Tertiary Winding

[sasalu]

Hi all,
I am going through a specification in which mentioned tertiary winding may be provided if it is a five limb construction and trans former vector group is YNyn0, if tertiary not considered what are the consequences for the system.

 

[cranky108]

The tertiary winding, provides a path for harmonics, and a zero sequence path. Not connected, or burried is not a problem.

 

[RRaghunath]

Tertiary winding provides is inevitably Delta connected, in case of Auto transformers and YNyn0 connected transformers. This is recommended for transformer rated > 100MVA.
Tertiary Delta provides path for third harmonics and zero sequence currents. Thus, helps reduce interference with Telecom lines (running close to power lines) and improves the earth fault current magnitudes by reducing transformer impedance for earth fault currents.

 

[davidbeach]

A five limb YNyn0 doesn't need a tertiary. What happens, now and then, is that when a YNyn0 is loosely specified is the manufacturer will go with a three limb core and throw in a buried tertiary rather than coming up with all the steel needed to make a five limb core. For load currents a 5 limb YNyn0 and a 3 limb YNyn0d are probably indistinguishable. But under unbalanced conditions, whether loss of phase or SLG fault, they can respond very differently. Know what you have and be prepared for what happens there's no problem. But think it's one, prepare for that, and then find it's the other can even result in law suits. No, I wasn't involved in the law suits, but they meant that a large facility was once looking for a new consulting engineer, and when I explained the problems that the three limb YNyn0d could cause without even knowing the history I wound up with a few years of steady work.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[prc]

1) In simple English- a YNyn connected transformer with five limbed core will have 10-20 % more zero-sequence impedance than one with three limbed core. With a 5-limbed core, zero sequence impedance will be the same as positive sequence impedance. Adding a stabilizing delta-connected tertiary will bring down the zero-sequence impedance almost to the same low level. So on the conservative side, many countries ask for a stabilizing tertiary in 5 limbed cored transformers. ( eg. India)
2)Nothing will happen with 5 limbed core units without tertiary esp when neutrals of both Y and y are solidly grounded. In certain situations, conditions like neutral shifting under unbalanced loading or lower current under LG fault conditions (to actuate relays) may result without delta tertiary.

 

[sasalu]

Thanks all for your valuable feedback and prc sir for your explanation in simple words.