Design and simulation of a solar-wind stand-alone system with a seven-level inverter

Renewable energy sources (RES) have become very important in recent years owing to their nondegradability, eco-friendliness and self-sufficiency. Systems using RES are reliable and can replace conventional generation techniques. Fossil fuels are degradable, non-renewable and contribute to climate change and ecological imbalance. Solar, wind and geothermal energies, as well as other renewable energy resources, are forms of RES. Solar and wind energy are the most commonly used RES [1]. The benefits of a solar photovoltaic systems over wind turbines include their minimal maintenance, lack of moving parts and ease of installation. Wind-turbine systems are less expensive than solar panels, especially when used in large quantities. However, they require a professional staff for operation and maintenance. The two technologies are typically combined using multiple RES so that they may deliver continuous power under various situations [2].

Since the amount of energy provided by renewable systems changes throughout the day, it is critical to maximize the delivered energy [3]. The output power of wind turbines and photovoltaic arrays is influenced by wind speed and solar irradiation. As a result, changes in these parameters must be handled appropriately by system control mechanisms. With variable wind, the turbine speed must be adjusted to optimize the generation of power to ensure that the system runs at its maximum power point (MPP). Similarly, the DC voltage and current at the output of the PV system must be modified to run the PV systems at their MPP [4]. The biggest issue that some RES encounter is they produce DC electrical energy. This requires equipment to convert DC to AC power. Inverters serve this role. However, there are switching components utilized during power conversion that reduce the stability and quality of the electricity [5].

Multilevel Inverter (MLI) technology is used at the DC output terminals of RES to convert the generated electricity into AC power and improve the power quality and stability. The benefits of MLI include improved voltage and current output waveforms, less electro-magnetic interference (EMI), their small size, and lower total harmonic distortion (THD) [6]. MLI switches are used to interrupt the DC so that it is produced at different levels. They are essential because they determine circuit size, installation dependability, control complexity and cost. In conventional MLIs, the size, cost and complexity of inverters increase with the output voltage level [7]. In this research, MLI switch reduction is used to provide a larger number of output levels with fewer switching components, thereby lowering costs. The proposed MLI circuit reduces the voltage stress on the switches, enhancing protection against overvoltage [8].

There are three main types of reduced switch MLIs that can be used. The first is a reduced switch symmetrical MLI (RSS-MLI) that produce many DC levels with equal magnitude. This method is a lower cost option. A reduced switch symmetrical MLI (RSA-MLI) produces DC of different magnitudes. Typically, this technology is used in a cascaded H-bridge method. Finally, a reduced switch modified MLI (RSM-MLI) does not use a cascaded H-bridge configuration. It cannot be used for high power applications due to its excessive voltage stress on H-bridge switches [9]. Fig. 1 shows the conventional and reduced switch MLIs.

Design and implementation of a single-phase switching reduction MLI in a stand-alone system fed from multiple individual RES are presented in this work. A new logical switching method, pulse width modulation (PWM), is used to create a seven-level output voltage. In this research, two PV-solar systems and one wind-turbine generator with a permanent magnet synchronous generator (PMSG) are offered as distinct energy sources. The proposed method is used to reduce the number of switching devices, thereby decreasing losses and THD.

The MATLAB Simulink program is used to simulate a seven-level inverter for use with renewable

Fig. 1. Conventional and reduced switch MLIs [9]

energies. RES continue to be more dominant in local power production for stand-alone networks than fossil fuel units. In this research, PV and wind turbine technologies are used to create a self-contained system with high reliability and security that can work under a variety of operational conditions. The contingency of reduced PV solar system performance (due to cloudy winter days, nighttime or fault event) is taken into consideration in this work. Multilevel inverters are an ideal way to employ inverters for stand-alone use in systems that produce just a few kilowatts.