Distributed MPPT design to improve the efficiency of solar photovoltaic system-EEWORLD

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This paper describes the problem of power generation degradation in solar PV systems due to partial shading of the panels, the advantages of using a distributed maximum power point tracking (MPPT) system at the panel level, and discusses the results of various case studies using SolarMagic technology.

Solar energy is one of the most promising renewable energy sources on the market. Due to government incentives and the rising cost of traditional electricity, more and more households are turning to solar energy and installing photovoltaic (PV) systems on their roofs. At the current price of PV systems, users usually get a return on their investment after 7-8 years. Government incentives and the service life of PV systems must last 20 years or more. The return on investment of a solar PV system depends on the amount of electricity the system generates each year, so users need a PV system that is efficient, reliable, and easy to maintain to maximize power generation.

Today, many users who install solar photovoltaic systems have realized that partial or intermittent shading will affect the system's power generation.

Impact of partial shading on solar photovoltaic systems:

When shadows cast by trees, chimneys or other objects block a PV system, it causes a "mismatch" problem in the system. Even a small amount of shadow blocking a PV system can result in a significant drop in power generation. The actual impact of system mismatch caused by partial shading on power generation is difficult to obtain with a simple calculation formula. This is because there are many factors that affect the power generation of the system, including the interconnection between internal battery modules, module orientation, series and parallel connection between photovoltaic battery groups, and inverter configuration. PV modules are connected to each other through multiple battery strings, each of which is called a "string". Each string is protected by a bypass diode to prevent the entire battery string from being damaged due to overheating when one or more batteries are shaded or damaged. These series or parallel battery strings enable the solar panels to generate relatively high voltages or currents.

The PV array is composed of PV modules connected in series and connected in parallel. The maximum voltage of each string of PV modules must be lower than the maximum input voltage rating of the inverter.

When a PV system is partially shaded, the current in the unshaded cells flows through the bypass diodes of the shaded part. When the PV array is shaded and this happens, a VP electrical curve with multiple peaks is generated. Figure 1 shows a standard grid-connected configuration with a centralized maximum power point tracking system (MPPT) function, where two panels of one string are shaded. The centralized MPPT cannot set the DC voltage and therefore cannot maximize the output power of both strings. At the high DC voltage point (M1), the MPPT maximizes the output power of the unshaded string. At the low DC voltage point (M2), the MPPT will maximize the output power of the shaded string: the bypass diodes bypass the shaded panels and the unshaded panels of this string will provide full current. Multiple MPPs of the array may cause additional losses in the centralized maximum power point tracking (MPPT) configuration, because the maximum power point tracker may get the wrong information to stop at the local maximum point and stabilize at a sub-optimal point with VP characteristics.

Figure 1: Standard grid-connected configuration with centralized MPPT functionality, where two panels in one string are shaded.

Different case studies and field tests have proven that partial shading has a serious impact on the power generation of PV systems. The adverse effects of shading on the system can be mitigated by using distributed MPPT control.

Use distributed MPPT to minimize system mismatch problems:

In order to maximize the power output of each solar photovoltaic panel in the array, National Semiconductor has developed SolarMagic™ technology. With this technology, each panel can still output the maximum power even if there is a mismatch problem with other panels in the array. SolarMagic technology uses advanced algorithms and advanced mixed signal technology to monitor and optimize the production capacity of each solar photovoltaic panel, thus compensating up to 50% of the power loss caused by mismatch problems. SolarMagic power optimizers can be quickly and easily installed in traditional solar photovoltaic systems.

Figure 2 shows a typical PV system using SolarMagic™ technology:

The system has two strings formed by n modules connected in parallel. For ease of demonstration, only three photovoltaic modules are shown in each string in the figure, but a string is usually composed of 5 to 12 modules connected in parallel to obtain a string voltage of 500-800V.

All modules in column A have no irradiation misalignment problem, each module has the same characteristics and irradiation is uniform.

All modules in String B have different characteristics or irradiance misalignment due to shading, directional tilt or more dust accumulation. The output of each module is connected at the input point of the SolarMagic™ Optimizer (SMO) module. The output of each SMO is connected in series in the same way as the modules in String A.

Figure 2: Simplified PV wiring diagram of a PV system using a SolarMagic power optimizer.

SolarMagic™ optimizer modules have highly efficient integrated power circuits and use a maximum power point algorithm to maximize the output power of each PV module. Therefore, the entire string has the same output current, greatly reducing hot spot problems and adopting internal bypass mode. Each SMO module will regulate its output voltage to match the overall bus voltage.

The result is that the entire PV system will present an IV curve with a single maximum power point, simplifying the operation of the central inverter and minimizing the power generation losses caused by mismatch.

The table below summarizes the results of field tests of solar PV systems subjected to partial shading, with the last column showing the percentage of lost energy that was replenished by SolarMagic™ technology.

time	Shaded array (%)	Power loss due to shading	Solar Magic's replenishment of lost energy (%)
9:30am	13%	44%	50%
10:30am	11%	47%	58%
11:30am	9%	54%	66%
12:30pm	6,5%	44%	65%
2:30 p.m.	3%	25%	40%

Keywords：MPPT Reference address：Distributed MPPT design to improve the efficiency of solar photovoltaic system

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