In a double busbar, the bus selector disconnector of a bay is operated off-load (bay breaker open and the other selector open) or on-load (coupler and its disconnectors closed and the other selector already closed). The coupler closes only with its two disconnectors in the same position and is not opened while a bay ties the two buses. Earthing a bus requires every disconnector to that bus to be open. The same positions feed the busbar protection replica.
Why the double busbar needs more interlocks
In a double busbar configuration, each bay has a breaker (52) and two selector disconnectors, one to each bus (89B1 and 89B2). A bus coupler (52A, with its disconnectors 89A1 and 89A2) ties the buses, and each bus has its earthing switch (57B1 and 57B2). That flexibility brings operations that do not exist in a single bus: choosing the bus for each bay, moving a bay from one bus to the other without interrupting it, and freeing a whole bus for maintenance. Each has its risk, and interlocking covers it.
The conditions in this guide are those of a typical high-voltage double busbar. The table of a real project depends on the utility’s switching philosophy and on the equipment.
Conditions per device
| Device and operation | Allowed if |
|---|---|
| Bay selector 89B1, off-load | Bay 52 open, 89B2 open, bay earthing switch open and 57B1 open |
| Bay selector 89B1, on-load | Coupler closed (89A1, 52A and 89A2), 89B2 already closed and 57B1 open |
| Coupler disconnectors 89A1 and 89A2 | 52A open; to close, the earth of the corresponding bus also open |
| Closing the coupler 52A | 89A1 and 89A2 both open or both closed, and both bus earths open |
| Opening the coupler 52A by command | No bay with both selectors closed |
| Bus earthing switch 57B1 | Every bay 89B1 open and 89A1 open |
Selector 89B2 and earth 57B2 are symmetrical. The conditions within the bay itself (line disconnector, line and breaker earthing switches) are the same as in any bay and are explained in the general interlocking guide.
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.
On-load transfer of a bay
Moving a bay from bus 1 to bus 2 without interrupting it requires both buses to be at the same potential, so that the selectors operate without a significant change of voltage across their terminals, which is the condition under which a disconnector may operate according to IEC 62271-102.
- 01Coupler closed89A1, 52A and 89A2 closed: the buses are at the same potential
- 02Coupler blocked against openingThe interlock prevents opening 52A by command while the transfer lasts
- 03Close bay 89B2Allowed because the coupler is closed and 89B1 is still closed
- 04Open bay 89B1Allowed because the coupler is closed and 89B2 is already closed
- 05End of the bridgeThe bay is on bus 2 and the coupler may be opened again
While the bay has both selectors closed, it ties the buses in parallel with the coupler. If the coupler opened at that moment, all the current between buses would flow through the selectors of that bay, and opening one of them would mean breaking load. That is why the coupler cannot be opened by command during the bridge. Protection, however, does open it: a trip never goes through the interlock.
Closing the coupler with a single disconnector
The coupler has a condition of its own: it closes with both disconnectors open (to test it) or both closed (to tie the buses), never with only one. With a single disconnector closed, 52A would energize the section between the breaker and the open disconnector, and the next operation would be closing that disconnector with voltage on one side and the bus on the other, without the coupler as backup.
Earthing a bus
To earth bus 1, every disconnector connecting it must be open: the 89B1 of each bay and the coupler 89A1. It is the most extensive station condition in the substation, because it depends on one signal from each bay. When a new bay is added, its 89B1 must be added to the condition: an omission makes it possible to close the earth with a bay connected.
The other way round, no selector can close onto a bus with its earth closed.
Replica for busbar protection
The selector positions do not only feed the interlocking. The bus differential protection 87B uses them to assign the current of each bay to the zone of its bus, and breaker failure protection uses them to know which breakers to open. During the transfer, with both selectors closed, the two zones merge. If the replica takes a different contact from the interlock, or if its timing does not match the disconnector travel, the 87B can operate during switching, as explained in disconnector replica for the 87B.
Classic mistakes
- Transferring with the coupler open: the interlock prevents it; if it was bypassed, the selector closes tying two buses at different voltages.
- Opening the coupler in the middle of a transfer from a control level that does not go through the interlock.
- Bus earth without the new bay in its condition.
- Selectors in intermediate position treated as open.
- Hardwired station interlocking left open when a bay was removed.
How it is verified
The test goes through each condition of the table positive and negating each term, with special attention to the station conditions: the bus earth against each bay, the coupler opening block with each bay bridging, and the on-load transfer from each control level. It also checks that the 87B replica follows the same positions.
Are you extending a double busbar or migrating its interlocks to bay controllers? At Tiza Group we configure and test bay and station interlocking and the disconnector replica, hardwired or over IEC 61850. 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 the controllers remotely and test on site in Colombia, or elsewhere on request. Contact us with the single-line diagram of your substation.
Related guides
- Bus differential 87B (PDIF): what it is and how it works
- Interlock testing: what is checked at FAT and SAT
- Bus disconnector interlocking: conditions
- Disconnect replica for bus differential protection
- Double bus with transfer and bypass: interlocking
- Automatic switching sequences in substations
References
- IEC 62271-102:2018, Alternating current disconnectors and earthing switches (definition of disconnector; bus-transfer current).
- IEC 61850-7-4, Compatible logical node classes and data object classes (CILO, CSWI, XSWI).
- IEC 61936-1:2021, Power installations exceeding 1 kV AC and 1,5 kV DC – AC (interlocking).
- IEEE C37.234, Guide for Protective Relay Applications to Power System Buses.
Frequently asked questions
Why will the bus coupler not open during a transfer?
Because a bay has both selector disconnectors closed and ties the two buses. If the coupler opens, the current it was sharing flows through those disconnectors, and the next step of the transfer would be opening a disconnector under load. The interlock blocks opening the coupler by command while that bridge exists.
Can a bay be moved to the other bus with its breaker closed?
Yes, that is the on-load transfer, and it is only safe with the coupler and both of its disconnectors closed. The two buses are then at the same potential and the selector disconnectors operate without a significant change of voltage across their terminals.
What happens to the 87B while a bay has both selectors closed?
The busbar protection can no longer tell through which bus the current of that bay enters, so it merges the two zones into one while the bridge lasts. If a fault occurs at that moment, both buses trip.