As a rule of thumb for transmission lines, we know that short lines (shorter than 80 km) are modeled as series impedance, medium lines (80-250 km) are modeled as PI section, and long lines (>250km) are modeled using distributed line models.
Now, when we perform harmonic (higher frequency) analysis on distribution network with underground cables (which have large parasitic shunt capacitance), the wavelength dramatically decreases.
Is there any study to estimate in what frequency or line length (or in general in what multiplication of the wavelength) we consider distributed model for distribution lines for harmonic analysis?
An under ground distribution network practically have localized Distribution transformer and 11 kV network on the primary side of it. Its better to study power quality issues including harmonics or even the sag, swell, transient disturbances etc. at the distribution network itself considering it as point of common coupling (PCC). distribution cable length from the transformer secondary to its consumer (which are modeled to produce harmonics for example) should be assumed in terms of few hundred meters to few kms only.
I would like to give the following explanation to your case.
All the lengths of waves and their shares (half, quarter etc) including ones determined use of modeling way (lumped or distributed parameters, pi-sections etc) are given for industrial frequency. In my opinion it should be more correct to determine these lengths for the highest frequency appeared in your problem.
Please also note that for cable lines takes place less propagation velocity of electromagnetic waves in comparison with aerial ones. This also leads to decreasing of line's wave length.
Thank you Dr. Tahir, and Dr. Naik.
Your responses were very helpful.
Now, we know that if the distribution line length is comparable with the wavelength, we should consider distributed model for the lines instead of lumped model. My original question could be restated as:"what do we consider comparable?" Assume, for a distributed line the wavelength could be written as 100/n km (where n is the harmonics number). In what length shall we use distributed model for such network?
Dear Amin,
both models, lumped and distributed parameters, are accurate and reliable depending on the wavelengths, as the other comments have already noted. You can get accurate results using lumped parameter models if the wavelength is at least 10 times the line length. The same thumb rule can be applied for choosing the distributed parameters model, if the signals have wavelengths below one tenth of the line length, you should employ a distributed model.
Thank you Dr. Pavas,
Could you please provide me with any document that I can cite for this rule of tenth of wavelength?
Bests,
Amin
Recently I have been involved in a project where modeling of power line communication devices was required. I asked some experts on communications, they suggested such criterion and seemed to be reasonable, but I have not checked references. However, I would always suggest Paul Clayton's book on modeling multitransmission lines or his book "Introduction to Electromagnetic Compatibility". Probably you will not find such thumb rules anywhere, the best would be to try some simulations and pick the best length partition.
I hope you find the best solution for your problem.
Regards, AP
Dear Dr. Pavas,
I found your response very helpful and complete.
Thanks,
Amin
- Badges
- Science topic
- Similar topics
- Computer Communications (Networks)
- Simulators