"Scientific design according to local conditions" is the rational choice of photovoltaic power plant inverter selection

At present, there is an understanding of the selection of inverters in photovoltaic power plants: centralized inverters for large photovoltaic power plants and string inverters for distributed photovoltaic power plants. The author believes that this is an amateur argument.

In fact, the selection of inverters is not simple and cannot be generalized. “Scientific design” is the rational approach. That is, according to the installed capacity of the photovoltaic power station, the environment in which it is installed, and the grid access requirements, a reasonable choice of inverter types will enable the participants of the power station to maximize their mutual benefits in terms of safety, stability, and benefits.

As one of the two major components of the photovoltaic power generation system, the core task of the photovoltaic inverter is to track the maximum output power of the photovoltaic array and feed its energy into the grid with the minimum conversion loss and the optimal power quality. Since the inverter is connected in series between the photovoltaic square array and the grid, the choice of inverter will be the key to the long-term reliable operation of the photovoltaic power plant and realize the expected return.

The current photovoltaic power station in China is divided into desert power station, hill power station, and roof power station due to different installation environments. Photovoltaic inverters can be divided into three types: centralized inverters, string inverters and micro-inverters, depending on their power levels, internal circuit structures, and applications.

Among them, the main characteristic of the centralized inverter is that the single-machine power is large, the maximum power tracking (MPPT) is small, and the cost per watt is low. At present, the mainstream models in China are mainly based on 500KW and 630KW. In Europe and North America, mainstream models have a stand-alone power of 800KW or even higher, and the power level and integration are still increasing. Centralized inverters are the first choice for most large-to-medium-sized photovoltaic power plants. In the global 5MW or more photovoltaic power plants, the percentage of their choice is more than 98%.

The unit power of the string inverter is between 3-60KW. The mainstream model single machine power 30-40KW, single or multiple MPPT, generally 6-15KW all the way MPPT. This type of inverter has a higher cost per watt and is mainly used for small and medium-sized power plants. The utilization rate of power plants with a capacity of 1MW or less in the world exceeds 50%.

The single inverter power of the micro-inverter is below 1KW, single MPPT, and the application is mostly 0.25-1KW all the way to MPPT. The advantage is that it can perform independent MPPT control for each or several panels, but this type of inverter is per watt. High cost. At present, there are many applications in home photovoltaic power stations below 10KW in North America.

Desert power stations have obvious advantages in using centralized inverters. Solar Power Technology experts said that for desert power stations, the advantages of centralized inverters are obvious.

First, the initial investment is lower. According to a comparative analysis, the centralized solution saves approximately 260,000 yuan per megawatt on the initial investment compared with the string inverter solution. Second, the amount of electricity produced is the same as the string type. In the desert power station, the concentration of centralized and string power generation is basically the same. The integrated concentration type has the advantages of the highest efficiency and overload capacity. The concentrated power generation capacity is slightly higher than the string type. Third, operation and maintenance are more convenient and more economical. By comparing the centralized and clustered mainstream models in the operation and maintenance data of the 100MW power plant, the loss of power generation is equivalent to both; since the string-type equipment is the maintenance of the whole equipment, the centralized equipment is the maintenance of the equipment and the maintenance cost of the equipment. The concentrated advantage is obvious. At the same time, in the large-scale 100-MW-class power station that covers an area of ​​several thousand acres, the fully-distributed string inverters will be replaced, and the maintenance staff will spend more time on the road than the equipment replacement time, which is also One of the disadvantages of the string type of large-scale power plant applications.

Most importantly, the centralized plan is more in line with grid access requirements. It is understood that high-voltage transmission grids have strict requirements on grid-connected photovoltaic power generation in terms of dispatch response, fault traversal, limited emission, super-transmission, smoothing, harmonic limitation, power change rate, and emergency start-stop. Fault traversal means that the inverter must be able to output a certain amount of active and reactive power within 625 milliseconds to a few seconds in the event of a short circuit, surge, or phase loss in the power grid, ensuring that the power system relay protection can operate normally. Because the centralized inverter has a small number of stations in the power station, the stand-alone function is powerful, and the communication control is simple. The probability of being able to cross the fault during the fault is far greater than that of the string inverter.

Hill Power Station: Mainly focused on multi-MPPT. The hill power station can be seen as a desert power station with uneven terrain. It is also mainly fed into the power transmission network, with a size of more than 5MW. In the hill power station project, usually more than 100 KW or so capacity components are planned in a coordinate system (for example, 125 KW components are laid in the same orientation), and the optimal ratio of power generation and investment and maintenance costs is achieved.

Sunlight Power's multi-MPPT module-based centralized inverters developed for this application track more than 100 KW components per MPPT, reducing the design footprint of the same orientation components to approximately 1,000 square meters, greatly improving ease of construction. And effectively solve the problem of facing and shielding, at the same time a total of AC bus output, with the characteristics of centralized inverter friendly network, is the preferred program for hill power station.

Roof power station: recommended string type, can also be concentrated. The design of the rooftop power station is relatively complex. Affected by the roof size, layout, material weight bearing, and shading, etc., it is necessary to maximize the revenue through component placement and inverter selection planning. At the same time, components are installed on the roof, and fire safety and other safety issues need to be considered. Access to the distribution network, directly close to the user load, need to consider the user's electricity safety, power quality meets the requirements, and the relay protection coordination with the original distribution. The selection of rooftop PV system solutions requires a balance of safety, grid friendly, investment return, maintenance and other factors.

Solar power technicians recommend the use of string inverters. The roof structure is complex. In order to simplify the design, it is recommended to use a string inverter and select the inverter according to the actual roof and grid point location and the voltage level of the grid point. The string inverter needs to have the ability to monitor and shut down the arc, in order to effectively prevent the occurrence of fire, with PID elimination function, with high-precision leakage current protection and island protection.

However, for large-scale factories, centralized solutions can be used in consideration of roof load-bearing and maintenance convenience. The industrial plant has a flat roof, a large scale, less shade, and a simple orientation, and is mostly a 10kV medium voltage distribution network. Taking into account that most of the factory buildings are colored steel roofs, the load-bearing capacity cannot be installed on the string inverters, and the routine maintenance is convenient and does not affect the normal production operations. The centralized inverter may be used.

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