Dc voltage pulse injected through an energized ac cable

[Amryan]

Hi guys,

I want to ask a silly question but i don't get it yet. So for example I have a 10 kV ac voltage cable. And suddenly I applied a dc voltage pulse signal (like TDR). Is the signal will disrupt the ac voltage?

 

[FvM]

Not silly, just unclear. What do you mean with "apply a DC voltage pulse"? Disconnect the AC power source before? Superimpose the pulse to the AC voltage? Or something different?

By the way, a TDR pulse isn't a "DC voltage".

 

[Amryan]

Not silly, just unclear. What do you mean with "apply a DC voltage pulse"? Disconnect the AC power source before? Superimpose the pulse to the AC voltage? Or something different?

By the way, a TDR pulse isn't a "DC voltage".

No without disconnect the ac power... Like superimpose the pulse to the ac voltage...

I see the TDR is not a dc pulse. Because I looked into this website

https://www.epanorama.net/circuits/tdr.html

They use dc voltageas an input and i think the output is dc as well ( i don't see any inverter).

So if I want to use tdr circuit on that website but using ac voltage can I just simply change the input?? Or I should change some parts as well

 

[FvM]

I suggest to distinguish two points:
- the operation of the circuit in the link
- is there any way to perform TDR measurements on a powered 10 kV cable

The circuit generates short rectangular pulses of a few volts, as shown in the waveform diagrams. There's no way to operate this simple circuit on a powered AC line.

It's most likely possible to make TDR measurements on a powered HV AC cable. Besides a suitable signal coupler, you'll need a sufficient high signal voltage that can be detected over the AC voltage and respective noise level. Instead of a simple rectangular pulse, the measurement would use a complex signal that's optimized both to the limited cable bandwidth and expectable noise spectrum.

 

[Amryan]

I suggest to distinguish two points:
- the operation of the circuit in the link
- is there any way to perform TDR measurements on a powered 10 kV cable

The circuit generates short rectangular pulses of a few volts, as shown in the waveform diagrams. There's no way to operate this simple circuit on a powered AC line.

It's most likely possible to make TDR measurements on a powered HV AC cable. Besides a suitable signal coupler, you'll need a sufficient high signal voltage that can be detected over the AC voltage and respective noise level. Instead of a simple rectangular pulse, the measurement would use a complex signal that's optimized both to the limited cable bandwidth and expectable noise spectrum.

Here is another question. My supervisor ask me to build a line distance measurement in ac power cable (without shutting down the line). So i have reading for couple weeks. So I came up with using a pulse to measure the line through a contactless capacitive coupling device. So the question is, should I build a pulse generator or a tdr for measure that??

 

[FvM]

Your latest question makes much more sense than the confuse first post.

Yes, you'll need a coupling device, typically capacitive.

Defing a measurement setup would be a first necessary point. You won't get a reasonable measurement when tapping the cable in the middle, because waves are propagated towards both ends. Tapping it at the end or possibly at a transformer should work.

Secondly define the distance range and identify the transmission parameters of the cables of interest (line impedance, bandwidth, frequency dependent attenuation).

Finally determine the expectable cable noise level and necessary pulse respectively RF generator voltage.

Consider that sensitivity can be increased by repeated measurements and digital signal processing. Or perform a reflection measurement in frequency domain and transform to TDR.

I presume, there will be some literatur related to this application.

 

[Amryan]

Your latest question makes much more sense than the confuse first post.

Yes, you'll need a coupling device, typically capacitive.

Defing a measurement setup would be a first necessary point. You won't get a reasonable measurement when tapping the cable in the middle, because waves are propagated towards both ends. Tapping it at the end or possibly at a transformer should work.

my supervisor told me that there are 2 capacitive coupling sensor installed in ther overhead transmission line. So all I need is to add a pulse into it

Secondly define the distance range and identify the transmission parameters of the cables of interest (line impedance, bandwidth, frequency dependent attenuation).

about that, I don't know the cable yet because, he said I need to prove that the pulse can successfully transferred to the cable using the contactless sensor through simulation. So that's why I need to decide whether to use TDR or a voltage pulse generator

Finally determine the expectable cable noise level and necessary pulse respectively RF generator voltage.

Consider that sensitivity can be increased by repeated measurements and digital signal processing. Or perform a reflection measurement in frequency domain and transform to TDR.

The supervisor specifically told me to use Time of flight method to measure the line distance.

I presume, there will be some literature related to this application.

For line distance measurement, I couldn't find any literature about it. I only found about capacitive coupling, pulse generator(I still need to learn about the basic pulse generating circuit) and TDR(but as you can see I found the wrong sample about TDR).

 

[FvM]

Overhead transmission line is a bit different from cable, e.g. in terms of impedance. You have been asking for cable TDR.

Overland-line impedance can be estimated from basic text book formulas. Distance range and coupler capacitance should be known.

I don't see a difference between "TDR" and "voltage pulse" generator. A TDR measurement circuit is comprised of a pulse generator, a splitter like resistive divider in the link and a receiver circuit. For HV application, two separate coupler insted of a splitter might be preferable. The question is about required pulse voltage and a suitable receiver circuit.

 

[Amryan]

Overhead transmission line is a bit different from cable, e.g. in terms of impedance. You have been asking for cable TDR.

Overland-line impedance can be estimated from basic text book formulas. Distance range and coupler capacitance should be known.

I don't see a difference between "TDR" and "voltage pulse" generator. A TDR measurement circuit is comprised of a pulse generator, a splitter like resistive divider in the link and a receiver circuit. For HV application, two separate coupler insted of a splitter might be preferable. The question is about required pulse voltage and a suitable receiver circuit.

That's the one I have to find out because the voltage pulse must not triggered the over voltage relay or over the cable specification. That's why I asked about dc pulse voltage injected to a ac power cable (without shutting down the line) because all of the theory that I could find is in dc... Is there any ac schematic about TDR or pulse generator??