Defects in Each Solar Panel Pinpointed by Using Pulse Signals

About 4,000 solar panels are installed at a mega (large-scale) solar power plant of 1MW output, and the number exceeds 10,000 at a mega solar power plant with an output of 3MW or more. Defects and deterioration are observed at a certain rate in line with the passage of time after start of operation.

It is important to detect defective panels at an early stage and take appropriate action for stable operation and maintain feasibility. One of the challenges in O&M (operation and maintenance) is an effective and accurate defect discovery method.

It is said that monitoring of the power generation state in the string stage is effective. Compared with monitoring of the power generation state of each PV inverter, it is easier to detect strings containing defective panels, because the number of solar panels in each string is smaller.

However, it is also known that it is not so easy to detect defects or deterioration of panels even if this method is used unless the difference from other panels is extreme, because power generation amounts are compared relatively in this method, making the difference in generated power less noticeable.

As such, development of a method was required to detect defective solar panels in a short period and accurately.

One method that satisfies these requirements is that utilizing pulse signals, which has been widely introduced to many mega solar power plants in Japan.

The method is used to inspect crystal silicon solar panels.

All related persons at power producers that handle solar power plant O&M, O&M companies and the Electrical Safety Services Foundation in each region say that defects of each panel can be detected in a short time by a simple method, which is the advantage of the system. The system was developed by System JD Co Ltd based in Sawara Ward, Fukuoka City (video below)

Defect detection using pulse signals is called TDR (time domain reflectometry). TDR is used in the semiconductor integrated circuit (IC), communication and power fields. In the power and communication fields, TDR is used to discover failures in long-distance transmission lines. In the semiconductor field, it is widely used for inspection of conduction and insulation.

System JD was originally engaged in the development of tests targeting semiconductors. The company designed semiconductors that facilitate tests and handled automatic creation of test programs.

Defects in solar panels are detected via string input/output terminals in combiner boxes, incorporating conventional technologies for string monitoring and detection of abnormalities in the entire string.

In conventional string monitoring, the power generation amount is measured to detect abnormalities in the entire string by detecting abnormalities in the current and voltage of the entire string. In the system developed by the company, pulse signals are transmitted to each string to identify defective panels.

When pulse signals are transmitted to a string, reflected signals are returned. If the string contains panels with defects such as disconnection, the resistance value at the defective portion is extremely high. When pulse signals pass through areas with an abnormal resistance value, signals with deformed waveforms are returned.

The time required for the return of reflected signals varies depending on the distance from pulse signal transmission/reception points.

Defective panels in a string, as well as disconnected portions between panels, are identified and the operation state of bypass diodes inside panels is grasped based on the time difference.

In normal defect detection using pulse signals, comparison with the reference value is needed. Because it is difficult to set a standard value for a string, the company transmits and receives pulse signals from both the positive pole and negative pole in the same manner to solve this issue. The company obtained a patent for this technology.

With such a method, defects can be detected without being affected by the sunlight condition Detection is possible even at night. Influences of external factors such as power generation state and PV inverter operation state are limited at night when power is not generated. In addition, there is no need to worry about power generation loss because power generation does not have to be stopped.

Operation time per string is short, and the results are obtained in just about 20 seconds.

The company commercialized “SOKODES” by systemizing these related technologies. It includes a mobile terminal or a substrate fixed in a combiner box, and a remote monitoring system.

The substrate fixed in a combiner box is used for periodic daily inspection. Inspection is carried out every night at one-hour intervals. Inspection at a high frequency like this is not required if detection of defects in solar panels is the only purpose. Detection of theft at an early stage is also the purpose. A warning will be transmitted if cables inside mega solar power plants are cut and stolen. The system can also be used for security management.

The substrate is mounted with a temperature sensor, and overheating in combiner boxes due to unknown defects is detected immediately.

The other method is to check the combiner boxes while visiting the solar power plant, and inspect each string manually by using a mobile terminal.

The company initially sold mobile terminals exclusive for disconnection detection, and so forth, but it developed and released terminals that can also detect ground faults later.

A frequently detected defect at solar power plants is a soldering defect inside solar panels, according to the company. Solder inside panels sometimes starts to become loose several years after the start of power generation, resulting in disconnection, if the moisture content is excessively high or low quality solder is used.

Cutting of thick electrodes (bus bars) on the solar cell (power generation element) surface, burning or disconnection of bypass diodes and displacement or disconnection of connectors between solar panels are also detected frequently.

These defects are often generated after a certain period due to repeated thermal expansion and contraction of solder and other materials caused by repeated temperature changes after the start of operation, according to the company.

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