Circuit Breaker

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

Comparison of SF6, CB & ABCB for Number of Interrupters per Pole:

Rated Voltage KV	145 KV	245 KV	420 KV	765 KV
SF6  CB	1	1 or 2	2 or 4	4
ABCB	4	4 or 6	8 or 10	10 or 12

            The piston is pushed down by high pressure oil on top area of the piston 

            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 continuously without 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 pushed-up 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

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.

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.

In this mode, NC/NO “start” and NC/NO “stop” switches should be in ‘No’ 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 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.

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.

In this mode, NC/NO “START” switch should be in “NO” position and NC/NO “STOP” switch 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).

            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.