Differences between Central Inverter and String Inverter

The National grid has the following requirements to the distributed photovoltaic power station: The single grid connection point is less than 6MW, the annual self-use power consumption is greater than 50%. 8kW or less 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 model selection method of the inverter of the PV station is: The 220V project selects the single-phase string inverter, the 8kW-500kW selects the three-phase string inverter, and the project above 500kW can select the string inverter and central inverter according to the practical condition.

Inverter scheme comparison

Central inverter: The power is between 100kW and 2500kW. With the development of power electronics technology, the string inverter is having an increasingly bigger market, and the central inverters below 500KW have already basically been eliminated from the market. The power device adopts high-current IGBT. The system topology adopts DC-AC first-level power electronic device to transform the full-bridge inverter. The latter stage is generally connected to the double-split power-frequency boost isolation transformer. The protection level is generally IP20. It has a large size and is vertically installed indoor.

String inverter: The power is from 1kW to 80kW. The low-power inverter switch tube generally uses the MOSFET with a small current. The medium-power inverter generally uses a power module that integrates multiple discrete devices. The topology uses DC-DC-BOOST boost and DC-AC full-bridge inverter two-stage power electronic device for transformation, and the protection level is generally IP65. It has a small size and is suspended outdoor.

Comparison of main system components

Central inverter: PV module, DC cable, combiner box, DC convergence power distribution, inverter, isolation transformer, AC power distribution, power grid.

String inverter: Component, DC cable, invertes, AC power distribution, power grid.

Main advantages and disadvantages, applied occasions

The central inverter is generally used in large power generation systems of the desert power plants and ground power stations. The total power of the system is large, generally above megawatts.

Its main advantages include:

  1. Fewer inverters, can be centralized installed, easier maintenance.
  2. Fewer inverter elements, fewer fault points and higher reliability.
  3. Fewer harmonic contents, less dc component, higher power quality.
  4. High inverter integration level, higher power density, lower cost.
  5. Complete protection functions of the inverter, higher safety of the power station.
  6. Possess the power factor adjustment function and low-voltage crossing function, good grid adjusting performance.

Main disadvantages:

  1. High fault rate of the DC convergence box, affect the entire system.
  2. Narrow MPPT voltage scope of the central inverter, generally 500-820V, inflexible component configuration, short power generation time in rainy or misty days.
  3. Difficult inverter machine room installation and deployment, require the dedicated machine room and equipment.
  4. The inverter itself consumes power and the machine room ventilation and dissipation consumes power, thus having rather complicated system maintenance.
  5. In the centralized grid-tied inverter system, the component square array reaches the power inverter through two convergences, and the maximum power tracking function (MPPT) of the inverter cannot monitor the operation of each component, so it is unable to make each component in the best working condition. If a component fails or is blocked by shadows, the power generation efficiency of the entire system will be affected.
  6. The centralized grid-connected inverter system has no redundancy ability. If it stops because of a fault, the entire system will stop power generation.

The string inverter is suitable for small and medium-sized rooftop photovoltaic power generation systems and small ground power stations.

The main advantages include:

  1. The string inverter adopts the modular design. Each photovoltaic string corresponds to one power 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 affected by the difference between the modules and shadow sheltering. While reducing the optimal operating point of the photovoltaic module and the inverter does not match, the generated power will be increased to the maximum.
  2. The string inverter has a wide MPPT voltage range, generally 200-800V, and the component configuration is more flexible. In areas of rainy and misty days, the power generation time is long.
  3. The string grid-tied inverter is small in size and light in weight, thus being easy to be handled and installed. It does not require professional tools and equipment, and does not require a dedicated power distribution room. It can simplify construction and reduce occupation in various applications. The DC line connection does not require any DC combiner box or 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. The power equipment and signal circuit are on the same panel, thus having difficult design and manufacturing, and poorer reliability.
  2. The inverter using the discrete power device has the small electrical clearance, thus not being suitable for high-altitude areas. The power inverter using the power module is not affected. Outdoor installation, wind and sun exposure can easily lead to aging of the outer casing and heat sink.
  3. The design with no isolated transformer, and the electrical safety is rather poor. The film component negative earthing system must be increased with the isolation transformer, thus having great dc component and affecting the power grid greatly.
  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 100 inverters are connected in parallel, the total harmonics will be superimposed and it is difficult to suppress.
  5. If there are many power inverters, the total fault rate will rise and the system monitoring is difficult.
  6. It has no dc circuit breaker and ac circuit breaker. If there is no dc fuse protector, in case of a system fault, it is not easy to be disconnected.
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