Contents
Chapter 2 Technical feasibility of the existing power source.
Chapter 3 Investigates several different approaches to carry out the project
Chapter 4 Examines the labour and equipment requirements
Abstract
The following report presents a final chosen project proposal for DC to AC inverter. Three project ideas were put forward and thoroughly discussed. Project proposal one was a Robot controlled by a TV/DVD player remote. Project proposal two was DC to DC Inverter with PWM. Third project proposal was a Robot following black and white lines with microcontroller.
Upon very much thought and discussion it was decided that project proposal two would be the final project. It is an inverter that converts DC power into AC power.
Introduction
In recent years, pollution free green energy demand has increased dramatically due to the change of the world climate.
Pakistan is an undeveloped country, which has an electricity problem due to the vast population and lack of energy resources. Mainly the electricity is provided by the local government via water dams. The dams are not able to produce the required amount of electricity needed for the current population. As the dams are not able to produce the required electricity the government limits the electricity load. It is limited by turning off the electricity for several hours throughout the day.
This has a major effect not only on the residential areas but also on the industrial areas. Because of the limited loads the industrial areas are forced to use generators.
Generators are not good for the environment, as they pollute the air because of the burning fuel. Burning fossil fuels results in environment problems. The emissions of carbon dioxide, methane and many other green house gases have already resulted in global warming problems. Because of the global warming problems, it has given an opportunity for the renewable energy resources and technology to take over the electricity process. Therefore, renewable energy sources play a very crucial role in electric power generation due to its environmentally friendly and pollution free energy.
All over the world the preferred pollution free energy is solar power and wind power, both generate DC power. Home appliances and many industrial machines need AC power. Due to the difference between generated DC power and the required AC power a device is needed to change the generated DC power into AC power. The device needed is called DC to AC Inverter.
The inverter requires DC batteries or any other DC source, which supplies DC power to the inverter which then the inverter will convert the DC power into AC power allowing the home appliances and the industrial machines to be operated on the AC power using the inverter.
The feasibility study of the project has been outlines in several chapters within the report. Chapter one is a Project meeting between the stakeholder of the project and a manager of the wheat industry in Pakistan. Chapter two examines the technical feasibility of the existing power source. Chapter three investigates several different approaches to carry out the project. Chapter four examines the labour and equipment requirements needed. Chapter five considers the initial costing and profit margins. Chapter six looks at the initial project planning with the inclusion of a Gannt Chart. Chapter seven includes project planning amendments and a revised Gannt Chart. Chapter eight examines project issues and problems. Chapter nine examines the exclusions of the generator. Chapter 10 examines implementing and testing of the project. Chapter eleven discusses about generator issues. Chapter twelve examines the health and safety considerations for the wheat industry team.
The feasibility study will investigate several options to upgrade from a generator. A method of removing the generator and installing batteries as a source of power, so the DC to AC inverter changes to AC power for the industrial machines.
Once the full installation has been completed, the DC to AC inverter will be commissioned and tested.
Chapter 1 Project Meeting
At the outset of the project, meeting has been scheduled between the Chief Engineer, Manger of Kings Flour Mill, Health and Safety Manager and the Accountant, this meeting will take place in Pakistan. The meeting is an opportunity to discuss how the project should be implemented and what the Manager of KFM needs and the expectations. The meeting also presents an opportunity to discuss the cost implications and the scope of works involved.
Figure 1.1 illustrates the scope of the participants of the meeting.
Hashsham Abdul Aziz Abdul Hakim
Chief Engineer Kings Flour Mill Manager
Mrs Munawar Akhtar Shahjeel Khan
Accountant Health and Safety Manager
Figure 1.1 Architecture of the representatives at the project meeting
Hashsham Abdul Aziz is the Chief Engineer (CE) for the project and is responsible for all work carried out on site.
Shahjeel Khan is the Health and Safety Manager (HSM) for Kings Flour Mill (KFM) and is responsible for ensuring the compliance of Health and Safety regulations. The CE and the HSM shall liaise with each other to manage the risks associated with installing the project.
Abdul Hakim is the Manager of Kings Flour Mill and is responsible for all changes made. The CE and the HSM will liaise with the manager to discuss problems, issues or any other installation requirements to make sure that the manager is kept up to date in the project at every stage of the installation.
Mrs Munawar Akhtar is the Accountant for KFM and is responsible for making sure that the budget of the project is within the company’s agreed budget. Also, if the project goes beyond the budget the company has all rights to abort the project at any time. The CE and the Manager will liaise with the accountant to ensure that all purchases are within the budget. The accountant is also responsible for releasing the funds to the CE upon successful project installation.
The informal minutes of the meeting are a list of the most important aspects discussed between the participants of the meeting. The bullet point for the informal meeting streamline the way the project moves forward.
- HSM points of view
- Concerns regarding the impact of removing the generator to install the project. The HSM would like a proposal of how KFM will still be in production during the maintenance and installation of the project.
- Proposal requested that works commences outside of the factory production hours, when there are no productions being carries out. The installation must not impact on the flour mill’s production.
- Risk assessments are to be submitted by KFM before work can proceed.
- Electrical equipment is to be inspected and tested prior to use in the mill.
- CE points of view
- Working at weekends is the favourable option.
- Large area required for the full installation of the project.
- Preferred to be in a secluded area of the factory, in case of any problems it wont effect the workers and the mill.
- Managers point of view
- Work should be safe, efficient and be completed as soon as possible.
- Requested that work should commence outside of factory production hours, to prevent disruption to the workers.
- Requested project should be cost effective.
- Accountants point of view
- Requested that the project stays within the budget agreed.
- Requested that any items purchased should be shown directly to the accountant for proof of purchase.
- Requested that items are guaranteed or under a valid warranty, in case of any returns or damages.
Chapter 2 Technical feasibility of the existing power source.
A thorough understanding of the current power sources is required to understand the implications of installing the project. The main aspect of the technical feasibility study is to decide if it is technically possible to install the project. The feasibility will examine how the existing power sources function and what technologies are utilised.
Each power source has been investigated as part of the feasibility study.
The investigations have been separated into two categories.
- A generator is used during the limited hours of electricity that the government supplies. Can be used for as long as the fuel is available for.
- Government electricity is limited and very unreliable as it can turn off at any given time without any prior notice.
The current power source from the government means that KFM has automatic switches placed on the power lines so that it detects when the government electricity is powered on then the generator will automatically power off and when the government electricity is powered off then the automatic switches will power on the generator to provide power. This results in the generator being powered on and off throughout the day which reduces the life of the generator.
- Three phase fuse board
The three-phase fuse board is distributed into single phases in different areas of the factory.
Phase one is supplied to the wheat cutting machine area which has four AC motors installed. The motors require 220V, 8A input so the four motors require a total of 220V, 32A input supply or 7040W.
Phase two is supplied to the separation wheat machine area which has two AC motors installed. The motors require 220V, 6A input so the total for the two motors require 220V, 12A input supply or 2640W.
Phase three is supplied to the wheat packing area which requires one AC motor installed. The motor runs on 220V, 6A input power or 1320W.
The total power required for the factory is more than 11,000W.
Figure 2.1 has been included below as a reference to the information in this section.
Figure 2.1 Diagram of the three-phase layout
Chapter 3 Investigates several different approaches to carry out the project
The CE and the Manager discussed several methods that can be considered to remove the generator and the government power supply.
There are two main factors that affect the costing and efficiency of the removal of the existing power supply.
- Keeping the existing power supply to operate some of the machines and installing the project to operate the remaining machines. This will reduce the costs of the project. However, the efficiency will be reduced due to a part installation.
- Removal of the existing power supply by installing the full project to operate all the machines. This will increase the installation costs, but the efficiency will be increased.
3.1 Methods of DC to AC Inverter
There are a few methods discussed which are listed below:
3.1.1 Method 1 – Batteries are required for this method. The batteries will then be connected to a DC to DC Boost Converter which will lead to a DC to DC Buck Converter and then connect to an inverter to change the Sine Waves to AC power. This method is called DC to AC Inverter Transformerless.
3.1.2 Method 2 – Batteries are also required for this method. The batteries will be connected to DC to AC Inverter with Transformer.
3.2 Ways the batteries can be charged
The CE and the Manager discussed a possible two ways of charging the batteries.
3.2.1 Charge with Solar
Install the solar panels on the roof of the factory and connect the voltage regulator to the solar panels, then connect the batteries to the voltage regulator to charge the batteries.
3.2.2 Charge with DC Motor
Install the DC motor which will be connected to the alternator, then the alternator will connect to the batteries to give charge to the batteries.
3.3 First initial design
The first initial design is based on method one as mentioned in 3.1.1.
As illustrated in the simulation circuit below, there is a 9V power supply but on the output of the circuit there is a voltmeter, which is measuring the climbing voltage of this circuit. As shown in the circuit the voltage is at 68V from a 9V power supply. This could be a 9V battery or any other type of 9V DC power supply.
Before this circuit is explained any further it is advised that because the circuit is now using higher voltages that when it is copied to a circuit board that the installer must stay safe, as high voltages like this can produce quite nasty shocks and it only takes 20mA to stop the human heart. So, make sure everything is safe and do not touch any high voltage contacts if planning on using any high circuitry.
The circuit below represents a boost converter otherwise known as a step-up converter. It is a type of switch mode power supply because it uses a type of switch in this case it is to turn up part of the circuit on and off at a fast speed.
Switching transistors are the main component in any type of switch mode power supply. Now to explain how this circuit is taking 9V and stepping it up to a much higher voltage. This circuit is running a lot slower than the eye can see, this is because every circuit cannot process the speed of which everything happens because of the lack of operating power. As it is shown here for every one second it is only viewed three hundred microseconds of time, which is a very tiny portion of a second. This means that in real life this voltage would climb almost instantly. If the circuit is slowed down even more then stopped and restarted it is now restarted at 0V and as it is shown it climbs very slowly, closing in at 25V
If current passes through an inductor it creates a magnetic field when the voltage is changed the voltage passes through an inductor, the inductors magnetic field will start to collapse and thus inducing current through the inductor as it creates a high voltage spike as the magnetic field collapses. This means that this circuit is taking advantage of that very property of an inductor to step-up voltage.
There is a MOSFET transistor, so imagine that this is a normal on and off switch but can be switch on and off very fast. It can be switched on and off hundreds of thousands of times a second. It can be electronically controlled to turn it on an off with this power supply installed. Basically, it is supplying power to the gate of this transistor which just turns a switch on and off every one hundred microseconds. When the switch is turned on current can flow from the 9V power supply through the inductor, through the switch and back to the 9V power supply. As the switch is turned on the current starts to flow through the inductor, a magnetic field then builds up in the inductor. Now when the switch is turned off current can no longer flow from the inductor through to the switch because the magnetic field in this inductor needs to collapse and induce current through the circuit. As current can no longer flow through the switch when it is off, the current quickly flows through the rest of the circuit. It is also visible to see glimpses of it as the switch is open or off current quickly shoots through this part of the circuit.
There is a capacitor in parallel so at any time the current shoots through part of the circuit because of the collapsing magnetic field of the inductor creates a high voltage spike it gets pushed through the diode and gets stored into the capacitor. Due to having the diode this stops current flowing backwards from the capacitor through to the switch when its on and back to the power supply. Every time when there is a voltage spike from the inductor this adds to the voltage of the capacitor and therefore steps up the voltage, so the voltage that is shown is the voltage of the capacitor. Every time the switch is open or off there is a voltage spike from the inductor flowing through to the capacitor. This circuit can step-up to around 100 V. Now as the voltage is supplied to the inductor, the inductors polarity on the left side is positive and on the right side is negative, when the switch is off the polarity of the inductor changes instead negative is now on the left side and positive is now on the right side, which enables the current to flow to the rest of the circuit because positive is connected to positive.
3.4 What is an inductor and the basic principles of inductance
Inductor Toroid
Figure 3.4.1 – is a less common Inductor Figure 3.4.2 is a very common Inductor
Inductors are found in almost all electronic devices and they come in all kinds of shapes and colours.
The inductor illustrated in figure 3.4.1 is a traditional inductor. Nowadays the most common used inductor is a ring-shaped inductor, known as a Toroid, which is commonly used in switch mode power supply (SMPS) and DC to AC Inverters. Thorus means doughnut hence the ring shape.
To make an inductor like the toroid a magnetic core is needed. The green ring in figure 3.4.3 is the magnetic core, then copper wire is needed to be wound around the magnetic core.
Figure 3.4.3- Green magnetic ring
After the copper wire has covered the full core the inductor is ready for use.
If the copper wire is removed from the inductors magnetic core and a magnet is placed near the copper wire, then the wire is not attracted to the magnet because the copper is non-ferrous magnetic. However, if the magnet is placed over the core then the core is attracted to the magnet because the core is ferrous magnetic. This is called a magnetic core in electronics.
Different types of inductors
It has been described above that the magnetic core inside the inductor plays a major role when it comes to the functionality of the DC to AC Inverter.
Comparing an inductor and a transformer. The transformer is different as it has a different material. The inductor has ferrite and the transformer is made with laminated steel sheets which have silicon in it. The transformer is very much cheaper to use than the inductor.
The ferrite in the inductor contains very rare earth metals which can be expensive. For example:
- Cobalt
- Neodymium
- Plutonium
- Rhodium
The bigger the transformer the lower the characteristic frequency of oscillations. This means that this can be fed 50-60Hz of Sine Waves directly in the primary of such transformer. So, this means that a smaller transformer is not needed. If a smaller transformer was needed, then a more complicated circuit would also be needed. The steel sheets contain silicon which modifies the magnetic properties of the core. Also, steel is quite cheap. A magnetic core of an inductor must be made from a special magnetic material which makes this expensive to use.
Figure 3.4.8 different inductors and the uses
In the illustration above is some examples of common inductors that are used for various applications.
Below, all the above inductors have been described and explained what the uses are for each inductor.
Black box shaped inductors are most of the time used for low frequency applications because they are known as generic cores.
Green and red inductors are intended to be used for SMPS or DC to AC Inverters up to about 500 kHz.
Yellow and grey inductors are also used in SMPS and DC to AC Inverters but up to about 5 MHz.
Purple and blue inductors are the ones that are used for high frequency applications.
All are expensive and are not easy come by for a private user.
DC Boosting using Maxium 1771.
This is a boost converter it takes an input of 12V DC and can produce more than 250V output DC. It works with a Maxium 1771 chip. There is a 100
μHinductor. Also, a MOSFET transistor IRF740 is used. If IRF740 is not available, then it is recommended to use IRF634 which is like the IRF740. This MOSFET transistor is the voltage regulator. It is connected to the input side of the DC booster.
A 5K Ω variable resistor is used to regulate the voltage. On the input side there is two to three series of capacitors which have values of 100V and 0.1 – 0.4
μF. Two small resistors are connected to the emitter of the MOSFET and the other side is connected to the GND of the Max1771 chip pin 7. Pin 7 in the chip is the GND. Pin 1 of the chip is connected to the base of the MOSFET, pin 2 is connected to the 10 Ω resistor and this resistor is connected to the series of capacitors. Pin 3 is connected to the EMI filter capacitor. Pin 4, 5 and 6 are all connected to GND. Pin 8 is not connected, due to this pin being the sensing pin.
Two more resistors are in parallel which are connected to the MOSFET which have small values. The values are 0.1 Ω each, they have been put in parallel to give a combined value of 0.5 Ω.
Situated after the MOSFET there are two out capacitors, one is an electrolytic capacitor and the other one is a ceramic capacitor. Both have a value of 250V and _ and _
μF.
Between the MOSFET and the output capacitor there is a diode, which is very important. The value of this diode is UF-4004. If the UF-4004 diode is not available, then it is recommended that the alternative of 1N4007 or 1N4004 are used.
In figure ____ 1N4004 diode is used. The 1N4004 can go up to 400V and the 1N4007 can go up to 1000V.
The first time that the circuit was tested it gave an output of 40V when the input was 12V and 13A.
Figure _____ illustrates the tested results.
The 40V was static before the installation of EM capacitor. After installing the EM capacitor, the variable 5Ω resistor then began to work as it should. When the resistor was turned to below 5Ω it started to produce less voltage and when the resistor was to up to full 5Ω it started to produce 40V.
When the 5Ω variable resistor was not working the MOSFET became very hot, so for a good boost converter the variable resistor is recommended to control the voltage.
The 5Ω resistor has three pins. The first pin is the wiper pin which is connected to the EM capacitor, the second pin is connected to the GND with 450Ω resistor and the third pin is connected to the positive side of the output capacitor with a resistor value of 1MΩ.
The first time when the 5Ω resistor was not working even when the value was changed up and down, it was due to the inductor producing too much EMI interference. When the inductor pulses, it makes interferences because the resistor wiper pin connected directly to pin 3 of the chip without an EMI filter capacitor. Due to the interference the chip was picking up signals, but it did not understand what to do with the signals. The EMI filter or EMI capacitor used in the circuit is manufactured by Murata. It has three legs, leg 1 or leg 3 can be connected to pin 3 of the chip and the other leg can be connected to the upper resistor pin. The third leg which is the middle leg is connected to the GND of the chip. The filter capacitor filters the interferences and sends them to the GND.
Below is an illustration of how the circuit design is installed.
Without load the value tested is 40V, with load the value tested is 38V and the load is with a 10kΩ resistor.
The important parts of the circuit or the basic theory of this circuit is the same as described above.
The main components of the circuit are listed below:
- Inductor
- Diode
- MOSFET
- Output Capacitor
All other remaining parts are for other purposes, for example: using a chip to allow the MOSFET to work as an on and off switch. Some of the other components are to help the stability of the chip, so that the chip does not get any interference, over heat or burn out, and to ensure that the MOSFET turns on and off as it is set to do.
12V voltage is coming through the input and going straight to the inductor which creates around it a magnetic field. Then when the MOSFET is on the inductor starts to save the energy inside it and when the MOSFET is off the energy saved inside the inductor is then released and goes to the capacitor. 12V is already available inside the capacitor so the stored voltage that comes from the inductor increases the voltage inside the capacitor and when the MOSFET is on the capacitor starts to discharge and goes to the GND.
The diode is to prevent the discharge voltage from the capacitor to go back to the inductor.
This process continuously repeats.
To drive this circuit using another chip for example: NE555 timer it will work the same way as the Max1771. But the efficiency of the Max1771 is more than that of the NE555 timer. The purpose of the chip is to drive how many cycles the MOSFET needs to open and close.
The result of the above circuit is 40V which should go to 250V, but due to the input voltage being 12V with 13A it is not increasing to 250V. If the input was 12V with 50-60A then it can increase to the output voltage up to 250V.
In this circuit if the output capacitor is replaced with a 400V capacitor with any capacitance the output voltage can increase up to 400V, but the frequency must be considered as the higher the frequency the higher the voltage is gained at the output. If the higher frequency is needed, then the higher the MOSFETs are needed.
DC to AC Inverter using boost converter and H Bridge
A boost power converter is recommended instead of a transformer to achieve low THD as well as high efficiency. The boost power converter converts a large scale of voltage like the grid value, but a single stage boost converter requires a high duty cycle which is inconvenient for MOSFETs switching. Therefore, the DC- DC power conversion is employed through a dual stage boost converter to obtain suitable duty cycle for MOSFETs switching. Likewise a voltage divider circuit is also used to synchronize the output frequency with the grid. Instead of using a conventional low-pass LC filter, a T-LCL immittance converter is employed in the proposed inverter, which not only suppresses the harmonics contained in the inverter output but also maintains a constant output current for any type of load, and thus stabilises the inverter output.
The proposed inverter configuration consists of four main parts. They are:
- Deep Cycle Batteries
- DC to DC boost Converter in Dual Stage
- DC to AC Inverter with H Bridge
- T-LCL Immittance Converter for filter output
Deep Cycle Batteries
A true deep cycle battery is a battery that is designed and manufactured to store a large quantity of energy that discharges deeply and recharges repeatedly. A deep cycle battery design ensures that a steady amount of power is being delivered to applications over a long period of time without interruption or failure. A deep cycle battery is constructed with thicker plates and a denser active material ratio. Because of these specific construction features deep cycle batteries achieve greater cycling capacity, greater run times and an increased life.
Here is a list of some applications that require a deep cycle battery.
- Floor Machines
- Electric Vehicles
- Materials Handing Vehicle
- Renewable Energy Sources
- Aerial Work Platforms
- Commercial Transit
- RV and Marine Vehicles
- Mobility Vehicles
- Telecom UPS
- Security Electronics
All the applications above require batteries that offer greater run times, deeper discharging, a longer life and the reliable and constant delivery of power over a long period of time.
First there are two types of deep cycle batteries on the market which are flooded lead-acid batteries and a valve regulated lead-acid batteries, also known as VRLA maintenance free batteries.
There are three types of VRLA batteries which are:
- Gel
- AGM
- EV traction Dry Cell
Comparing performances of the batteries:
| Performance, Value and ROI | Flooded | Sealed Gel | AGM | EV Traction Dry Cell |
| Longer Run Times | Y | N | N | Y |
| Longer Duration Discharges | N | N | N | Y |
| Longer Life | Y | N | N | Y |
| Good Charge Retention | N | Y | Y | Y |
| Maintenance | Often Inspection, cleaning and watering | Periodic Inspection | Periodic Inspection | Periodic Inspection |
| Short Charging Time | N | N | Y | Y |
| Initial Purchase Price | £ | ££ | £££ | £££ |
| Lower Cost to Own | N | N | N | Y |
Y = Yes
N = No
£ = Less Purchase Price
££ = Average Purchase Price
£££ = Higher Purchase Price
Flooded and EV Traction Dry Cell batteries are designed for longer run times, whereas the Sealed Gel and AGM batteries are not. The EV Traction Dry Cell battery is the only technology that offers long duration discharges. Both the Flooded and the EV Traction Dry Cell batteries offer a longer life than a Sealed Gel and AGM battery. Because the Gel, AGM and EV Traction Dry Cell battery are all VRLA they have a greater charge retention than the Flooded Lead-acid battery. The Flooded battery has the greatest time and financial investment when it comes to maintenance. Operators are required to perform regular maintenance which involves inspection, cleaning and watering.
The Gel, AGM and the EV Traction Dry Cell are maintenance free which requires no maintenance other than a periodic inspection. The AGM and the EV Traction Dry Cell battery have shorter charging time whereas Flooded and Gel batteries require exact and precise charging. The Gel battery requires a slower recharging voltage rate and time to prevent battery damage and over charging. The EV Traction Dry Cell battery and the AGM batteries have a higher initial purchase price than the other Flooded and Gel batteries. But look at the maintenance requirements in the table above, charging practices, replacement blocks, repairs and performance, the EV Traction Dry Cell battery offers the best value and return on investment for use in industrial applications that require regular deep cycling and discharging. A true deep cycle battery will reduce the risk of battery failure and applications not reaching their intended performance outcomes.
Battery size calculation and specification
Batteries are rated in ampere-hour (Ah) and the sizing depends on the what is needed: how long the inverter needs to work for relative to the loads that is provided.
The formula below shows the required battery size.
Battery size =
load Volt-Ampere x duration (hour)battery voltageVolt x discharge capacity
Discharge capacity arises from the fact that one does not use complete battery capacity. Only a certain percentage (discharge capacity) of the battery would be used. A deep cycle battery can be discharge up to 80 % (actual value depends on the low voltage disconnect) of its capacity.
Below is a table of battery parameters.
| Parameter | Values |
| Manufacturer | Trojan |
| Model | |
| Current | |
| Voltage | 12V |
| Power |
Design of DC Boost
This section describes the design of a dual stage DC to DC boost converter for converting battery voltage to a fixed high level regulated voltage which is the same as the grid value of 220V. A dual stage boost converter is proposed since the duty cycle of a single stage boost converter would be large approximately 90%, which is not convenient for MOSFET switching. Dual stage converter provides a more symmetrical duty cycle and reduces the voltage stress on the MOSFETs. The first stage of the booster is from 12V to 86V and the second stage of the booster is from 86V to 220V.
The estimated design specification of the first and second stage of the boost converter are listed in table two and three.
| Symbol | Description | Value |
| Vin | Input voltage | 12V |
| Vout | Estimated Output Voltage | 86V |
| fs | Minimum Switch Frequency | 20 kHz |
| ILMax | Maximum Inductor Current | 260A |
| ∆IL | Estimated Inductor Current Ripple | 4.55A |
| ∆Vout | Estimated Voltage Ripple | 44mV |
| Iout | Current Output | 4.3A |
| Symbol | Description | Value |
| Vin | Input voltage | 86V |
| Vout | Estimated Output Voltage | 220V |
| fs | Minimum Switch Frequency | 21 kHz |
| ILMax | Maximum Inductor Current | 230A |
| ∆IL | Estimated Inductor Current Ripple | 60A |
| ∆Vout | Estimated Voltage Ripple | 0.35V |
| Iout | Current Output | 10.4A |
Duty Cycle
Maximum duty cycle for the first stage is,
D1,boost=
1VinVout=
11286= 0.72
Maximum duty cycle for the second stage is,
D2,boost
=
1VinVout=
186312= 0.72
Inductor Selection
In the boost converter, a smoothing inductor is used in series with the input voltage to limit the current ripple of converter. In conventional processes, inductor value is usually chosen from recommended data sheets. But here voltage conversion is occurred at a large scale 24V DC to 86V DC and hence no inductor value is available in the data sheets for this voltage conversion range. Therefore, the following equation is an estimate for choosing the right output value for the first stage boost converter.
L1,boost=
Vin(Vout-Vin)∆IL X fs X Vout=
24 X(86-24)4.55 X 20000 X 86 ≈190µH
Similarly, the same method can be used to choose the inductor value for the second stage boost converter.
L2,boost=
Vin(Vout-Vin)∆IL X fs X Vout=
86 X(312-86)60 X 21000 X 312 ≈50µH
Capacitor Selection
The following equation is used to adjust the output capacitor value for a desired output voltage ripple. The estimated capacitor value for the first stage is:
C1,boost=
Iout X Dfs X ∆Vout=
4.3 X 0.7220000 X .044 ≈3.5mF
And the estimated capacitor value for the second stage is:
C2,boost=
Iout X Dfs X ∆Vout=
10.4 X 0.7221000 X .35 ≈1mF
The designed 12-312V DC to DC Boost Converter
The power converter which consists of two boost converters and two PWM gate pulses to drive the MOSFETs is shown in figure____. The output of the designed boost converter simulates using Multisim is shown in Figure____, which indicates that the output of the first stage is 86V and the second stage is 312V DC and which is later converted to 312V AC using an H-Bridge inverter.
Power Circuit
The proposed inverter circuit has employed four members of MOSFETs for switching purposes. The circuit is employed a DC to DC boost power converter, AC to DC voltage divider and H-Bridge inverter as shown in figure ____.
A DC to DC boost converter, the design of which is shown in section_____, is used to step up the unregulated input voltage from 24V to regulated 312V, which is finally converted to 312V pure AC applicable to grid by using inverter.
Switching Circuit
An alternating current periodically reverses its direction, for this reason the average value of an alternating current over a cycle will be zero. Before proceeding to sine wave production, let’s see how a square wave alternating current is produced. In face the old type inverters used to produce simple square waves as its output. Let’s build an interesting circuit as shown in figure___, with four switches and one input voltage. This circuit is known as full-bridge or H-bridge inverter. The output is drawn between points A and B. To make this circuit analysis easier let’s replace this actual load with a hypothetical load, take notice of the current flow when switches S1 and S4 are on and S2 and S3 are off. Now just do the reverse and observe the current flow, the current flow is the opposite in this case as is the output voltage across the load. This is the basic technique that produces a square wave alternating current.
The frequency of AC supply available is 50Hz or 60Hz, this depends upon the country. The countries that use 50Hz have 220 – 240V AC and the countries that use 60Hz have 110 – 120V AC. For example, if a country that uses 50Hz was chosen this means that the switches would need to be turned on one hundred times in a second. If a country that uses 60Hz was chosen then the switches would need to be turned on one hundred and twenty time in a second, which is not possible whether manually or by using mechanical switches. That is why semiconductor switches such as MOSFETs have been introduced for this purpose, MOSFETs can turn on and turn off thousands of times per second with the help of control signals. Transistors can turn on and turn off very easily. The square wave output is a high approximation of sine wave output.
A technique called Pulse Width Modulation is used for this purpose. The logic of PWM is simple, generate the DC voltage in form of pulses of different widths in regions where higher amplitudes are needed, it will generate pulses of larger width. The pulses of the sine wave look like this, as illustrated in figure_____.
Now here is the tricky part, what will happen if the pulses are averaged in a small-time interval. It is surprising to see that the shape of the averaged pulses looks very similar to the sine curve. The fine the pulse is used the better shape the sine curve will be. Now the real question is how to make theses pulses and how are they averaged in a practical way. Let’s see how they are implemented in an actual inverter. For this purpose, two comparators are used, which compare sine waves with triangular waves. One comparator uses a normal sine wave and the other comparator uses an inverted sine wave. The first comparator controls S1 and S2 switches and the second comparator controls S3 and S4 switches. S1 and S2 switches determine voltage level at point A and the other two switches determine voltage level at point B. As shown, one branch of comparator output is filtered with a logic not gate, this will make sure that when S1 is on S2 will be off and vice versa. This also means that S1 and S2 can never be turned on at the same time which will cause the DC circuit to short-circuit. Turning on S1 gives cell voltage at point A and turning on S2 gives zero voltage at the same point. This is the same case for point B. The switching logic for PWM is simple, when the sine wave value is more than the triangular wave, comparator produces one signal otherwise zero signal. Now observe voltage variation. At first comparator, according to this logic control signal of one turns on the MOSFET. The voltage pulses produced at point A are shown. Apply the same switching logic and observe the voltage pulses generated at point B. Since output voltage is being withdrawn between point A and B the net voltage will be the difference between A and B. This is the exact pulse train that is needed to create this sine wave. The finer the triangular wave the more accurate the pulse train will be. Now the next question is how to practically implement the averaging to make it exactly sinusoidal energy storage elements. Such as inductors and capacitors are used to smooth the power flow, they are called passive filters. Inductors are used to smoothen the current and capacitors are used to smoothen the voltage. With an inverter bridge, a good PWM technique and a passive filter its possible to generate sinusoidal voltage and operate all appliances without any fuss.
Filter Circuit
To eliminate harmonics from the inverter output, a filter circuit is employed. In conventional inverters, an LC filter is used. This circuit consists of two inductors
L1and
L2as well as a capacitor, C in T shape. The equations of the output current of the filter are found as ___.
I2≅ F1Z0[1-1Z2QZo] (a)
Where,
V1is the input voltage,
L2is the load impedance, Q is the quality factor
Q= ωLr (b)
Here
ω=
2πfis the angular frequency,
ris the internal resistance of the inductor and
Z0is the characteristic impedance determined by the filter components,
Land
C:
Z0=LC (c)
When the internal resistance of the inductor is negligible or zero, the quality factor becomes infinity. Under this condition, the second term becomes zero, giving the ideal condition:
I2≅V1Z0 (d)
From Eq. (d) it is observed that the output of T-LCL filter is independent of load. Therefore, in the proposed inverter, a T-LCL immittance converter is applied as a filter circuit because it is not only capable in reduction of harmonic but also helpful to maintain constant current at the load.
The value of
Cand
Lof T-LCL filter (considering Butterworth type) is calculated using the condition of cut-off frequency of low pass filter i.e
Z0=XC=12πfCC (e)
In the proposed design, the cut-off frequency,
fC= 50Hz and characteristic impedance is assumed as 20Ω. Therefore, the value of
Cand
Lis calculated using Eqs. (5) and (3) as,
C=12 X π X fC X Z0= 12 X π X 50 X 20 ≈0.159mF
L= CZ02=0.159 X 10-3 X (20)2≈63.60mH
