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



Variable Frequency Drive

Variable frequency drives are widely used to control the speed of ac motors.  This note looks at the mechanisms which enable drive units to control the...

Operational Amplifier

The fundamental component of any analogue computer is the operational amplifier, or op amp. An operational amplifier (often called an op-amp,) is a high...

Cable Insulation Properties

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

Laplace Transform

Laplace transforms and their inverse are a mathematical technique which allows us to solve differential equations, by primarily using algebraic methods...

Difference Between Live and Dead Tank Circuit Breakers

A quick post in connection with an email question: Live Tank - the circuit breaker the switching unit is located in an insulator bushing which is live...

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...

GE's Shingijutsu Factory

GE's latest thinking on product manufacturing is he Shingijutsu philosophy or Lean production system. They have started applying this at the Louisville...

Material Properties

Everything physical in electrical engineering from insulations to conductors revolves around materials. Here we are listing common materials along with...

Harmonised Cable Codes and Colours

Within Europe the European Committee for Electrotechnical Standardization (CENELEC) has standardised the both the designation and colour of cables.   ...

Motor Starting - Introduction

Motor starting and its associated problems are well-known to many people who have worked on large industrial processes. However, these things are, of course...

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