Transformer impedance

[CuriousElectron]

Hi All,
When specifying 1MVA transformer or above, do you usually specify its impedance based on short circuit-study performed to determine what the impedance should be, or do you go with OEM recommended standard value for the size transformer and then specify appropriate kAIC and protective device interrupting ratings of equipment located downstream?
Just curious about what a typical procedure is here.
Thank you,
EE

 

[bacon4life]

My utility specifies an allowable range for each size of transformer size when buying transformers. This allows us to then publish a table of max kAIC for customers to use in their designs. Here is an example:

When designing an installation, it is import for a customer to design for the worst case kAIC rather than using the specific impedance of the transformer that happens to get installed. This allow for future replacement of a failed transformer with any transformer in our warehouse. It can be very problematic if an installation is designed with a requirement for a higher than typical impedance. Note that is opposite from arc-flash stuff, where you need to use the impedance of the transformer that is actually installed.

Check out this previous thread as well for some links. https://www.eng-tips.com/threads/transformer-impedance.283766/

Also, specifying a higher than typical impedance may cause problems with voltage drop. Sometimes voltage regulation calculations are based on typical design assumption, and selecting a high impedance transformer will cause higher than typical voltage drop. Sometimes fthe 5.3% impedance of large service transformers ends up with loading being limited by voltage drop rather than by transformer thermal capacity.

 

[dpc]

For larger transformers (above 500 kVA iirc), ANSI standards provide a standard impedance as a function of the voltage and BIL. You can specify something other than this, but you should discuss with transformer manufacturers regarding the tradeoffs. The temptation to specify higher impedance to limit fault current can result in voltage drop issues as noted in the previous comment. Non-standard impedances will also be more difficult to replace in the event of failure. Transformer efficiency will also be impacted.

 

[waross]

You can specify something other than this, but you should discuss with transformer manufacturers regarding the tradeoffs.

My feeling is;
Go with standard transformers and limit ASCC with series reactors or with cable impedance.