Measuring currents in single core HV cables

[AusPowEng]

In a recent discussion with a colleague, we were looking at ways to measure the magnitude of armour current in a numner of parallel single-core HV cables with having to break-in between the armour and the earthed gland.

With the use of flex-CT's (i.e. rogowski coils) below (CT2) and above (CT1) the cable screen-to-earth lead we wondered if we could calculate by waveform substraction (CT1-CT2) the magnitude of screen current (perhaps?). The terms 'Above' and 'below' are in relation to a bottom entry cable in this description.

The conversion then went on to placing another flex-CT around the entire cable (CT3) and subtracting the CT2 waveform to determine armour current.

My colleague then said 'hang-on CT3 will measure nothing because currents on the conductor, screen and armour will balance' so we abandoned the idea.

Thinking about it again, I do not think this is true. The currents in the screen and armour depededant on cable profile, the specific materials used, current in the conductor and and very complex to determine. Furthermore, I would expect phase shift and of course there are different voltages associated with each conducting layer.

Nevertheless, from a practical understanding there cannot be magnetic field cancellation because that same single cable must be installed carefully (i.e. cannot be taken through steel gland plates) due to eddy currents.

Any comments would be welcomed.

 

[jghrist]

The currents on the conductor, screen and armour will only balance out if there is no current in the other phases.

 

[AusPowEng]

jchrist,

In this instance there are 3 per phase 3 x 1C cables without transposition or cross-bonding of screens along the run and with both screen and armour earthed at each end.

 

[waross]

This raises another question.
Will the currents in the shield and armour be at the same phase angle as the currents in the cable?
respectfully

 

[AusPowEng]

Waross,

Excellant question. I was alluding to this also and I think the answer is no, the are not in phase. My computer-based analysis reveals quite different phase angles and I supposed this makes sense when you consider the screen current has components that are induced from the current flowing the primary conductor and capacitive current coming across the insulation.