CIRCUIT BREAKERS
The function of a Circuit breaker is to isolate the faulty part of the power system in case of
abnormal conditions.
A Circuit breaker has two contacts - a fixed contact and a moving contact. Under normal
conditions these two contacts remain in closed position. When the circuit breaker is required to isolate the faulty part, the moving contact moves to interrupt the circuit. On the separation of the contacts, the flow of current is interrupted, resulting in the formation of arc between the contacts. The contacts
are placed in a closed chamber containing some insulating medium (liquid or gas) which extinguishes
the arc.
Arc Interruptions
There are two methods of arc interruption
i. High Resistance Interruption
ii. Current Zero Interruption
(i) High Resistance Interruption
In this method of arc interruption, its resistance is increased so as to reduce the current to a value insufficient to maintain the arc. The arc resistance can be increased by cooling, lengthening, constraining and splitting the arc. This method is not suitable for a large current interruption.
(ii) Current Zero Interruption
In case of a.c supply, the current wave passes through a zero point, 100 times per second at the supply frequency of 50 Hz. This feature of arc is utilised for arc interruption. The current is not interrupted at any point other than the zero current instant, otherwise a high transient voltage will occur across the contact gap.
Restriking voltage and Recovery Voltage
After the arc has been extinguished, the voltage across the breaker terminals does not normalise instantaneously but it oscillates and there is a transient condition. The transient voltage which appears across the breaker contacts at the instant of arc being extinguished is known as restriking voltage.
The power frequency rms voltage, which appears across the breaker contacts after the arc is finally extinguished and transient oscillations die out, is called recovery voltage.
Classification of Circuit Breakers
Depending on the arc quenching medium employed, the following are important types of
circuit breakers.
i. Oil Circuit Breakers
ii. Air Blast Circuit Breakers
iii. Sulphur Hexafluoride (SF6) Circuit Breakers
iv. Vacuum Circuit Breakers
During closing stroke high pressure oil is admitted from the bottom of piston. Piston bottom area being larger than that on the top the differential pressure acts and the piston is pushed upwards for closing stroke.
Rating of Circuit Breakers
Circuit breakers have the following important ratings
i. Breaking Capacity
ii. Making Capacity
iii. Short-time Capacity
Breaking Capacity
The breaking capacity of a circuit breaker is of two types.
(i) Symmetrical breaking capacity
(ii) Asymmetrical breaking capacity
(i) Symmetrical breaking capacity
It is the rms. Value of the ac component of the fault current that the circuit breaker is capable
of breaking under specified conditions of recovery voltage.
(ii) Asymmetrical breaking capacity
It is the rms value of the total current comprising of both ac and dc components of the fault current that the circuit breaker can break under specified conditions of recovery voltage.
Making Capacity
The rated making current is defined as the peak value of the current (including the dc
component) in the first cycle at which a circuit breaker can be closed onto a short circuit. Ip in Figure. is the making current.
Short-time Current Rating
The circuit breaker must be capable of carrying short-circuit current for a short period while another circuit breaker (in series) is clearing the fault. The rated short-time current is the rms value (total current, both a.c. and d.c. components) of the current that the circuit breaker can carry safely for
a specified short period.
Rated Voltage, Current and Frequency
The rated current is the rms value of the current that a circuit breaker can carry continuouslywithout any temperature rise in excess of its specified limit.
The rated frequency is also mentioned by the manufacturer. It is the frequency at which the
Circuit breaker has been designed to operate. The standard frequency is 50 Hz. If a circuit breaker is to be used at a frequency other than its rated frequency, its effects should be taken into consideration.
Operating Mechanism
Different manufacturers adopt one of the following:
1. Pneumatic
2. Hydraulic
3. Spring
1. Compressed air system for pneumatic mechanism: -
The pressure of the operating air produced by the self-enclosed motor compressor unit is
supervised by a pressure gauge and controlled by a pressure switch (located in the breaker
control cabinet). The pressure switch has four sets of contacts each of which operates at a
different pressure level for the following functions: Start/stop, low pressure indication,
reclosure interlock, closing and opening interlock.
This type of system is provided for each pole of 420 KV and 765 KV CB.
2. Hydraulic Mechanism: -
The hydraulic cylinder is mounted at the base of interrupter unit and the hydraulic rod is pushedup during closing stroke and pushed down during opening stroke. Moving contact and puffer cylinder are linked with the hydraulic rod.
The hydraulic oil at atmospheric pressure from the low pressure tank is pumped into
accumulators at high pressure (310 kg/cm2
). High pressure oil is used for opening and closing operations.
During opening operation high pressure oil from lower area of hydraulic piston is discharged into low pressure tank by opening appropriate hydraulic valve.
3. Spring Mechanism: -
For CB upto 245 KV, spring operating mechanism is used. The closing spring gets charged by geared motor. During closing stroke the closing spring is discharged. It closes the breaker and also charges the opening springs.
The Circuit breaker mechanism is basically very simple since it comprises a motor-reducer and two springs (one for closing and one for opening), plus their solenoid operated latching
devices.
The closing spring after having been charged by the motor-reducer is kept ready for closing by its latching device.
As soon as energised, the closing control coil friggers the closing latch thereby causing the
closing spring to operate the breaker to the closed position and simultaneously charge the
opening spring.
From now on, trip latch keeps the breaker in the closed position and the opening springs ready for opening.
The compact design of the mechanism avoids the need for the opening springs to be located
close to each pole and permits the opening operation to be performed by only one spring
accommodated in the control cabinet.
Reasons for preference of SF6 Breakers
Less number of interrupters per pole, hence cheaper.
Maintenance free. Very long contact life.
Reliable
Non-explosive
Does not require compressed air system
Silent operation
Technically superior
Performs all the required duties including line switching, transformer switching, reactor switching, etc.
without excessive over voltages.
Easy to install, operate & maintain.
OPENING & CLOSING TIMES
a) Closing Time Measurement of Circuit Breaker using Digital Time Internal Meter
To measure the closing time of circuit breaker (i.e. the time elapsed from the movement of
the extending control supply to the Closing coil to the movement of actual closing of the circuit
breaker), the following circuit is made use of.
The counter starts when the two terminals of “START” switch are shorted by closing the
D.P.D.T switch. Simultaneously the D.C supply is extended to the closing coil of the breaker. The counter stops counting when the two terminals of ‘STOP’ switch are shorted through the ‘main contact’ of circuit breaker, as soon as it closes and the timer displays the time interval, which is the closing time of circuit breaker.
Hence the principle is that the energisation of the closing coil of the circuit breaker and
starting of the counter should be simultaneous.
In this mode, NC/NO “START” switch should be in “NO” position and NC/NO “STOP” switch
b) Measurement of Circuit Breaker Opening time
To measure the opening time of circuit breaker, (i.e., the time elapsed from the movement of
extending control supply to the trip coil of the circuit breaker to the movement of actual tripping of
circuit breaker), the following circuit is made use of.
should be in ‘NC’ position.
The counter starts when the two terminals of “START” switch are shorted by closing the
D.P.D.T switch.
Simultaneously the D.C supply is extended to the trip coil of the breaker.
The counter stops counting when the two terminals of “STOP” switch are opened with the opening of the circuit breaker “MAIN CONTACTS”. The time internal displays the time internal,which is the “Opening time” of circuit breaker.
Pole Discrepancy Relay:
Two cases are to be studied:
1) Three Phase Tripping.
All the thee poles should simultaneously trip or close. While tripping,if any one or more of
the poles do not open simultaneous with the other, after a set time (of 0.1 Sec.), the pole
discrebancy relay will give a trip command again to trip the same. Similarly while closing the breaker, if any of the three poles do not close simultaneous with the other, the pole discrebancy relay, after a preset time (of 0.1 Sec.) will give a trip command to trip the closed pole(s).
2) Single phase tripping and Auto Reclosure:
While the application principle is same as with the three phase tripping, the time setting of
the pole discrebancy relay shall be more than the dead time (of 1 Sec.) of the auto reclosure relay.
Circuit breaker operation Analyzer: -
This instrument helps in measurement of contact make and break times, contact travel
speeds.
