Guidelines on MV core-balance CT installation 1

[LiteYear]

What guidelines do you follow for the proper installation of a core-balance CT in MV (~4kV) environments? I appreciate the general advice of centring the phases in the window, using the smallest window possible, twisting the secondary wiring, maintaining separation between CTs and sources of electromagnetism etc., but what I don't appreciate is how much any of these measures are likely to contribute to the quality of the measured signal.

1) Are there any good published guidelines on measures to take and their criticality?
2) Any suggestions from field experience on what separates a good installation from a bad one?
3) Any data on the effect of: primary current magnitude; split core vs solid core; symmetry of phase conductors; routing of conductor shields; loop area of secondary wiring; shielding of secondary wiring; orthogonality of primary to secondary wiring; influence of chassis elements; any other significant factors?

 

[LiteYear]

Should I try rephrasing my question, or does there just not happen to be experience in this area amongst those reading the forum at the moment?

FWIW, I found two IEEE articles that were relevant, unfortunately they constrain themselves to installations that are either right or wrong, not good or bad. Maybe this is indicative: the main industry focus is on making sure the installation matches the schematic, rather than details about physical separation, symmetry and alignment. The two articles are:

IEEE Protective Relaying on Medium and High Voltage Systems, some lessons to be learned. Dudor and Padden.
IEEE Recommended Practice For Protection and Coordination of Industrial and Commerical Power Systems. IEEE Std 242-1986.

Perhaps I should cast my net a little wider: has anyone ever heard of a core-balance CT installation, correctly wired, that nonetheless produced a false reading? If so, any word on how it was fixed?

 

[ScottyUK]

I can think of two problems I have seen in twenty-odd years:

One instance gave false trips during motor starting. The motor was connected to the MCC by two big parallel feed cables - likely 185mm² or 240mm² - which are far from easy to manipulate and terminate. Someone had decided to make the installation easier by putting a CBCT over each set of three conductors, then paralleling the CBCT secondaries. Under starting conditions the current didn't split perfectly between parallel conductors in different cables and trips resulted. By little other than luck this arrangement worked for a while, and I guess that over time one of the paths must have increased a little in resistance causing unequal sharing of current which was when the problems started. The techs struggled a bit initially because 'it had worked originally so it must be ok' and therefore there 'must' be a fault on the drive.

The other that I can recall was due to an earth passing through the CBCT aperture. An earth fault occured in an HV motor and without the sensitive E/F protection we damaged a stator quite badly and had to scrap the motor as not economically viable to repair. We also lost supply to a fair amount of plant when the E/F element on the switchboard incomer cleared the fault.

The latter was a simple installation mistake and should have been seen by the commissioning guys.

 

[oldfieldguy]

If you have access to a manual for one of the GE motor protection relays, the installation instructions has a neat little picture that tells you how the shield ground wire needs to be routed through the window of the GFCT.

That's the biggest trick to the installation: All phase cables go through the window, the shield ground has to be routed properly. This usually means that the conductor used to ground the shields must be routed back through the window to ground. It's more difficult to explain than it is to look at the picture in one of GE's manuals.

Installations requiring multiple large conductors may require a large window-type GFCT. These are available, but may be relatively long lead-time items. I would do this rather than try to parallel CT's.

old field guy

 

[ScottyUK]

To be clear - paralleling CBCTs to accommodate parallel cables is a bad idea and will either not work or will work down to luck. Don't do it - it is just bad engineering.

 

[dpc]

This might help a bit:

http://www.gedigitalenergy.com/smartgrid/Mar07/article4.pdf

Expand your Google search to include "zero sequence" and "flux summation" CTs as well as just "window" CTs.

I have found these CTs to be pretty forgiving - I do advise at least a little time delay if these are seeing large transformer inrush current or motor starting current. Of course for medium voltage shielded cables, the shield wires must be properly dealt with. If the shield goes through the CT, the drain wire must be brought back out.

 

[LiteYear]

Thank you for the very valuable responses.

ScottyUK, I agree the parallel arrangement would fall into the "wrong" category. And I did see lots of reference to making sure the earths are/are not passed back through as appropriate. Clearly an error that occurs in practice!

oldfieldguy, yep, in fact both the references I mentioned in my second post have a diagram on just this issue. Figure 4 (pg 55) of the first reference and Figure 130a (pg 222) of the second.

dpc that reference was a huge help. Describes a highly practical method of calculating saturation error due to off-centre primary conductors. And further suggests that this is the dominant cause of measurement error (given a "correct" installation). Exploring this new information and your collective experiences, I also found this very generous reference, which extends dpc's reference (and I just noticed, shares an author!) to explore in simulation and experiment, how relays typically react to the measurement error.

GE Multilin: CT Saturation in Industrial Applications - Analysis and Application Guidelines. Kasztenny, Mazereeuw, Jones.

 

[Kartikkapil]

These are taken from my (very!) old lecture notes on motor protection.
Common mistake made in armor earthing during CBCT installation is explained.
K Rajamani

 

[ssantoshr]

Here is a good reference document for proper cable installation of Ground Sensor CT (Core Balance CT).

 

[LiteYear]

Nice, succinct document. Interesting that it doesn't have anything to say about secondary wiring.

FWIW, I completed the job that prompted this question last week. Long story short, something we changed had the effect of reducing unfaulted earth leakage reading from ~600mA to <30mA. In other words, error in the core-balance reading reduced by at least 550mA. The major aspect we were able to change was from a split core CT to a solid core of the same diameter. We also made improvements to the shielding and orientation of the secondary wires, but it's likely that the majority effect was the CT itself. Primary currents were ~200A per phase @ 3.3kV. Very similar results on two installations. Details on brands, loads and relays deliberately omitted.