The great German philosopher Hegel once said that "existence is reasonable." The emergence and continuation of any thing has its inevitable truth. This is true of all things in the world. With the increase in the capacity of large-scale ground power stations and the increasing construction volume, the requirements for the efficiency of power station construction are getting higher and higher. The traditional method of building stations cannot meet the general requirements for rapid establishment of stations, so a kind of container-type inverter The solution has been rapidly promoted and applied. It is a megawatt box-type inverter station. The megawatt box-type inverter station solution is widely used in large-scale ground power stations. Because of its simple construction, easy wiring, and high degree of protection, it has been widely recognized by many investment companies and EPC manufacturers.

Megawatt box type inverter station solution

I. The development of megawatt box-type inverter station

As an upgraded version of the high-power centralized inverter, the megawatt box-type inverter station subverts the construction mode of traditional photovoltaic power plants and brings more convenient power plant solutions to customers. In 2011, SMA introduced a 1MW and 1.25MW inverter unit solution in the country. In the same year, domestic solar power and TBEA also launched a 1MW box-type inverter station with color steel housing as the carrier in 2011. The strength and protective performance of the steel room are poor and cannot adapt to the harsh natural environment of large-scale ground power stations. Therefore, with the development of technology, Solar Power and TBEA have successively launched megawatt-level inverters using standard containers as their carriers. unit. The improved box-type inverter station has been redesigned for heat dissipation and corrosion protection. The steel plate has higher strength, better anti-corrosion design, and better heat dissipation effect. It can be fully applied to poor photovoltaic projects.

Caigang megawatt box-type inverter station Container megawatt box-type inverter station

II. Misunderstandings of MW Level Box Inverter Stations

The megawatt box-type inverter station integrates grid-connected inverters (integrated DC distribution cabinets), communication cabinets, and integrated cabinets in standard containers. After the engine room arrives at the project site, the user only needs to complete simple foundation construction, on-site installation, and external wiring. Compared with the traditional scheme, it can effectively reduce the on-site construction time, reduce on-site construction, wiring, and installation difficulties, and improve the efficiency of the establishment of the station.

Mega-level box-type inverter station interior layout

The emergence of mega-level box-type inverter stations is an appearance of the maturing of photovoltaic grid-connected power generation equipment, and it is also the key to the rapid and rapid development of photovoltaic power plants. Although the large-scale application of megawatt-level box-type inverter stations has been for many years, many designers and investors still have misunderstandings about this product.

(1) Misunderstanding 1: The megawatt box-type inverter station cannot adapt to the harsh operating environment

The existence of this misunderstanding stems from two aspects, one is the lack of understanding of the container material and surface treatment methods, and the other is due to uneven quality of the products within the industry, resulting in a point to face the conclusion. In fact, excellent inverter manufacturers have very high requirements on product quality and product technology, especially on the outdoor equipment, material strength, protection and corrosion requirements are very high.

a. Cabinet material selection

The container body of the megawatt-level box-type inverter station adopts a corrugated steel plate of not less than 2mm (the cold-rolled steel plate is formed by bending and the strength of the steel plate is increased). The container is weighed to be about 3 tons, and the strength can be Anti-magnitude 8 earthquake and 16 strong wind damage.

b. Protection scheme

The container body of the megawatt-level box-type inverter station is integrally welded and molded, and the box body itself will not leak or enter the ash. In addition, careful and detailed design is provided for the inlet and outlet ports that may enter the ash and leak the rain. .

Air inlet design: The inlet of the megawatt box type inverter station is usually made of rainproof, sand-proof shutters. The shutter can be used to protect the spray room from entering the equipment room in any direction within 180°. At the same time, large sand particles can be prevented from entering. The polyurethane dust-proof net with a PPI of 30 on the grid is fully capable of preventing rain and sand.

Outlet design: The air outlet of the mega-level box-type inverter station adopts the ventilation elbow and dust-proof net protection method. The air outlet of the ventilation elbow on the outside of the machine room faces downward, and the dust-proof net with a grid PPI of 20 is effective. To prevent the strong wind from directly blowing out the air outlet and causing the wind direction to reverse flow, prevent wind and sand from entering the equipment room from the air outlet.

c. Anti-corrosion program

The surface of the container body of the megawatt box-type inverter station adopts a sand blasting rust removal method and a three-layer paint spraying multi-treatment scheme to prevent the sand falling and corrosion caused by sand blowing and salt spray.

Sandblasting and rust removal: sandblasting removes dirt, grease, rust, oxide scale, and welding slag from the surface of the container body. After sandblasting, the adhesion of the paint surface is better, and the paint surface is not affected by the service life of the engine room. Fall off.

Three-layer paint spraying: the surface of the box body is treated with three layers of anti-corrosion paint spray. The primer is made of epoxy zinc-rich paint with strong adhesion to provide the most direct corrosion protection for the box; the middle paint has good adhesion and Enclosed performance of epoxy cloud iron paint, increase the paint surface water, electrolyte shielding; topcoat using UV resistant polyurethane paint, to prevent paint aging, maintain gloss.

Through the above protection and anti-corrosion treatment program, it can completely ensure that the box type inverter station operates in various harsh environments.

(2) Misunderstanding 2: The poor thermal performance of the megawatt box-type inverter station

The existence of this misunderstanding stems from the lack of understanding of the heat dissipation design of the box-type inverter station. It is wrong to think that the room is small in size, compact in space, and unfavorable to heat dissipation. In fact, thermal design of power electronic equipment is the most important and most important part of all inverter manufacturers. Excellent large-scale machine manufacturers can fully guarantee the thermal performance of their products.

a. Inlet and outlet design

At present, mainstream domestic mainstream manufacturers basically use 10-foot standard containers as carriers, and the air intake and outlet designs are basically the same. Both sides adopt single-side air inlet and two-way air inlet and outlet air designs. The design scheme is reversed from the box type. Since the birth of the transformer station, the distance between the cold air and the air inlet of the inverter has become shorter and shorter, and it is more conducive to the heat dissipation of the internal equipment. In terms of ventilation, the internal inverter of the equipment room is directly placed against the wall. The outlet of the inverter is directly aligned with the exhaust ports on both sides of the room. There is no thermal resistance and hot air is directly discharged from the equipment room.

Computer room inlet and outlet

b. Full thermal simulation

Thermal Simulation Objective: To determine the elimination of heat dissipation problems before the sample and product are produced, and to ensure that the product's thermal design is at an excellent level. The thermal simulation can clearly see the temperature and heat flow of the critical heating device under the set conditions.

Thermal simulation conditions: ambient temperature 50 °C, altitude 3000m, inverter overload 1.1 times.

Thermal simulation conclusion: All critical devices are below the derating point temperature to ensure reliable operation of the system.

Megawatt box type inverter station thermal simulation

c. Strict thermal test verification

The megawatt box inverter station is placed in an environment of 50±5°C, the AC output voltage is controlled at 315V±10V, and the DC side voltage is controlled at 645V±20V. Run for 6 hours and record the temperature at each test point on the sample under test.

After actual test and verification, the internal temperature of the engine room is only 2°C~3°C higher than the external temperature, and the temperature rise of internal critical devices is below the derating design standard.

In summary, the reduction of the volume does not mean that the heat dissipation conditions are poor, and an excellent heat dissipation design can completely ensure the reliability of the heat dissipation of the device while increasing the power density of the device.

(3) Misunderstanding 3: Poor maintainability of MW-level box-type inverter station

The existence of this misunderstanding stems from the lack of understanding of the product structure design, and the current structural design of the inverter is not clear. At present, high-quality inverters in the industry all use modular design of devices. Key components such as IGBT modules, control modules, and cooling fans are all modularized with drawers. After fault location, faulty module replacement can be completed within 20 minutes. In addition, the current megawatt-level box-type inverter stations are equipped with maintenance doors, even if the whole machine replacement is not to mention, there is no problem of internal equipment maintenance difficult.

Maintenance of MW Level Box Inverter Station

Third, the conclusion

The existence is reasonable, and the emergence and continuation of MW-level box-type inverter stations are in line with the development trend of large-scale and rapid construction of photovoltaic power plants. After years of development, it has the characteristics of mature technology, wide application, high reliability and long service life. At present, excellent inverter manufacturers both at home and abroad have introduced box-type inverter station solutions and have been promoted and applied globally, and have received good customer feedback. In the near future, box-type inverter stations will occupy more market share and provide photovoltaic power plants with reliable revenue protection for 25 years. (The author is Shao Yabin, Senior Electrical Design Specialist, Hebei Electric Power Survey & Design Institute)

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