DC rating of AC wire

[thinker]

If we have a wire rated for AC 600V(RMS), can this wire be used for DC application with the DC voltage not exceeding the peak value of AC voltage (846V)? Is there any reference (beyond common sense) to allow to do such re-rating and to prove that this usage is correct and acceptable?

 

[glory102]

Well an RMS value is the equivalent steady DC (constant) value for AC voltage/current. Using this knowledge it would let one believe if the wire is rated for 600VRMS you can put a maximum of 600VDC on the line.

I would think anything over the RMS rated value would cause problems.

 

[davidbeach]

Unless there is a specific DC rating different from the AC rating, just use the AC rating. The instantaneous peak of a 600V RMS AC voltage may well be in the 850V range, but it is only there for a brief instant before reducing. A DC voltage in that same range would be there continuously. Big difference.

 

[electricpete]

If it was something like XLPE cable and had the possibility to become wet at some point, dc might not be such a good idea.

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(2B)+(2B)' ?

 

[electricpete]

I should add that there are some who think that we should never put dc on our cables because they might get damaged. I'm not talking about anyone on the forum. But a guy from our plant came back from a seminar. There is this scare we have talkd about regarding dc testing of cable and all kinds of various concerns have been raised about dc being potentially destructive at normal megger test (no hi-pot) values (separate from discussion of whether dc testing is effective). I always understood the concern related only to xlpe cables that may become wet (water treeing), but this guy was told it was pretty much any cable. Maybe the truth is somewhere in between.

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(2B)+(2B)' ?

 

[jghrist]

The potential problem with dc hi-potting of cable is that if there is a breakdown, indicating a failure at one point, the rapid change in dc field on the insulation can increase the size of voids or trees in another part of the cable insulaion. This could cause a reduction in the life of the cable, after the initial cable failure point is repaired or replaced. Applying dc voltage to a cable at a level that does not cause a breakdown does not cause a reduction in life.

 

[7anoter4]

If the Electric Field is applied in a slow rate rise method the dielectric break down intensity will be more elevated than
that when the ac field is applied. So, the dielectric strength will depend on voltage time rise. The insulation needs time to
withstand the field stress. The voltage time rise depends on resistance, inductance and capacitance.
Usually cable of 0.6/1 kV 50 Hz complying with IEC 60502-1 can be used for 0.9/1.5 kV dc. See:

http://www.dcc-cables.com/Media/Uploads/PDX-Sun.pdf

 

[electricpete]

jghrist - thanks. I tend to agree with you. The particular statement I mentioned from a colleauge applied to insulation resistance/ megger test voltages (typically dc voltage less than peak nominal ac line to ground). It may have also considered the possibility the cables may be wet. I was a little bit removed from the source of that info... figured someone here may have heard it as well. But it does seem pretty ridiculous... if it were true, then how could we ever use epr and xlpe cables in dc systems.

If the Electric Field is applied in a slow rate rise method the dielectric break down intensity will be more elevated than that when the ac field is applied.

I didn't see much to support that in the attachment. I saw max dc rating for the cable was just slightly more than sqrt(2) times the ac rms value which seems to roughly equate the stresses to the instantaneous volage.

I do find this excerpt from IEEE 400-2001 (the most recent rev of that standard!?!... even though a lot has changed since then) which tends to suggest that cables have equivalent breakdown stress to ac as to dc when they are uniform, but the relationships change where there are defects, moisture, terminations, etc.

Under ideal, homogeneously uniform insulation conditions, the mathematical formulas governing the
steady-state stress distribution within the cable insulation are of the same form for dc and for ac, resulting in
comparable relative values; however, should the cable insulation contain defects in which either the conductivity
or the dielectric constant assume values significantly different from those in the bulk of the insulation,
the electric stress distribution obtained with direct voltage will no longer correspond to that obtained with
alternating voltage. As conductivity is generally influenced by temperature to a greater extent than the
dielectric constant, the comparative electric stress distribution under dc and ac voltage application will be
affected differently by changes in temperature or temperature distribution within the insulation. Furthermore,
the failure mechanisms triggered by insulation defects vary from one type of defect to another. These
failure mechanisms respond differently to the type of test voltage utilized. For instance, if the defect is a void
where the mechanism of failure under service ac conditions is most likely to be triggered by partial discharge,
application of direct voltage would not produce the high partial discharge repetition rate that exists
with alternating voltage. Under these conditions, dc testing would not be useful. However, if the defect triggers
failure by a thermal mechanism, dc testing may prove to be effective. For example, dc can detect the
presence of contaminants along a creepage interface.
In the case of joints and accessories, their dielectric properties may differ from that of the cable with regard
to conductivity. This may result in a dc stress distribution at the interfaces between the cable and the accessory
that is very different from the stress under ac voltage. A careful examination of the system is necessary
prior to a dc test in order to avoid difficulties.
Testing of cables that have been service aged in a wet environment (specifically, XLPE) with dc at the currently
recommended dc voltage levels (see IEEE P400.1?) may cause the cables to fail after they are
returned to service (see Fisher, et al. [B23], and Stennis, et al. [B48]). The failures would not have occurred
at that point in time if the cables had remained in service and not been tested with dc (see Eager, et al. [B21],
and Srinivas, et al. [B47]). Furthermore, from the work of Bach, et al. [B7], we know that even massive insulation
defects in extruded dielectric insulation cannot be detected with dc at the recommended voltage levels.

When they talk about testing wet cables at currently recommended levels resulting in failures, I assume they are talking about hi-pot. I don't have access to IEEE P400.1.


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(2B)+(2B)' ?

 

[7anoter4]

I have to apologize. I agree with you electricpete it is no connection between first and second sentence in my above post.
Further more it is no connection between break-down test and hi-pot test since the first is executed on insulation specimen
and the second test on finished cable.
I agree with electricpete and with jghrist and I don't recommend any hi-pot test on cable already in operation- nor dc neither ac.
Hi-pot test has to be done on new installed cable according to standard requirements. They are authors sustaining the dc test is less destructive due to less corona or partial discharging phenomena.