Paste your essay in here…INTRODUCTION1.1 Back groundThe widespread use of non-linear masses is resulting in a range of undesirable phenomena within the operation ofpower systems. The harmonic elements in current and voltage waveforms area unit the foremost necessary among these. Conventionally, passive filters are wont to eliminate line current harmonics. However, they introduce resonance within the power grid and have a tendency to be large. So, active transmission line conditioners became additional fashionable than passive filters because it compensates the harmonics and reactive power at the same time. The active power filter topology may be connected asynchronous or shunt and combos of each. Shunt active filter is additional fashionable than series active filter as a result of most of the economic applications need current harmonic compensation. differing kinds of active filters are planned to extend the electrical system quality. The classification relies on followingcriteria.
Power rating and speed of response needed in salaried system.
System parameters to be salaried (e.g. current harmonics, power issue and voltage harmonics)
Technique used for estimating the reference current/voltage.
Current managementled voltage supply inverters may be used with associate degree acceptable control strategy to perform active filter practicality. The electrical grid can embody a really sizable amount of little producers that use renewable energy sources, like star panels or wind generators.1.2. Literature surveyJohan H. R. Enslinand Peter J. M. Heskes,[1] “Harmonic interaction betweena large number of distributed power inverters and the distribution network,”
In this paper discussed the harmonic interaction between a large number of distributed power inverters and the distribution network. This paper is to analyze the observed phenomena of harmonic interference of large populations of these inverters and to compare the network interaction of different inverter topologies and control options.
Uffe Borup, Frede Blaabjerg and Prasad N. Enjeti ,[2]
“Sharing of nonlinear load in parallel-connected three-phase converters,”
Presented about the sharing of linear and nonlinear loads in three-phase power converters connected in parallel, without communication between the converters. The paper focuses on solving the problem that arises when two converters with harmonic compensation are connected in parallel.
Pichai Jintakosonwit Hideaki Fujita, Hirofumi Akagi and Satoshi Ogasawara, [3]
“Implementation and performance of cooperative control of shunt active filters for harmonic damping throughout a power distribution system,”
This paper proposes cooperative control of multipleactive filters based on voltage detection for harmonic damping throughout a power distribution system. The arrangement of a real distribution system would be changed according to system operation, and/or fault conditions. In addition, shunt capacitors and loads are individually connected to, or disconnected from, the distribution system.
Pedro Rodríguez, JosepPou, Joan Bergas,J. Ignacio Candela , Rolando P. Burgos and DushanBoroyevich ,[4]
“Decoupled double synchronous reference frame PLL for power converters control,”
Presented the detection ofthe fundamental-frequency positive-sequence component of theutility voltage under unbalanced and distorted conditions. Specifically, it proposes a positive-sequence detector based on a new decoupled double synchronous reference frame phase-locked loop (PLL), which completely eliminates the detection errors of conventional synchronous reference frame PLL’s. This is achieved by transforming both positive- and negative-sequence components of the utility voltage into the double SRF, from which a decoupling network is developed in order to cleanly extract and separate the positive- and negative-sequence components.
SoerenBaekhoejKjaer, John K. Pedersenand Frede Blaabjerg,[5]
“A review of single-phase grid-connected inverters for photovoltaic modules”
Presentsa Review of Single-Phase Grid-Connected Inverters for Photovoltaic Modules. This paper focuses on inverter technologies for connecting photovoltaic (PV) modules to a single-phase grid. The inverters are categorized into four classifications: 1) the number of power processing stages in cascade; 2) the type of power decoupling between the PV module(s) and the single-phase grid; 3) whether they utilizes a transformer (either line or high frequency) or not; and 4) the type of grid-connected power stage.
F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus,[6]
“Overview of control and grid synchronization for distributed power generation systems,”
This paper gives an overview of the structures for the DPGS based on fuel cell, photovoltaic, and wind turbines. In addition, control structures of the grid-side converter are presented, and the possibility of compensation for low-order harmonics is also discussed. Moreover, control strategies when running on grid faults are treated. This paper ends up with an overview of synchronization methods and a discussion about their importance in the control.
J. M. Carrasco, L. G. Franquelo, J. T. Bialasiewicz, E. Galván, R. C. P. Guisado, M. Á. M. Prats, J. I. León, and N. M. Alfonso,[7]
“Powerelectronicsystems for the grid integration of renewable energy sources: A survey,”
This paper proposes about distributed energy resource is increasinglybeing pursued as a supplement and an alternative to large conventional central power stations. The specification of a power electronic interface is subject to requirements related not only to the renewable energy source itself but also to its effects on the power-system operation, especially where the intermittent energy source constitutes a significant part of the total system capacity.
1.3 Problem Formulation
One of the foremost common issues once connecting tiny renewable energy systems to the electrical grid considerations the interface unit between the facility sources and also the grid, as a result of it will inject harmonic elements that will detoriate the facility quality.
The increasing use within the trade of non-linear hundreds supported the facility electronic components additionally introduced serious perturbation issues within the electrical power distribution grids. Also, regular increase within the harmonic emissions and current unbalance additionally to high consumption of reactive power will be noticed . The flow of harmonic currents within the electrical grids may also cause voltage harmonics and disturbance. These harmonic currents will move adversely with a good vary of grid equipment’s, management systems, protection circuits and alternative harmonic smart hundreds. The energy distributers like customers were involved by imposing some regulation protection against the enlargement of harmonic downside.
In order to face the matter of harmonics, several solutions are projected. These solutions enclosed modifications on the load itself for fewer harmonic emissions just like the case of special structure single section and 3 section rectifier and PWM rectifiers or the affiliation on the contaminated power grids of alternative ancient or fashionable compensation systems.
Many types of APF are projected and employed in harmonic compensation. Series APF is employed for voltage harmonics compensation. Shunt APF was projected for current harmonics and reactive power compensation. The Unified Power Quality Filter or Conditioner combines the 2 varieties Shunt and Series APF in one device accountable for the synchronous compensation of voltage, current harmonics and reactive power. though there ar differing kinds of APF, the Shunt APF continues to be the foremost noted and used kind APF.
The main perform of Shunt Active Power Filter is to cancel harmonic currents occurring in power grids. The principle of SAPF is to get harmonic currents equal in magnitude and opposite in section to those harmonics that flow into within the grid. The non-linear masses absorb non-sinusoidal currents from the grid. Whereas, the SAPF current is generated in a very manner that grid current keeps the curving kind. SAPF is controlled to be seen with the non-linear load by the grid either as linear resistive load.
There ar 2 main structures for the management of Shunt Active Power Filter; these ar the direct management and therefore the indirect management of APF. within the direct management the most plan is to get filter current references victimisation the suitable ways. The generated reference currents ar then to be compared with the measured APF currents. The error is then wont to turn out management signals of the filter. The indirect management interests in dominant the grid currents rather than filter currents. It compares the measured grid currents with their generated references. The error is then sent to the negative feedback circuit that determines the management signal of the APF.
1.4 Objective of Thesis
The main objective of my thesis work is to include the options of APF within the typical electrical converter interfacing i.e., four leg voltage supply electrical converter that interfaces the Renewable Energy Sources like wind energy and also the picture electric cell at the Distributed Grid, with none further hardware price. Here the grid interfacing electrical converter is most utilised to perform following vital functions:
To inject most offered power of Renewable Energy Sources to the AC grid.
To offer load Active power.
To compensate load Reactive power and current harmonics at PCC.
To compensate current unbalance and neutral current just in case of 3-phase 4-wire system.
In order to guage the performance of the projected approach, a model of the AC gird, non-linear load, structure electrical converter and renewable energy supply are going to be developed in MATLAB/SIMULINK 2012a package setting.
1.5. Organization of Thesis
Chapter 1
Includes the introduction, literature survey and brief analysis of thesis work.
Chapter 2
Discusses different power quality problems and their associated solutions.
Chapter 3
The study is pointed toward the shunt active power filter and its control methods.
Chapter 4
This chapter deals with the Renewable Energy Sources and their types.
Chapter 5
This chapter deals with the fuel cell and their types.
Chapter 6
The modeling of the system is illustrated.
Chapter 7
Thesimulation results of all studied control strategies
Chapter 8
Conclusion and future scope
Bibliography
CHAPTER-II
POWER QUALITY AND ITS PROBLEMS
Electric systems and grids square measure complicated dynamic systems. These systems suffer typically from surprising or abrupt changes of the currents and voltages. These changes square measure due in the main to the various kinds of linear and non-linear masses to that they're connected. additionally, to differing kinds of accidents which may intervener into the grid. With the increasing use of power semiconductors within the most of business and domestic procedures, the electrical grids square measure contaminated with totally different harmonic currents and voltages. These harmonics have an effect on the traditional operate of the foremost of the grid connected devices; additionally to sizable economic losses. several classic and trendy solutions are projected within the literary for the harmonic issues. during this chapter, the harmonic drawback collectively of the foremost common power quality issues are conferred. the various trendy and ancient solutions can then be mentioned.
2.1 Definition of Power Quality
Power quality may be a term which means various things to totally different individuals. Institute of Electrical and Electronic Engineers (IEEE) customary IEEE1100 defines power quality as “The conception of powering and grounding sensitive instrumentation|equipment} during a manner appropriate for the equipment.” As applicable as this description may appear, the limitation of power quality to “sensitive electronic equipment” may be subject to disagreement. Electrical instrumentality vulnerable to power quality or additional fitly to lack of power quality would fall inside a on the face of it infinite domain. All electrical devices ar liable to failure or malfunction once exposed to 1 or additional power quality issues. The device may be an electrical motor, a electrical device, a generator, a computer, a printer, facility or a home appliance. All of those devices et al react adversely to power quality problems, reckoning on the severity of issues.
A simpler and maybe additional brief definition would possibly state: “Power quality may be a set of electrical boundaries that permits a bit of kit to perform in its meant manner while not important loss of performance or life.” This definition embraces 2 things that we tend to demand from associate electrical device: performance and life. Any power-related drawback that compromises either attribute may be a power quality concern.
Power quality also can be outlined as a group of electrical boundaries that permits a bit of kit to perform in its meant manner while not important loss of performance or life. Power distribution systems ought to offer their customers with associate uninterrupted flow of energy at swish curving voltage at the contracted magnitude level and frequency. However, in power systems, particularly the distribution systems have several nonlinear masses, that considerably have an effect on the standard of power provides. As a results of the nonlinear masses, the pure curving wave is lost. This lands up manufacturing several power quality issues.
2.2 Power Systems Distortion and Problems
In power systems, totally different voltage and current issues is two-faced. the most voltage issues is summarized in brief period variations, voltage interruption, frequency variation, voltage dips and harmonics. Harmonics represent the most drawback of currents of power systems.
2.2.1 Voltage Variation for Short Duration
The short length voltage variation is that the results of the issues within the operate of some systems or the beginning of the many electrical hundreds at identical time. The defaults will increase or decrease the amplitude of the voltage or perhaps cancel it throughout a brief amount of your time. the rise of voltage may be a variation between 10-90% of the nominal voltage. It will hold from half a amount to one minute in keeping with the IEEE 1159-1995. in keeping with identical reference, the rise in voltage is outlined once the amplitude of the voltage is regarding 110-180% of its value.
2.2.3 Frequency Variations
In the traditional conditions the frequency of the distribution grid should be among the interval 50±1 rate. The variations of the frequency of the grid will seems to the purchasers WHO area unit victimization auxiliary electrical supply (solar system, thermal station…etc.). These variations area unit rare and happen within the case of outstanding conditions just like the defaults within the turbines.
2.2.4 Unbalance in Three Phase Systems
The 3 part system is unbalanced once the currents and voltages don't seem to be identical in amplitude; or once the phase between every 2 phases isn't 120°. within the ideal conditions, the 3 part system is balanced with identical hundreds. In reality, the masses don't seem to be identical, additionally to the issues of the distribution grids which may interfere.
2.2.5 Voltage Dips
The voltage dips area unit periodic perturbations. they seem as a natural result of the shift of the transistors. they're due additionally to the beginning of massive hundreds like motors. Lifts, lights, heaters…etc. this phenomena causes unhealthy functioning of the protection equipment’s.
2.2.6 Harmonics
Power systems square measure designed to work at frequencies of fifty or sixty cycles/second. However, sure forms of hundreds produces currents and voltages with frequencies that square measure whole number multiples of the fifty or sixty cycles/second first harmonic. These frequencies elements square measure a kind of electrical pollution referred to as harmonic distortion. There square measure 2 forms of harmonics which will be encountered during a power grid.
Synchronous harmonics.
Asynchronous harmonics.
Synchronous harmonics square measure sinusoids with frequencies that square measure multiples of the elemental frequency. The multiplication issue is usually remarked because the harmonic range. The synchronous harmonics will be divided into 2 classes.
Sub-harmonics: once the harmonic frequency is a smaller amount than the elemental frequency.
Super harmonics: once the harmonic frequency is quite the elemental frequency.
Harmonics square measure acquainted to the musicians because the overtones from associate instrument. {they square measure|they're} the whole number multiples of the instrument’s elementary or natural frequency that are made by a series of standing waves of upper and better order.Exactly the same issue happens in power circuits once non-linear hundreds produce harmonic currents that square measure whole number multiples of the availability first harmonic. The rising of solid-state power physics has greatly enlarged the quantity and size of those hundreds.
The conception of harmonics was introduced within the starting of the nineteenth century by Joseph Fourier. Fourier has incontestable that every one periodic non-sinusoidal signals will be described by infinitive total or series of sinusoids with discontinuous frequencies as given by Equation (2.1).
i(t)=I_0+∑_(h=1)^∝▒〖I_h cos〖(hωt 〗+〗 φ_h) …… (2.1)
The component I0 in the Fourier series is the direct component. The first term of the sum with the index h=1 is the fundamental of the signal. The rest of the series components are called the harmonics of the range h. Fig. 2.1 Shows the form of a wave containing the third harmonic (h=3). In the three phase electric grid, the principle harmonic components are the harmonics of ranges (6*h±1).
Fig. 2.1:Harmonic Content of a Signal and its Fundamental.
Transformer exciting current, arc furnaces, rectifiers and many other loads will produce harmonics in the utility lines. Most utilities limit the allowable harmonic current levels to the values shown in IEEE 519.
2.2.6.1 Total Harmonic Distortion (THD)
The total harmonic distortion of a symbol could be a measuring of the harmonic distortion gift in current or voltage. it's outlined because the quantitative relation of the add of the abilitys of all harmonic elements to the power of the elemental frequency. Harmonic distortion is caused by the introduction of waveforms at frequencies in multiplies of the elemental
THD(%)=√(∑_(i=2)^α▒x_i^2 )/|x_1 | (2.2)
The THD could be a terribly helpful amount for several applications. it's the foremost ordinarily used harmonic index. However, it's the limitation that, it's not a decent indicator of voltage stress at intervals a condenser as a result of that's associated with the height worth of voltage wave form.
2.2.6.2 Distortion Factor
The distortion factor Fd is defined as the ratio between the fundamental and the signal in RMS values. It is given by:
F_(d=) I_L1/I_rms (2.3)
It is then equal to unity when the current is purely sinusoidal and decreases when the distortion appears.
2.2.6.3 Crest Factor
The crest factor of a signal Fc is defined by Equation (2.4):
F_c=(crest value )/(effectivevalue ) (2.4)
For sinusoidal waves, the crest factor is 1.41. It can achieve the value of 5 in the case of highly distorted waves.
2.2.6.4 Effects of Harmonics
Harmonic currents can flow into the utility feeder and should produce variety of issues in therefore doing. they will be treed by power issue correction capacitors and overload them or cause resonant over-voltages. they'll distort the feeder voltage enough to cause issues in computers, phonephone lines, motors, and power provides, and should even cause electrical device failures from eddy current losses. The harmonic currents is also treed by putting in series LC filters resonant at the offensive frequencies. These filters ought to be designed to supply low electrical phenomenon at the resonant frequency compared to the supply electrical phenomenon at that frequency. But, again, there's a hidden “gotcha.” If a filter is put in that incorporates a series resonance at the seventh harmonic, it'll even have a parallel resonance with the utility at a lower frequency once the supply inductance is else to the filter inductance. If this parallel resonance ought to lie on or close to the fifth harmonic, there's the likelihood of the resonant over-currents represented earlier. The installation of series resonant entices can continually introduce parallel resonances at frequencies below the trap frequencies. sensible apply dictates that multiple resonant traps be put in 1st at all-time low harmonic frequency of concern so in sequence at the higher-frequency harmonics. If switched, they ought to be switched on in sequence beginning with all-time low frequency entice and switched move into sequence ranging from the very best frequency entice.
The voltage or current distortion limit is decided by the sensitivity of hundreds (also of power sources), that square measure influenced by the distorted quantities. the smallest amount sensitive is heating instrumentation of any kind. the foremost sensitive reasonably equipment’s is those electronic devices that are designed forward a perfect (almost) curved fundamental voltage or current waveforms. electrical motors square measure the foremost standard hundreds that square measure set between these 2 classes.
2.2.6.5 Power issue
Power issue is outlined because the magnitude relation of real power to volt-amperes Associate in Nursingd is that the cos of the phase between the voltage and also the current in an AC circuit. These square measure showing neatness outlined quantities with curved voltages and currents. Power issue may be improved by adding capacitors on the facility line to draw a number one current and provide insulant VArs to the system. Power issue correction capacitors may be switched in and out as necessary to take care of power unit and voltage management.
For a curved signal, the facility issue is given by the magnitude relation between the active and also the apparent power. Electrical equipment’s’ parameters square measure unremarkably given below nominal voltage and current. an occasional power issue will indicate unhealthy use of this equipment’s. The apparent power may be outlined by:
S=V_rms.I_rms=V_rms.√(1/T) ∫_0^T▒i_L^2 dt (2.5)
The active power P can be given by the relation:
P=V_rms.I_rms.cos〖(∝1)〗 (2.6)
The reactive power Q is defined by:
Q=V_rms.I_rms.sin〖(∝1)〗 (2.7)
The power factor in this case can be given by Equation 2.8.
P.F=P/S=P/√(P^2+Q^2 ) (2.8)
In the case where there is harmonics, a supplementary power called the distorted power D appears. This power can be given by the relation.
D=V_rms.√(∑_(n=2)^α▒I_Ln^2 ) (2.9)
The apparent power can then be expressed as:
S=√(P^2+Q^2+D^2 ) (2.10)
The power factor is then given by:
P.F=P/√(P^2+Q^2+D^2 ) (2.11)
From equation (2.11), we can notice that the power factor decreases because of the existence of harmonics in addition to the reactive power consumption. The Fresnel diagram of the power is given in Fig. 2.2.
Fig. 2.2: Fresnel Representation of the Power
2.3 Harmonic Currents Sources
The main explanation for harmonics is that the injection of harmonic currents by the non-linear hundreds. The bridges of diodes ar the foremost non-linear hundreds gift within the power applications as a result of they don’t want an impression and that they have long life length with low price. There are several alternative harmonic manufacturing hundreds such as:
Industrial equipment’s (welding machines, arc furnaces, induction furnaces, rectifiers).
Offices equipment’s (computers, photocopiers,…etc.).
Domestic devices (TVs, micro-wave furnaces, Ne lightening,…etc.).
Power inverters.
Power transformers once operating within the saturation zone are also thought of as non-linear hundreds that manufacture harmonics.
The feeding of non-linear hundreds generates harmonic currents that unfold into the electrical grid. The unfold of current harmonics into the feeding impedances (transformers and grid) creates harmonic voltages in these feeders. memory that the conductor electrical phenomenon will increase with the frequencies of the currents that withstand it, totally different electrical phenomenon can seem for every vary of current harmonics. The harmonic current of vary h can produce through the electrical phenomenon harmonic voltage. All the hundreds connected to identical purpose are fed with identical flustered voltages. The equivalent circuit per section of a non-linear load connected to the grid is given by Fig.2.3.
Fig. 2.3: Equivalent Circuit Per Phase of a Non-Linear Load Connected to the Grid.
The spread of harmonic currents from different loads can be represented as in Fig.2.4.
Fig. 2.4: Spread of Harmonic Currents into the Grid
2.4 Economic effects of harmonics
Premature aging of materials that forces its replacement, additionally to AN initial over size of those materials.
The overloading of the grid which means to extend the nominal power and to outsized the installations, inflicting additional and additional losses.
The current distortions cause abrupt triggers and also the stop of production equipment’s.
These material prices, energetic and production losses have an effect on the aggressiveness and also the productivity of factories and firms.
2.5 Solutions for the Harmonics
The filtering of the grid currents and voltage could be a cloister drawback for the distributer as like because the consumer. as a result of the bounds on harmonic emission don't seem to be equally applied within the low of the various countries, the producers of the various electrical devices attempt to construct devices that satisfy for the conditions and limits of the international standards. the electrical firms, from its aspect, use completely different filtering equipment’s and encourage the researches toward finding new economical solutions for the ability quality issues. The shoppers install conjointly generally reactive power and harmonic compensation batteries to ameliorate the ability issue and cut back the energy consumption bill.
Many ancient and fashionable solutions for harmonics mitigation and power quality improvement were planned in literary. a number of these solutions investigate within the load to reduce the harmonic emission whereas the others propose the employment of external filtering equipment’s that stop the unfold of harmonics into the grid.
2.5.1 In-Line Reactors
In-line reactor or choke could be a straightforward resolution to regulate harmonic distortion generated by adjustable speed drives. the answer is return up with inserting a comparatively little reactor, or choke, at the input of the drive. The inductance prevents the capacitance to be charged in an exceedingly short time and forces the drive to draw current over a extended time and reduces the magnitude of this with a lot of less harmonic content whereas still delivering a similar energy.
2.5.2 Transformers with Passive Coupling
Some styles of triangle zigzag coupling of transformers permit the elimination of the harmonics of order three and its multiples. the value of those coupling sorts is that the augmentation of the supply resistance, and so the augmentation of voltage harmonic distortion.
2.5.3 Passive Filters
Passive filter, that is comparatively cheap compared with the opposite harmonic reduction ways, is that the most used technique. Inductance, capacitance and also the load as a resistance ar tuned during a thanks to management the harmonics. However, they suffer from meddlesome with the facility systems. Actually, passive filters ar designed to shunt harmonics from the lines or block their flow through some elements of the systems by standardization the weather to make a resonance at the chosen frequency. These filters ar tuned and stuck in step with the ohmic resistance of the purpose at that they'll be connected and thence can not be adjusted instantly in accordance to the load. As a result their cutoff frequency modifications unexpectedly when any change within the load ohmic resistance leading to manufacturing a resonance with alternative parts put in within the system.
2.6 fashionable Solutions for Harmonic issues
Modern solutions were projected as economical solutions for the elimination of electrical grid harmonics so as to defeat the disadvantages of the standard ways like passive filters. Between these solutions we discover 2 classes that ar the foremost used:
Active filters (series, parallel, or a mix of each of them in Unified Power Quality Conditioner (UPQC)).
Hybrid filters composed of active and passive filters directly.
2.6.1 Active Power Filters
The perform of the active power filters (APF) is to get either harmonic currents or voltages during a manner such the grid current or voltage waves conserve the curving type. The APFs may be connected to the grid serial (Series APF), shunt (SAPF) to compensate voltage harmonics or current harmonics severally. Or may be related to passive filters to construct the hybrid filters (HAPF).
Active filters ar comparatively new sorts of devices for eliminating harmonics. this type of filter is predicated on power electronic devices and is way costlier than passive filters. they need the distinct advantage that they are doing not resonate with the facility system and that they work severally with relevancy the system ohmic resistance characteristics. they're utilized in troublesome circumstances wherever passive filters don’t operate with success thanks to resonance issues and that they don’t have any interference with alternative parts put in anyplace within the facility.
The active filters gift several alternative blessings over the standard ways for harmonic compensation such as:
Adaptation with the variation of the hundreds.
Possibility of selective harmonics compensation.
Limitations within the compensation power.
Possibility of reactive power compensation.
2.6.1.1 Series Active Power Filter (Series APF)
The aim of the series APF is to regionally modify the electric resistance of the grid. it's thought of as harmonic voltage supply that cancel the voltage perturbations that return from the grid or these created by the circulation of the harmonic currents into the grid electric resistance. However, series APFs can’t compensate the harmonic currents made by the masses.
Fig.2.5: Series Active Power Filter Connected to the Grid
2.6.1.2 Shunt Active Power Filter (SAPF)
The SAPFs area unit connected in parallel with the harmonic manufacturing hundreds. they're expected to inject in real time the harmonic currents absorbed by the waste product hundreds. Thus, the grid current can become curved .
Fig.2.6: Shunt APF Connected in Parallel with Non-Linear Load
2.6.1.3 Combination of Parallel and Series APF (UPQC)
Fig.2.7 explains the combination of two APFs parallel and series, called also (Unified Power Quality Conditioner). This structure combines the advantages of the two APF type’s series and parallel. So it allows simultaneously achieving sinusoidal source current and voltage.
Fig.2.7: Unified Power Quality Conditioner’s Diagram
2.6.2 Hybrid Filters
Hybrid filter may be a filter topology which mixes the benefits of the passive and active filters. For this reason, it's thought of because the best answer to eliminate the harmonic currents from the grid. The principal reason for the employment of hybrid filters is that the development of the ability semiconductors like MOSFETs and IGBTs. Over more, from a cheap purpose of read, the hybrid power filters permit reducing the value of APF.
Hybrid power filters will be classified consistent with the quantity of parts utilized in the topology, the treated system (single section, 3 section 3 legs or four legs) and therefore the used electrical converter sort (current supply electrical converter or voltage supply inverter).
2.7 Non-Linear Loads
When the input current into the electrical instrumentation doesn't follow the affected voltage across the instrumentation, then the instrumentation is claimed to possess a nonlinear relationship between the input voltage and input current. All equipment’s that use some type of rectification square measure samples of nonlinear hundreds. nonlinear hundreds generate voltage and current harmonics which will have adverse effects on instrumentation designed for operation as linear hundreds. Transformers that bring power into associate degree industrial surroundings square measure subject to higher heating losses attributable to harmonic generating sources (nonlinear loads) to that they're connected.
2.8 Shunt Active Power Filter
The conception of using active power filters to mitigate harmonic issues and to compensate reactive power was projected over twenty years past. it's well-tried its ability to regulate the grid current and to ameliorate the ability quality. The theories and applications of active power filters became additional fashionable and have attracted nice attention. while not the drawbacks of passive harmonic filters, like element aging and resonant issues, the active power filter seems to be a viable answer for reactive power compensation still as for eliminating harmonic currents. As we have a tendency to mentioned earlier, the SAPF is connected in parallel with the non-linear load to behave as another controlled non-linear load. The system of the non-linear load and also the SAPF are seen by the grid as a linear load connected to the PCC. within the case of compensation of reactive power this load are resistive. Otherwise it'll be either inductive or electrical phenomenon linear load.
CHAPTER-III
SHUNT ACTIVE POWER FILTER
3.1 Overview
Shunt active power filter compensates current harmonics by injecting equal-but-opposite harmonic compensating currents into the grid. during this case the shunt active power filter operates as a current supply injecting the harmonic elements generated by the load however section shifted by 180°. This principle is applicable to any sort of load thought of as harmonic supply. Moreover, with AN acceptable management theme, the active power filter may compensate the load power issue. during this approach, the ability distribution system sees the non-linear load and therefore the active power filter as a perfect electrical device. this compensation characteristics of the shunt active power filter is shown in Fig. 3.1
Fig.3.1: Compensation Characteristic of Shunt Active Power Filter
3.2 Harmonic Current Extraction Methods
The aim of active power filtering is to compensate the harmonic currents made by the non-linear hundreds, and to make sure the curved kind of grid currents and voltages. the primary step in active filtering is that the harmonic currents extraction to be injected into the grid. the great extraction of harmonics may be a keyword for a decent active power filtering. several extraction ways were projected in literary. they will be divided into 2 families: the primary family uses the quick Fourier remodel (FFT) within the frequency domain to extract the present harmonics. the most disadvantages of this technique ar the dangerous leads to transient, the significant quantity of calculations, and also the use of goodly memory. additionally to a delay within the extraction of harmonics which may be a minimum of one amount.
The second family is predicated on the time domain calculations within the extraction of harmonics. a number of its ways ar supported the instant active and reactive power. Others ar supported the calculation of direct and indirect current elements. Recently, the neural networks and also the adaptive linear neural networks are utilized in the extraction of harmonic elements of current and voltage.
3.2.1 instant Active and Reactive Power Theory
Most APFs are designed on the idea of instant active and reactive power theory (p-q), initial projected by Akagi et al in 1983. Initially, it absolutely was developed just for three-phase systems while not neutral wire, being later worked by Watanabe and Aredes for three-phase four wires power systems. the tactic uses the transformation of distorted currents from 3 part frame ABC's into bi-phase stationary frame αβ. the essential plan is that the harmonic currents caused by nonlinear hundreds within the facility is stipendiary with different nonlinear controlled hundreds. The p-q theory is predicated on a group of thirty one instant powers outlined within the time domain. The three-phase offer voltages (ua, ub, uc) and currents (ia, ib, ic) ar remodeled victimization the Clarke (or α-β) transformation into a unique reference system yielding instant active and reactive power elements. This transformation is also viewed as a projection of the three-phase quantities onto a stationary two-axis frame of reference. The Clarke transformation for the voltage variables is given by
[■(u_∝@u_β@u_0 )]=√(2/3) [■(1&-1/2&-1/2@0&√3/2&-√3/2@1/√2&1/√2&1/√2)][■(u_a@u_b@u_c )] (3.1)
Similarly, this transform can be applied on the distorted load currents to give:
[■(i_(l∝)@i_lβ@i_l0 )]=√(2/3) [■(1&-1/2&-1/2@0&√3/2&-√3/2@1/√2&1/√2&1/√2)][■(i_la@i_lb@i_lc )] (3.2)
The instantaneous active power p(t) is defined by:
p(t)= u_a i_la+u_b i_lb+u_c i_lc (3.3)
This expression can be given in the stationary frame by:
{█(p(t)=u_∝ i_(l∝)+u_β i_lβ@p_o (t)=u_o i_lo )┤ (3.4)
Where, p(t) is the instantaneous active power, p0(t) is the instantaneous homo-polar sequence power. Similarly the instantaneous reactive power can be given by:
q(t)=-1/√3 [(u_a-u_b ) i_lc+(u_b-u_c ) i_la+(u_c-u_a ) i_lb ]=u_∝ i_lβ-u_β i_(l∝) (3.5)
It is important to notice that the instantaneous reactive power q(t) signify more than the simple reactive power. The instantaneous reactive power take in consideration all the current and voltage harmonics, where as the habitual reactive power consider just the fundamentals of current and voltage.
From equations (3.4) and (3.5) the instantaneous active and reactive power can be given in matrix form by:
[■(p@q)]=[■(u_∝&u_β@-u_β&u_α )][■(i_lα@i_lβ )] (3.6)
In general, each one of the active and reactive instantaneous power contains a direct component and an alternating component. The direct component of each presents the power of the fundamentals of current and voltage. The alternating term is the power of the harmonics of currents and voltages.
In order to separate the harmonics from the fundamentals of the load currents, it is enough to separate the direct term of the instantaneous power from the alternating one. A Low Pass Filter (LPF) with feed-forward effect can be used to accomplish this task. Fig. 3.2 shows the principle of this extraction filter.
Fig. 3.2: Diagram of the Low Pass Filter with Feed-Forward.
After the separation of the direct and alternating terms of instantaneous power, the harmonic components of the load currents can be given using the inverse of equation (3.6) which gives:
[■(i_lα@i_lβ )]=1/(v_(s∝)^2+v_sβ^2 ) [■(v_(s∝)&〖-v〗_sβ@v_sβ&v_sα )][■(p ̃_l@q ̃_l )] (3.7)
Where, the "~ "sign points to the alternating.
The APF reference current can be then given by:
[■(i_fa^*@i_fb^*@i_fc^* )]=√(2/3) [■(1&0@-1/2&√3/2@-1/2&-√3/2)][■(i ̃_lα@i ̃_lβ )] (3.8)
Fig. 3.3 presents the principle of the active and reactive instantaneous power. This method offers the advantage of the possibility of harmonic compensation and/or reactive power compensation. In the case of reactive power compensation it is enough to send the reactive power q(t) directly to the reference current calculation bloc without the use of any extraction filter.
Fig. 3.3:Principle of Instantaneous Active and Reactive Power Theory.
3.3 Voltage Source Inverter
Voltage supply inverters (VSI) square measure one in every of the foremost vital applications of power natural philosophy. the most purpose of those devices is to supply a three-phase voltage supply, wherever the amplitude, phase, and frequency of the voltages must always be manageable. The vital development of VSI may be a result, from the one hand to the event of quick, manageable, powerful, and strong semi-conductors, from the opposite hand to the employment of the alleged pulse dimension modulation (PWM) techniques. within the high power applications, the 3 level VSIs square measure the foremost adopted as compared with 2 levels ones. as a result of the doctorate of the output voltage and current of the 3 levels VSI is clearly lower.
The standard three-phase VSI topology is shown in Fig. 3.4. it's composed of 3 legs with current reversible switches, controlled for the open and shut. These switches square measure complete by controlled switches (GTO or IGBT) with anti-parallel diodes to permit the flow of the free-wheeling currents.
The switches of any leg of the electrical converter (T1 and T4, T2 and T5, T3 and T6) can't be switched on at the same time as a result of this could end in a brief circuit across the dc link voltage provide. Similarly, so as to avoid undefined states within the VSI, and therefore undefined ac output line voltages, the switches of any leg of the electrical converter can't be converted at the same time as this can end in voltages which will rely upon the various line current polarity.
Fig. 3.4: Three-phase Two Levels VSI Topology
3.3.1 Modeling of Voltage Source Inverter
The output of the VSI which is shown in Fig. 3.4 can take two levels of voltage (+Vdc, – Vdc) dependent on the dc source voltage and the switches states. Actually, the control of the two switches on the same leg is complementary: the conduction of one of them implies the blocking of the other.