The question is related to calculating the current rating from heat flow equations in IEEE Std. 738-2012.
I = sqrt((q_c+q_r-q_s)/(R))
If the solar heat gain (q_s) is greater than q_c and q_r the resulting value of 'I' will become imaginary. In such a situation, how should I make sense of this imaginary value? What approach should I consider to calculate the line current rating under such a scenario?
Thanks
You should make sense of this imaginary value using analytic approach.
Best regards,
Syahrial Shaddiq
Mathematical formula was developed for, short time over loading(based on life consuming cost) on measured temperature(hot spot cooper wire(winding)) could be used in your case, refer my papers.... ,
1. Dynamic line ratings is evaluated using worst-case parameter values.
2. Dynamic line rating strategy is quite different since the predetermination of the worst-case weather condition need only apply for the next 5 to 15 minutes. Clearly, dynamic ratings are likely to be higher than the static rating but have the disadvantages of volatility and unpredictability
The physical meaning is that the conductor temperature will increase and therefore the line ampacity will decrease due to the low effect of convection cooling.
Also emissivity and absorptivity values are generally correlated, so I would suggest to refer to IEEE-738 and CIGRE-299 to have a better understanding.
As the conductor temperature increases, qr&qc increase, until the heat equation (qr+qc=qs+ri2) is balanced.
To put it briefly, the chosen maximum allowable conductor temperature (TS) should probably increase in order to result a real value for the ampacity.
Dear Arif, It may be mistake in your calculation. The Qc+Qr-Qs relation is a thermal balance equation. If Qs is bigger than Qc+Qr, it shows that the conductor equilibrium temperature is bigger than rated temperature. (I=0) This means even without current flowing, the conductor has a temperature more than 90 or 100C. It is very strange.So I think you have a mistake in your calculation.
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