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Introduction of Photovoltaic Inverter

Release time:

2023-07-06 15:27

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1. Preface

In the future, clean energy transformation has become an inevitable development trend. According to a report released by Ember, an independent British climate think tank in 2022, in order to achieve the global temperature control target of 1.5 ℃, wind and solar power generation needs to maintain an annual growth rate of 20% by 2030. In 2021, the proportion of global scenery power generation exceeded 10% for the first time, and there is still a lot of room for development in the future. As a result, major regions around the world have proposed future clean energy development plans.

In June 2022, China promulgated the "14th Five-Year Plan for Renewable Energy", proposing that by 2025, renewable energy power generation will increase from 0.22 billion kWh in 2020 to 0.33 billion kWh, of which wind power and photovoltaic power generation will double. On the power generation side, focus on the construction of "nine" clean energy bases, using UHV transmission to the load center, the total installed capacity is expected to exceed 500GW in 2025; on the power side, promote the new plant and public buildings to carry out the integrated development of photovoltaic buildings, and implement the "thousands of households to soak the light action", which has become an important measure to implement the above goals.

With the world's attention to energy issues, people's expectations of green energy are getting higher and higher, which promotes the development and utilization of renewable energy, especially solar energy. The photovoltaic power generation system based on solar power generation has many advantages such as system safety, no pollution and high reliability. One of the key components, the photovoltaic inverter, converts the DC current output from the array into alternating current and uploads it to the grid.

This article focuses on the classification of photovoltaic inverters, working principle and its test requirements in the field of electromagnetic compatibility.

2. Introduction of photovoltaic inverter

The photovoltaic inverter is the heart of the solar power generation system, and its main function is to convert the direct current generated by the photovoltaic module into alternating current, which is fed back to the commercial transmission system or used by the off-grid grid grid. It consists of a logic control circuit, a filter circuit and an inverter circuit, where the inverter circuit is the core of the inverter. (See Figure 1 Physical Diagram)

Figure 1 Photovoltaic inverter

Working principle of 2.1 photovoltaic inverter

The direct current generated by the photovoltaic module first passes through the direct current filter circuit to remove current fluctuation and electromagnetic interference, and enters the inverter circuit. In the inverter circuit, the direct current is converted into alternating current, and then the irregular alternating current is rectified into sine wave alternating current. The filter circuit at the output end filters out high-frequency interference signals generated in the inverter process, thus being incorporated into the power grid or directly supplying the load. (See the working principle diagram of Figure 2)

Fig.2 Working principle diagram of photovoltaic inverter

2.2 PV Inverter Classification

There are many ways to classify inverters, for example: according to the number of phases of the inverter output AC voltage, it can be divided into single-phase inverter and three-phase inverter; according to the different types of semiconductor devices used in the inverter, it can be divided into transistor inverter, thyristor inverter and turn-off thyristor inverter. According to the different principles of the inverter circuit, it can also be divided into self-excited oscillation inverter, step wave superposition inverter and pulse width modulation inverter. According to the application in the grid-connected system or off-grid system can be divided into grid-connected inverter and off-grid inverter. In order to facilitate the photoelectric users to choose the inverter, here only the inverter application of different classification.

① Centralized inverter

The photovoltaic inverter mode of the centralized inverter is to connect many parallel photovoltaic groups in series to the DC input end of the same centralized inverter, do maximum power peak tracking, and then merge into the power grid after inversion.

The single capacity of the centralized inverter is usually above 500kW, the single power is high, the cost is low, and the grid regulation is good, but it requires a good match between the photovoltaic strings. Once cloudy, partially shaded or a single string fault occurs, it will affect the efficiency and electric capacity of the entire photovoltaic system. The maximum power tracking voltage range of centralized inverter is narrow, the flexibility of component configuration is low, the power generation time is short, and a special room with ventilation and heat dissipation is required, which is mainly suitable for large-scale photovoltaic power stations on the ground with uniform illumination.

② Group string inverter

Group string inverter is to several groups (generally 1-4 groups) of photovoltaic string for individual maximum power peak tracking, and then after the inverter into the AC power grid, a group string inverter can have multiple maximum power peak tracking module. The single capacity of string inverter is generally below 100kW. Its advantage is that there may be mismatch of voltage and current between strings of different maximum power peak tracking modules. When a component fails or is shaded, it will only affect the power generation of a few strings of its corresponding maximum power peak tracking module and has no impact on the whole system. The maximum power tracking voltage range of the inverter is wide, the component configuration is flexible, the power generation time is long, and it can be directly installed outdoors.

Compared with centralized inverters, string inverters are slightly more expensive, and when a large number of string inverters are connected in parallel, it is necessary to technically suppress the occurrence of resonance, which is mainly applied to distributed power generation systems, and can also be applied to centralized photovoltaic power generation systems.

③ Micro-inverter

Micro-inverter is a separate maximum power peak tracking for each photovoltaic module, and then after the inverter into the AC grid.

The single capacity of the micro inverter is generally suitable for low power, and its advantage is that it can carry out independent maximum power tracking control for each component, and improve the overall efficiency in the case of partial shading or component performance differences. In addition, the micro-inverter has only a few tens of volts of DC voltage, all of which are connected in parallel, which reduces the potential safety hazard to the greatest extent. It is expensive and difficult to maintain after failure.

2.3 PV inverter connection diagram

Fig. 3 Schematic diagram of connection

3, Domestic photovoltaic inverter electromagnetic compatibility test requirements

At present, the domestic photovoltaic inverter industry standard is NB/T32004-2018 "Technical Specification for Photovoltaic Grid-connected Inverter".

3.1 Scope of application

The standard is applicable to photovoltaic grid-connected inverters connected to PV source circuit voltage not exceeding 1500V DC and AC output voltage not exceeding 1000V. The pre-installed photovoltaic grid-connected inverter device with integrated step-up transformer connected to the power grid with voltage level of 35kV and below can be implemented by reference.

3.2 inverter classification: according to the access voltage level is divided into class a inverter and class B inverter,

Class A: refers to photovoltaic inverters used in photovoltaic power stations connected to the grid through voltage levels of 35kV and above, or connected to the public grid through voltage levels of 10kV and above.

Class B: refers to the photovoltaic inverter used in the photovoltaic power generation system connected to the power grid through 380V voltage level and connected to the user side of the power grid through 10kV and below voltage level.

3.3 Test Items and Requirements

Class A inverters are used in photovoltaic power stations, and Class B inverters are used in user-tested photovoltaic power generation systems. Most EMC laboratory testing capabilities can only test Class B inverters. The following test items and test requirements are only introduced for Class B inverters.

① Conducted emission

One group of Class B limits in AC port and DC port GB4824 of Class B inverter, wired network port and signal/control port shall meet Class B limits in IEC62920:2017, as shown in the following figure

Figure 4

② Radiated emission

Class B inverter shall meet the Class B limit value of Group 1 in GB4824, and the limit value is shown in the figure below.

Figure 5

③ Immunity items and requirements, see Fig. 6 and Fig. 7:

Figure 6

Figure 7

I hope everyone can basically understand the photovoltaic inverter and its test requirements in the field of electromagnetic compatibility through this article.

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