Inversion is the opposite process to rectification, which is the process of converting DC power into AC power. Photovoltaic inverter refers to a circuit that completes the inverter function or a device that implements the inverter process.

The main components of the inverter:

  • Shell and terminals: used for junction box protection
  • Radiator: used for system heat dissipation of the inverter
  • Display: inverter status and data
  • Control board: the core component of the inverter, used for inverter Power control of the inverter and various algorithm control
  • Power supply board: Internal power supply for the inverter
  • Power board: The core component of the inverter, the main circuits are concentrated on the power board.

Classification of inverters

Divided by function: Grid-connected inverters and off-grid inverter

Divided by the frequency of output AC power: industrial frequency inverter (frequency: 50-60Hz), medium frequency inverter (frequency: 400-20kHz) and High frequency inverter (frequency: 20kHz-10MHz).

Divided according to the number of inverter output phases: single-phase inverter, three-phase inverter and multi-phase inverter.

Divided according to the access status of photovoltaic modules: centralized inverters, string inverters and micro (module) inverters.

Centralized inverter

String inverter

Microinverter

Definition

A square array composed of multiple photovoltaic strings is centrally connected to a large inverter.

Based on the concept of modularity, each photovoltaic string in the photovoltaic array is input into an inverter, and multiple photovoltaic strings and inverters are combined together in a modular manner.

Each photovoltaic module corresponds to a micro-inverter, which has independent variable speed and MPPT functions and can be directly fixed behind the photovoltaic module.

Advantage

  1. High power, small quantity, easy to manage, few components, good stability and easy maintenance.
  2.  Low harmonic content, high power quality; complete protection functions and high safety.
  3.  It has power factor adjustment function and low voltage ride-through function, and has good grid regulation.
  1. It is not affected by module differences and shadow occlusion between photovoltaic strings, reducing the mismatch between the optimal operating point of photovoltaic modules and the inverter, and maximizing power generation.
  2. MPPT has a wide voltage range and the photovoltaic module configuration is more flexible; on rainy days and in areas with a lot of wind, the power generation time is long.
  3. Small size, small floor space, no need for a dedicated computer room, flexible installation; low self-consumption, and small impact of failures.
  1. Maximize power generation.
  2. Adjust the voltage and current of each row of photovoltaic modules until they are all balanced to avoid mismatch in the system.
  3. Each module has a monitoring function, which reduces the maintenance cost of the system and makes the operation more stable and reliable.
  4. There is no high-voltage direct current, which means it is safer. It is simple to install, fast and easy to maintain. It reduces dependence on installation service providers, allowing users to DIY the solar power generation system.
  5. The cost is equivalent to or even lower than that of centralized inverters.

Shortcoming

  1. The MPPT voltage range is narrow and the operation of each photovoltaic module cannot be monitored. Therefore, it is impossible to keep each photovoltaic module at the optimal operating point, and the configuration of photovoltaic modules is inflexible.
  2. It occupies a large area, requires a dedicated computer room, and is inflexible in installation.
  3. Its own power consumption and the ventilation and heat dissipation of the computer room consume a lot of power.
  1. The electrical gap of the power device is small and is not suitable for high altitude areas; there are many components integrated together and the stability is slightly poor.
  2. Outdoor installation, exposure to wind and sun can easily cause aging of the casing and heat sink.
  3. If the number is large, the total failure rate will increase, making system monitoring difficult.
  1. Same installed capacity, larger quantity.
  2. Characteristics such as safety, stability and high power generation still need to be tested by engineering projects.

Main parameters of the inverter

1. Technical parameters of DC input side

  • Maximum allowable access to photovoltaic string power

The maximum allowable connected photovoltaic string power is the maximum DC connected photovoltaic string power allowed by the inverter.

  • Rated DC power

The rated DC power is calculated based on the rated AC output power divided by the conversion efficiency, plus a certain margin.

  • Maximum DC voltage

Taking the temperature coefficient into consideration, the maximum voltage of the connected photovoltaic string should be less than the maximum DC input voltage of the inverter.

  • MPPT voltage range

The MPPT voltage of the photovoltaic string taking into account the temperature coefficient must be within the MPPT tracking range of the inverter. A wider MPPT voltage range enables more power generation.

  • Starting voltage

When the starting voltage threshold is exceeded, the inverter starts to start, and when it is lower than the starting voltage threshold, the inverter shuts down.

  • Maximum DC current

When selecting an inverter, focus on the maximum DC current parameter. Especially when connecting thin-film photovoltaic modules, it is necessary to ensure that the photovoltaic string current connected to each MPPT is less than the maximum DC current of the inverter.

  • Number of input channels and MPPT channels

The number of input channels of the inverter refers to the number of DC input channels, while the number of MPPT channels refers to the number of maximum power point tracking channels. The number of input channels of the inverter is not equal to the number of MPPT channels. If the inverter has 6 DC inputs, the inputs of every three inverters are used as one MPPT input. The inputs of several photovoltaic groups under 1 MPPT need to be equal, and the inputs of photovoltaic strings under different MPPTs do not need to be equal.

Main parameters of the inverter

2. Technical parameters of AC output side

  • Maximum AC power

Maximum AC power refers to the maximum power that the inverter can emit. Generally speaking, inverters are named according to their AC output power, but they are also named according to their DC input rated power.

  • Maximum AC current

The maximum AC current refers to the maximum current that the inverter can emit, which directly determines the cross-sectional area of the cable and the parameter specifications of the power distribution equipment. Generally speaking, the circuit breaker specifications should be selected to be 1.25 times the maximum AC current.

  • Rated output

Rated output has two types: frequency output and voltage output. In China, the frequency output is generally power frequency 50Hz, and the deviation should be within +1% under normal working conditions. Voltage outputs include 230V, 400V, 480V, etc.

  • power factor

In an AC circuit, the cosine of the phase difference ( Φ ) between voltage and current is called the power factor, represented by the symbol cosΦ. Numerically, power factor is the ratio of active power to apparent power, that is, cosΦ=P/S. The power factor of resistive loads such as incandescent light bulbs and resistance furnaces is 1, while the power factor of circuits with inductive loads is less than 1.

3. Efficiency

There are four commonly used efficiencies for inverters: maximum efficiency, European efficiency, MPPT efficiency and overall efficiency.

  • Maximum efficiency:

refers to the maximum instantaneous conversion efficiency of the inverter.

  • European efficiency:

It is the weight of different power points based on European lighting conditions at different DC input power points, such as 5%, 10%, 15%, 25%, 30%, 50%, and 100%. Used to estimate the overall efficiency of the inverter.

  • MPPT efficiency:

 refers to the accuracy of the inverter's maximum power point tracking.

  • Overall machine efficiency:

refers to the product of European efficiency and MPPT efficiency under a certain DC voltage.

4. Function protection parameters

  • Island protection

When the power grid loses voltage, the photovoltaic power generation system continues to supply power to a certain part of the power grid. The so-called islanding protection is to prevent this unplanned islanding effect from occurring and ensure the personal safety of grid operators and users. And reduce the occurrence of power distribution equipment and load failures.

  • Input overvoltage protection

Input overvoltage protection means that when the DC input side voltage is higher than the maximum DC array access voltage allowed by the inverter, the inverter shall not start or stop.

  • Output side overvoltage/undervoltage protection

Output side overvoltage/undervoltage protection means that when the output side voltage of the inverter is higher than the maximum allowable output voltage of the inverter or lower than the minimum allowable output voltage of the inverter, the inverter starts the protection state. The abnormal voltage response time on the AC side of the inverter must comply with the specific provisions of the grid connection standards.

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