About mains transients and how they can happen? - Page 1
Forum section: electronics / projects

author: IanB
author_rank: Super Contributor
timestamp: July 28, 2017, 02:26:43 am
content: There are lots of discussions in other threads about high voltage mains transients, surges, CAT ratings on meters to resist them, and so on.I have a simple, naive question:If the mains supply has a very low impedance, which decreases as you get closer to the source, making it more and more like a short circuit, then how do multi-kV mains transients actually occur? Why do those transients not get shorted out and damped by the low impedance of the mains supply before they can grow so large?I can imagine maybe it relates to the inductance of the circuit, which permits voltages to grow before they can be conducted away?Are there any data for how frequently such transients occur in practice, and their magnitude? I suspect there is a statistical distribution, with small transients being observed more frequently, and extreme transients less frequently.

author: Brumby
author_rank: Unknown
timestamp: July 28, 2017, 04:41:49 am
content: I have no real knowledge of the industry, but I always imagined such spikes were the result of switching transients, with inductive loads being a significant factor.As I understand it, they can occur with supply switching or with customer usage - especially those customers with huge industrial motors and the like. While the further from the generation point we go, the higher the circuit impedances will be - they are still pretty low. This makes me wonder what the transients from an industrial customer might be if the impedancesweren'tas low.I once walked past a linesman who was about to re-energise a residential circuit and commented that there would be a bit of a display when the blades of the switch closed. He responded that the real display occurs when the switch gets opened. It only took a second to imagine all the inductive loads with collapsing magnetic fields that would be generating back emf. I would have liked to have seen that.

author: ConKbot
author_rank: Super Contributor
timestamp: July 28, 2017, 05:07:16 am
content: http://www.st.com/resource/en/application_note/dm00080038.pdfyou're looking at a 1.2us rise time on a surge waveform, putting the ballpark bandwidth at 300KHz.The impedance is low for mains at 50/60 Hz, but I imagine it's quite poor at 300KHz in comparison to the nominal operating frequency.

author: Simon
author_rank: Unknown
timestamp: July 28, 2017, 05:51:43 am
content: Think of the impedance of a 24V car battery, a fan having power removed just 2m from the battery can still induce a -400V spike in the system.

author: Berni
author_rank: Super Contributor
timestamp: July 28, 2017, 06:20:56 am
content: I would imagine the worst case is when a factory full of giant motors is disconnected from the grid.Not only does this produce a inductive kickback in all those giant coils, but the impedance is larger than usual as the grid has just been disconnected, this leaves the impedance to be determined by any other loads on the same line. I wouldn't expect it to go to crazy high voltages of a few kV since there are likely surge arestors at the factories local transformer facility that shunt the current to ground safely.Over here factories tend to have there own transformer on site so the near by residents don't get hit by that kick.CAT ratings are probably more of an issue with localized fast transients, like you are probing right on the terminals of a giant electromagnet where the switch next to it disconnects it and reconnects it. The short spike might strike an arc inside the meter before the mains comes back and sends thousands of amps trough it. Also the CAT rating includes the amps range. A properly CAT III rated meter can be put in to amps mode and stuck across the bus bars of two phases rated for carting perhaps 1000A. Yes there will be quite the fireworks display but the meter should not explode in the users hand.At the factory complex i had a summer job a long time ago there was a electrician that came to fix a industrial lift. He did something stupid with his cheap multimeter and it all turned in to a ball of fire where the meter ended up a pile of molten plastic on the ground and the electrician suffered major burns and needed to have an arm amputated. A good meter might have saved him there.

author: dmills
author_rank: Super Contributor
timestamp: July 28, 2017, 10:55:52 am
content: The mains is low resistance, but the supply can obviously be quite inductive, which causes it to appear as a very much higher impedance by the time you are looking at tens of microsecond rise time events.The lines are also long enough to be transmission lines of significant length when considering those time scales.The overall effect is that switching off a heavy load near the end of a long line can cause both a voltage excursion at that point and a reflected wave to travel back along the line powered by the energy stored in the magnetic fields around the lines, classic transmission line stuff.This is easy to model in spice, but do not forget to include parasitic and you may need a resistor or two to get the maths to converge.The other ugly is when a breaker between the generator and a large inductive load opens with your victim device on the inductive load side of the breaker, many motors can be convinced to generate is disconnected under load and a synchronous machine will (briefly) supply a lot of current if suddenly deprived of its reactive power source and over excited under some circumstances.Obviously the real danger (and whole reason behing the CAT rating) is that a short fast rise time event will start an arc which will be be sustained by the full might of the grid....Regards, Dan.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 28, 2017, 12:22:50 pm
content: Quote from: IanB on July 28, 2017, 02:26:43 amNightmare....I suspect that if you had such a global data collection system that had real time feedback of the mains you could name your price for it. I would imagine you would see trends for countries, population, climate. If you are a dishwasher repair man or someone who's only data point is what has gone on inside their house since they were born, it would be naïve to try and extrapolate a global view with any sort of confidence.

author: Jeroen3
author_rank: Super Contributor
timestamp: July 28, 2017, 01:17:20 pm
content: You'll only find transients in high impedance building wiring. Or, high impedance street poles.On the grid, you'll have lightning strikes. Or perhaps, when an entire part of the grid is disconnected you get a surge in voltage. Because the production suddenly is much higher than the load. Because there is a lot rotating energy on the grid, these magnetic fields in the rotors isn't suddenly going away, that is a relatively slow process. The result is a raise in voltage for a short time. This is a serious problem on ships with ancient regulators.What we sometimes see with cogeneration plants is that when they have weak grid the voltage is unstable.A weak grid is often caused by a small feeder transformer. When the generator is on, typically 1 MW, the voltage climbs.When multiple companies in the neighborhood (greenhouses) have these, the voltage is often pushed up above 420V. Then they go offline on over-voltage. Solar power on some locations has the same issue.

author: Vtile
author_rank: Super Contributor
timestamp: July 28, 2017, 10:11:39 pm
content: What Berni said about meter between live bushbars with ampere connection... I wouldn't want to test it. The 1kA bushbars will deliver so much prospective shortcircuit current that the meter will just go vapor from the arc that will be lit when you are creating the shortcircuit with your meter. Remember that the nominal current rating is only nominal and the sort circuit current is in magnitude of 10x nominal minimum. Ie. your typical 16A Diazed bottle fuse needs way over 100 A of current to work properly. This is one of the central design factors of mains networks.. that the wire inductance does allow fuses to burn when you short the circuit at the end. Also one should note that line voltages are nominal RMS values. Ie. 230 Vrms / 50Hz euro line do have peak voltage of aprox. 325Vp and 650 Vp-p and this between phase and ground. If you are dealing with phase to phase then these are 400V / 560Vp / 1120 Vp-p without transients.One typical source for transients in household is ie. induction motors. Every time the cage bar cuts the magnetic field it creates transient. Unfortunately the transient analysis were shit where I studied distribution stuff and the teacher(s) would have bootheel where the sun doesn't shine if I could decide.

author: Berni
author_rank: Super Contributor
timestamp: July 28, 2017, 10:50:04 pm
content: Yes i really wouldn't want to be anywhere near something like that happening. But a high CAT rated and well designed electricians meter might have a chance staying in one piece with there large HRC fuses inside. While doing the same stunt using a cheap multimeter with normal glass fuses makes it even more dangerous. Not only will the glass fuse have no chance in interrupting a short circuit current even on a weaker mains supply, but also draw an arc that destroys the cheap weak plastic housing, exposing the live arcing insides to your hand if you are holding it.While the multimeter can't keep you from doing idiotic mistakes, but it can make the result of said idiotic mistake less deadly.Im not an electrician that works with these dangerous high powers, i work with nice safe powers on a workbench. But knowing what an inferno can happen with one wrong move around these big mains systems makes me really not want to ever work on them.

author: saturation
author_rank: Super Contributor
timestamp: July 29, 2017, 02:36:18 pm
content: https://www.nist.gov/pml/quantum-measurement-division/surge-protection-low-voltage-ac-power-circuitsIn toto, Martzloff and other workers showed its overall a phenomenon of AC power distribution, so protection should be a norm for all mains derived power supplies, not just industrial locations. Simultaneously, was also the development with studies in transients was also the development of the varistor as a practical device.https://www.nist.gov/sites/default/files/documents/pml/div684/Superalpha.pdfhttp://www.arresterworks.com/history/pdf_files/History_of_Arresters_on_Power_Systems_1965-Present.pdfQuote from: joeqsmith on July 28, 2017, 12:22:50 pm

author: Zero999
author_rank: Super Contributor
timestamp: July 29, 2017, 03:32:46 pm
content: Other than the inductive issues mentioned above. I imagine transients can occur on the mains because it takes time for the grid to stabilise itself. The field current in an alternator or taps on a transformer need to change according to the load and if this doesn't happen quickly enough, the voltage can be higher than expected for awhile.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 29, 2017, 05:54:13 pm
content: Typical mains inductance is a few hundred uH.Low impedance, sure, but at what frequency?(Surge generators are usually around 2 ohms.)Tim

author: joeqsmith
author_rank: Super Contributor
timestamp: July 29, 2017, 06:11:46 pm
content: Quote from: saturation on July 29, 2017, 02:36:18 pmI'm not sure if you are repeating IanB's question by highlighting it or just forgot to add a comment. I am only suggesting that you may find a correlation based on other factors such as location if you had a way to collect the data in the first place.If everyone in this forum worked for a company who sold to the global market. And everyone of us had to go into the field to investigate AC line problems. This was our entire career for the last 40 years. Now we combine everything that we have learned. Maybe, just maybe we could start to paint some sort of picture of what goes on. My take anyway. Lots of variables.

author: saturation
author_rank: Super Contributor
timestamp: July 29, 2017, 08:55:08 pm
content: Hi Joe, my links were to answer the question.Transients are a well studied phenomenon, the frequency they occur, causes, modifying conditions, the locations etc., even the shape of the pulses used for testing that is now an IEEE standard was designed around the field data, analyzed statistically and then modeled, this is between 1970-1990. The NIST link has actual reports of many field studies covering over 20 years of reports. A bottom line is you'd don't need real time tracking as transients are as ubiquitous as noise so it has to mitigated in most all equipment as much as fuses are in ammeters; its caused by natural events like lighting strikes, static discharge, as well as man made such as inductive feedback, capacitive discharge etc., field data determined what type of protection is needed for a specific location [ marine vs commercial vs residential vs power vs military etc.,] and how much energy they are designed to absorb. In power electronics, its a specialty:https://www.amazon.com/dp/0071622489/ref=rdr_ext_tmbWhat enters a home, or a lab is usually what escapes the power providers intervention for that type of use and thus the market for home arrestors, and mains SPD etc..https://www.nist.gov/sites/default/files/documents/pml/div684/Surges_happen.pdfQuote from: joeqsmith on July 29, 2017, 06:11:46 pm

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 30, 2017, 12:49:50 am
content: Quote from: saturation on July 29, 2017, 08:55:08 pmNote that transients aren't much beyond a few kV, because that is the point where unintentional voltage limiters come into play: terminals and receptacles themselves begin to spark over.A good illustration of this was that video of a Russian guy tazering his house wiring: zapping an outlet here caused a lamp over there to spark, and such.The power company does indeed provide transient limiting; everywhere you see a high voltage power line, with a somewhat funny insulator attached to it, and the insulators are grounded, each one is actually a rubber boot over a stack of MOVs, for lightning arresting. That limits peak voltage (at the arrestor) to maybe three times the nominal voltage. The peak can still be quite high at a distance, but only for a short time. This is why the traditional surge pulse is brief: a lightning strike on a pole will be arrested within a modest distance. That pulse also needs to pass through transformers, which may saturate due to the huge flux of a direct surge, thus limiting the energy further. Transformers also have a low-pass filtering effect (hence the microseconds rise time).So, what's left, by the time it gets to your house -- assuming it wasn't a direct hit on residential mains -- is a whole lot easier to deal with.And if it's direct, or very nearby anyway, you might be screwed not just from conducted surge, but from induced EMP as well. (This spring, my parents had a lightning strike on a tree in their backyard -- most things were okay, but some lights and Ethernet routers were trashed. Routers make sense, because although they're isolated, it's only to a few kV, and the wires going every which way means a large loop area for the EMP to act on.)Tim

author: joeqsmith
author_rank: Super Contributor
timestamp: July 30, 2017, 03:34:37 am
content: Quote from: saturation on July 29, 2017, 08:55:08 pmThere is some VERY GOOD reading material in there for sure.Transients are studied, sure but not on a large scale. The standards continue to evolve as we learn more. Having more data would help but there is really no way to collect it on a large scale, yet.And while you may not see a use for such data beyond what the latest IEC standards have to offer, I do.We did not start building the grid in 1970. There was a lot of testing prior to this. A good source of info if you are a bit of a history buff can be found at:http://ethw.org/Archives:Transformers_at_Pittsfield,_part_1http://ethw.org/Archives:Transformers_at_Pittsfield,_part_2#Transient_Analysis

author: SeanB
author_rank: Super Contributor
timestamp: July 30, 2017, 06:08:56 am
content: Look for a mains transient adaptor, or just use a simple one consisting of a 100n class X2 capacitor in series with the primary of a 115VAC to 12VAC transformer, with a 10k resistor connected across the primary as damping, with the secondary connected to a small 8R speaker, and listen to the noise as you turn assorted loads in the house on and off. Cheap and gives an idea of the general noise, and you can even go active and place a 120Hx high pass filter on the output to only hear the harmonics and noise, though the capacitor will tend to make a high pass filter on the incoming side.

author: Circlotron
author_rank: Super Contributor
timestamp: July 30, 2017, 06:36:00 am
content: Quote from: Simon on July 28, 2017, 05:51:43 amMakes me think of 1 metre of battery cable leading to the starter motor solenoid. Some cars actually had the rest of the 12 volt feed for the car wiring attached directly to that starter motor stud! When the solenoid contacts interrupt several hundred amps the inductance of the 1 metre of battery cable makes that stud swings a long way positive!!!

author: saturation
author_rank: Super Contributor
timestamp: July 30, 2017, 01:45:34 pm
content: You are right Joe, I stand corrected. Research is always ongoing. In my electronics perspective, there is less to consider, although who knows what the future will bring?Quote from: joeqsmith on July 30, 2017, 03:34:37 am

author: Fungus
author_rank: Super Contributor
timestamp: July 30, 2017, 02:22:34 pm
content: Quote from: IanB on July 28, 2017, 02:26:43 amNope. It's totally dependent on where you live, the local weather and what your neighbors are doing.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 30, 2017, 06:34:57 pm
content: Quote from: Fungus on July 30, 2017, 02:22:34 pmA perverse crossing of perspectives.You've brought the perspective from "average over decades of experience across the industry" down to "uh, ask your neighbors".The "average" already includes all such "dependencies". Why would you special-case it into uselessness?Tim

author: floobydust
author_rank: Super Contributor
timestamp: July 30, 2017, 07:58:37 pm
content: There is a need to have some numbers on transient voltages, spacings etc. otherwise it's impossible to design and test anything safe.This is important anywhere insulation breakdown can endanger people and property; anything related to electrical power systems: your wall outlet, the equipment plugged in, your electrical panel and breakers, energy meter/solar modules etc.This also applies to your multimeter that happens to be connected to any of this while making measurements on the power system.I can go into the transient voltage/energy numbers and where they came from, but the second part is the probability of them occurring.Walk into a casino, hit the roulette table and spin it. Will you get a 4kV hit today on your TV or multimeter? What are the odds? Roulete has 3-11% chance of winning; also known as "not likely".The overvoltage numbers are at the "not likely to occur" probability level but gamblers may choose to ignore them.