Comparison of selection between central inverter and string inverter

　　The National Energy Administration issued a notice that the total capacity of newly installed domestic photovoltaic power plants reached 14G in 2014, including 8G distributed and 6G ground power plants. Distributed photovoltaic power plants will usher in an unprecedented development opportunity. The State Grid has the following requirements for distributed photovoltaic power plants: a single grid-connected point is less than 6MW, and the annual self-generated and self-used electricity is greater than 50%; 8KW and below can be connected to 220V; 8KW-400KW can be connected to 380V; 400KW-6MW can be connected to 10KV. According to the characteristics of the inverter, the selection method of photovoltaic power station inverter: 220V projects use single-phase string inverters, 8KW-30KW use three-phase string inverters, and projects above 50KW can use string inverters and centralized inverters according to actual conditions.

　　Inverter solution comparison:

　　Centralized inverter: The power of the equipment is between 50KW and 630KW. The power device uses high-current IGBT. The system topology adopts DC-AC primary power electronic device conversion full-bridge inverter and power frequency isolation transformer. The protection level is generally IP20. It is large in size and installed vertically indoors.

　　String inverter: The power is less than 30KW, the power switch tube uses a small current MOSFET, the topology adopts DC-DC-BOOST boost and DC-AC full-bridge inverter two-stage power electronic device conversion, and the protection level is generally IP65. It is small in size and can be installed outdoors in an arm-mounted manner.

　　Comparison of main components of the system:

　　Centralized inverter: photovoltaic panels, DC cables, combiner boxes, DC cables, DC power distribution, DC cables, inverters, isolation transformers, AC power distribution, power grid.

　　String inverter: components, DC cables, inverters, AC distribution, grid.

　　Main advantages and disadvantages and applicable occasions:

　　1. Centralized inverters are generally used in large power generation systems such as large factories with uniform sunlight, desert power stations, and ground power stations. The total system power is large, generally above megawatt level.

　　The main advantages are:

　　(1) Small number of inverters, easy to manage;

　　(2) The inverter has a small number of components and high reliability;

　　(3) Low harmonic content, low DC component and high power quality;

　　(4) The inverter has high integration, high power density and low cost;

　　(5) The inverter has complete protection functions and the power station is highly safe;

　　(6) It has power factor regulation function and low voltage ride-through function, and the grid regulation is good.

　　The main disadvantages are:

　　(1) The failure rate of DC combiner boxes is high, affecting the entire system.

　　(2) The MPPT voltage range of the centralized inverter is narrow, generally 450-820V, and the component configuration is not flexible. On rainy days and in foggy areas, the power generation time is short.

　　(3) The inverter room is difficult to install and deploy, and requires a dedicated room and equipment.

　　(4) The inverter itself consumes power, as does the ventilation and heat dissipation in the equipment room. System maintenance is relatively complex.

　　(5) In a centralized grid-connected inverter system, the component array is converged twice before reaching the inverter. The maximum power point tracking (MPPT) function of the inverter cannot monitor the operating status of each component. Therefore, it is impossible to make each component at the optimal working point. When a component fails or is blocked by a shadow, the power generation efficiency of the entire system will be affected.

　　(6) There is no redundancy in the centralized grid-connected inverter system. If a failure occurs, the entire system will stop generating electricity.

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

　　The main advantages are:

　　(1) The string inverter adopts a modular design. Each photovoltaic string corresponds to an inverter. The DC end has a maximum power tracking function and the AC end is parallel and grid-connected. Its advantage is that it is not affected by module differences between strings and shadows. At the same time, it reduces the mismatch between the optimal working point of the photovoltaic cell module and the inverter, thereby maximizing the power generation.

　　(2) The MPPT voltage range of string inverters is wide, generally 250-800V, and the component configuration is more flexible. On rainy days and in foggy areas, the power generation time is long.

　　(3) The string-type grid-connected inverter is small in size and light in weight, and is very convenient to carry and install. It does not require professional tools and equipment, nor does it require a dedicated distribution room. It can simplify construction and reduce floor space in various applications. The DC line connection does not require a DC combiner box and a DC distribution cabinet. The string type also has the advantages of low self-consumption, small impact of failures, and easy replacement and maintenance.

　　The main disadvantages are:

　　(1) There are many electronic components , and the power devices and signal circuits are on the same board, which makes design and manufacturing difficult and has poor reliability.

　　(2) The electrical clearance of power devices is small, which is not suitable for high altitude areas. Outdoor installation, wind and sun exposure can easily cause aging of the casing and heat sink.

　　(3) The design is not equipped with an isolation transformer, so the electrical safety is slightly poor and it is not suitable for the negative grounding system of thin-film modules. The DC component is large and has a great impact on the power grid.

　　(4) When multiple inverters are connected in parallel, the total harmonics are high. The THDI of a single inverter can be controlled to more than 2%, but if more than 40 inverters are connected in parallel, the total harmonics will be superimposed and difficult to suppress.

　　(5) If there are a large number of inverters, the total failure rate will increase, making system monitoring more difficult.

　　(6) There are no DC circuit breakers and AC circuit breakers, and no DC fuses. When a system failure occurs, it is not easy to disconnect.

　　(7) A single inverter can achieve zero voltage ride-through function, but when multiple machines are connected in parallel, it is difficult to achieve zero voltage ride-through function, reactive power regulation, active power regulation and other functions.

　　Comparison between 20KW string inverter and 500KW centralized inverter

　　Comparison of 1MW PV 10KV Grid-connected System Costs

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