Generator tripping on ground fault during load test

[knightsof3]

well i think you're almost there...

with that resistor missing it'll be spectacularly oversensitive.

If you can hang a simple battery handheld 'scope across it it'd sure be interesting to see the waveform . 60 hz, 180 hz, or a mix ?

old jim

So we finally got the vendor back with the load bank. We reinstalled the burden resistor across the CT and viola we cranked the generator right up to 570 kW with no issues. Problem solved! Thanks everyone! Now what's going to eat away at me is the electrical theory aspect of why with no ground fault current and with the CT core both not a fully rectane and also installed correctly fully enclosed, how was a voltage being induced onto the secondary winding of the CT? I guess without the burden resistor the secondary circuit is an open circuit but how was anything at all being induced into secondary circuit to mess with the solid stare ground fault relay? And why was at always when we went over 350 kW, 400+ amps? It consistently tripped at the exact point. My mind would think that if the CT secondary open circuit is screwy it would just trip the unit as soon as a load is applied? Any electrical theory thoughts on why an open circuit CT gets an output at with no magnetic field from the load cables?

 

[Windadct]

The short answer is - the world and the things we build are not perfect.. in that there are a few issues. As I mentioned these large window CTs are not very accurate, and the location of the cable in the rectangle (window) affects the magnetic linkage between magnetic field created by the current in each cable, and how much of this is established in the core of the CT. Example : Cable A : 99.5%, Cable B 99%, Cable C 98.5% = at some point in every cycle it can look to the CT as 1% current "missing" in the A+B+C = Ground Current equation.

Additionally there may - or probably is some Ground leakage, but how this would be load dependent? -- requires more input.

The CT is Current to Current device, or a current source... it will "push" current out to match the ratio ( probably 600:1, maybe 600:5A ) ... the relay is high impedance like > 10K ohms, this makes the system far more sensitive (> 50x ) to even the slightest output.

In reality - this system likely operated according to "theory" - it was just not configured properly. This is a human error, not an electrical system error. All of the system was static - not changing, the cable location, the CT Secondary Burden, etc... so the trip point was consistent.

Unfortunately - this could be pretty easily demonstrate on site, with typical field test equipment. Even measuring the V in the leads to the Meter, before and after the correction would have shown this. Also - HI-Z died long ago...

 

[jim hardy]

Any electrical theory thoughts on why an open circuit CT gets an output at with no magnetic field from the load cables?

EDIT oops i see @Windadct answered whilst i was typing (and drawing) . He's correct of course.

Here is my two cents ...i think the answer is local coupling into only part of the CT winding.

A CT with closed core is a different animal from one with an open core.

(i probably should have said for this scenario "..whichever conductor is nearest the core dominates voltage.."

I'll wager if you look you'll see one of your big wires right against the core
where its "d" is just its insulation thickness
airgap of "d" is still a lot of reluctance compared to a closed core but not to an open one...

So when the core is open
your three conductors contribute to total flux(and secondary volts) not equally but in proportion to their closeness to the sides of the core, 1/d .

So i'd guess
with its burden resistor unconnected,
your CT was sensitive enough to develop tripping voltage because the three phase wires are not equidistant from the sides of the core.

Make sense ? Corrections welcome.

Even when the core is closed, manufacturers recommend routing the conductors toward the center of the core.
That's because of what i'd call ' the 'unequal d effect ' ..
https://w3.usa.siemens.com/us/internet-dms/btlv/PowerDistributionComm/PowerDistribution/docs_MV/TechTopics/ANSI_MV_TechTopics50_EN.pdf

The cables should be located toward the center of the CT opening, and should not be allowed to contact the CT case. If the cables are directly against the CT case, it could lead to localized saturation of the CT core under throughfault conditions, leading to false operation. Typically, the mounting plate for the zero-sequence ground CT can be used to restrict the cables so that they pass through the approximate center of the CT window.

If the CT opening is much bigger than the conductor, position the conductor in the center in the CT opening. If that is not possible, try to position the conductor in the bottom of the ‘U’ shaped half of the CT, away from the opening end where there is magnetic flux leakage.

and from a really good paper at https://www.gegridsolutions.com/smartgrid/Mar07/article4.pdf (ahhhh , good old GE i sure hate to see them go)

4. Localized CT Saturation
Situations that result in localized CT core saturation can be divided into two categories: Lack of concentricity of fields that should couple to the CT core and the presence of fields that are in proximity to the CT that should not couple. Concentricity problems result when the primary current carrying conductors are not centered in the CT window, or the CT window is irregular in shape. It is common to use bus conductors that are rectangular in shape, which inherently brings the edge of the bus closer to one side of the CT, but more serious problems result when, out of convenience, the CT is allowed to rest on the face or the edge of a bus bar. It is also common to use more than one cable to carry current, and installers are usually content to simply verify that all conductors pass successfully through the CT window. In situations where differential current is to be measured, as in the case of ground fault detection, users often fail to group conductors to cancel magnetic fields that should not couple to the CT core. Finally, and probably the most detrimental situation, is the practice of abruptly turning the primary conductor that passes through a window CT. For example, it is common practice to mount CTs on low voltage bus bars and slide them back against a 90° turn within power handling distribution equipment.

old jim

 

[jim hardy]

Problem solved! Thanks everyone!

Thanks @knightsof3 for letting us know.

And - congratulations !
We all enjoy hearing of somebody's victory.