We want to build an open source micro-inverter.
Comparison of micro-inverters with rated output power between 350VA and 400VA:
| Model | HM-3501 | HM-4001 | IQ7A2 | EVT3003 | TSOL-M8004 |
|---|---|---|---|---|---|
| Manufacturer | Hoymiles | Hoymiles | Enphase | Envertech | TSUN |
| Number of solar panels | 1 | 1 | 1 | 1 | 2 |
| Recommended input power (W) | 280-470+ | 320-540+ | 295-460 | 180-420+ | 2 |
|
|
33 | 34 | 38 (18) | 24 | 33 |
|
|
48 | 48 | 43 (58) | 45 | 48 |
| Start-up voltage (V) | 22 | 22 | 22 | - | - |
| Operating volage range (V) | 16-60 | 16-60 | 16-58 | 18-54 | 16-60 |
| Maximum input current (A) | 11.5 | 12 | 12 | 12 | 11.5 |
| Maximum input short circuit current (A) | 15 | 15 | 20 | 15 | 15 |
| Rated output power (VA) | 350 | 400 | 349 | 300 | 600 |
| Peak efficiency (%) | 96.7 | 96.7 | 97.7 | 95.4 | 96.7 |
| CEC weighted efficiency (%) | 96.5 | 96.5 | 97.0 | 95.0 | 96.5 |
Subject to further modifications, the micro-inverter should have the following specifications:
- Efficiency > 90%
- Operating range: 16V - 58V
- Input power from solar panel: 350W - 550W
- Power: 400VA with possibility of software limitation.
- Power factor ≈ 1
- Total Harmonic Distortion (THD) < 5%.
- Electrical isolation between solar module and grid voltage
- Temperature range: -40 °C to 60 °C
- Interfaces:
- WIFI with SunSpec Modbus
- Powerline Communication (PLC)
Optional features:
- Adjustable Power Factor
The technical implementation of the micro-inverter will be continuously revised and iteratively improved during the course of the project. Comments and suggestions for improvement are welcome here!
During basic research, we came across the application note 5. The application note describes the implementation of a 250W grid-connected micro-inverter. The design is based on 2 power stages, namely an interleaved isolated DC-DC boost converter and a DC-AC converter.
The application note provides a detailed description of the operation and component selection.
The system presented is relatively simple and requires relatively few components. It has an efficiency $ > 90 % $ and avoids flux-walk problems due to the DC-DC boost converter being current-fed 6. The capacitors required are of such low capacitance that they can be implemented as film capacitors, which avoids the eventual lifetime issues with electrolytic capacitors.
For these reasons, we decided to adopt and extend the design.
The following table shows a comparison of different solar modules and their technical data, which were adopted as a guide for designing the microinverter.
| Model | WS350M7 | Meyer Burger White8 | JAM72S-30-550-MR9 |
|---|---|---|---|
| Manufacturer | Wattstunde | Meyer Burger White | JA Solar |
| Power (Wp) | 350 | 400 | 550 |
| Short Circuit Current (A) | 9.68 | 10.9 | 14.00 |
| Open Circuit Voltage (V) | 46.7 | 44.6 | 49.9 |
|
|
38.1 | 38.6 | 41.96 |
|
|
9.19 | 10.4 | 13.11 |
The required capacitance of the capacitor
Where
-
$P_0$ is the output power, -
$f$ the line frequency, -
$V_{DC}$ the voltage of the DC bus and -
$\Delta V$ is the allowed peak-to-peak voltage variation.
This gives the required capacitance of the capacitor
$P_0 = 400W$ $f = 50Hz$ $V_{DC} = 380V$ $\Delta V = 40V \Rightarrow V_{DC_{min}} = 360V; V_{DC_{max}} = 400V$
The calculation was verified with simulation dc-bus-power-decoupling.
ESP32
The circuit design of the micro inverter was simulated in LTspice. All LTspice simulations are stored in the simulation folder. Since the entire circuit design is quickly complex and time-consuming to simulate, the individual building blocks of the circuit were first built and simulated individually.