Current Monitoring is a Vital Technology for the Protection, Control and Supervisory facilities in power utilities. Traditionally, current measurement is performed using current transformers designed with iron cores and copper windings that have several technical and operational shortcomings. Current transformers tend to be heavy and bulky, which makes them difficult to install or attach to existing conductors. Their accuracy is also influenced by electromagnetic induction noise and high currents that include low-frequency components.

In the 1980s, a method employing an optical fibre — such as the Faraday sensor element (optical-fibre current sensor) — was proposed that prompted long-term research. The principal problem was that the polarization of light in the fibre used as the sensing element was affected by mechanical stress due to the photoelastic effect. Tokyo Electric Power Co. (TEPCO; Tokyo, Japan) solved this problem by developing a novel optical fibre that offered stable current detection by encircling the light and flexible sensor fibre around the current conductor. This development led TEPCO to develop a fault location system for 66-kV, 154-kV and 275-kV underground transmission cables using these sensors.


A cable fault section location system employing optical-fibre current sensors was developed by a joint project team that included TEPCO, Kansai Electric Power Co. (Osaka, Japan) and Takaoka Electric Manufacturing Co. Ltd. (Tokyo).

On a fault section location system, current sensors are installed at each end of the cable section to detect the zero-phase fault current. The signal light from the sensor head, whose intensity is modulated by the fault current, is transmitted by the optical fibres to the detection panel in the substation. The detection panel measures the fault current with the O/E conversion board, and the detector output for the current differential relay decides when the fault is within or outside the protected section. In the event the fault is within the cable section, the relay output will activate the locking mechanism of the reclosing circuit breaker.

Two different types of fault section location systems were developed, the first for 66-kV and 154-kV systems with resistance-grounded neutral systems, and the second for 275-kV systems with solidly grounded neutral systems. These two systems were required because the value of fault current is dependent on the method of system grounding and the differences in the cable terminations used for these voltages.

The sensor installation is shown in Fig. 1, the fibre loop enclosing the three single-core cables provides accurate zero-phase fault current detection. The sensor head is waterproof and is suitable for underground installation.

This system fault location method offers the following advantages over conventional systems:

  • No power source or electronic equipment is required at the sensor installation.

  • It can be used on circuits above 20 km (12 miles), which is the limit applicable to systems relying on traditional signal transmission.


    Installation is easy with the reflection-type flexible sensors.

  • No spurious operations occur due to magnetic saturation of the iron core used in current transformers.

Since 2004, Kansai Electric Power Co. has commissioned six systems, and in early 2007, TEPCO installed this fault location systems to a 66-kV circuit that included teed connections.


Previously, TEPCO relied on surge-receiving-type fault point location systems in the event of a fault on a 275-kV system, but a joint research project with Toko Electric Corp. (Tokyo) and Fujikura Ltd. (Tokyo) has now developed a location system using optical-fibre sensors.

Figure 2 shows the fault point location system that comprises two optical-fibre sensors, two local stations and signal transmission lines. Each optical-fibre sensor detects the fault current and transmits a signal to the local station; the arrival time of the current is recorded via GPS. The master station is then able to determine the fault position from the difference in the arrival time data. Note that surge or fault current velocity is 50% to 60% the speed of light.


The field tests of the fault location system were conducted by TEPCO on a 275-kV fluid-filled underground cable some 15.6 km (9.7 miles) long. The accuracy of the fault location system is ±50 m (165 ft). In March 2007, following the successful field tests, a system was designed and manufactured for installation on a 275-kV fluid-filled cable circuit 13.9 km (8.6 miles) long. Figure 3 shows the sensor fibre attached to the 275-kV cable.

TEPCO plans to increase the number of installations. Furthermore, a system developed primarily for the Japanese market is already installed in the United States. The cost of the system is dependent on the size of the cable system to be protected.

The joint research project initiated by TEPCO has resulted in the successful development of a cable fault location system for extra-high-voltage and high-voltage underground cable systems. This new, easy-to-install technology eliminates the shortcomings of using current transformers, thus offering utilities a superior method of identifying faults that may occur on key transmission systems.

The authors wish to thank T. Yamaguchi (Toko Electric Corp.), K. Amano (Fujikura Ltd.) and T. Yamada (Takaoka Electric Mfg. Co. Ltd) for their contributions and cooperative development of the systems.

Shinsuke Nasukawa ( received his BSEE in 1996 and MSEE in 1998 in from Tohoku University Japan. Since joining the Underground Transmission Group of Tokyo Electric Power Co., Nasukawa has been engaged in all sections involved with the design, construction and maintenance of underground transmission lines.

Reishi Kondo ( graduated from the engineering course of Japan's Tokyo Electric Power Academy in 1992. He has been engaged in the research and development of optical-fibre sensors for power systems at the R & D Centre of Tokyo Electric Power Co.

Kiyoshi Kurosawa ( graduated from the engineering course of the Tokyo Electric Power Academy in 1980 and received his Doctorate of Engineering from Tokyo University, Japan in 1998. Kurosawa has been engaged mainly in the research and development of optical-fibre sensors for power systems at the R&D Centre of Tokyo Electric Power Co.


When a beam of light is passed through a transparent medium in a magnetic field, polarization of the light is rotated through an angle that is directly proportional to the strength of the magnetic field. This effect is known as the Faraday effect and forms the basic principle of the optical-fibre current sensor. The angle or rotation èF is determined by the following equation:

èF = VHL

where: H = strength of the magnetic field (amperes/meter)

L = length of the Faraday sensor (meter)

V = Verdet constant (degree/ampere)

This equation confirms that the angle of rotation can be used to measure conductor current. The material used for the optical fibre can have a marked impact on the polarization of light. TEPCO, in cooperation with the HOYA Corp. (Tokyo), overcame the errors associated with fibres made with silica by developing an optical fibre made from flint glass. Further development work on a signalling process resulted in improved output signal stability and noise reduction.

The application of these key technologies is shown in the sensing-device configuration.

Table 1. Specification for the optical-fibre sensor for the fault section location system.
Applicable circuits Resistance grounded neutral system - 66 kV and 154 kV
Measurement data Zero-phase current
Frequency 50 Hz to 60 Hz
Measurement range 40 A to 2000 A
Ratio - Error Current 40 A Less ± 4.0%
Current 2 kA Less ± 2.0%
Signal transmission distance Maximum 20 km (12.4 miles)
Round trip 40 km (25.8 miles)
Table 2. Specification for the optical-fibre current sensor for the fault position locator.
Item Specification
Number of channels Three channels
Usage environment Sensors - outdoors; electronics - indoors
Current-measurement range AC 100 A to 2 kA (peak value) accuracy: less ± 5%
Frequency response 50 Hz to 250 kHz (flat)
Response time Raise time: under 1 microsecond
Power source AC 100 V: Frequency 50 Hz