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Three Winding Transformer Impedance Question

cyriousn

Electrical
Jul 19, 2017
27
From reviewing some responses on other threads from forum user PRC and also reviewing some modeling data I have access to, it appears that the impedance between two secondary windings of a 3 winding transformer is roughly 0.95 the summation of the impedances between the primary and each of the secondary windings. Does that sound right and is there any way to calculate this value instead of using a rule of thumb?

The reason I ask is we are looking at trying to model a three winding transformer in the hopes of reducing the fault current on the secondary side to a reasonable level below 40kA since that is the limit for commercially available 34.5kv gear. Transformer is currently a two winding 210/280/350MVA 345kv Wye-G / 34.5kv Delta with Z1 of 8.5%. The low impedance seems to be the issue along with the fact that the project is located near a generating facility only a few buses down on the 345kv network.

If we move towards a 3 winding transformer with 8.5% between the primary and each of the secondary windings and we use the 0.95 rule of thumb for the impedance between the two secondary windings this yields 16.15% impedance which drops the fault current down to 38.1kA for my scenario. This is still probably too close to the 40kA limit which means we can also try and increase the impedance of the transformer to within a limited that the Independent System Operator will allow without dinging the project for a Material Change.

Thanks in advance.
 
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I used to see air core reactors in sub stations.
Cheap and easy and relatively small I2R losses.
 
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Cheap and easy and relatively small I2R losses.
Sounds like another good way to reduce the fault current. It would also be subject to the ISO's review of the overall impedance change of the system just like changing the transformer impedance. We are also dealing with a limited footprint for the stie so I'm not sure we would have the space for adding more equipment.
 
Don't forget that for high X/R, you must de-rate your breakers. Simply adding reactance might not get you there.
 
Hi cyriousn,

when specifying such a large transformer with the objective of limiting the short-circuit current to a reasonable value for CBs, a three-winding transformer is undoubtedly the best solution. We are currently manufacturing several dozen transformers for renewable energy collection, rated at 250 MVA, with a three-winding (YNdd) configuration, a transformation ratio of 400 kV / 36 kV / 36 kV or 230 kV/ 36 kV / 36 kV, and a short-circuit impedance of 18%. Regarding the 0.95 rule, it is not necessarily a strict requirement; it is entirely feasible to design three-winding transformers with windings that are completely magnetically decoupled.
 
Don't forget that for high X/R, you must de-rate your breakers. Simply adding reactance might not get you there.
Thank you. Your comment has led me down a rabbit hole of trying to figure out if we are going to be okay due to this. The Aspen model says yes but Easypower model is saying maybe.
 
Don't forget that for high X/R, you must de-rate your breakers. Simply adding reactance might not get you there.
Good point Stevenal. But when adding reactors, rather than just increasing reactance, the reactors may be designed to add some resistance also.
Adding resistance may affect regulation, but there may be an inescapable compromise between reduced fault current and regulation.
 
Stevenal is definitely on to something. Had an application where an air coil reactor was already being used between bus and 115kV cap bank to limit fault current to what a breaker with both cap switching ratings and fault interrupting ratings could handle. As available fault current went up I looked at increasing the X of the reactor. Fault current went down, X/R went up, and the capability of the breaker, relative to the available fault current, didn't change much.

I didn't look at adding R, but any useful amount of R, in terms of lowering X/R, would have created so much additional I2R that it would have been hard to justify.
 

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