Photovoltaic inverters are a crucial component of photovoltaic (PV) power generation systems. Compared to conventional inverters, PV inverters possess maximum power point tracking (MPPT) capabilities and low-voltage ride-through capabilities for grid safety. Currently, common PV inverters are mainly divided into centralized PV inverters and string PV inverters. So, which is better, a centralized PV inverter or a string PV inverter? Let's take a look at the advantages and disadvantages of centralized PV inverters and string PV inverters.
Centralized photovoltaic inverter
Centralized photovoltaic inverters convert the direct current generated by photovoltaic modules into alternating current, then boost the voltage and connect to the grid. Therefore, the power of these inverters is relatively large, and centralized inverters of 500kW or more are generally used in photovoltaic power plants.
Centralized photovoltaic inverters have a power output between 50kW and 630kW, using high-current IGBTs as power devices. The system topology employs a DC-AC first-level power electronic device conversion full-bridge inverter with a power frequency isolation transformer. The protection level is generally IP20. They are relatively large and designed for indoor vertical installation.
Advantages of centralized photovoltaic inverters:
1. High power, small quantity, easy to manage; fewer components, good stability, easy to maintain;
2. Low harmonic content, high power quality; comprehensive protection functions, high safety;
3. It has power factor regulation and low voltage ride-through functions, and good grid regulation.
Disadvantages of centralized photovoltaic inverters:
1. Centralized inverters have a narrow MPPT voltage range, making it impossible to monitor the operation of each module. Therefore, it is impossible to ensure that each module is at its optimal operating point, resulting in inflexible module configuration.
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 also consume a lot of power.
String photovoltaic inverter
String photovoltaic inverters directly convert the direct current generated by photovoltaic modules into alternating current, which is then boosted and connected to the grid. Therefore, the power of these inverters is relatively small, and string photovoltaic inverters of 50kW or less are generally used in photovoltaic power plants.
String inverters with a power rating of less than 100kW use low-current MOSFETs or IGBTs as power switches. Their topology employs a two-stage power electronic conversion process: DC-DC-BOOST boost and DC-AC full-bridge inverter. The protection rating is typically IP65. They are compact and can be wall-mounted outdoors.
Advantages of string photovoltaic inverters:
1. Unaffected by differences between modules in the same string and by shading, it also reduces the mismatch between the optimal operating point of the photovoltaic module and the inverter, maximizing power generation;
2. MPPT has a wide voltage range, allowing for more flexible module configuration; it also provides longer power generation time in areas with cloudy, rainy, or foggy conditions.
3. Small size, small footprint, no need for a dedicated server room, and flexible installation;
4. Low self-power consumption and minimal impact from malfunctions.
Disadvantages of string photovoltaic 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 larger number of inverters increases the overall failure rate and makes system monitoring more difficult;
4. The design without an isolation transformer has slightly lower electrical safety and is not suitable for thin-film module negative grounding systems.
The above is a comparison of the advantages and disadvantages of centralized photovoltaic inverters and string photovoltaic inverters. Compared with centralized photovoltaic inverters, string photovoltaic inverters have advantages in terms of failure rate, system security, and operation and maintenance costs. They also have better system reliability and can ensure the long-term safe and reliable operation of the power station. This is why buyers of large-scale photovoltaic systems are increasingly favoring string photovoltaic inverters over centralized photovoltaic inverters.
