Determination of point and nature of transmission lines’ damage by pulse method

During the transmission line exploitation because of different causes faults arise, which lead to connection malfunction. Therefore one of the most important task to attain at TL exploitation is a determination of point and nature of damage for the purpose of its instant correction.

Line faults according to their nature are broken into the following categories:

– conductor’s break;

– conductor’s fault;

– conductors’ insulation resistance reduction;

– conductors’ connection to the earth.

On revealing faults their nature should be found out, and then the distance to the place of the damage is determined by measuring instruments.

In this work a pulse method is examined to determine a location and nature of TL fault.

A pulse method of TL measurement is based on a phenomenon of electromagnetic pulse reflection at locations of obstacles through the changes in a TL surge impedance. TL obstacles appear as a consequence of its quality or being damaged.

The method is implemented in Р5-10 and its analogues and consists of the following: in a TL circuit, for instance in a cable, voltage pulses are transmitted (monitoring pulses), which propagating through the line partially reflect from a surge impedance obstacles and return to the sending end where they are displayed on the CRT screen with a time sweep. Signals which are reflected from obstacles are time-shifted and the size of the shift between the reflected signal and the monitoring pulse on the screen is proportional to the distance to the obstacle.

A measurement by a pulse method allow to get fast and accurate results especially in case of break, short, conductor’s connection in multiple-wire systems.

An obstacle is characterized by a reflection factor:

, (2.1)

Here U _ref, U _mon – amplitudes of reflected and monitoring pulses;

Z_sim – rated value of TL surge impedance;

Z – value of the surge impedance in the place of the obstacle.

A reflected signal’s absence on the CRT display indicates obstacles’ absence in the line. In this case the reflection coefficient is equal to zero in every point of the line.

If the obstacle is caused by an increase in the surge impedance then the pulse reflected from the obstacle has the same direction with the monitoring one . In case of line’s break (the extreme case) in the point of break the total reflection without the pulse direction reversion takes place .

If the obstacle is caused by decrease in line’s impedance then on reflecting the pulse reverses its direction to the opposite one . In case of short circuit the fault total reflection of the pulse with reversing direction to the opposite one takes place .

It is clear that the reflected signal’s amplitude doesn’t exceed the monitoring pulse’s one therefore a reflection coefficient can only possess the value from –1 to +1.

A distance to the obstacle (short, break, cable insertion, smooth variation of impedance) is determined by a latency time t _lt of the reflected signal relative to the leading edge of the monitoring pulse.

Each line has its own propagation velocity of a pulse signal. It is defined by primary transmissions parameters, which depend on a type of dielectric, a cross-section and a material of conductors.

Knowing (or having already measured) pulse propagation velocity n and having measured latency time tlt, the distance to the obstacle can be determined as:

. (2.2)

Depending on a length of a line being measured and its attenuation, a duration of a monitoring pulse, which is sent to line, can be chosen by use of a switch «Mon. pulse μs».

Use of narrower pulses makes it possible to study a line more thoroughly; in this case an accuracy of distance measurement is improved (an increase in device’s resolution). However it is known that narrow pulses have wide spectrum and attenuate highly during propagation. Therefore narrow pulses are not applicable to study lines with high self-attenuation, because propagating in line they will be received too damped, which may complicate their detection against noise.

Thus decrease in duration of pulses leads to accuracy improvement but it restricts distance. Increasing duration of pulses lowers accuracy of measurement but distance of measurements increases.

Reading the distance is carried out taking into account a velocity of pulse propagation in the line. The propagation velocity is connected with a wavelength shorting coefficient ξ through an expression

, (2.3)

Here с is the electromagnetic constant, c = 300000 kmps;

n – pulse propagation velocity in line of a certain type, kmps.

You can set the wavelength shorting coefficient by the knob «Shorting».

Taking pulse measurements on lines with unknown shorting coefficient it can approximately be calculated as

, (2.4)

Here e is dielectric constant of conductor’s insulation.