Different power-factor rectification methods are reviewed, every bit good as the dorsum land to the power-factor. Problem is originating in modern electrical distribution systems due to the connexion of quickly increasing Numberss of non-linear electronic tonss. The basic rules of harmonic coevals and restriction in power systems are first discussed. The chief portion presents a critical reappraisal of normally used power-factor rectification techniques that have been identified in a literature reappraisal, and highlights the advantages and disadvantages of these techniques. After the analysis of methods and their working rules, the development of the most promising systems such as the boost-type PFC convertors is considered. Finally, a undertaking program is proposed for the following stage of the thesis work. This will affect look intoing the operation, dynamic control and public presentation of the most promising systems by carry oning a theoretical survey and puting up and running a figure of simulation theoretical accounts utilizing the MATLAB/SIMULINK package tools.
Key-words: Power factor rectification, harmonic extenuation, PFC convertors
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List of Abbreviations and Principle Symbols
AC Alternating Current
APF Active Power Filter
CCM Continuous Conduction Mode
DC Direct Current
DCM Discontinuous Conduction Mode
DF Distortion Factor
FFT Fast-Fourier analysis
IGBT Insulated Gate Bipolar Transistor
PF Power Factor
PFC Power Factor Correction
PWM Pulse Width Modulation
RMS Root Mean Square
THD Total Harmonic Distortion
TDD Total Demand Distortion
H Harmonic contents
RMS value of the line-current cardinal constituent
RMS value of the line-current harmonic constituents
Entire RMS value of the line-current
It is now clearly seeable from power systems diaries and general literatures that power-factor rectification is now an of import research subject in the power systems country. As non-linear power electronic systems are progressively being connected to power systems in greater measures every bit good as capacities for such applications as power quality control, adjustable velocity thrusts, uninterruptible power supplies, renewable energy-source interfacing, and so on [ 1 ] [ 2 ] . The power quality regulators of those systems are extremely concerned now, because some of their drawbacks, such as harmonics coevals and decreased power factor can botch their advantages [ 3 ] . Power electronic systems are effectual because their high efficiency and quickly adjustable end product. However, when processing and commanding the input electric energy suited for users [ 4 ] , power electronic systems frequently operate at a low power-factor, and that may do serious jobs to power system operators by cut downing distribution constituent RMS current capacity and to other users on the same web by falsifying the sinusoidal supply electromotive force seen by other user connected at the same point of common yoke as a heavy electronic or power electronic load.. Of all power line perturbations, harmonics are likely the most serious one for power users because they exist under steady province conditions.
This literature reappraisal considers harmonic coevals anticipation of power electronic systems and examines the effectivity of harmonic extenuation methods. The boost-type power factor rectification convertors will be taken as the nucleus power factor rectification method for future research. The bing publications originating from research in this country and their decisions have set a good foundation for this study. Consequences in this study will be based on a theoretical survey and simulation surveies utilizing package MATLAB/SIMULINK power-factor rectification system theoretical accounts which be developed.
This subdivision provides treatment on the cardinal rules of power-factor rectification, including definitions of power-factor footings and a consideration of the common criterions which affect how harmonics controlled in power system. Besides, the harmonic coevals anticipation of ideal power electronic systems is discussed at the terminal of this subdivision
2.1 Important definitions and aim of power-factor rectification
The power factor ( PF ) is the ratio of the existent power to the evident power [ 5 ] and gives a step of AC supply use on how efficient that the energy is supplied and can be converted into effectual work end product. The definition of power factor is as shown below:
( 2.1 )
In the definition, the value of the power factor is ever between 0 and 1, and can be either inductive or capacitive. That means mean power is ever lower than evident power. The ground is harmonic constituents and phase-displacement angle, .
Hence, the power factor equivalent can be described as below:
( 2.2 )
is termed the ( current ) deformation factor ( DF ) and stand for the harmonic constituents in the current and comparative to beckon form [ 6 ] . DF is defined as the ratio of the cardinal current constituent to the RMS current value [ 4 ] .
is termed the supplanting factor and defined as the current and electromotive force wave form stage angle [ 6 ] . Displacement factor has unity value for in-phase current and electromotive force. The addition of displacement angle will do larger reactive current in the power system [ 4 ] .
Hence, the aim of power-factor rectification is to diminish the current deformation or harmonic content and increase the displacement factor or convey the current in stage with the electromotive force. The closer power factor is to the integrity value, the higher efficiency and lower energy loss. And the power system will run at a lower supply electromotive force.
Another normally used index for mensurating the harmonic content of a wave form applied for current deformation degree is entire harmonic deformation, THD. THD is the deformation current as a per centum of the cardinal current. The equation of THD is given by:
or ( 2.3 )
In AC supply uses, power factor, , can be expressed in footings of THD and the displacement factor:
( 2.4 )
With these equations, it is easy to see that high THD leads to low power factor and even damaging of the power web. THD and power factor will be used together in the undermentioned work as of import index in mensurating public presentation of the harmonic extenuation techniques.
2.2 Effectss and restriction of harmonic deformation on power system
In any power transition procedure, to acquire high efficiency and low power loss are of import for two grounds: the cost of the otiose energy and the trouble in taking the heat generated due to disperse energy [ 4 ] . The public presentation of power end product efficient is defined by several factors. The power factor and harmonic deformation are the most of import 1s.
Mentions [ 8 ] [ 9 ] show the chief issues of harmonics within the power system include the possibility of them exciting series and parallel resonances which cause a farther addition of harmonic degrees, low efficiency caused in coevals, transmittal, and use of electric energy, increasing thermic losingss in the electrical constituents and shortening their utile life and doing malfunction of motors and other constituents in the power system.
Those effects can be divided into three general classs: “ Thermal emphasis, Insulation emphasis and Load break ” [ 10 ] . Those represent effects on increasing equipment losingss and thermic losingss, increased value of current drawn from the power system and insularity emphasis and failure to action and malfunction of some electrical devices and systems.
The IEEE Standard 519-1992 recommended harmonic current bounds with an extra factor, TDD. This is really same as THD except the deformation factor is expressed by burden current alternatively of cardinal current magnitude [ 11 ] . Hence, the equation of TDD is given by:
( 2.5 )
Therefore, IEEE Standard 519-1992 restriction for harmonic current in power system expressed with TDD is shown below:
Maximum harmonic current deformation in per centum of
Individual harmonic order ( Odd harmonics )
& lt ; 20
20 & lt ; 50
50 & lt ; 100
100 & lt ; 1000
& gt ; 1000
Even harmonics are limited to 25 % of the uneven harmonics bounds above.
Table 2.1 IEEE 519-1992 Standard for harmonic current bounds [ 12 ] .
Besides, there are restrictions for power system harmonic electromotive force and power factor ordinance, like IEC 61000-3-2 criterion. The methods for power factor rectification should non do perturbations for other facets of public presentation.
2.3 Harmonics coevals in power electronic systems
Power electronic systems may of course run at low power-factor due to big harmonic coevals and stage shifting in controlled devices like controlled rectifiers. Understanding features of the harmonic current is indispensable for harmonic extenuation research. Based on the signifier on the two sides, convertors can be divided into four classs [ 4 ] including:
1. AC to DC ( rectifier )
2. DC to AC ( inverter )
3. DC to DC
4. AC to AC
Power electronic systems ever draw high quality of low frequence harmonic current from the public-service corporation and hence cause jobs for other users. Take an ideal single-phase rectifying tube span rectifier as illustration, the entire harmonic deformation can be up to 48.43 % [ 4 ] and the 3rd harmonic current can be every bit big as one tierce of the cardinal current. If a non-linear burden is considered, the supplanting factor will fall down from unity value and do a lessening of power factor. This is certainly over the harmonic criterions restriction and needs to be corrected.
Theoretically, Rectifiers and choppers end product DC and pull a cardinal AC beginning current and big low frequence harmonic content. On the other manus, inverters end product low frequence AC and provide cardinal current and harmonic content normally at higher frequence. Harmonic contents can be reduced by harmonic extenuation techniques and therefore increase power factor.
Take Fourier analysis consequence diagram of single-phase rectifying tube span rectifier and PWM control Buck convertor as illustration.
( a ) ( B )
Figure 2.1 Fourier analysis diagram for input current of ( a ) single-phase rectifying tube span rectifier and ( B ) PWM control Buck convertor.
2.4 Software tools for harmonic extenuation rating
To filtrating harmonic current in the power system, the frequence of harmonic contents is indispensable. However, in pattern, the harmonic frequence is non perfectly equal to the theoretical value and that makes analysis of harmonic frequences really hard. The ground is isolated induction and electrical capacity in the system and change by reversal recovery clip and frontward voltage bead of non-ideal devices [ 1 ] . To analyse harmonic contents, appropriate package can be helpful. In this undertaking, the package chosen to assist analysing harmonic current drawn by power electronic systems is MATLAB/SIMULINK.
Taking the three-phase rectifying tube span rectifier as an illustration, a simulation theoretical account can be established as shown below. In the theoretical account, a three-phase 50Hz AC power supply is used for a resistive burden and most devices are non ‘ideal ‘ . The theoretical account is followed by the diagram of input current wave form and frequence spectrum of AC input current. Valuess of each order harmonic content and entire THD are given by Fast-Fourier ( FFT ) analysis in powergui analysis tools. With the aid of Fourier analysis, the public presentation of harmonic extenuation techniques can be evaluated and compared rapidly.
Figure 2.2 Simulation theoretical account for three-phase rectifying tube span rectifier.
Figure 2.3 Wave form of rectifier input current ( phase A ) .
Figure 2.4 Frequency spectra of AC input current of three-phase rectifier.
3. Power Factor Correction Techniques
After 10s of old ages developing and bettering, assorted types of power factor rectification techniques or harmonic extenuation techniques can be chosen to work out power factor job. Those techniques can be divided into five classs [ 11 ] [ 13 ] as shown below:
1. Passive filters
Passive filters can better power factor with low cost and cut down high frequence harmonics efficaciously. However, they are ever in big size and can non change flexibly with system alterations [ 4 ] [ 14 ] . If tuning reactors are non used, parallel resonance may happen in operation [ 15 ] .
2. Active filters
Active filters improve power factor and supply stable end product even under changing supply status, and cut down harmonics in the end product current efficaciously and expeditiously [ 4 ] [ 16 ] . These, nevertheless, ever requires much higher costs and the harmonic currents they injected may flux into other system constituents [ 13 ] [ 14 ] .
3. Hybrid systems
Hybrid active filters combine active and inactive filters together in assorted signifiers [ 17 ] . Hence they can cut down initial and running costs and better public presentation of the filter [ 11 ] [ 13 ] . Smaller filter inductance, smaller dimension, light weight and better filter public presentation intercrossed system take advantages of both inactive and active filters [ 18 ] . However, the complexness of operation is the chief drawback of intercrossed systems.
4. Phase generation
Increasing the pulse figure of power convertors can raise the lowest harmonic order generated by the convertor [ 2 ] . Typically, 6-pulse convertor has the lowest harmonic order of 5 [ 1 ] . When lifting pulse figure to 12, the lowest harmonic order can increase to 11. As value of harmonic current are ideally relative to cardinal current value [ 4 ] , the sum deformation of the power system can de cut down to a low degree. On the other manus, the effectivity of this technique is based on balanced burden [ 13 ] which seldom happens in pattern.
PWM convertors have much better public presentation compared to traditional convertors like rectifying tube rectifiers and square-wave control inverters [ 4 ] . As a control scheme betterment, PWM harmonic extenuation technique can even used with some devices for traditional convertors and hence get wide application chance [ 11 ] . However, the topology complexness and hard on planing accountants [ 19 ] makes the usage of PWM is limited.
The aim of these techniques is to do the input current about a pure sinusoidal wave form and hence to better the power factor in electrical supply system. All these five techniques are discussed individually in the undermentioned work.
3.1 Passive filters
Passive filters have widely been used to absorb harmonics generated by the power electronic systems, chiefly due to their simpleness, low cost and high efficiency [ 20 ] . Passive filters are ever consists inductances, capacitances and muffling resistances [ 21 ] . The aim of the inactive filter is to halt the flow of the harmonic current from upseting power system, either by forestalling them with the use of series filters or deviating them to a shunt way [ 9 ] [ 11 ] . That is the different between series filter and shunt filter, excessively.
Seriess filters can be tuned LC system or merely a individual inductance in the system. Parallel induction and electrical capacity are tuned to supply low electric resistance for cardinal frequence current and high electric resistance for a selected frequence current, ever high degree harmonic current. The series tuned filters are simple and dependable to utilize. The circuit constellation can be shown as below.
Figure 3.1 Series LC tuned filter.
The series tuned filters are ever used as input filter for power electronic systems. However, a large drawback limits the utilizing. If the series tuned filter is used in a VSI system as the input filter for the inverter, several order harmonic current demand to be filtered, 5th, 7th, and so on. Each order harmonic current required an single filter, and therefore the size of the system can be unbearable.
On the other manus, shunt filter have much more types including shunt-tuned filter, double-band base on balls filter and 1st, 2nd and 3rd -order damped filters. Besides, broadband filters are good solution for filtrating broad scope of harmonics [ 22 ] . The circuit constellations of these widely used inactive filters are like shown below.
( a ) ( B ) ( degree Celsius ) ( vitamin D )
Figure 3.2 Typical harmonic filters: ( a ) Single-tuned filter ( B ) Double tuned filter
( degree Celsius ) High-pass parallel filter ( vitamin D ) C-type high-pass filter [ 5 ] [ 27 ] .
A few individual tuned filters cope with big degree harmonic contents and a high-pass ( 2nd order ) filter filtrating high frequence harmonics is the typical theoretical account for shunt inactive filters and can acquire better characteristic than series filters [ 24 ] . Take the individual tuned filter as illustration, single-tuned filter besides called the band-pass filter as merely a selected frequence of current can go through in low electric resistance. The tuning frequence of the single-tuned filter could be:
( 3.1 )
And at this frequence, the electric resistance of the filter is:
( 3.2 )
where s is the Laplace operator, L represents value of induction and C represents the electrical capacity value.
However, largely inactive filters can merely filtrate 30 % of harmonic current in the power system [ 23 ] and can non fit IEEE 519-1992 criterion well. Even the broadband filter, which can filtrate a scope of harmonic contents and cut down system THD to about 10 % , the resonance caused by the filter and the large size of inductance and capacitance still limit the use of the filter.
So we can acquire the list of advantages and disadvantages for inactive filters shown in table 3.1.
Efficaciously for filtrating high frequence harmonics
Low handiness for low frequence harmonic filtering
Very low cost and dependable
Bulky devices and inflexible devices parametric quantities
Individual subdivision is necessary for each dominant harmonics in the system
High chance resonance
Table 3.1 List of inactive filter public presentations [ 4 ] [ 14 ] [ 25 ] [ 29 ] .
3.2 Active power filters ( APF )
The basic thought of an active filter is to counterbalance current or electromotive force perturbation so as to cut down the reactive power electronic systems drawn from the power system [ 23 ] . The active filters utilizing in power system are non the same as what we use in electronic circuits. The active filters conventional agencies combined operational amplifiers and inactive constituents like inductances and capacitances, and ever been used in electronic circuits runing under low electromotive force. That is the beginning of the active compensation applications and came out earlier than active filters utilizing in power systems. The active filters which are used in power system for active power compensation and harmonic compensation are ever called Active Power Filter ( APF ) [ 30 ] . The ‘active ‘ in APF means the filters are act as power beginnings or generators and supply compensation currents which have opposite stage angle with the harmonic currents in power system [ 30 ] . Similarity between electronic circuit active filters and power system active filters are the demand of external power supply. The active filters which are talked in the undermentioned parts are all agencies APF.
With the active power filters, the compensation for reactive power and for harmonic current can de done at the same clip, therefore efficiency on harmonic compensation and besides dynamic response are all be improved [ 23 ] . The tendency of active power filters began in 1970s and was introduced by Mr. Akagi. The inducement for active filters is the inductance is non appropriate to utilize under high frequence, so the tendency is to replace the inductance with active constituents. As the harmonic contents in the power system assorted often, fast response of active filters required a good control scheme to do active filters ‘smarter ‘ and faster. But more complex devices and sophisticated control scheme are required, that all makes active filters more expensive and difficult to utilize [ 26 ] .
Active filters can besides be classified by convertor type as shunt-type active filters and series-type active filters. The diagrams of two basic types of active filters are shown below. The other manner to sort active filters is the phase figure of filters which will be discussed subsequently.
( a ) ( B )
Figure 3.3 Diagrams of ( a ) Shunt-type active filter and ( B ) Series-type active filter [ 11 ] [ 28 ] .
Series active filters are good at compensate electromotive force harmonics and capacitive, voltage-source tonss. When applied to an inductive or current-source burden, a low electric resistance analogue subdivision is necessary. Similarly, shunt active filters are ever used with inductive, current-source tonss and high current deformation conditions. Sometimes over current status occurs with the usage of shunt-type active filters [ 31 ] .
Typical working rule of the active power filter is:
The detector detects the wave form of the instantaneous burden current and feedback to the accountant, which is typically a digital processing block.
Load current is ever high deformation current including cardinal current and many orders of harmonic current. The processor must separate the cardinal current with the harmonic currents and give out the information including frequence, value, and stage angle of harmonic contents, so as to command the power beginning inverter supplying opposite phase current of harmonic current.
The power beginning inverter draws current from single DC electromotive force supply and change overing to required current to call off harmonic currents. Like the diagram shown below.
Figure 3.4 Diagram of compensation features [ 31 ] .
Therefore, we can pull a decision of advantages and disadvantages of active power filters shown in the tabular array below.
High compensation efficiency and high ability on harmonic compensation
Low dependability with sophisticated control system and devices
Small size constituents
Difficult to build a big rated current beginning with a rapid current
Fast action on harmonic current fluctuation makes good dynamic response
High initial costs and running costs
No resonance doing
Complex control scheme and accountants are necessary
Suitable for widely supply and burden conditions, like imbalanced power supply
Table 3.2 List of active power filter public presentations [ 13 ] [ 22 ] [ 30 ] [ 31 ] .
3.3 Hybrid systems
Hybrid filters comes from the thought to unite the advantages of both inactive filters and active filters together hence to acquire superb public presentation on harmonic extenuation [ 17 ] . Combine inactive filters and active filters can significantly cut down costs and better the compensation features in the power system. Besides, assorted types of intercrossed systems of inactive and active filters can acquire better public presentation than merely inactive or active filters.
Like the mention [ 18 ] and [ 20 ] , little evaluation active power filter and inactive filter connected in consecutive or shunt type. Smaller filter inductance, smaller dimension, light weight and better filter public presentation intercrossed system take advantages of both inactive and active filters [ 18 ] . However, as the cellar of the intercrossed power filters are ever active power filters, the initial costs and command complexness is still large disadvantages of intercrossed systems.
3.4 Phase generation
The intent of stage generation is to increase the pulse figure of the convertor and hence to increase the harmonic order and frequence [ 4 ] . The low frequence harmonics can be mitigated efficaciously and phase generation technique does non do serious resonance and other bad effects on power system performances [ 13 ] . The practical application of stage generation technique, the multipulse convertors, have the ability to pull low deformation current from power beginning and bring forth DC current with low degree rippling [ 32 ] .
Typically, 6-pulse convertor has the lowest harmonic order of 5 [ 1 ] . When lifting pulse figure to 12, the lowest harmonic order can increase to 11. As value of harmonic current are ideally relative to cardinal current value [ 4 ] , the sum deformation of the power system can de cut down to a low degree. Besides, the multipulse thyristor convertors can end product assorted value current by commanding the thyristor fairing angle ( ) [ 32 ] .
The drawbacks of stage generation technique are largely the contradiction between the cost and end product characteristic. If controlled end product is required, the multipulse convertor should incorporate at least 12 exchanging devices and that can be a large sum of costs. On the other manus, multipulse convertor merely use rectifying tubes may run on low efficiency [ 11 ] .
PWM ( Pulse Width Modulation ) is a modern control technique for power electronic systems. PWM convertors have much better public presentation compared to traditional convertors like rectifying tube rectifiers and square-wave control inverters [ 4 ] . Like the stage generation technique, PWM control can raise the frequence of harmonic contents of current so as to cut down the consequence caused by harmonics. Besides, convertors utilizing PWM control can hold high efficiency and little size. With all these advantages, PWM control absorbed great concern in modern power transition systems.
However, the topology complexness and hard on planing accountants [ 19 ] makes the usage of PWM is limited.
3.6 Power factor rectification convertor
Power factor rectification ( PFC ) convertor is a typical active power factor rectification method. As a mature technique for power factor rectification, PFC convertors have been widely used in power electronic systems to accomplish high power factor ( PF ) and low harmonic deformation [ 33 ] . PFC convener forces the input current follow the input electromotive force, which makes the input current drawn from power supply about in a unity power factor [ 34 ] . The Boost-type PFC convertors are the most used topology which have many advantages, such as low degree rippling in the input current, high power factor, little size and simple circuit construction [ 35 ] . A typical circuit diagram of Boost-type PFC convertor is as shown below from mention [ 36 ] .
Figure 3.5 Typical circuit diagram of Boost-type PFC convertor [ 36 ] .
As we seen in the diagram before, conventional PFC convertor consists two chief phases [ 33 ] – [ 37 ] :
Power factor rectification phase.
This phase is combined with a rectifying tube rectifier and a DC/DC convertor and used to rectify power factor of the input current drawn from the power system. The most used type of chopper is Boost chopper. Besides, the new Buck and Cuk type PFC convertors are progressively being used now. The exchanging working rule can be divided into two types, DCM and CCM.
2. DC/DC convertor
The chopper here is used to change over the power end product electromotive force and current lucifer the user ‘s demand. Since choppers merely drawn low deformation power from supply, the typical filter on the utilization terminal is ever a inactive filter.
This is the working rule for conventional PFC convertors, the two-stage DCM/CCM Boost-type PFC convertor. However, this type of PFC convertor has some disadvantages and necessitate to be improved [ 33 ] – [ 39 ] :
1. Phase figure
Individual control system and shift devices are required for each phase of PFC convertor, therefore increasing the costs of the whole system and do some other jobs, such as power denseness, transmittal efficiency and control response [ 38 ] . Besides, the design of control system can be a challenge.
A new one-stage PFC convertor topology has been introduced to power factor rectification research country. The circuit diagram is as shown in figure 3.6 [ 36 ] . The combination of the power factor rectification convertor and the frontward convertor may convey many advantages point as below [ 36 ] :
1. High power factor rectification public presentation
2. Reduced value of rippling in the DC end product
3. Low initial cost and running cost
4. High efficiency and easy control system
And so on.
Figure 3.6 Circuit diagram of individual phase PFC convertor [ 36 ] .
2. Converter type
Like shown in figure 3.6, Buck convertor is progressively being used in PFC convertors. Besides, Cuk convertor and other type of choppers are going good pick for PFC convertors [ 36 ] – [ 39 ] . The Buck type PFC convertor was seldom used since its high input current deformation. However, with the characteristic improving of the Buck type PFC convertor, it can make good public presentation with specific double manner responsibility rhythm control strategy [ 36 ] . The chief advantage of Buck type PFC convertor is easy to cut down the phase figure to one phase.
3. Devicess and control scheme
One of the most of import purposes in the design of power electronic systems is the decrease of the size of the inactive devices, since it allows addition on the power denseness and the decrease in the initial and running cost. As inductance and capacitance are still utilizing in the PFC convertor, the decrease of them can be really of import [ 33 ] [ 37 ] .
However, the betterment of devices must establish on the development of the control scheme [ 37 ] . With a good detect and control system, the size of the inductance and capacitance can be reduced while the harmonic content can still run into the demand [ 33 ] .
The farther analysis and betterment of PFC convertor based on this literature reappraisal will be an of import work in the last phase of undertaking.
This literature reappraisal provides a critical survey on power factor issues and power factor rectification techniques. A theoretical reappraisal of power factor definitions and harmonic coevals by power electronic systems are presented at the beginning of the paper. The public presentation of five basic types of harmonic extenuation techniques has been discussed with the support of many old research publication and their consequences. The PFC convertor is chosen as the promising system for power factor rectification after the analysis and comparing. The simulation theoretical account constitution and simulation comparing of power factor rectification techniques will be of import plants for the following period of the undertaking. Besides, design regulations and counsel of PFC convertors will be designed in the following period, excessively.