Triple-Bus Substation: Operation, Faults and Protection

What a triple-bus configuration is

The International Electrotechnical Vocabulary (IEC 60050-605, term 605-01-18) defines the triple busbar substation as one in which lines and transformers are connected to three busbars by means of selectors. Each circuit has a breaker, its current transformers (CTs), an outgoing disconnector and three selector disconnectors, one toward each bus. The buses are tied together with couplers: one coupler per pair of buses, or a coupler with selectors that can join any pair.

It is the natural extension of the double bus: the operating and protection logic is the same, with one more bus. In the diagram of this series on substation configurations, Line 1 and Transformer 1 go to bus 1, Line 2 and Transformer 2 to bus 2, and bus 3 stays as the reserve.

How it operates in normal service

There are two ways to use it:

• Reserve bus. Two buses carry the circuits and the third stays available. The same standard (605-02-04) calls a reserve bus the one used only under abnormal conditions and notes that it is usually less equipped than the main buses.
• Three active nodes. The circuits are distributed over the three buses, with the couplers open or closed according to the load flow and short-circuit study, to separate sources or limit the fault current.

Transferring a circuit from one bus to another is done live, as in a double bus: with the coupler between the two buses closed, the selector of the new bus is closed and that of the old one is opened.

What happens during faults and maintenance

• Fault on a bus. The 87B trips the circuits connected to that bus and the couplers that join it to the others. The other two buses stay in service.
• Fault on a line. The line breaker and the one at the remote end trip.
• Breaker failure (50BF). The 50BF trips the bus to which the circuit is connected at that moment, and the remote end with transfer trip if it is a line. The failure of a coupler takes out the two buses it joins.
• Maintenance of a circuit breaker. The circuit goes out of service, as in a double bus, unless there is a bypass or transfer arrangement.
• Maintenance of a bus. Its circuits move to another bus and work proceeds without outages; the other two keep the flexibility of a double bus.

Reliability, flexibility, cost and footprint

A triple bus gives more operating flexibility than a double bus: one bus can be maintained while still having two, or the substation can be split into three nodes. It does not improve what happens on a fault or maintenance of a circuit breaker, which is the weakness of every one-breaker-per-circuit configuration.

Cost and footprint grow: one more selector per circuit, more couplers, one more bus and more interlocks. That is why it is uncommon. To limit what is lost on a bus fault, many utilities prefer the sectionalized double bus; to maintain breakers without outages, the breaker-and-a-half or the double bus with transfer.

Protection and control implications

• 87B with disconnector replica. Each bus is a zone; the protection decides which circuits belong to each zone by reading the auxiliary contacts of three selectors per circuit. With more selectors there are more combinations and more risk of a wrong replica. A check zone that does not depend on the replica adds security.
• Bus transfer. While a circuit has two selectors closed, the two buses are tied through it and the 87B must treat them as a single zone.
• Couplers. Each coupler has its CTs, its dead-zone logic if the CTs are on one side only, and its synchronism check 25 between the buses it joins.
• 50BF and voltages. The 50BF matrix and the selection of the bus voltage for each circuit follow the replica, as in a double bus.
• 89 interlocking. A selector closes only if the circuit is open or if the two buses involved are tied by their coupler; never two selectors closed toward separate buses. The logic lives in the bay controller. The logic of each device is in interlocking in sectionalized double bus and triple bus, and the sequence can be practiced in the switching simulator.

Typical use

It is found in some large high- and extra-high-voltage substations, where the owner wants more operating nodes or a reserve bus. Its use depends on each utility’s practice. The guide on the conventional AIS substation summarizes the criteria for choosing the arrangement.

Commissioning tests

• Ratio, polarity and excitation of the CTs of circuits and couplers.
• 87B with secondary injection for each zone, running through the valid positions of the disconnector replica, including the bus transfer.
• 50BF matrix with each circuit on each bus and transfer trip tested end to end.
• Voltage selection for the 25 and for the protections that use bus voltage.
• Complete interlocking in the yard, in the bay controller and from the control center.

Interlocking logic in gates

These are the conditions that must be met to enable operation of the key devices in 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 triple-bus substation

With three buses, the disconnector replica is the heart of the protection. At Tiza Group we carry out the protection coordination study for high- and extra-high-voltage substations, in utilities and in industry, remotely and worldwide.

References

• IEC 60050-605: International Electrotechnical Vocabulary, substations (605-01-18, triple busbar; 605-02-04, reserve busbar).
• IEEE C37.234: guide for protective relay applications to power system buses.
• IEEE C37.119: guide for breaker failure protection of power circuit breakers.
• CIGRE, Technical Brochure 161: General guidelines for the design of outdoor AC substations.

Frequently asked questions