Design of grid-connected photovoltaic system

Design of grid-connected photovoltaic system

Through a successful on-site assessment, all the required information can be collected, after which the system design can be started. The selection and design of photovoltaic system components will be described below. Most of the devices described here have safety and system life guarantees. It must be noted that in most countries, their national standards and specifications have covered the key points of photovoltaic system design, and designers need to be familiar with these standards and specifications.

Before proceeding with component selection, it is necessary to consult the customer’s needs and expectations for the photovoltaic system. The amount of funds that customers invest in the system will affect the scale of the system, as well as the measurement system. Different metering systems: When total electricity metering is adopted, it is usually necessary to increase the installed photovoltaic system as much as possible so that the funds invested in total electricity metering can be recovered as soon as possible. When using net electric energy metering, users may choose a smaller system and invest more money in on-site energy efficiency measurement devices, thereby minimizing local power consumption and maximizing power output.

When designing a system, it is very useful for photovoltaic system owners and installers to go to the site to see what specific site problems they may encounter and how to deal with these problems. Some areas experience harsh climatic conditions, some areas may have destructive wildlife, and some areas may experience high humidity throughout the year. If these factors are not considered, these phenomena may affect the life of the system. Communicating with local experienced technicians as much as possible can help designers carry out good system design according to local conditions and comply with local standards and regulations.

An important part of system design is the selection of components suitable for the system and environment. The important considerations when selecting system components are listed below.

①Photovoltaic module
It is very important to choose and buy the right photovoltaic modules. Components are the most expensive part of the system. If you choose the wrong product, it is an expensive mistake. Component selection should not only be determined by performance, efficiency and cost, but also its operating conditions. The following discusses the factors affecting component selection.
The special environmental conditions that affect the selection of components should be identified in the on-site assessment stage, and briefly summarized as follows:
●Local temperature range: systems working in hot climates require photovoltaic modules to have a low temperature coefficient to reduce the drop in output power caused by temperature rise. The PV module data sheet specifies the working temperature range of the module. It is very important to ensure that the temperature of the installation site is within the range of the selected PV module.
●Coastal salt fog atmospheric environment: modules installed within 1km of the coast should comply with IEC 61701 “Photovoltaic (PV) module salt spray corrosion test”.
●Heavy snowy weather: In areas with heavy snowfall, the rated load capacity of photovoltaic modules reaches 5400Pa, which is very important, and the relevant regulations are given in the module data sheet.
The aesthetics of the components is also important to the system owner. The system uses exactly the same components and generally looks better. Usually there are not many options for component colors, but new components with better aesthetics are currently being developed.
Finally, national standards and specifications may indicate the requirements of photovoltaic modules, for example, modules must be affixed with a mandatory CE performance mark before they can be sold in the European Union.

Design of grid-connected photovoltaic system
Figure1 There are several components on the market that are all (or most) black. It should be noted that the crystalline silicon components on the black backplane are more affected by the battery temperature than the same components on the white backplane, so the output power is lower.

②Installation structure
After the photovoltaic modules are selected, the designer must determine how the modules are to be installed. This depends not only on the component type, but also on the installation site and local environment. In the process of determining the type of installation system, environmental characteristics such as heavy rain frequency, coastal distance (corresponding to a corrosive environment), and local wind load requirements play an important role. The designer may also have to consider the owner’s aesthetic requirements for the photovoltaic system.

The main types of inverters include: centralized type, multi-branch type, branch type and component type. When choosing an inverter, the following considerations are important:
●Rated peak power of the pv array: The maximum rated power of the pv array that the inverter can be applied to is given in the inverter data sheet. The rated peak power of pv arrays is similar, but in order to ensure safety, more complicated calculations are required.
●Whether all photovoltaic modules have the same inclination angle and direction: If the inclination or orientation of the photovoltaic modules are different, divide the photovoltaic system into a series of different branches and use multi-branch inverters or use a single-branch inverter for each branch Generally, the effect is better. In a typical situation, a large inverter is replaced with several smaller inverters, although its output power is higher, which may balance the increase in capital investment, but the cost is higher
●Inverter efficiency: The efficiency of most modern inverters is not much different, but this may affect the choice of system designers. Transformerless inverters are generally more efficient and are now widely used in Europe and Australia. However, since the National Electrical Code did not allow the use of transformerless inverters before 2005, such inverters are not common in the United States.
●Inverter location: The inverter data sheet specifies the rated value of electronic protection. The inverter installed outdoors should not be invaded by water and dust. The protection level 65 (IP65) or the National Electrical Manufacturers Association 3R (NEMA3R) is used for this purpose. IEC 60529 “Protection Degree Provided by Enclosure (IP Code)” highlights the IP rating. IP rating standards are internationally recognized and are commonly used in Europe and Australia; however, NEMA is mostly used in the United States.
●The investment cost of different inverters.
●Expected average annual electric energy output.
Designers need to determine the advantages and disadvantages of each solution in terms of investment cost and system performance. These considerations and the corresponding costs should be clearly explained to the customer, and the final decision should be made on the basis of consultation with the customer.
There are many kinds of software to assist in the design of photovoltaic systems. Most of this kind of software saves and accesses real product information such as photovoltaic modules and inverters, and provides users with hundreds of options from different manufacturers and different models. Designers can use software to simulate different systems composed of different products and compare their outputs to choose the system scheme that best matches their design. Then, the software can be used to determine the applicable array capacity of the inverter, saving the designer’s time, otherwise the designer will spend a lot of time on complex calculations. Most software can access weather data and use factors to simulate the output performance of the system in the field. In addition, users can input local solar resources and shading information. Commonly used software includes PV”SOL, PVsyst, Sunny Design and Homer of SMA. These softwares are all placed on the website. Note that the software listed above, only Sunny Design is free software.

It is very important that the cables used in the photovoltaic system should be selected appropriately (such as UV protection requirements for outdoor use), and the appropriate cable capacity should be selected according to the expected current and voltage of the system, which is very important. Local regulations cover the detailed process of cable capacity selection. Although the regulations of different countries are very different, these regulations should be followed. Cable capacity selection usually varies with its location (for example, the capacity of a branch cable is different from that of a pv array cable). The cable capacity in the system installation project must be correctly selected to ensure that the voltage drop (power loss) between the pv array and the inverter is not too high, and the cable capacity is sufficient to conduct current. If the current flowing through the cable is greater than the current-carrying capacity of the cable, the cable will overheat and damage the circuit components. The cable should be as short as possible, and the cable rating must be selected according to temperature and environment (such as UV protection).
⑴Voltage constant capacity
Each cable has a maximum voltage rating (provided by the manufacturer), which cannot be exceeded, and the voltage rating standards of different countries are different. In Australia, DC cables must be rated for 1.2x component Voc (such as branch, sub-array, or array).
⑵Current constant capacity
Each cable also has a maximum current rating, which must not be exceeded from a safety point of view. For example, in the standards of the United Kingdom and Australia, the DC cables of photovoltaic systems must be rated to carry a minimum current of 1.25 × ISC. This means that branch cables must be calibrated according to 1.25 × component ISC, and array DC cables must be calibrated according to carrying 1.25 × array ISC (array ISC = component ISC × number of parallel branches in the array). In the United States, the standards are slightly different. DC cables must have a minimum current rating of 1.5625×ISC. For example, branch cables must be rated at 1.5625 × component ISC, and array DC cables must carry 1.5625 × array ISC.

Inverter selection also includes monitoring options, such as wireless displays that allow users to monitor system performance indoors, or on-site monitoring of power output and power consumption in large power stations that use multiple inverters. Bluetooth is also a feature required by some customers now, and customers can use wireless monitoring on computers, PDAs or mobile phones.

⑥System protection
The design of the protection system is essential to ensure the safe operation of the photovoltaic system. There are many potential causes of photovoltaic system failure. Natural threats such as lightning, floods, or high winds can damage system components or cause the system to operate in dangerous conditions. In typical cases, national regulations require photovoltaic systems to be equipped with many types of protection for safety reasons: overcurrent protection, lightning and surge protection, and a series of disconnection methods are common requirements.