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Delta Connected CTs

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arrehman1

arrehman1

Electrical
Apr 7, 2013
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While doodling delta connected CTs on engineering paper (we've all done this), I realized that the CT secondary line current which is going into the relay input terminal has no return path back to the CT secondary circuit... the line current just goes to ground. There is no circulation of line current... please see attached doodle. Is this a correct way of thinking about it?

Just as a comparison, for WYE connected CTs, we'll often see the non polarity side (X2 bushings) of the CT connected together which is also connected to the non-polarity side of the relay input terminal... and grounded in only one location of the secondary circuit. We can easily see the circulating path for secondary phase current.

Thoughts?
 
 http://files.engineering.com/getfile.aspx?folder=752c44c9-2d4d-4691-ae2f-f1622e9f3752&file=delta_connected_Cts.pdf
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So, if Ila=Ia-Ib, Ilb=Ib-Ic, and Ilc=Ic-Ia then Ila+Ilb+Ilc=Ia-Ib+Ib-Ic+Ic-Ia=0. No current to flow to ground. Pick a set of primary currents and do the vector math to determine what's on the secondary. It will always add up, no current to ground. Ground is just there for safety.
 
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When I have used delta connected CTs it was to meter three phase loads with a two element KWHr meter.
The meter elements have much less impedance than the CTs and the current from each CT tends to pass through the meter element rather than through the other CTs..
The current from A phase CT goes through the A phase element and returns to the A phase CT.
The current from C phase CT goes through the C phase element and returns to the C phase CT.
The current from the B phase CT passes through both the A phase element and the C phase element.
B phase values may be resolved into a combination of A phase values and C phase values.
The A phase element registers the A phase component of the B phase current and the C phase element registers the C phase component of B phase.
This metering scheme assumes that the phase voltages are equal and gives accurate metering when the phase voltages are equal.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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I'm still looking for an on-line diagram but I'll give you what I have found:
Here is a form 5S meter such as we used for delta CT connections.
This meter measures each 120 volt circuit independently.
This meter is suitable for measuring a 120/240 Volt single phase circuit or two phases of a 120/208 Volt three phase circuit.
If a CT is added to the third phase of a three phase circuit and connected between the ungrounded ends of the existing CTs the current will flow through both current elements. The CTs will be in a corner grounded delta configuration.
Accuracy is based on the assumption that the phase voltages are equal.
Here is a form 14S meter:
You can see current elements for the third phase arranged with both potential elements.
The 14S meter uses discrete coils to pass the third phase current past each stator.
The form 5S meter may be considered as a form 14S meter with the third phase current elements missing. The delta CT connection forces the third phase current past both the potential elements or stators through the first and second phase current elements.
The meter sees the third phase current and doesn't really care how it got there.
If I can find the complete diagram I will post it.
I hope that this development of the circuit will help with your understanding.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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The currents flow as they would in any Delta connected circuit.
The return path for line current Ia would be through line Ib and Ic.

If you ask questions, others may think of you as foolish or ignorant.
If you never ask questions, you will remain foolish and ignorant.
 
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Referring to the diagram of the 5s meter: The current produced by the third delta connected CT flows into the upper left meter terminal, through the element to the bottom left terminal, through the jumper to the the lower right terminal, through the right element and back to the CT from the upper right terminal of the meter.
The delta connected CT arrangement is used to meter wye circuits with both line to line and line to neutral loads.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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To point out the obvious - There is no path for current flow when there is just a single connection to ground.

I've never seen that scheme in any practical use. It must be only used for special metering cases as Waross has posted.
 
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Delta connected CTs were and still are used for transformer differential current relaying. Prior to the common use of microprocessor relays, a properly connected delta CT on the wye side of a delta wye transformer will provide the proper phase shift and zero sequence filtering needed. Even for microprocessor relay retrofits, it may be easier to leave the three conductor cable in place rather than to pull a new 4-wire and unwrap the delta. Very practical.
 
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I have located my old metering book. It is undated. I have had it for about 40 years.
The origin of the delta metering circuit is a publication from Westinghouse.

Meter and Instrument Transformer Application Guide and condensed metering course
ALL IN ONE
Westinghouse
Fourth Edition
(undated)
The application is measuring all three phases of a 120/208 Volt three phase circuit.
The same result may be obtained by using two CTs and passing the B phase conductor through both the A phase and the C phase CTs.
Our little utility serviced about 5000 subscribers. I didn't have a lot of resources.
A phrase in the Canadian Electric Code caught my attention years ago:

Rule 18-000
Through the exercise of ingenuity in the layout of electrical installations for hazardous locations, it is frequently
possible to locate much of the equipment in a reduced level of classification or in a non-hazardous location and
thus to reduce the amount of special equipment required.
With a limited budget both in money and spare parts, I fell back on that phrase a lot during my time with the little island utility.
To those of you who are skeptical:
I was skeptical myself the first time I used delta connected CTs to measure a wye circuit. The load was primarily three phase with fairly good balance. I connected a single phase KWHr meter to one phase. The single phase meter tracked well at close to 1/3 of the consumption of the three phase meter. Any connection mistakes generally cause a metering error of 1/3, 2/3 or greater. The single phase check meter helped me gain confidence in the delta CT configuration.
Today, all the three phase metering and much of the single phase metering on the island is done with electronic meters.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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I like it with two quibbles.
You call this a Dy1 connection. Since your primary is wye while the secondary is delta, I think it would more properly be called YD11.

You left out the zero sequence filtering this connection provides. Perhaps sequence components theory might be a bit advanced for your intended audience, but the simple Ia=Ib=Ic analysis could be pretty instructive on why this connection might be handy.
Nice job.
 
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