock shaft assembly block & move the handle upward till it comes to FREE position & then pull out Breaker from Service position to Test position.(This is to be done by the concerned S/S operator)
3. Disengage the Multi-pin plug from the socket.
4. Rack out the breaker from the cubicle.
5. Remove the front cover of breaker
6. Clean the breaker with cotton linen cloth.
7. Tightness of hardwares to be done
8. Lubricate the operating mechanism such as Drive lever, coupling rod connected at mechanism shaft, latches, dashpot, opening for manual charging, etc i.e. all sliding & rotational parts.
9. Charge the closing spring manually for checking free movement of Gear.
10. Check the plunger of Oil dashpot. Verify that plunger lift up during closing operation & damping action is OK while pressing it down.
11. Check the trip & closing coil plunger movements are free & the coil casing is properly tightened to frame.
12. Check ON/OFF indicator, spring charged position indicator is operating correctly.
13. Check & tighten all the relevant wirings
14. Measure the resistance of operating coils. If resistance not within +/-5% of the rated value, replace the coil/coils.
15. Check the contact erosion mark on Vacuum Interrupter, (Bottle). If visible in breaker closed condition, then its OK and if not visible change Vacuum Interrupter.

Parts of VCB

  

(Note: This contact erosion mark is painted either a red line or white dot on the moving stem of vacuum interrupter (lower side)

16. Check Insulation resistance by 2.5/5KV megger in breaker open condition. (IR>10,000 M-ohms)---This will indicate healthiness of Vacuum Interrupter.
17. Check motor cut-off switch/aux contacts
18. Close the spring manually.
19. Check breaker operation three –four time thru manual closing & tripping.
20. Fix the front cover.
21. Insert the breaker in TEST position and electrically charge the spring and check that motor supply is automatically cut off by limit switch.
22. Check Electrical closing & tripping in test position.
23. Return LC.

DONT’S:

1.  Do not disturb the various settings marked with red paint.

2.  Do not leave any tools & tackles on the breaker

  

Electrical Operation :

DO’s:

1. Withdrawal of Breaker from Service to Test position.

A.    Ensure breaker is Tripped condition in service position.

B.     Insert the handle in to hole in interlock shaft assembly block & move the   handle   upward till it comes to FREE position & then pull out Breaker from Service position to Test position.

C.     I it is to be kept in Test position , again insert the handle and move it  downward to its locked position.

2.  Withdrawal of Breaker out of cubicle from Test position:-

A.    Ensure Plug & socket is disengaged

B.     Interlock is in Locked position

C.     Now, Breaker can be withdrawn

3. To insert the breaker back into the test position:

A.    Ensure breaker is in Tripped condition

B.     Ensure that interlock is in fully LOCKED position.

C.     Push the breaker till interlock cam is stopped at stop pin of breaker chamber. This is the TEST position of Breaker

4. To insert the breaker from TEST to Service position:

A.    Ensure breaker is in Tripped condition

B.     Insert the handle & rotate the interlock shaft to FREE position.

C.     Engage the plug & socket

D.    Push the truck further inside till interlock cam is stopped by interlock block

E.     Insert the handle & rotate the interlock shaft to LOCKED position. This will        further push the breaker into its SERVICE position.

NOTE:- Doubly ensure that breaker is to be closed when interlock shaft is in fully LOCKED position. Then only operate the breaker for closing and tripping.

DONT’S:

Don’t operate the breaker unless interlock shaft is in fully LOCKED position. If it is not in fully LOCKED position in SERVICE/TEST position/Outside, while closing the breaker, this will damage the driving/interlocking mechanism ,will disturb the various settings and it can cause single phasing.