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 PCB’s design and its control.
Forms of Contact
Before explaining what each device in the scheme does, understanding the different forms of auxiliary contact is necessary.
A form ‘a‘ contact represents a Normally Open (N.O.) contact. Thus when the breaker is open, its 52a contacts are open. When the breaker is closed, the 52a contacts are closed. The 52a contact follows the status of the breaker.
A form ‘b‘ contact represents a Normally Closed (N.C.) contact. It operates exactly opposite of what an ‘a’ does. When the breaker is open, the 52b contacts are closed. When the breaker is closed, the 52b contacts are open.
Apart from breaker contacts, you will see relays such as anti-pump relay 52Y, low gas relay 63X, under voltage relay 27 etc in the breaker scheme. The contacts from each of these relays are interlocked with other relays in such a way that they either permit or not permit the breaker operation.
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 breaker 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 any one relay contact closes, thus completing the circuit, the breaker trips.
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 breaker. When the breaker opens 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 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.
There are two relaying elements in the close circuit. There is the close contact from a breaker control relay. This contact is then tied in series with a string of lockout relay (LOR) contacts (all in series). The lockout relay contacts are all ‘b’ contacts. Read more on lockout relays here. The ‘b’ contacts are normally closed during normal operation. However when an abnormal condition exists, the ‘b’ contact opens up, preventing the breaker from closing.
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 open and close multiple times. Since the motor in the breaker is not rated for continuous duty, serious damage can occur.
Modern breaker control relays are programmed to check for synchronism. That is, before the breaker is closed, the relay checks the phase angles of source and load side voltages. This will require PT inputs from the source side (typically a line PT) and from the load side (typically a bus PT).
The close circuit also has contacts from the Motor Switch (MS). They open-close based on motor operation.
On a final note, keep in mind that not all relay contacts can handle the momentary trip/close coil currents of the breaker. For example SCADA board control relays. Interposing relays like those manufactured by Potter-Brumfield are typically installed to handle these currents. So in our case the SCADA control relay trips an aux relay and the aux relay trips the breaker.
Most modern microprocessor relays especially ones made by Schweitzer can handle inrush currents upto 30Amps and thus can be wired directly to the breaker coils.
- Breaker scheme is a web of interlocked relays and switches.
- Breaker operation is controlled by relays and switches.
- Trip contacts are wired in parallel.
- Close contacts are wired in series i.e. breaker control relay ‘a’ contact followed by a series of LOR ‘b’ contacts.