Line current differential protection, device 87L, protects a transmission or distribution line by comparing the current entering one end against the current leaving the other. Two relays, one at each end of the line, measure their local currents and exchange the values over a communication channel, usually fiber. As long as what goes in equals what comes out, the line is healthy; when the two no longer balance, current is escaping into a fault somewhere on the line and both relays trip. Because it directly compares the two ends, 87L is a form of unit protection with a precisely bounded zone.
Line Differential (87L) in one line: Line current differential protection (87L) compares the current at both ends of a line by having a relay at each end exchange its measured current over a communication channel. If the currents do not sum to zero, a fault exists on the line and both relays trip. It is unit protection with a sharply defined zone that needs no coordination with adjacent protection.
The principle of 87L is the same Kirchhoff current balance that underlies all differential protection: the sum of currents entering a healthy zone is zero. The difficulty for a line is that its two ends can be many kilometers apart, so unlike a transformer whose terminals sit in one cubicle, the two current measurements are physically remote from each other. Line differential solves this by putting a relay at each end and having them communicate. Each relay digitizes its local three-phase currents and sends that information to the relay at the far end.
Each relay then has both its own current and the far-end current, and it evaluates whether they balance. Two common decision methods exist. A percentage or biased characteristic computes a differential current from the vector sum of the two ends and compares it to a restraint derived from the through-current, tripping when the difference exceeds the bias. The alpha-plane method instead looks at the complex ratio of the two end currents: for through-load or an external fault the currents are essentially equal and opposite, placing the ratio at a specific point, whereas an internal fault moves the ratio away from that point into a trip region. The alpha plane is prized for its tolerance of CT saturation and channel time error.
The communication channel is typically dedicated fiber, either direct fiber between substations or a channel over a multiplexed network, carrying the current data in each direction continuously. The relays exchange their measurements every few milliseconds, so the differential comparison is effectively continuous and both ends reach the trip decision at nearly the same instant, clearing line faults quickly from both terminals.
For the comparison to be valid, both relays must compare current samples taken at the same instant, even though the data from the far end arrives after a channel delay. Getting this alignment right is central to 87L. One approach uses the ping-pong or echo method, which assumes the channel delay is the same in both directions and measures the round-trip time to work out the one-way delay, then shifts the far-end data accordingly. This is simple and needs no external clock, but it breaks down if the two directions have different delays, as can happen when a routed or switched network path changes.
The more robust approach uses external time synchronization, typically GPS, so each relay timestamps its own samples against a common absolute time and the far-end data is aligned by timestamp rather than by assuming symmetric delay. GPS alignment handles asymmetric channels correctly but introduces a dependency on the time source; well-designed relays fall back to a channel-based method or an appropriate secure mode if GPS is lost, so a clock outage does not silently corrupt the differential.
A second correction 87L must make is charging current. A transmission line has capacitance to earth, and energizing it draws a steady charging current that flows in at one end without flowing out the other. To a naive differential element that unbalanced current looks like a permanent internal fault. Line differential relays compensate for the known charging current, subtracting the expected capacitive component so the scheme stays stable on a healthy energized line, which matters most on long or cable circuits where charging current is large.
Because 87L compares the two ends of exactly one line, its protected zone is bounded precisely by the two sets of CTs. A fault anywhere between them is inside the zone and trips; a fault beyond either end is outside the zone, keeps the currents balanced, and is ignored. This makes 87L inherently unit protection, in the same family as transformer and bus differential, and gives it the same defining advantage: it does not have to coordinate its timing with the protection on adjacent lines because it never responds to anything outside its own zone.
That freedom from coordination is a significant operational benefit. Distance protection has to grade its zones and time steps against neighboring lines, accepting delayed clearing for faults near the far end; line differential can clear a fault anywhere along the line at high speed from both terminals, with no time-delayed second zone. It is especially valued on short lines, cables, and multi-terminal or tapped lines where distance settings become awkward, and on critical circuits where fast clearing from both ends improves stability.
The trade-off is the dependence on the communication channel. Unlike distance protection, which is self-contained at each relay, 87L cannot make its differential decision if the channel fails, so utilities provide a backup protection element - often a distance or overcurrent function in the same relay - that takes over if communications are lost. The channel therefore has to be engineered for reliability and low, stable latency, but in return the scheme delivers clean, coordination-free, high-speed protection of the whole line.
Because 87L compares the current at both ends of one specific line, its zone is bounded exactly by the two sets of CTs. It only responds to faults between those CTs and stays balanced for anything outside, so it never reacts to faults on adjacent lines. Since it never trips for external faults, there is no neighboring device it has to grade against, and it can clear line faults at high speed with no coordination delay.
It must compare samples taken at the same instant despite the channel delay. The ping-pong or echo method measures round-trip time and assumes equal delay each way to estimate the one-way delay. The more robust method uses external time synchronization such as GPS, timestamping local samples against common time so far-end data is aligned by timestamp, which correctly handles channels whose two directions have different delays.
A transmission line has capacitance to earth, so an energized healthy line draws a steady charging current that enters at one end but not the other, appearing as unbalance to a differential element. Line differential relays compensate by subtracting the expected capacitive charging current so the scheme stays stable on a healthy line. It matters most on long overhead lines and cables, where charging current is large enough to otherwise cause misoperation.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.