When selecting a photovoltaic inverter, generally select the DC input voltage of the inverter according to the DC voltage determined by the system design, determine the capacity and phase number of the inverter according to the total power and type of the load, and then consider the instantaneous impact of the load. The power margin of the inverter, usually the continuous power of the inverter is greater than the power of the load, and the maximum impact power of the inverter is greater than the starting power of the load. In addition, when selecting inverters, it is also necessary to consider leaving some room for expansion of the photovoltaic power generation system.
In the independent photovoltaic power generation system, if it has an AC load, it must be equipped with an inverter; if the voltage of the DC load is inconsistent with the voltage of the battery pack, it needs to be equipped with a DC converter.
Inverters are used wherever DC is required to be converted into AC, so the inverter itself is very versatile. However, inverters with AC loads in stand-alone PV systems have some specific requirements, such as:
① High inverter efficiency within the operating range (that is, not only under full load, but also under light load);
② safe and reliable operation;
③ It can adapt to the wide variation of the DC voltage of the battery of the photovoltaic power generation system;
④It is resistant to instantaneous large current impact and can be used continuously for a long time;
⑤ For inverters with inductive loads, the high-order harmonic components of the AC output are required to be small;
⑥ The performance and price are relatively good.
The inverter is the last level device of an independent photovoltaic system, and it is also the key balance component with the third largest investment in the system. Its performance directly affects the investment, performance and reliability of the system. Therefore, for different application systems, the selection of suitable inverters is also an important task for design users.
There are many kinds of inverters, which can be generally classified according to their output waveform, whether the load is active, the number of output current phases, and the topology of the main circuit, as shown in Table 1.
Of course, there are other classification methods, but for independent photovoltaic power generation systems, passive sine wave inverters are generally required.
The main technical parameters of the selected inverter are as follows.
(1) Inverter efficiency
Inverter efficiency indicates its own power loss. Generally, inverter efficiency can be required according to the following standards:
For inverters with a capacity of 100~1000kW, the efficiency should be above 96%~98%:
For inverters with a capacity of 10~100kW, the efficiency should be above 90%~93%; for inverters with a capacity of 1~10kW, the efficiency should be above 85%~90%; for inverters with a capacity of 0.1~1kW, The efficiency should be above 80%~85%.
It should be noted that the efficiency referred to here refers to the efficiency achieved by the inverter under the condition of full load, and the inverter with good quality is also more efficient under light load.
(2) Rated output voltage
The photovoltaic inverter should be able to output a constant voltage value within the allowable fluctuation range of the specified input DC voltage. For medium and small independent photovoltaic power stations, the general transmission radius is less than 2km, and the output voltage of the inverter is selected as single-phase 220V and three-phase 380V, and the voltage is no longer boosted and transmitted to the user. At this time, the voltage fluctuation range is as follows:
①When running in a steady state, the passive voltage range does not exceed ±5% of the rated value; ②When there is an impact load, the voltage fluctuation range does not exceed ±10% of the rated value;
③ During normal operation, the unbalance of the three-phase voltage output by the inverter shall not exceed 8%;
④ The sine wave distortion of the output voltage is generally less than 3%;
⑤ The frequency fluctuation of the output AC voltage should be within 1%, and the output voltage frequency specified in GB/T19064-2003 should be between 49~51Hz.
(3) Rated output power
The rated output power refers to the product of the rated output voltage of the inverter and the rated output current when the load power factor is 1, and the unit is kV·A.
(4) Overload capability
The overload capacity is the time required for the inverter to work continuously under the condition of rated output power. The standard is as follows:
①When the input voltage and output power are rated values, the inverter should be able to work continuously for more than 4 hours;
②When the input voltage and output power are 125% of the rated value, it should be able to work continuously for more than 1min;
③ When the input voltage and output power are 150% of the rated value, it should be able to work continuously for more than 10s.
(5) Rated DC input voltage and range
The rated DC input voltage refers to the DC voltage value input to the inverter in the photovoltaic power generation system. The input voltage of low-power inverters is generally 12V, 24V, 48V, and the medium and high-power inverters are usually 110V, 220V, 500V, etc.
Since the voltage of the energy storage battery bank of the independent photovoltaic power generation system is changing, this requires the inverter to be able to meet the input current and voltage can be changed within a certain range without affecting the output voltage change, usually this value is 90%~120% .
(6) Protection function
The inverter should have the following main protection functions to ensure the safe and reliable operation of the photovoltaic power generation system: ①Overvoltage and undervoltage protection; ②Overcurrent protection; ③Short circuit protection; ④Reverse connection protection; ⑤Lightning protection and grounding protection, etc.
(7) Security requirements
①Insulation resistance The insulation resistance between the DC input of the inverter and the casing, and the insulation resistance between the AC output and the charger should be ≥50MΩ.
② Dielectric strength The DC input of the inverter and the casing should be able to withstand a dielectric strength test with a frequency of 50Hz, a sine wave AC voltage of 500V, and a duration of 1min. There is no breakdown or arc phenomenon. It should be able to withstand the dielectric strength test with a frequency of 50Hz and a sine wave AC voltage of 1500V, which lasted for 1min, without breakdown or arc phenomenon.
(8) Other requirements
①Using environmental conditions The normal operating conditions of the inverter for photovoltaic systems are: ambient operating temperature – 20~+50℃.
Relative humidity ≤93%, no condensation and altitude limit, etc. When the working environment exceeds the above conditions, consider reducing capacity usage or redesign customization.
②Electromagnetic interference and noise The switching circuit of the inverter is very prone to electromagnetic interference, and the iron core transformer generates noise due to vibration. Electromagnetic interference and noise must be controlled in design and manufacture to meet relevant standards and user requirements.