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Does this "pattern" have a name? (Bad current regulation on AC inverter)
Posted by ivanjh on 05 Sep, 2023 13:08 -
Background:
I recently had a grid tie solar system installed.
Intermittently (maybe when limited by available PV power?) it's generating noise on the mains lines that I can audibly hear in some devices and it's causing other devices to "malfunction" (relays in the microwave cycle when not actually "cooking").
Luckily, I'm equipped to trace the voltage/current for the inverter (images attached)
It seems to just "lose" its current regulation ability for periods.
The underlying 20kHz switching seems to stay during these events.
Current spikes correlate with voltage spikes - so I'm pretty sure the inverter is the source.
I'm not looking to repair the unit, or diagnose the internals.
I'm looking to provide this information to the supplier/distributor/manufacturer.
The question:
Does this "pattern/failure" have a name?
I'd label it "poor current regulation" but it seems more specific - suffering constant overshoot+undershoot.
Targeting a 1.0 Power Factor, it should be a sinusoidal current wave - so is this "tracking error"?
It's not really regular so I don't think "harmonics" would suit.
Or does the irregular nature just make it "noise"?
My googling has failed me, and any help is appreciated.
Warm Fuzzies,
Ivan
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I would call it "intermitently noisy output". Inverters aren't simple devices and finding out wether the problem is in current or voltage regulation is hard. The reason being that, if you are using mostly resistive loads the current will always mirror the voltage.
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This looks weird. Not like an inverter at all.
Isn't it just arcing somewhere? -
That's certainly very unusual, the name is probably somewhere along the lines of kaput!
You say installed, was that a known qualified PV installation company ?
What is the inverter make and model ?
What country are you talking about (local regulations) ?
My initial reaction would be a grid that is to soft, high impedance connection or even worse some kind of reactive connection.
I take it you have not fitted some kind of filtering components in your house ?
Do you have any nasty reactive loads connected ?
How are you measuring current, if with a shunt, what value ?
Sorry for all the questions but your post is a little bare
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Wow. I wonder if the "bad" waveform is related to specific devices in the house operating. I might guess that some unit with a nonlinear power supply might cause this. Possibly a refrigerator or air conditioner with an inverter driven motor, or maybe an induction stovetop. Also, think about LED lights and SCR dimmers.
Jon -
That's certainly very unusual, the name is probably somewhere along the lines of kaput!
Sorry for the belated reply.
You say installed, was that a known qualified PV installation company ?
What is the inverter make and model ?
What country are you talking about (local regulations) ?
My initial reaction would be a grid that is to soft, high impedance connection or even worse some kind of reactive connection.
I take it you have not fitted some kind of filtering components in your house ?
Do you have any nasty reactive loads connected ?
How are you measuring current, if with a shunt, what value ?
Sorry for all the questions but your post is a little bare
The device in question was a Sunways STS-10KTL Grid tie solar inverter (Australia Region A).
Known qualified company, who'd never seen anything like this before (not that they typically look that hard).
This would only happen sporadically during low-light, producing 1-2kW. Never seen during "sunny" 4-11kW output.
The unit was replaced, and the replacement acted the same. The issue was beyond the manufacturer's ability to support and they eventually refunded the unit.
Yes, the grid is "somewhat soft". With 200m of old aluminum overhead 240V lines to a 1960s HV transformer. The transformer was "tapped down" as far as it would go by the utility. The attending technician ran a simple impedance test which reported 0.25Ω to which he said "That ain't great. But there's zero chance we'll upgrade any of our equipment.".
This would still happen with everything else at the house isolated, and only at lower power output levels.
Current was measured with a Hantek CC-65 current clamp.
For the morbidly curious or someone with the same issue some of my experience is recorded here - https://youtu.be/a8pmicR1z_A
I no longer have the "noisy" Sunways unit, it was replaced with a Fronius PRIMO GEN24 10.0 solar inverter. The waveform produced by the Primo is always clean... but seems to have a separate software bug that triggers "pulsing" after a high AC voltage event (typically local load switching off - e.g. oven element). Pulsing for... at times hours. Yup - I'm one lucky guy with this stuff.
I'm finding that my tech support journey isn't necessarily that much better. It's just too far away from what they typically deal with. Somewhere in the depths of the organization's head-office hides a grey-beard who would shit himself if he saw this trace. But coming via first level support in another country - it's hard to get past the gate-keepers.
What do I call this? 50Hz AC pulsed at 7Hz?
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Hi.
That looks interresting.
Some syncron pulse?
Is the solar system connected to mains or feeding your house only? -
Hi.
That looks interresting.
Some syncron pulse?
Is the solar system connected to mains or feeding your house only?
It's grid tied. Functions perfectly 99.9% of the time.
That is the moment it switches from perfect functioning to "pulsing". (Usually triggered by high AC & local load switch off).
Those pulses peak at 45A (raising voltage enough to be visible in incandescent lighting).
It can remain in this state for hours.
No pattern like this is exhibited at any other time (e.g. initial connect).
Feels like a firmware issue around grid rules and power control (maybe an unstable control loop?) - you can get it out of this state by setting power throttling to 99% (from 100% - which power output can't reach anyway). -
FYI, both look like control instability.
The first, some sort of chaotic failure mode where the current regulation, or whatever it's controlling on, goes not just unstable but bonkers and makes trash. Current is still roughly following the line voltage, so power factor is okay, but the error is bounded rather than asymptotic. That is, it bounces around some range above and below the setpoint, rather than approaching it smoothly as a stable and linear controller would.
The new one looks far better behaved, but is subject to its programmed limits -- namely that it must shut down in brownout and overvoltage conditions. So it starts up, puts some current into the line, line voltage overshoots, it scares itself, shuts off, oh would ya look at that line voltage is nominal, starts up again, and so on. Probably this continues until either a load switches at the same time it does (canceling out the bump), or until enough load pulls down the line voltage generally to where it can start without hiccuping.
AFAIK, the behavior is required by rules, though whether the particular limit here is overly tight or something, I don't know. It is Bad Design, from the standpoint of, it's not only a wildly nonlinear scheme (it's switching on and off suddenly over not much change in voltage), but it's a negative dynamic resistance (higher voltage --> lower current) so is predisposed to oscillate. Perhaps they could've decided upon a more stable set of rules, but, it is what we have now.
Possibly there is a startup speed or power limit option for it that you could adjust, or maybe the voltage threshold, or calibration even (but, the latter would mess up your (onboard) metering, so beware of that).
Possibly, you could add some rotating mass at your end of the line, to act as a sort of rotating capacitor to smooth out fluctuations -- this could be as simple as a large flywheel on an induction motor with power factor correction capacitors, I think, but is more often a synchronous machine; and perhaps a rotary phase converter would perform a similar job, if you happen to also need one for other purposes (good way to run higher powered machine tools from single phase ). This wouldn't be a cheap solution, though.
You could also run everything through a power bank, if you have one (or are interested in so investing). This would be charged gently by solar and/or mains, as needed, or return to the grid likewise. Perhaps throughout the day as demand and grid pricing varies (you'd combine this with a time-varying pricing model from your utility), or just slowly as the solar/consumption balance varies from day to day. The lower load on the grid would also reduce how much and how often it changes, mitigating your impedance problem.I'm finding that my tech support journey isn't necessarily that much better. It's just too far away from what they typically deal with. Somewhere in the depths of the organization's head-office hides a grey-beard who would shit himself if he saw this trace. But coming via first level support in another country - it's hard to get past the gate-keepers.
FYI, as an engineer myself, I have 0.01 reason (not exactly zero, but low!) to believe there is such a person -- in many projects, I might be that very person, or I might be cleaning up after someone hacking their way through a project. Especially from China, designs are highly evolved -- copied from western designs, reproduced and modified, parts substituted with local variants, and lightly tested before release to production and sales. Only shallow understanding is needed in the process; and even those doing original from-scratch work, can get by on account of low wages and cheap parts/proto costs, with evolving rather than synthesizing their designs.
Compounding that, our most frequent interaction in the west, with such designs from the east, is through budget-priced wares -- which, well, what more do I have to say, y'know?
And just to make that clear, this isn't at all unique to China, it's just more prominent there, given their problematic educational standards/methods, and the above bias. There are also plenty who are highly capable -- they have an IC industry for example, many have tried to do that throughout history and most have failed, it is truly one of the most advanced multidisciplinary industries to pursue -- but top performers are in short supply and, I assume, eagerly sought out by companies in most dire need. This is all true of western countries, just differing by degree: there might be more and better here, but the average engineer still amounts to a technician with a leveled-up degree.
Tim -
So, for those interested in the second issue (and not the original issues' pattern naming)...
From some testing today - it looks like it's a Voltage Fault Ride Through (VFRT) setting.
Interestingly, it's on the high voltage region, and instead of just avoiding tripping out - it is also set to cut current to zero in this voltage region.
Unfortunately, it appears to come back on "too" hard and re-enters the region under its own exported power... cutting power again, etc, etc.
It feels like some dampening was missed here.
Anyway... if you arrived here as a search result on a Fronius Primo GEN24 10.0 unit pulsing power on a high voltage / high impedance Australian grid connection - trying setting VFRT Region 1 Current Calc Mode to Passive. -
You could maybe add some simple loads on your end, some a line reactor or inductive load. Something with iron.
Nothing likes driving capacitances, and most loads today are capacitive. And with your high mains impedance it almost gets into the territory what I often see, when people use inverter based devices on small generators. The impedance of the genset is too high causing severe harmonics when for example VFD's start pulling high frequency currents.