Main and Transfer Bus Interlocking: 89T and Bypass

Short answer

In a main and transfer bus arrangement the interlocking enforces three rules: only one circuit at a time on the transfer bus, the transfer disconnector 89T operates only in parallel with a closed circuit breaker, and the protection follows the current. The 89T closes with the transfer breaker closed, no other 89T closed, the transfer bus earthing switch open and the protection transferred.

Main and transfer bus interlocking solves the problem that gives the arrangement its name: maintaining a circuit’s breaker without taking the circuit out. In the main and transfer bus arrangement each bay has, besides its bus disconnector (89B), its circuit breaker (52) and its outgoing disconnector, a transfer disconnector (89T) that connects the outgoing side to the transfer bus. A transfer breaker (52T), between disconnectors 89TP and 89TT, ties the two buses.

The bay logic is that of a single busbar. What belongs to this arrangement are three rules: one circuit at a time on transfer, the 89T operates only in parallel with a closed breaker, and the protection follows the current.

Conditions per device

The table summarizes typical logic for this arrangement.

Device	Opens if	Closes if
89TP and 89TT (transfer coupler)	the 52T is open	the 52T is open and the bus earthing switch on its side is open
Transfer breaker 52T	free	89TP and 89TT both open or both closed; earthing switches of both buses open
Transfer disconnector 89T	the 52T is open, or the circuit’s 89B, 52 and outgoing disconnector are closed	transfer coupler closed, no other 89T closed, transfer bus earthing switch open and protection transferred
Transfer bus earthing switch	free	every 89T and the 89TT open
Main bus earthing switch	free	every 89B and the 89TP open
Line earthing switch 57L	free	the circuit’s 89L and 89T open

The last row is easy to forget: the 89T is a second voltage path to the line. With the 89L open but the 89T closed, the line is still in service through the transfer bus.

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.

Why the 89T can operate live

A disconnector does not make or break load current. The 89T is the recognized exception: when it closes, the circuit breaker and the transfer breaker are already closed and both buses are at the same potential, so the voltage across its contacts is very small. This is the bus-transfer current switching duty that IEC 62271-102 allows a disconnector to perform, and it depends on the size of the loop and on the bus-transfer rating assigned to the device. That is why the interlocking requires the 52T closed to close the 89T, and the circuit’s own path closed to open it.

Transferring the protection

While the circuit goes through the 52T, its protection has to trip the 52T. Depending on where the current transformers are, the transfer switches only the trip or also the currents, the reclosing and the teleprotection channels. The interlocking treats it as one more state:

the protection is transferred before the 89T closes, and only if no other protection is transferred;
it is returned to the circuit’s own 52 only with the 89T already open.

In the field, the transfer is done with test/transfer switches, a transfer selector switch or a dedicated relay in the transfer bay with one settings group per circuit. If currents are switched, the sequence must never leave a current transformer secondary open.

Critical switching sequence: moving a circuit to transfer

Maintaining a line circuit breaker without taking the line out

01Energize the transfer busClose 89TP, 89TT and 52T: the breaker proves the bus is healthy
02Transfer the protectionThe trip, and the currents if needed, move to the 52T
03Close the 89TIn parallel with the line 52: no significant current is made
04Open the line 52The current now flows through the 52T
05Isolate the circuit breakerOpen and lock out the 89B and the 89L, verify absence of voltage, close earthing switches 57-1 and 57-2

The return is the reverse sequence: remove the earths, close the 52 disconnectors, close the 52, open the 89T, return the protection and open the 52T.

Bay level and station level

The logic of this arrangement is almost all station level:

the 89T of each bay depends on the transfer coupler and on the 89T of every other bay;
the protection transfer depends on the transfer state of the others;
both bus earthing switches depend on every bay.

Each bay controller needs the state of the others. Hardwired, that is a contact network that grows with every circuit; by GOOSE, each controller publishes its 89T and its transfer state, and the others subscribe. The project defines what to do when a message is lost: treat the remote 89T as «not open» and block.

Medium voltage vs high voltage

High voltage: the classic switchyard arrangement, with motor-operated disconnectors and logic in the bay controller. The protection transfer is usually a command from the same controller or from a selector switch on the panel.
Medium voltage switchgear: with the breaker on a withdrawable truck, the breaker comes out of the panel for maintenance, so a transfer bus is more typical of switchyards. Where it exists at medium voltage, the logic is the same.

Typical mistakes

89T without the condition on the other 89T. The substation is extended and the new bay is left out of the exclusivity condition.
Line earthing switch without the 89T. The 57L closes with the 89L open while the line is still live through the transfer bus.
Protection returned with the 89T closed. The circuit is left on the 52T with the protection tripping an open breaker.
Teleprotection not transferred. The line is left with only distance zones during the maintenance.
Open current secondary while the currents are being switched.

How it is tested

The full sequence is run with each circuit, checking that the controller rejects the 89T without protection transfer, with another 89T closed and with the 52T open, and that the transferred protection really trips the 52T. End-to-end tests confirm the transferred channels.

We can help with your interlocking logic

At Tiza Group we design the transfer scheme as part of secondary engineering: interlocking, protection transfer logic 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 and CSWI.
IEC 62271-102:2018: disconnectors and earthing switches; bus-transfer current switching.
IEC 61936-1:2021: power installations exceeding 1 kV AC.
EN 50110-1: operation of electrical installations (five safety rules).
J. Gastón Ortega and E. Nelson, Preserve High-Speed Protection During Bypass Operations, Western Protective Relay Conference, 2006.
C. Labuschagne, N. Fischer and B. Kasztenny, Simplifying and Improving Protection of Temporary and Unusual Bus Configurations With Microprocessor-Based Relays, Western Protective Relay Conference, 2012.

Frequently asked questions

Why won't the transfer disconnector close even though the transfer bus is live?

First check that the transfer breaker and its two disconnectors are closed, that no other circuit has its 89T closed, that the transfer bus earthing switch is open and that the circuit protection shows as transferred. Any one of those conditions blocks the close.

Can two circuits be moved to the transfer bus at the same time?

No. There is one transfer breaker and one transferred protection. With two circuits in parallel behind that breaker, a fault on one would take out both, and the protection would be set for only one of them. The interlocking prevents a second 89T from closing.

What happens if the transfer disconnector is opened with the circuit breaker open?

All the circuit current flows through the 89T with no other path, and the disconnector would interrupt it with an arc. That is why the 89T only opens with the circuit's own path closed, breaker and disconnectors, or with the transfer breaker already open.