Busbar Protection Schemes: High and Low Impedance

What a busbar protection scheme is

A bus fault affects every connected bay and can carry the highest fault current in the substation. If it is cleared slowly, by the remote backup of the lines, the damage and the disturbance to the system are severe. If the protection trips falsely, it takes the whole bus out of service. The scheme has to be both very fast and very secure.

The core function is the bus differential 87B. This guide, part of the substation protection schemes, covers how the scheme is built around it: zones, disconnector replica, check zone and breaker failure.

The protection functions and how they are coordinated

• Bus differential per zone (one per bus section).
• Check zone in configurations with bus selector disconnectors.
• CT circuit supervision, which blocks or alarms on an open CT.
• Breaker failure 50BF of each bay, which trips the zone to which the failed breaker is connected. In low-impedance schemes 50BF is usually integrated in the same relay, because it already knows the disconnector replica.
• Dead-zone protection for faults between the breaker and its CT. See dead zone and stub protection.
• Reclose blocking of the lines tripped by the bus protection.
• At medium voltage, simpler alternatives such as reverse interlocking or arc flash detection.

High and low impedance

High impedance. The CTs of all bays are connected in parallel across a voltage-operated relay with a series stabilising resistor and a voltage limiter (varistor). On an external fault with a saturated CT, current takes the low-impedance path through the saturated CT and the voltage across the relay stays below the setting. On an internal fault, all CTs drive current into the high impedance and the voltage rises. It is simple and very stable, but it needs dedicated CTs of equal ratio with sufficient knee-point voltage, and it does not adapt well to buses with selector disconnectors.

Low impedance. A numerical relay receives the current of each bay, compensates different ratios, applies a percentage restraint characteristic and detects saturation. It builds the zones from the dynamic replica of the disconnectors. It can be centralised (all CT wiring goes to one panel) or distributed (bay units connected by fibre to a central unit).

Logic and sequence

For a fault on bus 1 of a double busbar:

1. Each bay measures its current; the replica shows which bays are on bus 1.
2. The bus 1 zone detects differential current; so does the check zone.
3. The relay trips only the breakers connected to bus 1, including the bus coupler.
4. Breaker failure 50BF is initiated for each tripped breaker.
5. Bus 2 stays in service with its bays.

If the coupler fails to open, its 50BF also trips bus 2. If a bay is on transfer or has both bus disconnectors closed, the replica merges the zones.

Variants by voltage level

• Extra-high voltage: distributed or centralised low-impedance schemes, sometimes duplicated, with check zone and integrated 50BF. Breaker-and-a-half configurations, where each bus is a single zone.
• High voltage: double busbar with coupler and disconnector replica, high or low impedance depending on the owner.
• Medium voltage: high-impedance differential in important switchboards, reverse interlocking between overcurrent relays, and arc flash detection.
• Industry: the same options in the plant’s main switchboards.

Design criteria

• CTs: ratio, class and knee-point voltage suited to the chosen scheme; in high impedance, dedicated CTs of equal ratio.
• Zone overlap with the lines and transformers around each breaker.
• Disconnector replica: reliable auxiliary contacts, logic for intermediate states and a discrepancy alarm.
• Tripping: direct or through lockout relays, with reclose blocking.
• Settings from the short-circuit study and the protection coordination study, with no universal values.

How each manufacturer implements it

The best-known low-impedance bus protections are Siemens SIPROTEC 5 7SS85 (DIGSI 5), SEL-487B (AcSELerator QuickSet), Hitachi Energy REB670 (PCM600), GE Vernova Multilin B90 (EnerVista) and MiCOM P746, sold today by GE Vernova (MiCOM Agile P746) and Schneider Electric (Easergy MiCOM P746). The 7SS85 supports centralised, distributed or hybrid architecture; the REB670, B90 and P746 are centralised, and Hitachi Energy offers the REB500 as a distributed system with bay units. What changes is how the topology is modelled, how couplers and transfers are handled, and how 50BF is integrated. A high-impedance differential, on the other hand, can use an overcurrent or voltage relay of any brand with its stabilising resistor.

Why it fails or misoperates

• Wrong disconnector replica: an auxiliary contact that did not change over, reversed wiring or a switching operation stopped halfway.
• CT open or shorted during maintenance without blocking the zone.
• CT saturation on a close-in external fault, with a poorly set characteristic.
• New bay not integrated into the scheme, or with reversed polarity.
• 50BF test without isolation that trips the whole bus.

The first step is to check the per-bay currents in the event record and the replica status at the instant of the trip.

How it is tested

Each zone is tested by secondary injection: restraint characteristic, stability on external faults, check zone, CT supervision and 50BF. The scheme is checked by operating disconnectors and verifying that the replica builds the right zones, and with functional trips to each breaker. At commissioning, an on-load check confirms that the sum of currents in each zone is practically zero.

Do you need to integrate a new bay into the bus differential or review a trip? At Tiza Group we configure and review busbar protection remotely. See our remote relay configuration service or contact us.

References

• IEEE C37.234-2021, Guide for Protective Relay Applications to Power System Buses.
• IEEE C37.119-2025, Guide for Power System Circuit Breaker Failure Protection.
• IEEE C37.110-2023, Guide for the Application of Current Transformers Used for Protective Relaying Purposes.
• J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions