Main problems of on grid photovoltaic system installation

Main problems of on grid photovoltaic system installation

The installation guidelines for photovoltaic systems in different countries are different, because different standards and regulations exist in different regions. Special permits, requirements, metering policies, on grid access standards, installer authorization, and other special national laws and regulations will determine the installation procedures of photovoltaic systems. During installation, the manufacturer’s instructions should be followed carefully. The following focuses on the main issues of the installation of system components and the interconnection requirements of the photovoltaic system and the public grid.

① PV array installation
We strongly recommend that PV system installers take time to carefully plan the precise location of the PV array before deciding on the installation method. Usually need to measure the effective installation area, and use a rope or chalk line to mark the array boundary and the fixed point position of the bracket system on the installation area (such as the roof). The next step is to install the fixtures (array bracket structure) and fasten the bracket system to the roof. If installed on a roof, care should be taken to ensure that the fastening screws are firmly embedded in lemons or other structural support elements to provide maximum fastening strength.
When the fixing points are established, the bracket system should be assembled. There are many types of photovoltaic module mounting systems on the market, which can be selected according to specific applications. If the mounting system is fully assembled, the photovoltaic modules can be installed. Many special bracket systems rely on compression fixtures to fix the module frame on the mounting rail. Therefore, there is an urgent need for the component fixture to be fully compatible with the bracket system.
②DC wiring
For photovoltaic modules used in on grid systems, a complete interconnection cable is typically provided, including a sealed junction box and plug-in connectors at the end of each cable. Adjacent components are connected in series (the positive pole is connected to the negative pole, or the negative pole is connected to the positive pole) to form a component string. After the required number of modules are connected in series to form a module string, the circuit needs to be connected to a centralized location, usually a photovoltaic combiner box, where it is connected in parallel with other module strings. The fuses of all module strings will also be installed in the photovoltaic combiner box. The DC link is a very important component of the photovoltaic system, and many key factors must be considered during the design and installation.
③System grounding
Grounding is used to ensure the equipotential of the exposed part of the system (ie, the array frame), which means that there is no voltage difference between the component and the ground. The metal frame and metal support structure of photovoltaic modules are grounded, so that the voltage on the surface of these structures will not reach dangerous levels. This ensures that personnel will not suffer electric shocks when touching conductive elements. One important point here is that people who may come into contact with the photovoltaic array (that is, when the system owner cleans the components) have almost no electrical system experience, and ensuring its safety is the first priority. The grounding rules and standards of different countries are very different, but national standards all stipulate that the system and individual components should be grounded. Certain component types require grounding to ensure maximum performance of components (such as the SunPower component series), which is a requirement from component manufacturers. The installer shall comply with the requirements of the inverter and component manufacturers and national regulations. see figure 1

Main problems of on grid photovoltaic system installation
Figure 1 It is a common practice to ground each component of a photovoltaic system separately. This makes it possible to remove a grounding point while other components remain grounded, that is, the grounding point of the array is not connected to the grounding wire of the inverter. The techniques shown in this figure are compatible with American National Standards. There are various technologies, so local standards should be consulted frequently .

④Inverter installation
The inverter should be installed as close to the components as possible to shorten the DC cable length (the longer the cable, the greater the power loss). The inverter should be placed in a cool, concealed, well-ventilated place, and should not be exposed to the temperature range specified in its data sheet (generally -25-60°C). In addition, the installation wall of the inverter must be able to support these loads. The overcurrent protection and breaking switch should be installed close to the inverter. The inverter can be installed at any time during the system installation process. If necessary, it can be installed at the same time as the support system.

⑤Installation checklist
The installer should ensure that all tools and corresponding equipment are in place and available for installation.

⑥Public grid access system
For systems where small generator sets (such as rooftop photovoltaic systems or small wind turbines) are connected to the grid, it is also called interactive distributed power generation.
Consumers use electricity from two sources (photovoltaic system and public grid) (as opposed to a standalone system that only relies on photovoltaic arrays). According to the metering scheme, this will be slightly different: when the net energy metering method is adopted, the output power of the photovoltaic system is directly used at the grid connection point, and the remaining power is output to the grid, and when the output power of the photovoltaic system is insufficient, the remaining power is purchased from the grid. If the total electric energy measurement method is adopted, all electric power is output to the grid, and the load power demand is obtained from the grid, so there is no direct power flow between the photovoltaic system and the load.

Main problems of on grid photovoltaic system installation
Figure 2 In the grid interactive photovoltaic system, the photovoltaic array is connected to the inverter, then to the switchgear, and then connected to the grid. National norms and regulations cover this situation and these norms should be observed
Main problems of on grid photovoltaic system installation
Figure 3 Summary of connection elements of grid-connected photovoltaic system using net energy measurement

In order to connect the photovoltaic system to the public grid, a license contract for on grid access is typically required. The public grid systems of different countries are different, and so are the grid connection permits and policies. This permission usually depends not only on local standards and regulations, but also on the grid, which must agree to the injection of electricity from the photovoltaic system into the grid. Some local laws require public grids to purchase electricity from photovoltaic systems (such as the United Kingdom). However, in other regions, the decision is made by the grid company. Regarding the security issues caused by multiple power sources connected to the power grid, power suppliers all over the world have different rules and regulations. Some safety issues are overcurrent and island operation. If power is still sent to the grid during the grid loss period, this is a serious threat to electrical engineers working on power lines. Today’s inverters have an anti-islanding function. When the grid fails, the inverter should shut itself down. It is not uncommon for local grid companies to insist on system inspections before PV systems are put into operation.

Main problems of on grid photovoltaic system installation
Figure 4 Wiring diagram of the net electric energy measurement method, the power that cannot be used in the building is output to the grid
Main problems of on grid photovoltaic system installation
Figure 5 Summary of connection elements of grid-connected photovoltaic system using total energy measurement
Main problems of on grid photovoltaic system installation
Figure 6 Wiring diagram of the total electric energy measurement method, all the electricity of the photovoltaic system is output to the grid

⑦ Information requirements for installation engineering
In order to install a photovoltaic system, the local authority responsible for the installation of the photovoltaic system generally requires schematic diagrams and drawings. These drawings depict the location of equipment components on site and the electrical configuration of the system. Here are some examples of drawings that should be completed:
·Electrical schematic diagram: A simplified schematic diagram showing the configuration of the photovoltaic array, wiring system, overcurrent protection, inverter, breaking device, required identification and AC line connection with the building. The wiring schematic diagram should show detailed information about the electrical components, wire types and specifications, the number of wires, and the type of conduit (if required). It should also include electrical information about photovoltaic modules and inverters. In addition, it should also include information on the disconnection method of the public grid (required by many grid companies).
·Site planning drawing: shows the structural schematic diagram showing the location of the main components on the building. The main components of a photovoltaic system can include photovoltaic arrays, inverters, isolation/disconnect switches, and access points to power distribution panels. It is a good practice to include the main buildings/structures and asset boundaries of the installation site. The drawing does not need to be accurate to scale, but it should show the relative position of the components on the installation site.
·Calculation table: Includes calculations and notes related to photovoltaic array design, such as temperature-corrected maximum power and open circuit voltage, maximum rated power, maximum power, and short-circuit current. It should also list inverter-related information, such as voltage, current, and rated power, as well as calculations related to overcurrent protection devices.

Main problems of on grid photovoltaic system installation
Figure 7 Example of electrical schematic diagram

In addition to these drawings, there should be a licensed application package, including data manuals and installation manuals (if any) for all components, including but not limited to photovoltaic modules, inverters, photovoltaic combiner boxes, isolation/breaking devices, and support systems. Local regulations and national standards will stipulate the required documents and shall be implemented in accordance with them.

Main problems of on grid photovoltaic system installation
Figure 8 Typical calculation table of photovoltaic system

The safety of photovoltaic array installation
Installing a photovoltaic system is a risky thing, so taking appropriate safety measures is critical.
Before installing a photovoltaic system, risk assessment should be part of the on-site assessment: according to the likelihood and severity of the risk, the risk needs to be identified and classified according to low, medium, high or extreme. Also need to identify and take risk control measures.