Household photovoltaic system generally includes ① solar panel, ② lightning protection box, ③ cable, ④ grid-connected inverter, ⑤ metering instrument. But for the general public, how to build a suitable distributed photovoltaic grid-connected power generation system?

1. How to apply for building your own household photovoltaic system?

The commitment of State Grid: State Grid will provide the whole process services such as access system scheme formulation, electric energy metering device installation, grid connection acceptance, and commissioning for the owners of distributed photovoltaic power generation projects without any charge;

Support distributed photovoltaic power generation decentralized access to the low-voltage distribution network, and fully purchase surplus electricity in accordance with national policies;
All distributed photovoltaic power generation projects are exempted from the system reserve capacity fee. The part of the project connected to the public power grid is also paid for by the power grid. Users only need to invest in photovoltaic power generation equipment and a small number of cable lines for their own use. So how do we apply?

In the first step, it is better for residents to have an independent site (such as a roof) for installation. If it is a public roof, it is necessary to seek the consent of neighbors.

In the second step, residents should seek permission from the industry committee or neighborhood committee of the self-built power station community.

In the third step, residents go to the business hall of the power company to apply for the grid connection of distributed photovoltaic power generation projects. The power supply department will carry out the on-site investigation in the future and organize the review of the 380V (220V) V access scheme.

Step 4: if there are problems in the design of the access scheme and equipment, the power supply department will provide an access scheme and comment letter on access to the power grid, and residents will transform the facilities according to the scheme and comment letter.

The fifth step is to install or find a professional solar system installation company to install the grid-connected system.

Step 6: After installation, inform the power company to come for acceptance. They will install metering devices in residents’ homes, sign power purchase agreements and complete the final grid connection procedures.

2. Capacity design of solar power generation system

Capacity, that is, the power generation of the photovoltaic power generation system, is generally designed according to the constructive area of residents. The area of 1 square meter can generate about 150W at present. Ordinary residents can build 3kw-5kw power stations, which requires more than 20 square meters of installation space, and so on. The installation space can be a sloping roof or a flat roof.

3. Selection of solar panels

Solar panels are divided into monocrystalline silicon solar panels, polycrystalline silicon solar panels, and amorphous silicon solar panels. Monocrystalline silicon solar panels have the highest power generation efficiency and can generate more electricity under the same area. For a relatively small installable area, monocrystalline silicon solar panels are the most appropriate choice, but the unit price will be higher. The efficiency of polysilicon solar panels is the second.

For those with rich installation areas, polysilicon solar panels are more cost-effective and the price is relatively low. Amorphous silicon solar panels have the lowest efficiency and are generally not recommended.

The five parameters of solar panels are peak power, open-circuit voltage, short circuit current, working voltage, and working current. The selection of these parameters is very important.
Solar panels can be divided according to their power. The power of each solar panel ranges from 10W to 300W. The size of solar panel power is directly proportional to the area, so when choosing power, you should also choose the most appropriate size of solar panel to meet your installation space requirements.

The solar panel under sunlight (1000W / m2): ① the voltage measured without load is the open-circuit voltage; ② the current measured by directly short-circuiting the positive and negative electrodes of the solar panel is the short circuit current; ③ the voltage measured with load is the working voltage and the current measured is the working current.

The working voltage is generally proportional to the open-circuit voltage, and the working current is proportional to the short-circuit current. The selection of these four parameters is closely related to the inverter to be selected later. Generally speaking, the open-circuit voltage of grid-connected solar panels is generally about 45 volts, and the working voltage is generally about 35 volts. The working current and open-circuit current change with the change of solar panel.

Series and parallel of solar panels: multiple solar panels can be used in series or parallel, or mixed in series and parallel. For the grid-connected system, the series connection is to obtain the voltage suitable for the grid-connected inverter, and a parallel connection is to increase the output power under the same voltage level.

4. Selection of grid-connected solar inverter

As the interface device between solar panels and power grid, the grid-connected solar inverter converts the DC energy of solar panels into AC energy and transmits it to the power grid. It plays a vital role in photovoltaic grid-connected power generation systems.

There are two kinds of household grid-connected inverters. One is a centralized grid-connected photovoltaic inverter, with a minimum power of 1kW and an input voltage ranging from 150V to 550V. The other is a micro grid-connected photovoltaic inverter, which is generally used in the voltage level of AC220V, with power ranging from 200W to 500W and input voltage ranging from 12V to 28vdc.

The usage of the two grid-connected inverters is very different. The centralized grid-connected inverter is generally large and suitable for wall mounting. The micro grid-connected photovoltaic inverter is small and generally installed nearby with the solar panel (can be installed on the support below the solar panel). Both households distributed grid-connected photovoltaic power generation systems can be selected.

The main parameters of the grid-connected inverter are maximum input power, maximum input voltage, rated input voltage, starting voltage, MPPT voltage range, and output grid voltage. The maximum input power is a limit value, and the peak power of solar panels should be about 90% of this value;

The maximum input voltage corresponds to the open-circuit voltage of the solar panel; The rated input voltage corresponds to the working voltage of the solar panel, which can have a certain range of deviation; The starting voltage refers to the voltage point at which the grid-connected inverter should start working. When the sunlight is too weak, the grid-connected inverter does not work;

The MPPT voltage range is a function of a grid-connected inverter set according to the characteristics of solar panels. It will automatically adjust the input voltage and current of the inverter to make the product of voltage and current, that is, the power reaches the maximum value; The output grid voltage refers to the voltage of the grid to be connected (single-phase 220V / three-phase 380V).
The selection of grid-connected inverter is mainly to select the rated power and input and output voltage level.

Then the user can easily determine the input voltage range of the grid-connected inverter according to the voltage level and series-parallel mode of the selected solar panel and can choose single-phase or three-phase output according to the power consumption of his home.

For example, the peak power of solar panels is 200W, the open-circuit voltage is 45V, and the working voltage is 35V. To form a 2kW system, 10 solar panels need to be connected in series, so the maximum input voltage of grid-connected inverter must be greater than 45V / piece * 10 pieces = 450V, and the rated voltage is 350V left and right.

The output voltage can be determined according to the specific power supply situation of the family, and single-phase 220V or three-phase 380V can be selected. As the most important part of the whole system, the grid-connected inverters must choose products that have passed relevant national certification.

5. Selection of other components

1) Cable. Special photovoltaic cables can be selected, but the price is high and the quantity is small, which is not easy to buy. Bv cable is still used for a short distances, which can be calculated according to the safety current of 1mm2 as 6A.

For example, for the system with a power of 3KW, for the photovoltaic input cable, the rated input voltage of the inverter is dc350v, the current is 3000W / 350V = 8.57a, and bv2.5 cable is selected. For the inverter output cable, the rated output voltage of the inverter is AC220V, and the current is 3000W / 220V = 13.64a. Theoretically, bv2.5 cable can also be selected, but for safety reasons, bv4 cable can be selected.

2) Lightning arrester. The lightning arrester is different from the ordinary lightning arrester. The special photovoltaic high-voltage DC lightning arrester should be selected, because the continuous working voltage of the ordinary lightning arrester is AC220V or AC380V, and the continuous working voltage of the special photovoltaic lightning arrester can be as high as DC 1000V.

3) Meter power companies will install two meters for users free of charge. One is the meter for measuring photovoltaic power generation, and the other is the two-way meter, that is, the meter for users’ own power consumption and sold to the power grid. In this way, users can see their daily power consumption, power generation, and power transmitted to the power grid in real-time.

6. Installation and commissioning

If you are not familiar with the electrical system, you’d better ask qualified personnel to install it. The series-parallel connection after the installation of solar panels is best completed after the sun sets to avoid high-voltage electric shock, and do a good job in lightning protection and grounding. In addition, pay attention to the wind resistance grade of the support, and the users in the North should also pay attention to the snow resistance grade. Commissioning can cooperate with the personnel of the power company.