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Functions And Classification Of Photovoltaic Inverters

An inverter is a device that converts DC power to AC power. Solar cells produce DC electricity in daylight and stored in batteries, but DC power supply system has great limitations, and daily household appliances AC loads cannot be powered by DC power. For PV power generation to be widely used in our daily life, an inverter that can turn DC power into AC power is indispensable. As an important part of PV power generation, the main function of PV inverter is to convert the DC power from PV modules into AC power.
PV inverters are generally classified into four categories: centralized inverters, distributed inverters, string inverters and micro inverters
1.Centralized inverter
The centralized inverter is a PV inverter that connects many parallel PV strings to the DC input of the same centralized inverter, and after doing the maximum power peak tracking, it is then inverted and connected to the grid.
The power range of centralized inverter is 250KW-10MW, with high single power, low cost and good grid regulation, but it requires good matching between PV strings, once there is cloudy, partial shading or single string failure, it will affect the efficiency and electric capacity of the whole PV system. Centralized inverter maximum power tracking voltage range is narrower, module configuration flexibility is lower, short power generation time, requires a dedicated with ventilation and heat dissipation of the special room, mainly suitable for uniform light centralized ground large-scale photovoltaic power plant, etc.

PV inverters in solar farm
PV inverters in solar farm

2.Collective inverter
The power range is 1MW-10MW, which is similar to the centralized inverter, mainly used in complex large-scale ground power station, which can be applied to projects with different terrain, with high adaptability and good flexibility.
3.String Inverter
String inverter is to track the maximum power peak of several groups (usually 1-4 groups) of PV strings individually, and then connect to AC grid after inverting. The power range of string inverter is 1.5KW-250KW. The advantage of string inverter is that there can be voltage and current mismatch between strings of different maximum power tracking modules, when one module fails or is shaded, it will only affect the power generation of a few strings of its corresponding maximum power tracking module, and has no effect on the system as a whole.

The inverter has a wide range of maximum power tracking voltage, flexible module configuration and long power generation time; it can be installed directly outdoors.Compared with centralized inverters, string inverters are slightly more expensive and require technical suppression of resonance when a large number of string inverters are connected in parallel, which are mainly used in distributed power generation systems, but can also be applied in centralized PV power generation systems.

Household PV Energy Storage system
Household PV Energy Storage system

Micro-inverter is to track the maximum power peak of each PV module individually, and then after inverting it, it is connected to the AC grid.
The power range of micro-inverters is generally below 4kW. The advantage of micro-inverters is that they can control the maximum power tracking of each module independently, which can improve the overall efficiency in case of partial shading or differences in module performance. In addition, microinverters have only a few tens of volts of DC voltage and are all connected in parallel to minimize safety hazards, they are expensive and difficult to maintain when they fail.
Inverter Functions
1.Automatic operation and shutdown function
After sunrise, the intensity of solar radiation gradually increases, and the output power of PV array increases, and the inverter starts to run automatically when it reaches the output power required for the inverter to work. When the sun sets gradually, the output power of the PV array becomes smaller and the inverter output is close to 0, the inverter is in standby mode.
2.Anti-islanding effect function
In the process of grid-connected photovoltaic power generation, as the photovoltaic power generation system and the power system are running in parallel, when the public grid is out of power due to abnormalities, if the photovoltaic power generation system cannot stop working or disconnect from the power system, it will continue to supply power to the power output line and connect with the local load in an independent operation state, this operation state is called the islanding effect.

The inverter has an internal anti-islanding protection circuit, which detects the voltage and frequency of the grid to be connected in real time, and compares it with the preset voltage and frequency values in real time, if the actual values detected are within a reasonable range, the inverter will work normally; otherwise, it will disconnect the current and stop the output within the corresponding time according to the different measured values, and report the fault.
3.Maximum power point tracking control function
MPPT technology is the core key technology of the inverter, which refers to the ability of the inverter to track and find the maximum output power of the module (square array) in real time. The output power of PV modules is affected by various factors such as irradiation and temperature, and does not always output the nominal rated power. The task of the inverter is to track in real time the maximum power that the modules can output under each momentary condition to maximize the power generation.

By measuring the current, voltage and power, the inverter determines the relationship between the current operating point and the peak point, and intelligently adjusts the voltage (or current) at the operating point to bring it closer to the peak power point, so that the PV system always runs near the peak power point.
4.Intelligent string monitoring
With the development of technology, the inverter realizes intelligent string-by-string monitoring on the basis of the original MPPT monitoring. Compared with MPPT monitoring, the string monitoring of voltage and current is accurate to each branch string, so users can clearly view the real-time operation data of each string.
5.Intelligent I-V curve scanning diagnosis
PV module I/V curve refers to a correspondence relationship between PV module output current and output voltage, and researchers have expressed it in the form of a curve. The I/V curves of both modules and strings change significantly when shading, damage or poor contact occurs in the module.
6.Intelligent anti-PID effect
PID effect of PV module is a condition that the performance of the module will gradually decay after a long time of work. the direct harm of PID effect is that a large number of charges gather on the surface of the cell, which makes the passivation phenomenon on the surface of the cell, making the filling factor (FF), open circuit voltage and short circuit current of the cell module decrease. Reduce the output power of solar power plant, reduce the power generation, and reduce the revenue of PV power plant.
7.Intelligent air cooling
The 10-degree rule for electronic devices means that for every 10°C increase in temperature, the life of the device will be reduced by half. At present, inverters generally adopt intelligent air cooling, with external high-performance fans, protection level up to IP67, built-in temperature sensors and drive circuits to monitor the device temperature in real time and set the appropriate threshold value. When the temperature exceeds the threshold value, the circuit automatically drives the fan to run, and when the temperature returns to normal, the drive circuit is closed and the fan gradually stops running.
8.Smart recovery grid connection
The reason why the inverter can operate automatically is that it has the function of intelligent restoration of grid connection. The normal operation of a power station depends on various factors, such as string voltage, grid voltage, frequency, etc. The inverter may be in standby or down because the grid-connected output conditions are not available. Once it detects that the operating conditions are normal, it will automatically resume the grid connection output after self-test, and no longer needs human operation such as re-commissioning and restarting.
9.Intelligent reactive power compensation
The PV inverter only generates active power under normal power generation conditions. If the power factor is lower than the requirements of the grid company, the grid company will charge a punitive power transfer fee. Commercial and industrial owners usually use reactive power compensation devices at the load side to ensure the power factor is up to the standard in order to avoid the power transfer charge.
Where to install the inverter?
1.Mounting options for common ground-based distributed projects
The inverter can be installed under a string of modules nearby, using fixed brackets or using hoop mounting to fix the equipment directly to the column. This option requires attention to the strength of the brackets and columns and the height of the PV inverter from the ground to prevent the equipment from being placed too low and flooded by rain on stormy days.

PV inverters in solar farm
PV inverters in solar farm

2.Large-scale commercial and industrial plant installation plan
Large commercial and industrial rooftops generally have open roofs, no roofs or daughter walls, where PV inverters can be hung. For the sake of reducing cable costs, it is generally necessary to make your own mounting brackets and hang the inverters on the homemade brackets, which need to consider the inverter load-bearing and fixing forms as well as typhoon resistance in coastal areas.
3.Common home distributed project rooftop power plant installation scheme
The PV inverter should be installed on the south wall as much as possible, and its panel should face north with less sunlight. The machine should be installed at a suitable height from the ground to observe and read the LED display. For outdoor installation, a rain and sun canopy should be installed on top of the inverter to avoid direct sunlight and rain soaking.

Photovoltaic Inverter
Photovoltaic Inverter

4.Systems with microinverters
Microinverters can be installed directly on the bracket under the module or integrated with the module to achieve a modular design and plug-and-play. Microinverters are small in size and basically do not occupy the installation space independently. The distributed installation is easy to configure and can make full use of space and adapt to applications with different installation directions and angles.
What is MPPT?
MPPT, short for Maximum Power Point Tracking, refers to the inverter's ability to adjust the output power of the PV array according to different ambient temperatures and light intensities, so that the PV array always outputs the maximum power.

Under the same input power, the number of MPPT paths of the inverter affects the power generation of the PV plant, especially when there are problems such as shading, different module orientation and inconsistent module performance, etc. The longer the life of the plant (module decay), the more the number of MPPT paths, the higher the accuracy of MPPT single path tracking, the faster the dynamic response, the ability to adapt to rapid changes in light and improve power generation.
Microinverter products are fully parallel structure, each module has independent MPPT, and the power generation between modules does not affect each other. Since there is no short board effect, factors such as shadow shading and dust of some modules do not affect the power generation of the whole string of modules, which makes each module output maximized and enhances the power generation of the power plant.


Boland Renewable Energy Co.,LTD As an integrated new energy power company, providing you with high quality integrated wind energy , solar energy and energy storage system solutions.Boland is now a subsidiary of CRRC, and is responsible for the overseas expansion of  CRRC's wind power & solar power business. We have a relatively complete internal supply chain, service network and excellent product quality and technology.

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