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What is a solar inverter and what are the functions of an inverter
What is a solar inverter
The solar AC power generation system is composed of solar panels, charge controller, inverter and battery; the solar DC power generation system does not include the inverter. Inverter is a power conversion device. Inverters can be divided into self-excited oscillation inverter and separately excited oscillation inverter according to the excitation method. The main function is to invert the DC power of the battery into AC power. Through the full-bridge circuit, the SPWM processor is generally used to undergo modulation, filtering, voltage boosting, etc. to obtain sinusoidal AC power that matches the lighting load frequency, rated voltage, etc. for system end users. With an inverter, a DC battery can be used to provide AC power to appliances.
- Type of inverter
(1) Classification by application scope:
(1) Ordinary inverter
DC 12V or 24V input, AC 220V, 50Hz output, power from 75W to 5000W, some models have AC and DC conversion, that is, UPS function.
(2) Inverter/charger all-in-one machine
In this type of inverter, users can use various forms of power to power AC loads: when there is AC power, the AC power is used to power the load through the inverter, or to charge the battery; when there is no AC power, the battery is used to power the AC load. . It can be used in conjunction with various power sources: batteries, generators, solar panels and wind turbines.
(3) Special inverter for post and telecommunications
Provide high-quality 48V inverters for post and telecommunications, communications. Its products are of good quality, high reliability, modular (module is 1KW) inverter, and have N+1 redundancy function and can be expanded (power from 2KW to 20KW ).
4) Special inverter for aviation and military
This type of inverter has a 28Vdc input and can provide the following AC outputs: 26Vac, 115Vac, 230Vac. Its output frequency can be: 50Hz, 60Hz and 400Hz, and the output power ranges from 30VA to 3500VA. There are also DC-DC converters and frequency converters dedicated to aviation.
(2) Classification by output waveform:
(1) Square wave inverter
The AC voltage waveform output by the square wave inverter is a square wave. The inverter circuits used by this type of inverter are not exactly the same, but the common feature is that the circuit is relatively simple and the number of power switch tubes used is small. The design power is generally between one hundred watts and one kilowatt. The advantages of square wave inverter are: simple circuit, cheap price and easy maintenance. The disadvantage is that the square wave voltage contains a large number of high-order harmonics, which will produce additional losses in load appliances with iron core inductors or transformers, causing interference to radios and some communication equipment. In addition, this type of inverter has shortcomings such as insufficient voltage regulation range, incomplete protection function, and relatively high noise.
2) Step wave inverter
The AC voltage waveform output by this type of inverter is a step wave. There are many different lines for the inverter to realize step wave output, and the number of steps in the output waveform varies greatly. The advantage of the step wave inverter is that the output waveform is significantly improved compared to the square wave, and the high-order harmonic content is reduced. When the steps reach more than 17, the output waveform can achieve a quasi-sinusoidal wave. When using transformerless output, the overall efficiency is very high. The disadvantage is that the ladder wave superposition circuit uses a lot of power switch tubes, and some of the circuit forms require multiple sets of DC power inputs. This brings trouble to the grouping and wiring of solar cell arrays and the balanced charging of batteries. In addition, the staircase wave voltage still has some high-frequency interference to radios and some communication equipment.
(3) Sine wave inverter
The AC voltage waveform output by the sine wave inverter is a sine wave. The advantages of the sine wave inverter are that it has good output waveform, low distortion, little interference to radios and communication equipment, and low noise. In addition, it has complete protection functions and high overall efficiency. The disadvantages are: the circuit is relatively complex, requires high maintenance technology, and is expensive.
The classification of the above three types of inverters is helpful for designers and users of photovoltaic systems and wind power systems to identify and select inverters. In fact, inverters with the same waveform are still very different in terms of circuit principles, devices used, control methods, etc.
- Main performance parameters of the inverter
There are many parameters and technical conditions that describe the performance of an inverter. Here we only give a brief explanation of the technical parameters commonly used when evaluating inverters.
- Environmental conditions for use of the inverter
Normal usage conditions of the inverter: the altitude does not exceed 1000m, and the air temperature is 0~+40℃.
- DC input power conditions
Input DC voltage fluctuation range: ±15% of the rated voltage of the battery pack.
- Rated output voltage
Under the specified input power conditions, the inverter should output the rated voltage value when outputting the rated current.
Voltage fluctuation range: single-phase 220V±5%, three-phase 380±5%.
- Rated output current
Under the specified output frequency and load power factor, the rated current value that the inverter should output.
- Rated output frequency
Under the specified conditions, the rated output frequency of the fixed frequency inverter is 50Hz:
Frequency fluctuation range: 50Hz±2%.
- Maximum harmonic content of the inverter
For sine wave inverters, under resistive load, the maximum harmonic content of the output voltage should be ≤10%.
- Inverter overload capability
Under specified conditions, the inverter output capability exceeds the rated current value in a short period of time. The overload capacity of the inverter should meet certain requirements under the specified load power factor.
- Inverter efficiency
Under the rated output voltage, output, current and specified load power factor, the ratio of the inverter output active power to the input active power (or DC power).
- Load power factor
The allowable variation range of the inverter load power factor is recommended to be 0.7-1.0.
- Load asymmetry
Under 10% asymmetric load, the asymmetry of the fixed frequency three-phase inverter output voltage should be ≤10%.
- Output voltage asymmetry
Under normal operating conditions, the load of each phase is symmetrical, and the asymmetry of the output voltage should be ≤5%.
12． Starting characteristics
Under normal operating conditions, the inverter should be able to start normally 5 times in a row under full load and no-load operating conditions.
- Protective function
The inverter should be equipped with: short-circuit protection, over-current protection, over-voltage protection, under-voltage protection and phase loss protection.
- Interference and anti-interference
The inverter should be able to withstand electromagnetic interference in general environments under specified normal working conditions. The anti-interference performance and electromagnetic compatibility of the inverter should comply with relevant standards.
- noise
Inverters that are not frequently operated, monitored and maintained should be ≤95db;
Inverters that are frequently operated, monitored and maintained should be ≤80db.
- show
The inverter should be equipped with data display for parameters such as AC output voltage, output current, and output frequency, as well as signal display for input live, energized, and fault status.
- Determine the technical conditions of the inverter:
When selecting an inverter for a photovoltaic/wind power complementary system, the first thing to do is to determine the following most important technical parameters of the inverter: input DC voltage range, such as DC24V, 48V, 110V, 220V, etc.;
Rated output voltage, such as three-phase 380V or single-phase 220V;
Output voltage waveform, such as sine wave, trapezoidal wave or square wave.