5. Modeling transmission lines, cables and transformers
5.1 Transmission lines

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One of the EMTP's special features is to efficiently calculate phenomena in transmission lines. For actual multiphase overhead transmission lines, the following models are applicable.:
(Multi-stage) lumped PI model
Features:

Highly reliable
Only for relatively low frequency of phenomena or for short lines
No limitation for calculation time step
Effect by lumped capacitance may be introduced.
No frequency-dependent feature (For fixed frequency only)

Distributed parameter, Transposed type model
Features:

Computationally efficient calculation
Practically enough accuracy for general transient calculations of limited frequency range.
Limitation for calculation time step (shall be shorter than the travel time.)
No frequency-dependent feature (For fixed frequency)
Nothing for un-transposed feature --- only averages of (three) phases are applied.

Distributed parameter, Un-transposed type model
Features:

Same as the before except for un-transposed feature
Useful where line unbalance is important

Frequency dependent model --- J. Marti model
Features:

Accurate for wide frequency range
Computationally relatively efficient calculation
Limitation for calculation time step
May be inaccurate for very low frequency range

In EMTP, these model data are directly calculated in the sub-routine named LINE CONSTANTS, introducing geometrical parameters of transmission lines etc. The data are directly applicable to calculations. In the example attached, square shaped voltage wave transmitting responses are calculated for four kinds of line models, --- One stage PI, Five stage PI, Distributed parameter untransposed and Frequency dependent (J. Marti) models of 210km UHV (1100kV) transmission line.
In EMTP'.s distributed (constant) parameter model, a transmission line is modeled by two pieces of loss-less lines and loss equivalent resistors connected in series at the middle and the both ends. Therefore in the example, step-shaped voltage appears. For making clear the difference an extreme case, i.e. applying square shaped wave, is shown. Applying more low frequency of wave, the difference is less.
Appropriate model should be applied. At the same time care should be taken that each version of EMTP may still have each bug.

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This page is based on Prof. E.Haginomori's lectures in Tokyo Institute of Technology, and edited by Japanese ATP User Group. Copyright (C) Eiichi Haginomori and Japanese ATP User Group.