The industry challenge

Circuits that combine overhead line and underground cable are becoming common at 110 to 400 kV. Undergrounding programmes, planning constraints on new overhead routes, weather resilience, and new loads such as metros and data centres beside existing corridors all add cable sections, often as loop-in, loop-out cable tails.

Most overhead line faults are transient, so auto-reclose restores supply in under a second. Cable faults, on the other hand, are almost always permanent. Reclosing onto a faulted cable cannot restore supply: instead, it adds damage at the sealing ends and joints, can spread it to adjacent plant, and puts people near the cable at risk. The repair then takes weeks.

Distance protection cannot reliably tell which section is faulted. A cable’s impedance to a fault does not vary linearly with distance, its charging current is high, its sheath bonding adds circulating currents, and every transition reflects travelling waves. Differential protection at each transition would solve it, but traditionally needs a relay, a power supply, a communications link, and a time source at each transition tower.

How it works

Passive sensors

At each transition tower, a split-core current transformer fits around each cable below the termination bushing, and a single-phase Passive Secondary Converter (PSC-1-C), rated IP68, connects to its secondary. Where measurements are required in the substation, a rack-mounted three-phase PSC-3-C connects to spare cores of the existing line CTs. Each PSC encodes the CT secondary current as a shift in the wavelength of light, which electromagnetic interference cannot alter, and returns it on standard single-mode fibre. There is no fibre to wind around the conductor and no specialist work at the tower.

DES Interrogator

A single 3U rack-mounted DES Interrogator in the protection and control room reads every sensor on the fibre, sampling at 4.8 kHz. All measurements are synchronised to within one microsecond by the Interrogator itself, with no satellite or network time source at any sensor. Synaptec’s patented time-of-flight correction keeps phase and time-stamp accuracy within IEC limits at any practical fibre length.

The Interrogator runs line differential protection (ANSI 87L) on every zone, with overcurrent and earth fault (ANSI 50 and 50G) where required. It issues trip and auto-reclose block signals by dry contact or IEC 61850 GOOSE, publishes IEC 61850-9-2 Sampled Values to your relays, and records time-synchronised fault data at every sensor.

Zones that follow your network

A zone is formed between any two sets of sensors, so it can hold one cable section or several, in series or in parallel. Single cable tails, double circuits, multi-ended circuits, and routes with several cable sections are all protected from one Interrogator, and zones are added by adding sensors. Commissioning is carried out centrally from the substation, without test sets at the tower.

>1.5 million

sensor-hours of operation (October 2026)

<10 ms

to identify a faulted zone on mixed circuits in service

In service: Tata Power, 110 kV Mandale substation, Mumbai

MCP was commissioned on two 110 kV mixed circuits in January 2026, protecting the loop-in, loop-out cable sections that feed the Mumbai metro traction supply. By June 2026 the circuits had seen three overhead line faults. Each time the scheme remained stable and the line auto-reclosed successfully.

“The deployment of Synaptec’s MCP system has demonstrated reliability and effectiveness in providing auto-reclose functionality for the 110 kV Mandale–Dharavi–Vikhroli hybrid circuit.”

Girish Jawale, Head, Technical Services (T&D, MO), Tata Power, June 2026

Read the Tata Power case study