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Free Standing Fiber Optic Current Sensor FOCS-FS for digital substations

Non-conventional current transformer enabling digital HV substations and smart grids with an optical IEC 61850-9-2LE Ethernet output

Beside inductive current transformers, oil or SF6 insulated, Hitachi Energy has developed since 1990s optical current sensors, based on the Faraday effect principle, whereby light is used to deduce the precise magnitude of current that is creating the magnetic field. As a result, the design is inherently free of magnetic saturation, therefore suitable for capturing fast transient currents, short circuit currents, and alternating current (AC) with DC-offset.

FOCS-FS is a 3-phase sensor system consisting of:

  • three hollow insulators filled with nitrogen at ambient pressure and supporting the sensor heads
  • one outdoor kiosk, installed on the steel structure of the central phase and connected via optical fiber to the three HV columns and to the relay house via a duplex ethernet optical cable (IEC61850-9-2LE protocol).

Any optoelectronic module located in the kiosk does:

  • send polarized light to the sensor
  • receive the reflecting polarized light from the sensor
  • compare in close-loop control the phase displacement in the polarized light in proportion to the magnetic field and the primary current
  • convert the result into an optical IEC 61850 Ethernet output.

The level of redundancy that can be specified is given by the number of optoelectronics units integrated into the system.

All solutions can be equipped on request with merging units, which digitalize analog signals coming from voltage transformers, synchronize those signals with the digital signal coming from FOCS-FS and consolidate both signals into a digital output stream complying with IEC61850-9-2 LE protocol.


  • Protection and metering for digital substations

Why Hitachi Energy?

  • Hitachi Energy has been producing inductive instrument transformers for more than 70 years and optical current sensors for more than 20 years
  • Hitachi Energy can provide the complete system (sensors > merging units > relays)
  • FOCS-FS is a “Plug & Play” solution: Fully redundant system with “hot swappable” opto-electronics
  • FOCS-FS is intrinsically safe and eco-efficient, having digital output and being filled with nitrogen gas at ambient pressure

Brief performance data

Installation Outdoor
Design Optical
System voltage 50 – 60 Hz
72.5 ÷ 800 kV
Current ratings Up to 4800 A and 63 kA
Insulators Silicon rubber
Specific creepage distance: 31 mm/kV
Withstand load: up to Class II
Ambient conditions Outdoor operating temperature : [-40;+45°C]
Air pollution: very heavy
Seismic level: 0,5g
Accuracy Protection:
IEC: class 5P, 5TPE
IEEE: 10%
IEC: 0.2s
IEEE: 0.15s
Interfaces Digital output (IEC 61850 9-2 LE)
Optical ethernet cable: Duplex MM 62.5/125 with ST connector
Optical 1PPS cable: Duplex (or Simplex) MM 62.5/125 with ST connector
Options Redundancy of electronics
Merging unit for VT signals

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