What Are The Main Technical Indicators Of Solar Photovoltaic Inverters?
Jul 12, 2024
Photovoltaic inverters are one of the important system balances (BOS) in photovoltaic array systems and can be used with general AC-powered equipment. Solar inverters have special functions for photovoltaic arrays, such as maximum power point tracking and islanding protection. So, what are the main technical indicators of photovoltaic inverters?
1. Output voltage stability
In photovoltaic systems, the electric energy generated by solar cells is first stored by batteries and then converted into 220V or 380V AC power by inverters. However, due to the influence of its own charging and discharging, the output voltage of batteries has a large range of variation. For example, the voltage value of a nominal 12V battery can vary between 10.8 and 14.4V (exceeding this range may cause damage to the battery). For a qualified inverter, when the input voltage changes within this range, the change in its steady-state output voltage should not exceed ±5% of the rated value. At the same time, when the load changes suddenly, the output voltage deviation should not exceed ±10% of the rated value.
2. Output voltage waveform distortion
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% (single-phase output allows 10%). Since the high-order harmonic current output by the inverter will generate additional losses such as eddy current on the inductive load, if the inverter waveform distortion 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 operating efficiency of the system.
3. Rated output frequency
For loads including motors, such as washing machines and refrigerators, since the optimal frequency operating point of their motors is 50Hz, too high or too low frequency will cause the equipment to heat up, reduce the operating efficiency and service life of the system, so the output frequency of the inverter should be a relatively stable value, usually 50Hz, and its deviation should be within ±1% under normal working conditions.
4. Load power factor
Characterizes the ability of the inverter to carry inductive loads or capacitive loads. The load power factor of the sine wave inverter is 0.7~0.9, and the rated value is 0.9. Under the condition of constant 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 increase the apparent power of the AC circuit of the photovoltaic system, increase the circuit current, increase the loss, and reduce the system efficiency.
5. Inverter efficiency
The efficiency of the inverter refers to the ratio of its output power to input power under the specified working conditions, expressed as a percentage. Generally, the nominal efficiency of the photovoltaic inverter refers to the efficiency under pure resistance load and 80% load. Due to the high overall cost of the photovoltaic system, the efficiency of the photovoltaic inverter should be maximized to reduce the system cost and improve the cost performance of the photovoltaic system. At present, the nominal efficiency of the mainstream inverter is between 80% and 95%, and the efficiency of the small power inverter is required to be not less than 85%. In the actual design process of the photovoltaic system, not only should a high-efficiency inverter be selected, but also the system should be reasonably configured to try to make the photovoltaic system load work near the optimal 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 the rated output capacity, which 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 (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 components of the system are protected 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 be able to ensure timely action when the load is short-circuited or the current exceeds the allowable value, so as to protect it from damage by surge current. When the working current exceeds 150% of the rated value, the inverter should be able to automatically protect.
(4) Output short-circuit protection: The inverter short-circuit protection action time should not exceed 0.5s.
(5) Input reverse connection protection: When the positive and negative poles of the input terminal are reversed, the inverter should have protection functions and display.
(6) Lightning protection: The inverter should have lightning protection.
(7) Overtemperature 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
Characterizes 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
Transformers, filter inductors, electromagnetic switches, fans and other components in power electronic equipment will generate noise. When the inverter is operating normally, its noise should not exceed 80dB, and the noise of small inverters should not exceed 65dB.
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