Key Factors in Selecting Grid-connected Photovoltaic Inverters: Reliability

In the life cycle of a photovoltaic power station, the quality of the inverter largely determines the power generation income of the photovoltaic power station. In the last issue, Wang Changjun, a technical expert from Shengnengjie Technology, shared with us the selection of inverter specifications. This issue will explain in detail how to evaluate the reliability of the inverter.

In the initial investment of the entire photovoltaic power station, the proportion of inverters is about 7%-10%. However, in the actual operation of the power station, the loss of revenue caused by a failure of the inverter is much greater than that of other equipment failures. Therefore, the reliability of the inverter is a key consideration for the revenue of the power station. The SE series inverter of Shengnengjie Technology is a high-reliability string-type grid-connected inverter based on a mature overseas platform and designed with domestic practical application details. It includes two types: household and industrial and commercial. Through the SE series inverter, let's take a look at how to choose a high-reliability inverter.

1. Structural appearance

Shell material: String-type grid-connected inverters need to run outdoors for a long time, so the outer box material of the inverter must be rust-proof and corrosion-resistant. It also needs to undergo harsh salt spray operation tests to ensure that the inverter can be used for a long time in acid rain areas, offshore areas or some chemical plant areas. Shengnengjie SE series inverters all use aluminum boxes, with imported powder coating, and have undergone rigorous outdoor harsh environment application tests to ensure a 25-year service life for the structural box.

Waterproof and dustproof: The working environment of the inverter is inevitably exposed to the sun and rain, and it must be guaranteed to reach the IP65 protection level during its life cycle. The key components of the inverter's waterproof and dustproof design are the box structure and sealing gaskets. The SE series' die-casting mold integrated structure design, combined with radiation-proof, corrosion-resistant, and high-heat-resistant sealing gaskets, allows the SE series to be used outdoors without fear of wind and rain.

Wiring port: The trick to choosing the AC and DC wiring port of the inverter is to remove the cover as much as possible, and it is best to use a direct plug-in terminal to avoid damaging the airtightness of the inverter when it leaves the factory. In addition, when constructing the direct plug-in port, you can use its fool-proof wiring method to prevent the cable from being connected incorrectly or in poor contact, and eliminate the hidden dangers of fire caused by short circuit and heating.

2. Electrical safety

Key components: Generally, the life of a photovoltaic power station is 20 to 25 years, and the inverter will be replaced twice during the life of the power station. The basis for the inverter to ensure a life of more than ten years is the life of its internal key components. Therefore, when we choose an inverter, we must consider the selection of key components inside the inverter. The key components used in the SE series inverter are all provided by world-class manufacturers, listed below:

Heat dissipation method: Generally, there are three ways to dissipate heat for inverter equipment: natural heat dissipation, fan forced heat dissipation, and water circulation heat dissipation. The first two methods are the most common. Currently, most inverters use fan forced heat dissipation. As the main component of inverter heat dissipation, the fan has a low failure point and life span, which greatly reduces the heat dissipation reliability of the inverter.

The impact is as follows:

Dust is adsorbed on the fan blades, reducing the fan's heat dissipation efficiency or even causing it to fail, causing the inverter to operate at a derating and affecting the power generation of the photovoltaic power station.

Foreign matter can get stuck in fan blades, causing inverter failure and resulting in power generation loss.

The fan has a short service life in harsh outdoor environments, which affects the service life of the entire inverter.

Considering the above factors, inverters with natural heat dissipation are more conducive to the stable operation of power stations.

Built-in lightning protection: The photovoltaic power station system is installed outdoors and is very susceptible to lightning strikes, which can cause power station failures and reduce the returns of power station investors. Lightning rods are installed during power station design and surge protectors are installed in AC and DC distribution boxes to prevent lightning from affecting power station equipment.

Surge protectors are also required on both AC and DC sides of the inverter to achieve multiple protection purposes. Some inverters on the market do not add this component, leaving hidden dangers for the reliable operation of the inverter. The SE series inverter has built-in photovoltaic-specific lightning protection devices.

UL/CE certified dedicated lightning protection module with up to 40KA discharge current

Secondary lightning protection level

Built-in fuse: Each MPPT of the string inverter will have 1 to 4 photovoltaic strings connected in parallel. According to the principle and characteristics of the components, when the number of strings connected in parallel on the same MPPT is greater than 2, a fuse must be installed in each string to prevent heating and fire in the event of a photovoltaic string failure. There are currently several ways to connect strings on the DC side of the inverter:

Solution 1

Solution 1 is to add a DC fuse to each parallel string. When one string fails, it can prevent the other two strings from being faulted by the backflow of working current. However, the contacts of the DC fuse installation terminals will increase the access resistance in the line and increase the line loss. Long-term operation may cause oxidation, and the fuse terminals may overheat and catch fire.

Solution 2

Solution 2 does not take into account the possibility that when one of the strings fails, the backflow of working current in the other two strings may cause failure, which poses a great safety hazard.

Option 3

Safe string access method

Shengnengjie SE series inverter adopts scheme 3 design. When one group of strings fails, the reverse current of the other group of strings is less than the safety current of the failed string. The safe string access method ensures reliable operation during the life cycle of the inverter.

Three: Display and communication

Display mode: There are three types of human-machine interface modes for the inverter, namely LED display, LED+LCD display, and LED+intelligent wireless connection. The LED display can intuitively show the operating status of the inverter, but the inverter operating data cannot be viewed. The LCD can view the real-time operating data of the inverter and can operate and set the inverter. However, it will bring several potential faults:

The LCD display content is not clear due to strong light or the installation position is not within the viewing angle range

In extremely cold weather, startup or display problems are likely to occur

LCD quality issues: white/black screen, bright spots/lines, uneven brightness

Susceptible to electromagnetic interference, resulting in poor display

Long-term exposure to sunlight, rain, dust, salt spray and other conditions will reduce reliability

Shengnengjie SE series inverter adopts LED + intelligent wireless connection method, which can solve the above problems and make it convenient for customers to analyze the inverter data and set parameters on their smartphones, which is simple and reliable.

Communication method: In photovoltaic power stations, the conventional communication access methods for inverter data collection are wired RS485, LAN and wireless WIFI, GPRS, PLC, etc. Each communication access method has its own suitable scenario. It is worth noting that when both wired and wireless methods are available, it is recommended to choose the wired method, which is more reliable.

Conclusion:

The reliability of the inverter involves all aspects, from design, testing, manufacturing to shipping and transportation, which are all key quality control points. The above is to share with you a few points about the reliability of the inverter that can be intuitively understood on the client side, hoping to provide a reference for you when choosing an inverter.