Delta and star concept for the CT

[bejoyengineer]

CTs are connected in star on T/F delta connected winding side & in delta on T/F star connected winding side for T/F differential protection and the reason
is to eliminate zero sequence current for avioding the differential relay from operating.but i am confused when we say that the Wye connected formula is
Iline=Iphase and Delta connected formula is Iline = 1.73 * Iphase
so if these formulas are applied ,then will this actually effect the current value of the secondary CT there by activating the differential relay. Thanks in advance

 

[LarryFine]

Allow me to be the first: What?



Seriously, what is the question?

 

[jim dungar]

CTs are connected in star on T/F delta connected winding side & in delta on T/F star connected winding side for T/F differential protection and the reason
is to eliminate zero sequence current for avioding the differential relay from operating.but i am confused when we say that the Wye connected formula is
Iline=Iphase and Delta connected formula is Iline = 1.73 * Iphase
so if these formulas are applied ,then will this actually effect the current value of the secondary CT there by activating the differential relay. Thanks in advance

You often have different CT ratios for the wye and delta circuits and the settings of the differential relay need to take these ratios into account.

 

[ohmhead]

Well i see different voltages to ct ratios will effect ct balance , currents flowing in restraint coils must be balanced at load or common peaks or normal minor increases in loads wye /delta connections help to keep this in balance in phase or ct correction taps a adjustments can be made . Does phase shift have a part in this also i think so ? so a wye ct /delta ct or a delta ct / wye ct connection to keep the current from going into the restraint coils unbalanced is what i always thought . How does a formula change the flow from a zone to b zone just asking ? take care best to yas

 

[richxtlc]

The primary concern in using a delta ct on the wye side and a wye ct on the delta side (ratios adjusted) is to insure that the secondary ct currents entering the differential relay are equal in value and phase relationship. the use of the opposite connections for the cts is to correct for the 30 degree phase shift that takes place in a delta/wye transformer.
If the currents cannot be matched exactly, most differential relays have taps to compensate for the difference, if it is still off, due to an odd ratio or the transformer have an LTC (load tap changer) the relay has a slope adjustment that will allow (in some cases) from 15 to 40 percent mismatch without operating.

 

[rcwilson]

In any delta connection the line current is = 1.73 x phase current or winding current. It's the same for a delta connected power transformer winding or current transformers connected in delta. The delta connected CT's on the trasnformer's wye winding take care of the phase shift and add/subtract the winding currents so the relay compares the correct signals.

Draw the three line diagram for the power transformer and CT's. Assume that Ia, Ib, Ic currents are flowing in the wye power transformer winding. The currents in the delta windings (transformer windings, not line currents) will be ia, ib, ic. The line currents on the delta side will be (ia-ib), (ib-ic), (ic-ia). The differential relay needs to compare (ia-ib) with (Ia-Ib). To get Ia-Ib connect the CT's onthe transformers Wye winding in delta.

I recommend you get a good elementary book on Protective Relaying where a lot of this is explained.

 

[mjoneill]

In any delta connection the line current is = 1.73 x phase current or winding current. It's the same for a delta connected power transformer winding or current transformers connected in delta. The delta connected CT's on the trasnformer's wye winding take care of the phase shift and add/subtract the winding currents so the relay compares the correct signals.

Draw the three line diagram for the power transformer and CT's. Assume that Ia, Ib, Ic currents are flowing in the wye power transformer winding. The currents in the delta windings (transformer windings, not line currents) will be ia, ib, ic. The line currents on the delta side will be (ia-ib), (ib-ic), (ic-ia). The differential relay needs to compare (ia-ib) with (Ia-Ib). To get Ia-Ib connect the CT's onthe transformers Wye winding in delta.

I recommend you get a good elementary book on Protective Relaying where a lot of this is explained.

Mr. Wilson's response is well done and correct based on my meager knowledge of the subject. The only portion missing was a discussion of blocking of the zero sequence components by the delta connections made for this transformer differential protection scheme.