Correct Zero Sequence Wiring 2

[Mbrooke]

When would option B be used? Would it be applied with a low Z meter (amps) or High Z meter (volts)?

 

[davidbeach]

Option A is what should be done. Option B should be avoided under all situations as the CTs will be saturated under any load.

 

[xnuke]

CTs are current sources, therefore they should never be connected in series. They must always be connected in parallel as shown in Option A.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Mbrooke]

Thanks for the replies. Scratching option B out of my notebook.

 

[che12345]

Dear Mr. Mbrooke

Q1. "...When would option B be used? Would it be applied with a low Z meter (amps) or High Z meter (volts)? "
A1. a) Option A is more widely used. The detection relay would be a [current] detector, e.g. a low Z meter (amps).
b) Option B is widely used in MV systems. This is called open-delta formation. It is usually connected across a resistor. The detection relay would be a [voltage] detector, e.g. a high Z meter (volts).
Che Kuan Yau (Singapore)

 

[waross]

CTs are current sources, therefore they should never be connected in series.

Yes, CTs don't like to push current through each other.
This leads to an interesting exception to the general rule that CTs should not be connected in series.
There is a recognized four wire, three phase metering scheme that uses three CTs in closed delta to meter unbalanced three phase currents with a two element meter.
(Apologies to Blondell).
A phase CT is connected to the A phase meter element.
B phase CT is connected to the B phase meter element.
C phase CT is connected in closed delta so that C phase current passes through both A phase and B phase meter elements.
The current in C phase may be resolved into an A phase component and a B phase component and C phase components are metered by the A and C phase meter elements respectively.
Does this contradict the statement by xnuke?
Actually this connection only works because CTs will not push current through each other.
All of the C phase current passes through the A phase and B phase metering elements rather than through the other CTs.
This unique connection may be viewed as a proof of xnuke's statement.
The accuracy is comparable to other metering schemes with the assumption that the phase voltages are equal.
If that assumption is not true, then the metering errors will be closely related to 1/3 x the amount of voltage unbalance.
Option B is a common connection for Potential Transformers, not Current Transformers.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

A broken delta is a connection of three VTs with the primaries in wye and the secondary in delta, with one corner broken for the insertion of a resistor and a meter across the resistor. Used for measuring zero sequence voltage.

An open delta is a connection of two VTs with the primaries connected phase-phase, generally A-B and B-C. The secondaries form two sides of the open delta. B-phase of the secondary is grounded.

CT secondaries should never be connected in series if the CTs are not also connected in series on the primary side, then only if the CTs are matched.

CTs can be connected in delta, but only if there are instrument connections at each corner of the delta, as used to be the case when connecting a transformer differential. No zero sequence current would go to the instrument.

 

[rockman7892]

I've always heard that CT's connected in series such as on the same bus, cable etc...(primary's in series)can increase the voltage rating of the CT's and improve saturation?

 

[davidbeach]

Yep, that's the reason to series up CTs. Two CTs in series, in that configuration, each have half the total voltage, greatly reducing the amount of saturation, or eliminating the saturation.

 

[waross]

Respectfully, DavidBeach.
Mr. Che is referring to IEC standards where the connection that we call a broken delta is called an open delta.
A confusion of terms.
Nobody is incorrect.

CTs can be connected in delta, but only if there are instrument connections at each corner of the delta, as used to be the case when connecting a transformer differential. No zero sequence current would go to the instrument.

Yes, that is the case in the delta metering scheme.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

It's just a windmill I tilt at now and then. Wye and star, earth and ground, that's just different names for the same thing - no big deal. But using the name "open delta" for two distinctly different things does seem to create unnecessary confusion. I think the IEC area may refer to an open delta as a V connection, but there's a bunch of different connections of two transformers that could be described as a V but which can't be used interchangeably.