Sectionalized Double Bus and Triple Bus Interlocking

Sectionalized double bus and triple bus interlocking is where station-level logic stops being one more condition and becomes the centre of the design. In a sectionalized double bus each bus is split into two sections tied by bus section breakers, with a bus coupler in each section. In a triple bus each circuit has three bus selectors and there are two couplers, with a reserve bus. In both, knowing whether an operation is safe requires the state of devices spread across the whole substation.

The logic of each bay is that of the double bus: bus selectors operate off load with the circuit breaker open, or on load with both buses tied by a closed coupler. What is new is what «tied» means.

Sectionalized double bus: the coupler of the circuit’s own section

With couplers and bus sections closed, the four bus sections form a ring. Opening coupler A does not separate buses 1A and 2A, which are still tied through BS1, coupler B and BS2. Even so, the usual logic requires the coupler of the circuit’s own section for an on-load transfer. The long path depends on three circuit breakers that can open halfway through the operation, by command or by protection.

Triple bus: buses tied by more than one coupler

In a triple bus, coupler 1-2 ties buses 1 and 2, and coupler 2-3 ties buses 2 and 3. There is no direct coupler between 1 and 3. Typical logic handles it this way:

• On-load transfer between buses j and k: only if they are tied by closed couplers. To move a circuit from bus 1 to bus 3, both couplers have to be closed.
• Opening a coupler: blocked while a circuit has two selectors closed towards buses that only that coupler ties. Coupler 1-2 does not open with a circuit on buses 1 and 3, because that tie goes through it.
• Reserve bus: energized with coupler 2-3, never with a bus selector.
• Earthing switch of each bus: with every selector towards it and the coupler disconnectors on that side open.

Why GOOSE beats hardwiring here

In a single busbar, the only station-level condition is the bus earthing switch. Here almost all of them are:

• each bus selector depends on the couplers and, in a triple bus, on the combination of couplers that ties its buses;
• each coupler depends on the selectors of every circuit on its buses;
• each bus section earthing switch depends on every bay that can reach it.

Hardwired, that logic is a network of auxiliary contacts and contact multiplier relays running across the switchyard. Each new bay forces rewiring of couplers and earthing switches across the substation, and each contact is a failure point. With GOOSE, each bay controller publishes the position of its devices and the one that decides subscribes. In IEC 61850-7-4, the CILO node of each switching device computes the enable from those positions and passes it to CSWI; the standard leaves the algorithm to the implementer. Extending the substation means adding publishers and subscriptions in the SCL file, which you can review with our free SCL viewer.

The price is defining data quality carefully: what the logic does when a message arrives late, arrives with invalid quality or with the test bit set. The safe choice is to block and alarm.

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.

Critical switching sequences

Maintenance of a bus section in a sectionalized double bus. The circuits of that section move on load to the other bus of the same section; the section coupler and the bus section breaker of that bus are opened; their disconnectors on the bus section side are opened and, once absence of voltage is verified, the bus section earthing switch closes.

Using the reserve bus in a triple bus. The disconnectors of coupler 2-3 are closed and then its circuit breaker, which energizes the reserve bus and proves it is healthy. Only then can circuits move to it on load.

Typical mistakes

• Assumed bus tie. Logic that only looks at the direct coupler and allows a transfer between buses 1 and 3 with a coupler open.
• Coupler released on the long path. A section coupler is opened relying on the other section and on the bus section breakers.
• Incomplete bus section earthing condition. A selector or one end of a bus section is missing from the condition.
• GOOSE quality ignored. The logic takes as valid the last position received from a controller that stopped publishing.
• 87B disconnector replica out of step with the interlocking logic. See 87B tripping during disconnector switching.

How it is tested

The matrix for a substation like this is large, and it is tested at two levels: at the factory, with the controllers and a position simulator, covering combinations of couplers and bus sections that are hard to reproduce in the switchyard; and on site, with the real operations and forced loss of GOOSE messages. FAT and SAT are the natural place for that verification.

We can help with your interlocking logic

At Tiza Group we design substations with several couplers as part of secondary engineering and communications engineering: interlocking table, IEC 61850 GOOSE engineering and the disconnector replica for the 87B, and we program the controllers through remote configuration, for high and extra-high voltage. On-site testing is done in Colombia, or elsewhere on request.

Related guides

• Bus differential protection 87B
• Bay-level and station-level interlocking
• Bus coupler interlocking: when it must not open
• Disconnector replica for the bus differential
• Double bus interlocking: coupler and bus transfer
• Interlocking tests: what is checked at FAT and SAT

References

• IEC 61850-7-4: logical nodes CILO, CSWI and XSWI; one CILO instance per switching device, algorithm defined by the implementer.
• IEC 61850-8-1: GOOSE messages.
• IEC 62271-102:2018: disconnectors; bus-transfer current switching.
• IEC 60050-605: substation vocabulary (double busbar, triple busbar, reserve busbar).
• IEC 61936-1:2021: power installations exceeding 1 kV AC.
• EN 50110-1: operation of electrical installations (five safety rules).

Frequently asked questions