Main and Transfer Bus Configuration: How It Operates

What the main and transfer bus configuration is

It is a single bus to which a transfer bus and a transfer breaker joining both buses are added. In our example, two lines and two transformers connect to the main bus, each with its own breaker.

Each bay has a main bus disconnector, a breaker, current transformers (CT), an outgoing disconnector and a transfer disconnector that connects the circuit output, on the outer side of its breaker, to the transfer bus. On the lines, the voltage transformer (VT) is on the line side. The transfer bay has its breaker, its disconnectors to each bus and its own CTs.

The position of the circuit CTs relative to the point where the transfer is connected is decisive:

• Inner CT (between the breaker and the tap to the transfer bus): with the circuit on transfer, the current no longer flows through it.
• Outer CT (beyond the tap, on the line side): the circuit current keeps flowing through it during the transfer.

How it operates in normal service

All circuits are on the main bus with their breakers closed. The transfer bus stays de-energized and the transfer disconnectors open. It works the same as a single bus.

To take the breaker of a line out for maintenance, the general sequence is:

Each utility defines its own sequence; the essential point is that a disconnector is never left to open or close load.

What happens on faults and maintenance

Fault on the main bus. The 87B trips all breakers connected to it, including the transfer breaker: the whole substation is lost. Against bus faults, this configuration is as reliable as the single bus.

Fault on a line. The breaker of that line trips, or the transfer breaker if the line is on transfer.

Breaker failure (50BF). As in the single bus, the 50BF trips all main bus breakers and sends a transfer trip to the remote end when needed. If it is the transfer breaker that fails while replacing a line, the response is the same: the main bus and the remote end are tripped.

Breaker maintenance. It is the reason for this configuration: the circuit stays in service through the transfer breaker. Only one circuit at a time. Maintenance of the main bus, on the other hand, takes out the whole substation.

Comparison with neighboring configurations

• Versus single bus: adds a bus, a breaker and a disconnector per circuit; it solves breaker maintenance, but not the bus fault.
• Versus sectionalized single bus: the sectionalized scheme limits the loss from a bus fault; the transfer scheme limits the unavailability from maintenance. They can be combined.
• Versus double bus: in a double bus both buses carry load and circuits can be distributed between them; the transfer bus works only during maintenance. The double bus with transfer combines both advantages at the cost of more equipment and area.

Implications for protection and control

• 87B of the main bus. The zone is bounded by the circuit CTs and by the CT of the transfer breaker. With inner CTs it stays balanced when a line goes on transfer, because the current leaves through the transfer CT. With outer CTs, the circuit current passes through two CTs and the assignment must change: the transfer bus can form its own zone with the circuit CT and the transfer breaker CT.
• Moving the circuit protection. With inner CTs, the line relay must receive the current from the transfer breaker CT and trip that breaker. This is done with auxiliary relays or switching blocks, with the risk of opening a CT secondary if operated wrongly. With outer CTs it is enough to move the trip. The alternative is a dedicated relay in the transfer bay with one settings group per circuit.
• Teleprotection and 87L. If the line uses line differential or teleprotection, the channel must be moved as well; otherwise the line is left with only stepped distance protection during the transfer.
• Reclosing and synchronism. Reclosing and synchronism check 25 must act on the transfer breaker while it replaces the line breaker.
• 89 interlocks. Only one transfer disconnector closed at a time; the transfer disconnector of a circuit closes only with the transfer breaker closed and the protection already moved. The logic of each device is in interlocks in main and transfer bus, and the sequence can be practiced in the switching simulator (available in Spanish).

The bus transfer current that the disconnector must switch is explained in bus transfer current.

Where it is used

It is found in distribution and subtransmission substations where breaker maintenance must not interrupt customers, but the cost of a double bus is not justified.

Tests at commissioning

• Full transfer sequence, circuit by circuit, verifying interlocks, signals to the control center and the transfer of trips, currents, reclosing and channels.
• 87B stability in normal service and with a circuit on transfer, with primary injection or with load.
• End-to-end tests of each line also in transfer mode. See end-to-end tests.
• CT switching: check that no secondary is left open during the transfer.

Interlocking logic in gates

These are the conditions that must be met to enable operation of the key devices of this configuration, drawn with IEC 60617-12 logic gates (AND ”&”, OR “≥1”). An intermediate position or an invalid quality signal counts as “not met”. The interlocking table generates the diagram for each device and command, and in the switching simulator you can test each condition.

We can help with your substation

At Tiza Group we carry out protection coordination studies, including settings groups and transfer logic, for medium-, high- and extra-high-voltage substations, in utilities and in industry. Studies are done remotely, worldwide.

Related guides

• Moving protection to the transfer breaker
• Bus transfer current of the disconnector

References

• IEEE C37.234: guide for protective relay applications to power system buses.
• IEC 61850-7-4: compatible logical nodes (XCBR, XSWI, CILO).
• C. Martin et al., Bus Protection Considerations for Various Bus Types, Western Protective Relay Conference, 2013.
• 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.
• J. Gastón Ortega and E. Nelson, Preserve High-Speed Protection During Bypass Operations, Western Protective Relay Conference, 2006.
• J. D. McDonald (ed.), Electric Power Substations Engineering, CRC Press, chapter on bus arrangements in AIS substations.
• CIGRE, Technical Brochure 585, Circuit Configuration Optimization.

Frequently asked questions