Photovoltaic inverters have strict technical standards like ordinary inverters. Any inverter must meet the following technical indicators to be considered a qualified product.
1. Output Voltage Stability
In the photovoltaic system, the electric energy generated by the solar cell is first stored by the battery, and then converted into 220V or 380V alternating current through the inverter. However, the battery is affected by its own charge and discharge, and its output voltage varies widely. For example, for a battery with a nominal 12V, its voltage value 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 of the steady-state output voltage should not exceed ±5% of the rated value, and when the load changes suddenly, the output voltage deviation should not exceed ±10% of rated value.
2. Waveform Distortion of Output Voltage
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, 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 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 system. operating efficiency.
3. Rated output frequency
For loads including motors, such as washing machines, refrigerators, etc., because the optimal frequency of the motor is 50Hz, the frequency is too high or too low, which will cause the equipment to heat up and reduce the operating efficiency and service life of the system. The output frequency should be a relatively stable value, usually the power frequency 50Hz, and its deviation should be within ±1% 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 the sine wave inverter is 0.7 to 0.9, and the rated value is 0.9. In the case of a certain load power, if the power factor of the inverter is low, the required capacity of the inverter will increase, which will increase the cost and increase the apparent power of the AC circuit of the photovoltaic system. As the current increases, the losses 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, expressed as a percentage. In general, the nominal efficiency of the photovoltaic inverter refers to pure resistance load, under 80% load. s efficiency. Since the overall cost of the photovoltaic system is high, the efficiency of the photovoltaic inverter should be maximized, the system cost should be reduced, and the cost-effectiveness of the photovoltaic system should be improved. 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 photovoltaic system, not only should high-efficiency inverters be selected, but at the same time, the system should be reasonably configured to make the photovoltaic system load work near the optimal efficiency point as much as possible.
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 when the output power factor is 1 (ie pure resistive load), the rated output voltage is the product of the rated output current.
7. Protective measures
An inverter with excellent performance should also have complete protection functions or measures to deal with various abnormal conditions during actual use, so that the inverter itself and other components of the system are not damaged.
(1) Input undervoltage policyholder:
When the input voltage is lower than 85% of the rated voltage, the inverter should have protection and display.
(2) Input overvoltage insurance account:
When the input voltage is higher than 130% of the rated voltage, the inverter should have protection and display.
(3) Overcurrent 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 as to prevent it from being damaged by the surge current. When the working current exceeds 150% of the rated value, the inverter should be able to automatically protect.
(4) Output short-circuit guarantee
The inverter short-circuit protection action time should not exceed 0.5s.
(5) Input reverse polarity protection:
When the positive and negative poles of the input terminals are reversed, the inverter should have protection function and display.
(6) Lightning protection:
The inverter should have lightning protection.
(7) Over temperature protection, etc.
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 load and the performance during dynamic operation. The inverter should be guaranteed to start reliably under rated load.
Transformers, filter inductors, electromagnetic switches and fans in power electronic equipment all generate noise. When the inverter is in normal operation, its noise should not exceed 80dB, and the noise of a small inverter should not exceed 65dB.