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Integration of Fiber Optic Element in Armoured 3-core submarine cables – success for more than 30 years

 

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Summary  |  Introduction  |  Background  |  Factory acceptance test  |  Operation |

 

Summary

This web page addresses potential failure modes of the integrated FOC (Fiber Optic Cable) element in 3-core submarine export cables which, in some cases and specific designs, may lead to a fatal cable failure and power loss.
Nexans cannot comment on why some other manufacturer’s integrated FOC designs have failed but can provide supporting documentation as to why failure has been avoided in the past 30 years and will be avoided in the future for Nexans supplied cables.
This statement is supported by a 30 year long track record of integrated fiber elements in three core submarine cables, which is achieved by a robust cable design and a controlled production process. It is shown that with the Nexans design, the voltage and current in the FOC steel tube is insignificant during all relevant scenarios, eliminating the possible mechanisms that may cause FOC related High Voltage (HV) insulation failure.

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Introduction

Nexans Norway has utilized integrated fiber optic elements in armoured three core submarine cables since 1985 with no registered failures (failures in FOC causing damage to other parts of the cable than to the FOC itself). Nexans design philosophy for submarine cables is to ensure semiconducting contact between armour, lead sheaths and fiber tube, eliminating voltage differences between the elements within the cable during testing and operation. If such contact is not ensured, severe local heat development and arcing may occur, ultimately causing High Voltage (HV) insulation failure.

Nexans FOC element comprises fibers inside a steel tube covered by a semi conductive jacket (see Figure 1 left). To ensure continuous semiconducting contact, the FOC element is placed inside of plastic filler profiles with semiconducting walls (see Figure 1 right). Nexans patented plastic fillers with semi conductive walls, combined with the fiber optic cable’s semi conductive jacket, provide continuous electrical connection between the steel tube and the power cable’s screens.

During lay-up at Nexans cable factory, special means such as back-twist feeding and strain control is incorporated in the FO pay-off system when inserting the FOC into the 3-core construction. Also, continuous monitoring of the optical signal may be utilized, detecting if high strain occurs somewhere along the FOC in the production line. These methods ensure the integrity of the fiber tube during production.

FOC figure 1

Figure 1: (Left) Cross section drawing of optical fiber element, (right) picture of three core cable with integrated Fiber Optic element (FOC).

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Background

Due to the close proximity of the FOC's steel tube and the power phases, there is a mutual inductive coupling between them. To show how this induced electromotive force will influence the FOC steel tube during testing, operation, and if there is a fiber tube break, a 50 km long armoured three core 245 kV cable is used as an example, carrying 1000 A load current. To illustrate the difference between semi conductive and insulating FOC sheath, which is key to understanding the relevant fault mechanisms, the conductance of the fiber tube is varied from practically insulating outer sheath (G=1e-6 S/km) to continuously grounded (G=1 S/km). The conductance G is a measure of the electric contact between the FOC steel tube and the lead sheaths of the cable. 

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Factory acceptance test

During the factory acceptance test, each phase of the cable is tested with a voltage of 2.5U0 /3.5 U0 according to applicable standards. The armour, lead sheaths and FOC tube are grounded at both ends during testing. In Figure 2 the voltage difference between FOC metallic tube and lead sheaths/armour is plotted for different values of the tube conductance. As shown in Figure 2 the voltage difference is zero when there is good semi conductive contact between the metallic tube and lead sheaths (G=1 S/km), eliminating the risk of fault scenarios related to potential differences during testing.

FOC figure 2

Figure 2: Voltage difference between FOC steel tube and lead sheath/armour, with different values for the conductance of the FOC tube (G).

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Operation

It is known that failures to the fiber have caused HV insulation failure during operation, and the identified possible failure modes are:

  1. High induced voltage, combined with failure or breakdown of the FOC tube insulation, followed by local heat development and severe damages to the power phases.
  2. High induced current combined with a break in the FOC tube giving local heat development and arcing, with severe damages to the power phases.

In Figure 3 the induced voltage on the FOC steel tube is plotted without a break (left) and with a break (right), for different values of the steel tube conductance. With good semi conductive contact between the FOC metallic tube and the lead sheaths (Nexans design, G>>1 S/km), the voltage is practically zero for both steel and copper metallic tube (black line), with or without a break in the fiber tube. This is because the current is able to bypass the broken region of the metallic tube, through the semi conductive sheath.

On the other hand, if the outer FOC sheath is insulating, the current in the power phases will induce a significant voltage on the metallic tube. As shown in Figure 3, for a broken metallic tube (right plot), the induced voltage can reach up to 2 kV across the broken area. It is important to also note that a substantial voltage will be induced also without a break (see Figure 3 left), reaching a maximum at midpoint, similar to the voltage during testing (see Figure 2).

FOC figure 3

Figure 3: Induced voltage along the FOC tube for different values of the electric contact between FOC tube and lead sheaths/armour (conductance G). Left plot without fiber break and right plot with fiber break.

For a FOC with a broken tube, local heat development may eventually cause a HV insulation failure if the local heat dissipation is high enough to melt the surrounding materials. To investigate under which circumstances such high power dissipation is possible, a FOC tube with a break is modeled, assuming a local fault resistance (Rf) across the gap of the broken metallic tube. In Figure 4 the calculated power dissipation in the area with a tube break is plotted as a function of the fault resistance. With a steel tube it is practically impossible to create large damages to the cable even with an insulating sheath (red line), as the current flowing in the steel tube is so low (0.2 A per kA phase current), giving limited power dissipation. It is only with a copper tube (or similar) combined with poor semi conductive contact (or insulating sheath), that the resulting power dissipation is high enough to cause HV insulation failure. For a system with good semi conductive contact between FOC tube and lead sheath (black line), no heat is developed in the fault area for both tube materials, as the current may bypass the broken area through the semi conductive sheath.

FOC figure 4

Figure 4: Calculated power dissipation as a function of fault resistance, with a break in the FOC tube, (left) for stainless steel tube and (right) copper tube.

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