Solar system factory new energy photovoltaic inverter principle (||||)
Main technical indicators of solar PV inverter
1.Stability of output voltage
In the photovoltaic system, the electric energy emitted from the solar cell is first stored by the battery, and then inverted into 220V or 380V AC power through the inverter. But the battery is affected by its own charging and discharging, its output voltage changes in a wide range, such as the nominal 12V battery, its voltage value can change between 10.8 ~ 14.4V (beyond this range may cause damage to the battery). For a qualified inverter, when the voltage at the input side changes within this range, the steady-state output voltage variation should be no more than &Plusmn;5% of the rated value, and at the same time when the load changes abruptly, the output voltage deviation should not exceed ±10% of the rated value.
2. Waveform distortion of output voltage
For sinusoidal inverters, the maximum waveform distortion (or harmonic content) allowed should be specified. Usually expressed as the total waveform distortion of the output voltage, the value should not exceed 5% (l0% allowed for single-phase output). As the high harmonic current of the inverter output will produce 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 operation efficiency of the system.
3. Rated output frequency
For loads containing motors, such as washing machines and refrigerators, the best frequency working point of the motor is 50Hz. Too high or too low frequency will cause the equipment to heat up and reduce the system operation efficiency and service life, so the output frequency of the inverter should be a relatively stable value, usually 50Hz for industrial frequency, and the deviation should be within &Plusmn;l% under normal working conditions.
4. Load Power Factor
Characterize the ability of the inverter to carry inductive or capacitive loads. The load power factor of sinusoidal inverter is 0.7~0.9, and the rated value is 0.9. Under the condition of certain load power, if the power factor of the inverter is low, the capacity of the required inverter will be increased, which will increase the cost on one hand, and increase the apparent power of the AC circuit of the PV system, increase the circuit current, increase the loss, and reduce the system efficiency.
5. Inverter efficiency
Inverter efficiency is the ratio of its output power to input power under the specified operating conditions, expressed as a percentage, in general, the nominal efficiency of the PV inverter is the pure resistance load, 80% of the efficiency under load. Since the overall cost of PV system is high, the efficiency of PV inverter should be maximized to reduce the system cost and improve the cost performance of PV 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 the PV system, not only to choose a high-efficiency inverter, but also through the system reasonable configuration, as far as possible to make the PV system load work in the best efficiency point near.
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, whose unit is expressed in VA or kVA. The rated capacity of the inverter is the product of the rated output voltage for the rated output current when the output power factor is 1 (i.e. pure resistive load).
7、Protection measures
An inverter with good performance should also have complete protection functions or measures to deal with various abnormal situations that occur during actual use, so that the inverter itself and other parts of the system are protected from damage.
(1) Input undervoltage protector.
When the voltage at the input is lower than 85% of the rated voltage, the inverter should be protected and displayed.
(2) Input over-voltage bailout.
When the voltage at the input terminal is higher than 130% of the rated voltage, the inverter shall have protection and display.
(3) Over-current protection.
The over-current 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 that it is protected from damage by inrush current. When the working current exceeds 150% of the rated, the inverter should be able to protect automatically.
(4) Output short-circuit protector
Inverter short-circuit protection action time should not exceed 0.5s.
(5) Input reverse connection protection.
The inverter shall have protection function and display when the positive and negative terminals of the input are connected in reverse.
(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 the damage of overvoltage.
8. Starting characteristics
Characterize the inverter's ability to start with load and its performance during dynamic operation. The inverter should ensure reliable starting under rated load.
9.Noise
The transformer, filter inductor, electromagnetic switch and fan in power electronic equipment will generate noise. When the inverter is in normal operation, the noise should not exceed 80dB, and the noise of small inverter should not exceed 65dB.
V. Selection Techniques
For the selection of inverter, the first thing to consider is to have enough rated capacity to meet the requirements of electric power of the equipment under the maximum load. For the inverter with a single device as the load, the rated capacity is relatively simple to select.
When the power equipment is pure resistive load or power factor is greater than 0.9, the rated capacity of the inverter can be 1.1 to 1.15 times of the capacity of the power equipment. At the same time, the inverter should have the ability to resist the impact of capacitive and inductive loads.
For general inductive loads, such as motors, refrigerators, air conditioners, washing machines, high-power pumps, etc., when starting, its instantaneous power may be 5 to 6 times its rated power, at this time, the inverter will be subjected to a large transient surge. For such systems, the rated capacity of the inverter should be left with 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 without damaging the power devices. Small inverters sometimes need to adopt soft start or current-limited start for their own safety.
Sixth, installation precautions and maintenance
1. Before installation, you should first check whether the inverter is damaged during transportation.
2、When choosing the installation site, you should make sure that there is no interference from any other power electronic devices in the surrounding area.
3、Before making electrical connections, be sure to use opaque materials to cover the PV panels or disconnect the DC side circuit breaker. Exposure to sunlight, photovoltaic arrays will generate dangerous voltages.
4, all installation operations must be and only by professional and technical personnel to complete.
5, photovoltaic system power generation system used in the cable must be firmly connected, good insulation and appropriate specifications.
After reading the above, I believe that you have figured out the principle and characteristics of photovoltaic inverters, I hope the above content, you can have a comprehensive understanding of photovoltaic inverters.