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Talking about the function of photovoltaic inverter and how to choose photovoltaic inverter correctly

Sep 28, 2020

Talking about the function of photovoltaic inverter and how to choose photovoltaic inverter correctly

A. Function of photovoltaic inverter

The inverter not only has the function of direct-to-ac conversion, but also has the function of maximizing the performance of the solar cell and the function of system failure protection. In summary, there are automatic operation and shutdown functions, maximum power tracking control function, anti-single operation function (for grid-connected systems), automatic voltage adjustment function (for grid-connected systems), DC detection function (for grid-connected systems), DC grounding detection Function (for grid-connected system). Here is a brief introduction to the automatic operation and shutdown functions and the maximum power tracking control function.

1. Automatic operation and shutdown function

After sunrise in the morning, the solar radiation intensity gradually increases, and the output of the solar cell also increases. When the output power required by the inverter is reached, the inverter automatically starts to operate. After entering operation, the inverter will always monitor the output of the solar cell components. As long as the output power of the solar cell components is greater than the output power required by the inverter, the inverter will continue to run until sunset and stop, even if it is rainy or cloudy. The inverter can also operate. When the output of the solar cell module becomes smaller and the output of the inverter approaches 0, the inverter enters a standby state.

2. Maximum power tracking control function

The output of the solar cell module varies with the intensity of solar radiation and the temperature of the solar cell module (chip temperature). In addition, because the solar cell module has the characteristic that the voltage decreases with the increase of the current, there is an optimal operating point that can obtain the maximum power. The intensity of solar radiation is changing, and obviously the best operating point is also changing. Relative to these changes, the operating point of the solar cell module is always at the maximum power point, and the system always obtains the maximum power output from the solar cell module. This kind of control is the maximum power tracking control. The biggest feature of inverters used in solar power generation systems is that they include the maximum power point tracking (MPPT) function.

B. How to choose photovoltaic inverter correctly

Due to the diversity of buildings, it will inevitably lead to the diversity of solar panel installations. In order to maximize the conversion efficiency of solar energy while taking into account the beautiful appearance of the building, this requires the diversity of our inverters to achieve the best way of solar energy Conversion. The most common solar inverter methods in the world are: centralized inverters, string inverters, multi-string inverters and component inverters. Now we will analyze the applications of several inverters.


   Centralized inverter


Centralized inverters are generally used in systems with large photovoltaic power stations (>10kW). Many parallel photovoltaic strings are connected to the DC input of the same centralized inverter. Generally, three-phase IGBT power modules are used for high power. The lower power uses field-effect transistors and the DSP conversion controller to improve the quality of the generated electric energy, making it very close to the sine wave current. The biggest feature is the high power and low cost of the system. However, it is affected by the matching of photovoltaic strings and partial shading, resulting in the efficiency and power capacity of the entire photovoltaic system. At the same time, the power generation reliability of the entire photovoltaic system is affected by the poor working status of a photovoltaic unit group. The latest research direction is the use of space vector modulation control, and the development of new inverter topology connections to obtain high efficiency under partial load conditions.


On the SAFESAVE centralized inverter, you can attach a photovoltaic array interface box to monitor each photovoltaic windsurfing string. If one of the strings is not working properly, the system will transmit this information to the remote controller At the same time, this string can be stopped through remote control, so that the failure of a string of photovoltaic strings will not reduce and affect the work and energy output of the entire photovoltaic system.


   string inverter


   string inverter has become the most popular inverter in the international market. The string inverter is based on the modular concept. Each photovoltaic string (1kW-5kW) passes through an inverter, has maximum power peak tracking at the DC end, and is connected in parallel at the AC end. Many large photovoltaic power plants use string inverters. The advantage is that it is not affected by module differences and shadows between strings, and at the same time reduces the optimal working point of photovoltaic modules


  The situation that does not match the inverter, thereby increasing the power generation. These technical advantages not only reduce system costs, but also increase system reliability. At the same time, the concept of "master-slave" is introduced between the strings, so that when the system is unable to make a single inverter work with a single string of electric energy, several sets of photovoltaic strings are linked together and one or several of them work , Thereby producing more electricity. The latest concept is that several inverters form a "team" to replace the "master-slave" concept, which makes the reliability of the system a step further. Currently, transformerless string inverters have taken the lead.


  Multi-string inverter


  Multi-string inverter takes the advantages of centralized inverter and string inverter, avoids its shortcomings, and can be applied to photovoltaic power stations of several kilowatts. In the multi-string inverter, different individual power peak tracking and DC-to-DC converters are included. These DCs are converted into AC power by an ordinary DC-to-AC inverter and connected to the grid. Different rated values of photovoltaic strings (such as: different rated power, different number of components per string, different manufacturers of components, etc.), photovoltaic modules of different sizes or technologies, and strings of different directions (such as : East, South and West), different inclination angles or shadows, all can be connected to a common inverter, and each string is working at their respective maximum power peak.


  At the same time, the length of the DC cable is reduced, the shadow effect between the strings and the loss due to the difference between the strings are minimized.


  Component inverter


   component inverter is to connect each photovoltaic component to an inverter, and each component has a separate maximum power peak tracking, so that the component and the inverter are better matched. Usually used in 50W to 400W photovoltaic power plants, the total efficiency is lower than string inverters. Because it is connected in parallel at the AC, this increases the complexity of the wiring on the AC side and makes maintenance difficult. Another problem that needs to be solved is how to connect to the grid more effectively. The simple way is to directly connect to the grid through an ordinary AC socket, which can reduce the cost and equipment installation, but often the safety standards of the grid may not allow it. In doing so, the power company may object to the power generation device being directly connected to the ordinary socket of ordinary household users. Another factor related to safety is whether an isolation transformer (high or low frequency) is required, or a transformerless inverter is allowed. This inverter is most widely used in glass curtain walls.