It is recommended to always use the default parameters of the Frequency Dependent (Phase) Model for setting curve fitting options.
However, there may be occasions where it is required to change the default parameters for certain overhead line/underground cable configurations. These should be changed very carefully. A poor parameter selection may cause inaccuracies in the simulations, or sometimes instabilities due to passivity violations. The following table gives a practical guideline to selecting the proper curve fitting parameters.
Parameter |
Typical Values |
Possible consequences as a results of significant deviation from the typical values |
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An increase |
A decrease |
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Curve fitting starting frequency |
1 Hz 0.5 Hz 0.1 Hz 0.01 Hz |
(e.g. 10 Hz)
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(e.g. 0.01 or 0.001 Hz)
(A very low value, e.g. 1e-4 Hz)
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Curve fitting end frequency |
1 MHz |
(e.g. 10MHz)
|
(e.g. 10kHz)
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Total number of frequency increments |
100 |
(e.g. 500)
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Maximum order per delay group for propagation function |
20-40 |
When plotted as a function of frequency, the angle of the propagation function shows an oscillatory behaviour (due to the transportation delay of the transmission line). Hence a higher order approximated function may be required to obtain an accurate fitting (compared with Yc). |
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Maximum order of fitting for Yc |
20-30 |
Compared with propagation function, curve-fitting of Yc is relatively easy and hence accurate fitting may be achieved less than 20 poles for many transmission line configurations. These values may be increased based on fitting results. |
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Maximum fitting error for Prop. function or Yc |
1%-0.1% |
(e.g. 10%)
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(e.g. 0.01%, 0.001%)
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A common issue using any phase domain model is that the possibility of unstable simulations due to passivity violations. This is mainly caused by the error introduced by the curve fitting of the transfer function. In such cases, the below suggestions may help to obtain the stable simulation.