Fluid Filled Cables

Alan Archer CMIOSH, Asset Lifecycle Engineer

Overview

• Function of cable fluid in fluid filled cables

• Types of fluids used

• Movement of fluid in cable circuits

Fluid Filled Cable Design

• For fluid filled cables of all voltages (33, 66, 132, 275 and 400kV) to operate reliable they must be filled with a cable fluid which is maintained under an appropriate positive pressure.

• 1920 – Luigi Pirelli produce first pressurised fluid filled cable which operated at 80kV

• 1928 – First fluid filled cable in UK which operated at 66kV

• 1969 – First 400kV fluid filled cable

Fluid Filled Cable Design

• The fluid forms an integral part of the cables insulation and prevents the formation of voids in the lapped papers around the conductor.

• In addition the fluid aids the conduction of heat away from the conductor allowing the cable to be run more efficiently than a cable without it.

Conductor

Metallic Sheath

Insulation

Butt Gap

Cable Fluid

Types of Cable Fluid• Fluid filled cables have been used in the UK for over 80 years with some circuits installed in the 1930’s still operating successfully today without any deterioration.• Over the years a number of different type of fluid have been used:

– Before 1965 - a heavy mineral fluid with low biodegradability;– 1965 to 1985 – a synthetic fluid made as a by product of the detergent industry;– 1986 to today – a light synthetic biodegradable fluid.

• Today most Distribution and Transmission Operators use a single product (T3788) produced in the UK by a company called H & R Chempharm Ltd.• Most cable circuits, except those recently installed contain a mix of all three of the above fluids.

System Design and Fluid Movement• The fluid contained within a fluid filled cable has to be kept at a positive pressure under all conditions of loading and ambient temperature change.

• The pressure used depends on a number of factors:–Voltage and size of cable;–The length of the cable;–The route the cable follows;–Whether the cable has a leak.

•The pressure in the system is designed to ensure that the whole cable route is maintained at an adequate pressure ( 4 to 70 PSI) to ensures that it operates correctly.

System Design and Fluid Movement

• The type of fluid filled cable used in the UK is termed a “Static” system.• This means that under normal conditions the fluid in the system is total

contained.• When the cable is heated either by the amount of current passing through it

or the ambient air temperature the fluid expands and consequently moves.

Typical fluid filled cable circuit

Oil Feeding Equipment Joint Stop Joint

Termination

System Design and Fluid Movement

•To cope with changes in pressure a fluid reservoir is fitted to the ends of each section of the circuit.•Fluid movement = •[Cexp x (δTcxVc)x(δTixVi)x(δTdxVd)]xL•Where•Cexp = Coefficient of expansion of fluid•δTc, δTi, δTd = Temperature rise in conductor, insulation and ducts•Vc, Vi, Vd = Volume of fluid in conductor, insulation and ducts•L = Length of cable

Pressure Gauge

To Alarms

Valve

InsulatedLink

To CableSystem

Pressure element

De-gassed

Cable Fluid

System Design and Fluid Movement

•This graph illustrate the changes of pressure that can be seen in a cable.•At point A (08:00AM) the pressure measured is approximately 18PSI.•At point B (12:30PM) the pressure measured is approximately 21PSI.•This change in pressure 3 PSI which equates to 15 litres of fluid moving from the cable into the tanks.

A

B

System Design and Fluid Movement•This same graph also illustrate the changes of pressure that can be seen in a leaking cable that is pumped.•At point C (07:00AM) the pressure measured is approximately 12PSI.•At point D (13:30PM) the pressure measured is approximately 24PSI.•This change in pressure 12 PSI which equates to 26 litres of fluid being put into the cable via the tanks. C

D

Summary of cable design•Fluid filled cables rely on the presence of pressurised able fluid to work efficiently.

• Three different types of cable fluid have been used over the years, with the material used since 1986 being the least harmful to the environment.

•The fluid within a cable system normally only moves within the confines of the cable and the pressure tanks that feed it.

•Fluid movement is as a result of the cable being heated by the current it is passing or the ambient temperature in the environment.

•Under normal conditions the amount of fluid that moves can be measured in 10’s of litres.

Typical installation profile•This diagram shows the cross section of a typical cable installation.•Today cables are installed within ducting to allow easier replacement.•Existing FFC were mostly installed direct to ground, in a bed of sand protected by concrete cable markers or tiles.•Cables can be buried from 1m to more than 4m.•Access is difficult due to other services and utilities.•Access impeded by red routes and road opening notices etc.

• Thank You

Any Questions