The bus coupler breaker ties the two buses of a double bus. Its interlocking has two sides: its disconnectors may only be operated with the coupler open, and the coupler itself must be blocked against opening while any bay has both bus disconnectors closed, because at that moment it carries the on-load bus transfer. When it stands in for a bay breaker, its logic changes again.
What the bus coupler does
In a double bus, the bus coupler is a breaker with a disconnector toward each bus and its own current transformers. It ties the two buses to share load, separates them when the short-circuit level has to be limited or half of the substation isolated, and makes on-load transfer of circuits between buses possible.
Its interlocking differs from that of a line or transformer bay because it does not only protect its own operation. It protects the operation of the disconnectors of the other bays. That is why it combines local conditions with station conditions.
Its disconnectors
The coupler disconnectors behave like any bus disconnector: they may only be operated with the coupler breaker open and with the adjacent bus and bay earthing switches open. The reverse relationship is stated in the National Grid interlocking specification (TS 3.01.01): the coupler breaker may only close with both of its associated disconnectors open or both closed. With both open the breaker can be tested without tying the buses; with both closed, it ties them. With only one closed there is no operational reason to close it.
Blocking against opening
An on-load transfer of a circuit goes like this: with the coupler closed, the bay disconnector toward the new bus is closed and the one toward the previous bus is opened. While both bay disconnectors are closed, there are two parallel paths between the buses: the coupler and the bay itself.
If the coupler opens at that moment, all the current exchanged between the buses flows through the bay disconnectors. The next step of the transfer, opening one of them, no longer commutates a small current between two low-impedance paths: it interrupts a current that can be large. IEC 62271-102 recognizes bus-transfer current switching with a rated current and voltage (clause 5.108 and annex B), and that capability assumes the coupler stays closed.
The key condition of the coupler is therefore:
- Does any bay have both bus disconnectors closed?
- YesOpening blocked while the transfer lasts
- NoAre the positions of all bus disconnectors valid and received from every bay?
- NoOpening blocked until the position is clarified
- YesOpen permitted
The blocking applies to opening by command, not to opening by protection. If there is a fault, the bus protection or the 50BF must be able to trip the coupler. Some utilities, on top of the logic interlock, remove the coupler control supply during the transfer as a procedural measure. The National Grid specification lists among the purposes of interlocking ensuring the correct sequence of on-load bus transfer.
Relationship with the bus protection
While a bay has both disconnectors closed, the two buses are tied through it. The bus differential protection 87B knows this from the disconnector replica: it merges the two zones into one and leaves the coupler current transformer out. A fault on either bus during the transfer then trips both. It is a risk accepted because the operation is short, and one more reason not to leave a bay on both buses longer than necessary.
If a disconnector auxiliary contact is badly adjusted, the error shows up at the same time in the coupler interlocking and in the 87B replica. The guide on the disconnector replica for the 87B explains how it appears.
The coupler’s breaker failure protection 50BF has a special destination: if the coupler does not open, both buses have to be tripped.
The coupler standing in for a bay
In a double bus with transfer and in the variants with a bypass disconnector, the coupler can take the place of a bay breaker while that breaker is under maintenance. In that state the logic changes completely:
- The coupler no longer ties buses: it carries the current of one circuit.
- Its protection must be that of the circuit it replaces, which means transferring trips or settings.
- Only one circuit can be substituted at a time, and the interlocking must prevent a second one from being transferred.
- The bus tie function is lost during the maintenance: an on-load transfer of another bay is no longer possible.
Some designs have separate coupler and transfer breakers, and others a single breaker that performs both functions depending on the position of its disconnectors. In the second case the logic must tell the mode from the topology, not from a manual selector that can be left in the wrong position.
Implementation
The blocking condition is station-wide: it depends on the bus disconnectors of every bay. In a hardwired substation it is built with contacts from every bay in series and in parallel. In a numerical one, each bay controller publishes the position of its disconnectors by GOOSE and the coupler controller evaluates them in its CILO logical node (IEC 61850-7-4). If a message is lost, the safe design blocks the opening and alerts the operator; it does not allow it. The alarm must say which bay stopped reporting, so the operator does not have to guess why the coupler rejects the command; an unexplained rejection is behind many requests to bypass the interlocking.
How it is verified
During commissioning each bay is taken in turn with both disconnectors closed, checking that the coupler does not accept an open command but does accept a protection trip. The loss of each bay’s message is simulated, and the substitution mode is checked if there is one. The rest of the test plan is in the guide on interlocking tests.
We configure it with you
Our team designs, configures and tests the bus coupler logic, the replica for the 87B and the 50BF trip destinations in high and extra-high voltage substations and in industry. Field testing is done in Colombia, or elsewhere on request. We load and change the logic, interlocks and signals remotely through our remote relay and controller configuration service, without waiting for a site visit. See secondary engineering or contact us with your arrangement.
Related guides
- Lockout relay 86
- Double bus interlocking: coupler and transfer
- Bus disconnector interlocking: conditions
- Bypass and transfer disconnector interlocking
- Disconnector replica for bus differential protection
- Double bus with transfer and bypass interlocking
References
- IEC 62271-102:2018, Alternating current disconnectors and earthing switches, clause 5.108 and annex B (bus-transfer current switching).
- National Grid, TS 3.01.01 (RES), Substation Interlocking Schemes, issue 1, 2018.
- IEC 61850-7-4, logical nodes CILO, CSWI, XCBR and XSWI.
- IEC 61936-1:2021, Power installations exceeding 1 kV AC, 6.2.16.
- IEEE C37.234, Guide for Protective Relay Applications to Power System Buses.
Frequently asked questions
Why must the bus coupler not open during a bus transfer?
Because while a bay has both bus disconnectors closed, the coupler is the main path between buses. If it opens, the current exchanged between the buses flows through the bay disconnectors, and the one operated next interrupts or makes that current with an arc.
Can the bus coupler be closed with only one disconnector closed?
Usual practice avoids it: the coupler is closed with both disconnectors closed or with both open, as in a breaker test. Closing it with only one ties nothing and can leave a section energized without clear protection.
What happens to the 87B when the coupler is blocked and a bay is on both buses?
The bus protection sees the two buses tied through the bay and treats them as a single zone, leaving the coupler current transformer out. A fault on either bus then trips both, until the transfer is over.