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Power Circuit Breaker Operation and Control Scheme

BYADMINON MARCH 7 , 2012 1 COMMENT

Power Circuit Breakers (PCB) break an electrical circuit to isolate faults. They also re -close to

make a circuit after the fault is removed. To enable this open and close operation, it is operated

by either a remote relay or a local switch. A remote relay is located at a remote location such as

a control room while the switch is located inside the circuit breaker junction box.

Understanding the breaker scheme is important if you plan on designing a substation. Quite

often, it is overwhelming to make sense of the entire scheme at a glance. Therefore, the figure

below depicting a circuit breaker scheme will be used to simplify and explain various elements

of the PCBs design and its control.

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Figure 1: Close and Trip Circuit of a Breaker

http://peguru.com/wp-content/uploads/2012/03/CB1-2.jpg

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Forms of Contact

Before explaining what each device in the scheme does, understanding the different forms of

contact is necessary. A form a contact represents a Normally Open (N.O.) contact while a form

b is a Normally Closed (N.C.) contact. Thus when a breaker is de -energized, its 52a and 52b

contact position stay true to the statement above and as shown in Figure 1. However, when

PCB is energized, these contacts switch their state i.e. 52a contact will be closed while 52b is

open. Contact positions of all relays, switches, auxiliary relays and switches remote or local

stay unchanged.

Circuit Breaker Trip Coil

Figure 1 depicts a trip coil of the breaker. For brevity, I will cover the trip coil no.1 with trip coil

no.2 identical.

From the diagram, the breaker is fitted with a 43 switch that toggles between local trip and

remote trip. Positioning it in local allows the persons at the breaker junction box to trip the

circuit by closing the Control Switch (CS). Switching it to remote position permits the relays in

the control house to close their contact and trip the breaker.

Modern PCBs employing Sulfur Hexa -Flouride (SF6) gas to extinguish an arc are fitted with

ANSI 63 relay. To prevent breaker damage due to flash-overs during low gas conditions,

tripping of breaker is cut-out by this relays contact. Notice in Figure 1 how the contacts from

this relay are strategically placed in the close and trip circuit to cut out any signal from the

relays or switches.

At this point, the reader should realize the importance of contact development. All contacts

operate only when the trip coil of their respective relay is energized. For instance, consider the

63 relay and its contacts shown in in figure 1. This relay is energized by the same DC source as

the one supplying the breaker. However its trip coil is actuated by a transducer that can sense a

fall in SF6 gas pressure. When this occurs, it switches its contacts located in different circuits to

prevent any breaker operation. Similarly, the 27 undervoltage relay trip coil is connected across

the DC source. When this supply is interrupted, the relay switches its contact position. This

change can be relayed to an alarm or initiate some other action.

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To trip the breaker from a remote location, all contacts from relays at the remote location shall

be hard-wired. Yes, this means laying a lot of copper from the breaker cabinet to the relays.

Further, all tripping contacts are wired in parallel. When either relays contact close and thus

complete the circuit, the breaker trips.

Target Devices

Now, you may notice the red target lamp is connected in a way that will essentially short out the

remote relays and trip the breaker. Not surprisingly, this is not the case. The target lamps

shown in the scheme have enough resistance in them (~200 ohms), limiting the current that can

energize the coil.

Target lamps are used in circuits to convey certain conditions. With the breaker closed and

energized, the red lamp illuminates to indicate a live circuit. When the breaker opens (due to a

fault) the green lamp illuminates the circuit complete with 52b contact switching from open to

close.

Most modern circuit breakers are specified with two trip coils. Energizing either one leads to

breakers trip. Since a good amount of redundancy is built into the protection and control of a

power system, it is not too uncommon to see all primary relaying in the sy stem tripping trip coil

1 and the back-up tripping trip coil 2.

Circuit Breaker Close Coil

This coil when energized actuates a lever that engages the closing mechanism (like a spring).

A close circuit is optionally fi tted with both 43 loca l/remote switch and a local tr ip switch.

Remote relays are wired in as shown in Figure 1. Unlike the trip circuit, the relay contacts in the

close circuit are always connected in series and present in normally closed position. Thus, when

a relay trips, it also blocks closing of the breaker. Until the relay is reset, either manually or

remotely, the breaker will not be operational.

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Anti -Pump Relays

To prevent inadvertent multiple closing operation, breakers are fitted with anti-pump relay.

Assume a scenar io where a fault persists on a line and a person is looking to close a breaker on

it. Although the person presses the close button for a second or two, for the breaker which

operates in cycles, this duration is an eternity. With the close button pressed, the breaker

attempts to close but because of the fault in the system it trips again, then closes, then trips.

This trip/close operation repeats for the second or two the button is pressed. Since the motor in

the breaker is not rated for continuous duty, serious damage can occur to it.

Modern breaker control relays are programmed to check for synchronism and also to reclose a

breaker. A single contact from this relay is all that is needed to initiate one -shot, two-shot, or

three-shot scheme. In old breaker schemes, 25 relay contacts and reclosing relay (79) contacts

from auxiliary relays are typically wired into the breaker close scheme.

On a final note, keep in mind that not all relays can handle the momentary trip/close coil

currents. Auxiliary switches like those manufactured by electro-switch inc. are typically installed

to handle these currents.

Summary

Breaker scheme is a web of interlocked relays and switches.

Breaker operation is controlled by relays and switches.

The switches can be operated remotely or locally.

TAGGED WITH Anti pump re laycircuit breaker DC schemesf6 gas relay

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