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Dave van Dongen avatar image

How does frequency shift work in a grid-coupled ESS system - my clock is wrong

I have a grid-coupled ESS system, with PV on the AC output of the MultiPlus and feed-in to the grid enabled. So I have frequency shifting enabled, in case I get more PV power than I use in a power cut situation. But normally the frequency should never shift, when the grid is working, right?

Well, my alarm clock is off ALL the time. Around 10 minutes per 24 hours, I guess.

How can this happen? And what can I do to solve this?

ESSfrequency
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Some pictures of the frequency measurements I did. The Victron system itself says 50.1 Hz on the AC input and a frequency varying between 49.9 and 50.2 Hz on the AC output. My multimeter says 49.9 Hz and sometimes 50.0 Hz, both on the AC input and the AC output. VRM shows the variation in AC output frequency, while the input stays at 50.1 Hz. One time the AC output is down to 49.15 Hz. The VRM can be accessed here: https://vrm.victronenergy.com/installation/12864/share/0d3b30f2

Here are the pictures of the digital alarm clock and my battery-driven analog clock at 13:25h, when I set the time correctly on the digital alarm clock and at 19:37h, where the digital alarm clock is already about 4-5 minutes, compated to 13:25h. To doublecheck I will create a AC socket at the grid connection of my house, so before the Victron ESS system. I will connect the digital alarm clock there and check how it behaves then.

It probably IS your clock. Most digital clocks that sync to the AC grid have a circuit that converts the analog 50 or 60 Hz sine wave to a digital triangle wave or square wave. This circuit is called a "Schmitt trigger " circuit. Less expensive digital clocks use a simple resistor/transistor/capacitor arrangement. All grid-tied Inverters sell back using a modified square wave which has multiple "peaks" that are apparently being detected by your clocks synchronization circuit thus "adding" counts. In reality it is both the clock and Inverters fault.

3 Answers
Wizbandit avatar image
Wizbandit answered ·

It probably IS your clock. Most digital clocks that sync to the AC grid have a circuit that converts the analog 50 or 60 Hz sine wave to a digital triangle wave or square wave. This circuit is called a "Schmitt trigger " circuit. Less expensive digital clocks use a simple resistor/transistor/capacitor arrangement. All grid-tied Inverters sell back using a modified square wave which has multiple "peaks" that are apparently being detected by your clocks synchronization circuit thus "adding" counts. In reality it is both the clock and Inverters fault.

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In electronics, a Schmitt trigger is a comparator circuit with hysteresis implemented by applying positive feedback to the noninverting input of a comparator or differential amplifier. It is an active circuit which converts an analog input signal to a digital output signal.

Thanks for your input!

Ok, but the Victron MultiPlus is supposed to generate a TrueSine Wave, right? So then even a "cheap" digital alarm clock should hold the right time, right? And how could it be that the digital alarm clock DOES run fast during the day and does NOT run fast during the night? In both cases it is behind the same inverter... Or is the signal different? (less of the peaks you mention during the night and more during the day? What could be the cause of that ?)

And does the effect that you describe also happen in a grid-tied system, when the AC input and AC output of the inverter are coupled/connected, supposedly? I am not sure, but please advise.

"my digital alarm clock ONLY runs faster during the day. It does NOT run faster during the night."

You will find that your PV inverter is your problem.

Hi @Dave van Dongen

If you try shutting down your PV inverter for a day, and checking your clock, I think you'll see it is running correctly.

In other words: it's not the Victron that confuses your clock, but the PV inverter.

That's why it only happens during the day (when your PV inverter is producing power).

Dave van Dongen avatar image Dave van Dongen Daniël Boekel (Victron Energy Staff) ♦♦ ·

Thanks Daniel (and klim8skeptic). I was going to try without the PV inverter tomorrow. I think you might be right. But the PV inverter can not/does not influence the net frequency, so the mechanism that makes my alarm clock runs faster is the mechanism mentioned by Wizbandit, then? PV inverter making square waves and the alarm clock seeing this as extra frequency ticks?

If the alarm clock doesn't run faster with the PV inverter, I can do another check: taking out the ESS (this will be bit of a thing...) and having my house on the grid, but with the same PV inverter feeding in. And then seeing if the alarm clock also runs fast.

Thanks for this link! Good information. I install solar panels for a living, but I didn't know this. I am wondering if another PV inverter would do this better. Maybe I can experiment.

Concerning the digital alarm clock, I haven't been practically using it since I have my ESS and PV system. But it turns out to be a good measuring tool to check if the PV inverter leaves a clean grid! ;-)

JohnC avatar image
JohnC answered ·

If your Hz is varying on your 'house' load it isn't connected to the grid. Can't happen. Like Guy sez.

What may be occurring is that when your ac pv winds up past your house/batt? loads your system is disconnecting. For some reason (ESS?). Not my place/expertise to comment there..

Test your Hz under those conditions. My cheapo multimeter can't do that, but GX/VRM graphs can if you have that available.

Also check your exports then too. If nil, then expect your clock to gain some time..

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"Can't happen.", you write. I know. But this seems to be happenening and that is why I am reporting/checking here. If you look at the "evidence" above, you see the AC output frequency of my MultiPlus is varying from the AC input frequency. I don't know why, it's also my question how this is possible on a grid-connected system. Don't shoot the messager...

You can check the VRM of this system here:
https://vrm.victronenergy.com/installation/12864/share/0d3b30f2

You say: " If nil, then expect your clock to gain some time.. " What do you mean with "nil", I don't understand.

Ha no, not aiming at the messenger, just shooting for a useful answer..

By "nil" I meant nil export, with the only real possibility of serious Hz deviation occurring is when your system is isolated from the grid.

I can't grab your VRM , just says "no data". No big deal. But the other shots only show minor Hz deviations and are unlikely to account for serious clock errors. I can't explain your clock observations..

Don't write off reporting errors either. This screenshot is of my own Multi with a generator running, no grid involved. In & Out. An old genny due for replacement, but the Multi reporting a much more serious deviation despite being a simple passthrough setup. I just call it 'best effort' reporting tolerance, and ignore it. Also the sampling time will vary, so they can rarely be matched, yet they must be the same.

And of course I go back to take a look at my 50Hz 'chattering' without the genny. Never seen that before. I don't believe it, but I can't get there anyway - redirected to your site (can't see it, don't worry). Now need to find the way back to my own.. ?

1561973171740.png (24.0 KiB)
Guy Stewart (Victron Community Manager) avatar image
Guy Stewart (Victron Community Manager) answered ·

When connected to the grid in an ESS system, the Multi is synchronised to the grid waveform and the grid sets the frequency.

The AC input transfer switch closes and then the grid is actually connected to your loads.

The multi is unable to shift the grid frequency that is now provided to the loads, it’s not powerful enough. It will make small shifts to its own frequency to keep the waveforms in sync and to check to make sure the grid is still connected (+ - 0.1hz) . But the grid is setting the frequency.

Check what the grid and multi frequency is in the Multi menu of the GX device. If it’s not normal, you will need to contact your power supplier.

If it is normal, it might be time for a new alarm clock?

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You are not understanding...

I live in The Netherlands, with a VERY stable grid. I have had this alarm clock for many, many years and never a problem.

Ever since I installed my ESS system between my house and my grid connection (which has an uptime of 99,9999%, so it never failed during all this time with the ESS installed.), my alarm clock is off BIG TIME. Several minutes ahead (too fast) a day. That's a lot. If I measure the frequency in my house (so on the AC output of the MultiPlus) I get a neat 50 Hz.

You days it is an almost impossibility that I have this problem. I KNOW, that's why I am reporting this. This should not be happening and I couldn't think of a cause of this happening as it is. That is why I am resorting to your help...


See the results of my measurements above. What do you make of that?

Hello Guy, you say I might need a new alarm clock. But I have new information: my digital alarm clock ONLY runs faster during the day. It does NOT run faster during the night. (So I would say the alarm clock is not in need of replacement, but there is a issue with my Victron setup.) I already had a suspicion, but now I carefully checked it. So during daytime (with PV production on the AC output of the MP) my alarm clock runs around 10 minutes ahead in about 16 hours time. But in the 8 hour night, it doesn't run ahead at all. I feed-in my PV excess, so the AC output should be and is connected to the AC input all the time. What more can I check/doublecheck to solve this mystery?

The key to troubleshooting is isolation. From easiest to most difficult.

Try turning off your AC PV panels, does that change it?

Then try another alarm clock. Is it also effected? Or are others ok.

Try changing the frequency ramping settings in the AC PV assistant, does that have any effect? Eg faster, slower, no change?

Try the AC PV on the input side of the Multi, is it still affected?

Once on input side, you could disable the AC PV assistant, does that make a difference?

See if you can narrow it down to 1 element.

How much time you put into diagnosis depends how much you like that alarm clock, vs one with an independent quartz crystal timer.