In breaker-and-a-half, each breaker has its two disconnectors and its earthing switches, with the same logic for the three breakers of the diameter. The line disconnector depends on two breakers, the bus breaker and the center breaker, and the line earth depends on that disconnector and on the absence of voltage. That is why the logical unit of interlocking is not the bay but the diameter, and the earth of each bus depends on one disconnector from each diameter.
Why the diameter changes the logic
In a breaker-and-a-half configuration, each diameter has three breakers in series between the two buses and feeds two circuits: one between the bus 1 side breaker and the center breaker, and the other between the center breaker and the bus 2 side breaker. Each breaker has a disconnector on each side, and each circuit has its line disconnector or transformer disconnector (89L) with an earthing switch.
In a single bus or a double bus, each circuit has its own breaker and its logic fits in one bay. Not here: the center breaker belongs to both circuits at once, and the line disconnector depends on two breakers. The logical unit of interlocking is the diameter, and the conditions that are station level in other configurations are shared here between diameters and buses.
Conditions for each breaker
The three breakers of the diameter follow the same logic, with their side 1 disconnector, their side 2 disconnector and an earthing switch on each side of the breaker:
| Device and operation | Allowed if |
|---|---|
| Open or close a breaker disconnector | The breaker open |
| Close a breaker disconnector | Also, the earthing switch on its side open and, if it connects to a bus, the earth of that bus open |
| Close a breaker earthing switch | Both breaker disconnectors open |
| Close the breaker | Its earthing switches open, plus the breaker permissives and synchronism |
These conditions allow any breaker of the diameter to be isolated for maintenance without touching the circuits: open the breaker, then its two disconnectors, and then close its earths.
Circuit conditions
| Device and operation | Allowed if |
|---|---|
| Open or close 89L | Both breakers feeding the circuit open: the bus breaker and the center breaker |
| Close 89L | Also, the line earthing switch open |
| Close the line earth 57L | 89L open and, depending on the philosophy, no voltage on the line |
The 89L condition is what sets this configuration apart. With only one breaker open, the circuit is still fed by the other path and the line disconnector would be breaking load. With 89L open, the diameter can be closed again to keep the buses tied; the section between the breakers and the disconnector is then covered by stub bus protection, enabled by the 89L position.
- 01Open both breakersThe bus breaker and the center breaker of the circuit
- 02Open 89LAllowed because both breakers are open
- 03Verify absence of voltageOn the line, with the remote end already open
- 04Close 57LAllowed with 89L open and the line dead
- 05Close both breakersThe diameter ties the buses again; stub protection covers the section up to 89L
The full table for this configuration, with the CILO equations and the list of test cases, can be generated with our free interlocking table tool.
Line earth with voltage detection
An open line disconnector isolates the line from the substation, but not from the remote end. Closing the earthing switch with the line energized from the other side is a bolted short circuit. That is why many specifications add to the 57L permission the absence of voltage measured on the line voltage transformer, or a signal from the remote end. The fifth rule of EN 50110-1 also requires earthing on every side from which the section can be fed, which on a line means coordinating with the other substation.
Bus earth
The earthing switch of bus 1 closes only if the bus 1 side disconnectors of every diameter are open. It is the station condition of this configuration: it depends on one signal from each diameter, and when a new diameter is added it must be added to the condition.
Center breaker and voltage selection
The center breaker ties the two circuits of the diameter. When it closes between two energized parts it needs a synchronism check 25, and the reference voltage on each side depends on what is connected: the line voltage transformer of each circuit, or that of the bus if the circuit is out and its bus breaker is closed. These voltages are selected from the disconnector positions, just like the interlocking, and a selection error shows up as a close rejected by synchronism check for no apparent reason.
Reclosing of lines with two breakers uses a leader and follower scheme, and a failure of the center breaker is the most severe case for breaker failure protection 50BF, because it takes out both circuits of the diameter.
What belongs to the bay and what to the diameter
| Condition | Scope |
|---|---|
| Disconnectors and earths of one breaker | The breaker (within the diameter) |
| 89L and 57L of a circuit | Both breakers of the circuit: diameter |
| Voltage selection for the center breaker | Both circuits of the diameter |
| Bus earth | Every diameter: station |
With one controller per circuit, the center breaker forces positions to be exchanged between the two controllers of the diameter, by wiring or over GOOSE. Many architectures assign a controller to the center breaker or integrate it into one of the two circuits; that decision defines which signals travel between devices.
How it is verified
The test goes through each condition negating each term, for each breaker of the diameter and for each circuit. It includes the line earth with voltage present, the bus earth against each diameter, the voltage selection of the center breaker with each combination of disconnectors, and enabling the stub protection with 89L open. End-to-end tests also cover the transfer trip of the center breaker 50BF.
Are you designing or extending a breaker-and-a-half substation? At Tiza Group we configure and test interlocking logic, voltage selection and stub protection per diameter, in high and extra-high voltage substations. We load and change the logic, interlocks and signals remotely through our remote relay and controller configuration service, without waiting for a site visit. We prepare the secondary engineering, configure remotely and test on site in Colombia, or elsewhere on request. Contact us with the single-line diagram of your substation.
Related guides
- Breaker failure 50BF (RBRF): what it is and how it works
- Synchronism check 25 (RSYN): what it is and how it works
- Bay and station interlocking: what each level solves
- Interlock testing: what is checked at FAT and SAT
- Breaker-and-a-half reclosing: leader and follower
- Breaker-and-a-third interlocking
- Voltage selection for synchronism check in double bus
References
- IEC 61850-7-4, Compatible logical node classes and data object classes (CILO, CSWI, XSWI, XCBR, RSYN).
- IEC 62271-102:2018, Alternating current disconnectors and earthing switches.
- IEC 61936-1:2021, Power installations exceeding 1 kV AC and 1,5 kV DC – AC (interlocking and earthing).
- EN 50110-1, Operation of electrical installations – General requirements (five rules).
- IEC 60050-605, International Electrotechnical Vocabulary – Generation, transmission and distribution of electricity – Substations.
Frequently asked questions
Why does the line disconnector need two breakers open?
Because the circuit is fed by two paths: from its bus through its bus breaker, and from the other bus through the center breaker. With only one open, the line disconnector would still carry the circuit current.
Can the diameter be closed with the line disconnector open?
Yes, it is a usual operation to keep the buses tied through the diameter while the circuit is out. In that case there is a section between the breakers and the line disconnector, which is covered by stub bus protection, enabled by the disconnector position.
Does the line earth need voltage detection?
Many philosophies require it, because an open line disconnector does not guarantee that the line is dead: it may be energized from the remote end. The permission combines the open disconnector with the absence of voltage measured on the line voltage transformer, or with confirmation from the remote end.