cable AC resistance 1

[hhhansen]

Hello
I have performed measurements on a 50 kV, 3x1x1200 mm2 AL PEX cable with 96 mm2 CU screen.

The cable system has a length at 6700 m. At the midpoint the screens are earthed to reduce screen voltage the system has earthed screen at both ends. Cables are laid in flat formation.

A VLF test has been made indicating no abnormalities.

The AC resistance is specified as 0,24 Ohms/phase.

We measure a value of 0,75 Ohms/ phase that is 3 times specified value !

The DC-resistance is measured to 0,4 Ohms corresponding to specified value.

What might be the reason that we measure 3 times AC resistance compared with cable supplier spec.

Best regards
HH

 

[waross]

Grounded screens mid point.
This allows induced current to flow affecting the AC resistance test.
Screens should be rolled or transposed in equal thirds and grounded at one point only.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[hhhansen]

Grounding at the 2 end point are also just fine allrhough circulating screen current affects the AC-resistance to a much less extend compared with this problem.

Waross are you able to make a equivalent circuit diagram which explains the principle of the problem with midsection earthed.

Thank you

B.R. Hans-Henrik

 

[dpc]

How can the specified ac resistance be lower than the specified dc resistance?

 

[jghrist]

The OP said that the measured dc resistance was higher than the specified ac resistance. This indicates either an error in the specification or in the measurement.

 

[waross]

This allows induced current to flow affecting the AC resistance test.

Read my blips.

Grounding at the 2 end point are also just fine allrhough circulating screen current affects the AC-resistance to a much less extend compared with this problem.

Not much effect on the actual resistance but more effect on the measurement.
Imagine testing a single turn, air core transformer (which this is.)
Do you expect the same readings with the secondary shorted?

Frequent practice #1. For shorter cables:. Ground the supply end and insulate the load end of the screens.
Frequent practice #2. For longer cables where high screen voltage may be an issue: At the 1/3 distance and at the 2/3 distance;
Cut the screen on all phases.
Connect incoming A phase screen to out going B phase screen.
Connect incoming B phase screen to out going C phase screen.
Connect incoming C phase screen to out going A phase screen.
On extremely long cables, the cable may be divided into more than one section and then each section treated in thirds.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[7anoter4]

In my opinion the d.c. measurement was done using 2 phases: one for + and second for -.
According to IEC 60228 Table II class 2, at 20oC, the maximum resistance has to be 0.0247 ohm/km for 1200 mm^2 aluminum. Then 0.16549 for 6.7 km. 0.4 ohm it is more than twice.
The a.c. resistance[skin effect and proximity effect calculated according to IEC 60287-1-1] it is 0.033298 ohm/km [0.0319 according to a manufacturer] or 0.223 for 6.7km.
Since the resistance depends on charging current the capacitance is very high here and the current decrease very slowly -in the d.c. case.

 

[Nayeem1993]

HV cable screens should be grounded at one end, not both ends. If it's grounded at both ends then there will be an induced current flowing through the screen which will heat up the screen- resulting in deteriorating the insulation of cable.
Normally AC resistance is 60% higher than DC resistance due to skin effect. but as you have grounded the both end of cable screen, due to higher reluctance between cable screen and conductor, reactance will be higher-increasing in AC impedance.

 

[waross]

And skin effect may drive more of the current to flow in the screen then is calculated by the parallel resistance formula, further degrading the accuracy of the test measurement.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!