The invisible killer of photovoltaic power station safety——Connector mixed insertion

2021-01-21

The solar cell is one of the core components in the solar power generation system, and a solar cell can only generate a voltage of about 0.5-0.6 volts, which is far lower than the voltage required for actual use. In order to meet the needs of practical applications, multiple solar cells need to be stringed into solar modules, and the multiple modules are then formed into an array through photovoltaic connectors to obtain the required voltage and current. As one of the components, the photovoltaic connector is also affected by factors such as the use environment, use safety, and service life. Therefore, the connector is required to have high reliability.

Photovoltaic connectors, as a component of solar cell modules, should be able to be used under harsh environmental conditions with large temperature changes. Although the environmental climate in different regions of the world is different, and the environmental climate in the same area varies greatly, the impact of environmental climate on materials and products can be summarized by four major factors: first, solar radiation, especially ultraviolet rays. The impact on polymer materials such as plastics and rubber; followed by temperature, among which high and low temperature alternation is a severe test for materials and products; in addition, humidity such as rain, snow, frost, etc. and other pollutants such as acid rain, ozone, etc. Impact on materials. Furthermore, the connector is required to have high electrical safety protection performance, and the service life must be more than 25 years. Therefore, the performance requirements of photovoltaic connectors are:

(1) The structure is safe, reliable and easy to use;
(2) High environmental and climate resistance index;
(3) High tightness requirements;
(4) High electrical safety performance;
(5) High reliability.

When it comes to photovoltaic connectors, one has to think of the Stäubli Group, where the world’s first photovoltaic connector was born. “MC4“, one of Stäubli’s Multi-Contact full range of electrical connectors, has experienced 12 years since its introduction in 2002. This product has become a norm and standard in the industry, even synonymous with connectors.

solar power station

Shen Qianping, graduated from the University of Stuttgart, Germany with a master’s degree in electrical engineering. He has been engaged in the photovoltaic industry for many years and has rich experience in the field of electrical connection. Joined the Stäubli Group in 2009 as the head of technical support for the photovoltaic products department.

Shen Qianping said that poor-quality photovoltaic connectors are likely to cause fire hazards, especially for rooftop distributed systems and BIPV projects. Once a fire occurs, the loss will be huge. In western China, there is a lot of wind and sand, the temperature difference between day and night is large, and the intensity of ultraviolet radiation is extremely high. The wind and sand will affect the maintenance of photovoltaic power plants. Inferior connectors are aging and deformed. Once they are disassembled, it is difficult to insert them again. The roofs in eastern China have air-conditioning, cooling towers, chimneys and other pollutants, as well as the salt spray climate by the sea and the ammonia produced by wastewater treatment plants, which will corrode the system, and poor-quality connector products have low corrosion resistance to salt and alkali.

In addition to the quality of the photovoltaic connector itself, another problem that will cause hidden dangers to the operation of the power station is the mixed insertion of connectors of different brands. In the process of photovoltaic system construction, it is often necessary to purchase photovoltaic connectors separately to realize the connection of the module string to the combiner box. This will involve the interconnection between the purchased connector and the module’s own connector, and due to the specifications, size and Tolerance and other factors, connectors of different brands cannot be matched well, and the contact resistance is large and unstable, which will seriously affect the safety and power generation efficiency of the system, and it is difficult to obtain the manufacturer to be responsible for quality accidents.

The following figure shows the contact temperature rise and resistance obtained after TUV mixed and inserted connectors of different brands, and then tested TC200 and DH1000. The so-called TC200 refers to the high and low temperature cycle experiment, in the temperature range of -35℃ to +85℃, 200 cycle tests are carried out. And DH1000 refers to the damp heat test, which lasts for 1000 hours under high temperature and high humidity conditions.

photovoltaic connector

Connector heating comparison (left: temperature rise of the same connector; right: temperature rise of connectors of different brands)

In the temperature rise test, connectors of different brands are plugged into each other, and the temperature rise is obviously greater than the allowable temperature range.

solar power generation system

（Contact resistance under mixed insertion of connectors of different brands）

For contact resistance, if no experimental conditions are applied, there is no problem with connectors of different brands plugging into each other. However, in the D group test (environmental adaptation test), the connectors of the same brand and model maintain stable performance, while the performance of connectors of different brands varies greatly.

photovoltaic connectors

For connectors of different brands that plug into each other, its IP protection level is more difficult to guarantee. One of the main reasons is that the tolerances of different brands of connectors are different.

Even if connectors of different brands can be matched when installed, there will still be traction, torsion, and material (insulating shells, sealing rings, etc.) mutual contamination effects. This will not meet the standard requirements and will cause problems in the inspection.

Consequences of mixed insertion of connectors of different brands: loose cables; a significant increase in temperature rises and leads to a risk of fire; deformation of the connector leads to changes in airflow and creepage distance, resulting in a click hazard.

In current photovoltaic power plants, the phenomenon of inter-plugging of connectors of different brands can still be seen. This kind of wrong operation will not only cause technical risks but also legal disputes. In addition, because the relevant laws are still not perfect, the photovoltaic power station installer will be responsible for the problems caused by the mutual insertion of different brands of connectors.

At present, the recognition of “interplugging” (or “compatible”) of connectors is limited to the use of the same series of products produced by the same brand manufacturer (and its foundry). Even if there are changes, each foundry will be notified to make synchronous adjustments. The current market results of tests on connectors of different brands that are mutually inserted, only illustrate the situation of the test samples this time. However, this result is not a certification that proves the long-term validity of interplug connectors.

Obviously, the contact resistance of connectors of different brands is very unstable, especially its long-term stability is difficult to guarantee, and the heat is greater, which may cause a fire in the worst case.

Regarding this, the authoritative testing organizations TUV and UL have issued written statements that they do not support the application of connectors of different brands. Especially in Australia, it is mandatory not to allow mixed connector insertion behavior. Therefore, the connector purchased separately in the project must be the same model as the connector on the component, or the same series of products of the same manufacturer.

photovoltaic power plants

In addition, the photovoltaic connector on the module is generally installed by the junction box manufacturer through automated equipment, and the inspection project is complete, so the installation quality is relatively reliable. However, at the project site, the connection between the module string and the combiner box generally requires manual installation by workers. According to estimates, at least 200 sets of photovoltaic connectors must be manually installed for each megawatt photovoltaic system. As the professional quality of the current photovoltaic system installation engineering team is generally low, the installation tools used are not professional, and there is no good installation quality inspection method, the connector installation quality at the project site is generally poor, which becomes the quality of the photovoltaic system Weak point.

The reason why MC4 is admired by the market is that in addition to high-quality production, it also integrates Stäubli’s patent: Multilam technology. Multilam technology is mainly to add a special metal shrapnel shaped like a strap between the male and female connectors of the connector, replacing the original irregular contact surface, greatly increasing the effective contact area, forming a typical parallel circuit, and having high current carrying capacity , Power loss and minimum contact resistance, impact resistance, corrosion resistance and high temperature resistance, and can maintain such performance for a long time.

Photovoltaic connectors are an important part of the internal connection of photovoltaic power generation systems, not only in large numbers, but also involving other components. Due to the quality of the product itself and the quality of installation, compared with other components, photovoltaic connectors are the most frequent source of system failures, and have an important impact on the power generation efficiency and economic benefits of the entire system. Therefore, the photovoltaic connector selected must have a very low contact resistance, and can maintain low contact resistance for a long time. For example, the Slocable mc4 connector has a contact resistance of only 0.5mΩ and can maintain a low contact resistance for a long time.