Dielectric loss in cables 

By on

cableSection
Cable cross section showing
insulation
 
Dielectrics (insulating materials for example) when subjected to a varying electric field, will have some energy loss.   The varying electric field causes small realignment of weakly bonded molecules, which lead to the production of heat.  The amount of loss increases as the voltage level is increased.  For low voltage cables, the loss is usually insignificant and is generally ignored.  For higher voltage cables, the loss and heat generated can become important and needs to be taken into consideration.

Dielectrics (insulating materials for example) when subjected to a varying electric field, will have some energy loss.   The varying electric field causes small realignment of weakly bonded molecules, which lead to the production of heat.  The amount of loss increases as the voltage level is increased.  For low voltage cables, the loss is usually insignificant and is generally ignored.  For higher voltage cables, the loss and heat generated can become important and needs to be taken into consideration.

Dielectric loss is measured using what is known as the loss tangent or tan delta (tan δ).  In simple terms, tan delta is the tangent of the angle between the alternating field vector and the loss component of the material.  The higher the value of tan δ the greater the dielectric loss will be.  For a list of tan δ values for different insulating material, please see the Cable Insulation Properties note.  

Note: in d.c. cables with a static electric field, there is no dielectric loss.  Hence the consideration of dielectric loss only applies to a.c. cables.

Cable Voltage

Dielectric loss only really become significant and needs to be taken into account at higher voltages.  IEC 60287 "Electric Cables - Calculation of the current rating", suggests that dielectric loss need only be considered for cables above the following voltage levels:

  Cable Type   U0, kV
Butyl Rubber 18
EDR 63.5
Impregnated Paper (oil or gas-filled) 63.5
Impregnated Paper (solid) 38
PE (high and low density) 127
PVC 6
XLPE (filled) 63.5
XLPE (unfilled) 127

 

Cable Dielectric Loss

Cable Capacitance

Cable capacitance can be obtained from manufacturers or for circular conductors calculated using the following:

  C= ε 18ln( D i d c ) 10 9 F. m 1

Given the tan δ and capacitance of the cable, the dielectric loss is easily calculated:

  W d =ω C U 0 2 tan δ

It is possible to use the above for other conductor shapes if the geometric mean is substituted for Di and dc.

Symbols

dc - diameter of conductor, mm
Di - external diameter of insulation, mm
C - cable capacitance per unit length, F.m-1
U0 - cable rated voltage to earth,  V
Wd - dielectric loss per unit length, W.m-1
tan δ - loss factor for insulation
ε - insulation relative permitivity
ω - angular frequency (2πf)

See Also



Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

comments powered by Disqus



How D.C. to A.C. Inverters Work

Traditionally generation of electricity has involved rotating machines to produce alternating sinusoidal voltage and current (a.c. systems). With the development...

Frame Leakage Protection

While not as popular as it once was, frame leakage protection does still have some use in some circumstances.  In essence frame leakage is an earth fault...

Contribute to myElectrcial

Have an opinion or something to say, want to ask or answer questions, share your knowledge then use our site to do it . As a community of people interested...

Occam's Razor

I was reminded of Occam's Razor while reading a book. It's quite a simple principal of logic which has stood the test of time and is accepted as central...

Wiki Depreciation

We have had the Wiki with us for a long time now, but at last I have decided to say bye bye – more details on why below.

Copyright Infringement

myElectrical does not support or promote the use of copyrighted material without the copyright owner's consent. If you believe that material for which...

Cable Insulation Properties

Cable insulation is used to provide electrical separation between conductors of  a cable.  During the historical development of cables, numerous types...

Gas Insulated or Air Insulated Switchgear

Various arguments exist around SF6 Gas Insulated (GIS) and Air Insulated (AIS) medium voltage switchgear. Recently we had to change a GIS design to AI...

How Electrical Circuits Work

If you have no idea how electrical circuits work, or what people mean then they talk about volts and amps, hopefully I can shed a bit light.  I’m intending...

ABB Technical Guides - Motor Operation

ABB has produced a range of technical guides that offer concise explanations of the major technologies and technical issues in low voltage AC drives. ...

Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.  

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note