Double Bus Double Breaker Interlocking and Stub Zone

Double bus double breaker interlocking is among the simplest of the high-voltage arrangements, and that simplicity is part of its value. In the double bus double breaker arrangement each circuit has two breakers of its own, one towards each bus, and leaves from the node between them. There are no bus selectors and no coupler: with everything closed, the buses are tied through the circuits.

Since no disconnector moves a circuit from one bus to the other, none needs to operate in parallel. Bus transfer, which in a double bus concentrates the station-level conditions, does not exist here.

Conditions per device

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

In a transformer bay, the earthing switch on the HV terminals requires the transformer disconnector and the low-voltage breaker open.

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

Almost everything is bay level. There are two station-level conditions:

• the earthing switch of each bus, which needs the bus-side disconnectors of every circuit;
• the synchronism check of each breaker, which takes the voltage of its bus and of the line.

The first is solved as in a single busbar, with auxiliary contacts or with GOOSE. The second belongs to the close command: the function 25 of each breaker compares the voltage transformer of its bus with that of the line. With two breakers per circuit, reclosing is usually arranged with a leader and a follower breaker, so that the second one closes only if the first was successful.

The stub zone

To work on a line without losing the tie between buses, both circuit breakers are opened, the line disconnector is opened and both breakers are closed again. The stretch between them and the open 89L, the stub zone, is energized but outside the zone of the line protection, which no longer sees the line. The protection of that stretch, an overcurrent or a differential between the two sets of current transformers, is enabled by the 89L position.

That position signal is part of the control logic. If the 89L auxiliary contact fails, stub protection is disabled with the stretch energized, or enabled with the line in service. That is why normally open and normally closed contacts with discrepancy supervision are used, as explained in the substation disconnector article.

Critical switching sequences

Circuit breaker maintenance. The breaker and its two disconnectors are opened, and its earthing switches are closed. The circuit stays in service through the other breaker, although it now depends on a single bus: a fault on that bus takes it out.

Bus maintenance. Every breaker towards that bus and its bus-side disconnectors are opened; then the bus earthing switch closes. No circuit goes out of service.

Intermediate position and remote earth

Two details apply to any arrangement, but here they weigh more because the logic of each device is short and an error is not covered by another condition:

• Double-point position. Each disconnector and earthing switch reports open, closed, intermediate or invalid. The logic must treat intermediate and invalid as «not open» and «not closed» at the same time, so that it blocks both operations that depend on it.
• Remote-end earth. The bay controller cannot see the earthing switch at the other substation. Before closing the 89L or energizing the line, the procedure confirms with the control centre that the remote earth has been removed. If the line has direct transfer trip, its enabling also depends on the position of the devices.

Medium, high and extra-high voltage

This arrangement belongs to high and extra-high voltage, because of the cost of two breakers per circuit. In SF6 gas-insulated substations the logic is the same, with disconnector and earthing switch combined in a single three-position device depending on the manufacturer; each position is interlocked as if they were separate devices.

Typical mistakes

• 89L with only one breaker open. The condition is written with the bus 1 breaker only.
• Unsupervised stub. The enabling depends on a single 89L contact.
• Bus earthing switch on open breakers. The condition looks at the breakers instead of their bus-side disconnectors.
• Sync check on the wrong voltage. The breaker towards bus 2 compares against the bus 1 voltage transformer.
• Both breakers reclosing at once, with no leader and follower logic.

How it is tested

The matrix of each breaker and of the 89L is covered, stub protection enabling is tested with the 89L in each position, including intermediate, and the test verifies that the 25 of each breaker takes the correct voltages. Secondary injection testing covers the stub and the sync check.

We can help with your interlocking logic

At Tiza Group we design two-breaker schemes as part of secondary engineering: interlocking, stub logic, sync check and leader and follower reclosing, and we program them in the bay controllers and relays through remote configuration, for high and extra-high voltage. On-site testing is done in Colombia, or elsewhere on request.

Related guides

• Automatic reclosing 79
• Bay-level and station-level interlocking
• Line disconnector interlocking: conditions
• Breaker-and-a-half interlocking: diameter and lines
• 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, RSYN and RREC.
• IEC 62271-102:2018: alternating current disconnectors and earthing switches.
• IEC 61936-1:2021: power installations exceeding 1 kV AC.
• EN 50110-1: operation of electrical installations (five safety rules).
• IEEE C37.234: guide for protective relay applications to power system buses.
• M. Lampe, G. Corpuz, S. Das and A. Hargrave, Line Current Measurement at Dual-Breaker Terminals: Challenges and Solutions, Conference for Protective Relay Engineers, 2021.

Frequently asked questions