Protection transfer to the transfer bus breaker

Short answer

When a circuit moves to the transfer bus, its own breaker is taken out and the transfer breaker replaces it. The circuit's protection must be transferred to that breaker: its trips always, and its currents, reclosing, synchronism check and teleprotection channels depending on where the CTs are and how the bay is designed. An incomplete transfer leaves the circuit unprotected or trips the wrong circuit.

What protection transfer is

In a substation with a main and transfer bus, or a double bus with transfer bus, a circuit can be fed by the transfer breaker while its own breaker is under maintenance. The physical connection is made by the disconnects: the circuit’s transfer disconnect joins its outgoing terminal with the transfer bus, and the transfer breaker joins that bus with the main bus.

But the circuit still needs its protection. Protection transfer is the switching that makes the circuit’s protection measure what it should and trip the breaker that now feeds it. Without it, the circuit stays connected to the network with its own breaker open and its relay acting on a device that no longer operates.

What is transferred

Trips. Always. The trip outputs of the circuit protection are switched from its own breaker to the transfer breaker. The same applies to the initiation of breaker failure 50BF, which must supervise the transfer breaker.

Currents. It depends on where the current transformers are:

Line-side CTs (beyond the tap to the transfer bus): the circuit current still flows through them. Transferring the trips is enough.
CTs between the breaker and the tap: with the circuit on transfer they no longer see current. The circuit relay must receive the current from the transfer breaker CT, and that requires switching current circuits.

CT switching is done with auxiliary relays or switching blocks designed to short-circuit the secondary before opening it. An open CT secondary under load produces voltages dangerous to people and to insulation.

Voltages. If the voltage transformer is on the line side, nothing changes. If the protection takes bus voltage, it is still the same main bus.

Reclosing and synchronism check. Automatic reclosing 79 and synchronism check 25 must act on the transfer breaker, with its closing time and its voltage selection.

Teleprotection and line differential. If the line uses a channel-aided scheme or 87L, the channel must follow the relay that protects the line. If it is not transferred, the line is left with stepped distance protection, which is slower, for the whole duration of the transfer.

Bus differential. The 87B of the main bus must keep seeing the boundaries of its zone: with line-side CTs, the circuit current flows through two CTs and the zone assignment changes for as long as the transfer lasts.

Two design philosophies

Transfer of the circuit protection: each circuit’s relay is switched to the transfer breaker. It keeps the circuit’s settings and channel, but needs trip switching and sometimes CT switching.
Dedicated transfer relay: the transfer breaker has its own relay with one settings group per circuit. There is no CT switching; in exchange, the operator must select the right group, and the teleprotection channel does not follow the circuit unless it was designed to.

In both, the change of state must be indicated in station control and at the control centre: the operator needs to know which protection is acting on which breaker.

How it fits into the sequence and the interlocks

The transfer is a step of the switching sequence, not an option. In the classic main and transfer bus arrangement:

Moving a circuit to the transfer bus

01Energize the transfer busCoupler disconnects and transfer breaker closed
02Transfer the protectionTrips and, if needed, currents, reclosing and channel
03Close the circuit's transfer disconnectInterlocked: requires the transfer done and no other circuit on transfer
04Open the circuit breakerThe load moves to the transfer breaker
05Isolate the circuit breakerBus and line disconnects open

The return follows the reverse order: the protection is restored only after the transfer disconnect is open. The interlocks ensure that a circuit’s transfer disconnect only closes with the transfer breaker closed, with the protection already transferred and with no other circuit connected to the transfer bus, because the transfer breaker can only replace one circuit at a time.

What goes wrong in practice

Incomplete transfer: the trips were transferred but not the 50BF initiation or the reclosing, and the transfer breaker is left without backup or without reclosing.
Open CT because of a wrong switching operation or a switching block without prior short-circuiting.
Wrong settings group in the transfer relay: the circuit is protected with another circuit’s settings.
Channel not transferred: the line clears faults with zone 2 times for days.
Early restoration: the protection returns to the circuit’s own breaker before the transfer disconnect opens, and during that interval the circuit has no effective trip.
87B zones not updated when the circuit CTs are left outside the main bus zone.

How it is tested

At commissioning the full transfer of each circuit is run through, generally with secondary injection in each state: each protection must trip the right breaker, the 50BF, the reclosing and the 25 must follow the transfer breaker, no CT secondary may be left open during switching, the interlocks must reject out-of-order operations and the signals must reach the control centre. If there is a channel, it is tested end to end with the circuit on transfer.

We can help with your transfer bus

At Tiza Group we do the remote relay configuration of the settings groups and the transfer logic, and the secondary engineering of the interlocks and transfer signals, for medium, high and extra-high voltage substations and for industry.

References

IEC 61936-1:2021, Power installations exceeding 1 kV AC and 1.5 kV DC – Part 1: AC.
IEEE C37.234, Guide for Protective Relay Applications to Power System Buses.
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

What happens if the transfer disconnect is closed without transferring the protection?

The circuit is connected to the transfer bus and fed by the transfer breaker, but its protection still acts on its own breaker, which is already open or isolated. A fault on the circuit would only be cleared by the backup protection of the transfer breaker or, worse, by the remote protection. That is why the interlock requires the transfer to be made before the transfer disconnect closes.

Do the currents always have to be transferred as well as the trips?

No. If the circuit CTs are on the line side, beyond the tap to the transfer bus, the current still flows through them and transferring the trips is enough. If they are between the breaker and the tap, the current stops flowing through them and the relay must receive the current from the transfer breaker CT.

Can the transfer be avoided with a dedicated relay in the transfer bay?

Yes. It is a common alternative: the transfer breaker has its own relay with one settings group for each circuit it can replace. CT switching is eliminated, but the operator must activate the right group, and the circuit's teleprotection is lost for as long as the transfer lasts unless the channel is also transferred.