November 28, 2023|General

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

Does a smart meter show wrong measurement data due to self-consumption solutions? We address this issue and explain which technologies might create this impression and why this can be avoided with my-PV solutions.

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!

1. Unregulated switching devices

These typically only switch the controllable resistive load on or off. A fully regulated surplus management is virtually impossible. So-called SG-Ready heat pumps, controlled by inverters, usually behave in a similar manner (except for inverter heat pumps within certain limits).

2. Step-changer switching devices

These devices switch heat sources in steps, typically in increments of 500 to 1,000 watts, and provide much better control compared to (1). The average control error is half the step power, which means a 250-watt control error for a 500-watt step. This results in an energy loss of 875 kWh over 3,500 operating hours. With an energy price difference of 35 cents per kilowatt-hour, this translates to an additional cost of 300 euros per year compared to continuously regulated systems.

3. Thyristor Controllers

These operate with a so-called phase-fired control, similar to what is used in low-power light dimmers. According to the technical connection requirements of Austrian and German grid operators, heat sources regulated by phase-cut control can only be connected up to 200 watts without approval. This limitation is due to network feedback and interference with other consumers. Additionally, the installer is responsible for ensuring compliance with all electromagnetic compatibility (EMC) standards. EMC compliance can only be verified through on-site measurements (as is customary with individual system approvals), as thyristor controllers are components and not certified finished devices. Typically, without additional filtering measures, these standards are far from being met.

4. Pulse packet controls

These controllers regulate power by releasing half or full wave packets. Due to non-compliance with the allowed switching frequencies in the grid connection conditions of Austrian and German grid operators, they cause flickering of lighting fixtures.

5. HF-clocked power controller with HF output

These power regulators can be described as true "high-frequency transmitters." At their output, they deliver unfiltered high-frequency voltages of up to 300 volts peak ("electrosmog"). Such devices do not comply with radiation limit values. The distributing party is responsible for ensuring compliance with these limits. High-Frequency Controlled Power Regulators with HF Output:

6. HF-clocked power controllers with 50 Hz sinusoidal output and HF-clocked compact devices:

These power controllers operate at a high frequency with pulse-width modulation, feature sophisticated input and output filters, and comply with the current regulations concerning EMC, TAB, and TAEV in terms of both network feedback and HF-emissions.

Good to know

PV products exclusively belong to category (6). We prioritize quality, and we encourage you to do the same!

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.

Good to know

In most cases, the power control in my-PV solutions is carried out every second, providing users with a very fast, highly accurate, and above all, grid-compliant solution for feed-in optimization with hot water storages!