%Z0 vs %Z 1

[stretched2thin]

Is there a generally accepted relationship between a transformer's positive-sequence impedance and it's zero-sequence impedance? I believe that manufacturer's usually only test positive sequence impedance. I'm not aware of any conversion, but there must be some relationship.

Any suggestions?

 

[jghrist]

For solidly grounded 2-winding transformers, Z0=Z1.

 

[stevenal]

According to Blackburn, X0=0.85 to 0.9*X1 for three leg core form transformers.

IEEE C57.12.00 lists the test as an "other" (rather than "routine" or "design&quot test for 501 kVA and larger transformers. If you want it you need to specify it.

 

[jbartos]

Suggestion: Reference:
1. Electrical Transmission and Distribution Reference Book by Central Station Engineers of the Westinghouse Electric Corporation, East Pittsburgh, Pennsylvania, 1964
Table 5 etc. in the rear of the book shows relationships between the positive sequence transformer impedance and zero sequence transformer impedance. The relationships depend on the transformer winding connections, turns ratio and system grounding.

 

[jghrist]

jbartos,
In the same edition of the Westinghouse T&D Book, Chapter 5, XXI.56, p. 138 says "The impedance to the flow of zero-sequence currents in three-phase core-form units is generally lower than the positive-sequence impedance." This doesn't show up in Table 5 where, for instance, Connection A-1, Star/Star Solidly Grounded shows both Z1% and Z0% = Z45%.
Stevenal is correct.

 

[stretched2thin]

Thanks for the information. This has been a big help.

 

[jwerthman]

The zero sequence impedance of a three-legged core form transformer will be 85-90 percent of the positive sequence impedance as stated previously. For five-legged core form or shell form or any configuration that has a complete magnetic circuit through the core for zero sequence flux (i.e. no air gap), the zero sequence impedance will be nearly equal to the positive sequence impedance.

 

[jbartos]

Suggestion to jghrist (Electrical) Sep 15, 2003 marked ///\\jbartos,
In the same edition of the Westinghouse T&D Book, Chapter 5, XXI.56, p. 138 says "The impedance to the flow of zero-sequence currents in three-phase core-form units is generally lower than the positive-sequence impedance."
///Yes, agree. However, the original posting calls for relationships. The relationship does not necessarily mean the equality.\\ This doesn't show up in Table 5 where, for instance, Connection A-1, Star/Star Solidly Grounded shows both Z1% and Z0% = Z45%.
///Agree.\\\
Stevenal is correct.
///Yes, for the three-leg core-form transformer. However, the original posting does not indicate a type of the transformer. Therefore, I use the very same page of the cited reference excerpt:
The impedance of a three-phase bank of two-winding transformer to the flow of zero-sequence currents is equal to the positive-sequence impedance for three-phase shell form units (or for a bank made up of three-single phase units) if the bank is star-star with both star points grounded.
If the bank is connected star-delta, with the star point grounded, the zero-sequence impedance viewed from the star-connected terminals for shell-form transformer units, or banks of three single-phase units, is equal to the positive-sequence impedance; the zero-sequence impedance viewed from the delta-connected terminals is infinite.

It is clearly seen what I have stated in my posting in terms of dependency on the transformer winding, grounding, etc. is correct.

The steneval posting indicates just a portion of the sought information in the original posting.\\\

 

[stevenal]

The steneval posting indicates just a portion of the sought information in the original posting.

Maybe stretched2thin is best qualified to determine that.

 

[stevenal]

Since I've been accused of being incomplete:

I was surprised when factory testing of a delta/wye three legged core form transformer Showed Z0 to slightly exceed Z1. When I asked the testing techs about this, they did not find the readings to be unusual. They told me the presense of the low side LTC was the reason. As complete as I can get without delving into all the permutations of connections, grounding, etc.