Solar Panel Monitoring System Status

Regarding the status of the Japanese solar panel monitoring system, the reporter interviewed Ikeda Makoto, who runs the EPC service company in Yokohama and also served as a director of the Japan PV Planners Association.

- Japan has successively launched a megawatt (MW) class photovoltaic power station. Has it established a system for monitoring the amount of electricity generated by solar panels, detecting unqualified products in time, and detecting deterioration in performance?

Ikeda: Japan's MW-class photovoltaic power plants generally accumulate data by individually monitoring the amount of power generated by each photovoltaic inverter (PCS). Three 500 kW PCSs are to be installed in a 1.5 MW (1500 kW) class power station. A PCS can monitor the power generation of 3000-4000 solar panels. The amount of electricity generated per panel is inherently more or less different, and will change with the flow of clouds and shadows. Even if an abnormality occurs in more than a dozen solar panels, the amount of power generation is drastically reduced, and it is difficult to find each other by monitoring only the amount of power generated by each PCS.

Shortly after being put into use, some 40 panels were found to be abnormally output—that is, unless there were several hundred solar panel abnormalities that caused the output power to drop significantly, they could not be found.

Ikeda: You can reduce the number of panels that a PCS monitors, so that you can find unqualified products in time. In less than one year after the company started building a 1.5MW photovoltaic power station, it discovered through the monitoring system that there were abnormalities in the output of about 40 solar panels, and contacted the panel manufacturer in a timely manner for replacement. The reason why unqualified products can be found is because this plant uses a monitoring system called "string monitoring."

The "string" refers to a circuit of a plurality of solar battery modules (solar battery modules) connected in series, and generally connects 10 to 20 battery boards in series. MW-level photovoltaic power stations connect multiple strings in parallel in the junction box under the platform and connect multiple junction boxes to the PCS. "String monitoring" measures the amount of power generated by each string of circuits and monitors and accumulates data at any time. To achieve specific implementation, install the sensor for the line box.

In this PV power plant, it is through comparison of the power generation data of each string that several circuits with low power generation are found. Then, the string of panels is checked one by one with the inspection equipment and the unqualified product is finally found. After locking up the "suspicious" string, it is not difficult for an experienced expert to find out which panel in the string is abnormal. In this case, the output power of about 40 panels fell below 10% of the rated value and almost no power was generated.

- Even if only some of the battery plates in the series are abnormal, resistors are formed, resulting in a drastic reduction in the output power of the entire string circuit. In this case, individual monitoring of each PCS should also be able to find it.

Ikeda: In the circuit of the solar panel, a device named “bypass diode” is embedded, and a circuit design is adopted in which the output power is not reduced even if a part of an abnormality generates a resistance and the current can be avoided. Even if a battery panel does not generate electricity at all, its string circuit will ignore this panel and continue to generate electricity. In large-scale systems such as the MW-class photovoltaic power station, bypass diodes are a barrier in a sense and will increase the difficulty of finding substandard solar panels. Of the thousands of panels, even if there are a few pieces of electricity that do not generate electricity, it is very difficult to find anomalies in the overall power generation. The station just mentioned is the same. If you do not use the string monitoring method, you will not find 40 abnormal panels.

String monitoring has become popular in Germany. What is the popularity of string monitoring in Japan's MW-class PV power plants?

Ikeda: In a photovoltaic power plant of class 1M, there are as many as 400 to 500 string circuits. Monitoring all the circuit installations and measuring devices requires high costs. However, in Germany and other places, string monitoring has basically become common, and Japan has successively adopted MW-class photovoltaic power plants. Initially, string monitoring systems were mostly produced overseas and the cost was high. However, recently the inspection equipment manufacturers in Japan have also launched such products in succession, and the choices have become more. Since these monitoring devices must have a communication function, a Japanese system designed on the premise of a Japanese communication standard is more convenient to use. With the increase in the number of systems manufactured by Japanese manufacturers, more and more PV power stations will adopt string monitoring in the future.

——What do you think is the percentage of defective solar panels?

Ikeda: About 40 unqualified products discovered by the company in that PV power plant were found in about 10,000 panels. In view of this ratio, we should pay attention to it. And they are all brands of large Japanese manufacturers. However, it is actually an OEM product that purchases solar battery modules from overseas solar panel manufacturers. With the rise of the MW-level photovoltaic business with Japan's fixed-price acquisition system (FIT), its basic policy is to minimize construction costs and improve investment efficiency. Therefore, they all want to adopt lower-cost overseas products and simplify the monitoring system.

I don't mean to deny the panels of overseas manufacturers. However, when used in Japan's climate and environment, whether or not we can guarantee quality for 20 years is unknown. In the period when the roof was set, there were also Japanese companies that had appeared unqualified products, but they had sincerely replaced them. After these lessons, the product quality of Japanese manufacturers in the past 10 years has been very stable and their reliability has been rising. Although there are companies overseas that claim to be able to guarantee quality for 20 years, there is no guarantee that these companies will remain alive 20 years later.

However, in the construction market of Japan's MW photovoltaic power plants, overseas manufacturers, including OEMs, make up more than half of solar panels. After the implementation of the FIT promotion policy, there will be more and more unqualified MW-class photovoltaic power stations that contain a considerable number of overseas unqualified panels. At that time, there may be cases where panel manufacturers and design and construction companies have disappeared. In that case, there is no way that the power generation company can only afford it in silence. Because of this, it is more and more important to build a meticulous operation and maintenance system through string monitoring and the timely detection of problems and timely responses.