Distributed maximum power point tracking system can improve photovoltaic system efficiency

According to the current price of photovoltaic systems, users usually get a return on investment after 7-8 years. Government incentives and the service life of photovoltaic systems must last for 20 years or more. The return on investment of solar photovoltaic systems depends on the annual power generation of the system, so users need photovoltaic systems that are efficient, reliable and easy to maintain so as to obtain maximum 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 a shadow cast by a tree, chimney or other object blocks a PV system, it causes a "mismatch" problem in the system. Even a small amount of shadowing 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 many factors affect system power generation, including internal battery module interconnection, module orientation, series and parallel connection between photovoltaic battery groups, and inverter configuration. PV modules are connected to each other through multiple strings of cells, each of which is called a "string". Each string is protected by a bypass diode to prevent the entire string from being damaged due to overheating if one or more cells are shaded or damaged. These series or parallel battery strings enable the panel to produce 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 photovoltaic system is partially shaded, the current in the unshaded cells flows through the bypass diodes of the shaded portion.

When the PV array is shaded and the above conditions occur, 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 diode bypasses the shaded panel and the unshaded panel 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 a PV system. The adverse effects of shading on the system can be mitigated by using distributed MPPT control .

Minimize system mismatch issues using distributed MPPT:

To maximize the power output of each solar PV panel in the array , National Semiconductor has developed SolarMagic™ technology. This technology allows each panel to output maximum power even when there is a mismatch in 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 PV panel, thereby compensating up to 50% of the power loss caused by mismatch. SolarMagic power optimizers can be quickly and easily installed in traditional solar PV 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. As a result, the entire string has the same output current, greatly reducing hot spot issues 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.