Flex_CT uses 2

[heberovchem]

I would like to know what would be the possible safety issues in using a common CAT IV flex-CT (Rogowski) around a 34.5kV insulated cable (not exposed). Theoretically I understand that the amount of current through a 34.5kV is not so high, but during a fault this may change. Is it more of an accuracy issue with the reads, BIL , or is it really a hazard? I understand the cable might be punctured too. I would like to thank you in advance for any help.

 

[GroovyGuy]

Howdy,
It is very common to utilize CTs that are not fully rated for the "primary circuit' voltage. Most donut-type MV switchgear CTs are nominally rated 600V (by themselves), but rely on other means to provide the required insulation. As long as the Rogowski coils are installed around the non-shielded but insulated portion of the power cable, and not the bare conductors, you will be OK. I assume that the coils has UL or some other recognized form of approval.
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[bacon4life]

At 34.5 kV, the cable better be shielded rather than just insulated! If you are installing the CTs while the cable is energized, there would be the routine safety issues of handling live equipment.

 

[heberovchem]

Greetings,

Thank you @GroovyGuy, this was what I imagined. Now bacon4life got me curious:
@bacon4life Aren't most of UG cables shielded, though? Reading upon it, I found that it is to decrease inductive reactance. Follow-up: Do you know how shielding decreases the inductive reactance (I have no clue, maybe reduce the magnetic flux)? And how is that a big safety hazard (not being shielded) in the Flex-CT scenario?

Thanks!

 

[GroovyGuy]

Obviously any 35kV UG must be shielded. This is a code requirement. BTW, is this application for a single-conductor cable or a 3-conductor cable? Is this a concentric neutral cable?

I was also pondering my earlier statement wrt placement of the CT, on a non-shielded portion of the cable (ie between the stress-cone and the lug). I believe that you can install the CT on any shielded or non-shielded portion of the cable. I recall that we routinely install a zero-sequence CT (ZSCT) around the shielded portion of a 3-conductor MV TECK-type cable assembly. This ZSCT can pick up ground-faults currents of 1 ampere. Note, in this case the shield drain wires are passed back through the ZSCT.
Comments anyone?
GG


"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[heberovchem]

@GroovyGuy: This is to test 34.5kV three-phase cables (3 wires, delta configuration, and there is no neutral involved). And surely not near lugs or stress-cones (it would be interesting to be sure if it could be done around them, as you brought up). I'd love to hear what bacon4life has to say, since he came up with the comment on shielded cables.

Regards,

 

[ScottyUK]

In switchgear it is common to provide a section of conductor with an earthed electrostatic screen which does not carry any image current as a cable shield might. Standard low voltage CT's (and in principle Rogowski coils) can be fitted over the earthed electrostatic screen. The presence of the screen prevents any capacitive coupling from the HV cable conductor to the LV devices, while having no influence on the magnetic field around the conductor and hence not degrading the accuracy. Without the screen it is entirely possible for direct capacitive coupling to occur with potentially dangerous results.

 

[heberovchem]

@ScottyUK so is the capacitive coupling a possibility in the testing scenario I presented due to the image current the shielding can carry even if the cable is insulated? Could share a diagram or nice website with information on capacitive coupling so I could understand better about the potentially dangerous results?

 

[ScottyUK]

Well if you want to measure the conductor current you'd need to mount the coil closer to the end of the cable than where the bonding conductor earths the shield. With most terminations this will mean mounting the coil over the insulated conductor but without any earthed material between the Rogowski coil and the metal conductor - only insulation either as the cable insulation itself or as the air gap. The problem isn't the image current, it's just capacitance between an energised HV conductor and the coil. What prevents the Rogowski coil rising to a non-zero potential?

 

[heberovchem]

@ScottyUK Let me see if I understood this correctly: usually mirroring currents through shielding would decrease or change the magnetic flux, which in turn, not only reduces the inductive reactance, but also the magnetic fluxes which will affect the reads on the Rogowski coil. That is why you would suggest near the testing near the ends. At the same time, the capacitance between HV insulated cables and the Rogowski coil will have a voltage difference across the terminals which could be a potential above the ground by several volts (which could be dangerous). Hopefully I understood most of what you said correctly (please let me know) . I will read on image currents later on.Thank you for your input: much appreciated !

 

[ScottyUK]

Yeah, that's pretty much it. The capacitive charge from the cable could be a bit more than several volts, maybe enough to cause some harm. In my relatively limited experience of Rogowski coils and their integrators, the coil isn't generally connected to a power earth in the way that a CT secondary is always connected to earth so the coil can rise to an elevated potential due to capacitve coupling.

 

[JensenDrive]

Above 600V the insulation is always sheilded to equalize and define the voltage gradient across the insulation. It ensures the volts per meter/inch is uniform in the insulation is uniform and defined. The cable manufacture has to know the volts per meter/inch to design and guarantee the insulation.