CT Misconnection
CT Misconnection
(OP)
I saw a case where a set of there CTs had one wye-point jumper removed. The B-phase and C-phase CTs wound up connected non-polarity to non-polarity and the B- and C-phase polarity sides brought into the control house while the A-phase CT had its polarity wire and the wye-point wire brought into the control house. So the current supplied by the A-phase CT would have been a good replica of the primary current, but what would have been seen on the B- and C-phase circuits?
Nothing was open circuited, so no damage from that. But the B- and C-phase CTs each had to have the negative of the current in the other but that's not a replica of what was happening on the primary. Are there any good references for what happens in a situation like this? I'm guessing there would have been a fair amount of saturation during that time but I'm not aware of the math necessary to figure it out. To what extent could this situation influenced the primary current? There were multiple alternate paths for the primary current to redistribute. There was nothing on this CT circuit that had any recording capability so there are no oscillography records to look at.
Thanks.
Nothing was open circuited, so no damage from that. But the B- and C-phase CTs each had to have the negative of the current in the other but that's not a replica of what was happening on the primary. Are there any good references for what happens in a situation like this? I'm guessing there would have been a fair amount of saturation during that time but I'm not aware of the math necessary to figure it out. To what extent could this situation influenced the primary current? There were multiple alternate paths for the primary current to redistribute. There was nothing on this CT circuit that had any recording capability so there are no oscillography records to look at.
Thanks.






RE: CT Misconnection
RE: CT Misconnection
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. ____________________________________________ ( ( C (___________ | . ________|_______________________________ ( | ( B | (________| . ______________________________________ ( ( A (______________________________________A bit hokey, that should convey the idea. If wired correctly the vertical line would have extended down to the non-polarity of A.
RE: CT Misconnection
that is a good question.
my guess is that the current that ended up in B (and hence the negative value in C) would be a harmonic laden current that is not representative of either the original B or C phase primary current. As you suggest, the secondary would probably be driven into saturation as the main driving force is the primary current, which would overcome any chance that the secondary windings had of trying to drive the secondary circuit (through a relay or metering unit or something).
we have used series connections of CTs to balance up loads for temperature rise tests across different rated outputs from a 11 kV / 415 V distribution substation (ie haveing 800 A in one circuit, 1200 A in another circuit). You use a 800/5 and 1200/5 CT and series up the secondaries so they both carry 5 A. The concept works well, but you need to have enough VA (and hence CTs in parallel) to ensure that the secondaries can operate with the correct current without saturating. Without enough VA, you cant drive the primaries to balance up to the 800 A and 1200 A values that they need to be.
I see this as a similar situation to the question you ask about what influence could there be on the primary. If you only have a small VA rating on the secondary of the CT, then it would not be enough to drive back into the primary. hence here it would be a fight between the driving force from the primary side and the driving force from the secondary, most likely driving it into saturation.
ausphil
RE: CT Misconnection
For your case we must add the phase difference. I suspect that the output of the "A" CT will be choked off by the "B" CT, and vice versa.
If that is the case then I would expect fairly high voltages across each CT. The CTs will probably be producing knee point voltages.
I would expect that the combined voltage of both CTs would be 1.73 times the knee point voltages.
I don't know what this circuit is connected to. A load across the open circuit may result in a current of 1.73 times equal line currents, but CTs don't like to pass out of phase secondary currents.
The current from "A" CT will be trying to induce a primary current in the "B" CT primary line conductor that is out of phase with the line current. I doubt that it will be successful.
For support I cite the delta metering connection to meter three phase power with a two element meter.
"A" phase CT is connected to one current element in the meter.
"B" phase CT is connected to the other current element in the meter.
The CTs are connected in an open delta. Generally the "V" point is grounded for safety.
The third, "C" CT is connected across the open delta, forming a closed delta.
The current developed by the "C" CT travels through both the metering elements of the meter. Each element meters a component of the "C" phase current.
The accuracy of this scheme depends on the three phase voltages being equal as the "C" phase current components are metered against both the "A" phase voltage and the "B" phase voltage.
The point of this anecdote is that the out of phase CTs choke off the "C" phase current so that all of The "C" phase CT secondary current is forced to pass through the meter elements rather than through the other CTs.
My metering handbook assures me that this scheme gives good accuracy with balanced voltages. That would not be the case if any significant portion of the "C" phase CT current was passing through the "A" & "B" CTs rather than through the metering elements.
The two meter elements in series provide a low impedance path for the "C" phase CT current so that high voltages are not produced.
Bill
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"Why not the best?"
Jimmy Carter
RE: CT Misconnection
RE: CT Misconnection
Bill
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"Why not the best?"
Jimmy Carter
RE: CT Misconnection
Similar discussion at thread238-394712: Ungrounded wye connected CTs
RE: CT Misconnection
Have you been able to inspect the CTs for thermal damage? I suspect the heating pattern under saturation would be quite different than normal losses. With saturation, almost all the losses would be in the iron core rather than the copper winding.
It might also be worth doing a Megger test on all insulation to make sure overvoltage didn't damage anything. If the two CTs were at different points on their hysteresis curves, would it result in damaging voltages during system transients?
RE: CT Misconnection
The current in line 2 is metered by the elements for line 1 and line 3. The impedance of the CT secondaries is so high, relative to the impedance of the metering elements that not enough of the current from the line 2 CT is diverted through the other CTs to affect the metering accuracy.
Bill
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"Why not the best?"
Jimmy Carter
RE: CT Misconnection
The metering diagram is slightly different to davidbeach's issue.
The metering circuit doesn't have secondary windings in series, even though it sort of looks like it. What goes into the two meter elements in essentially the vector sum of B-A and C-B, so this doesn't put any of the 3 windings in series.
RE: CT Misconnection
The point is that all of the current goes into the meter elements. Virtually no current is forced back through the other CTs. Either one of Davids CTs may not be able to pass much current through the other CT.
Bill
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"Why not the best?"
Jimmy Carter
RE: CT Misconnection
RE: CT Misconnection
It depends on the secondary burdens. CTs are transformers but special ones. Let us assume two extreme cases:
1) B and C secondary windings are open (of course induced high voltage, but let us park it aside)
RE: CT Misconnection
RE: CT Misconnection
RE: CT Misconnection
RE: CT Misconnection