Differential protection 87 (PDIF): principle and types

What 87 differential protection covers

Differential protection covers a zone: the part of the network enclosed by the current transformers (CTs) around it. It can be a transformer, a bus, a line, a generator, a motor or a reactor. Inside that zone it detects phase-to-phase faults and, depending on the variant, ground faults, and clears them with no intentional delay.

It prevents the damage a sustained internal fault causes: burnt windings, damaged cores, melted bus bars and, in generators, damage to the stator core that forces a rewind. Since it responds only to internal faults, it also prevents healthy circuits from being lost through the operation of slower backup protection.

• ANSI/IEEE C37.2: 87, differential protective relay; by custom the letter of the equipment is added.
• IEC 61850: logical node PDIF.
• IEC 60617: the relay symbol with differential current; its exact form depends on the drafter.

Where it is applied

Each piece of equipment has its own 87 and its own particular problem:

At low voltage the differential is hardly used: faults are cleared by fuses and trip units. At medium voltage the decision depends on the size and criticality of the equipment.

Measuring principle

The basis is Kirchhoff’s current law applied to the zone. The relay receives the current at each boundary and calculates:

• Differential current (or operate current): the phasor sum of all currents with the polarity referred to the zone, Id = |I1 + I2 + … + In|. With no internal fault, it is almost zero.
• Restraint current (or bias current): a measure of the current flowing through the zone. Each manufacturer defines it in its own way: average, sum or maximum of the magnitudes.

There are four ways to build the 87:

1. Low-impedance percentage differential. The numerical relay receives each CT separately and compares Id against a characteristic that rises with the restraint. It is the most common.
2. High impedance. The CTs are paralleled into a voltage relay with a stabilizing resistor. Stability under saturation comes from the circuit, not from the algorithm. It is the classic form of restricted earth fault 87N and of many bus differentials.
3. Over a channel (87L). The ends are far apart, so each relay sends its currents to the other over fiber or a communications network and both calculate the same differential.
4. Restricted earth fault. Compares the residual current of the phases with the neutral current of a winding; it sees faults the 87T cannot reach.

The characteristic

In the restraint current–differential current plane (Ires–Idif), the percentage characteristic separates the restraint region, below, from the operate region, above:

• Minimum pickup: the lowest differential current accepted as a fault. It covers magnetizing current and errors at low load.
• First slope: covers errors proportional to current (CT accuracy class, tap changer range).
• Second slope: steeper, from a breakpoint onwards. It gives stability on severe external faults in which a CT may saturate and produce a false differential.
• Unrestrained stage: above a high level of Id, it trips without looking at restraint or blocking.

An external fault stays below the curve even if the through current is huge. An internal fault drives the differential upwards into the operate region. That is the logic the diagram of this guide draws.

Setting criteria

There are no universal values. The general criteria are:

• Sensitivity: the minimum pickup must be below the minimum internal fault to be detected, with margin.
• Security: the slopes must absorb the errors of the CTs and of the protected equipment, and stability is checked with the maximum external fault.
• CT performance: ratio, class and burden must ensure the CTs do not saturate unevenly before the relay operating time, as described in IEEE C37.110. You can check it with our free CT saturation tool.

The calculation for each piece of equipment is in its own guide: 87T, 87B, 87L, 87G and 87M.

Interaction with other functions

• Lockout relay 86: transformer, bus and generator differentials usually trip through a lockout relay 86, which prevents reclosing until someone has checked the equipment.
• Breaker failure 50BF: the 87 trip starts the 50BF of each breaker in the zone.
• Reclosing 79: blocked by transformer, bus or cable 87; on overhead lines with 87L reclosing is allowed.
• CT supervision: an open CT circuit produces differential current under load. Many relays block or alarm the 87 through CT supervision.
• Backup protection: overcurrent or distance keep covering the zone if the 87 is out of service.

How each manufacturer implements it

• Siemens SIPROTEC (DIGSI): organizes the differential by protected object (transformer, bus, line, generator) and measuring sides.
• SEL (AcSELerator QuickSet): restrained differential elements with harmonic blocking, with matrix compensation for transformers.
• Hitachi Energy and ABB Relion (PCM600): differential blocks per application in the 670, 650, 615 and 620 series.
• GE Vernova Multilin (EnerVista): defines the current sources before enabling the element.
• Schneider MiCOM and Easergy: relay families per application, each with its own characteristic.

What changes is the definition of the restraint, the shape of the slopes and the handling of saturation.

IEC 61850 logical node

The function is modeled with PDIF, which publishes the start (Str) and the operate signal (Op). The trip goes through PTRC, which groups it with the other functions and takes it to the physical output or to a GOOSE message towards the XCBR of each breaker in the zone. In a distributed bus differential, the bay units exchange currents and trips over the substation network. The chain is explained in IEC 61850 GOOSE.

How it is tested

In general, secondary injection verifies:

• Compensation: through currents at two boundaries that the relay must see with almost zero differential.
• The minimum pickup and the points of the characteristic, on several slopes and phases.
• The blocking specific to the equipment: harmonics in transformers, saturation in buses, channel in lines.
• Stability on an external fault and the unrestrained stage.
• Operating times and the actual trip to every breaker.

At commissioning it is completed by measuring the differential current under load, which must be small. What the procedure must cover is in the relay test plan.

Typical mistakes

• Reversed CT polarity: an external fault or the load appears as internal. It shows up under load, if it is measured.
• Wrong CT ratio entered in the relay, or wrong vector group in the 87T.
• Badly defined zone: a misplaced CT leaves a blind zone between the CT and the breaker.
• CT with insufficient class or burden for the maximum external fault.
• Test switch not properly closed after maintenance: it leaves a CT open or shorted.
• Testing only the curve points and not the stability on an external fault.

Did your differential relay trip with no apparent fault, or do you need to review its settings? At Tiza Group we review the events and settings remotely, worldwide. See our remote relay configuration service or contact us.

Related guides

• Transformer differential 87T
• Bus differential 87B
• Line differential 87L
• Generator differential 87G
• Motor differential 87M
• Restricted earth fault 87N
• High-impedance differential: what it is and how it works

References

• IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations: number 87.
• IEC 60255-187-1, Functional requirements for differential protection: requirements for motor, generator and transformer differentials.
• IEEE C37.91-2021, Guide for Protecting Power Transformers: application to the transformer.
• IEEE C37.234-2021, Guide for Protective Relay Applications to Power System Buses: bus differential.
• IEEE C37.243-2015, Guide for Application of Digital Line Current Differential Relays Using Digital Communication: line differential.
• IEEE C37.110-2023, Guide for the Application of Current Transformers Used for Protective Relaying Purposes: CT performance.
• IEC 61850-7-4, Compatible logical node classes and data object classes: PDIF and PTRC nodes.

Frequently asked questions