When smart meters go silent

Link failure in a PV system

Roman Rubanovich

Speech is silver, silence is golden. This truth we have long since learned. But sometimes our «virus of wisdom» reaches electronics designed for data transmission.

In photovoltaic systems, we often need so-called smart meters — «intelligent» energy measurement devices. They can serve different purposes, but most commonly they’re installed at the grid connection point of a site to measure energy import and export.

For example, at a given moment a PV inverter produces 50 kW. If the site is consuming 80 kW at that time, the full 50 kW from the inverter will be used to power the internal loads, and the missing 30 kW will be drawn from the grid (this is import). If the consumption is only 20 kW, then the inverter covers that demand, and the «excess» 30 kW is sent to the grid (export).

Principle of operation of a smart meter

The inverter knows how much it’s generating — but not where the energy is going. That’s what the smart meter measures at the grid input. It usually transmits this data to the inverter via the RS-485 interface. Knowing the inverter’s output and the current import/export value, one can calculate the site’s actual consumption. For example: we produce 50 kW, send 40 kW to the grid — so the local load is 10 kW. All of this is visualized as energy flow in the monitoring system, which is very convenient and useful — though not strictly necessary.

But if there’s no agreement in place for selling excess energy to the grid, then in most cases you can’t do without a smart meter. In such setups, the system operates in zero-export mode, and as soon as the smart meter detects export to the grid, the inverter immediately reduces its power. In this way, the system literally «tracks» internal consumption to prevent overproduction. The same applies to limited export, when the system is allowed to send, say, no more than 30 kW to the grid.

In practice, things get a bit more complex, especially in three-phase systems, where there may be import on one phase and export on another — and this introduces nuances depending on the metering method. But that’s a story for another time. The point is: a smart meter is a truly useful thing. These meters come in single- and three-phase variants, for direct metering, CT-based metering, and other setups.

We usually work with smart meters from Carlo Gavazzi (Switzerland), Victron Energy (Netherlands), Fronius (Austria), and Eastron (China), though not exclusively. We use them in PV system design and also supply them to our clients in Bulgaria.

Recently, one of our partners who had installed 22 Eastron meters in their PV systems returned three of them to us — they «stopped talking to the inverters». The client tried different communication cables, tested with other inverters — still silence. Replaced the meters — everything worked. Asked us to investigate.

I connected them to a compatible logger — indeed, all three were silent. The screen does show the phone icon, which indirectly suggests there’s some connection. I connected them to a computer — they send all measured data. I sent them commands — they respond without any issues. Strange.

Eastron’s European office is in the UK, but they also have a warehouse in Ireland, which helps with logistics after Brexit. I explained the situation to them, but they didn’t believe it. They said: «If the phone icon is there and the computer connection works — everything’s fine.» If we want a warranty replacement — we have to send them to the UK for inspection. We said: «It doesn’t make sense — shipping costs more than the meters.» They replied: «Then send them to Ireland.»

That’s certainly cheaper, but still not economically viable — especially if such cases start happening more often. We need a fast and reliable way to diagnose issues on site.

So I hooked up an oscilloscope and ran a comparative test, including a known-good unit for reference.

Testing smart meters

Turns out two of the «faulty» meters had a weak signal — but at least with correct shape and duration. The third one was just sending a short pulse and then nothing. And yet, even in this state, it somehow reliably sends data to the computer, including all measurements and its serial number. What’s more — it successfully receives all commands, like address change, baud rate, CT ratio, and so on.

Properly working smart meter on the left

A rather odd behavior — seemingly a degraded or partially damaged interface. I made a video of the test and sent it to Eastron. They replied that they would send a courier at their expense to collect and examine these quiet guys.