MODELING OF OVERHEAD TRANSMISSION LINES FOR LIGHTNING OVERVOLTAGE CALCULATIONS PDF FREE DOWNLOAD LINK

ABSTRACT

Several parts of an overhead transmission line have to be included in a model adequate for lightning overvoltage studies:  wires (shield wires and phase conductors), towers, grounding, insulator strings and air clearances. An additional component,  the surge arrester, should be added if the study is aimed at determining the lightning performance of a transmission line  protected by surge arresters. This latter aspect is important since it can affect the model to be used for representing some  parts of the line.

Lightning is a physical phenomenon of random nature. The calculation of the lightning flashover rate  must be performed taking into account this aspect, as well as the random behavior of some line component; e.g., insulator  strings. The aim of this paper is twofold: on one hand, it presents a discussion about modeling guidelines proposed to  date for representing overhead transmission lines in lightning studies; on the other hand, it proposes a methodology for selecting the most adequate model of an overhead transmission line in lightning overvoltage calculations, when using a time-domain simulation tool, e.g. an EMTP-like tool. This methodology is applied to a test transmission line.

PREVIEW

MODELING GUIDELINES FOR LIGHTNING OVERVOLTAGE CALCULATIONS
Modeling guidelines for representation of overhead transmission lines in lightning overvoltage simulations have been proposed in many references [1, 5-7]. These guidelines, irrespectively of the study goal, can be summarized as follows (see Figure 1):
1) Shield wires and phase conductors of the transmission line can be modeled by several spans at each side of
the point of impact. A rigorous representation of each span should be based on a multi-phase frequency dependent untransposed distributed-parameter line model [8].

However, for lightning overvoltage calculations, a constant-parameter line model can be accurate enough, and parameters are usually calculated at 400-500 kHz [5]. A line termination at each side of this model is needed to avoid reflections that could affect the simulated overvoltages around the point of impact.

This termination must be represented accordingly to the model chosen for the line spans; for instance, if the line spans are represented by line sections with constant and distributed parameters calculated at 500 kHz, the line termination could by a long enough section, whose parameters are also calculated at 500 kHz.

CONCLUSION

Models to be used to determine arrester energy stresses must have some differences with respect the models to be developed for flashover rate calculations.

-incorporate more spans than models required for flashover rate calculations. A frequency-dependent model is required when the goal is to estimate arrester energies, although a constant parameter model will suffice for flashover rate calculations.

-various approaches developed to date can have some influence on both flashover rates and arrester energy
stresses.

-aspect for calculation of overvoltages and arrester energy stresses caused by strokes to a shield wire or a tower. Differences obtained with different modeling approaches (constant vs. nonlinear and variable) can
be very significant.

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