How the replacement of solar panel micro inverters lead to 50 dB less QRM

Past time my focus was on resolving the interference caused by neighbors’ two solar panel installations. Last year I bought a QRM eliminator with which I, in combination with a Miniwhip active antenna as auxiliary antenna, achieved good results on a decent part of the HF spectrum. But the interference on my beloved 10 meter band (up to over S9 +10 dB) could not be eliminated, it was just too loud. Because past three months I had been working from home due to Covid-19, I also noticed that my wireless mouse regularly went out and the WiFi was very unstable on sunny afternoons. Time to take action.

So-called “plug & play systems”

The neighbors’ solar panel installations consist of two pairs of panels. The older pair of panels are equipped with two micro inverters type GT260 from the (now bankrupt) manufacturer i-Energy. The newer pair of panels features one micro inverter type YC500i from AP Systems. Both pairs were purchased as so-called “plug & play systems”. Completely assembled, all you have to do is plug it in and start supplying the panels directly to the mains. A method that is no longer allowed in the latest directives.

Invested in solar panels ‘to make money’

I went to my neighbor with this and explained that there was something wrong with his installation. But he was initially inadmissible. How is it possible that my installation causes radio interference, was what he said. Until I emailed him several publications by email about solar panels that interfere with police radio channels. He suggested to take a look at the installation. But initially he had no intention of incurring any costs, because more costs would mean a longer time for return on investment.

50 dB interference

After the neighbor unplugged both installations, the interference level dropped to about S3. In densely populated urban district this is still acceptable. When we reconnected the installation with the AP Systems inverter, the S-meter rose to S4. When we then connected the other installation, there was again S9 + 10 dB on the meter. A quick calculation shows that these i-Energy inverters produce 50 dB interference!

What could be the cause?

The installation itself didn’t look as if it was malfunctioning. The i-Energy inverters are built into a fairly heavy cast aluminum housing. DC cables from the panels, which can act as antennas, lay neatly side by side. Because micro inverters were mounted right under the panels, the DC cables were at most one meter (3 ft) long. What could be the cause of that abnormally high interference level?

Protective earth not the best option

Correct grounding is often a problem with inverters for solar panels. An ungrounded or poorly grounded installation does not provide good shielding on the inverter and will therefore irrevocably radiate. Micro inverters are grounded through the AC protective earth conductor. That in itself is a questionable method if you also need that to make the inverter housing work as a shield to prevent it from radiating. Radio amateurs generally know that the protective earth is not the best option for this.

A bad protective earth?

So I started to doubt whether the neighbors had a decent protective earth. In a 100-year-old house, wiring is also often a weak point. Well, it turned out that precisely that pair of panels with the i-Energy inverters that were plugged into a wall socket without protective earth. Well, why would you do that if there is nothing about it in the English manual? The other pair of panels with the AP Systems inverter were connected to an earthed socket in the attic.

Huge risk of short circuit

But the inverters causing interference wasn’t the biggest problem for the owner. An ungrounded installation outside poses a huge risk of short circuit and therefore fire. Something that leads to very dangerous situations on the roof of a 100-year-old house like ours. Where the neighbor was initially hardly receptive to my complaints about the radio interference, this danger made him realize that action had to be taken.

Rerouting gave some relief

To test whether interference would decrease if we pluggen the i-Energy inverters into a socket with earth contact, we rolled out an extension cord to the kitchen, where there was an earth contact socket. That rerouting gave some relief, but the S-meter nevertheless remained hanging around S9. We then also grounded both inverters via the earth lip of the AP Systems inverter, but that also didn’t lower the interference.

Two options remained

I seemed to be dealing with a problem that could not be resolved easily. The two options that remained were:

Making better (new) grounding:
This meant that we had to lay a grounding cable from the roof for a long time, then through a ventilation grid under the floor to the house earth, in the hope that it was still acceptable. Or otherwise have a whole new earthing system made in the garden.

Replace inverters:
Replace the two i-Energy inverters with a non-disturbing inverter. The YC500i from AP Systems, which is under the other installation, is less disturbing. Only this is no longer made. Its successor, the YC600 from AP Systems, is reported on various forums that it produces quite some interference.

See if the inverters could be replaced

Because the first option is quite drastic and the result uncertain, I decided to see if the inverters could be replaced by another type that does not produce strong interference. I had the strong impression that the problem was literally in the inverters itself. Just poor quality by poor design or poor parts. Perhaps the reason that the Chinese manufacturer went bankrupt.

Share the costs

The range of micro inverters is not large and the types out there are also reported to produce interference. Unfortunately, the only manufacturer with a good reputation among radio amateurs, German manufacturer SMA, no longer makes micro inverters. But then I found a webshop that still offers the YC500i for a dump price. I propose to the neighbor that we share the costs and he agrees.

Replacing the inverters

We replace the old inverters with the new ones. We ground the frame of both panels through the inverter earth lip. We twist the DC cables and I place a mains filter on the AC output. The AC cable goes to the shed where there is a socket with earth protection. We put the plugs back in the socket. There is an S5 on the meter.

QRM eliminator suddenly more effective

Of course I would rather have the S-meter needle not passing S1. But if you are realistic, that is simply not feasible in a dense populated area. The cacophony of rattling power supplies, crackling LED lamps and buzzing chargers simply cannot be controlled. But I still come a long way. Where my QRM eliminator was previously hardly effective on 15 and 10 meter band, it suddenly turned out to be a lot more effective. I was able to reduce the S5 interference could to almost S2.

An identical interference signal

My suspicion is that the QRM eliminator is now more effective, because there are now two sources (exact the same inverters) that produce an identical interference signal. The QRM eliminator is not nearly as effective with multiple interference sources, because the produced interference signals arrive at the receiving antenna with different phases. That makes it impossible to cancel them by shifting the phase.

Conclusion

It seems that there are not much micro inverters that make no interference. I do know there is a lot of junk on the market. After all, many people see a solar panel installation as an investment to make more money, rather than investment in sustainable energy. Then it’s likely they demand as cheap as possible products to have a quick return on their investment.

If you ever see a neighbor putting up a solar panel installation, you know that the chances of saying goodbye to your hobby and all your expensive equipment are big. So take action and help your neighbor choosing decent products and with a safe and interference free installation.