Line Differential Protection (ANSI 87L) — current comparison over a communication channel

The guide on distance protection (21) and the guide on teleprotection (85) cover how a distance relay recognises a fault from an estimated impedance, optionally accelerated via a permissive or blocking scheme between both line ends. ANSI 87L (line differential protection) works on a fundamentally different principle: instead of estimating impedance, the relay directly compares the current flowing in and out at both ends of the protected line or cable connection — the same basic principle as transformer differential protection (87T) and busbar differential protection (87B), now applied across a connection that can be kilometres long.

The basic principle: what goes in must come out

For a healthy line, Kirchhoff's law applies: the sum of the currents entering the line at one end equals (apart from charging current, see below) the sum of the currents leaving at the other end. During an internal fault — a short circuit somewhere between the two measuring points — part of the current leaves the line via the fault location itself, creating a measurable difference between the two ends. Because this difference by definition only arises for an internal fault, 87L is inherently selective for its own protection zone and — unlike distance protection — the relay does not need to be set in time-graded zones: an 87L relay can trip instantaneously for the full line length.

Why channel delay must be compensated

With transformer or busbar differential protection, all current measuring points are physically close together, usually in the same switchgear installation, so a single relay can sample all current samples at the same instant. With a line, the two measuring points lie kilometres apart, and the current samples from both ends must be compared with each other over a communication channel — a channel that itself has a propagation delay (roughly 5 µs per kilometre for fibre optics). Without correction, that propagation delay would cause an apparent difference between both ends, even for a fully healthy line. Modern 87L relays compensate for this with one of two methods:

• Channel-based compensation ("ping-pong"): the relays at both ends continuously measure the propagation delay of the communication channel itself by sending timestamps back and forth, and correct the received current samples for that measured delay.
• GPS time synchronisation: both relays timestamp their own current samples with a GPS time reference, so the comparison is independent of the exact channel delay — provided both GPS receivers remain correctly synchronised.

Charging current: a disturbing factor for long cables

A long cable or overhead-line connection has significant inherent capacitance between the phase conductors and earth, which under normal operation draws a continuous charging current — current that does enter one end but never leaves at the other end, because it is "consumed" within the line itself by the capacitance. For a long, heavily loaded cable, this charging current can produce an apparent differential signal that, without compensation, can lead to an unwanted trip. Modern 87L relays explicitly compensate for this charging current based on the known capacitance of the connection, particularly relevant for long, high-voltage cable connections where the charging current can make up a substantial part of the rated current.

Comparison with distance protection (21) and teleprotection (85)

Why a channel failure requires backup protection

Because 87L is entirely dependent on a working communication channel for its core function, loss of that channel means loss of the primary protection function itself — not merely a delay, as with a teleprotection scheme layered on top of an already independently functioning distance protection. An 87L application is therefore almost always combined with independent backup protection (typically distance protection or time-graded overcurrent protection) that automatically becomes active as soon as the relay detects a channel fault.

Note: this guide covers the principle. The exact channel requirements (bandwidth, maximum permissible asymmetric delay between both directions), the settings for charging-current compensation, and the precise backup-protection strategy follow from the system study and manufacturer specification of the chosen relay.

Practical relevance

When assessing the protection philosophy of a critical cable or line connection — for example a long, high-voltage cable connection between two switchgear installations, or a short line for which distance protection is difficult to set selectively — it is relevant to check whether the communication channel is actually monitored, whether charging-current compensation is correctly set for the actual cable length, and whether backup protection automatically takes over on channel failure.

Common mistakes

1. No supervised backup protection provided for channel failure — 87L is entirely dependent on the communication channel for its core function; without a backup, the primary protection is completely lost as soon as the channel fails.
2. Underestimating charging-current compensation for long cable connections — for a lightly loaded, long cable the charging current can make up a disproportionately large part of the measured current.
3. Treating 87L as a full replacement for distance protection instead of as a complementary, faster primary protection — the two functions are complementary, not interchangeable.
4. Not correctly compensating channel delay during commissioning — an uncompensated or asymmetric channel delay can create an apparent differential signal leading to an unwanted trip.

Related

• Distance protection (21) — impedance protection and zones
• Teleprotection (85) — permissive/blocking schemes
• Transformer differential protection (87T)
• Busbar differential protection (87B)