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Self-healing dielectric fluids for fluid filled cables (Anagen™)


Fluid filled cables (FFCs) are legacy elements of power transmission and distribution networks. These were deployed prior to the development of effective polymer insulation layers. FFCs contain paper insulation layer, impregnated with a dielectric fluid held under a positive pressure. This fluid enhances thermal transport and prevents the formation of voids which would degrade the paper insulation.

Damage to the cable sheath can result in oil leaking to the surrounding environment. If a sufficient volume of oil is leaked, the cable loses pressure and the insulation degrades, eventually to failure. The insulation integrity can be maintained by replenishing oil, at a cost to the cable operator and it has environmental implications. Operators are required to record leakage rates and if these exceed specific thresholds action must be taken. This can include locating and repairing the leak and cleaning up the lost oil, which are expensive and time consuming.


Kinectrics are materials experts with recognised expertise in the field of self-healing materials. We were approached by DNOs to develop a solution to the problem of leaking FFCs.

Following a research programme, Kinectrics developed a self-healing dielectric fluid (SHF). This SHF functions as a dielectric fluid under normal operation but will form a gel upon exposure to oxygen. In the context of a cable, this will seal a defect in the cable sheath, on joints, or within cable accessories.

The fluid has demonstrated leak-healing capabilities for leaks of up to 40L/month and has shown the ability to attenuate leaks of up to 70L/month. If leaking into backfill the fluid can function synergistically to form a hardened oil-resistant barrier. This can contain leaked SHF close to the cable, limiting environmental impact. In addition, SHF-treated backfill can form a sealing layer on the defect, reducing or eliminating the leak.

The fluid has been trialled on decommissioned circuits in the UK. All tests were successfully completed, and they have included:
  • Direct sheath leak healing
  • Backfill leak reduction
  • Electrical pressure testing
  • PFT compatibility trials


Kinectrics has developed a self-healing dielectric fluid that can be retrospectively applied to leaking FFCs. Circuit trials have been successfully completed, and live-circuit trials are currently being undertaken prior to rollout across the UK.

The advantages

  • Autonomously resolves leaks of up to 40L / month, reducing costs associated with cable maintenance, oil replenishment, and environmental remediation.
  • Capable of autonomously containing spillages and severe leaks within backfill materials, preventing contamination and the costs of remediation.
  • Can be stored and handled as common cable dielectric fluids, with small changes in procedure to account for self-healing function.
  • Equivalent or superior breakdown voltages to currently used mineral oils.

  • Fluid Filled Cables

    Fluid Filled Cables (FFCs) are part of legacy transmission and distribution systems, dating before the deployment of extruded polymer insulation layers. Also, most were replaced, some cannot be replaced (inaccessible) or are functioning without issue and so are low priority.

    FFCs are insulated by a layer of tightly lapped cellulose paper, impregnated with a low viscosity dielectric fluid (commonly T3788). This is held under a positive pressure (3-5 bar) to improve thermal transport and prevent the formation of voids. If oil pressure drops, thermal transport is reduced and voids form. Overheating and partial discharge (which occurs in voids) accelerates paper breakdown and so degrades the insulation.

    Fluid Loss

    Oil loss occurs primarily through defects in the sheath. These commonly arise through physical damage to the sheath, which is made of either lead or aluminium. Leaks can also form on plumb joints (through poor plumbing or damage) or other cable accessories.

    Lost cable oil can result in a reduction in cable pressure; results in overheating and voids (see above). Although lost oil can be replenished, this carries a financial cost and environmental impact. DNOs must report leaks and resolve those that breach specific thresholds (~40L/month).

    Locating and resolving a leak is a time-consuming, expensive process that often requires the use of tracer compounds and freezing lengths of cable. In lead sheaths, aging can lead to crystallisation, resulting in a very fragile sheath that cracks readily and cannot be repaired. 400,000L of oil are lost p.a. across the UK.

    Current solutions include:
    • Replenishment of leaked cable oil
    • Location and repair of damaged leak sites through tracer compounds and cable freezing
    The solution Kinectrics have developed consist in replacing current cable dielectric fluids with a self-healing fluid (AnagenTM). This functions as a standard dielectric fluid under normal operation, but gels upon exposure to oxygen to seal defects in the cable sheath or other accessories.

    This solution will reduce both oil loss and environmental impact. Also, the self-healing fluid can be purified, processed, and deployed according to standard cable oil processing procedures.