This installation tip was provided by David Bromberg, senior scientist at Aurora Solar
When choosing an inverter for a project, always consider whether the inverter can perform global maximum power point tracking. While most inverters on the market today perform maximum power point tracking (MPPT), not all perform global MPPT. If shade is present on the project site, choosing an inverter that performs global MPPT can help increase energy production.
In addition to converting DC power to AC power, another key function of string inverters is determining the voltage and current levels at which the array operates (its operating point). MPPT refers to an inverter’s ability to identify the operating point that maximizes the output power of a PV array (i.e., it finds the maximum power point of the array).
When conditions are the same across all modules (identical irradiance, temperature and components), an array will have a single maximum power point. However, if part of the array is shaded, there will be multiple operating points that maximize power output. This is due to the impacts of bypass diodes within modules that allow the inverter to “skip over” shaded sections instead of operating at their lower current.
As a result of the behavior of bypass diodes, there are two distinct operating points at which power is “maximized” for shaded arrays. Global MPPT refers to the ability of an inverter to sweep across the full range of current and voltage levels (within its operating voltage limits) to find the point at which power output is globally maximized and avoid picking local maximum power points.
Inverters without global MPPT functionality can make sense for sites without shade, because during the times that the inverter is searching for the maximum power point, it is not actually operating at that maximum power point and the array is not producing as much energy as it could. If the maximum power point is unlikely to change because bypass diodes will not be activated due to shade, then using an inverter without global MPPT can be beneficial. However, for sites with shade, global MPPT capability can help increase the system’s energy production. Analysis of a shaded solar installation by Aurora Solar found that global MPPT increased production by over 5% annually.
Whether the selected inverter performs global MPPT should also be factored into estimates of how much energy the system will produce. Modeling energy production based on the assumption that an inverter performs global MPPT when it does not can lead to underperforming systems. Similarly, failing to account for global MPPT when the inverter does perform this behavior can lead to oversizing a system relative to the customer’s needs. When using software to simulate energy production, it is important that the software models inverter behavior in accordance with the manufacturer specifications for each inverter model.