Single Busbar Interlocking: Bay Logic and Bus Earthing

Single busbar interlocking is the reference logic for any substation: every other arrangement starts from it and adds conditions. In the single busbar arrangement every bay hangs from one bus, each with its bus disconnector (89B), its circuit breaker (52) and its earthing switches. With no bus selectors and no bus coupler, almost everything is solved inside the bay.

What the logic protects against

Interlocks exist so that a wrong command cannot be executed. In a single busbar substation they prevent four situations:

• a disconnector opening or closing load current, since it has no arc-quenching chamber;
• an earthing switch closing onto a live section;
• a circuit breaker closing onto an earthed section;
• earthing a section that is separated from the source only by an open circuit breaker, which does not provide isolation.

In IEC 61850-7-4 each switching device has its CILO logical node, which delivers the open and close enables (EnaOpn and EnaCls) to the CSWI control node. The standard defines those signals, not the conditions: the conditions are set by the project engineering.

Conditions per device

The table summarizes the typical logic of a line bay and of the bus. Each project has its own.

In a transformer bay the outgoing side changes: the earthing switch on the HV terminals (57T) also requires the low-voltage circuit breaker (52LV) open, so that nothing back-feeds voltage from the LV side. And the 52LV does not close with the 57T closed.

When a disconnector is in an intermediate or invalid position, the logic treats it as «not open» and «not closed» at the same time. That way it blocks both operations that depend on it. This handling of the double-point position of substation disconnectors is the first source of rejected commands in the field.

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.

Bay level and station level

A condition is bay level when it only uses devices of the same bay, and station level when it needs the state of other bays. In a single busbar substation every condition is bay level except one: the bus earthing switch. To close it, the bay controller that commands it has to know the position of the 89B of every bay.

That information travels in two ways:

• Hardwired: auxiliary contacts of each 89B in series up to the closing circuit of the earthing switch. Simple with few bays, cumbersome when the substation is extended.
• By GOOSE: each controller publishes the position of its 89B and the one in charge of the bus earthing switch subscribes to all of them. Extending the substation means adding a subscription, not rewiring. See GOOSE in IEC 61850.

With GOOSE, the project decides what to do when a message is lost. The safe choice is to treat the position as unknown and block.

Critical switching sequences

Taking a line out for work. This is the sequence that goes through the most conditions, and it shows why the order matters.

The disconnector order is operating practice, not a requirement of the standard: if the circuit breaker stayed closed because of a mechanical failure, an arc at the 89L is cleared by the line protection, whereas an arc at the 89B is cleared by the bus differential, taking out the whole substation.

Bus maintenance. The whole substation goes out: every circuit breaker opens, then every 89B, and only then does the bus earthing switch close.

Medium voltage vs high voltage

• High voltage, conventional switchyard: motor-operated disconnectors and earthing switches, electrical interlocking in the bay controller or in hardwired logic, and a mechanical lock or padlock for lockout/tagout. The earthing switch usually also has a mechanical interlock with its disconnector.
• Medium voltage, metal-enclosed switchgear: isolation is provided by the withdrawable truck or by a three-position switch. IEC 62271-200 requires interlocks inside the panel: the truck cannot be moved with the breaker closed, and the earthing switch cannot close with the truck in service. Anything that depends on other panels is still electrical logic, a trapped key or GOOSE.

In both, interlocking does not replace the five safety rules of EN 50110-1: disconnect, secure against reconnection, verify absence of voltage, earth and short-circuit, and protect against adjacent live parts.

Typical mistakes

• Bus earthing switch without every bay. When a new bay is added, its 89B is left out of the bus earthing switch condition.
• Circuit breaker as isolation. Logic that allows the 57-1 with the 52 open and the 89B closed.
• Invisible remote earth. The controller cannot see the 57 at the other end of the line; if the procedure does not confirm it, the line is energized onto an earth.
• Intermediate position treated as open. A single 89b contact, without the 89a, lets the command through with the disconnector halfway.
• Permanent interlock bypass. The interlock bypass key is left in place after a test.

How it is tested

During commissioning the full matrix is covered: for each device and each condition, the forbidden operation is attempted and the test checks that the controller rejects it, from local control, from the HMI and from the control centre. It is also tested with simulated intermediate positions and with the loss of an interlocking GOOSE message.

We can help with your interlocking logic

At Tiza Group we design the interlocking table as part of secondary engineering and program the bay controller equations and the interlocking GOOSE through remote configuration, for medium, high and extra-high voltage substations and for industry. Matrix testing on site is done in Colombia, or elsewhere on request.

Related guides

• Lockout relay 86
• Bus differential protection 87B
• Bay-level and station-level interlocking
• GOOSE interlocking with CILO and CSWI (IEC 61850)
• Sectionalized busbar interlocking
• Main and transfer bus interlocking
• Interlocking tests: what is checked at FAT and SAT

References

• IEC 61850-7-4: logical nodes CILO (EnaOpn, EnaCls) and CSWI; one CILO instance per switching device, algorithm defined by the implementer.
• IEC 62271-102:2018: alternating current disconnectors and earthing switches.
• IEC 62271-200: AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV (panel interlocks).
• IEC 61936-1:2021: power installations exceeding 1 kV AC.
• EN 50110-1: operation of electrical installations (five safety rules).
• IEEE C37.2: device function numbers 52, 57 and 89.

Frequently asked questions