knightsof3 said:
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.
https://ctlsys.com/support/ct_installation_and_wiring/ said:
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
