How It Works Real Time Sag

Mar 2011

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AMPACIMON for Real Time Monitoring

Dynamic Rating Ampacity

Definition of Sag

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The sag “f” is the largest vertical distance between the conductor and its (imaginary) chord

Definitions : Horizontal distance between two attached ends: a [m], difference level h [m], sag f [m], lowest point abscissa xA [m], conductor mass per unit of length: mC [kg/m], gravity g [m/s²], chord angle ψ[dimensionless], H: horizontal tension, tension component along x axis [N].

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Sag – Cable Vibrations Conductors of overhead lines, like any

cables may vibrate and they do because of wind excitation.

Now, just like a music instrument, the vibration frequency spectrum is a mix of harmonics or octaves, i.e. the sum of frequencies that are exact multiples of the cable fundamental frequency, with varying amplitudes.

And as for music instruments, the cable fundamental frequency depends on tension, length and mass. Typically the fundamental frequency is 0.15 Hz.

The nice thing is that sag can be expressed in relation with fundamental frequency without any other parameter !

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Sag – Vibration Signature

In order to extract the fundamental frequency from the vibration spectrum, a Span Vibration Signature is needed. This signature is constructed over a period of typically 2 weeks under normal line operating conditions. There is absolutely no need for calibration.

The fundamental frequency is obtained by matching the actual vibration spectrum to the vibration signature. Note that as a result of the constantly varying wind excitation, the amplitudes of the frequencies composing the spectrum may vary a lot or even disappear. Thanks to the richness of the signature this is not a problem.

The span sag is calculated simply from a direct relation with the fundamental span frequency not needing any other parameter.

Note that the monitors work fine both on single conductor spans or on bundles.

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Sag – Vibration Signature

FFT Fast Fourier Transform

Conductor Vibration

RT Span Sag =

Ampacimon Monitor

Frequency Spectrum Analysis

Acceleration Sampling

Fundamental Frequency

Span Frequency Signature

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Vibration Analysis => Sag

No conductor data

No topological data

No sagging data

No weather data

= No Calibration !

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Live !

Topographic validation of Sag

Mar 2011

Typical: < 10 cm Max : 20 cm

No calibration needed !

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Sag Evolution over one Month

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