The National Energy Administration issued a notice that in 2014, the total installed capacity of domestic PV new equipment reached 14G, including distributed 8G and 6G ground power station. Distributed photovoltaic power plants will usher in an unprecedented development opportunity. The requirements of the national grid for distributed photovoltaic power plants are as follows: a single grid connection point is less than 6MW, the annual self-use power consumption is greater than 50%; 8KW can access 220V; 8KW-400KW can access 380V; 400KW-6MW can access 10KV. According to the characteristics of the inverter, the PV power plant inverter selection method: 220V project selects single-phase string inverter, 8KW-30KW selects three-phase string inverter, 50KW or more projects, according to actual conditions The string inverter and the centralized inverter are selected.
Inverter solution comparison:
Centralized inverter: The power of the equipment is between 50KW and 630KW. The power device adopts high-current IGBT. The system topology adopts DC-AC first-level power electronic device to transform full-bridge inverter, power frequency isolation transformer, and the protection level is generally For IP20. Large size, indoor vertical installation.
String inverter: power less than 30KW, power switch tube uses small current MOSFET, topology uses DC-DC-BOOST boost and DC-AC full-bridge inverter two-stage power electronic device conversion, the protection level is generally IP65 . Small in size, it can be mounted on an outdoor arm.
System main device comparison:
Centralized inverters: PV modules, DC cables, combiner boxes, DC cables, DC bus power distribution, DC cables, inverters, isolation transformers, AC power distribution, power grids.
String inverters: components, DC cables, inverters, AC power distribution, power grids.
Main advantages and disadvantages and adaptation occasions:
1. Centralized inverters are generally used in large-scale power plants with uniform sunshine, desert power plants, ground power stations, etc. The total power of the system is large, generally above MW.
The main advantages are:
(1) The number of inverters is small and easy to manage;
(2) The number of components of the inverter is small and the reliability is high;
(3) Less harmonic content, less DC component and higher power quality;
(4) Inverter has high integration, high power density and low cost;
(5) The inverter has various protection functions and the safety of the power station is high;
(6) With power factor adjustment function and low voltage ride-through function, the power grid is well regulated.
The main disadvantages are:
(1) The DC combiner box has a high failure rate and affects the entire system.
(2) The centralized inverter MPPT voltage range is narrow, generally 450-820V, and the component configuration is not flexible. In the rainy days, the foggy area is short, and the power generation time is short.
(3) The installation and deployment of the inverter room is difficult, and a dedicated machine room and equipment are required.
(4) The power consumption of the inverter itself and the ventilation and heat dissipation of the equipment room are relatively complicated.
(5) In the centralized grid-connected inverter system, the component square matrix reaches the inverter through two convergences, and the maximum power tracking function (MPPT) of the inverter cannot monitor the operation of each component, so it is impossible to make each channel Components are at the optimum working point. When a component fails or is blocked by shadows, it will affect the power generation efficiency of the entire system.
(6) There is no redundancy capability in the centralized grid-connected inverter system. If there is a fault shutdown, the entire system will stop generating electricity.
2. The string inverter is 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 one inverter. The DC terminal has the maximum power tracking function, and the AC terminal is connected in parallel. The advantage is that it is not subject to the difference between the modules. The effect of shadow occlusion, while reducing the optimal operating point of the PV module and the inverter does not match, maximizing the amount of power generation.
(2) The string inverter has a wide MPPT voltage range, generally 250-800V, and the component configuration is more flexible. In the rainy days, the foggy area is long and the power generation time is long.
(3) The string-type grid-connected inverter is small in size, light in weight, easy to handle and install, does not require professional tools and equipment, and does not require a dedicated power distribution room. It can simplify construction in various applications. Reduce the land occupation, DC bus connection does not need DC combiner box and DC power distribution cabinet. The string type also has the advantages of low self-consumption, low impact, convenient replacement and maintenance.
The main disadvantages are:
(1) There are many electronic components, and the power device and the signal circuit are on the same board. It is difficult to design and manufacture, and the reliability is slightly poor.
(2) The power device has a small gap and is not suitable for high altitude areas. Outdoor installation, wind and sun exposure can easily lead to aging of the outer casing and heat sink.
(3) Without isolation transformer design, the electrical safety is slightly poor, it is not suitable for the negative electrode grounding system of thin film components, and the DC component is large, which has a great influence on the power grid.
(4) When multiple inverters are connected in parallel, the total harmonics are high, and the single inverter THDI can be controlled to more than 2%. However, if more than 40 inverters are connected in parallel, the total harmonics will be superimposed. And it is more difficult to suppress.
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