Line Current Differential 87L (PDIF): How It Works

What 87L line differential protection is

Line differential protection applies Kirchhoff’s current law to a line: under normal conditions, or with a fault outside the line, the current entering one terminal is the same as the current leaving the other. If a difference appears, there is a current path inside the line, that is, an internal fault.

In IEEE C37.2 the function is device 87, and the suffix L indicates that it protects a line. In IEC 61850 it is modeled with logical node PDIF, the same as the other differentials. What sets it apart from the transformer differential 87T or the bus differential is distance: the current transformers (CTs) at each terminal are kilometers apart, so each relay measures its own currents and exchanges them with the other over a communications channel.

Its great advantage is absolute selectivity. The 87L needs no time coordination with neighboring protections and does not depend on the line impedance, so it trips without delay for faults anywhere on the line, including resistive faults, short lines and cables, where distance protection 21 has a harder time.

Where it is applied

• Transmission lines at high and extra-high voltage, as main protection, often with distance in the same relay or in a second relay.
• Short lines and underground or submarine cables, where the impedance is so low that a distance element has little margin.
• Three-terminal and multi-terminal lines, with tapped generation or load, as long as there is a CT and a relay at each terminal.
• Interconnection of generating plants and industrial substations to the grid, when the utility requires fast tripping at both ends.
• Medium-voltage lines in industrial or distribution networks where optical fiber is available.

How it works

Differential and restraint current

Each relay calculates, phase by phase, the differential current: the vector sum of the currents of all terminals. It also calculates a restraint current, which grows with the current flowing through the line. The relay operates when the differential current exceeds a threshold that increases with restraint. This tolerates CT and channel errors, which grow with the through current, without losing sensitivity for internal faults.

The characteristic usually has a minimum pickup and one or two slopes. Some manufacturers also use an alpha plane, which compares the ratio and angle between the remote and local currents; each brand defines restraint in its own way.

The communications channel

The channel is part of the protection. Typical options are direct optical fiber between relays, or a multiplexer over a transport network, with interfaces such as IEEE C37.94 or G.703. What matters about the channel is:

• Delay: how long the data takes to arrive; it adds to the trip time.
• Symmetry: whether the go and return paths take the same time.
• Availability and errors: the relay supervises the channel and blocks the 87L when the data is not reliable.

Time alignment

To compare currents, the samples from both terminals must correspond to the same instant. There are two main methods. The first measures the round-trip time of the message (the ping-pong method) and assumes the delay is equal in each direction. It works well over direct fiber, but in multiplexed networks the routes can change and become asymmetrical, and that difference turns into an angle error that shows up as false differential current. The second uses an external time reference, normally GPS, at each terminal, which removes the asymmetry problem at the cost of depending on time synchronization.

Charging current

A long line or a cable draws capacitive charging current even with no load. To the relay, that current enters one terminal and does not leave the other, so it looks like a small fault. Relays handle it by raising the minimum pickup or by compensating it with the measured voltage. On cables and long lines it is a central part of the setting.

Other design situations

• Transformer inside the zone: requires ratio and vector group compensation, and harmonic blocking during energization, as in the 87T.
• Series compensation and parallel lines: they do not affect the differential principle the way they affect distance, but they do affect the backup functions.
• Single-pole tripping: the 87L selects the faulted phase naturally, which makes single-pole reclosing easier.
• Transfer trip: the same channel usually carries direct transfer trip and teleprotection scheme signals.

Setting criteria

The settings come from the protection study and the installation data, not from universal values:

• CT ratio at each terminal, which may differ; the relay normalizes them.
• Minimum pickup, above the charging current and the expected errors.
• Slopes and breakpoint, according to the class and performance of the CTs on external faults with a DC component.
• Channel supervision: maximum delay, permissible asymmetry and what to do if the channel is lost (block the differential and enable backups).
• CT supervision, so that an open secondary circuit does not cause a trip.

IEEE C37.243 covers these application topics: principles, synchronization, channel and CT requirements, line charging current, multi-terminal lines and testing.

How each manufacturer implements it

Every brand applies the same principle, but the shape of the characteristic, the definition of restraint, the synchronization method and the setting names change. Siemens offers it in its SIPROTEC line relays, configured in DIGSI; SEL has dedicated line current differential relays set with AcSELerator QuickSet; Hitachi Energy (formerly ABB) includes it in the Relion 670 family with PCM600; GE Vernova includes it in its Multilin line relays with EnerVista. The relays at both terminals must be compatible with each other: in practice, the same manufacturer with consistent firmware and communication versions. That is why settings that work on one brand are not copied to another without checking its manual.

Why it trips or blocks

How it is tested

In concept, the 87L is tested at two levels. The local test checks the relay with secondary injection, with the channel looped back or the remote terminal simulated: pickup, points on the characteristic, times and logic. The end-to-end test injects at both terminals at the same time with GPS-synchronized test sets and simulates internal faults, which must trip both sides, and external faults, which must not trip. The channel behavior is also checked (delay, alarms, blocking when it is lost) and, once in service, that the differential current under load is practically zero. The details are in the guide on 87L line differential testing and in our end-to-end testing service.

We review your line differential

Did your 87L trip on an external fault, show differential current with load or block because of the channel, and the cause is not clear? At Tiza Group we review the events, settings and channel supervision remotely, for substations and industry at medium, high and extra-high voltage. See remote relay configuration or contact us.

Related guides

• 87L line differential testing
• Distance protection 21
• Line protection: main 1 and main 2
• Phase comparison
• Teleprotection schemes
• IEEE C37.94
• Multiplexer and optical fiber network

References

• IEEE Std C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.
• IEEE Std C37.243-2015, Guide for Application of Digital Line Current Differential Relays Using Digital Communication.
• IEEE Std C37.94, Standard for N Times 64 Kilobit Per Second Optical Fiber Interfaces Between Teleprotection and Multiplexer Equipment.
• IEC 61850-7-4, Communication networks and systems for power utility automation – Basic communication structure – Compatible logical node classes and data object classes.
• IEC 60255-187 series, functional requirements for differential protection (part 3 covers line differential).

Frequently asked questions