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



Tips for a better Low Voltage Protection Discrimination Study

Carrying out a protection system discrimination study is critical to ensure the correct functioning of  the electrical system in the event of faults. ...

8 Steps to Low Voltage Power Cable Selection and Sizing

A recurring theme on our forums is cable sizing. Now many installations are unique and require special consideration. However, a lot of the time things...

Generation of a Sine Wave

A fundamental concept behind the operation of alternating current systems is that voltage and current waveforms will be sinusoidal – a Sine Wave. This...

Write your best report

Years ago I was told that you should always try to write the best report you can.  Many years later I still think on this as one of the better pieces of...

IEC 60287 Current Capacity of Cables - An Introduction

IEC 60287 "Calculation of the continuous current rating of cables (100% load factor)" is the International Standard which defines the procedures and equations...

Getting Started with Patents

If you have a great idea or invent something the last thing you want is someone to steal the idea. One of the things you can do is protect the intellectual...

Sony Pocket eBook Reader

For the past few years I have reading eBooks on my HTC touch phone. On and off I have debated buying an eReader and recently purchased a Sony PRS-300 ...

IEC Reference Designations

The IEC publishes a series of documents and rules governing the preparation of documents, drawings and the referencing of equipment.   Depending on country...

Tech Topics/Application Notes - Siemens

There are a lot of interesting two page type notes on various medium voltage topics – switchgear, circuit breakers, bus systems etc. It is on the Siemens...

Dielectric loss in cables

Dielectrics (insulating materials for example) when subjected to a varying electric field, will have some energy loss.   The varying electric field causes...

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