Hi Dave:
The voltage drop fluctuations due to load fluctuations is a function of the source impedance of the supply. This applies to both the secondary side of the distribution transformer as well as the supply to the primary side of the distribution transformer. The 120/208 distribution within a building is typically designed for no more than 3% voltage drop at rated load. So the source impedance of a 15-amp branch circuit from a 200 amp panel on the secondary would not be more than 3.6/200 or 0.018 ohm. On the primary side, there are a number of distribution transformers. The supply to all of these transformers should be such to support the 3% maximum voltage drop at the secondary of each distribution transformer. So, if you know the primary voltage, the number of distribution transformers, the transformer kVA, then you can calculate the source impedance for the primary supply. I think you can see that the source impedance of the distribution system is quite low, and your load would unlikely cause significant voltage fluctuations. On the other hand, if you were running an electrically- heated smelter furnace, you could easily cause voltage fluctuations on the utility distribution system. Or, as my cousin once described to me... At Ames Research in Mountain View, California, they ran the trans-sonic wind tunnel during the wee hours of the morning due to the required power. If they needed to shut down the wind tunnel in a hurry, they turned the motors into generators and back-fed the power grid, which messed up the power factor in all of Northern California. Regards, Richard Nute San Diego From: [email protected] [mailto:[email protected]] On Behalf Of Spencer, David H Sent: Tuesday, May 15, 2007 5:35 AM To: [email protected] Subject: Voltage Fluctuations Howdy All, I'm trying to get an idea of what sort of voltage fluctuations(AKA Flicker) would be possible back through a step-down transformer onto the HV power grid based on known voltage fluctuations generated the secondary. I'm not sure what the HV feed is (7200, 4000, or 480VAC). The "EUT" in this case has the potential to generate between 3%-15% (at 120Vac) voltage drops based on the AC mains impedance. I would like to calculate how the primary of the HV step down transformer is changing based on instantaneous changes on the secondary. Is it as simple as n= Vs/Vp = Ip/Is ? Thanks for any pointers on this Regards Dave Spencer Xerox Corp - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
