Centralized inverters and string inverters

According to the decentralization notice issued by the National Energy Administration, the total installed capacity of domestic photovoltaics reached 14G in 2014, including distributed 8G and 6G ground power station. Distributed photovoltaic power plants will usher in an unprecedented opportunity for development. The requirements of the State Grid for distributed PV power plants are as follows: a single grid point is less than 6MW, and annual self-use electricity 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, PV power plant inverter selection method: 220V single-phase string inverter is selected, 8KW-30KW three-phase string inverter, 50KW or more, according to the actual situation Select string inverters and centralized inverters.

Inverter program comparison:

Centralized inverter: The equipment power is between 50KW and 630KW. The power device adopts high-current IGBT. The system topology uses DC-AC power electronics to transform the full-bridge inverter. The power-frequency isolation transformer is used, and the protection level is general. For IP20. Larger size, indoor vertical installation.

String inverters: The power is less than 30KW, the power switch tube adopts the MOSFET of small current, the topological structure adopts DC-DC-BOOST step-up and DC-AC full-bridge inverter two-level power electronic device transformation, the protection level is generally IP65 . Smaller size, outdoor arm-mounted installation.

System main device comparison:

Centralized inverters: photovoltaic modules, DC cables, combiner boxes, DC cables, DC bus distribution, DC cables, inverters, isolation transformers, AC distribution, and power grids.

String inverters: components, DC cables, inverters, AC distribution, power grids.

Main advantages and disadvantages and adaptation occasions:

1. Centralized inverters are generally used in large-scale powerhouses with uniform sunlight, desert power stations, ground power stations, and other large-scale power generation systems. The total power of the system is large, generally more than megawatts.

The main advantages are:

(1) Small number of inverters for ease of management;

(2) Small number of inverter components and high reliability;

(3) less harmonic content, less DC component, higher power quality;

(4) The 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) Power factor adjustment function and low voltage ride-through function are available, and the grid regulation is good.

The main disadvantages are:

(1) The high failure rate of the DC combiner box affects the entire system.

(2) The MPPT voltage range of the centralized inverter is narrow, generally 450-820V, and the component configuration is inflexible. In areas that are cloudy and rainy, there are many areas where there is much fog, and the power generation time is short.

(3) The installation and deployment of the inverter room is difficult and requires a dedicated machine room and equipment.

(4) The power consumption of the inverter itself and the ventilation and heat dissipation in the equipment room are relatively complex.

(5) In a centralized grid-connected inverter system, the component squares reach the inverter after two confluences, and the inverter maximum power tracking function (MPPT) cannot monitor the operation of each component. Therefore, it is impossible to make each path. The components are at the best working point. When a component fails or is shaded, it will affect the overall system's power generation efficiency.

(6) There is no redundancy in the centralized grid-connected inverter system. If any shutdown occurs, 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 modular design, each PV string corresponds to one inverter, the DC end has the maximum power tracking function, and the AC end is connected in parallel to the grid. The advantage is that it is not affected by the module differences between strings, and The effect of shadow shading, while reducing the mismatch between the best operating point of the PV module and the inverter, maximizes the amount of power generated.

(2) The MPPT voltage range of the string inverter is wide, generally 250-800V, and the component configuration is more flexible. In cloudy and rainy days, there is a long period of power generation in areas where there is much fog.

(3) The series-connected grid-connected inverter is small in size and light in weight, easy to handle and install, requires no specialized tools and equipment, and does not require a special power distribution room, which can simplify construction in various applications. To reduce land occupation, DC link boxes and DC distribution cabinets are not required for DC link connections. Strings also have the advantages of low self-consumption, small impact of faults, and easy replacement and maintenance.

The main disadvantages are:

(1) There are many electronic components. The power device and the signal circuit are on the same board. It is difficult to design and manufacture, and the reliability is somewhat poor.

(2) The power device has a small electrical clearance and is not suitable for high altitude areas. Outdoor installation, wind and sun can easily lead to aging of the shell and heat sink.

(3) Without isolation transformer design, the electrical safety is slightly poor, and it is not suitable for the negative-electrode grounding system of the membrane module. The DC component is large, which has a great impact on the power grid.

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

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

(6) There is no DC circuit breaker and AC circuit breaker, no DC fuse, when the system fails, it is not easy to disconnect.

(7) A single inverter can achieve zero voltage ride through, but when multiple inverters are connected in parallel, the functions of zero voltage ride through, reactive power adjustment, and active power adjustment are difficult to implement.