VACUUM CIRCUIT BREAKER

VACUUME CIRCUIT BREAKER (VCB)

MAKE	Jyoti Ltd. Vadodara
Type	UY – 30M25
Rated frequency	50Hz
Rated Voltage	36000 Volts(36KV)
Rated Current	1250 Amps
Ms Level Imp	170KV P, PF-75 kv
Rated Breaking current	25 KA
Rated Making Current	63 KA peak
Sr. No.	B-242, Year- 2001
Supply Voltage Closing	110 VDC
Supply Voltage Tripping	110 VDC
Rated short time current	25 KA
Wt of Beaker	375 Kg
Standard	IS:13118/IEC-56
Making Capacity	2.55 x symmetrical ÷ Breaking capacity
Breaking Capacity	1000 x 106/√3 x 33000

ADVANTAGE OF VCB

1.    They are compact, reliable and have longer life

2.    There are no fire hazards

3.    There is no generation of gas during and after operation

4.    They can interrupt any fault current. The outstanding feature of a VCB is that it can break any heavy fault current perfectly just before the contacts reach the definite ope position

5.    They require little maintenance and are quiet in operation

6.    They can successfully withstand lightening surge.

7.    They have low arc energy

8.    They have low inertia and hence require smaller power for control mechanism.

DISADVANTAGE OF VCB

1.    Air chopping/current chopping

2.    Vacuum leakage cannot be indentified

3.    Costly

MAKING CAPACITY: Breaker closes on fault condition, contacts does not damage. Current on which a ckt breaker is capable of breaking under specified condition.

BREAKING CAPACITY: Breaking current capacity of circuit breaker, is the highest value of short circuit current which a circuit breaker is capable of breaking under specified condition.

In the circuit breaker, breaking capacity is less than making capacity: When short circuit occurs, there will be voltage drop. Thus voltage is less, current is also less. So Coil temperature in the circuit breaker = 80⁰.

We are having OCB.VCB with low resistance zero common method. For zero method, contact, material is used.

When breaker is switching on & off, spark takes place & due to hydrogen gas heat produced. To cool this heat and quenching of arc, vacuum is used.

High Vacuum : 10^-5 to 10^-8 Tor

PRINCIPLE OF VACUUM CIRCUIT BREAKER

The production of arc in vacuum circuit breaker and its extinction can be explains as follows:

When the contacts of the Breaker are opened in vacuum (10^-7 – 10^-5 torr) and arc is produced between the contacts by ionization of metal vapour of contacts. However, the arc is quickly extinguished because the metallic vapours, electrons  and ions produced during arc rapidly condenses on the surfaces of the circuit breaker contacts resulting in quick recovery of di-electric strength. The reader May note that the salient feature of vacuum as an arc quenching media. As soon as the arc is produced in vacuum, it is quickly extinguished due to the fast rate of recovery of di-electric strength in vacuum.

ANTI PUMPING RELAY.

The VCB is provided with an anti-pumping feature to prevent the re-closing of the breaker after opening in case of continuous electrical closing command. The anti pumping relay “Y” and “X” are mounted in the mechanism cabinet on the top left hand side.

The moving contact of the vacuum interrupter is connected to the bottom disconnecting arm through flexible jumpers. The fixed contacts of the vacuum interrupter are assembled with the top disconnecting arm.

The jaw contacts, fixed at the end of the disconnecting arms, are made of specially designed copper flat which are assembled together and are spring loaded. This ensures sufficient contact pressure when the jaw contacts get engaged with the contact blades in the panel while the VCB is in service.

The VCB has closing coil, tripping coil and breaker operated auxiliary switches.

Commissioning instructions

a)    Check manual spring charging, closing and opening operations.

b)    Check operation of all indicators (e.g. CHARGE/DISCHARGED. OPN/CLOSED)

c)    Check tightness of the all wire terminations.

d)    Check electrical operations of the VCB

e)    Check all interlocks for satisfactory working.

f)     Check and ensure that there are no loose connections in the power circuit.

g)    Clean the VCB with dry ling-free cloth and or vacuum cleaner.

h)   Check insulation resistance between phase and also phase to earth with the help of a megger.

i)     Conduct High Voltage test on the vacuum interrupters.

Maintenance and inspection

The stroke of vacuum Circuit Breakers being very small, the energy required to operate in vacuum interrupter at specified speed is very less as compared to other types of circuit breakers off the same rating.

CHECK ON VACUUM

Whether the interrupter maintains a high internal vacuum can be checked by applying a voltage of AC 70 kV rms as a voltage withstand test for about 10 seconds. Measurement is taken with the VCB OPEN and the voltage applied between the terminals of the vacuum interrupter.

When the internal pressure (Vacuum level) is not sufficiently low there is almost no delay in the breakdown. So a duration of 10 seconds of voltage application is enough.

If test set up trips, repeat the process trice. If test up trips all three times, the vacuum interrupters is deemed to the defective and needs to be replaced.

NOTE

Megger test (I.R. test) is not a method recommended for assessing integrity of vacuum in the interrupter.

MEASUREMENT OF CONTACT EROSION

The wipe spring (25) shown in figure 10 provide sufficient contact pressure to the vacuum interrupter when it is CLOSED. In case of excessive fault trippings, the main contacts may get eroded progressively. When the contact get eroded, the wipe of the wipe springs get reduced.

Wipe can be observed for each phase by removing the front cover  of the VCB. It can be seen by the red mark on the link of the wipe spring (25) with circuit breaker CLOSED as shown in figure 13. As a routine inspection, the presence of wipe can be seen by the red mark.

When the red mark is about to disappear with breaker CLOSEDC, the value of wipe reaches is minimum permissible value. Nominal value of wipe, when circuit breaker is supplied, is 3+0.5 mm. When the value of wipe reaches 1 mm or less, the vacuum interrupter is required to be replaced.