What causes disturbance in mains at zero cross?

[Faringdon]

What might cause electrical disturbance at the mains zero cross?
Is it mainly CCM PFC stages whereby they lack mitigation of the problem where at zero cross, the demand is very low, but because the inductor is rel. large, the current cant be raised up enough in the inductor near zero cross, so therefore the error amp saturates high, and then when the mains rises further in the half cycle, the current then overshoots, causing high di/dt  current spikeing which comes with V=Ldi/dt,  so a voltage disturbance in the mains at zero cross, would you agree this is the main causation?

[jonpaul]

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[floobydust]

Why rationalize that your design approach is wrong.
Anybody in playing in the big league of 3-phase power systems - knows the presence of mains noise and never uses a circuit such as zero-cross detection there.

[DavidAlfa]

Some parts of the statement could be true under certain conditions, but undetermined in the current present, being unable to predict the future at this stage of human development, thus might be also be false, variable, completely wrong or not exposed correctly.

[Faringdon]

Anybody in playing in the big league of 3-phase power systems - knows the presence of mains noise and never uses a circuit such as zero-cross detection there.

Thanks, we have a 3 phase heating system, its about turning resistive heaters on at the zero cross for x half cycles, then off for x half cycles, repeatedly....so we need a zero cross detector on each phase.
Is there any reason why we cant use a standard discrete R/C/BJT or R/C/diode type zero cross detector in the AC front end section?...on each phase.?

[floobydust]

How are you going to deal with noise - it's not a textbook sinewave and heaters with their cabling have inductance.
Why do you need to monitor all 3 phases, why not one phase? You want micro-degree accuracy for a heater?
In your old post of the zero-cross schematic on the other forum (before you deleted it), you need post-processing - a PLL is what bigger systems use, done in firmware.
Just use software to LPF the incoming zero-cross and sync the PLL.
If this notion is alien, there are many app notes on phase-control firmware out there that outline the technique.

[tggzzz]

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[BrokenYugo]

How are you going to deal with noise - it's not a textbook sinewave and heaters with their cabling have inductance.
Why do you need to monitor all 3 phases, why not one phase? You want micro-degree accuracy for a heater?
In your old post of the zero-cross schematic on the other forum (before you deleted it), you need post-processing - a PLL is what bigger systems use, done in firmware.
Just use software to LPF the incoming zero-cross and sync the PLL.
If this notion is alien, there are many app notes on phase-control firmware out there that outline the technique.

Easy: he's not going to deal with it.

Remember treez/faringdon is the guy with an impressive number of employers on his CV, and who repeatedly asks questions similar to Cap dropper capacitor fail?: "Would you expect C102 to fail? ... As you can see, it does have 235Vrms on it (which is above its rating)"

Even if you don't spot his name as the OP, a frequent giveaway in his posts is the phrase "would you agree that" - often indicating that he is asking us to justify his design decisions!

That's the pattern that jumps out at me too "Tell me it's ok to design this 240 volt power circuit to $1 phone charger standards." Always trying to do something cheaper and/or cruder than is safe/legal/practical. And it's not little details they're missing, it's basic stuff that often immediately jumps out even to a hobbyist like me, heat dissipation, insulation, false triggers on a zero cross detector are probably bad, etc. often at least safety adjacent. I still say it's some sort of trolling.

[floobydust]

OP had posted his zero-cross detector schematic, in the usual LT Spice pseudo-design he posts. Then he takes it down and says he can't post it. I didn't see anything great in the circuit, it's a copy of the EDN design but he screwed it up changing to using dropper capacitors.

It's trolling. We get withheld information - schematic or requirements, scope traces, the real problem - in every single thread that OP starts. It just is filling the Internet forums with garbage. There is no solution or outcome or closure ever.

[tggzzz]

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[f4eru]

What might cause electrical disturbance at the mains zero cross?

Technically correct answer: Harmonics 

[Faringdon]

Thanks,
Ayk, A  really good zero cross detector for mains can be had by literally three 1n4148's in series, and two such branches in reverse parallel, (so 6 1n4148's altogether), and this in series with a lowish value resistor.......you put it before or after an offline smps AC filter and it gives true zero cross all the time without jittering....it does need a comparator to look at it, and the isolated supply to feed that comparator...but it gives a magnificent zero cross....i cant immediately think of apps that would require a phase locked loop type instead, but of course there are many.....(i suppose GTI's?)
I once got a 2kW PFC'd SMPS to look at... (EV charger) it had two 1kW parallel interleaved PFCs...alternately on each 10ms...and they had just a cheap zero cross detector for that....so they knew when to switch over from one PFC to the other.

[floobydust]

I say it needs more than a comparator's hysteresis - any spike or noise can still false trigger that. Any filtering introduces a lag.
As well the problem is 3-phase transmission and distribution systems have failures such as loss of a phase, phase-phase short etc. as lines get tossed in the wind, that squirrel gets roasted, one of three fuses blows etc.
Breaker reclosing and tap changing voltage regulators also cause disturbances up to a few cycles.

If a phase drops out, gets lost, loads will backfeed and put voltage on the lost phase. I had one site missing a phase (broken wire at pole transformer) and the induction motors were not happy, growling a bit and through transformer-action they were putting voltage on the missing phase. You don't measure 0VAC. It's just a low voltage and the angle was lagging, way off. Power meter read -ve power for that phase on a motor, +ve high power on the other two. Had me totally confused.

Search for phase sequence detector or phase-loss detector these are used to protect motors and there are tons of weird circuits out there.
Yes you apparently have a resistive heater but what will it do with a missing or shorted phase?

If you have 3 zero-cross detectors, one for each phase, you still need to detect zero-sequence.

So all this ends up best implementing a PLL or oscillator in firmware.
The MCU has a sync'd oscillator and knows mains angle internal and compared to what is seen. You can wait for several seconds before syncing. Then a small correction for drift and small frequency changes.
Because mains absolute phase does not change unless there is a blackout or grid collapse, so the MCU will have an accurate mirror of it.

[fourtytwo42]

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[tggzzz]

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[Zero999]

I say it needs more than a comparator's hysteresis - any spike or noise can still false trigger that. Any filtering introduces a lag.
As well the problem is 3-phase transmission and distribution systems have failures such as loss of a phase, phase-phase short etc. as lines get tossed in the wind, that squirrel gets roasted, one of three fuses blows etc.
Breaker reclosing and tap changing voltage regulators also cause disturbances up to a few cycles.

If a phase drops out, gets lost, loads will backfeed and put voltage on the lost phase. I had one site missing a phase (broken wire at pole transformer) and the induction motors were not happy, growling a bit and through transformer-action they were putting voltage on the missing phase. You don't measure 0VAC. It's just a low voltage and the angle was lagging, way off. Power meter read -ve power for that phase on a motor, +ve high power on the other two. Had me totally confused.

Search for phase sequence detector or phase-loss detector these are used to protect motors and there are tons of weird circuits out there.
Yes you apparently have a resistive heater but what will it do with a missing or shorted phase?

If you have 3 zero-cross detectors, one for each phase, you still need to detect zero-sequence.

So all this ends up best implementing a PLL or oscillator in firmware.
The MCU has a sync'd oscillator and knows mains angle internal and compared to what is seen. You can wait for several seconds before syncing. Then a small correction for drift and small frequency changes.
Because mains absolute phase does not change unless there is a blackout or grid collapse, so the MCU will have an accurate mirror of it.

Do the spikes really get that big they spuriously set off the zero-crossing? I've not seen a mains waveform that was bad enough not to be able to detect the zero-crossing with a simple comparator with hysteresis?

[floobydust]

Just a note - these threads are proven doomed because too many details are missing or facticious and they might trickle in.
OP's question is a bit silly because who cares why it's happening - more important is a design being immune to it.

What voltage window is considered zero by the circuit? For a MOC3061 inhibit voltage is 12V which I calculate as 2° or 113usec (240VRMS, 50Hz). The EDN circuit author mentions 200usec for 21V?

I had seen industrial sites with sqiggles at zero-cross, looks like ringing and a few volts p-p. One had many VFD drives in the plant and the second was a large DC power supply, 3-phase with SCR rectifier (on the secondary) of the step-down transformer. That had a magic phase-angle when things went noisy. I wonder if anyone has looked at solar grid-tie inverters, they have the same challenge - to sync to the grid yet survive their own output hash. I changed to using a PLL.

I think the assumption is that things are quiet over a narrow portion of the sine wave around zero-cross. That's when in time its impedance is highest but dV/dt is highest as well.
Power-line communication or X-10 at zero-cross impose their HF carrier with TX data slots. I'm not saying that their (data) voltage is enough for false triggering, just that it seems to be most vulnerable time for noise to happen. Many industrial loads have energy storage and it takes just one to push around zero-cross and scuttle things.

Good info: Elliott Sound Products AN-005 - Zero Crossing Detectors and Comparators
TI AMC23C12 but hey where is the noise rejection?

[mtwieg]

Just a note - these threads are proven doomed because too many details are missing or facticious and they might trickle in.
OP's question is a bit silly because who cares why it's happening - more important is a design being immune to it.

I agree that the OP is vague on what they're asking about. No numbers, no waveforms... but it seems like this time they're not talking about zero cross detection, but about line current distortion in active PFC circuits. This isn't a disturbance originating from the mains, but from nonlinearity in the PFC itself.

[floobydust]

How do you scope the mains to get the zero cross? 
https://www.electro-tech-online.com/threads/how-do-you-scope-the-mains-to-get-the-zero-cross.165428/

Local interference is different than mains distribution-level disturbances and we'll never know what it was or what fix was found.