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 opening and closing, it is operated by either a remote relay or a local switch. A remote relay is located inside the control room while the switch is located inside the circuit breaker junction box.
Close and Trip Circuit of a Breaker
Understanding a 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. The figure below depicting a circuit breaker scheme will be used to explain various elements of the PCB’s design and its control.
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, the contacts switch their state i.e. 52a contact will be closed while 52b is open. Contact positions of all other auxiliary relays and switches – remote or local – stay unchanged unless, ofcourse, they operate on a fault or other desired condition.
Circuit Breaker Trip Coil
Figure above 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 PCB’s 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 relay’s 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.
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 relay’s 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 breaker’s 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 system 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 fitted with both 43 local/remote switch and a local trip 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.
Anti-Pump Relays
To prevent inadvertent multiple closing operation, breakers are fitted with anti-pump relay. Assume a scenario 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 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 relays like an electro-switches are typically employed to handle these currents.
