Ring Bus Interlocking: Open Ring and Voltage Selection

Short answer

In a ring bus each circuit hangs between two circuit breakers, so its disconnector operates only with both open. With one breaker out for maintenance the ring is open, and the logic should warn that a second opening splits the substation. Since there is no bus, the synchronism check of each breaker needs a voltage selection that picks, from the device positions, which voltage transformer represents each side.

Ring bus interlocking is simple device by device, but it requires thinking about the whole topology. In the ring bus arrangement the circuit breakers form a loop, with as many breakers as circuits, and each circuit leaves from the node between two of them. There is no bus, no bus selectors and no coupler.

That has three consequences for the logic: each circuit disconnector depends on two breakers, the open ring is a risk state the logic must make visible, and the voltage for synchronism check has to be found in the circuits.

Conditions per device

Each circuit breaker has a disconnector and an earthing switch on each side. Summary of typical logic:

Device	Opens if	Closes if
Disconnectors of a breaker	its breaker is open	its breaker open and the earthing switch on its side open
Ring breaker	free	its two earthing switches open
Earthing switches of a breaker	free	both disconnectors of that breaker open
Line disconnector 89L (or transformer disconnector)	both breakers the circuit hangs between open	both open and the circuit earthing switch open
Line earthing switch 57L	free	the 89L open
HV terminal earthing switch 57T	free	the transformer 89 and its low-voltage breaker open

There are no bus earthing conditions. Almost every condition is bay level, if each breaker with its disconnectors is taken as a bay; the circuit disconnector is the exception, because it uses the position of two neighbouring breakers.

The full table for this arrangement, with the CILO equations and the list of test cases, can be generated with our free interlocking table tool.

The open ring

With one breaker out for maintenance, every circuit stays in service, but the ring now behaves like a straight bus. A second opening, by fault or by command, splits the substation into two groups and can leave circuits without a source.

The interlocking logic does not block a breaker trip: protection must always be able to open it. What the control system can do:

show the «ring open» state on the HMI, and at which node;
ask for confirmation, or block by command, the opening of a second breaker that would leave circuits without a source;
change the reclosing priorities: with one breaker out, the other breaker of the circuit becomes the leader.

Those functions depend on the state of every breaker, so they are station level.

Voltage selection

Without a bus, there is no common voltage transformer. Each circuit has its own, on the circuit side, and the synchronism check of each breaker has to compare the voltage of the nodes on both sides. The voltage selection logic picks which transformer represents each node:

Which voltage does the 25 use for one side of the breaker?

Does the circuit at that node have a voltage transformer, with its disconnector closed?
- YesUse that voltage transformer
- NoIs the node tied to another circuit through closed breakers and disconnectors?
  - YesUse that circuit's voltage transformer
  - NoNo reference: the close permission is blocked or the node is treated as dead, as the project defines

In a common layout the lines have voltage transformers and the power transformers do not, so selection is essential for the breakers next to a transformer. The logic uses the same positions as the interlocking; a wrongly wired auxiliary contact makes the 25 compare the wrong voltage.

Critical switching sequences

Releasing a line and restoring the ring

01Open both neighbouring breakersThe line is de-energized from the substation
02Open the 89LWith both breakers open
03Enable stub protectionThe 89L position activates it
04Close both breakersThe ring closes again, with permission from the 25
05Isolate the remote end and earth the lineThe 57L closes only with the 89L open

Circuit breaker maintenance. It is opened, its two disconnectors are opened, absence of voltage is verified and its earthing switches are closed. The ring stays open while the work lasts.

Closing the ring. The last breaker that closes the loop ties two parts that may have an angle difference if the external network was split; its 25 makes the decision.

High and extra-high voltage

The ring bus is used mainly at high voltage when there are few circuits and the goal is to maintain any breaker without interrupting service with the fewest devices. It is common to plan for the ring to become a breaker-and-a-half later; if the logic is well organized per breaker and per circuit, that extension is done by adding devices and conditions, without redoing the existing ones. With IEC 61850 controllers, each breaker publishes its positions by GOOSE and the circuit disconnectors and the voltage selection subscribe to them.

The interlocking cannot see the earth at the remote end of a line either: before closing the 89L, the control centre confirms that the remote earth has been removed.

Typical mistakes

Circuit disconnector with only one breaker. The condition is written with one neighbour instead of both.
Static voltage selection. The 25 of a breaker always takes the same voltage transformer, even when its circuit is open.
Reclosing without a leader change when one breaker is out.
Unsupervised stub 89L position.
Transformer earthing switch with the LV breaker closed.

How it is tested

The matrix of each breaker and each circuit disconnector is covered. Then voltage selection is tested in each relevant combination of positions, injecting different voltages into each voltage transformer and checking which one the 25 takes. Reclosing with each breaker out and the stub are also tested. Secondary injection covers these logic functions.

We can help with your interlocking logic

At Tiza Group we design ring bus logic as part of secondary engineering: interlocking, voltage selection, reclosing and stub in the bay controller, and we program it through remote configuration, for high and extra-high voltage. On-site testing is done in Colombia, or elsewhere on request.

References

IEC 61850-7-4: logical nodes CILO, CSWI, RSYN and RREC.
IEC 62271-102:2018: alternating current disconnectors and earthing switches.
IEC 60050-605: substation vocabulary (ring busbar arrangement).
IEC 61936-1:2021: power installations exceeding 1 kV AC.
EN 50110-1: operation of electrical installations (five safety rules).
IEEE C37.104: guide for automatic reclosing of circuit breakers.

Frequently asked questions

Why does the synchronism check of a ring bus breaker use the voltage of another circuit?

Because a ring bus has no bus voltage transformer. Each side of the breaker is a circuit node, and if that circuit has no voltage transformer of its own, or its disconnector is open, the selection logic takes the one of a neighbouring circuit tied to that node through closed breakers and disconnectors.

Does the interlocking prevent opening a second breaker with the ring open?

A protection trip is never interlocked: protection must always be able to open. What good logic does is warn on the HMI, or ask for confirmation, when an opening by command would split the substation or leave circuits without a source.

Where does the bus earthing switch go in a ring bus?

There is no bus. Each breaker has an earthing switch on each side and each circuit has its own. To work on a section of the ring, the breaker is isolated with its two disconnectors and its earthing switches are closed.