Inverters are an important component of photovoltaic power generation, and their main function is to convert the direct current (DC) generated by photovoltaic modules into alternating current (AC). Currently, the most common inverters on the market are mainly divided into centralized inverters, string inverters, and the trendy distributed inverters.
1. Centralized photovoltaic inverter
Centralized inverters, as the name suggests, convert the direct current (DC) generated by photovoltaic (PV) modules into alternating current (AC) before stepping up the voltage and connecting it to the grid. Therefore, these inverters typically have relatively high power outputs. PV power plants generally use centralized inverters with a power rating of 500kW or higher.
The advantages of centralized inverters are as follows:
(1) High power, small quantity, easy to manage; fewer components, good stability, easy to maintain;
(2) Low harmonic content and high power quality; comprehensive protection functions and high safety;
(3) It has power factor regulation function and low voltage ride-through function, and good grid regulation.
Centralized inverters have the following problems:
(1) The MppT voltage range of centralized inverters is narrow, and the operation of each component cannot be monitored. Therefore, it is impossible to ensure that each component is at its optimal operating point, and the component configuration is inflexible.
(2) Centralized inverters occupy a large area, require a dedicated machine room, and are not flexible in installation;
(3) It consumes a lot of power itself and the computer room ventilation and heat dissipation.
2. String photovoltaic inverter
String inverters, as the name suggests, directly convert the direct current (DC) generated by photovoltaic (PV) modules into alternating current (AC), which is then boosted and connected to the grid. Therefore, the power output of these inverters is relatively small. PV power plants typically use string inverters with a power output of 50kW or less.
Advantages of string inverters:
(1) It is not affected by differences between modules in the string or by shading, and at the same time reduces the mismatch between the optimal operating point of the photovoltaic cell module and the inverter, thus maximizing power generation;
(2) MppT has a wide voltage range and more flexible module configuration; in areas with cloudy or foggy weather, the power generation time is longer;
(3) It has a small size and footprint, requires no dedicated computer room, and is flexible in installation;
(4) Low self-consumption and minimal impact from malfunctions.
Problems with string inverters:
(1) Power devices have small electrical clearances, making them unsuitable for high-altitude areas; they also have many components integrated together, resulting in slightly lower stability.
(2) Outdoor installations are easily affected by wind and sun, which can cause the casing and heat sink to age.
(3) A large number of inverters will increase the overall failure rate and make system monitoring more difficult;
(4) The design without an isolation transformer has slightly poor electrical safety and is not suitable for the negative grounding system of thin film modules.
3. Distributed photovoltaic inverter
Distributed inverters are a new type of inverter proposed in the last two years. Their main features are centralized inversion and distributed MPPT tracking. Distributed inverters combine the advantages of both centralized and string inverters, achieving the low cost of centralized inverters and the high power generation of string inverters.
Advantages of distributed inverters:
(1) Compared with centralized tracking, decentralized MPPT tracking reduces the probability of mismatch and increases power generation;
(2) Compared with centralized and string inverters, distributed inverters have a boost function and reduce line losses;
(3) Compared with string inverters, centralized inverters have a greater advantage in terms of construction cost.
Distributed inverter problems;
(1) Limited engineering experience. Compared to the previous two categories, this is a relatively new form with limited application in engineering projects;
(2) The safety, stability, and high power generation characteristics still need to be tested through engineering projects;
(3) Because of the use of centralized inverters, the disadvantages of large footprint and the need for a dedicated computer room also exist in distributed inverters.
