Sectionalized Busbar Interlocking: Bus Section Breaker

Short answer

In a sectionalized single busbar each bay keeps the single busbar logic and the bus section adds its own conditions: its disconnectors operate only with its circuit breaker open, the breaker closes with both disconnectors open or both closed, and each section has its own bus earthing switch. If both sections have their own source, closing the bus section goes through synchronism check.

Sectionalized busbar interlocking adds one new element to the single busbar logic: the bus section between the two halves of the bus. In the sectionalized single busbar arrangement each circuit is fixed to its section, and the bus section breaker (52S), between disconnectors 89SA and 89SB, ties or splits sections A and B. Each section has its own bus earthing switch.

What changes from a single busbar

Line and transformer bays are interlocked as in a single busbar: disconnectors with their breaker open, the breaker with its earthing switches open, earthing switches with their disconnectors open. What is new:

the bus section, a bay with two disconnectors, one circuit breaker and two earthing switches;
the bus earthing switch per section, which no longer depends on every bay but on the bays of its section and on the bus section disconnector on that side;
closing the bus section, which can tie two sources.

Conditions per device

The table shows typical logic. Each project adapts it to its own engineering.

Device	Opens if	Closes if
Disconnector 89SA (or 89SB)	the 52S is open	the 52S is open, the bus earthing switch of its section is open and the bus section earthing switch on its side is open
Bus section breaker 52S	free	89SA and 89SB both open, or both closed; earthing switches of both sections and of the bus section open
Bus section earthing switches	free	89SA and 89SB open
Bus earthing switch, section A	free	the 89B of the section A bays and the 89SA open
Bus earthing switch, section B	free	the 89B of the section B bays and the 89SB open

The «both open or both closed» condition on the 52S allows two uses: testing the isolated breaker and tying the sections. What it prevents is the in-between state, in which the breaker would energize the stretch up to an open disconnector.

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.

Closing between two sources

If each section has its own line, with the bus section open the two sections are separate sources. Closing the 52S at that moment makes a parallel, and the close permission does not come from the interlocking alone: it comes from the synchronism check 25, which compares the voltage of the two buses. For that the relay needs the voltage transformer of each section.

Close permission for the bus section breaker

Are both bus section disconnectors closed and the earthing switches open?
- NoThe interlocking blocks the close
- YesAre both sections live?
  - YesParallel close: function 25 gives permission only if magnitude, angle and frequency are within limits
  - NoDead-bus close: allowed by the dead-voltage mode enabled in the 25

A disconnector never closes that parallel. If the bus section is a disconnector only, with no circuit breaker, the interlocking must require one of the sections to be dead with no source connected.

Bay level and station level

Within the bus section, the conditions of its disconnectors, its breaker and its earthing switches are bay level. Station level are:

each bus earthing switch, which depends on every bay of its section and on the bus section;
the bus section disconnectors, which depend on the bus earthing switch on their side;
the 25 permission, which takes the voltage of both sections.

With two sections and few bays, wiring auxiliary contacts covers everything. As the substation grows, publishing the position of each 89B by GOOSE avoids rewiring the bus earthing switch every time a circuit is added.

Critical switching sequences

Maintenance of one section. The load is moved, if possible, to the other section; the bay breakers of the section and the 52S are opened; the 89B of the section and the 89S on that side are opened, and only then does the earthing switch of that section close. The other section stays in service with its line.

Losing the line of one section. Before taking out a line with the bus section open, the bus section must be closed so the other line picks up the section. Done the other way round, the section is left without a source.

Fault on one section. The bus differential protection of the section opens its bays and the 52S. Re-energizing the section by closing the bus section onto the fault repeats the short circuit: the fault is isolated first.

Medium voltage vs high voltage

Medium voltage: the bus section is a bus coupler panel with a withdrawable breaker, or a breaker panel plus a riser panel. The panel comes with its IEC 62271-200 mechanical interlocks; those that depend on other panels are solved in each panel’s relay or with trapped keys. An automatic transfer that closes the bus section when one section loses its source is common here, blocked if the section tripped on a fault.
High voltage: motor-operated disconnectors in the switchyard, logic in the bus section bay controller, and the 25 in the same controller or in the bay relay.

Typical mistakes

Section earthing switch wired to the wrong 89S. The condition for the section A earthing switch is wired with the 89SB instead of the 89SA.
52S without 25. The bus section close is enabled without synchronism check because «it is normally closed».
Transfer without fault blocking. The automatic logic closes the bus section onto a section that tripped on a bus fault.
A single voltage transformer. Without the voltage of the second section there is no possible 25 and no dead-bus detection.

How it is tested

The matrix of each device is covered by attempting the forbidden operation, with priority on the 52S: one disconnector closed and the other open, a section earthing switch closed, and the close with both sections live outside the 25 limits. If there is an automatic transfer, its blocking on fault and on disconnector position is tested.

We can help with your interlocking logic

At Tiza Group we design the bus section logic as part of secondary engineering: interlocking, synchronism check, transfer and GOOSE, and we program it in the bay controllers and relays through remote configuration, for medium, high and extra-high voltage substations and for industry. On-site testing is done in Colombia, or elsewhere on request.

References

IEC 61850-7-4: logical nodes CILO, CSWI and RSYN.
IEC 62271-102:2018: alternating current disconnectors and earthing switches.
IEC 62271-200: AC metal-enclosed switchgear and controlgear above 1 kV up to and including 52 kV.
IEC 61936-1:2021: power installations exceeding 1 kV AC.
EN 50110-1: operation of electrical installations (five safety rules).
IEEE C37.2: device functions 25, 52, 57 and 89.

Frequently asked questions

Why won't the bus section breaker close with only one disconnector closed?

Because it would energize a dead stretch between the breaker and the open disconnector, and the next operation would be closing that disconnector with voltage on one side. The usual interlock requires both disconnectors open, to test the breaker, or both closed, to tie the sections.

What happens if the bus section is closed between two sections fed from different sources?

A parallel is made. If the voltages differ in magnitude, angle or frequency, a closing current flows that can operate protections. That is why the close is supervised by the synchronism check function 25, which only gives permission within its set limits or with one of the buses dead.

Can a disconnector be used instead of a bus section breaker?

Yes, in simple installations, but then the bus section is only operated without current: with one of the sections dead and unloaded. The installation loses the ability to split the sections on a fault and to tie them live.