Many industrial applications have begun to require higher power apparatus in recent years. Some medium voltage motor drives and utility applications require medium voltage and megawatt power level. For a medium voltage grid, it is troublesome to connect only one power semiconductor switch directly. As a result, a multilevel power inverter structure has been introduced as an alternative in high power and medium voltage situations. A multilevel inverter is a power electronic device built to synthesize a desired AC voltage from several levels of DC voltages.
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The concept of multilevel converters has been introduced since 1975. The term multilevel began with the three-level converter. Subsequently, several multilevel converter topologies have been developed. Plentiful multilevel converter topologies have been proposed during the last two decades. Contemporary research has engaged novel converter topologies and unique modulation schemes. Moreover, there are three different major multilevel converter structures which are cascaded H-bridges converter with separate dc sources, diode clamped (neutral-clamped), and flying capacitors (capacitor clamped) [1] Although the diode clamped multilevel inverter is commonly discussed in the literature, there has been considerable interest in the series connected or cascaded H-bridge inverter topologies [2]. However, the elementary concept of a multilevel converter to achieve higher power is to use a series of power semiconductor switches with several lower voltage dc sources to perform the power conversion by synthesizing a staircase voltage waveform. Capacitors, batteries, and renewable energy voltage sources can be used as the multiple dc voltage sources [1].
Multilevel power conversion has become increasingly popular in recent years due to advantages of high power quality waveforms, low electromagnetic compatibility (EMC) concerns, low switching losses, and high-voltage capability. The primary disadvantage of multilevel power conversion technology is the large number of semiconductor devices required. This does not yield a significant cost increase since lower-voltage devices may be used. However, an increase in gate drive circuitry and more elaborate mechanical layout are required [3].
Project Overview
This project will involve in the design and construction of a single phase 3-level H-bridge inverter using the IGBTs. An H-bridge is an electronic circuit which enables a voltage to be applied across a load in either direction. These circuits allow DC motors to run forwards and backwards. H-bridges are available as integrated circuits, or can be built from discrete components.
In this single phase H-bridge inverter circuit, the IGBTs are used as power devices that will be operated as a switch by applying control signal to gate terminal of IGBTs. The insulated gate bipolar transistor or IGBT is a three-terminal power semiconductor device, noted for high efficiency and fast switching.
The software that will be used is MATLAB Simulink. Simulink is a commercial tool for modeling, simulating and analyzing multidomain dynamic systems. Its primary interface is a graphical block diagramming tool and a customizable set of block libraries.
The Aims and Objectives
The aim of this project is to simulate a single phase 3-level H-bridge inverter (DC to AC converter) using the MATLAB Simulink and constructed it.
The objectives of this project are as follows:
To investigate the application of H-bridge inverter.
To assemble using the software, circuit’s implementation, and troubleshoot for the hardware.
To analyze the operation of the single-phase 3-level inverter for software and hardware.
CHAPTER 2
LITERATURE REVIEW
Inverter
Power electronics converters may be classified into four categories based on the source and types of the desired output characteristics as shown in Figure 1.1 below:
OUTPUT
AC
DC
AC
INPUTRECTIFIER
REGULATORS
DC
CHOPPERS
INVERTERS
Figure 2.1: Converter Classification
DC-to-AC converter is known as inverter. The function of an inverter is to change a DC input voltage to a symmetrical AC output voltage of desired magnitude and frequency. The variable output voltage could be fixed or variable at a fixed or variable frequency. Inverter can be built in many output phases which is normally use in practice like single phase inverter and three phase inverter. The implementation of the inverter circuit must to be involved in application of the power devices like SCR, MOSFET, IGBT, GTO, and Forced-Commutated Thyristor which is controlled to turning ON and turning-OFF in its operation as a converter. This inverter generally use PWM control signal for producing an AC output voltage [3].
Single Phase H-Bridge Inverter Operation
The H-Bridge Inverter or sometimes called “Full Bridge” consists of four switches (see Figure 2.2). A boost converter is required as this system has no means of stepping up the input. Switches S1-S4, and S2-S3 make up two switch pairs. When S1and S4 are on, the output voltage is a positive pulse, and when S2 and S3 are on, the output is a negative pulse. The phase sequence, frequency, output magnitude and harmonics can be controlled through appropriate switching devices, in conjunction with other equipment.
Figure 2.2: Single phase H-bridge inverter
Single Phase Multilevel H-Bridges Inverter
There are two types of multilevel H-bridge inverter that can be selected in this project which are separated dc source and single DC source. These two types have its pros and cons. The advantages of separated DC source are:
The number of possible output voltage levels is more than twice the number of dc sources (m = 2s + 1).
The series of H-bridges makes for modularized layout and packaging. This will enable the manufacturing process to be done more quickly and cheaply.
while the disadvantage is:
Separate dc sources are required for each of the H-bridges. This will limit its application to products that already have multiple SDCSs readily available.
Each H-bridge cell requires an isolated dc source. The isolated sources are typically provided from a transformer/rectifier arrangement, but may be supplied from batteries, capacitors or photovoltaic arrays to add up the output voltages. This topology was patented by Robicon Group in 1996 and is one of the companies standard drive products.[2]
On the other hand, for the single DC source multilevel H-bridge inverter, the advantage of this type of connection is only one DC supply is used. This will not limit its application to products. And the disadvantage of single DC source is transformer is needed to add up the output voltages
Separated DC Source Multilevel H-Bridges Inverter
A single-phase structure of an m-level cascaded inverter is illustrated in Figure 2.3. Each separate dc source (SDCS) is connected to a single-phase full-bridge, or H-bridge, inverter. Each inverter level can generate three different voltage outputs, +Vdc, 0, and -Vdc by connecting the dc source to the ac output by different combinations of the four switches, S1, S2, S3, and S4. To obtain +Vdc, switches S1 and S4 are turned on, whereas -Vdc can be obtained by turning on switches S2 and S3. By turning on S1 and S2 or S3 and S4, the output voltage is 0. The ac outputs of each of the different full-bridge inverter levels are connected in series such that the synthesized voltage waveform is the sum of the inverter outputs. The number of output phase voltage levels m in a cascade inverter is defined by m = 2s+1, where s is the number of separate dc sources [1].
Figure 2.3: Single-phase structure of a multilevel cascaded H-bridges inverter
An example phase voltage waveform for a nine-level cascaded inverter and all H-bridge cell output waveforms are shown in Figure 2.4. In this thesis, all dc voltages are assumed to be equal. According to sinusoidal-liked waveform, each H-bridge output waveform must be quarter-symmetric as illustrated by V1 waveform in Figure 2.2. Obviously, no even harmonic components are available in such a waveform. To minimize THD, all switching angles must be numerically calculated.
Figure 2.4: Waveform showing a nine-level output phase voltage and each H-bridge output voltage.
One of the advantages of this structure is the number of possible output voltage levels is more than twice the number of dc sources (m = 2s + 1). The other advantage is the series of H-bridges makes for modularized layout and packaging. This will enable the manufacturing process to be done more quickly and cheaply. On the other hand, the main disadvantage of this topology is that separate dc sources are required for each of the H-bridges. This will limit its application to products that already have multiple SDCSs readily available. The sources are typically provided from a transformer/rectifier arrangement, but may be supplied from batteries, capacitors or photovoltaic arrays.
Single DC source Multilevel H-Bridges Inverter
Referred to Zhong Du1, Leon M. Tolbert, John N. Chiasson, and Burak Ã-zpineci thesis entitled “A Cascade Multilevel Inverter Using a Single DC Source”, a method is presented showing that a cascade multilevel inverter can be implemented using only a single DC power source and capacitors. Without requiring transformers, the scheme proposed allows the use of a single DC power source for examples a battery or a fuel cell stack while the remaining n−1 DC sources being capacitors.
Figure 2.5 shows the Single DC source Multilevel H-Bridges Inverter. The DC source for the first H-bridge (H1) is a DC power source with an output voltage of Vdc, while the DC source for the second H-bridge (H2) is a capacitor voltage to be held at Vdc/2. The output voltage of the first H-bridge is denoted by v1 and the output of the second H-bridge is denoted by v2 so that the output of this two DC source cascade multilevel inverter is v(t) = v1(t)+v2(t). By opening and closing the switches of H1 appropriately, the output voltage v1 can be made equal to −Vdc, 0, or Vdc while the output voltage of H2 can be made equal to −Vdc/2, 0, or Vdc/2 by opening and closing its switches appropriately.
Figure 2.5: Single DC source Multilevel H-Bridges Inverter
IGBTs Versus MOSFETs
The power MOSFET is a device that is voltage- and not current-controlled. MOSFETs have a positive temperature coefficient, stopping thermal runaway. The on-state-resistance has no theoretical limit, hence on-state losses can be far lower. The MOSFET also has a body-drain diode, which is particularly useful in dealing with limited free wheeling currents. All these advantages and the comparative elimination of the current tail soon meant that the MOSFET became the device of choice for power switch designs.
Then in the 1980s the IGBT came along. The IGBT combines the cross between the power MOSFET and a bipolar power transistor (see Figure 2.2). The IGBT has the output switching and conduction characteristics of a bipolar transistor but is voltage-controlled like a MOSFET. In general, this means it has the advantages of high-current handling capability of a bipolar with the ease of control of a MOSFET. However, the IGBT still has the disadvantages of a comparatively large current tail and no body drain diode. Early versions of the IGBT are also prone to latch up, but nowadays, this is pretty well eliminated. Another potential problem with some IGBT types is the negative temperature co-efficient, which could lead to thermal runaway and makes the paralleling of devices hard to effectively achieve. This problem is now being addressed in the latest generations of IGBTs that are based on “non-punch through” (NPT) technology. This technology has the same basic IGBT structure (see Figure 2.6) but is based on bulk-diffused silicon, rather than the epitaxial material that both IGBTs and MOSFETs have historically used [4].
Figure 2.6: NPT IGBT cross section
The comparisons between MOSFETs and IGBTs are as below:
Table 2.1: Comparisons between IGBTs and MOSFETs
IGBTs
MOSFETs
Characteristics
Low duty cycle
Low frequency (<20kHz)
Narrow or small line or load variations
High-voltage applications (>1000V)
>5kW output power
Operation at high junction temperature is allowed (>100°C)
Long duty cycles
High frequency applications (>200kHz)
Wide line or load variations
Low-voltage applications (<250V)
< 500W output power
Applications
Motor control: Frequency <20kHz, short circuit/in-rush limit protection
Uninterruptible power supply (UPS): Constant load, typically low frequency
Welding: High average current, low frequency (<50kHz)
Low-power lighting: Low frequency (<100kHz)
Switch mode power supplies (SMPS): Hard switching above 200kHz
Switch mode power supplies (SMPS): ZVS below 1000 watts
Battery charging [4]
Applications of Inverters
There are many application of inverter available today. Some of the applications are as follows:
DC power source utilization
An inverter converts the DC electricity from sources such as batteries, solar panels, or fuel cells to AC electricity. The electricity can be at any required voltage; in particular it can operate AC equipment designed for mains operation, or rectified to produce DC at any desired voltage.
Grid tie inverters can feed energy back into the distribution network because they produce alternating current with the same wave shape and frequency as supplied by the distribution system. They can also switch off automatically in the event of a blackout.
Micro-inverters convert direct current from individual solar panels into alternating current for the electric grid.
Electric vehicle drives
Adjustable speed motor control inverters are currently used to power the traction motor in some electric locomotives and diesel-electric locomotives as well as some battery electric vehicles and hybrid electric highway vehicles such as the Toyota Prius. Various improvements in inverter technology are being developed specifically for electric vehicle applications. In vehicles with regenerative braking, the inverter also takes power from the motor (now acting as a generator) and stores it in the batteries.
Uninterruptible power supplies
An uninterruptible power supply (UPS) uses batteries and an inverter to supply AC power when main power is not available. When main power is restored, a rectifier is used to supply DC power to recharge the batteries.
Variable-frequency drives
A variable-frequency drive controls the operating speed of an AC motor by controlling the frequency and voltage of the power supplied to the motor. An inverter provides the controlled power. In most cases, the variable-frequency drive includes a rectifier so that DC power for the inverter can be provided from main AC power. Since an inverter is the key component, variable-frequency drives are sometimes called inverter drives or just inverters.
Induction heating
Inverters convert low frequency main AC power to a higher frequency for use in induction heating. To do this, AC power is first rectified to provide DC power. The inverter then changes the DC power to high frequency AC power.
CHAPTER 3
METHODOLOGY
Introduction
This chapter exposes the proposed method of this project to built single phase multilevel H-bridge inverter. This project can be divided into two main parts of study which are software and hardware implementation. For the software part, the software used is PIC24 Compiler that used to do the programming for the microcontroller part and MATLAB to do the simulation of the inverter circuit before implemented it in hardware. In addition, Proteus 7 Professional is also used to simulate the driver circuit before do the hardware. The summary of the project is shown in Figure 3.1.
Software Part Prepared
(Microcontroller)
Hardware Part Prepared
Troubleshooting
Interfacing
Result
Figure 3.1: The project summary
Design of the H-Bridge Inverter System
The H-Bridge inverter system can be divided into three main stages that were constructed. It is consists of:
Microcontroller
Power electronics driver
Power electronics inverter
Each part was treated as a separate functional block system. Figure 3.2 below shows the block diagram of how each stage of the inverter system are organized. Power electronic driver circuit and microcontroller stage is the low voltage side and power electronics inverter circuit is the high voltage side.
DC Voltage Input
AC Output
Power Electronics Inverter Circuit
Microcontroller
Power Electronic Driver Circuit
Figure 3.2: The block diagram of the inverter system
Microcontroller
Microcontroller is a computer-on-a-chip optimised to control electronic devices. The microcontroller chip used for this project is PIC16F877A. In this project, microcontroller is used to develop the triggering signal for the IGBTs and interfacing to the single phase inverter circuit as a control signal for the gate driver.
To implement the microcontroller part, the program for triggering the IGBTs was written in assembly language using the PIC C Compiler. It is written in the text editor or notepad called as source code. It also can be written directly in the PIC C Compiler. Then the file saved is *file.c file. After the program is successfully compiled, the *file.hex file was generated. The hex file was tested by doing the simulation in the Proteus 7 Professional to see the output generated from the program. After got the correct output, the *file.hex file then was uploaded in the PIC16F877A using the PIC programmer. The process of implementing the microcontroller is shown in Figure 3.3. This microcontroller part is the first part that was implemented in hardware.
Figure 3.3: The process of implementing the microcontroller
Power Electronics Driver
A driver is an electronic component used to control another circuit or other component, such as a high-power transistor. Unlike the bipolar transistor, which is current driven, IGBTs, with their insulated gates, are voltage driven. It is allows user to speed up or slow down the switching speeds according to the requirements of the application.
The control circuitry supplied low current driving signals that are referenced to controller-ground. A logic one signal was applied to its gate with respect to its source to turn on an IGBT switch, and this signal needs to restrain sufficient power. These requirements can not be met by the control circuit. Figure 3.5 shows a diagram of how signals need to be applied to IGBT switches for effective operation.
Figure 3.4: Control signals need to be applied to the gate with respect to the source
The driver chose is IR2110 which is a dual driver. The IR2110 High Voltage Bridge Driver is a power integrated circuit that is designed to drive two insulated gate devices. The typical connection of the driver is shown in Figure 3.5. The two channels of the IR2110 are completely independent of one another. The HO output is controlled by the HIN input, and the LO output is controlled by the LIN input. The two inputs of the IR2110 are logically coupled to the shutdown (SD) pin through an AND gate. If HIN and LIN both go high, then the IR2110 will be shut down until one or both inputs go low. This measure helps prevent the catastrophic situation where both Q1 and Q2 turn on at the same time and short circuit the input source. [5]
Figure 3.5: Typical connection of IR2110 High Voltage Bridge Driver
Isolation using the optocoupler
An optocoupler or sometimes refer to as optoisolator allows two circuits to exchange signals yet remain electrically isolated. This is usually accomplished by using light to relay the signal. The standard optocoupler circuits design uses a LED shining on a phototransistor. The signal is applied to the LED, which then shines on the transistor in the ic. The optocoupler circuit is shown in Figure 3.6 below. In this project, the optocoupler is used as the source and destination are at very different voltage levels, where the source is the microprocessor which is operating from 5V DC but it being used to control the IGBTs which is switching at higher voltage. In such situations the link between the two must be an isolated one, to protect the microprocessor from overvoltage damage. The optocouplers can be used with following advantages for driving high side IGBT in any topology:
They can be used to give a very high isolation voltage
Signals from DC to several MHz can be handled by opto-couplers.
They can be easily interfaced to Microcomputers or other controller ICs or any PWM IC.
Figure 3.6: Optocoupler circuit
The circuit of low side voltage which consists of PIC, driver and optocoupler was first constructed in the Proteus 7 Professional to see the output generated to be compared with the hardware results. The circuit is as in Figure 3.7 below.
Figure 3.7: Low side voltage simulation
Power Electronic Inverter
The power electronics inverter part is the main part of the system. This is because this circuit will perform the conversion from DC to AC. The circuit consists of four IGBT that act as a switch, DC source and also the load. Figure 3.8 shows a diagram of the H-Bridge power electronics inverter stage.
Figure 3.8: H-Bridge power electronics inverter stage.
But for this project, the inverter circuit used is the 3-level H-bridge inverter circuit. The circuit was first constructed in the MATLAB as in Figure 3.9 and the simulation of the circuit was done to see the result of simulation.
Figure 3.9: 3-level H-bridge inverter circuit constructed in MATLAB
The block parameter for the IGBTs was set as in Figure 3.10. The switching frequency used for this circuit is 50Hz. So, the period of waveform can be calculated as below:
Switching frequency, f = N / Pf
Fundamental period, Pf = 1 / f fundamental = 1/ 50 = 0.02s
Figue 3.10: The block parameter setting for the IGBTs
In addition, the phase delay or switching times of the IGBTs were also set. Table 3.1 below shows the switching time of the IGBTs. After the simulation was success, the circuit of single 3-level H-bridge inverter was constructed.
Table 3.1: The switching time of the IGBTs
IGBTs
Switching Time
IGBT 1 and IGBT 3
10 ms (Ï€)
IGBT 2 and IGBT 4
0 ms (0 π)
IGBT 5 and IGBT 7
7 ms (Ï€/7)
IGBT 6 and IGBT 8
3 ms (Ï€/3)
For the switch, IRGB10B60KDPBF IGBT was selected for this design. It is very important to choose the correct switches for the inverter circuit because the performance of the design is directly depends on this. This IGBT was chosen because it has ultra fast recovery diode along, it offered benchmark efficiency for motor control and excellent current sharing in parallel operation. In addition the IGBT was selected as they are able withstand the power rating of the inverter. Table 3.2 shows some of the features of the selected IGBT.
Table 3.2: The features of IRGB10B60KDPBF IGBT
Characteristics
Value
Drain to Source Voltage (Vds)
600V
Drain Current (Id)
12A
Rise Time
20ns
Fall Time
23ns
Short Circuit Capability
10μs
Figure 3.4: 3-level H-bridge inverter circuit
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