Selectivity and fast tripping of protections in the event of a fault in medium‑voltage networks are now essential requirements. XMR‑L was developed to meet these needs: a multifunction protection relay that integrates line differential protection (87L) for two‑terminal MV lines, alongside the standard feeder protection functions.
Thanks to a distributed architecture, redundant fibre‑optic communications and advanced algorithms for comparing currents at both ends of the line, XMR‑L provides fast, selective and highly sensitive protection against phase‑to‑phase and earth faults.
How XMR‑L works
At each end of the line, an XMR‑L relay is installed and connected to the current transformers; each relay measures the phase currents and the residual current. The two relays exchange the sampled values in real time via a fibre‑optic link, synchronised through the IEEE 1588v2 (PTP) protocol to ensure comparison on the same time base.
The 87L protection logic operates on two fundamental quantities:
- the differential current, that is the vector sum of the currents measured at the two ends of the line;
- the ratio between the current phasors at the line ends, checking whether the representative point falls inside or outside a specific “stability zone” in the complex plane.
The protection trips the circuit‑breakers only when the differential current exceeds a set threshold and the phasor ratio clearly indicates a fault internal to the protected section.
XMR-L solution features
XMR‑L integrates a series of distinctive features that enhance its reliability and application flexibility:
- Distributed architecture One relay at each end of the line, installable close to the CTs, reduces performance requirements on the current transformers and limits the effects of saturation, especially on long lines.
- Redundant PRP communications Current samples (Sampled Values) are exchanged over fibre optics according to IEC 62439-3 PRP; each packet is sent on two parallel channels, ensuring service continuity without recovery time in the event of a link failure.
- Integrated PTP synchronisation Synchronisation between relays occurs via IEEE 1588v2 with hardware timestamping, without the need for external GPS devices, minimising errors due to time base differences.
- High-sensitivity 87LP and 87LE functions In addition to phase differential protection, XMR-L features a dedicated input for a summing toroidal CT to measure residual current, enabling highly sensitive and fast detection of single-phase-to-earth faults even in isolated, compensated or resistive earthed networks.
- Integration with traditional protections The same relay provides 50-51-51N-67-67N functions (67-67N optional protections), which can protect busbars and zones outside the line, while 87LP/87LE cover the line section with zonal selectivity.
- Autonomous tripping logic Each XMR-L independently processes the 87LP and 87LE functions and can directly open its own circuit-breaker, without the need for intertripping between the circuit-breakers at the two ends of the line.
Problems solved: stability, selectivity, trip times
The introduction of XMR‑L addresses some typical issues in modern MV networks, particularly in ring or meshed architectures.
- Stability even in severe conditions The stability zone in the phasor ratio plane is adjustable for radius and angular width, allowing the protection to remain stable for external faults even with heavily saturated CTs, with different characteristics at the two line ends, or in the presence of synchronisation jitter. In meshed networks, where the so-called "false residual current" can cause unwanted trips of 51N/67N protections, XMR‑L's 87L protection is inherently insensitive to this contribution, ensuring stability without having to desensitise the thresholds.
- Intrinsic zonal selectivity Differential protection trips only for faults internal to the section, remaining stable for external faults regardless of network configuration and earthing arrangement. This eliminates the need for time delays for time-graded selectivity with other protections and reduces the complexity of blocking or intertripping logics.
- Fast trip and high sensitivity Thanks to differential/ratio analysis of currents, XMR‑L detects internal faults even with currents comparable to or lower than the nominal current, where a traditional overcurrent protection might not trip. The typical trip time is 50 ms for both polyphase faults (87LP) and single-phase-to-earth faults (87LE), with the option to selectively delay only 87LE when needed to handle intermittent phenomena or transient errors.
- Better management of earth faults in compensated neutral networks In compensated neutral systems, the strong unidirectional fault current component can cause saturation of the toroidal CT, requiring minimum delays of about 380 ms for 67N directional earth protections (ref. CEI 0-16). XMR‑L, thanks to 87LE, maintains stability for external faults and trips nevertheless in 50ms for internal faults, significantly improving the clearance times for single-phase-to-earth faults.
Application contexts: where XMR‑L makes the difference
XMR‑L has been designed for all those infrastructures where reliability, service continuity and power quality are strategic. Among the main application areas:
- Renewable generation plants Photovoltaic and wind farms, where the high sensitivity of the 87L differential protection enables fault detection even at low levels of currents supplied by static generation.
- Critical facilities and data centres Hospitals, data centres and IT infrastructures, where fast fault clearance and protection stability in meshed networks are essential to ensure service continuity.
- High process continuity industries Semiconductor, pharmaceutical and plastic film production industries, and other processes where unplanned downtime has immediate impacts on product quality and costs.
- Public distribution networks MV distribution networks where increasing complexity of configurations (distributed generation, closed rings, parallel runs) requires more intelligent, stable and selective protection solutions compared to overcurrents alone.
XMR‑L as an enabler of safer and more flexible MV networks
With XMR‑L, line differential protection 87L becomes an integrated solution ready for modern, digital and high-availability networks.
The combination of distributed architecture, redundant PRP communications, PTP synchronisation and advanced protection functions allows reducing trip times, simplifying coordination logics while simultaneously increasing protection stability and sensitivity.
For MV network operators and end users, this means better protected lines, minimised outages and reduced fault location times.
