Discussion on centralized layout scheme of photovoltaic power station string inverters

What factors should be considered when choosing a photovoltaic inverter?

What kind of layout can reduce power generation losses?

Where can I find the latest and most authoritative solution reference?

They seem to be just details, but they are very important to structural design.

If you study it bit by bit on your own, it may take several years to accumulate experience and lessons. If you are lucky, you can probably master it halfway.

Today, the SMA technical team has tailored a fast track for you - "Discussion on the Centralized Layout of String Inverters in Photovoltaic Power Plants", which highly refines the industry's professional knowledge and project experience and presents them to you one by one. As we all know, different types of photovoltaic power plants use photovoltaic inverters of different powers. Large ground power plants recommend the use of centralized inverters. At present, the power of mainstream centralized inverters has reached more than 3MW; small ground power plants and industrial and commercial rooftop power plants recommend the use of string inverters. String inverters in the power range of 50KW-200KW have good applications; and for household photovoltaic power plants, because they are generally small in scale, string inverters below 25KW are generally used.

Among them, for the layout of inverters for small ground power stations and industrial and commercial rooftop power stations, compared with the traditional decentralized layout of placing string inverters in a decentralized manner close to the component end, SMA believes after a large number of project practices that the layout mode of centralized string inverter architecture has the advantages of reducing power generation losses, fast and stable communication, and easy installation and construction as well as later operation and maintenance:

Reduce power generation losses

Discussion on centralized layout scheme of photovoltaic power station string inverters

① Traditional decentralized architecture of string inverters

② SMA recommended string inverter centralized architecture

As shown in the figure above, if the string inverter adopts the traditional decentralized architecture layout, the DC cable used is short and the AC cable is long; if the centralized architecture layout is adopted, the DC cable is long and the AC cable is short. The DC cable loss is less than the AC cable loss. The total cable transmission loss is less when the inverter is arranged in a centralized architecture. On the other hand, in order to reduce LCOE (levelized cost of electricity), DC side over-allocation is increasingly used in today's power station design, from the previous over-allocation of 10% to over-allocation of 20%, and even to 50% in some areas. The higher the over-allocation ratio, the more time the inverter outputs peak shaving. In photovoltaic power stations with strong sunlight and high over-allocation ratios, the inverter can output full power for 80% of the sunlight time in a day.

▲ PV inverter output power peak shaving curve under high over-allocation ratio (click on the image to view the original image)

The centralized architecture layout of the inverter can better compensate for the power generation loss in the environment of over-capacity and peak shaving. Let's compare the power generation loss of the centralized architecture layout and the decentralized architecture layout under over-capacity. As shown in the figure below, the left figure is the power transmission path of the decentralized architecture, and the right figure is the power transmission path of the centralized architecture. For power station owners, what is more important is the power generation at the grid connection point POC, not just the power generation at the inverter output point.

▲ Comparison of power transmission between distributed inverter architecture and centralized inverter architecture

Let's assume that the DC line loss is 0.5% and the AC line loss is 1.5% in both decentralized and centralized architectures (in fact, the total line loss of the decentralized architecture is greater than that of the centralized architecture). When peaking, the inverter side maintains 100% power output. If a decentralized architecture is used, the power obtained at the AC grid connection point POC is 98.5%. If a centralized architecture is used, the inverter performs peak shaving after DC power transmission, and the power obtained at the grid connection point POC is 99.5%. It can be seen that the final online power will be higher if a centralized architecture layout is used.

Faster communication

The centralized architecture layout of the inverters can shorten the communication cables between the inverters, and the communication distance between the inverter and the data collector can generally be within 20 to 30 meters. The SMA SUNNY HIGHPOWER PEAK3 inverter uses Ethernet communication, and the Ethernet field communication speed can reach up to 100Mbit/s, which is enough to realize all power station control within 1S.

▲ Centralized architecture provides better robust control for power plants

For string inverters using PLC communication, the communication cable length saved is only tens of meters, but an additional adapter is usually required, and the maximum PLC communication speed is 115.2Kbit/s, which is still a bit slow for the increasingly complex and demanding power station-level control.

Installation, construction and subsequent operation and maintenance are more convenient

Since string inverters have low power per unit, there are often dozens or even hundreds of string inverters in a power station. Take a 20MW small- to medium-sized ground power station as an example, which covers an area of ​​more than 300 acres. If a decentralized layout is adopted, hundreds of string inverters are scattered in various corners, which is not only time-consuming and labor-intensive to install and debug, but also leads to embarrassing scenes of searching for inverters all over the mountains and fields during daily inspections and maintenance or when the inverter fails.

PEAK3 150kW String Inverter

SUNNY HIGHPOWER PEAK3 is a string inverter launched by SMA for small ground power stations and industrial and commercial fields. It inherits the traditional advantages of SMA inverters' high power density integration and adopts the latest SIC (silicon carbide) technology. The maximum efficiency can reach 99.1%, and the DC over-allocation ratio can reach 150%.

For the application of PEAK3 inverter, SMA recommends a typical design demonstration.

▲ PEAK3 typical design diagram

As shown in the design drawing, the maximum voltage of PEAK3 DC input is 1500V. The components are connected to the PEAK3 inverter through a DC combiner box, supporting up to 150% over-allocation. Copper or aluminum core cables can be used on the DC and AC sides of the inverter, and the AC side of the inverter is connected to the box-type transformer through a low-voltage panel.

PEAK3 uses Ethernet communication to upload data to the SMA Sunny Protal cloud platform through SMA Data Manager M (which can connect up to 50 inverters) for free data monitoring.

▲ PEAK3 typical design demonstration picture