First, the photovoltaic inverter works
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.
Inverter simple schematic
Second, the main technical indicators of photovoltaic inverter
1, the output voltage stability
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 a wide range of changes, such as nominal 12V battery, the voltage value can be varied between 10.8 ~ 14.4V (Beyond 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 ± 5% of the rated value. At the same time, when the load suddenly changes, the output voltage deviation should not exceed ±10% of 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 the system operating efficiency and service life, so the inverter The output frequency should be a relatively stable value, usually 50Hz, and the deviation should be within ±1% under normal operating conditions.
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 condition. s efficiency. Due to the higher overall cost of the photovoltaic system, in the photovoltaic system, the energy emitted by 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 a wide range of changes, such as nominal 12V battery, the voltage value can be varied between 10.8 ~ 14.4V (Beyond 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 ± 5% of the rated value. At the same time, when the load suddenly changes, the output voltage deviation should not exceed ±10% of rated value.
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, start 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 in normal operation, the noise should not exceed 80dB, and the noise of the small inverter should not exceed 65dB.
Third, the simple photovoltaic inverter
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.
In addition, the inverter must have a certain overload capacity, when the input voltage and output power are rated, the ambient temperature is 25 °C, the continuous and reliable inverter operating time should not be less than 4h; when the input voltage is rated, When the output power is 125% of the rated value, the safe working time of the inverter shall not be less than lmin; when the input voltage is the rated value and the output power is 150% of the rated value, the safe working time of the inverter shall not be less than 10s.
Application examples:
The main load in the photovoltaic system is a 150W refrigerator. When the AC inverter with a rated capacity of 180W is selected for normal operation, it can work reliably. However, since the refrigerator is an inductive load, its power consumption can reach 5 to 5 times the rated power at the moment of starting. 6 times as much, so the output power of the inverter can reach 800W when the load starts. Considering the overload capacity of the inverter, the 500W inverter can be used reliably.
When there are multiple loads in the system, the selection of inverter capacity should also consider the possibility of several power loads working at the same time, ie “load simultaneous coefficientsâ€.
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 a wide range of changes, such as nominal 12V battery, the voltage value can be varied between 10.8 ~ 14.4V (Beyond 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 ± 5% of the rated value. At the same time, when the load suddenly changes, the output voltage deviation should not exceed ±10% of rated value.
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