The function of a photovoltaic (PV) inverter is to convert the variable DC voltage generated by PV modules into AC power at the mains frequency, and it is one of the important systems for balancing PV array systems. Currently, the most common inverters on the market are centralized inverters and string inverters. Below, we will learn about the advantages, disadvantages, and suitable scenarios for centralized and string PV inverters.
I. Introduction to Two Types of Inverters
1. Centralized inverter: A centralized inverter converts the collected DC power into AC power. Therefore, the power of the inverter is relatively large. In photovoltaic power plants, centralized inverters with a power of 500KW or more are generally used.
2. String inverter: String inverters directly convert the DC power generated by the modules into AC power and then combine them. Therefore, the power of the inverters is relatively small. String inverters below 100KW are generally used in photovoltaic power plants.
II. Advantages, disadvantages, and suitable scenarios for centralized inverters
Centralized inverters are generally used in large-scale power generation systems such as large factories with uniform sunlight, desert power stations, and ground-mounted power stations. The total power of these systems is large, typically in the megawatt range.
1. Main advantages: (1) Easy to maintain and manage; (2) High integration and high power density of the inverter; (3) Complete protection functions of the inverter, and high power station safety; (4) Active power factor regulation function and high and low voltage ride-through function.
2. Main disadvantages:
(1) The DC combiner box has a high failure rate, affecting the entire system;
(2) Centralized inverters have a narrow MPPT voltage range, typically 450-820V, and inflexible component configuration. In cloudy, rainy, or foggy areas, the power generation time is short.
(3) The inverter itself consumes a lot of power and the computer room has high ventilation and heat dissipation noise. The system maintenance is relatively complex.
(4) In a centralized grid-connected inverter system, the photovoltaic module array reaches the inverter through two combiners. The inverter's maximum power point tracking function cannot monitor the operation of each module, so it is impossible to ensure that each module is at its optimal operating point. When a module fails or is shaded, it will affect the power generation efficiency of the entire system.
(5) There is no redundancy in the centralized grid-connected inverter system. If a fault occurs and the system stops generating electricity, the entire system will stop generating electricity.
III. Advantages, disadvantages, and suitable scenarios for string inverters
String inverters are suitable for various photovoltaic power station scenarios.
1. Main advantages
(1) The string inverter adopts a modular design, with each pair of strings corresponding to one MPPT. The DC end has maximum power point tracking function, and the AC end is connected to the grid in parallel. Its advantage is that it is not affected by the differences between photovoltaic modules and shading, and at the same time, it reduces the mismatch between the optimal operating point of the photovoltaic module and the inverter, thus maximizing the power generation.
(2) String inverters have a wide MPPT voltage range, generally 200-1000V, with a maximum of 600-1500V currently, allowing for more flexible module configuration. They also provide longer power generation time in areas with cloudy, rainy, or foggy weather.
(3) String inverters are small in size, light in weight, and very convenient to transport and install. They do not require professional tools and equipment or a special power distribution room. They can simplify construction and reduce the footprint in various applications. DC line connections do not require DC combiner boxes or DC distribution cabinets.
(4) String inverters have low self-power consumption, low failure rate, and are easier to maintain.
