Utility Crew Screw Up

Easy to do but that is no excuse.

For a transformer the input voltage to output voltage ratio is the same as the ratio of the number of turns of wire on the input to the nunber of turns on the output. Several different transmission line voltages are used and therefore several different winding ratio transformers are used. Install wrong transformer or connect it incorrectly and the voltage in the home will be way off.

For a given power demand the higher the transmission voltage the lower the current and therefore the lower the loss due to current multiplied by resistance. Therefore there is an economic push to use higher voltages.

My neighborhood used to have 13,100 volts on the other side of the transformer. We had enough pole fires caused by leakage across sea salt soaked insulators and wood that they dropped the transmission voltage to 7,800. Less fires, more loss, and the risk of household voltage problems during the transition.

One of our major roads used to be very noisy on summer nights. The warm salt air would cause so much leakage on the poles that all power poles on that street would crackle and glow. I don't know what voltage was on those lines but I would guess tens of thousands of volts.

A neighbor had an open neutral problem that burned some of the items in her entertainment center. She called the power company and they had a rather "don't care" approach when talking to her. I assisted by taking the phone and told the power company that both the police department and fire department were still on the scene and I would be happy to put them on the phone if it would help. The power company declined the offer and became very helpful. Moral of the story, document and involve any professional or agency that can provide proof that it was a power company problem. This will help you deal with damage claims to the power company and insurance claims.

The part I don't understand is that the physical connections of the Line and Load sides are altogether different. On the high-voltage side you have bushings. I don't recall whether they used one or two-bushing HV connections (we don't live there now). The low-voltage side used lugs on what appeared to be short porcelain feed-throughs.

My puzzlement really is more the physical hooking-up, if you will, of those connections so as to produce that voltage I measured. How on earth did they do it?

Well, one of Jeb Bush's goals was to 'transform' the educational system. He wanted to purge the schools of teachers who were 'burned out'. He also wanted the schools be a better 'outlet' for trade skill type jobs. I know his term expired but I don't know who the 'current' governor is.

They make Comfy Chairs for folks like you.

I'm not sure which topic it was but recently one of the contributors indicated that those pole transformers used an auto transformer configuration. While I find that hard to believe, it sets up the possibility for that very condition.

It may have been connected correctly on the outside while the inside was mis-wired. One would expect that something like that was checked with the quality control program where it was manufactured. But, perhaps the QC program only sampled part of the production and a mis-wired transformer got shipped. All it required was for some minimum wage person to be distracted or confused while attaching the leadwires inside the can. I would imagine the utility would have been very reluctant to explain the problem. The utility could afford to cover the damages more easily than the manufacturer. I've seen the bigger of two companies take a hit instead of the little one in other accidents. After all, the bigger company only has so many vendors and Radio Shack doesn't carry transformers that big.

How did you measure the 1320 VRMS ?

That is outside the range or normal professional equipment.

Do you have access to high voltage monitoring equipment or did the utility company told you this value?

I was wondering if you meant 132.0 VRMS...

Fluke HV probe. I routinely work with high voltage. This probe is good to 40 kV.

Well I can understand the possibility of the first screw up. Which is to wire up, install the wrong transformer or bad one. Which ever was the case.

It's the second one I don't understand. For the crew installing it not to check the output voltage before putting on line before they walked away from it.

This is just my guess:

Instead of a 100:1 transformer ratio, they used a 10:1 rated transformer. I can't see other explanation, because if they used an autotransformer and left the common connection H1-X1 at the high side, the output voltage would have been 13200 minus 127VAC Approx.

One thing is that they are not actually connected in an auto transformer configuration but they do share common and earth ground connections for both the primary and secondary winding sets which if one of the line workers did not connect the HV sides common and earth grounds properly or a connection was broken further down on the HV return circuit its more than possible to get a very high localized floating ground in the sub systems that could create very high and odd voltages.

Most likely they got a bad or damaged transformer and the line crews did not do a pre test of the output voltages before connecting the secondary leads.

Just a guess though.

"One thing is that they are not actually connected in an auto transformer configuration but they do share common and earth ground connections for both the primary and secondary winding sets"

I beg to differ, since the above statement actually characterized what and how an autotype transformer is electrically should be connected! The problem maybe caused by an internal "open ground connections" which in turn may have caused the distribution of a higher voltage to all houses served by that transformer..

The primary and secondary windings of the autotransformer may also have been connected series - opposing, rather than series - aiding, which by doing so caused a lower output potential than the actual high tension supply line..

An auto transformer has the primary and secondary in series. A normal transformer with independent and isolated primary and secondary windings that happen to share a common circuit connection are not an autotransformer.

In the second case the primary could loose its connection to the common point and the only effect would be the transformer turning off. Or the secondary could loose it connection to the common point and the only effect on its output would be that it is now floating or disconnected from the common point of the circuit. Its output voltage would still be the same.

Utility step down transformers for normal lower voltage residential and commercial power have independent primary and secondary windings and they do share an earthing ground point but they are not deliberatly configured in a series connection that makes them an autotransformer. Or at least not here in the USA.

There's no doubt that in an autotransformer the primary and secondary windings are tied together, in series with a common point shared between input and output.

Now the problem is in the statement, "A normal transformer with independent and isolated primary and secondary windings that happen to share a common circuit connection are not an autotransformer".

Visualize the two separate (coils) windings that shares the same magnetic flux, connect them together in series and then tie that junction to a common point that happened to be earthed to ground. This (earth/ground) point now becomes common to both input and output of the autotransformer. Similar to the schematic diagram of an automotive ignition coils!

If the coils are wound separately on two different cores, are not sharing any magnetic flux, then your assessment is correct! In this example, there will be no transformer action even their windings has been tied together and made it as a common point !

On the other hand, with the OP's described scenario, although externally the transformer may have been been tied to earth / grounded, but internally the windings happened to be floating, not connected together, it is still possible to get a much lower output than the input voltage measured with respect to ground. Lower voltage due to the primary/secondary turns ratio, via leakage current / induction, since the generator/ source is always referenced and earthed to ground... Otherwise it will be a catastrophy since the full high tension line will be inside those houses!

What's the possibility that there wasn't enough clearance from input connection to the transformer to the output? Either the two leads touched or they were in close proximity such that the input lead arced over to the output?

Doubtful they were touching, though they could have, I suppose (the transformer was a ways down the street and such wouldn't have been obvious from my vantage point), but the voltage I measured (1320 Vrms) seems awfully low for such a situation. I think the HV side for residential distribution is normally higher, and more like 7-9 kV? Just guessing here. Utility guys on the forum would know. And Arcing? Highly unlikely, I think, for several reasons: The LV side presents a very low impedance to ground and so the arc would've essentially been one of the HV arcing straight to ground through a few milliOhms. In other words, a pretty darned HOT arc and for two hours straight - and it would've been arcing before the crews left. Arcs are bright and noisy as a rule and I'm sure they would've noticed. Nor would the voltage at my outlets have been as measured, I think, but probably *lower*, as that end of things would've been arcing somewhere as well and I would've been measuring the voltage drop across *that*. Arcs present a very low impedance, as a rule, and so the voltage drop across a second arc to ground (elsewhere from the transformer) would probably measure less than a few hundred volts, if that. Nor did the voltage seem to fluctuate, really - it remained fairly steady for the duration. An arc would have produced a much more erratic reading methinks. My guess is that anything that was going to blow at the initial application of that voltage, did, and what I measured henceforth was the open-circuit voltage straight off the transformer. My assessment, at any rate. :)

I figured there might've been a twist in the wire that, as it relaxed or unwound, leaned from one terminal to the other. In looking at power lines, it looks as though linesmen are typically very conscious of things like that and try to run wire so as to avoid problems, but you never know.

Putting it on the wrong tap just seems all the more far-fetched. As soon as they closed the circuit, it would have lit up like fireworks. Hardly something they'd leave for a few hours.

The problem may have been caused by a combination of an internal "open ground connections" or shorted windings between the primary and secondary which in turn may have caused the distribution of a higher voltage to all the houses served by that transformer..

The primary and secondary windings of the autotransformer may also have been misconnected series - opposing, rather than series - aiding, which by doing so can cause a lower output potential than the actual high tension supply line..

Hi all,

I work for a utility, (not directly in the distribtution side of the business), and here's my $0.02 FWIW. It seems like the wrong pole top transformer was installed. I don't believe that inadvertently connecting the HV side of the transformer to the LV side would result in the conditions you measured, because the LV side is only insulated to approximately 600V, and any primary distribution voltage would ground out through the tank of the transformer and trip upstream fuses/protective devices (that's why the HV side has bushings, and the LV side has smaller lugs). You didn't mention whether you measured the 1320V between the two hot legs, or between one leg and ground. I'm guessing that you measured it between one leg and ground, since your neighbors' wiring insulation failed at 2x the rated voltage level. Many utilities have multiple distribution voltages - 25kV, 13.2kV, 8kV, 4.2kV and keep stock to replace failed transformers. It appears that the wrong one was installed, which is why the utility accepted responsibility and made amends.

Stilicho

One leg and ground, being as it was a standard U.S. wall outlet. This is not to imply that the voltage would measure twice that across both legs as this assumes the voltage was symmetric (rather, antisymmetric since they're 180° out-of-phase, but you know what I mean). Just seeing that 1320 Vrms on that outlet was enough for me!

(It did get me a nice new stereo system, though, and boosted considerably my respect for that utility company's professionalism, so all was not lost. )

Slightly off topic here, but we had a power outage recently and yet there was about 50VAC still on the line after about 1 hour after the outage. It was enough to power a router, keep a monitor on and some other low power devices including a CFL. Clearly things were eventually going to crap out and so I tripped the main breaker to the house.

Any explanations? Could it be bad residential solar inverters that are not shutting down properly and still back feeding full power? Can't imagine that the total capacitance in network could keep all these low power devices dribbling along?

Thanks!

We had that problem during and after a hurricane. Line voltage was nearly that - around 60 volts. Biggest problem with voltages that low is that it is one of the fastest ways known of turning motors into expensive repairs. Refrigerator compressors and such. When the voltage is too low motors just sit there and overheat. Best to unplug anything containing an automatically-controlled motor and cut the breakers on the rest.

I suspect (though I'm not sure) that two of the three phases in our area's distribution system were down. Why that didn't trip some system-wide breakers is a mystery to me. It should have, IMO.