Bypass and transfer disconnector interlocking

Why they exist

In a substation with one breaker per circuit, maintaining that breaker means taking the circuit out of service. Arrangements with a transfer path avoid it by adding an alternative route:

• Main and transfer bus. Each bay has a transfer disconnector (89T) toward an auxiliary bus, and a transfer breaker ties that bus to the main bus.
• Double bus with transfer. A transfer bus is added to the double bus, with its own breaker or with a bus coupler performing both functions.
• Double bus with bypass. There is no transfer bus: a bypass disconnector bridges the bay breaker and one of the buses acts as the transfer bus, with the coupler as the substitute breaker.

In every case, during maintenance the circuit is protected and switched by another breaker. The interlocking must make sure that change happens without any disconnector interrupting or making load, and that only one circuit is substituted.

The sequence the interlocking protects

There are two practices for moving a circuit to the transfer bus. Both keep the disconnector free of any significant change in voltage across its terminals, as the International Electrotechnical Vocabulary definition of a disconnector requires (IEV 441-14-05):

The variant closes the transfer disconnector with the transfer breaker already closed. Since the bay breaker is also closed, both terminals of the disconnector are at the same potential. IEC 62271-102 recognizes this switching between parallel paths, for example when bypassing a breaker, and the capability of the disconnector depends on the current being commutated and on the size of the loop.

Usual conditions

Bay transfer or bypass disconnector:

1. Transfer breaker (or coupler acting as transfer breaker) open, or that breaker and the bay breaker closed, with their disconnectors closed.
2. No other transfer or bypass disconnector closed in another bay.
3. Transfer bus earthing switches open.
4. Valid position of every device involved.

Transfer breaker:

1. Its disconnectors both open or both closed, as with any coupler.
2. While a circuit is transferred and its own breaker is open, blocked against opening by command, except for a protection trip. It is now the circuit’s only breaker.

Bay breaker: it should not open by command before the circuit is in parallel with the transfer breaker, if the aim is not to interrupt service. That is an operating condition more than a safety one, and many designs leave it to procedure.

Condition 2 of the disconnector is the one most often forgotten. If two bays close their 89T, the transfer bus ties them together. A fault on either opens the transfer breaker and leaves both out of service, and in some states it can form a loop between circuits through the auxiliary bus.

Transferring the protection

The transfer breaker needs a protection that suits the circuit it replaces. There are two solutions:

• Trip transfer. The bay protection keeps measuring, with its current transformers if they remain in the path, and its trip is switched to the transfer breaker. A selector switch or the logic does the switching, following the 89T position.
• Own protection with a settings change. The transfer breaker has its own relay, and a settings group suited to the transferred circuit is activated.

The choice depends on where the current transformers are. If they are on the bay breaker side, which is under maintenance, the bay protection loses its measurement and only the second solution works. The transfer also affects the bus differential protection 87B, which must know which current belongs to the transfer bus and which zone to assign it to, and the 50BF, whose trip destination changes. Both solutions are covered in the guide on transferring protection to the transfer breaker.

Double bus with bypass and a combined coupler

When the coupler also serves as the transfer breaker, the logic must tell the two modes apart. As a coupler, it ties buses and is blocked against opening during on-load transfers between buses. As a transfer breaker, it replaces a circuit and is blocked against opening while that circuit depends on it. The safe approach is to determine the mode from the topology, that is from the disconnector positions, and not from a manual selector that can be left in the wrong position. While a circuit is bypassed, the substation loses the ability to transfer other bays between buses on load.

Implementation and testing

Condition 2 of the disconnector and the protection transfer are station-wide: they depend on every bay. In numerical substations each bay controller publishes the position of its 89T by GOOSE, and the IEC 61850-7-4 CILO logical node of each disconnector evaluates it. The test takes each bay into transfer, tries to transfer a second one, checks the trip transfer by injection and verifies that the transfer breaker does not accept an open command.

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Our team configures the transfer logic, the trip transfer and the settings groups in bay controllers and protective relays of the main brands, in high and extra-high voltage substations and in industry. See remote relay configuration and secondary engineering, or contact us.

Related guides

• Lockout relay 86
• Transferring protection to the transfer breaker
• Double bus interlocking: coupler and transfer
• Main and transfer bus interlocking
• Double bus with transfer and bypass interlocking

References

• IEC 60050-441, International Electrotechnical Vocabulary, 441-14-05 (disconnector).
• IEC 62271-102:2018, Alternating current disconnectors and earthing switches, annex B (bus-transfer current switching).
• IEC 61850-7-4, logical nodes CILO, CSWI and XSWI.
• IEC 61936-1:2021, Power installations exceeding 1 kV AC, 6.2.16.
• IEEE C37.234, Guide for Protective Relay Applications to Power System Buses.

Frequently asked questions