IEC TR 61597-2021 pdf free download – Overhead electrical conductors – Calculation methods for stranded bare conductors.
Based on these values at 20°C, the DC resistances have been calculated for temperatures of50C.8o C and 1oo °c.
The AC resistance is calculated from the DC resistance at the same temperature. Calculationmethods are in [1],[2],[3].[4].Clause A.2 gives an example based on [1].
The AC resistance of the conductor is higher than the DC resistance at the same temperature.The cause of this phenomenon can be explained by the fact that the inner portion of theconductor has a higher inductance than the outer portion because the inner portionexperiences more flux linkages. Since the voltage drop along any length of the conductormust be necessarily the same over the whole cross-section, there will be a currentconcentration in the outer portion of the conductor,increasing the effective resistance.
Various methods are available for computing the ratio between AC and DC resistances ([1].[2],[3],[4] .
For conductors having steel wires in the core (Ax/Sxy or Ax/xySA conductors), the magneticflux in the core varies with the current, thus the AC/DC ratio also varies with it,especiallywhen the number of aluminium layers is odd,because there is an unbalance ofmagnetomotive force due to opposite spiraling directions of adjacent layers.
Although this magnetic effect may be significant in some single layer Ax/Sxy conductors andmoderate in 3-layer conductors, the values of AC resistances for these types of conductorshave been calculated without this influence. Further information and a more completecomparison and evaluation of magnetic flux and unbalance of magnetomotive force may befound in chapter 3 of [ 1].
There are other factors with minor influence on the conductor electrical AC resistance，e.g.hysteresis and eddy current losses not only in the conductors but also in adjacent metallicparts,and they can be estimated by actual tests.The method in [6] takes into account theabove factors.
6.3Inductive reactance
The inductive reactance of conductors is calculated considering the flux linkages caused bythe current flowing through the conductors. In order to make computations easier，theinductive reactance is divided into wo parts:
a) the one resulting from the magnetic flux within a 0,3 m radius;
b) the one resulting from the magnetic flux from 0,3 m to the equivalent return conductor.NOTE Exact number is 0.304 8.
This separation of reactances was first proposed by Lewis [1] and the 0,3 m radius has beenused by all designers and conductor manufacturers and is herein adopted in order to allow acomparison between the characteristics of the new conductor series and old ones.
The advantages of this procedure are that part a) above is a geometric factar (function ofconductor dimensions) while part b) depends only on the separation between conductors andphases of the transmission line. As stated earlier in this clause, only the first term a) is hereiniisted and part b) can be obtained from the usual technical literature.
For conductors with steel core (Ax/Sxy or Ax/xySA designations), the magnetic flux in the coredepends on the current and this influence,as far as the inductive reactance is concerned,canbe considered negligible for conductors with three aluminium layers and more or with evennumber of layers. For single-layer Ax/Sxy type, the effect is not negligible and X is usuallydetermined after tests on complete conductor samples.
As there are no available results from tests carried out on conductor samples of the new lECseries，X for single-layer conductors has been estimated in comparison with experimentalfigures obtained for usual Ax/Sxy designations (old ACSR) at 25 C,published by theAluminium Association.These values are accurate within 3 %.
6.4Capacitive reactance
The capacitive reactance can also be divided into two parts:
a) the capacitive reactance for 0,3 m radius;
b) the capacitive reactance from 0,3 m to the equivalent return conductor.IEC TR 61597 pdf download