Applications of power electronics technology: Advanced inverters

An inverter is a crucial component of renewable energy systems, converting direct current from solar panels and wind turbines into alternating current for use in homes and businesses. Inverters have a wide range of applications in power electronics technology, including electric vehicles, industrial equipment, and microgrid. Maximizing the efficiency of inverters is a hot research topic in the electronic and electrical fields. Engineers and researchers are interested in understanding the structure and parameters of inverters to optimize their performance. This article explores the basic inverter topology, presenting three circuit structures and their pulse-width modulation waveform modulation method. It also introduces a new type of inverter and explains its working state. The article concludes by examining potential future developments and research directions for inverters. As the world seeks to transition to cleaner energy sources, inverters will continue to play a vital role in making renewable energy systems more efficient and cost-effective.


Introduction
The inverter converts DC electrical energy from a battery into alternating current, generally 220 V & 50 Hz sinusoidal or square wave.An inverter, to put it simply, is a tool that changes direct current (DC) into alternating current (AC).It comprises an inverter bridge, control logic, and filter circuitry [1].The name "inverter" comes from its ability to invert the process of rectifying 220 V alternating current into direct current.In a mobile state, the inverter can meet the demand for the low-voltage direct current supplied by the battery or power grid and the 220 V alternating current that is indispensable in daily life.But in fact, inverters are still facing problems such as the efficiency problem: in converting direct current to alternating current, there is some energy loss in the energy conversion process in the inverter, which will lead to a decrease in output power and waste of energy.
Advanced inverters play an indispensable role in renewable power energy applications, especially in photovoltaic (PV) power generation [1].The voltage-source multilevel inverters (MLIs) are gaining popularity because it is well equipped with high-quality output voltage and reduced voltage stress in other power devices but also due to their practical low dv/dt on the output filter [2].MLIs are divided into neutral point inverters (NPC), fast capacitor inverters (FC) and cascaded MLIs.
Because conventionally the MLIs require a high-voltage DC link typically created by connecting several DC power sources in succession, such as a PV string, there is still a concerning problem that the PV strings might still be mismatched [3].According to previous studies and research, methods have been proposed to help solve this problem.For example, the cascaded full-bridge inverter, voltage boosting MLI through a single-stage power conversion and Z-source inverter [4,5].However, given that there still might be some problems or imperfections in these approaches, such as the unbalanced source voltage/power, low wave quality or irregular input current drawn by the Z-source network, some critical problems still exist.More efficient topologies and control algorithms are required to improve the efficiency of inverters.

Basic inverter topology
A variety of inverters are mentioned earlier; they all have different topologies and advantages and disadvantages.This section will list several inverter topologies and some of their modulation methods and working states.

Basic voltage source inverters
Voltage source inverters (VSI) and current source inverters (CSI) are two common inverter types, which are based on voltage control and current control principles, respectively [6][7][8][9].
A voltage source inverter is a kind of inverter based on voltage control, which converts DC power into AC power through an AC filter [6,7].In VSI, the magnitude and frequency of the output voltage is determined by the DC power supply's voltage and the inverter's switching cycle.The output voltage waveform of the VSI approximates a square wave, and its amplitude and frequency can be controlled by adjusting the switching period and the duty cycle.VSI is commonly used in industrial drives, UPS, and solar photovoltaics applications.As shown in figure 1, the topology of the basic voltage source inverter consists of three sets of switches (K1-K6), which are modulated and controlled by three modulation waves of s1, s2 and s3 [10].Three phase angles of 120 degrees separate the modulation waves of s1-s3, and the sine wave of the same frequency and amplitude is compared with the same triangular carrier and stored the signal to control its state in three sets of switches, and the specific PWM waveform of s1-s3 is shown in figure 2.

Basic current source inverters
The current source inverter is a kind of inverter based on current control, which forms a resonance circuit through the DC power supply through the inductor and capacitor and then outputs it through the AC power supply [6,7].In CSI, the magnitude and frequency of the output current are determined by the DC power supply's current and the resonant circuit's resonant frequency.The output current waveform of the CSI approximates a sine wave, and its amplitude and frequency can be controlled by adjusting the current of the DC supply and the resonant frequency of the resonant circuit.CSI is commonly used in induction heating, inverter air conditioning and power transmission.The basic current source inverter topology circuit structure is shown in figure 3. Unlike the voltage source inverter, the circuit's excitation has changed from a voltage source to a current source.But because the ideal current source in real life can be far more difficult than the ideal voltage source, the application frequency of the current source inverter in real life is far less than the voltage source inverter because of its high cost, complex technology structure and other reasons.The current source inverter is characterized by its unidirectional switches, which can block reverse voltages.
Although the controlling principle of voltage source inverter and current source inverter is different, they both realize DC to AC conversion is important, which means there is a wide range of applications in different application fields: Current source inverters, such as motors and transformers, are mainly used in applications to drive inductive loads.Current source inverters can provide high-quality alternating current, and the output voltage and frequency can be controlled by controlling the current and frequency.Common applications include inverter air conditioning, motor drives, electromagnet control, etc.
Voltage source inverters are mainly used to drive capacitive loads, such as electronic devices, lighting fixtures, etc. Voltage source inverters can provide a stable output voltage, which can be controlled by controlling the voltage and frequency.Common applications include UPS power supplies, solar inverters, household appliances, etc.

A new single-phase multilevel inverter
In addition to the PV series loss problem mentioned above, there are some problems, such as when people trying to use a cascaded full-bridge inverter for low-voltage power supply multi-stage operation, the DC power can be isolated but will receive a part of the unbalanced current, resulting in a decrease in the overall quality of the waveform [3].When trying to offset the voltage change with DC-DC conversion in front of all submodules, it is accompanied by the problem that many reactive components and multi-switch modulation signals are difficult to unify and achieve.
Thus, to fix some disadvantages, a single-phase, five-level boosting MLIs is invented by a team, as shown in figure 4.This inverter relies on a Full Bridge (FB) cell, which is supported by two diodes, D a and Db.The FB cell's bridge legs are cascaded by two symmetrical half-bridge cells.[3].The symmetrical cascaded configuration that results has several important characteristics: a) A solitary inductive DC-link that is created by connecting an inductor and a DC source in sequence powers it.b) This arrangement is a great fit for green energy sources because it creates a continuous intake current and a boosted output voltage.c) The HB (Half Bridge) capacitors are charged with inductive current.d) It can also be extended for higher voltage-level processes by using several HB cells and a single inductor [3].
The steady-state operation of the SCHB (Symmetrical Cascaded Half Bridge) inverter during the positive half-cycle is shown in figure 5, with the presumption that the output voltage of the voltages across the floating capacitors Ca and Cb keep constant in each fundamental cycle [3].As is depicted in figure 5, there are five working conditions: Figure 5 (a): the two HB (Half Bridge) cells as well as the FB (Full Bridge) cell's top switches are switched on, shorting the two output terminals (a and b) and producing a zero-output voltage.The capacitors do, however, keep the energy level regulated.Figure 5 (b): the output voltage is still negative while S2a and S2b are on and charging Lin through the two bridge legs.Figure 5 (c): installing capacitance Cb in the output AC route causes the SCHB inverter's active output voltage Vc, to be produced [3]. Figure 5 (d): similarly, in this active state, the FB cell's ST (SHOOT) operation can be initiated for charging the inductor.The capacitor Cb keeps maintaining the output voltage (Vab=Vc).Figure 5 (e): as diode Db is reverse-biased in this condition, the inductor only discharges to capacitance Ca through the forwardbiased diode Da.Therefore, Vab = 2Vc.The ST operation is not allowed in this operating state, in contrast to the prior operating states.
Remarkably, switching between any two of the five operating states in the SCHB inverter does not result in a stepped change in yield current (Iac) or input current (Iin).The state transitions occur smoothly, without disrupting the current flow or causing voltage spikes, especially when both currents are inductive, as demonstrated experimentally.Similarly, the symmetrical structure of the SCHB inverter allows for comparable descriptions in the negative half cycle (NHC).Controlling the ST inductivecharge duty ratio in Figures 5 (b) and (d) will increase and manage the capacitors' voltage by intermittently running the SCHB inverter.However, at least one of the FB cell's upper switches (S1a and S1b) must always be switched on in order to keep a constant input current.

Future development and application prospect
In today's increasingly severe energy situation, advocating green energy, optimizing the energy structure, and reducing the heavy dependence on environmentally polluting resources such as coal has been widely recognized by society [11,12].
As an indispensable part of the energy structure and power grid, inverters should pay more attention to their efficiency and environmental protection.And with the progress of society, new energy technology is constantly developing and improving.And new energy grid-connected inverters and their control technology are increasingly valued by actors in the field of power electronics [13].Therefore, this paper believes that the future research and development aspects of inverters can include the following aspects: i) High efficiency: improves the conversion efficiency of the inverter and reduces the energy loss, which can be achieved by improving the device material, topology, and control strategy.
ii) High reliability: improves the stability and reliability of the inverter and reduces the failure rate, which can be achieved by optimizing the design, improving the material quality and using highly reliable components.
iii) Miniaturization and integration: The size and weight of the inverter are further reduced to adapt to a wider range of application scenarios, and the integration of inverters and other power electronic devices is realized to reduce the complexity and cost of the system.iv) Multi-function: further expand the functions of the inverter, such as power factor correction, harmonic suppression, power grid interconnection and other functions.
v) Application of high-performance materials: using new materials, such as silicon carbide, gallium nitride, etc.Power density and temperature stability should be improved to improve the performance of the inverter.
For example, possible specific research directions are displayed here for the above directions.Design and manufacture of efficient inverters based on silicon carbide semiconductor materials.Research on multi-functional inverters for renewable energy such as solar and wind energy and realize functions such as power grid interconnection and power factor correction. Advanced manufacturing processes enable high-density integrated inverter designs.Develop intelligent control algorithms and chips to achieve more efficient and accurate inverter control.Develop dedicated inverters for specific application scenarios, such as electric vehicles, medical equipment, etc.

Conclusion
Inverters are essential in electronic power technology and grids as they convert DC electrical energy into AC energy.They have widespread applications in modern society, including solar power generation, electric vehicles, UPS power supplies, home appliances, industrial control and communication equipment, and many other fields.This paper overviews inverter applications and basic topologies and describes a new single-phase multilevel inverter and its modulation and working state.However, it is important to note that this article only covers a partial list of inverters, and many classic inverters are not mentioned here.This paper aims to help researchers and engineers in electronic power technology better understand and determine the basic inverter topology and to provide new solutions for existing problems.
Additionally, it aims to help those new to power electronics technology, and those interested in electrical electronics quickly understand the topology of some classic and basic inverters, their characteristics, control and modulation methods, and power parameters.Furthermore, this paper discusses the general direction of future research and development for inverters and their potential applications in daily life and industrial fields.Energy efficiency, function diversification, intelligence, integration, and the application of new materials and technologies are expected to be the focus of future inverter development.These developments will bring both opportunities and challenges to the field of inverters.In conclusion, inverters are crucial components in modern electronic power technology, and this paper provides an informative overview of their applications and basic topologies.By understanding the basic topology of inverters and their challenges, researchers and engineers can work towards creating more efficient and effective inverters that meet the needs of modern society.

Figure 1 .
Figure 1.Basic voltage source inverter topology.As shown in figure1, the topology of the basic voltage source inverter consists of three sets of switches (K1-K6), which are modulated and controlled by three modulation waves of s1, s2 and s3[10].Three phase angles of 120 degrees separate the modulation waves of s1-s3, and the sine wave of the same frequency and amplitude is compared with the same triangular carrier and stored the signal to control its state in three sets of switches, and the specific PWM waveform of s1-s3 is shown in figure2.

Figure 2 .
Figure 2. The waveform of the modulated signal.

Figure 3 .
Figure 3. Basic current source inverter topology.The basic current source inverter topology circuit structure is shown in figure3.Unlike the voltage source inverter, the circuit's excitation has changed from a voltage source to a current source.But because the ideal current source in real life can be far more difficult than the ideal voltage source, the application frequency of the current source inverter in real life is far less than the voltage source inverter because of its high cost, complex technology structure and other reasons.The current source inverter is characterized by its unidirectional switches, which can block reverse voltages.Although the controlling principle of voltage source inverter and current source inverter is different, they both realize DC to AC conversion is important, which means there is a wide range of applications in different application fields:

Figure 4 .
Figure 4. Topology of the proposed inverter.The symmetrical cascaded configuration that results has several important characteristics: a) A solitary inductive DC-link that is created by connecting an inductor and a DC source in sequence powers it.b) This arrangement is a great fit for green energy sources because it creates a continuous intake current and a boosted output voltage.c) The HB (Half Bridge) capacitors are charged with inductive current.d) It can also be extended for higher voltage-level processes by using several HB cells and a single inductor[3].The steady-state operation of the SCHB (Symmetrical Cascaded Half Bridge) inverter during the positive half-cycle is shown in figure5, with the presumption that the output voltage of the voltages across the floating capacitors Ca and Cb keep constant in each fundamental cycle[3].

Figure 5 .
Figure 5. Working conditions[3].As is depicted in figure5, there are five working conditions: Figure5(a): the two HB (Half Bridge) cells as well as the FB (Full Bridge) cell's top switches are switched on, shorting the two output terminals (a and b) and producing a zero-output voltage.The capacitors do, however, keep the energy level regulated.Figure5 (b): the output voltage is still negative while S2a and S2b are on and charging Lin through the two bridge legs.Figure5 (c):installing capacitance Cb in the output AC route causes the SCHB inverter's active output voltage Vc, to be produced[3].Figure5 (d): similarly, in this active state, the FB cell's ST (SHOOT) operation can be initiated for charging the inductor.The capacitor Cb keeps maintaining the output voltage (Vab=Vc).Figure5(e): as diode Db is reverse-biased in this condition, the inductor only discharges to capacitance Ca through the forwardbiased diode Da.Therefore, Vab = 2Vc.The ST operation is not allowed in this operating state, in contrast to the prior operating states.Remarkably, switching between any two of the five operating states in the SCHB inverter does not result in a stepped change in yield current (Iac) or input current (Iin).The state transitions occur smoothly, without disrupting the current flow or causing voltage spikes, especially when both currents are inductive, as demonstrated experimentally.Similarly, the symmetrical structure of the SCHB inverter allows for comparable descriptions in the negative half cycle (NHC).Controlling the ST inductivecharge duty ratio in Figures5 (b) and (d) will increase and manage the capacitors' voltage by