1. Output voltage stability
In photovoltaic systems, the electrical energy generated by solar cells is first stored in batteries and then converted into 220V or 380V AC power by an inverter. However, due to the effects of its own charging and discharging, the output voltage of a battery varies considerably. For example, a nominal 12V battery can have a voltage range of 10.8 to 14.4V (exceeding this range may damage the battery). For a qualified inverter, when the input voltage varies within this range, the change in its steady-state output voltage should not exceed ±5% of the rated value. Furthermore, when the load changes abruptly, the output voltage deviation should not exceed ±10% of the rated value.
2. Output voltage waveform distortion
For sinusoidal inverters, the maximum permissible waveform distortion (or harmonic content) should be specified. This is typically expressed as the total waveform distortion of the output voltage, and its value should not exceed 5% (10% for single-phase output). Because the high-order harmonic currents output by the inverter will generate additional losses such as eddy currents in inductive loads, excessive waveform distortion can lead to severe overheating of load components, jeopardizing electrical equipment safety and significantly impacting system operating efficiency.
3. Rated output frequency
For loads containing motors, such as washing machines and refrigerators, the optimal operating frequency of the motor is 50Hz. Too high or too low a frequency will cause the equipment to overheat, reduce system operating efficiency and lifespan. Therefore, the output frequency of the inverter should be a relatively stable value, usually the power frequency of 50Hz, and its deviation should be within ±1% under normal operating conditions.
4. Load power factor
The power factor of an inverter is characterized by its ability to handle inductive or capacitive loads. The load power factor of a sinusoidal inverter is typically 0.7 to 0.9, with a rated value of 0.9. Given a fixed load power, a lower inverter power factor necessitates a larger inverter capacity. This increases costs, increases the apparent power and current in the photovoltaic system's AC circuit, leading to increased losses and reduced system efficiency.
5. Inverter efficiency
Inverter efficiency refers to the ratio of its output power to its input power under specified operating conditions, expressed as a percentage. Generally, the nominal efficiency of a photovoltaic (PV) inverter refers to the efficiency under purely resistive load conditions at 80% load. Since the overall cost of PV systems is relatively high, the efficiency of PV inverters should be maximized to reduce system costs and improve the cost-effectiveness of the PV system. Currently, the nominal efficiency of mainstream inverters is between 80% and 95%, and for low-power inverters, an efficiency of no less than 85% is required. In the actual design of PV systems, not only should high-efficiency inverters be selected, but also reasonable system configuration should be used to ensure that the PV system load operates near its optimal efficiency point.
6. Rated output current (or rated output capacity)
This indicates the inverter's rated output current within a specified load power factor range. Some inverter products specify the rated output capacity, expressed in VA or kVA. The inverter's rated capacity is the product of the rated output voltage and rated output current when the output power factor is 1 (i.e., a purely resistive load).
7. Protection measures: A high-performance inverter should also have complete protection functions or measures to deal with various abnormal situations that may occur during actual use, so as to protect the inverter itself and other system components 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 inverter's overcurrent protection should be able to operate in a timely manner when a short circuit occurs in the load or the current exceeds the allowable value, so as to protect it from damage by surge current. When the operating current exceeds 150% of the rated current, the inverter should be able to automatically protect itself.
(4) Output short circuit protection: The short circuit protection action time of the inverter should not exceed 0.5s.
(5) Input reverse connection protection: When the positive and negative terminals of the input 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: Characterizes the inverter's ability to start under load and its performance during dynamic operation. The inverter should guarantee reliable starting under rated load.
9. Noise: Components in power electronic equipment, such as transformers, filter inductors, electromagnetic switches, and fans, all generate noise. During normal operation, the noise level of an inverter should not exceed 80 dB, and for small inverters, it should not exceed 65 dB.
