The Difference Between Central Inverters and String Inverters

The State Grid has the following requirements for distributed photovoltaic power plants: single-point grid connection capacity must be less than 6MW, and annual self-consumption must exceed 50%. Units under 8kW can be connected to 220V, units between 8kW and 400kW can be connected to 380V, and units between 400kW and 6MW can be connected to 10kV. Based on the inverter's characteristics, the following inverter selection method is used for photovoltaic power plants: single-phase string inverters for 220V projects, three-phase string inverters for units between 8kW and 500kW, and string and central inverters for units over 500kW, depending on the specific situation.

Inverter Solution Comparison

Centralized inverter: Power ranges from 100kW to 2500kW. With the advancement of power electronics technology, the market for string inverters is growing, and centralized inverters below 500kW have largely been phased out. High-current IGBTs are used as power devices.

This system topology utilizes a DC-AC first-stage power electronic device to transform a full-bridge inverter. The downstream stage is typically connected to a dual-split power-frequency step-up isolation transformer. The protection level is generally IP20. It is large and is mounted vertically indoors.

String inverter: Power ranges from 1kW to 80kW. Low-power inverter switches typically use low-current MOSFETs. Medium-power inverters typically utilize power modules integrating multiple discrete components. The topology utilizes two-stage power electronic devices: a DC-DC boost and a DC-AC full-bridge inverter for voltage transformation. The protection level is generally IP65. It is compact and can be suspended outdoors.

Comparison of Key System Components

Central inverter: PV panels, DC cables, combiner box, DC bus and distribution, inverter, isolation transformer, AC distribution, and power grid.

String inverter: PV panels, DC cables, inverter, AC distribution, and power grid.

Main Advantages and Disadvantages, and Applications

Central inverters are generally used in large-scale power generation systems in desert power plants and ground-based power stations. The total system power is relatively large, generally exceeding megawatts.

The main advantages include:

Fewer inverters, allowing for centralized installation and easier maintenance.

Fewer inverter components, fewer failure points, and higher reliability.

Lower harmonic content and DC components result in higher power quality.

High inverter integration, higher power density, and lower cost.

Complete inverter protection features enhance power plant safety.

Power factor regulation and low voltage ride-through (LVRT) provide excellent grid regulation performance.

Main Disadvantages:

The DC combiner box has a high failure rate, impacting the entire system.

Centralized inverters have a narrow MPPT voltage range, typically 500-820V, inflexible component configuration, and limited power generation time in rainy or foggy conditions.

Inverter equipment room installation and deployment are difficult, requiring a dedicated room and equipment.

The inverter itself consumes power, as does ventilation and cooling in the equipment room, making system maintenance more complex.

In a centralized grid-connected inverter system, the photovoltaic array is fed twice to the power inverter. The inverter's maximum power point tracking (MPPT) function cannot monitor the operation of individual modules, thus failing to maintain optimal performance. If a module fails or is shaded, the power generation efficiency of the entire system will be affected.

Centralized grid-connected inverter systems lack redundancy. If one module fails due to a fault, the entire system will cease generating power.

String inverters are suitable for small and medium-sized rooftop photovoltaic power generation systems and small ground-mounted power plants.

Key advantages include:

String inverters feature a modular design. Each photovoltaic string corresponds to a power inverter. The DC side features maximum power point tracking, and the AC side is connected in parallel. This advantage is that they are unaffected by differences in modules and shading. They minimize mismatches between the optimal operating point of the photovoltaic modules and the inverter while maximizing power generation.

String inverters offer a wide MPPT voltage range, generally 200-800V, allowing for more flexible module configuration. They also extend power generation time in rainy and foggy areas.

This string-type grid-connected inverter is compact and lightweight, making it easy to transport and install. It requires no specialized tools or equipment, nor a dedicated distribution room. This simplifies construction and reduces footprint in various applications. No DC combiner box or DC distribution cabinet is required for DC line connection. The string type also offers advantages such as low self-consumption, minimal impact, and easy replacement and maintenance.

Major disadvantages include:

It contains numerous electronic components. The power supply and signal circuits are located on the same panel, making design and manufacturing difficult and resulting in poor reliability.

Inverters using discrete power devices have limited electrical clearances and are not suitable for high-altitude locations. Power inverters using power modules are not affected. Outdoor installations are susceptible to weathering, which can easily cause degradation of the casing and heat sink.

The lack of an isolation transformer compromises electrical safety. Thin-film element negative grounding systems require an isolation transformer, resulting in significant DC components and a significant impact on the grid.

When multiple inverters are connected in parallel, total harmonics are high, but the THDI of a single inverter can be controlled to above 2%. However, if more than 100 inverters are connected in parallel, the total harmonics will be superimposed and difficult to suppress.

With a large number of inverters, the overall failure rate will increase, making system monitoring difficult.

It lacks DC and AC circuit breakers. Without DC fuse protection, it would be difficult to disconnect the fuse in the event of a system fault.