Difference between String Inverter and Micro Inverter

As the core equipment of the photovoltaic power generation system, the inverter is used to convert the variable DC voltage generated by the photovoltaic modules into the alternating current of the mains frequency, which is one of the important system balances in the photovoltaic array system. At present, the common inverters on the market are mainly string inverters and micro inverters. Now, let us compare and analyze between them.

String inverter

The string inverter is based on the modular concept. Each PV string (1-5kw) has a maximum power peak tracking at the DC side through an inverter, and is connected in parallel at the AC side. The most popular inverter on the market. String inverters are mainly used in small and medium rooftop photovoltaic power generation systems and small ground power stations.

Advantages

  • The string inverter adopts a modular design, each photovoltaic string corresponds to an inverter, the DC side has the maximum power tracking function, and the AC side is connected to the grid in parallel. The influence of shading, while reducing the mismatch between the optimal operating point of the photovoltaic cell module and the inverter, maximizes the power generation;
  • The string inverter MPPT voltage range is wide, generally 250-800V, the component configuration is more flexible, and the power generation time is long in rainy days and foggy areas;
  • The¬†string grid tied inverter is small in size and light in weight. It is very convenient to carry and install. It does not require professional tools and equipment, nor does it require a special power distribution room. It can simplify construction and installation in various applications. The floor space is reduced, and the DC line connection does not require DC combiner boxes and DC distribution cabinets. The string type also has the advantages of low self-consumption, small fault impact, and convenient replacement and maintenance.

Disadvantages

  • There are many electronic components, power devices and signal circuits are on the same board, which is difficult to design and manufacture, and the reliability is slightly poor.
  • The electrical clearance of the power device is small, which is not suitable for high-altitude areas. Outdoor installation, wind and sun exposure can easily lead to the aging of the casing and heat sink.
  • Without isolation transformer design, electrical safety is slightly poor, not suitable for the negative grounding system of thin-film modules, the DC component is large, and the impact on the power grid is large.
  • When multiple inverters are connected in parallel, the total harmonic is high, and 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 harmonic will be superimposed, and it is difficult to suppress.
  • The number of inverters is large, the total failure rate will increase, and the system monitoring is difficult.
  • There is no DC circuit breaker or AC circuit breaker, no DC fuse. When the system fails, it is not easy to disconnect.
  • A single inverter can realize the zero-voltage ride-through function, but when multiple inverters are connected in parallel, it is difficult to realize the zero-voltage ride-through function, reactive power regulation, active power regulation, and other functions.

Micro inverter

Micro inverters can achieve maximum power point tracking at the panel level, which has advantages over central inverters. In this way, the output power of each module can be optimized to maximize the overall output power.

Advantages

  1. When one or more modules fail, the system can continue to provide power to the grid, with high availability. Multiple redundant modules can be selected to improve system reliability.
  2. Flexible configuration. In the household market, the size of photovoltaic cells can be installed according to the user's financial resources.
  3. Effectively reduce the impact of shadows caused by local shading on output power.
  4. No high-voltage electricity, safer, simple and faster installation, low maintenance and installation cost.
  5. Improve the power generation of each inverter power module and track the maximum power. Due to the tracking of the maximum power point of a single module, the power generation of the photovoltaic system can be greatly increased by 25%.

Disadvantages

  1. Application is limited. The application of micro inverter is generally only suitable for roof households.
  2. The cost of micro inverters is higher than that of central inverters and string inverters.
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