Working principle and features
working principle:
The core of the inverter device is the inverter switching circuit, or simply the inverter circuit. The circuit completes the inversion function by turning on and off the power electronic switch.
Features:
(1) It requires higher efficiency.
Due to the current 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) It requires high reliability.
At present, the photovoltaic power station system is mainly used in remote areas, and many power stations are unattended and maintained. This requires a reasonable circuit structure of the inverter, strict component screening, and requires the inverter to have various protection functions, such as: input DC polarity reverse protection, AC output short circuit protection, overheating, overload protection, etc.
(3) It requires a wide range of input voltage.
Because the terminal voltage of the solar battery changes with the change of the load and the sunshine intensity. Especially when the battery ages, its terminal voltage has a wide range of variation, such as a 12V battery. The terminal voltage may vary from 10V to 16V, which requires the inverter to operate normally within a large DC input voltage range. .
PV inverter classification
There are many methods for classifying inverters. For example, depending on the number of phases of the AC voltage output from the inverter, they can be classified into single-phase inverters and three-phase inverters; depending on the type of semiconductor device used in the inverter, they can be different. Divided into transistor inverters, thyristor inverters and can turn off thyristor inverters. According to the principle of inverter circuit, it can be divided into self-oscillating inverter, step-wave superimposed inverter and pulse width modulation inverter. According to applications in the grid system or off-grid system can be divided into grid-connected inverters and off-grid inverters. In order to facilitate the selection of inverters for opto-electronic users, the inverters are only classified according to the application of the inverter.
1. Centralized inverter
The centralized inverter technology is where several parallel PV strings are connected to the DC input of the same centralized inverter. The general power is the use of three-phase IGB T power modules, and the smaller power uses field-effect transistors. The DSP conversion controller improves the quality of the power produced, making it very close to a sinusoidal current and is typically used in systems with large photovoltaic power stations (>10 kW). The most important feature is that the power of the system is high and the cost is low. However, because the output voltage and current of different PV strings are often not completely matched (especially when the PV strings are partially blocked due to cloudyness, shade, stains, etc.), the centralized inverse is used. Changing the way will lead to a decrease in the efficiency of the inversion process and a drop in electricity consumption. At the same time, the reliability of power generation of the entire photovoltaic system is affected by the poor working condition of one photovoltaic unit group. The latest research directions are the use of space vector modulation control and the development of new inverter topology connections to achieve high efficiency under partial load conditions.
2. String inverter
The string inverter is based on the modular concept. Each PV string (1-5kw) has a maximum power peak tracking at the DC end through an inverter and is connected in parallel at the AC end. The most popular inverter on the market.
Many large-scale photovoltaic power plants use string inverters. The advantage is that it is not affected by the module differences and shading between the strings, and at the same time, the mismatch between the best working point of the photovoltaic module and the inverter is reduced, 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 among the strings, making the system link several groups of photovoltaic strings together to make one or several of them work under the condition that a single string of electrical energy cannot make a single inverter work. , which will produce more electricity.
The latest concept is that several inverters form a "team" instead of a "master-slave" concept, making the reliability of the system one step further. At present, transformerless string inverters have taken a dominant position.
3, micro inverter
In the traditional PV system, the DC input of each string inverter is connected in series by about 10 photovoltaic panels. When one of the 10 series panels does not work well, this string will be affected. If the inverter uses multiple inputs with the same MPPT, each input will also be affected, significantly reducing power generation efficiency. In practical applications, the above factors are caused by clouds, trees, chimneys, animals, dust, ice and other obstructing factors. The situation is very common. In the PV system of a micro-inverter, each panel is connected to a micro-inverter. When one of the panels cannot work well, only one of the panels will be affected. Other photovoltaic panels will operate at optimal operating conditions, making the overall system more efficient and generating more power. In practical applications, if the string inverter fails, it will cause several kilowatts of panels to fail to function, and the impact of micro-inverter failures is quite small.
4, power optimizer
The addition of a power optimizer (OptimizEr) to the solar power system can significantly increase the conversion efficiency and simplify the function of the inverter to reduce costs. To realize a smart solar power system, the device power optimizer can really make every solar cell play its best performance and monitor the state of battery wear at any time. The power optimizer is a device between the power generation system and the inverter. The main task is to replace the original best power point tracking function of the inverter. The power optimizer performs extremely fast tracking of the best power point by analogy by simplifying the circuit and a single solar cell, that is, corresponding to a power optimizer, so that each solar cell can truly achieve the best power point tracking. In addition, the state of the battery can be monitored at any time by placing a communication chip at the same time, and the problem of immediate payback allows relevant personnel to perform repairs as quickly as possible.
PV inverter function
The inverter not only has a direct AC conversion function, but also has a function to maximize the performance of the solar battery and a system fault protection function. Summarized automatic operation and stop function, maximum power tracking control function, anti-separation function (for grid system), automatic voltage adjustment function (for grid system), DC detection function (for grid system), DC grounding detection Function (for grid system). Here is a brief introduction of automatic operation and shutdown functions and maximum power tracking control.
(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 for the inverter operation is reached, the inverter automatically starts operating. After entering the operation, the inverter always monitors the output of the solar cell module. As long as the output power of the solar cell module is greater than the output power required by the inverter, the inverter will continue to operate; until sunset, even if it is overcast, The inverter can also operate. When the output of the solar cell module becomes smaller and the output of the inverter approaches zero, the inverter forms a standby state.
(2) Maximum power tracking control function
The output of a solar cell module varies with the solar radiation intensity and the temperature of the solar cell module itself (chip temperature). In addition, since the solar cell module has a characteristic that the voltage decreases as the current increases, there is an optimum operating point for obtaining the maximum power. The intensity of solar radiation is changing, and obviously the best working point is also changing. With respect to these changes, the working 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. Such control is the maximum power tracking control. The biggest feature of inverters used in solar power systems is the inclusion of Maximum Power Point Tracking (MPPT).
The main technical indicators of photovoltaic inverter
1. The stability of the output voltage
In the photovoltaic system, the energy from the solar cell is first stored by the battery and then reversed to an alternating current of 220V or 380V through the inverter. However, the battery is affected by its own charge and discharge, the output voltage of the change range is relatively large, such as the nominal 12V battery, the voltage value can vary between 10.8 ~ 14.4V (out of this range may cause damage to the battery). For a qualified inverter, when the input voltage changes within this range, the steady-state output voltage variation should not exceed the rated value & Plusmn; 5%, and at the same time when the load changes abruptly, the output voltage deviation should not ±10% of the rated value.
2. The output voltage waveform distortion
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, the value should not exceed 5% (single-phase output allows l0%). Because the harmonic current output from 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 lead to serious heating of the load components, which is not conducive to the safety of electrical equipment, and seriously affects the system. The operating efficiency. 3. Rated output frequency
For loads including motors, such as washing machines, refrigerators, etc., because the motor's optimal frequency operating point is 50Hz, the frequency is too high or too low will cause the device to heat, reduce system operating efficiency and service life, so the inverter The output frequency should be a relatively stable value, usually 50Hz, and under normal operating conditions the deviation should be within plus plus 1â„2%.
4. Load power factor
Characterize the inverter's ability to carry inductive or capacitive loads. The sine wave inverter has a load power factor of 0.7-0.9 and a rating of 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 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. It is expressed as a percentage. Under normal circumstances, the nominal efficiency of the PV inverter refers to the purely resistive load and the 80% load. s efficiency. Because the overall cost of the photovoltaic system is high, the efficiency of the photovoltaic inverter should be maximized, the system cost reduced, and the cost-effectiveness of the photovoltaic system increased. At present, the nominal efficiency of mainstream inverters is between 80% and 95%, and the efficiency of low-power inverters is not less than 85%. In the actual design process of photovoltaic system, not only high-efficiency inverters should be selected, but also reasonable configuration of the system should be adopted to make the photovoltaic system load work near the best efficiency point.
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 give rated output capacity, and their units are expressed in VA or kVA. The rated capacity of the inverter is when the output power factor is 1 (that is, a purely resistive load), and the rated output voltage is the product of the rated output current.
7, protection measures
An inverter with good performance should also have complete protection functions or measures to deal with various abnormal conditions that occur during actual use, so as to protect the inverter itself and other components of the system from damage.
(1) Input undervoltage protection:
When the input voltage is lower than 85% of the rated voltage, the inverter should have protection and display.
(2) Input overvoltage protection:
When the input 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 ensure that the load can be operated in time when the load is short-circuited or the current exceeds the allowable value to protect it from the inrush current. When the operating current exceeds the rated 150%, the inverter should be able to automatically protect.
(4) output short circuit warranty
Inverter short-circuit protection operation time should not exceed 0.5s.
(5) Input reverse protection:
When the input terminals are positive and negative, the inverter should have protection function and display.
(6) Lightning protection:
The inverter should have lightning protection.
(7) Over-temperature protection.
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
Characterize the ability of the inverter to start with a load and perform dynamic operation. The inverter should ensure reliable starting under rated load.
9. Noise
Transformers, filter inductors, electromagnetic switches, and fans in power electronic equipment can generate noise. When the inverter is operating normally, the noise should not exceed 80dB, and the noise of the small inverter should not exceed 65dB.
Selection techniques
The selection of the inverter must first consider having enough rated capacity to meet the equipment's requirement for electric power under the maximum load. For a single device as the load of the inverter, the selection of its rated capacity is relatively simple.
When the electrical equipment is a purely resistive load or the power factor is greater than 0.9, the rated capacity of the inverter is selected to be 1.1 to 1.15 times the capacity of the electrical equipment. At the same time, the inverter should also have the ability to withstand capacitive and inductive load shocks.
For general inductive loads, such as motors, refrigerators, air conditioners, washing machines, and high-power pumps, the instantaneous power at start-up may be 5 to 6 times its rated power. At this time, the inverter will be subject to large transients. surge. For this kind of system, the rated capacity of the inverter should have sufficient margin to ensure that the load can be reliably started. The high-performance inverter can be started multiple times at full load without damaging the power devices. For their safety, small inverters sometimes need to use soft start or current limit start.
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 the electrical connection is made, be sure to use opaque materials to cover or disconnect the DC-side circuit breakers from the PV panels. Exposure to sunlight, PV arrays will generate dangerous voltages.
4. All installation operations must be completed by professional technicians only.
5. The cables used in the photovoltaic system power generation system must be connected firmly, well insulated and the specifications are appropriate.
development trend
For solar inverters, improving the power conversion efficiency is an eternal task, but when the efficiency of the system is getting higher and higher, and it is almost close to 100%, further efficiency improvement will be accompanied by low cost performance. Therefore, how to maintain A high degree of efficiency while maintaining a good price competitiveness will be an important issue at the moment.
Compared with efforts to improve the efficiency of inverters, how to improve the efficiency of the entire inverter system is becoming another important topic of solar energy systems. In a solar array, when a local shadow of 2-3% of the area appears, an inverter with an MPPT function is used. In this case, when the output power of the system is poor, even a power drop of about 20% occurs. Better adapt to situations like this For single or partial solar modules, using one-to-one MPPT or multiple MPPT control functions is a very effective method.
Because the inverter system is in the state of grid-connected operation, the system leakage to the ground will cause serious safety problems; in addition, in order to improve the efficiency of the system, solar arrays are mostly used in series with high DC output voltage; for this reason, Due to the occurrence of abnormal conditions between the electrodes, DC arcs are easily generated. Due to the high DC voltage, arc extinguishment is very difficult and fire is easily caused. With the widespread adoption of solar inverter systems, the issue of system safety 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, presenting new technical challenges to the stability of smart grid systems. Designing an inverter system that can be more quickly, accurately, and intelligently compatible with smart grids will become a necessary condition for future solar inverter systems.
In general, the development of inverter technology has evolved with the development of power electronics, microelectronics, and modern control theory. As time goes on, inverter technology is developing in the direction of higher frequency, higher power, higher efficiency, and smaller volume.
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