Do smart meters miscount in the context of self-consumption optimizations?

A few weeks ago, the VDE (Verband der Elektrotechnik Elektronik Informationstechnik e.V.) released a press statement titled "PV feed-in optimization with hot water storage: Meters measure unexpectedly, but correctly", (in German). For a manufacturer of photovoltaic thermal solutions like us, this raised concerns, prompting a closer examination of the issue.

In the first paragraph of the article, a crucial keyword emerges – "continuous"! According to our understanding, only a quickly and precisely modulated heat output from the heating element is suitable for photovoltaics. This is precisely what my-PV solutions ensure, although there are different technologies even within this category.

Preceding the VDE press release was an announcement from the DKE (AK 461) in May 2023, fortunately specifying in more detail which technical solutions cause this perceived mismeasurement. This concerns the so-called phase-cut control with thyristor controllers or the so-called zero-crossing control (or burst-fire control).

None of these two types are applied in my-PV products. Instead, the announcement makes a clear recommendation for the pulse-width modulation method and, consequently, for my-PV solutions!

Under what circumstances can grid feed-in occur even in Category 6?

Grid feed-in of surplus photovoltaic energy occurs when the set target temperature in the heat storage is reached, meaning that the entire "storage capacity" in the boiler or heating buffer has been utilized. If no other energy storage options, such as batteries or electric vehicles, are available, and the inverter does not reduce its power, there is no alternative for the solar power at this point but to flow back into the public power grid.

Those who scrutinize the household connection in detail and measure each phase separately rather than the total current across the three household connection phases, will quickly notice the potential for grid feed-in. Most electrical ballasts for power control operate single-phase, but they regulate according to the three phases at the household connection point. While this is acceptable for energy cost billing, there are different physical directions of energy flow. For instance, if Phase 1 draws 2 kW from the grid, Phase 2 draws 0.5 kW from the grid, and at the same moment, 2.5 kW is fed into the grid at Phase 3, the household connection point is considered balanced.

Lastly, there can be momentary grid feed-in (as well as grid consumption) when the regulation of the power control devices needs to readjust to the set point at the household connection. This practically happens continuously whenever there are load changes, meaning electrical consumers are turned on or off, or when clouds clear, causing a sudden increase in inverter power. However, the energy amounts involved are minimal and practically insignificant. The purpose of our solutions is to execute such regulation processes quickly and accurately. These so-called settling times typically last only a few moments. They should be as short as possible to continuously balance the household connection point as precisely as possible. The speed of these regulation processes in my-PV solutions depends on the type of signal source that detects the surplus at the grid access point.