Energy Efficiency Made Simple Vol IV 2015

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.

Preventing damage to underground cables JJWalker and TR Becker, WalmetTechnologies

T 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 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

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ENERGY EFFICIENCY MADE SIMPLE 2015