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Conductor calculations based on skin effect

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300.What method do you use for calculating the AC resistance of ACSR conductors?
The Method of calculation refers to IEC 61597. AC Resistance of the conductor is calculated from the DC Resistance at the same temperature. The DC resistance of a conductor increases linearly with the temperature , according to the following equation R
T2
=R
T1
[1+α(T
2
-T
1
)]
Where R
T1
is the DC resistance at temperature T
1
R
T2
is the DC resistance at temperature T
2
α is the temperature coefficient of electrical resistance at temperature T
1
In this case the temperature T
1
is considered as 20
o
C and hence α = 0.00403 K
-1
for Aluminum Al1350 and DC Resistance at 75
o
C is calculated using the DC Resistance of conductor at 20
o
C. The AC Resistance at 75
o
C is thereafter determined from the relation R
ac
=K
j
R
dc
Where K
j
is factor taking into account the increase in resistance due to skin effect. The value of Kj is estimated from the method given in Aluminum Electrical Conductor Hand book of Aluminum Association 1988, using the following reference curve.
301.For the ACSR AC resistance calculation, what factors are included?
As stated above, the main factors influencing the AC resistance are the overall conductor DC resistance , the thermal coefficient of electrical resistance for the conductor , power frequency and conductor dimensions.
302.What method do you use for calculating the short-circuit rating of the conductor?
The short circuit current is calculated based on the method given in Aluminum Electrical Conductor of Aluminum Association inc. from the heat capacity of the conductor, permitted maximum conductor temperature of the conductor. The general formula employed is For ACSR
I
= 0.0862 A/ √t & AAC
I
= 0.0671 A/√t where
I
= current in Ampers
A
Cross sectional area in cmil and
t
time in seconds. Please refer to the calculation template as attached.
303.What method do you use for calculating the continuous current rating of the conductor?
The continuous current rating of the conductor is calculated using the equation (8) given in IEC 61597 Clause 3.8. I
max
=[(P
rad
+P
conv
-P
sol
)/R
T
]
1/2
This equation is derived from the heat balance equation P
j
+P
sol
=P
rad
+P
conv
Where P
j
is the heat generated by Joule effect P
sol
is the solar heat gain by conductor surface P
rad
is the heat loss by radiation of the conductor P
conv
is the convection heat loss.
A typical calculation for AAC1350 Neptune19x3.25mm conductor is given as annexure to this document,

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