We have a number of recognized transmission and distribution experts serving on our board of "Grid Masters." Several times each month we’ll post what we judge to be the toughest questions that also have high interest to our readers. At least one of our experts will respond. Want to challenge our Grid Masters for a chance to win?

This week’s question and answer:

**Q: We have a step down transformer from the utility company - 15 KV to 480 volt. The nameplate reads "AIC at zero sequence impedance." I know what the AIC is, but what does the zero sequence impedance refer to?**

**--Anon (USA)**

A: The simplified answer without going into a lengthy technical discussion of symmetrical components is: Zero sequence impedance is the impedance offered by the system to the flow of zero sequence current. The zero sequence reactance of transformers depends on the construction of the core and winding connections. The zero sequence impedance is also affected by the transformer neutral grounding method, i.e. solidly grounded vs. resistance grounded.

*Pete SantoroStrategic Projects Account ManagerABB Inc.*

Symmetrical components are taught at the upper division or graduate level of electrical engineering. in Depending on your back ground, there’s quite a selection of resources on the web on the topic of symmetrical components (the methodology that defines sequence impedance). Here’s a fairly clear one: http://www.gedigitalenergy.com/smartgrid/Dec07/7-symmetrical.pdf

*Paul MauldinEditorGrid Optimization*

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