Energy Efficiency Made Simple Vol IV

his article introduces a model where the service to develop the

cable models in the software is done by a third party and the

client then performs the simulations by means of remote access

to the service provider server.

The calculation of cable ratings can be divided into two distinct

sections.

The first is the calculation of the sustained or continuous steady

state rating and the second, the calculation of cyclic or transient ratings.

The calculations for the continuous rating of a cable are performed

Preventing damage to underground

cables

JJWalker and TR Becker, WalmetTechnologies

The current ratings of cable installations are normally determined

following the guidelines given in the international standards for

steady state rating and emergency ratings. An in depth knowledge

of the standards is required, which has resulted in only specialists

being able to do the calculations. It has been proved that the ratings

can also be determined using Finite Element Simulation (FES)

software but this again requires the programs to be available as

well as the knowledge to apply these programs.

by developing a model of the cable on the principle that heat flow is

analogue to the electrical current flow in an electrical circuit [1]. This

is based on the methods given in IEC 60287. The calculation of the

transient rating of a cable system is normally based on the lumped

capacitance method which involves drawing up a ladder network of

a cable and then reducing it to a two loop circuit for analytical calcu-

lation purposes [2]. Both the steady state and transient methods rely

on mathematical calculations that require a sound knowledge of the

theories involved.

A number of different computer models have been developed

in the past to allow the operator to input certain variables; software

will then do the calculations. A Dynamic Feeder Rating (DFR) System

[3] requires a continuous input of cable system data (temperatures,

currents, etc) to be available before the required calculations of the

emergency rating can be performed. Another system [4] uses the

mathematical functions and relies on the availability of information on

the cable system prior to the application of the step load.

Although these systems and methods will give the required

results, they have some disadvantages, namely:

• The computer programs are not freely available

• They require an advanced data acquisition and library system

Any cable carrying a current dissipates energy as heat. Heating a cable

to temperatures outside of prescribed limits can result in permanent

damage and put your entire distribution system at risk. Techniques

exist that allow you to manage the life of your cables by properly

predicting both steady state and transient cable temperatures. This

requires a sound understanding of the cable rating.

ENERGY EFFICIENCY MADE SIMPLE 2015