Fluid-filled cables overview - Energy Networks Association

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
Metallic Sheath
Conductor
Butt Gap
Cable Fluid
Insulation
• 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.

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
Typical fluid filled cable circuit
Oil Feeding Equipment Joint Stop Joint
Termination
• 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.

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
To Alarms
Pressure element
Pressure Gauge
Valve
Insulated
Link
To Cable
System
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