Centralized inverter detailed introduction - string inverter vs centralized inverter
We have talked about micro inverter and string inverter before, this article will take you to understand another type of pv inverter – centralized inverter. In this article, we will discuss what is a centralized inverter, its advantages and disadvantages, and a comparison with mainstream string inverters.
What is a centralized inverter
The centralized inverter photovoltaic inverter mode is to connect many parallel photovoltaic groups in series to the same centralized inverter DC input terminal for maximum power peak tracking, and then invert and merge into the grid. The single capacity of the centralized inverter is generally greater than 500kW, and it has the characteristics of high single power, low cost, and strong grid regulation, but it requires good cooperation between photovoltaic strings.
Once cloudy, partial shading or individual string failures occur, the overall efficiency and power capacity of the photovoltaic system will be affected. The maximum power tracking voltage range of the centralized inverter is narrow, the flexibility of component configuration is small, and the power generation time is short, and a special machine room with ventilation and heat dissipation function is required. It is mainly used in large-scale ground photovoltaic power plants and other places where the light is evenly concentrated.
The working principle of the centralized inverter
There are a large number of centrally connected PV strings, and the individual capacity is usually above 500KW. The centralized inverter is a common type of pv inverter on the market. Its working principle is to combine the DC current generated by the operation of multiple photovoltaic modules and perform maximum power peak tracking (MPPT), and then concentrate the inverter to convert and boost the DC current to achieve grid-connected power generation.
The centralized inverter generally adopts a single-channel MPPT, and a single MPPT is equipped with 2-12 sets of photovoltaic strings. The power of each MPPT can reach 125-1000KW, and the single-channel capacity is usually above 500KW, which has the advantages of high power and large capacity.
Advantages and disadvantages of centralized inverter
Advantages of centralized inverter
● The number of centralized inverters is small, which is easy to manage;
● The centralized inverter has a small number of components and high reliability;
● Less harmonic content, less DC component, high power quality;
● The inverter has high integration, high power density and low cost;
● The centralized inverter has complete protection functions and high power station safety;
● With power factor adjustment function and low voltage ride-through function, the grid has good regulation
Disadvantages of centralized inverter
● Safety issues. The DC combiner box is used in the centralized solution. Due to the built-in DC branch fuse, there is a risk of continuous fire, because the solar panel will be in working condition as long as there is light. For distributed rooftop workshops, it brings serious safety hazards. Not only the economic benefits of the power station itself will be affected, but also other equipment in the plant will be critically affected.
● For irregular roofs, a single 500KW inverter cannot make full use of the roof area. Inverters are often overloaded or lightly loaded, or overconfigured or underconfigured.
● Multiple facing roofs, the solar panels are partially shaded, resulting in inconsistency of the strings, and the single-channel MPPT leads to relatively low power generation; At the same time, the mismatch loss of each string will also lead to the loss of power generation.
● The inverter needs to be maintained by professional engineers. The failure of a single inverter has a great impact on the power generation and also brings great challenges to the safety of maintenance personnel. At the same time, there are many types of spare parts, and it takes more than 3 days to locate and repair the fault, which seriously affects the power generation revenue of customers.
● The failure rate of the DC combiner box is high, and each string cannot be monitored, which increases the fault location time. Due to the volatilization of the fuse, the failure rate and maintenance cost are high, and regular replacement and maintenance are required; The circuit is complex, there are many joints processed on site, and the failure rate is high; after 1~2 years of operation of some projects, the effective power generation rate is lower than 90%;
● The centralized scheme requires an inverter room and corresponding civil works, and also needs to be equipped with corresponding fans, air ducts, smoke detectors, temperature detectors and other equipment. However, if a photovoltaic power station is installed on the roof of a traditional factory building, the power distribution room may not be able to accommodate the bulky 500K Wp centralized inverter. At the same time, it not only increases the construction complexity, but also increases the initial investment and operation and maintenance costs.
● The centralized inverter needs forced air cooling, and the power consumption of the equipment room is large, with an average of at least 300W. Regular dust removal, fan maintenance and dust filter replacement are required.
● The MPPT voltage range of the centralized inverter is narrow, generally 450-820V, and the component configuration is not flexible. On rainy days and foggy areas, the power generation time is short.
● There is no redundancy in the centralized grid-connected inverter system. If there is a failure and shutdown, the entire system will stop generating electricity.
Comparison of string inverter vs centralized inverter
In the article string inverter, we have learned that the inverter mode of string PV inverter is to track the individual maximum power point of a single string or several strings of PV modules. After the inverter is merged into the AC grid, a string inverter can have multiple maximum power peak tracking modules, and the single-unit capacity is generally below 200kW. Let’s compare string inverter and centralized inverter.
|Project||Centralized inverter||String inverter|
|Centralized large power station||Applicable||Applicable|
|Distributed large industrial and commercial rooftop power station||Applicable||Applicable|
|Distributed small and medium industrial and commercial rooftop power stations||Not applicable||Applicable|
|Distributed household rooftop power station||Not applicable||Applicable|
|Maximum power tracking corresponding number of components||A large number of strings||1-4 strings|
|Maximum power tracking voltage range||Narrow||Wide|
|System power generation efficiency||Commonly||High|
|Installation footprint||Requires a separate computer room||Unnecessary|
|Outdoor install||Not allowed||Allow|
|Maintainability||Commonly||Easy to maintain|
|Inverter cost||Micro Inverter>String Inverter>Centralized Inverter|
|System cost of applying various inverters||Micro inverter > string inverter/centralized inverter (the two are close)|
In 2016, the inverters on the market were mainly centralized inverters, with centralized inverters accounting for 63% and string inverters accounting for 32%. In 2021, the photovoltaic inverter market will still be dominated by centralized inverters and string inverters. Among them, string inverters accounted for 69.6%, and centralized inverters accounted for 27.7%. Various advantages make string inverters regain the mainstream market.
In the early days of photovoltaic development, centralized inverter has always been the first choice for photovoltaic project construction. It has many advantages such as fewer components, fewer failure points, high integration, and high power density, and its market share is also firmly in the top spot. However, with the in-depth development of the industry, string inverters have sprung up, and their market share has continued to increase, mainly for the following reasons.
First, centralized inverters are mainly used in large power generation systems such as desert power stations and ground power stations. With the development of distributed photovoltaics, string inverters have begun to emerge due to their small size, reducing costs and improving competitiveness in large-scale production.
Second, while maintaining its own volume advantage, the power of string inverters has become larger and larger, and the application scope has been continuously expanded, eroding the market of centralized inverters, and the natural market share will also increase.
The third is the broad market of string inverters, attracting more and more companies to participate, continuous improvement in technological innovation, and more and better solutions. In turn, it has been favored by more customers, forming a good cycle.
In the battle between centralized and string inverters, the biggest beneficiary is the user. In the future, in the selection and design of power plants, there are more options for inverters. The topology of the inverter is no longer the key. Simply saying that the inverter is good has no meaning. The technical strength, price, and after-sales of the pv inverter manufacturers are the decisive factors.