In the development and construction of ground photovoltaic power plants, the design work can be called the core work. The design affects the construction of the entire photovoltaic power plant and is directly linked to the benefits. Last week, I talked about the general drawing and some issues in the civil construction that need to be paid attention to during the construction of photovoltaic power plants. Then in the design process, what should be paid attention to in the electrical part? The following is a brief analysis for everyone.
1. Component selection
As we all know, the energy density of solar energy is low. Under this premise, how to use solar energy effectively is very important. At present, the module efficiency required by the National Leader Program is no less than 16.5% for polycrystalline silicon modules and no less than 17% for monocrystalline silicon modules. In terms of module conversion efficiency, monocrystalline silicon modules are better than polycrystalline silicon modules. However, since the price of monocrystalline silicon cell modules is slightly higher than that of polycrystalline silicon modules, it is not advisable to blindly select modules based on price alone when selecting modules. It is necessary to carry out technical and economic analysis in various aspects such as power generation calculation and selection and project income for different components, and select suitable battery components.
2. Inverter selection
At present, inverters are divided into two types: string inverters and centralized inverters.
1. String inverter
String inverters are mostly used in mountain photovoltaic power generation systems, small and medium-sized rooftop photovoltaic power generation systems, and small ground power stations. The power is less than 50kW. In the design scheme of the string inverter, the DC power generated by the photovoltaic modules is directly connected to the string inverter, converted into AC power, and then boosted by confluence.
The main advantages of string inverters are:
①It is not affected by module differences between strings and shadows, and at the same time reduces the mismatch between the best operating point of photovoltaic cell modules and the inverter, and maximizes power generation;
②The MPPT voltage range is wide, and the component configuration is more flexible;
③ Small size and flexible installation.
The main disadvantages of string inverters are:
① The electrical clearance of the power device is small, which is not suitable for high altitude areas;
②Outdoor installation, wind and sun exposure can easily lead to aging of the casing and heat sink.
③ The number of inverters is large, the total failure rate will increase, and the system monitoring will be difficult.
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2. Centralized inverter
Centralized inverters are generally used in large-scale power plants with uniform sunshine, desert power plants and other large-scale power generation systems. The total system power is large, generally above the megawatt level. Equipment power is between 50kW and 630kW. In the design of the centralized inverter, the DC power generated by the photovoltaic modules, after being combined by the DC combiner box, is connected to the inverter, converted into AC power, and then boosted.
The main advantages of centralized inverters are:
①The number of inverters used in the project construction is small, which is easy to manage;
② In terms of inverter performance, the harmonic content is low, various protection functions are complete, and the safety of the power station is high;
③It has power factor adjustment function and low voltage ride-through function, and the power grid has good regulation.
The main disadvantages of centralized inverters are:
① The MPPT voltage range of the centralized inverter is narrow, and the operation of each component cannot be monitored, so it is impossible to make each component in the best working point, and the component configuration is inflexible.
②The centralized inverter occupies a large area and is not flexible in installation.
③ The system maintenance is relatively complicated due to its own power consumption and power consumption for ventilation and heat dissipation in the equipment room.
When selecting an inverter, it is necessary to select a suitable inverter according to various factors such as the terrain and altitude of the project. For example, in the design of large-scale ground-based power stations in high-altitude deserts in Qinghai, centralized inverters are often selected; in mountain photovoltaic power stations, due to the different sizes of the installed component arrays and the relatively scattered arrangement of components, string inverters can be selected. And use multi-channel MPPT for tracking to maximize power generation.
3. Design of collector circuit
For the collector circuit design of the photovoltaic power station, for areas with thick soil layers that can be excavated, the cable direct burial solution is usually adopted, which is also the most economical solution; if the surface is rocky and cannot be excavated, the cable along the Bridge laying scheme. For complex ground conditions, large fluctuations, or scattered layout of photovoltaic arrays, overhead installation is generally adopted in the form of towers. In the design process of the collector line, it is necessary to select an economical and reasonable design scheme according to the detailed topographical mapping and topography of the construction site of the power station project, avoiding construction difficulties as much as possible.
4. Grounding design
In the grounding design of the photovoltaic power station, in addition to calculating the grounding resistance according to the resistivity provided by the geological survey unit, geological conditions such as local soil corrosion should also be considered. Grounding material with strong corrosion resistance. If the calculated grounding resistance does not meet the specification requirements, the economical resistance reduction measures should be selected according to the project conditions.