You may have designed a system with a small array – for example 3 panels into one MPPT on a H1-G2 inverter. The calculated Voc may exceed the inverter start-up voltage by 20-30V; however there is no output being recorded. It is important to note that the PV voltage will decrease when under load, so here are some important factors to consider:
1. Voltage Drop Under Load (Vmp vs. Voc):
- Voc (Open Circuit Voltage) is the voltage when there is no load connected to the PV modules (i.e., no current is flowing).
- Vmp (Maximum Power Point Voltage) is the voltage under load, at which the PV array produces its maximum power. The Vmp is always lower than the Voc, typically around 70-80% of the Voc, depending on the PV module specifications.
For example, if your PV module has a Voc of 39V, its Vmp might be around 31-33V under normal conditions. You should always consider the Vmp when designing the system (this is particularly relevant with small arrays), as it is the actual operating voltage under load. It is important that you use a design tool when sizing a system, we recommend Open Solar.
2. Temperature Effects on Voltage:
- Cold temperatures increase the Voc (open circuit voltage), and hot temperatures decrease the Vmp (voltage under load). For instance, in colder environments, the voltage could rise above the rated Voc, while in hot environments, the voltage could drop significantly.
- It’s essential to calculate the possible temperature impact using the temperature coefficient provided by the PV module manufacturer.
3. Inverter Operating Window:
- Ensure that the PV array voltage (Vmp) remains within the inverter’s MPPT (Maximum Power Point Tracking) operating window. The inverter typically has a minimum MPPT voltage and a maximum input voltage. Your array’s operating voltage (Vmp) must stay within this range for optimal performance.
4. Voltage Losses in Cables:
- Voltage losses can also occur in the wiring due to resistance, especially in long cable runs. Properly sizing the cables can minimize these losses.
5. Partial Shading or Mismatch:
- If any part of the array is shaded or experiences mismatches (e.g., different panel orientations or degradation), this can reduce the voltage and overall power output. Optimizers or microinverters can help mitigate these issues.
Conclusion
Ensure that your PV array’s Vmp is high enough to meet the inverter’s minimum start-up voltage, and also consider the effects of temperature, load, and wiring. If your system operates in a variety of environmental conditions, it’s essential to model or calculate the voltage drop under load for different scenarios to ensure that the array stays within the inverter’s operating range at all times.
