Date: 2023-05-06 hits: 777
The core of the inverter device is the inverter switch circuit, referred to as the inverter circuit. The circuit completes the inverter function by turning on and off the power electronic switch.
Features:
(1) Higher efficiency is required.
Due to the high price of solar cells at present, in order to maximize the use of solar cells and improve system efficiency, it is necessary to try to improve the efficiency of the inverter.
(2) High reliability is required.
At present, the photovoltaic power station system is mainly used in remote areas, and many power stations are unattended and maintained. This requires the inverter to have a reasonable circuit structure, strict component selection, and requires the inverter to have various protection functions, such as: input DC reverse polarity protection, AC output short circuit protection, overheating, overload protection, etc.
(3) The input voltage is required to have a wide range of adaptation.
Since the terminal voltage of the solar cell changes with the load and the intensity of sunlight. Especially when the battery ages, its terminal voltage varies widely. For example, for a 12V battery, its terminal voltage may vary between 10V and 16V, which requires the inverter to work normally within a relatively large DC input voltage range. .
Photovoltaic 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 inverters and three-phase inverters; according to the type of semiconductor devices used in the inverter, it can be divided into Divided into transistor inverters, thyristor inverters and turn-off thyristor inverters. According to the different principle of the inverter circuit, it can also be divided into self-excited oscillation inverter, ladder wave superposition inverter and pulse width modulation inverter. According to the application in the grid-connected system or the off-grid system, it can be divided into grid-connected inverters and off-grid inverters. In order to make it easier for photovoltaic users to choose inverters, here we only classify them according to the different applicable occasions of inverters.
The centralized inverter technology is that several parallel photovoltaic strings are connected to the DC input terminal of the same centralized inverter. Generally, the three-phase IGBT power module is used for the large power, and the field effect transistor is used for the small power. DSP conversion controller to improve the quality of the generated power, making it very close to the sine wave current, generally used in large-scale photovoltaic power station (>10kW) system. 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 blocked due to cloudy, tree shade, stains, etc.), centralized inverters are used. The way of changing will lead to the reduction of the efficiency of the inverter process and the decline of the power consumption. At the same time, the power generation reliability of the entire photovoltaic system is affected by the poor working status of a certain photovoltaic unit group. A recent research direction is the use of space vector modulation control and the development of new topological connections of inverters to obtain high efficiency at part loads.
The string inverter is based on the concept of modularization. Each photovoltaic string (1-5kw) passes through an inverter, which has the maximum power peak tracking at the DC end and parallel connection with the grid at the AC end. It has become an international The most popular inverter on the market.
Many large PV plants use string inverters. The advantage is that it is not affected by module differences and shading between strings, and at the same time reduces the mismatch between the best operating point of the photovoltaic module and the inverter, thereby increasing the power generation. These technical advantages not only reduce the system cost, but also increase the reliability of the system. At the same time, the concept of "master-slave" is introduced between the strings, so that when the power of a single string cannot make a single inverter work, the system connects several groups of photovoltaic strings together and makes one or several of them work. , so as to produce more electricity.
The latest concept is that several inverters form a "team" to replace the "master-slave" concept, which makes the system reliability go one step further. At present, transformerless string inverters have taken the dominant position.
In a traditional PV system, about 10 photovoltaic panels are connected in series to the DC input end of each string inverter. When one of the 10 battery panels connected in series does not work well, the string will be affected. If the same MPPT is used for multiple inputs of the inverter, each input will also be affected, greatly reducing the power generation efficiency. In practical applications, various blocking factors such as clouds, trees, chimneys, animals, dust, ice and snow will cause the above factors, and the situation is very common. In the micro-inverter PV system, each battery board is connected to a micro-inverter, and when one of the battery boards fails to work well, only this one will be affected. The other photovoltaic panels will all be running at their best, making the overall system more efficient and producing more electricity. In practical applications, if the string inverter fails, it will cause several kilowatts of battery panels to fail to function, while the impact caused by the failure of the micro-inverter is quite small.
The installation of a power optimizer (Optimizer) in a solar power generation system can greatly improve the conversion efficiency, and simplify the functions of the inverter (Inverter) to reduce costs. In order to realize a smart solar power generation system, the device power optimizer can indeed make each solar cell play the 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, and its main task is to replace the original optimal power point tracking function of the inverter. By simplifying the circuit and a single solar cell corresponds to a power optimizer, the power optimizer performs an extremely fast best power point tracking scan in an analog way, so that each solar cell can indeed achieve the best power point tracking , In addition, the battery status can be monitored anytime and anywhere by using a communication chip, and the problem can be reported immediately so that relevant personnel can repair it as soon as possible.
The inverter not only has the function of DC-AC conversion, but also has the function of maximizing the performance of solar cells and the function of system fault protection. In summary, there are automatic operation and shutdown functions, maximum power tracking control functions, anti-solitary operation functions (for grid-connected systems), automatic voltage adjustment functions (for grid-connected systems), DC detection functions (for grid-connected systems), and DC grounding detection function (for grid-connected systems). Here is a brief introduction to automatic operation and shutdown functions and maximum power tracking control functions.
(1) Automatic operation and shutdown function
After sunrise in the morning, the intensity of solar radiation increases gradually, and the output of solar cells increases accordingly. When the output power required by the inverter is reached, the inverter starts to run automatically. After starting to run, the inverter will monitor the output of the solar cell components all the time. 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 on cloudy and rainy days. The inverter also works. When the output of the solar cell module becomes smaller and the output of the inverter is close to 0, the inverter will form 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 itself (chip temperature). In addition, because the solar cell module has the characteristic that the voltage decreases with the increase of the current, so there is an optimal operating point that can obtain the maximum power. The intensity of solar radiation is changing, obviously the best working 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 the function of maximum power point tracking (MPPT).
The main technical indicators of photovoltaic inverter
1. Stability of output voltage
In the photovoltaic system, the electric energy generated by the solar cell is first stored by the battery, and then converted into 220V or 380V alternating current through the inverter. However, the battery is affected by its own charge and discharge, and its output voltage varies in a large range. For example, the nominal 12V battery, its voltage value can vary between 10.8 and 14.4V (beyond this range may cause damage to the battery) . For a qualified inverter, when the input terminal voltage changes within this range, the steady-state output voltage variation should not exceed ±5% of the rated value, and when the load changes suddenly, the output voltage deviation should not exceed ±10% of rated value.
2. Waveform distortion of output voltage
For sine wave inverters, the maximum allowable waveform distortion (or harmonic content) should be specified. Usually expressed as the total waveform distortion of the output voltage, its value should not exceed 5% (single-phase output allows l0%). Since the high-order harmonic current output by the inverter will generate additional losses such as eddy currents on the inductive load, if the waveform distortion of the inverter is too large, it will cause serious heating of the load components, which is not conducive to the safety of electrical equipment and seriously affects the system. operating efficiency.
3. Rated output frequency
For loads including motors, such as washing machines, refrigerators, etc., since the optimal frequency operating point of the motors is 50Hz, too high or too low frequencies will cause the equipment to heat up, reducing the system's operating efficiency and service life, so the inverter's The output frequency should be a relatively stable value, usually power frequency 50Hz, and its deviation should be within ±1% under normal working conditions.
4. load power factor
Characterize the ability of the inverter with inductive load or capacitive load. The load power factor of the sine wave inverter is 0.7~0.9, and the rated value is 0.9. In the case of a certain load power, if the power factor of the inverter is low, the capacity of the required inverter will increase. On the one hand, the cost will increase, and at the same time, the apparent power of the AC circuit of the photovoltaic system will increase. As the current increases, the loss will inevitably increase, and the system efficiency will also decrease.
5. Inverter efficiency
The efficiency of the inverter refers to the ratio of the output power to the input power under the specified working conditions, expressed as a percentage. In general, the nominal efficiency of the photovoltaic inverter refers to the pure resistance load. Under the condition of 80% load s efficiency. Due to the high overall cost of the photovoltaic system, it is necessary to maximize the efficiency of the photovoltaic inverter, reduce the system cost, and improve the cost performance of the photovoltaic system. At present, the nominal efficiency of mainstream inverters is between 80% and 95%, and the efficiency of low-power inverters is required to be no less than 85%. In the actual design process of a photovoltaic system, not only should a high-efficiency inverter be selected, but also a reasonable configuration of the system should be used to make the load of the photovoltaic system work near the best efficiency point as much as possible.
6. Rated output current (or rated output capacity)
Indicates the rated output current of the inverter within the specified load power factor range. Some inverter products are given the rated output capacity, and its unit is expressed in VA or kVA. The rated capacity of the inverter is the product of the rated output voltage and the rated output current when the output power factor is 1 (that is, purely resistive load).
7. Protection measures
An inverter with excellent performance should also have complete protection functions or measures to deal with various abnormal situations that occur during actual use, so as to protect the inverter itself and other components of the system from damage.
(1) Enter the undervoltage insurance account:
When the input terminal voltage is lower than 85% of the rated voltage, the inverter should have protection and display.
(2) Input overvoltage protector:
When the input terminal voltage is higher than 130% of the rated voltage, the inverter should have protection and display.
(3) Overcurrent protection:
The overcurrent protection of the inverter should be able to ensure timely action when the load is short-circuited or the current exceeds the allowable value, so as to prevent it from being damaged by the surge current. When the working current exceeds 150% of the rated value, the inverter should be able to protect automatically.
(4) output short circuit protection
The short-circuit protection action time of the inverter should not exceed 0.5s.
(5) Input reverse polarity protection:
When the positive and negative poles of the input terminal are reversed, the inverter should have protection function and display.
(6) Lightning protection:
The inverter should have lightning protection.
(7) Over-temperature protection, etc.
In addition, for inverters without voltage stabilization measures, the inverter should also have output overvoltage protection measures to protect the load from overvoltage damage.
8. Starting characteristics
To characterize the ability of the inverter to start with load and the performance during dynamic operation. The inverter should ensure reliable starting under rated load.
9. noise
Components such as transformers, filter inductors, electromagnetic switches and fans in power electronic equipment will generate noise. When the inverter is running normally, its noise should not exceed 80dB, and the noise of a small inverter should not exceed 65dB.
Selection skills
The selection of the inverter should first consider having sufficient rated capacity to meet the electrical power requirements of the equipment under the maximum load. For the inverter with a single device as the load, the selection of its rated capacity is relatively simple.
When the electrical equipment is a pure resistive load or the power factor is greater than 0.9, the rated capacity of the inverter can be selected as 1.1 to 1.15 times the capacity of the electrical equipment. At the same time, the inverter should also have the ability to resist capacitive and inductive load impacts.
For general inductive loads, such as motors, refrigerators, air conditioners, washing machines, high-power water pumps, etc., when starting, the instantaneous power may be 5 to 6 times the rated power. At this time, the inverter will bear a large instantaneous power surge. For this type of system, the rated capacity of the inverter should have a sufficient margin to ensure that the load can be started reliably, and a high-performance inverter can be started at full load for many times in a row without damaging the power device. Small inverters sometimes need soft start or current-limited start for their own safety.
Installation precautions and maintenance
1. Before installation, first check whether the inverter is damaged during transportation.
2. When selecting the installation site, it should be ensured that there is no interference from any other power electronic equipment in the surrounding area.
3. Before making electrical connections, be sure to cover the photovoltaic panels with opaque materials or disconnect the DC side circuit breaker. Photovoltaic arrays will generate dangerous voltages when exposed to sunlight.
4. All installation operations must be done by professional and technical personnel only.
5. The cables used in the photovoltaic system power generation system must be firmly connected, well insulated and suitable in size.
development trend
For solar inverters, improving the conversion efficiency of power is an eternal subject, but when the efficiency of the system is getting higher and higher, almost close to 100%, further efficiency improvement will be accompanied by low cost performance. Therefore, how to maintain A very high efficiency, while maintaining a good price competitiveness will be an important issue at present.
Compared with efforts to improve inverter efficiency, how to improve the efficiency of the entire inverter system is gradually becoming another important issue in solar energy systems. In a solar array, when a partial shadow of 2~3% of the area appears, for an inverter using an MPPT function, the output power of the system at this time may even drop by about 20% when the output power of the system is bad! In order to better adapt to situations like this for single or partial solar modules, it is very effective to use one-to-one MPPT or multiple MPPT control functions.
Because the inverter system is in grid-connected operation, the leakage of the system to the ground will cause serious safety problems; in addition, in order to improve the efficiency of the system, most of the solar arrays will be connected in series to use a high DC output voltage; for this reason, in Due to the occurrence of abnormal conditions between the electrodes, it is easy to generate a DC arc. Due to the high DC voltage, it is very difficult to extinguish the arc, and it is very easy to cause a fire. With the widespread adoption of solar inverter systems, the issue of system security will also be an important part of inverter technology.
In addition, the power system is ushering in the rapid development and popularization of smart grid technology. A large number of solar energy and other new energy power systems are connected to the grid, which poses new technical challenges to the stability of the smart grid system. Designing an inverter system that can be more quickly, accurately, and intelligently compatible with the smart grid will become a necessary condition for solar inverter systems in the future.
Generally speaking, the development of inverter technology is developed along with the development of power electronics technology, microelectronics technology and modern control theory. With the passage of time, inverter technology is developing towards higher frequency, higher power, higher efficiency and smaller size.
