Chugoku Electric Power Co. Ltd., Hiroshima, Japan, installed a 66-kV cross-linked polyethylene insulated (XLPE) submarine cable with a mechanical damage detection sensor using optical fiber. The transmission line, called Tonosho Line, was installed to supply electricity to Syodoshima Island in the Seto-inland Sea in Japan. The submarine cable portion of the line is subjected to impact by fishing equipment and the fiber-based detection system has been working successfully to alert operators of possible damage to the cable.

The Seto-inland Sea is a calm sea with many islands between Honshu-mainland and Shikoku Island. Shodoshima Island is the most famous tourist resort and popular sightseeing destination in the Seto-inland Sea area. Electric power demand on the island had been increasing, which required the installation of the new submarine cable transmission line. The same area is a rich fishing location with many fishing boats. Many submarine cables are installed in Seto-inland Sea. Often they are damaged by ship anchors or fishing equipment. To satisfy increased power demand and detect cable mechanical damage, Chugoku Electric Power Co. Ltd. and Fujikura Ltd. developed and installed a cable system to detect cable deformations by the increase in transmission loss in the optical fiber.

Development of the Cable System Installed in 1992, the submarine cable is a three-core, 66-kV, XLPE-insulated power cable with a conductor size of 500 mm2 (987 kcmil) and transmission capacity of 66 MW. The cable is treated with a waterproof compound and features a lead sheath to prevent longitudinal and radial water penetration. In addition, a double layer of steel wire armor is applied to protect the cable from anchors and other objects. The outermost layer has shorter pitch of wire armor to protect against damage by sharp objects. Three optical fiber units including the mechanical damage detection sensor are placed in the interstices between the three assembled power cables. Coated with silicon/nylon, the SM optical fiber, which has a 0.9 mm (0.035 inch) diameter, is employed for damage detection and communication. In the unit, four detection fibers are spaced 90 deg apart and located in shallow grooves for easy detection of external damage. The communication fibers are placed into the deep grooves. Lap sheath and lead sheaths are applied to protect the fibers from water penetration. External pressure increases the optical transmission loss in the fiber. We intended to utilize this feature for the submarine cable damage detection.

The cable is designed in such a way that cable deformation imposes the external pressure only on the detection fibers located in shallow grooves. It is also possible to detect the location of the deformation on the cable by observing transmission loss of the detection fiber with optical time domain reflectometer (OTDR) readings. During the development period, we constructed the optical fiber unit in the submarine cable and confirmed the ability of the detection system to detect deformation of the cable under compression tests.

Manufacturing and Installing Cable The Tonosho Line consists of submarine cables and overhead lines connected in series. The submarine cable extends between Ishima Island and Teshima Island. Located in the middle of the transmission route, the cable was developed and manufactured by Fujikura Ltd. It has an overall diameter of 181 mm (7.13 inches) and a net weight of approximately 100 kg/m (67 lb/ft). This cable was manufactured in continuous lengths of 2 km (1.24 miles) without any factory joints. The installation site is about 800 km (497 miles) from Fujikura's manufacturing facility. The cable was transported to the installation site by a cable laying barge. Maximum sea depth where the cable was laid is 45 m (148 ft), and the seabed is composed of rocky masses and sandy sediment. The cable was installed from a barge by an anchoring laying method, which consisted of casting successive anchors with attached wires along the cable route. The cable was then lowered into the sea while wires were winched aboard the barge. The advantage of this laying method is that the cable can be accurately installed along the planned cable route.

Outdoor cable sealing ends were constructed and all 12 lengths of optical detection fibers were fused together in series at both ends of the submarine cable. One end of the fused detection fiber was connected with the automatic monitoring system using OTDR. The system included a master workstation at Okayama control station and local workstations at the site. Dedicated circuits connected both the master workstation and the local workstations. The system continuously monitors the transmission loss in the detection fiber and keeps the data of losses in its memory. If the cable has been externally damaged, the system displays and records the location, waveform and time of damage. This system can locate the exact damage location to within 8 m (26 ft).

Cable Operational Experience Chugoku Electric has gained five years of experience since the Tonosho Line was installed in June 1992. The sea area between Ishima Island and Teshima Island, where the submarine cable was installed, is an abundant fishing location, and summer is the best season for fishing. The monitoring system has detected loss increases several times during summer operation. For instance, on August 24, 1994, the system started to detect external damage before sunrise. Then the loss in the detection fiber returned to normal about two hours later. The detection time corresponded with the local fishing hours.

The fishermen in this area use tidal current change to fish. Fish are caught in large nets fixed to the sea bottom when the tidal current is fast. We theorize the cable is often damaged as follows:

A large net with anchors to fix the net is cast into the sea. By accident, anchors hook themselves on the submarine cable. The tidal current increase imposes excessive force on the net. The force is then transferred to the submarine cable by the hooked anchors. Excessive force then deforms the cable and the losses in the detection fibers in the composite optical units are increased by the pressure. When the tidal current becomes smaller and fishing has been completed, the anchors are removed and the cable is released from the pressure. The optical losses in the detection fibers return to the same standard value. Distance from the end of TeshimaIsland to the damaged point on that day of August 24, 1994 was calculated to be 440 m (481 yards) according to the data from the OTDR monitoring system. Both power cable and communication fiber were operating with no problem while the cable suffered the deformation.

Investigation of Cable A few months after the damage event, we surveyed the submarine cable with a remotely operated vehicle. The polypropylene serving layer applied over the double steel wire armor had been cut off for a few meters at several locations by a sharp object, and we found a fishing anchor and a piece of net next to the cable. At the point of the damage we also found that the serving layer had been cut off and outer steel wires of the armor were bent and deformed by some object. We decided the damage to the cable would not significantly affect the cable's electrical performance since the deformation seemed small. To confirm the property of the insulation, we measured electrical resistance, capacitance and tan d and performed dc voltage withstand tests on the power cables at the time of regular inspection. Test results were good and we confirmed that the power cables were fit for commercial operation. Moreover, communication optical fibers demonstrated good transmission characteristics. The Tonosho Line has been operating with no problems ever since.

Conclusion We have successfully developed the damage-detecting cable system using optical fiber to detect deformation of the submarine cable. During cable operation, the system detected the deformation of the submarine cable caused by fishing equipment. According to the cable investigation, the cable damage was not significant and the cable line has been operating satisfactorily. As a result of this positive experience, Chugoku Electric Power Co. Ltd. installed a new 22-kV submarine cable with mechanical damage detection sensor using an optical fiber for the Osaki Line in April 1997

References (1)Development of 66 kV XLPE Submarine Cable Using Optical Fiber As a Mechanical Damage Detection Sensor (EKE 95 WM 021-6 PWRD) (2) XLPE Power/Optical Fiber Composite Submarine Cable (Fujikura Technical Review, No23 1994).

T. Nishimoto joined Chugoku Electric Power Co. Ltd. in 1963. Previously, he worked in the design and operation of power transmission lines. He is now the general manager of Power Transmission Department.