Differences Between Single and Dual Maximum Power Point Trackers

By Roy Allen, technical sales engineer at Power-One, with contributions from Cameron Stewart, academy technical trainer at Power-One

MPPT stands for maximum power point tracker. It is a (DC-DC converter) circuit employed in the majority of modern photovoltaic inverters; its function is to maximize the energy available from the connected solar module arrays at any time during operation.

A solar module is a limited DC energy supply and has internal impedances and efficiencies that vary throughout the course of the day. The variances depend primarily upon the level of solar irradiance and ambient temperature.
An inverter without an MPPT circuit would operate in a “non-optimal” condition. Unless the inverter can match the strings to extract maximum power the result is a lower efficiency operation for the connected strings. The MPPT circuit constantly monitors the array voltage and current as it attempts to drive the operation point to the maximum power point on the power, resulting in the highest energy harvest.

Why are inverters with dual MPPT functionality better than inverters with a single MPPT?
Simply put for the majority of applications with two strings or more, two MPPTs are better than one MPPT. To support this statement, review the table below.

Considering the entries in the table, an inverter with dual MPPT functionality allows much greater system design flexibility, significant cost savings and higher energy harvest.

You will receive the most design and installation flexibility with a dual MPPT inverter. Consider for a moment using an inverter with a single MPPT, connecting two arrays with different solar azimuths, different string lengths (Voc), uneven soiling and/or different PV modules would result in a highly inefficient and, in many instances, unsafe system. To accomplish these types of installations without dual MPPT functionality would require two separate inverters, adding significant cost to the installation in both material and labor. What dual MPPTS gives the installer is faster less expensive installations with the capability of handling large and small roof surfaces with different azimuths using a single inverter.

The ability to combine up to four strings of PV modules to a single inverter without the need for additional external combiner boxes saves time and materials. The exception of NEC section 690.9 allows connection of two PV strings on a single input of an inverter without the need for a combiner fuse in each string, if the string wiring is sized properly and there are no other current sources that can back-feed into the strings. Because Power-One inverters have dual independent MPPT channels, (i.e., the inputs are designed such that there is no back-feed current from the inverter into either channel), then up to two strings may be connected per MPPT channel without need of combiner fuses in each string.

— Solar Builder magazine

[source: http://www.solarbuildermag.com/featured/single-dual-mppt-power-one/]