Double Bus with Transfer and Bypass Interlocking

Short answer

In a double bus with bypass, the bus coupler plays two roles the logic must not mix: it ties the buses to transfer circuits and it replaces the breaker of a circuit under maintenance. A circuit's bypass disconnector closes only with the coupler closed, the protection transferred, no other bypass closed and the circuit alone on the bus that will act as transfer bus; while it lasts, no other circuit may select that bus.

Double bus with transfer interlocking has to settle a conflict of roles. In the double bus with transfer arrangement the bus coupler ties the two buses so circuits can change bus on load, and it also replaces a circuit’s breaker during maintenance. While it does the second job it cannot do the first, and the logic has to stop anyone from forgetting it.

This guide focuses on the variant with a bypass disconnector (89BP): each bay has its bus selectors 89B1 and 89B2, a disconnector on each side of the circuit breaker (89-1 and the outgoing one) and an 89BP that connects the selection node to the outgoing side, in parallel with the whole breaker. The double bus logic still applies; this is what is added.

Conditions per device

Summary of typical logic for this arrangement.

Device	Opens if	Closes if
Bus selector 89B1 (or 89B2)	off load: the 52 and the bypass open and the other selector open; on load: coupler closed and the other selector closed	the same, plus the earthing switch of that bus open, and no other circuit on bypass using that bus
Breaker disconnector 89-1	the 52 is open	the 52 is open and earthing switch 57-1 open
Bypass 89BP	the 52 and its two disconnectors closed, or the coupler open	coupler closed, no other 89BP closed, breaker earthing switches open, protection transferred and the circuit on one bus only
Bus coupler 52A	no circuit has both selectors closed	89A1 and 89A2 both open or both closed; bus earthing switches open
Line earthing switch 57L	free	the 89L and the 89BP open

Two conditions deserve attention:

«The circuit on one bus only». To close the bypass, the circuit has one selector closed, the other open, and no other circuit connected to that bus. If another circuit stayed on it, a fault on the bypassed circuit would trip the coupler, but the other circuit would keep feeding the fault through the bus.
The 89-1 is not the bypassing device. With the bypass closed, the breaker and its disconnectors carry part of the current. That is why the 89-1 and the outgoing disconnector still require the 52 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.

Priorities between coupling and transfer

While a circuit is on bypass, the substation works as a single busbar with that circuit hanging from the coupler. This has three consequences for the station-level logic:

No other circuit can move to the transfer bus. The selectors of the other circuits towards that bus are blocked.
The coupler must not open by command while the bypass is closed: opening it takes the circuit out of service. The project can block it in the logic or flag it as an alarm on the HMI. Protection can still open it: it is the circuit’s protection.
The coupler 50BF now clears the whole other bus and, if needed, the remote end. That is why the bypass should last as little as possible.

Critical switching sequence: bypassing a circuit breaker

Maintaining a circuit breaker with bypass

01Clear bus 1The other circuits move on load to bus 2, with the coupler closed
02Transfer the protectionThe circuit's trip, currents and 50BF initiation move to the coupler
03Close the bypassIn parallel with the circuit breaker: no significant current is made
04Open the circuit breakerCurrent flows through bus 2, coupler, bus 1 and bypass
05Isolate the circuit breakerOpen and lock out the 89-1 and the outgoing disconnector, verify and earth

The return requires closing the circuit’s own path first, then opening the bypass, returning the protection and moving the other circuits back to their bus.

Bay level and station level

In this arrangement the station-level logic outweighs the bay-level logic. A circuit’s bypass needs the position of the selectors and bypasses of every other circuit, and the coupler state. Each circuit’s selectors need every bypass. Each added bay multiplies the cross conditions. Hardwired, this becomes hard to maintain; with GOOSE, each bay controller publishes its selectors, its bypass and its transfer state, and the logic grows by adding subscriptions.

If a message is lost, the safe choice is to treat any device whose state is unknown as «not open», which blocks the bypass and the transfers.

Typical mistakes

Bypass without the cleared-bus condition. It closes with another circuit still on the transfer bus.
Bus differential protection unaware of the bypass. The 87B neither inhibits the zone nor moves the coupler CT to the check zone, and it trips during the operation. See 87B tripping during disconnector switching.
Line earthing switch with the bypass closed. The 57L is conditioned only on the 89L and closes with the line still live through the bypass.
Reclosing and sync check on the wrong breaker. While the bypass lasts they must act on the coupler.
Early return of the protection with the bypass still closed.

How it is tested

During commissioning the bypass sequence is run with each circuit and on both buses, attempting each forbidden operation: bypass with another circuit on the bus, a second bypass, opening the coupler during a bypass, another circuit’s selector towards the occupied bus. The test also checks that the transferred protection trips the coupler and initiates its 50BF.

We can help with your interlocking logic

At Tiza Group we design transfer schemes as part of secondary engineering: the interlocking table, station GOOSE and protection transfer, and we program them in the bay controllers and relays through remote configuration, for 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, CSWI and XSWI.
IEC 62271-102:2018: disconnectors; 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 bypass disconnector close if the bus coupler is closed?

Check that the circuit is on one bus only, with its other selector open and no other circuit connected to that bus; that no other bypass is closed; that its earthing switches are open; and that the protection shows as transferred to the coupler. If a single one is missing, the interlocking blocks it.

Why can't the bus coupler be opened while a circuit is on bypass?

Because at that moment the coupler is the circuit breaker of that circuit: opening it takes the circuit out of service. Outside a bypass, the coupler does not open either while a circuit has both bus selectors closed, because the selectors would have to carry all the current between buses.

How does the logic differ between the transfer bus variant and the bypass variant?

With a transfer bus, each circuit's transfer disconnector is interlocked with the coupler connected to that bus and with the other transfer disconnectors. With bypass there is no third bus: one main bus is cleared and acts as transfer bus, and the interlocking must also prevent any other circuit from selecting it.