Modeling and simulation of three-level inverter controller

Dissertation for receiving the master degree "M.Sc" in the field of power electrical engineering

Trend of power systems

Abstract

Today, significant advances in semiconductor technology and the emergence of high power switches that have the ability to turn off through a control signal on the one hand and better efficiency Multi-level converters compared to two-level converters, on the other hand, have led to an increase in the application potential of multi-level converters in various industrial fields. In multi-level converters, various pulse width modulation (PWM) techniques are used to control switches. . Space vector modulation (SVM), which was first proposed in 1986, is one of the newest types of high frequency modulation techniques, which is known as a superior technique compared to other modulation methods, especially in multi-level converters. The most important issue in space vector modulation (SVM) is to locate the reference vector in the diagram (SVM) in the shortest possible time in order to achieve the maximum switching frequency, to increase the quality of the output voltage.

In this thesis, the method of space vector modeling (SVM) has been chosen to control the three-level diode clamp inverter, and in addition to simulating and displaying the output voltages of the three-level inverter, a comparison has been made between three-level and nine-level inverters, and the output of each has been examined in terms of their harmonic spectrum and THD coefficient, and we conclude that with the increase in the number of inverter levels, this coefficient decreases and the output improves in terms of harmonic spectrum. 

1-1-Introduction

An inverter is an electrical device that can convert direct current (DC) to alternating current (AC). Using transformers, switches and control circuits, converted AC can have any voltage and frequency value. Static inverters have no moving parts and are used in a wide range of applications. From switching power supplies in computers to high voltage direct current applications in electrical installations for bulk power transmission. Inverters are usually used to feed AC power from a DC source, such as a solar panel or batteries. Electric inverters are high power electric oscillators. The reason for the name is that previously AC to DC converters were used in reverse to convert DC to AC. The inverter performs the opposite function of the rectifier.

1-2-Description:

A transformer converts the AC source to any desired voltage, but at the same frequency. Inverters, plus DC rectifiers, can be designed to convert from any voltage, AC or DC, to any other voltage, AC or DC, at any desired frequency. The output power never exceeds the input power, but the efficiency can be high, with a proportion of the power dissipated as wasted heat.

In a simple inverter circuit, the DC source is connected to a transformer through the middle end of the input winding. A switch is quickly switched between the top and bottom coils so that the DC source current flows alternately through one end of the primary coil and then the other. Alternating current in the primary winding of the transformer produces alternating current (AC) in the secondary winding.

The electromechanical type of switching equipment consists of two fixed connections and one movable connection with a spring retainer. A spring holds the moving link against one of the fixed links and a magnetic magnet pulls the moving link towards the opposite fixed link. The current of the magnetic magnet is interrupted by the operation of the switch. So that the key is constantly and quickly switched between the upper and lower coils. This type of switch is called an electromagnetic inverter, vibrator or buzzer.

AC-AC regulators, voltage and current cutters, inverters, power supplies, etc. pointed out Among them, inverters are considered as one of the most important and widely used devices. Various applications of inverters, including DC to AC conversion systems, in cases such as new energy, electric machine drives, FACTS devices and so on. It is the topic of the day]3[.

In recent years, the demand for high voltage inverters for systems such as renewable energy, HVDC links, static compensators, medium voltage variable speed drives and active filters has grown. To overcome the limitations of the existing power semiconductor devices, new structures such as diode-clamped structure, flying capacitor structure and series H-bridge structure were introduced. In these converters, as the voltage levels increase, the THD of the output voltage waveform decreases. But the number of output voltage levels is limited by issues such as voltage imbalance, the need for voltage clamps and packaging limitations. Therefore, the important point in designing an efficient and high-efficiency multi-level inverter is to ensure that the THD of the output voltage waveform is low enough. Despite the need for more power transistors, they work at a lower voltage level as well as a lower switching frequency, and another advantage is having better electromagnetic compatibility due to the lower transistors. take The output voltage can be at a variable or fixed frequency with a variable or fixed amplitude, and the variable output voltage can be obtained by changing the dc input voltage and keeping the input gain constant. On the other hand, if the dc input voltage is constant and uncontrollable, it can be used to have a variable output voltage by changing the amplification factor of the inverter, which is usually done by controlling the pulse width modulation (PWM) in the inverter. The inverter amplification factor is the ratio of the output ac voltage range to the input dc voltage.

Depending on the type of application, the type of switch, the type of network to which the inverter is connected and so on. Different inverters are used. In this section, we will briefly review these types.