Calculation of the cyclic and emergency current rating of cables. Part 2: Cyclic rating of cables greater than 18/30 (36) kV and emergency ratings for cables of all voltages
|Publication Date:||1 July 1989|
|ICS Code (Cables):||29.060.20|
Introduction and scope
This standard gives manual methods for calculating cyclic rating factors for cables whose internal thermal capacitance cannot be neglected; in general this applies to cables for voltages greater than 18/30 (36) kV. It also gives a method for calculating the emergency rating for cables of any voltage. Part I of the standard dealt with cyclic rating factors for cables of voltages not greater than 18/30 (36) kV where the internal thermal capacitance could be neglected.
To determine the cyclic rating factors where the internal thermal capacitance cannot be neglected it is necessary to calculate the transient temperature response of the cable and its environment.
The formulae recommended in this standard contain quantities which vary with cable design and materials used. The values given in the tables are either internationally agreed, for example, resistance temperature coefficients, or are those which are generally accepted in practice, for example, thermal resistivities and volumetric specific heats of materials. In order that uniform and comparable results may be obtained, the cyclic andíor emergency current ratings should be calculated with the values given in this standard. However, where it is known with certainty that other values are more appropriate to the materials and design, then these may be used, and the corresponding cyclic/emergency current rating declared in addition, provided that the different values are quoted.
No account has been taken at present of possible effects of moisture migration on the thermal resistivity of the soil. This is in line with the approach already adopted for the calculation of steady state ratings in uniform soil as given in IEC Publication 287. Methods for dealing with regions having different thermal characteristics and with drying out of the soil are under consideration.
A means of incorporating the effect of change in conductor resistance with temperature is included.