I am reviewing for the PE exam and came across "ferroresonance" is anyone familiar with this. The statement in which it is being used is "the possibility of ferroresonance should be examined in distribution transformers with ungrounded primaries"

If anyone is familiar with what it is, I could use a little insight.

When in doubt, try wikipedia -
http://en.wikipedia.org/wiki/Ferroresonance

This is mainly a power thing, so maybe the power people will have more info.

I'm by no means an expert on it, but here is a quick, shooting from the hip explanation: Ferroresonace occurs when the magnetizing impedance (almost all reactance) of a transformer resonates with the connected system impedance--i.e. it is a big RLC circuit. This is most likely to occur when the system has a high capacitance (e.g. underground cables). Usually some sort of trigger is required to start the resonant condition--such as a fault or a switching transient.

As for the details it's probably best to find a written reference. Let us know if you find a good one though; I've had a hard time finding good references on it.

I couldn't find any textbook references either, but the wiki notes cover it pretty well. Mudpuppy is spot-on with the analogy of an LC circuit (I'm taking out the R, since a strong R would dampen the resonance). Poor or non-existent grounding exacerbates the problem.

Years ago we had a large customer data center that was self supporting during power outage. When they lost the utility connection, some strange combination of the non-linear loads (overhead lighting, computers, and UPS), underground cables, generators, and an ungrounded delta transformer would lead to occasional burning up of the weakest link (a transformer winding). It took a while to figure out that there was some electrical voodoo going on, not just a single bad piece of equipment.

There are many IEEE papers on ferroresonance as well.

wow, I've never felt more clueless in my life than I did after reading these posts. I am so totally glad that roads don't resonate.

me too... I hated my one EE class I had to take

I assume some of those roads have bridges. This is resonance:Tacoma Narrows Bridge

I've seen the Tacoma Narrows Bridge before. I was making a poorly delivered joke.

I knew you were joking. Mine wasn't all that well delivered either. It's still pretty cool footage though.

Ferroresonance is just what Mudpuppy said - a big RLC resonant circuit. In an electric utility, which I am most familiar with, it mostly happens when you have a three phase padmount transformer fed by a short section of underground primary cable, and the transformer is energized with single phase primary switches, and there is little or no load on the transformer. It can happen otherwise, but I think this is the biggie for electric utilities. The transformer will shake and jump just like the Tacoma Narrows bridge To prevent ferroresonance when energizing a transformer, a three phase gang operated switch to energize helps, also putting some load on the transformer helps - tie small electric heaters to the secondary temporarily is one method.