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posted by martyb on Tuesday December 15 2015, @04:18AM   Printer-friendly
from the blackouts-and-brownouts dept.

Our power went down yesterday afternoon (December 12). The utility posted a message on their 1-800 number with expected repair time (a few hours later), but no explanation except that ~2000 customers were affected in our suburban area (Northeast USA).

Here's the weird bit -- LED bulbs stayed on, at reduced brightness. I got out a meter and measured 16 VAC in the house. This was enough to make useful light from "dimmable" LED bulbs (happened to be GE brand). After it got dark, we could also see that incandescent bulbs were giving off a faint reddish glow.

16VAC was also enough to keep a Netgear home router/Wi-Fi box going, it must have a switcher in the wall wart that accepts a really wide input voltage range?

Called a friend on the other side of the country who is an EE (with hardware background). He didn't have a good explanation, but suggested that in the process of bringing the grid back up there might be some big voltage swings--recommended unplugging everything we could. Went out to dinner and all was restored when we got home (no damage).

He also told a story from a rural area (near CA-Nevada border) where there was a power failure that upset the normally-balanced split phase -- instead of ~120V on both sides of neutral, the power went to 80V on one side and 160V on the other side of neutral. Equipment on the high voltage side failed due to extended over-voltage, seems that surge suppressors won't deal with this much energy.

Anyone? How does the grid fail-soft?


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  • (Score: 5, Informative) by LoRdTAW on Tuesday December 15 2015, @02:43PM

    by LoRdTAW (3755) on Tuesday December 15 2015, @02:43PM (#276656) Journal

    A more likely scenario would be an open neutral leg or failed voltage regulating transformer. Any properly built home in the last 60+ years would have its panelbox neutral and ground line connected together. The utility transformer would also be tied right to ground creating a return path but of higher impedance depending on moisture content of the earth resulting in a voltage drop. If the neutral opened before the earth return, all sorts of bad things could happen if there are no other transformers downstream as appliances would be in series with others and dividing the voltage. That could fry certain appliances. If there were downstream transformers, they could form an autotransformer setup and recreate the missing neutral.

    One type of regulating transformer that would cause low voltage during a failure are known as ferroresonant transformers a.k.a. constant voltage transformer or CVT. They use an LC tank circuit on the same core to regulate the output voltage. They absorb spikes, boost low voltages and sags due to varying load conditions all without a single moving part. The only external parts are the caps. The tank circuit regulates the flux of the core. Voltage drops? More flux is allowed to build. Higher voltage? Less flux is allowed to build. If the capacitors blow, they safely go into a low voltage output condition as they can't regulate. Beautifully simple. They are very useful as they can compensate or long runs in rural areas where voltages can swing widely due to line impedance and varying load. So if the factory down the road turn a few big motors on and the line voltage begins to drop, the transformers take up the slack and boost the voltage.

    I actually replaced a small one just last week in an electron beam welder which stabilized the voltage supplying the filament, focus and deflection coils. The foreman was confused as to why there was only 8 volts at the fuse from the output of the transformer. Turns out one of the old caps blew, the case was swollen a bit and was internally shorted. Repair was as simple as ordering new caps of equal voltage and value rated for inverter and line filtering duty. Loaded tested it and put it back on the shelf. Caps cost a grand total of 25 bucks. FYI, the input voltage is rated from 190 to 260V and the output is a constant 118V. We feed it from a 3 phase 120/208 Y using only two legs for single phase 208.

    Another method of line regulation is the use of multi tap autotransformers where they switch taps when needed to boost voltage. It's possible that the autotransformer opened up or one of the transformers failed in a way to become an inductor and simply insert resistance to drop the voltage.

    Either way, the fault was certainly one in a transformer or open neutral.

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  • (Score: 0) by Anonymous Coward on Wednesday December 16 2015, @01:58AM

    by Anonymous Coward on Wednesday December 16 2015, @01:58AM (#276932)

    IMHO ferroresonant transformers are underrated, meaning, they should be used more. They work by core saturation, BTW, so if input voltage increases, you just get more input current flow, but not more output voltage because the core is saturating (clipping). They're not as efficient as a good switching supply, but they sure live longer.

    • (Score: 0) by Anonymous Coward on Wednesday December 16 2015, @03:42AM

      by Anonymous Coward on Wednesday December 16 2015, @03:42AM (#276957)

      They're more expensive than regular old silicon steel transformers, bulkier per watt, and run hotter (less efficient).
      The only place I've seen them deployed where they were justified was in a product acceptance test setup where *repeatability* of results was paramount.

      ...then again, I live in a First World country with a pretty reliable electricity supply.

      -- gewg_

    • (Score: 1) by driverless on Wednesday December 16 2015, @05:13AM

      by driverless (4770) on Wednesday December 16 2015, @05:13AM (#276973)

      Ferrorresonant transformers are awesome, best power protection you can get (within sane price limits), they're almost indestructible. The downside to them is that they buzz like a small substation, and they're quite lossy, efficiency can go as low as 60-70% under low load, and you're throwing away this power all the time, i.e. even with no load connected or the load powered off.

      Oh, another issue is size and weight, a 1500kVA Sola weighs in at something like 40kg (which contributes to its resilience, your power spikes are being absorbed by 35+kg of iron and copper, not some woosy MOV, which shouldn't even be used in any application involving power protection).