sabotaging a transformer - Page 1
Forum section: electronics / beginners

author: hazuki
author_rank: Contributor
timestamp: December 17, 2014, 05:53:00 pm
content: Hi all.I'm trying to get a tighter grasp on transformer non-idealities, particularly saturation. I've read about saturable core transformers that change the DC level of flux in the core to throttle the power delivered from primary to secondary. Is this seen in the standard model of a transformer show below?My understanding is that during saturation, the magnetizing inductance (Xm) will decrease greatly such that Xm becomes a short. If the secondary leakage inductance and winding resistance are significant compared to the magnetizing impedance, due to voltage divider action, the voltage available to the input of the ideal transformer and thus the secondary decreases. Thus the power that can be delivered to the load will decrease. Now all that power being injected in the transformer is being burned up in the primary resistance. Is this correct?Also, a thought experiment. Say I were to walk up to a distribution transformer in my neighborhood and somehow attach an additional winding to it's core, and then connect a 9V battery to that winding. Would the DC flux in the core start increasing until saturation? Would the transformer then overheat and fail? Something tells me though that my 9V might explode. What if it the 9V were indestructible?How would you engineer a power system to monitor the flux in the core? I'm guessing that power systems out there can do this and then somehow apply a correction to remove the DC flux.Thanks!

author: SeanB
author_rank: Super Contributor
timestamp: December 17, 2014, 08:14:36 pm
content: Distribution transformer you would have to have a DC load about equal to the rated current to get it to fail. The core is run very conservatively so as to keep losses down to a low level, as the transformer will spend almost all of it's life at low power compared to peak, which means losses in the core must be as low as possible. Older transformers ( 50 years or so) only needed about 10% DC to fail them, and these were replaced very fast with the advent of television, which introduced the first generation of mains frequency switch mode power supplies. These drew a DC current pulse, and the cumulative addition of the loads in a street would lead to transformer failure. The replacements handled this by running the core at lower flux, and by distributing the core gap better around the core.Basically the DC current needed is such that the secondary fuses will fail from overcurrent first.Now consumer grade units there they run at high flux, and with the core already going into saturation, but then any DC over about 10% can lead to problems. A half wave rectifier is about the worst case, which requires a lot of derating of the transformer.

author: IanB
author_rank: Super Contributor
timestamp: December 17, 2014, 08:43:40 pm
content: I suspect, however, that grid transmission transformers that handle high loads in the 100's of MVA might be more sensitive. Suppose such a transformer has an operating efficiency of 99.97% and has cooling arrangements to handle this. You would only have to cause a small drop in efficiency to vastly increase the heat dissipation.

author: SeanB
author_rank: Super Contributor
timestamp: December 17, 2014, 08:51:32 pm
content: True, but those tend to be protected rather well, as the possibility of connecting anything to them live are almost sure to result in a flashover and some rather well charred "something possibly organic" material being all that is left after the trip, and they also tend to be within large live enclosures to keep unwanted guests out. As well the power level is rather much above a typical battery pack you could move without a truck.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 17, 2014, 11:55:46 pm
content: There are ways of applying flux to traditional transformer designs, which mostly or completely cancels the AC voltage on the normal windings. Such a configuration is known as a saturable reactor ('reactor' being a somewhat archaic term for 'inductor' or 'choke' -- namely, areactor results inreactive current). So, yes, it's possible to saturate a transformer, given suitable access, additional windings, and an "unlimited" 9V battery.Obviously, it doesn't saturate instantly with current. The amount of flux required depends on the core material (steel laminations, or strip, ferrite, etc.) and its construction (core length, area, and any air gap, intentional or otherwise).Offhand, I would guess a 12V car battery would offer enough ampacity to saturate the typical 'pad' transformer, assuming they aren't doing anything overly special in the construction.I wouldn't recommend attempting it. Dry type transformers are wound with heavy copper bar, and may not be very well insulated internally. 3.6kV and up is not friendly to things other than air... and lots of it, at that.Another matter: suppose you connected your ideal 9V to the 120V (or whatever) outlet. What happens? Well, obviously, the line voltage gets shorted through the battery, and out goes the line fuse.Could that be prevented? Perhaps. It's not practical to try bucking the AC voltage away (the buck transformer needs to tolerate all that DC, too!), but you could use a very large inductor (designed to handle the DC), which presents a moderate to high impedance to the AC, while allowing the DC to flow. (You'd probably need a choke as big as, or bigger than, the 'transformer under test' (pad or pole transformer), to handle all that AC voltage, while also not losing all your DC to winding resistance -- needless to say, the copper wire must be beefy, too.)The result would be excessive and harmonically unbalanced current draw through the transformer, which would cause it to heat up, and eventually open circuit (via thermal fuse or upstream breaker, hopefully). Which would take out maybe one side of a city block of houses, or at worst, the neighborhood (usually 10-20 blocks?).The primary concern these days is not so much from loads as from solar flares, which cause very low frequency AC induction in long distance power lines (mainly from line to ground, but also line-to-line to a lesser extent) -- saturating 100MVA transformers is a lot more spectacular than toasting a city block!Tim

author: hazuki
author_rank: Contributor
timestamp: December 18, 2014, 01:00:22 am
content: Well, what's the mechanism that prevents the DC flux slipping far to either side to cause the transformer to run into saturation? Is there negative feedback in the physics of the transformer to reset the transformer or is there some clever method that is applied externally?Thanks for your replies guys!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 18, 2014, 12:57:27 pm
content: Just operating? It's all transformer isolated, so there's no DC element to cause polarization in the first place. Indeed, as the core goes slightly into saturation, the input current is slightly increased, and voltage slightly decreased as a result, which acts to keep it centered. The same occurs with degaussing, when removing undesired magnetization of objects (the difference being, only the coercive force must be applied, not full saturation, to achieve degaussing).Tim

author: hazuki
author_rank: Contributor
timestamp: December 18, 2014, 11:13:30 pm
content: Ok I see. Upon reading more, I learned that if there is a DC load on the transformer, is has the POTENTIAL to saturation, but doesn't runaway. Runaway only occurs if there is a significant volt*second quantity. A DC load like a half wave rectifier will only change the operating point of the B-H curve. If that DC load becomes too great, the operating point can then shift into the region of saturation.Starting to get it now