Sectionalized Single Bus: Operation and Protection

What the sectionalized single bus configuration is

It is a single bus split into two sections (A and B) joined by a bus-section breaker, with a disconnector on each side to isolate it. Each circuit still has only one breaker and can connect only to its section.

In our example, line 1 and transformer 1 go to section A, and line 2 and transformer 2 to section B. Distributing sources and loads this way is the key: each section can keep operating alone if the other goes out.

Each bay has a bus disconnector, a breaker, current transformers (CT) and, on the lines, a line disconnector with an earthing blade and a voltage transformer (VT). The bus-section breaker has its own CTs, and their arrangement defines how the protection behaves at the boundary between sections. Each section needs its own VT if you want to measure voltage, check synchronism or supervise the protection with each section separate.

How it operates in normal service

There are two ways to operate it:

• Bus section closed: the two sections work in parallel and the transformers share load. The short-circuit level is higher.
• Bus section open: each section is independent and the short-circuit level drops. If the source of a section is lost, the bus-section breaker can be closed, manually or with an automatic transfer scheme, to supply it from the other.

If the two sections can end up fed from different sources, closing the bus section must be supervised with synchronism check 25.

What happens on faults and maintenance

Bus fault. The 87B zone of the faulted section trips the breakers of that section and the bus-section breaker. The other section stays in service. With sources and loads evenly distributed, on average half the substation is interrupted.

Fault between the bus-section CTs. With CTs on both sides of the breaker and overlapping zones, the fault lies inside both zones: both sections trip and everything is lost, although the fault is cleared quickly. With non-overlapping zones, a numerical relay can open the bus section first, remove its currents from the zones and trip only the section that remains unbalanced. With a single CT, a fault between the breaker and the CT is cleared late, by the bus-section breaker failure protection, and the whole substation is also lost.

Line fault. Only the line breaker trips.

Breaker failure (50BF). If a line breaker fails, the 50BF trips the other breakers of its section and the bus-section breaker; the other section continues. If the breaker does not open for a fault in its section, a transfer trip is sent to the remote end. If the bus-section breaker itself fails on a fault in section A, the 50BF must also trip section B: everything is lost.

Maintenance. Maintenance of a circuit breaker takes that circuit out, as in the single bus. Maintenance of a section takes out only its circuits. Maintenance of the bus-section breaker forces the sections to be operated separately.

Comparison with neighboring configurations

• Versus single bus: adds a breaker, two disconnectors and CTs; cost and area grow little and a bus fault no longer takes out the whole substation.
• Versus main and transfer bus: the sectionalized scheme does not solve breaker maintenance; the transfer scheme does, but it has a single main bus.
• Versus double bus: in a sectionalized bus each circuit is fixed to its section. The double bus allows moving circuits from one bus to another, at the cost of disconnectors, disconnector replica and more switching.

Implications for protection and control

• Two static 87B zones. Each circuit always belongs to the same section, so disconnector replica is not needed. A high-impedance scheme works with suitable CTs; a low-impedance one lets the bus-section CTs be assigned by setting. This is how the 87B bus differential treats it.
• Bus-section CTs. One CT on each side of the breaker is advisable to overlap zones. With a single CT you have to accept the blind zone or cover it with dead-zone logic that uses the breaker status.
• Selective 50BF. The trip matrix is no longer single: the bus-section breaker trips both sections, and each circuit only its own.
• Reclosing. Only on overhead lines, and blocked by the 87B or 50BF trip through the 86 lockout relay.
• 89 interlocks. The bus-section disconnectors operate only with their breaker open; earthing a section requires all its bus disconnectors and those of the bus section on that side to be open. The logic of each device is in interlocks in a sectionalized single bus, and the sequence can be practiced in the switching simulator (available in Spanish).

Where it is used

It is common in medium-voltage distribution substations with two transformers, in industrial plants with critical loads split over two half-buses and in high-voltage substations with few circuits.

Tests at commissioning

• Zone boundary: injecting through the bus-section CTs, verify that each 87B zone sees the current with the correct polarity and remains stable.
• Stability under load: with the bus section closed and load flowing, the differential current of the two zones and of the check zone, if there is one, must be practically zero.
• 50BF matrix: failure of the bus section trips both sections; failure of a circuit, only its section.
• Interlocks and automatic transfer, if present, with secondary injection and real switching.

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 for medium-, high- and extra-high-voltage substations, in utilities and in industry. Studies are done remotely, worldwide. If you are going to sectionalize a bus or review the logic of your bus section, write to us.

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.
• 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