Power Circuit Breaker – Operation and Control Scheme

Power Circuit Breaker – Operation and Control Scheme

Power Circuit Breakers (CB) break an electrical circuit to isolate faults. They also re-close to make/close 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/remote switch. 

A remote relay / remote switch is located at a remote location such as a control room while the local switch is located inside the circuit breaker junction box.

Understanding the circuit breaker scheme is important for correct operation & designing of 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 CB’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 '52a' contact represents a Normally Open (NO) contact while a form '52b' is a Normally Closed (NC) contact. Thus when a breaker is de-energized / off , its 52a and 52b contact position stay true to the statement above and as shown in Figure. However, when CB is energized / on, these contacts switch their state i.e. 52a contact will be closed / NC while 52b is open / NO. Contact positions of all relays, switches, auxiliary relays and switches – remote or local – stay unchanged.

Circuit Breaker Trip Coil

Figure depicts a trip coil of the breaker. Generally, the trip coil no.1 with trip coil no.2 are 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 / close, the circuit by operating, the Control Switch (CS). Switching it to remote position permits the remote closing / tripping operations from the control room to close and trip the breaker.

Modern CB’s employing Sulfur Hexa-Flouride (SF6) gas to extinguish an arc are fitted with ANSI ’63’ [Gas Pressure lockout] relay. To prevent breaker damage due to flash-overs during low gas conditions, tripping of breaker is cut-out by this relay’s contact. Note in Figure, 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.

One must realize the importance of contact development. All contacts operate only when the energizing coil of their respective relay is energized. For instance, consider the 63 relay [Gas Pressure lockout] and its contacts shown in figure. This relay is energized by the same DC source as the one supplying the breaker. However its 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 connected to CB. 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, one 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) or, opened for maintenance, 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. 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 electricaly, 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 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  are typically installed to handle these currents.