Bus Differential Protection 87B (PDIF): How It Works

What 87B bus differential protection is

The bus is the point where every bay of a substation comes together. Bus differential protection applies Kirchhoff’s current law: the currents entering and leaving a node add up to zero. If every bay connected to the bus is measured with CTs and the sum is not zero, the missing current is flowing into a fault on the bus, inside the zone bounded by those CTs.

It is identified as follows:

• ANSI/IEEE C37.2: device 87, with the suffix B by custom (87B).
• IEC 61850: logical node PDIF; some manufacturers model the zones and the disconnector replica with additional nodes.

A bus fault carries very high currents and affects everything connected to it. Fast clearing is the goal, but above all secure clearing: a false 87B trip takes the entire substation out of service. Zones and their overlap are explained in protection zones.

Where it is applied

• Transmission substations at high and extra-high voltage, with single bus, double bus, main and transfer bus, breaker-and-a-half or ring bus arrangements.
• Generating stations and solar plants connected to high-voltage buses.
• Medium-voltage switchgear in distribution and industrial substations, where simpler schemes are also used (for example, blocking logic between the feeder relays and the incomer relay).
• GIS substations, where a bus fault has severe consequences for the equipment.

How it works

There are two main families:

High impedance. The secondaries of all the CTs are connected in parallel across a measuring element with a stabilizing resistor in series. On an external fault, even if one CT saturates, the current circulates between the CTs and the voltage across the relay stays limited. On an internal fault the current is forced into the relay and a high voltage appears, which makes it operate. A voltage-limiting device (varistor) protects the circuit from overvoltages. The classic stability criterion compares the voltage setting with the voltage the maximum external fault would produce with one CT fully saturated, which depends on the CT winding resistance and the lead resistance. This scheme needs dedicated CTs of equal ratio and suitable characteristics, and it does not adapt well to topology changes.

Low impedance (numerical). Each CT is wired to the relay (centralized, or distributed in bay units). The relay compensates the ratio of each bay and calculates:

• A differential current per zone, as the phasor sum of the currents of the bays connected to that zone.
• A restraint current, from the magnitudes of the bay currents.

It trips when the differential current exceeds a percentage characteristic. It also includes:

• Disconnector replica: the relay reads the disconnector auxiliary contacts to know which bay is connected to which bus and assigns the currents to each zone dynamically.
• Check zone: an overall zone that does not depend on the disconnectors; tripping of a selective zone is supervised by operation of the check zone.
• CT saturation detection: algorithms that recognize an external fault in the first milliseconds, before the CT saturates.
• CT circuit supervision: blocking on a small, sustained differential current.

The same platform often includes breaker failure 50BF, which trips the bus when a breaker fails to open, and the dead-zone (end-zone) protection between the CT and the breaker.

Setting criteria

• Substation arrangement: number of bays, zones, bus couplers and their CT locations, which define the zone overlap.
• Ratios and performance of the CTs of every bay, and the maximum external fault, to check stability.
• Minimum bus fault, with minimum generation, to check sensitivity: the pickup must sit clearly below it. Some philosophies also place it above the load of the most heavily loaded bay, so that an open CT does not trip the bus.
• High impedance: voltage setting from the CT and lead resistances, CT knee-point voltage and the resulting primary operating current.
• Trip and replica logic consistent with the substation control scheme.

The values come from the short-circuit study and the protection coordination study, together with the relay manufacturer’s recommendations.

How each manufacturer implements it

• Siemens SIPROTEC (DIGSI) models the substation with its graphical topology; the disconnector replica is shown on that diagram.
• SEL (AcSELerator QuickSet) organizes zones and terminal assignment through logic equations and zone settings.
• Hitachi Energy REB670 (PCM600) works with zone and bay blocks in the application configuration.
• GE Vernova (Multilin, EnerVista) defines the sources and the dynamic bus configuration through settings and FlexLogic.
• Schneider MiCOM/Easergy offers centralized and distributed schemes with bay units.

The main differences are the architecture (a single device, one device per phase, or bay units with communication), the way the topology is described and the saturation detection algorithms. Moving a scheme from one brand to another means rebuilding the logic, not just copying values.

Why it trips or blocks

• False trip on an external fault: undetected CT saturation, a CT with reversed polarity, a bay assigned to the wrong zone or a wrong CT ratio entered.
• Disconnector replica error: a damaged auxiliary contact, crossed wiring or an intermediate position misread. The bay ends up in the wrong zone and a differential current appears with load.
• Block by CT supervision: an open CT, a test block not properly closed or a new bay not yet wired to the relay.
• Block by disconnector discrepancy or by loss of communication with a bay unit in distributed systems.
• 50BF trip of a bay, which arrives as a bus trip.

Start with the event report (which zone, which phase, which bays) and the differential current measured with load, which should be close to zero in normal service.

How it is tested

With secondary injection and several synchronized current sources, such as the Omicron CMC 356 (or several test sets for multiple bays), the usual checks are:

• Stability on an external fault, injecting current that enters through one bay and leaves through another.
• Pickup and percentage characteristic per zone and per phase.
• The replica: when the simulated disconnector position changes, each bay must move to the correct zone.
• The check zone, CT supervision, saturation detection and integration with 50BF.
• The actual trip to every breaker in the zone, ideally with the substation de-energized or with controlled test blocks.

During commissioning, the differential current of each zone is confirmed to be small under load, and polarities are checked bay by bay, as in the CT polarity test.

Did your relay trip or block on this function and the cause is not clear? At Tiza Group we review the events and settings remotely. See remote relay configuration or contact us.

References

• IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms and Contact Designations.
• IEEE C37.234, Guide for Protective Relay Applications to Power System Buses.
• IEC 61850-7-4, logical node models (PDIF).
• IEEE C37.110-2023, Guide for the Application of Current Transformers Used for Protective Relaying Purposes.
• IEC 61869-2, current transformers.
• J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions