New single-stage Buck-Boost inverter without electrolytic capacitors, suitable for small and medium power photovoltaic systems

The inverter has the ability to step up and down. Not only does the circuit itself not contain electrolytic capacitors, but it also has a strong ability to resist low-frequency pulsation on the input side, which is conducive to reducing the value of the input side filter capacitor, thereby realizing the whole system without electrolytic capacitors. The inverter has the advantages of low cost, long service life, high reliability, simple short circuit and open circuit protection, etc., which meets the requirements of small and medium power photovoltaic power generation systems.

This paper first introduces the working principle of the inverter, then establishes its mathematical model and designs a closed-loop regulator. Based on the theoretical analysis, simulation and experimental verification are carried out, and the simulation and experimental results prove the correctness of the theoretical analysis.

In photovoltaic power generation systems, solar photovoltaic cells are very sensitive to environmental factors. Affected by external factors, their output voltage has great volatility, that is, the input voltage of the photovoltaic inverter varies in a wide range. In small and medium power photovoltaic power generation systems, photovoltaic inverters mostly use voltage source bridge inverters, and their output voltage peak can only be less than the input side DC voltage value. This causes the voltage source bridge inverter to fail to meet the requirements when the input side voltage fluctuation range is large.

In addition, affected by external environmental factors, the DC voltage output by the photovoltaic array will have a small range of low-frequency pulsations. At the same time, considering the inverter's need to buffer the reactive energy, it is generally necessary to add an electrolytic capacitor for voltage stabilization and filtering. In order to achieve better voltage stabilization and filtering effects, parallel capacitors often need to use large-capacity electrolytic capacitors, which leads to problems such as large system size, low lifespan and reliability.

To solve the problem of wide range variation of input voltage, a common measure is to use an inverter with buck-boost capability. Traditional inverters with buck-boost capability can be divided into isolated inverters and multi-stage inverters. The existence of industrial frequency or high-frequency transformers in isolated inverters leads to large system size and high cost. The multi-stage inverter adds a boost DC-DC circuit to the front stage of the voltage-type inverter, and realizes buck-boost inversion through coordinated control of the front and rear stages. However, large-capacity electrolytic capacitors are used between each energy level for voltage stabilization and filtering, resulting in low system efficiency and poor reliability.

Due to the shortcomings of traditional solutions, more and more scholars have begun to study single-stage non-isolated inverters. Compared with traditional solutions, single-stage non-isolated buck-boost inverters have many advantages: no transformer is required, the topology is simple, and fewer components are required; compared with multi-stage topologies, it has only one energy link, so it has an advantage in efficiency. However, most of the single-stage buck-boost inverters that have been proposed have not solved the problem of large filter size.

Some scholars have proposed a Z-source inverter, which achieves buck-boost inverter by adding a passive network composed of passive energy storage elements into a bridge structure. However, the inductance and capacitance values ​​required by the passive network in the Z-source inverter are large, and its boost capability is weak. Although the boost capability can be improved by changing the passive network, the structure of the changed passive network is complex and the hardware parameter design becomes difficult.

Some scholars have proposed a single-stage non-isolated dual Cuk inverter. With the help of the buck-boost capability of the Cuk circuit, the inverter can achieve buck-boost inversion. However, the intermediate capacitor value of the inverter is relatively large, and the positive and negative half-cycles of the output voltage require two DC power supplies to be independently powered, which increases the system cost. Although a single power supply plus two capacitors can be used for voltage division, the voltage division capacitor often requires a large-capacitance electrolytic capacitor, which will further reduce the reliability of the system.

Some scholars have proposed an integrated inverter, which integrates the buck-boost converter and the full-bridge inverter by sharing power devices. Although the number of components is reduced, the inverter still requires an electrolytic capacitor with a large capacitance.

In view of the shortcomings of the existing solutions, this paper proposes a new inverter based on Buck-Boost converter, which has the advantages of low cost, simple topology, no need for electrolytic capacitors and high reliability. The inverter can realize the buck-boost function in a single stage, which is suitable for occasions where the input voltage changes in a wide range; it has a strong ability to resist low-frequency pulsation on the input side, which helps to reduce the value of the input side filter capacitor and realize the electrolytic capacitor-free of the entire circuit; when the inverter operates at a unity power factor, it does not need to add dead zone signals and overlapping current signals, so it has strong reliability and low total harmonic distortion (THD) of the output voltage. This paper theoretically analyzes the working principle and modulation method of the inverter, and verifies the correctness of the theoretical analysis through simulation and experiments.

in conclusion

This paper proposes a new type of single-stage non-isolated Buck-Boost inverter for small and medium power systems. Theoretical and simulation studies are carried out on its topology, working principle, modulation mode, mathematical model and control method. Finally, an experimental platform is built, and the correctness of theoretical analysis and simulation is verified through experiments. The following conclusions are obtained:

1) The new single-stage non-isolated Buck-Boost inverter does not contain electrolytic capacitors, requires fewer active and passive devices, is small in size, low in cost, and highly reliable. The inverter can achieve buck-boost function at a single stage and has strong adaptability to wide-range input occasions such as photovoltaic power generation.

2) The inverter has a certain resistance to low-frequency pulsation of input voltage, and has the advantages of simple short-circuit and open-circuit protection and high sinusoidal output waveform.

3) The mathematical model of the inverter was established and a closed-loop control system was designed. Simulation and experiments proved that under closed-loop control, the inverter can follow the given and resist sudden changes in input voltage and load disturbances, and has good dynamic and static performance.