One.
1. The working principle of the inverter of the photovoltaic power station.
Second, the main components of the power generation system of photovoltaic power station.
1. Photovoltaic power generation refers to the direct power generation mode that uses the p-n junction photogenerated volt effect principle of solar cells, which is a semiconductor electronic device, to effectively absorb sunlight radiation energy and convert it into electrical energy through a conversion device, which is the mainstream of today's photovoltaic power generation, with the advantages of renewable, pollution-free, etc.
The working principle of the inverter of the photovoltaic power station.
Definition of inverter: Generally, the process of converting alternating current energy into direct current energy is called rectification, the circuit that completes the rectification function is called the rectifier circuit, and the device that realizes the rectification process is called the rectifier equipment or rectifier. Correspondingly, the process of converting direct current energy into alternating current energy is called inverter, the circuit that completes the inverter function is called the inverter circuit, and the device that realizes the inverter process is called the inverter equipment or inverter.
The role of the inverter.
The grid-connected inverter is an important electrical equipment in the photovoltaic power station, and it is also the core equipment in the photovoltaic power generation system. The inverter inverts the direct current (DC) generated by the photovoltaic array into three-phase sinusoidal alternating current (AC), and outputs electrical energy that meets the requirements of the power grid.
According to the capacity of the inverter, it can be divided into three categories:
Centralized inverter.
String inverter.
Microinverters.
Functional requirements for the inverter.
The inverter not only has a direct AC conversion function, but also has the function to maximize the performance of the solar cell and the system fault protection function. In summary, there are automatic operation and shutdown functions, maximum power tracking control functions, anti-separate 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 functions (for grid-connected systems).
1) Automatic operation and shutdown function.
After sunrise in the morning, the intensity of solar radiation gradually increases, and the output of the solar cell also increases, and when the output power required for the inverter to work is reached, the inverter will automatically start running. After entering operation, the inverter monitors the output of the solar cell module at all times, and as long as the output power of the solar cell module is greater than the output power required by the inverter to work, the inverter will continue to operate; Shut down until sunset, the inverter can operate even on rainy days. When the output of the solar cell module becomes smaller and the output of the inverter approaches 0, the inverter will form a standby state.
2) Maximum power tracking control function.
The output of a solar cell module varies with the intensity of solar radiation and the temperature of the solar cell module itself (chip temperature). In addition, since the voltage of solar cell modules decreases as the current increases, there is an optimal operating point to obtain maximum power. The intensity of solar radiation is changing, and obviously the sweet spot 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, which is the maximum power tracking control. The most important feature of inverters for solar power generation systems is that they include the function of maximum power point tracking (MPPT).
3) Power grid detection and grid connection function.
Before the grid-connected inverter is connected to the grid for power generation, it needs to take power from the power grid, detect the voltage, frequency, phase sequence and other parameters of the grid power transmission, and then adjust the parameters of its own power generation, which are synchronized with the power parameters of the grid, and will be connected to the grid for power generation after completion.
4) Low voltage ride-through function.
When the power system accident or disturbance causes the voltage sag of the photovoltaic power station at the grid-connected point, the photovoltaic power station can ensure the continuous operation without going off the grid within a certain voltage drop range and time interval.
5) Detection and control of islanding effect.
During normal power generation, the photovoltaic grid-connected power generation system is connected to the large power grid and transmits active power to the grid, but when the grid loses power, the photovoltaic grid-connected power generation system may still work continuously and operate independently of the local load, which is called the island effect. When the inverter has an islanding effect, it will cause great safety hazards to personal safety, power grid operation, and the inverter itself, so the inverter access standard stipulates that the photovoltaic grid-connected inverter must have the detection and control function of the islanding effect.
The detection methods of islanding effect include passive detection and active detection, and the passive detection method detects the amplitude of the voltage and current at the output end of the grid-connected inverter. The inverter does not add interference signals to the grid, and judges whether the grid is out of power by detecting whether parameters such as current phase offset and frequency exceed the specified value, which will not cause grid fluctuations and no energy loss. The active detection refers to the grid-connected inverter actively, regularly to the power grid to apply some interference signals, such as frequency shift and phase shift, because the power grid can be regarded as an infinite voltage source, when there is a power grid, these interference signals will be absorbed by the power grid, if the power grid occurs, these interference signals will form positive feedback, and eventually the frequency or voltage will be exceeded, so that it can be judged whether the island effect has occurred.
The main technical indicators of photovoltaic inverters.
1.Stability of the output voltage.
In a photovoltaic system, the electrical energy emitted by the solar cells is first collected by the combiner box, and then inverted by the inverter into alternating current such as 270V and 315V. However, the output voltage of photovoltaic modules is affected by its own irradiance, and its output voltage changes in a large range, and for a qualified inverter, when the input terminal voltage changes within this range, the change of its steady-state output voltage should not exceed the rated value of plusmn; 5%, and its output voltage deviation should not exceed 10% of the rated value when the load is abruptly changed.
2.The waveform distortion of the output voltage.
For sine wave inverters, the maximum allowable waveform distortion (or harmonic content) should be specified. It is usually expressed as the total waveform distortion of the output voltage, and its value should not exceed 5% (l0% allowed for single-phase output). Because the high-order harmonic current output by the inverter will produce 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 operation efficiency of the system.
3.Rated output frequency.
For loads such as motors, such as washing machines, refrigerators, etc., because the optimal frequency working point of the motor is 50Hz, too high or too low frequency will cause the equipment to heat up, reducing the operating efficiency and service life of the system, so the output frequency of the inverter should be a relatively stable value, usually 50Hz.
4.Load power factor.
Characterize the ability of an inverter to carry an inductive load or a capacitive load. The load power factor of the sine wave inverter is 07～0.9. The rated value is 09。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, which will increase the cost on the one hand, and at the same time, the apparent power of the AC circuit of the photovoltaic system will increase, the loop current will increase, 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 its output power to the input power under the specified working conditions, expressed as a percentage, and in general, the nominal efficiency of the PV inverter refers to the efficiency of the pure resistance load and 80% load. Due to the high overall cost of PV systems, the efficiency of PV inverters should be maximized, the system cost should be reduced, and the cost performance of PV systems should be improved. 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 the photovoltaic system, it is not only necessary to select a high-efficiency inverter, but also to make the photovoltaic system load work near the best efficiency point through reasonable system configuration.
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 the rated output capacity, which is expressed in VA or KVA. The rated capacity of the inverter is the product of the rated output current when the output power factor is 1 (i.e., a purely resistive load).
The main protection measures of photovoltaic inverters.
An inverter with excellent performance should also have complete protection functions or measures to deal with various abnormal situations in the actual use process, so that the inverter itself and other parts of the system are protected from damage.
1) Input Under-Voltage Protection:
When the input voltage is less 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 be able to ensure that it can act in time when the load is short-circuited or the current exceeds the allowable value, so that it can be protected from the damage of inrush current. When the operating current exceeds 150% of the rating, the inverter should be able to protect itself.
4) Output short circuit protection:
The action time of the inverter short circuit protection should not exceed 05s。
5) Input reverse polarity protection:
When the positive and negative poles of the input terminal are reversed, the inverter should have a protection function and display.
6) Lightning protection:
The inverter should have lightning protection.
7) Over-temperature protection, etc
In addition, for the inverter without voltage stabilization measures, the inverter should also have output overvoltage protection measures to protect the load from overvoltage damage.
The development trend of photovoltaic inverters.
For solar inverters, improving the conversion efficiency of power supply is an eternal topic, 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 high efficiency, but also to maintain a good competitiveness will be an important topic at present.
Compared with efforts to improve the efficiency of inverters, how to improve the efficiency of the entire inverter system is gradually becoming another important issue for solar energy systems. In a solar array, when the shadow of the local area of 2 3% appears, for the inverter with an MPPT function, the power output of the system at this time will even drop by about 20%! In order to better adapt to situations like this, one-to-one MPPT or multiple MPPT control functions are very effective for single or partial solar modules.
Since the inverter system is in the condition of 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 used in series to a high DC output voltage; For this reason, due to the occurrence of abnormal conditions between the electrodes, it is easy to generate a DC arc, and due to the high DC voltage, it is very difficult to extinguish the arc, and it is very easy to cause fire. 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, which poses new technical challenges to the stability of smart grid systems. Designing an inverter system that is compatible with smart grids more quickly, accurately and intelligently will become a necessary condition for solar inverter systems in the future.
In general, the development of inverter technology has developed along with the development of power electronics, microelectronics and modern control theory. Over time, inverter technology is moving towards higher frequency, higher power, higher efficiency, and smaller size.
Introduction to the shape of the centralized inverter.
Power generation process. The photovoltaic power station is to use a certain number of solar cell modules in series to receive sunlight to convert radiant energy into direct current of a certain voltage and current, and then connect a number of battery groups in parallel in the combiner box to increase the current, and connect several combiner boxes with current to the rated current of the inverter after parallel connection to a grid-connected inverter, and invert the DC power emitted by the battery components into alternating current that meets the needs of the power grid through the grid-connected inverter, and connect to the distribution device through the box transformer step-up, and then connect to the main transformer of the power station. The voltage is raised to a voltage level that meets the requirements of the grid through a transformer and then connected to the grid.
Composition of the equipment. The main equipment includes battery components, combiner boxes, inverters, box transformers, high and low voltage distribution devices and main transformers.