Dual Solar MPPT Defined

 

Read an updated story from 2019 on inverter MPPT here.

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By Roy Allen, technical sales support engineer at Power-One, a member of the ABB Group

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MPPT is a four-letter acronym referenced in the solar industry by many, but understood by few.  It’s important to understand the definition of MPPT and its functionality, because doing so can help a user improve the energy harvest of his photovoltaic installation, thereby increasing profitability.

What Is A MPPT?
MPPT stands for Maximum Power Point Tracker. It is a circuit (typically a DC to DC converter) 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 its operation.

Why Is A MPPT Necessary?
A solar module is a limited energy DC supply and has internal impedances that vary throughout the course of the day, depending primarily on the level of solar irradiance impinging on the module face and the cell temperature.

An inverter without an MPPT circuit would result in sub-par or non-optimal operating conditions between any PV module (or string of modules) and the inverter. 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. It attempts to drive the operating point of the inverter to the maximum power point of the array, resulting in the highest energy harvest.

Dual vs. Single MPPT
Simply put, in the majority of applications with two strings or more, two MPPTs are better than one. 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 levels of harvested energy.

Connecting two arrays with different solar azimuths or tilts, different string lengths (Voc) or different PV modules to a single-channel MPPT inverter would result in a highly inefficient system and, in some instances, an unsafe one.

By accommodating two arrays mounted at different azimuth and/or tilt angles, different string lengths –— even different modules –— into a single inverter increases the design options for installers because it eliminates the need for a second inverter in many situations.

Additionally, even for PV systems with all strings facing the same direction, using the dual MPPT function is a better choice. Assume a system has four strings all on a flat roof. If a single MPPT channel is used to connect these to the inverter –— in addition to requiring an external combiner –— if one string is damaged or subjected to higher soiling rates or shading issues, this would affect the output of the entire array and result in a lower overall energy harvest.

Breaking the array into two segments on two MPPT channels will improve system harvesting, because if one string/array is damaged or soiled, the output power of the “good” array at the second MPPT will continue to provide full power, thus providing a higher yield than the single MPPT case. Furthermore, in systems requiring shade mitigation, one MPPT input could operate the shaded array with the other operating the unshaded array.

Historically, without dual-MPPT functionality, efficient interconnection of arrays on two different azimuths required two separate inverters, adding significant material and labor costs to the installation. Dual MPPT provides the installer with faster, less expensive system installation with the capability of handling large and small roof surfaces with different azimuths — all using a single inverter.

Combining up to four strings of PV modules to a single inverter without additional external combiner boxes saves time and materials. The exception of NEC section 690.9 allows connecting two PV strings to a single input of an inverter without a combiner fuse in each string. This is as long as the string wiring is sized properly and there are no other current sources that can back feed into the strings.

If an inverter has dual independent MPPT channels, then up to two strings may be connected per MPPT channel without combiner fuses in each string. Therefore, an inverter with dual-MPPT channels can have up to four strings connected without any external combining hardware.

Over the past few years, the output power rating of most PV modules available on the market has increased substantially such that today’s small residential systems don’t typically need more than two strings. Larger residential applications, however, typically require four strings. Commercial systems require a large number of strings and have historically used larger central inverters and external string combiners. But there is an industry trend of using a multiplicity of smaller inverters for these applications, so a dual MPPT inverter would be advantageous in these designs as well.

MPPT And Monitoring
Single MPPT channel inverters can only provide monitoring data at the entire array level. Whether one, two or four strings, data collection will be based on the overall array input. With independent dual MPPT channels, the inverter can provide monitoring information at the MPPT channel level. As a result, there is a finer granularity in the monitoring data, such as site status, energy production and troubleshooting data. This is important because, depending upon the system design, the loading of the two channels can be different. Therefore, for small systems (with one string per MPPT channel), data collection essentially happens at the string level. For larger residential systems (with up to two strings per MPPT channel), data collection is reported at the two-string level. Therefore, in addition to providing proper energy harvest values per channel, the user can understand what is happening at each input at any time. This can be helpful in troubleshooting abnormal conditions at a given inverter input. 

Understanding all these aspects of MPPT can help provide better knowledge of how energy harvesting works and therefore help solar installations become more profitable.