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Advanced understanding and future development trend of grid-connected inverter(1)

Jan 06, 2021

Advanced understanding and future development trend of grid-connected inverter(1)

Grid inverter

Grid-connected inverters are generally divided into grid-connected inverters for photovoltaic power generation, grid-connected inverters for wind power generation, grid-connected inverters for power generation equipment, and grid-connected inverters for other power generation equipment. The biggest feature of the grid-connected inverter is the high power and low cost of the system.


Grid-connected inverters are generally used in the system of large-scale photovoltaic power stations. Many parallel photovoltaic strings are connected to the DC input of the same centralized inverter. Generally, three-phase IGBT power modules are used for higher power. The small ones use field effect transistors and DSP conversion controller to improve the quality of the generated electric energy, making it very close to the sine wave current.


The component inverter is to connect each photovoltaic component with 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.


Since it is connected in parallel at the AC, this increases the complexity of the wiring on the AC side and is difficult to maintain. 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 frequency or low frequency) is required, or a transformerless inverter is allowed. This inverter is most widely used in glass screen solar grid-connected inverters and photovoltaic grid-connected inverter walls.


Working principle of grid-connected inverter


Working principle and characteristics

working principle:


The core of the inverter device is the inverter switch circuit, referred to as the inverter circuit for short. This circuit completes the function of inverter by turning on and off the power electronic switch.


Features:


(1) High efficiency is required.


Due to the high price of solar cells, in order to maximize the use of solar cells and improve system efficiency, we must try to improve the efficiency of the inverter.


(2) High reliability is required.


At present, photovoltaic power station systems are mainly used in remote areas. Many power stations are unattended and maintained. This requires the inverter to have a reasonable circuit structure, strict component selection, and require the inverter to have various protection functions, such as: input DC polarity reverse protection, AC output short circuit protection, overheating, overload protection, etc.


(3) The input voltage is required to have a wide range of adaptation.


Because the terminal voltage of the solar cell changes with the load and the intensity of sunlight. Especially when the battery is aging, its terminal voltage varies widely. For example, the terminal voltage of a 12V battery may vary from 10V to 16V, which requires the inverter to ensure normal operation within a larger DC input voltage range. .


Photovoltaic inverter classification

There are many ways to classify inverters. For example, according to the number of phases of the AC voltage output by the inverter, it can be divided into single-phase inverters and three-phase inverters; according to the different types of semiconductor devices used in the inverter, Divided into transistor inverter, thyristor inverter and turn-off thyristor inverter, etc. According to the principle of the inverter circuit, it can be divided into self-excited oscillation inverter, stepped wave superposition inverter and pulse width modulation inverter. According to the application in grid-connected system or off-grid system, it can be divided into grid-connected inverter and off-grid inverter. In order to facilitate optoelectronic users to choose inverters, here are only classified according to the application occasions of inverters.


1. Centralized inverter


Centralized inverter technology is that a number of 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, field effect transistors are used for low power, and DSP is used at the same time. The conversion controller is used to improve the quality of the generated electric energy, making it very close to the sine wave current, which is generally used in the system of large photovoltaic power stations (>10kW). The biggest feature is the high power and low cost of the system, but because the output voltage and current of different photovoltaic strings are often not completely matched (especially when the photovoltaic strings are partially shaded due to cloudy, shade, stains, etc.), a centralized The method of change will lead to a decrease in the efficiency of the inverter process and a decrease in the energy of the electricity users. 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.


2. String inverter


The string inverter is based on the modular concept. Each photovoltaic string (1-5kw) passes through an inverter, has maximum power peak tracking at the DC end, and is connected in parallel at the AC end. It has become an international The most popular inverter on the market.


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 mismatch between the optimal working point of the photovoltaic module and 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, which makes the system connect several PV strings together and let one or several of them work when a single string of electric energy cannot make a single inverter 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.


3. Micro inverter


In the traditional PV system, the DC input end of each string inverter will be connected in series by about 10 photovoltaic panels. If one of the 10 panels connected in series does not work well, this string will be affected. If the inverter uses the same MPPT for multiple inputs, each input will also be affected, greatly reducing the power generation efficiency. In practical applications, various sheltering factors such as clouds, trees, chimneys, animals, dust, ice and snow will cause the above factors, and the situation is very common. In the PV system of the micro-inverter, each panel is connected to a micro-inverter. When one of the panels does not work well, only this one will be affected. All other photovoltaic panels will run in the best working condition, making the overall system more efficient and generating more power. In practical applications, if the string inverter fails, it will cause the panels of several kilowatts to fail to function, and the impact of the failure of the micro-inverter is quite small.


4. Power optimizer


The installation of a power optimizer (OptimizEr) in the solar power generation system can greatly increase the conversion efficiency and simplify the inverter (Inverter) function to reduce costs. In order to realize a smart solar power generation system, the device power optimizer can ensure that each solar cell exerts its best performance and monitor the battery consumption status at any time. The power optimizer is a device between the power generation system and the inverter. Its main task is to replace the original best power point tracking function of the inverter. The power optimizer uses analogy to perform extremely fast best power point tracking scans by simplifying the circuit and a single solar cell corresponds to a power optimizer, so that each solar cell can indeed achieve the best power point tracking In addition, you can also monitor the battery status anytime and anywhere by inserting a communication chip, report problems in real time and allow relevant personnel to repair them as soon as possible.


Working principle of grid-connected inverter


Function of photovoltaic inverter

The inverter not only has the function of DC-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, independent operation prevention 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 run. After entering operation, the inverter will monitor the output of the solar cell components at all times. 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; it will stop until sunset, even if it is rainy 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 control is the maximum power tracking control. The biggest feature of the inverter used in the solar power generation system is that it includes the function of maximum power point tracking (MPPT).