Hi guys,
I'm investigating a transformer failure. Apparently, the transformer got shot through electrically, primary to secondary.
The funny thing is, that when an even higher DC voltage than the value that was (probably) present on the transformer is applied onto the transformer, there is no problem, even for extended periods of time.
Is it possible, that when dealing with epoxy, a dielectric breakdown can occur at a lower voltage, provided that a high dV/dt pulse is applied (such as 50kV/us)?
If so, what parameter should I look out for in the material datasheet? Is there any non-experimental way to determine what kind of dV/dt will the material survive?
Thanks,
David
From what I know, it's not a dielectric problem (that you can find in some dielectric datasheet), but related to physical construction and geometry of the transformer.
With a pulse of very high dV/dt, you can in the first very short time have uneven potential distribution over the winding, leading to voltage "hotspots".
As an example, you can buy pulse rated capacitors, where both the construction as well as self-healing properties play a role.
I agree with Benta. Physical layout matters a lot
Question from me:
Have you double or tripple insulated the transformer?
(or, have you designed and had it wound 'yourself' or are you using a standard part?)
Try look at the datasheet for Mylar polyester film:
http://usa.dupontteijinfilms.com/informationcenter/downloads/Electrical_Properties.pdf- a quarter mil thickness only gives a Dielectric strength of 20kV (and it's not a standard thickness to my knowledge!)
Parameters such as temperature, frequency and moisture can also influence the transformer.
If purchasing generic devices, I recommend looking for the UL/CUL marked as a start, as they tend to be better isolated than most low quality/cost ones -
https://en.wikipedia.org/wiki/UL_(safety_organization)If this is for a commercial/safety product, talking to a transformer sales representative/tech person might be worth it. My experience is that they know a lot, and are willing to help you out - especially if they are looking at a sale
Yes. The isolation varnish on and epoxy have a limit dV/dt. However, only really really picky people care about this. Because the conditions in which these occur are rather rare.
Those transformers obviously were hit by lightning or a tester, or driven by unfiltered cheap choppers.
When the dv/dt gets high enough, the L in the wire starts acting up and raising the voltage between two individual windings. When a partial discharge occurs (corona, irrc), even trough the isolation, the material will degrade. They test high voltage stuff with test equipment sensitive enough to detect single electron discharges or so they claim.
(it's very expensive)
This is why you can't use VFD's on pre-vfd era motors.
I did some googling and found an paper on it. Apparently there are also resonances.https://www.ab.com/support/abdrives/documentation/techpapers/ieee/05_7.pdf
Look up "partial breakdown".
ordinarily, for DC, extended exposure time drives the safe operating V down
The 10:1 attenuator in a scope probe definitely had a dV/dt rating. A normal 400V rated probe doesn't like zero to 400V in 1us. Different kinds of capacitors have different dV/dt ratings, so why not transformer insulation?
Thanks for the info guys!
Those transformers obviously were hit by lightning or a tester, or driven by unfiltered cheap choppers.
naah, they were hit by my silly experiments.
Yes, this is a common issue with motors driven by inverters. The insulation of a standard motor, say an asynchronous machine driven by direct line connection may not be able to stand the dU/dt of the output of an variable frequency inverter. So you must specify this when you look for a motor to be used as VFD.
The effect behind this is to my knowledge:
- ringing inside the winding (caused by high dU/dt) causes local overvoltages inside the winding
- degradation of the insulation material due to high dU/dt, higher dielectric current, ...
What is frequenzy? dV/dt or dA/dt
What is edge? dV/dt or dA/dt
How ideal capacitors reactive resistance works. It drops while rate of change goes up.
How ideal inductors reactive resistance works. It goes up while rate of change goes up.
Our typical circuit models doesn't take account of the rapid edges (high frequenzies) as they are derived from DC era (as far as I know) and do not take account the wave lenghts etc.
High vd/vt can result in higher leakage currect through the interwinding capacitance. This can eventually result in breakdown of the insulation.
paul
