circuit breaker VCB and SF6 operation and its inside construction detail with practical aspects

1. Sub-Station Equipments
CIRCUIT BREAKER

2. INTRODUCTION
High voltage circuit breakers are mechanical switching
devices, which should be able to do following performances
under normal circuit conditions & also for a limited time
under abnormal circuit conditions.
Switching operations of:
Over headlines.
Cable branches (Lines).
Power Transformers.
Reactors.
Shunt Capacitor Banks.

3. Functions
• Bus sectionalizes in multiple bus-bar
installations so that power can be
transmitted from one bus to another
bus smoothly with out interruption.
• Make and break the circuit under
normal conditions.
• Make and break the circuit under
abnormal conditions such as those of
a maximum short circuit current.
• Perform the specified Rated
operating duty cycle satisfactorily.

4. 33KV VCB

5. Construction
• Each CB comprises of: 3 metal clad breaker poles, each
pole being actuated by its operating mechanism, one
supporting frame for the three poles.
• Each pole is provided with one single break interrupt of the
single pressure puer type with separate contact system for
carrying continuous current and for arching whereby
control erosion is reduced to a negligible level ensuring
long life.
• Simplicity of interrupt operation: the moving contact with
a compression cylinder, which, during tripping operation
generates the pressurized SF6 gas, required for arc
quenching Only minor over voltage of switching of small
inductive currents, owing to optimized interruption process
which prevents current chopping.

6. 132KV SF6 CB

7. Reasons for preference of SF6 Breakers
Þ Less number of interrupters per pole, hence cheaper.
Þ Maintenance free. Very long contact life.
Þ Reliable
Þ Non-explosive
Þ Does not require compressed air system
Þ Silent operation
Þ Technically superior
Performs all the required duties including line switching,
transformer switching, reactor
switching, etc. without excessive over voltages.
Easy to install, operate & maintain.

8. •The circuit breaker mainly consists of fixed contacts and moving contacts.
In normal "on" condition of circuit breaker, these two contacts are physically
connected to each other due to applied mechanical pressure on the moving
contacts.
•There is an arrangement of stored potential energy in the operating
mechanism of circuit breaker which is released, if switching signal given to
the breaker.
•The potential energy can be stored in the circuit breaker by different ways
like by deforming metal spring, by compressed air, or by hydraulic pressure.
•But whatever the source of potential energy, it must be released during
operation. Release of potential energy makes sliding of the moving contact
at extremely fast manner.
OPERATION

9. Mechanical Characteristics
•All circuit breaker have operating coils (tripping coil and closing coil),
whenever these coils are energized by switching pulse, the plunger inside
them displaces
•This operating coil plunger is typically attached to the operating
mechanism of circuit breaker, as a result the mechanically stored potential
energy in the breaker mechanism is released in forms of kinetic energy,
which makes the moving contact to move as these moving contacts
mechanically attached through a gear lever arrangement with the
operating mechanism.
•After a cycle of operation of circuit breaker, the total stored energy is
released and hence the potential energy again stored in the operating
mechanism of circuit breaker by means of spring charging motor or air
compressor or by any other means.

10. Working Principle of CB
Electrical Characteristics
•The circuit breaker has to carry large rated or fault power. Due
to this large power, there is always dangerously high arcing
between moving contacts and fixed contact during operation of
circuit breaker.
•The arc in circuit breaker can be quenched safely if the
dielectric strength between the current carrying contacts of
circuit breaker increases rapidly during every current zero
crossing of the alternating current.
•The dielectric strength of the media in between contacts can
be increased in numbers of ways, like by compressing the
ionized arcing media since compressing accelerates the
deionization process of the media, by cooling the arcing media
since cooling increase the resistance of arcing path or by
replacing the ionized arcing media by fresh gasses.

11. The closing and tripping circuits shall be
designed to permit the use of momentary
contact switches and push buttons.
Two electrically independent trip
circuits with two trip coils for each
tripping circuit.
Local remote selector switch is
required for operating the breaker
manually or electrically.
Pole discrepancy feature to trip
closed poles in the event of pole
discrepancy for single pole
operating device.
Independence of trip circuit from
local remote selection switch.
Control circuits

12. » Alarms: The following alarms and
indications are required
• General lockout for SF6
gas/air/oil/Nitrogen gas.
• Low pressure of SF6 gas /low oil
level.
• Auto re-closing lockout (for low
pressure of gas/air/oil)
• Pole discrepancy.
• Auxiliary AC/DC supply failure.
• Low operating air pressure.
• Low operating gas pressure.
• Low hydraulic oil pressure.

13. • Indications
• Breaker on-off.
• Spring charged.
• Mechanical indication:
• The operating mechanism housing shall be provided with the mechanical
indication/counters.
• Breaker “ON” - “OFF”.
• Operation counter to register the number of breaker operations.

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