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Essay: Power quality problems

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Source current distortion has become a major power quality problem in industries and utility power system because of higher usage of nonlinear loads in the system. Voltage harmonics in power distribution systems arise due to current harmonics produced by the nonlinear loads. The non-sinusoidal currents through electrical systems and the distribution-transmission lines, additional voltage distortions are produced due to the impedance associated with the electrical network. The presence of harmonics in the power system cause greater power loss in distribution, interference problems in communication system and sometimes results in operational failure of electronic equipment which are more sensitive because it contains microelectronic controller systems which work at very low energy levels. There are many ways and means to mitigate the harmonic distortion in the supply system.
The Power Quality (PQ) problem is the occurrence manifested in voltage, current or frequency deviations that results in damage, upset, failure or disoperation of end-use equipment. Almost all PQ issues are closely related with power electronics in every aspect of commercial, domestic and industrial applications. There are power quality standards that define the maximum allowable limit of distortions in voltage and current waveforms in the power supply. Power electronic circuits are the most important cause of harmonics, notches and neutral currents. Harmonics are produced by rectifiers, adjustable speed drives, soft starters, electronic ballast for discharge lamps and switched mode power supplies. Equipment affected by harmonics includes transformers, motors, cables, relays and capacitors. Notches are produced mainly by converters and they principally affect the electronic control devices. Neutral currents are produced by equipment using switched mode power supplies such as personal computers, printers, photocopiers. Neutral currents seriously affect the neutral conductor temperature and transformer capability.
Most of the electrical equipment and circuits create harmonics. If a circuit converts AC power to DC power (or vice versa) as part of its steady-state operation, it is considered to be a harmonic current-generating device. Harmonics cause overloading of conductors, transformers and overheating of utilization equipment. Triplen harmonics can especially cause overheating of neutral conductors on 3-phase, 4-wire systems. While the fundamental frequency and even harmonics cancel out in the neutral conductor, odd-order harmonics are additive. Even under balanced load conditions, neutral currents can reach magnitudes as high as 1.73 times the average phase current. This additional loading creates more heat, which breaks down the insulation of the neutral conductor. In certain cases, it breaks down the insulation between windings of a transformer. But, one can reduce this potential damage by using sound wiring practices. To be on the safe side, more engineers are doubling the size of the neutral conductor from feeder circuits to panel boards and branch circuit partition wiring to handle the additive harmonic currents.
To improve the power quality, traditional compensation methods such as passive filters, synchronous capacitors and phase advancers were employed. However traditional compensators include many disadvantages such as fixed compensation, bulkiness, electromagnetic interference, possible resonance. These disadvantages urged power system and power electronic engineers to develop adjustable and dynamic solutions using custom power devices. Custom power devices are power conditioning equipment using static power electronic converters to improve the power quality of distribution system customers. These include Active Power Filter (APF), Dynamic Voltage Restorer (DVR) and Unified Power Quality Conditioner (UPQC). APF is a shunt compensator used to eliminate the disturbances in current, whereas DVR is a series compensator used to eliminate the disturbances in voltage. Recently UPQC, which consists of both shunt and series compensators, is proposed as a one shot solution for power quality problems. Active power filters are one of the most important remedial measures to solve power quality problems. Nowadays Shunt Active Power Filters (SAPFs), due to their flexibility and reliability are one of the most versatile and efficient solutions in the compensation of the load power factor and current harmonics. The SAPF concept is used in power electronic switching, generating harmonic components to cancel the harmonic components of the nonlinear loads. The SAPF can raise the power factor and improve the quality of power system. A key point in SAPF control is to calculate the compensating reference current value from the nonlinear load current. The maintenance of constant voltage in the DC link capacitor of SAPF is necessitated to reduce the reactive power requirement. The proposed work is to develop a voltage source inverter based SAPF for a three phase three wire system, focusing on reduction of the Total Harmonic Distortion (THD) and for the improvement of power factor using different control techniques and soft computing technique.
There are two approaches for the mitigation of power quality problems. The first approach is called load conditioning, which ensures that the equipment is made less sensitive to power disturbances, allowing the operation even under significant voltage distortion. The other solution is to install line-conditioning systems that suppress or counteract the power system disturbances. Following are some of the power quality improvement solutions that have evolved over the years ever since the power industry established. Four of the most recommended solutions include:
‘ Increasing the size of the neutral conductor
‘ Decreasing the load of delta-wye transformer
‘ Replacing the delta- wye transformer with a k-factor transformer
‘ Installing a harmonic filter at the power source or equipment location
The first three solutions are to cope with the problem; the fourth actually eliminates the problem. A harmonic filter can eliminate the potentially dangerous effects of harmonic currents created by nonlinear loads. It traps these currents and through the use of a series of capacitors, coils, and resistors shunts them to ground. A filter unit may contain several of these elements, each designed to filter a particular frequency. Filters can be installed either between the device to be protected and the load’s power source, or between the device causing the condition and its power source. There are several types of harmonic filters:
‘ Passive Filter
‘ Fixed Capacitors
‘ Switched Capacitor Banks
‘ Synchronous Condensers
‘ Static VAR Compensator
1.3.1 Passive Filters
The passive filters are inexpensive compared with most mitigating devices. Internally, the filters cause the harmonic current to resonate at its frequency. This prevents the harmonic current from flowing back to the power source and causing problems with the voltage waveform. A disadvantage of the passive filter is that it cannot be perfectly tuned to handle the harmonic current at a significant frequency.
1.3.2 Fixed Capacitor
The use of the fixed capacitors is the first attempt towards reactive current compensation. Figure 1.1 shows a system employing fixed capacitor. When a capacitor is placed in shunt, the voltage at the Point of Common Coupling (PCC) forces a fixed leading current to flow. This compensates for the lagging current drawn by the load. Obviously, the disadvantage is that the leading current drawn is fixed by the PCC voltage and when the load is varying, there will always be improper compensation.
Figure 1.1 Reactive current compensation using fixed capacitor
1.3.3 Switched Capacitor Banks
The switched capacitor banks came up as a solution to the problem of improper compensation under varying load conditions. Capacitor banks were used with manual switches as shown in Figure 1.2. Capacitors will be switched in and out of the system manually, according to the load. However, the compensation was still not exact.
Figure 1.2 Reactive current compensation using switched bank capacitors
1.3.4 Synchronous Condensers
Synchronous machines, when connected to the grid can be used to absorb or deliver reactive current. Actually, it acts as a variable voltage source behind a reactance as shown in Figure 1.3. The advantages of synchronous condensers over fixed and switched capacitors are:
1. They can be controlled to follow the reactive current demand of the system
2. The reactive current compensation is not fixed entirely by the PCC voltage (though PCC voltage also influences it)
3. They can provide both lagging and leading reactive currents
Figure 1.3 Reactive current compensation using synchronous condensers
Though synchronous condensers were a step forward in the area of reactive current compensation as they are dynamic unlike the capacitor based compensation, they still had the following disadvantages:
1. They cannot respond quickly to varying load conditions. The response is
sluggish owing to the inertia of the mechanical parts involved.
2. They cannot be used to compensate for harmonic currents absorbed by the
1.3.5 Static VAR Compensator (SVC)
Static VAR Compensator (SVC) is one of the first attempts in using power electronics for power quality compensation which is shown in Figure 1.4. As shown, the compensating system can be controlled to follow the load’s reactive power requirement by controlling the thyristors. The major disadvantage is that the compensating system itself will inject harmonics into the system due to switching action.
Figure 1.4 Static VAR Compensator
Harmonic distortion due to power electronic loads and nonlinear electronic devices deteriorate the power quality of the system. Conventional passive filters have been used to eliminate current harmonics. These filters work well if the harmonics of interest are well known and relatively invariant over time. However, the passive filter must be tuned to the frequencies of the harmonics to be removed and when the harmonics change the filter effectiveness is reduced. If multiple harmonics are to be removed, the passive filter will require multiple stages, increasing size and cost. Passive filters can also produce harmonics due to resonance between the filter and source impedance.
For overcoming the drawbacks of passive filters and reducing power quality problems, a number of attempts have been made on the design, analysis and optimal control development of Active Power Filter (APF). The APF is a power electronic device that can dynamically suppress harmonics and compensate reactive power regardless of the changes of their frequencies and amplitudes. Active filters act as a harmonic attenuation device. The capacity and performance of the active power filter is determined by the choice of components and the execution of the power circuit.
The various compensation methods discussed so far were in use when there was no problem of harmonics. With the advent of harmonic loads, the capacitor based compensation techniques had a set back as the capacitors got heated up due to large harmonic current flows and sometimes there was failure due to resonance phenomenon too. The synchronous condensers and SVCs also could not compensate for the harmonic currents and hence there was a need for developing newer methods of power quality compensation. Active power filters were the result of research in this direction and they aid in dynamic compensation of reactive and harmonic currents demanded by the load. As shown in Figure 1.5, Active Power Filters are basically Voltage Source Inverters (VSI) behind a reactance. They can be controlled to continuously track and compensate for the reactive current demanded by the load.
Figure 1.5 Active Power Filter
Controlling the active power filter involves making the choice between open loop and closed loop current control. In open loop mode, the harmonic currents are measured on the load side of the active filter, which are used to calculate the required compensating current to be injected into the network. In closed loop control, the resulting current in the network is measured and the active filter injects a compensating current in order to minimize it. The open loop control system is easier to implement but is less efficient and require higher accuracy current sensors. The closed loop control is more precise.
The reference current generator and the control circuit are the two most important parts of an active power filter. The IGBT bridge uses a dc voltage source in the form of a capacitor and the generated voltage is coupled to the network via reactors. Changes in the network or the addition of new loads will not cause failure of well designed active power filter which may be designed to function to its full capacity without being overloaded. The active power filter provides power factor correction, which increases the efficiency of the load and eliminates problems such as voltage sags and power line flicker.
1.4.1 Various configurations of active power filter
Depending on the particular application or electrical problem to be solved, active power filters can be implemented as shunt type, series type, or a combination of shunt and series active filters (shunt-series type). These filters can also be combined with passive filters to create hybrid power filters.
‘ Series Active Power Filter
‘ Shunt Active Power Filter
‘ Unified Power Quality Conditioner (UPQC)
‘ Hybrid Filter Series Active Power Filter
Series active power filter is connected before the load in series with the mains using a matching transformer as shown in the Figure 1.6, to eliminate voltage harmonics and regulate the terminal voltage of the load or line. It is able to compensate for distortion in the power line voltages making the voltage applied to the load sinusoidal. The filter consists of a VSI, behaving as a controlled voltage source and is interfaced with the power system through transformer. The series active power filter does not compensate for load current harmonics but it acts as high impedance to the current harmonics coming from the power source side.
Figure 1.6 Series Active Power Filter Shunt active power filter
Shunt active power filter is most widely used to eliminate current harmonics, reactive power compensation and balancing the unbalanced currents. It is mainly used at the load end as shown in the Figure 1.7, because nonlinear loads inject current harmonics. Shunt active power filter injects equal compensating currents opposite in phase to cancel harmonics produced by the nonlinear load. It can also be used for stabilizing and improving the voltage profile.
Figure 1.7 Shunt Active Power Filter Unified power quality conditioner
Unified power quality conditioner shown in Figure 1.8 is a combination of shunt and series active filters. The DC link storage element is shared between two current source or voltage source bridges operating as series and shunt compensators. It can balance and regulate terminal voltage and eliminate negative sequence currents. Its main drawbacks are its high cost and control complexity because of the large number of solid state devices involved.
Figure 1.8 Unified Power Quality Conditioner Hybrid filter
The hybrid filter which is a combination of an active and passive filter is quite popular because the solid state devices used in the active series part can be of reduced size and cost and a major part of the hybrid filter is made of the passive shunt L-C filter used to eliminate lower order harmonics. A hybrid filter is shown in Figure 1.9. It has the capability of reducing voltage and current harmonics at a reasonable cost. The hybrid approach can decrease the size of both active and passive filters and this approach is desired in high power applications.
Figure 1.9 Hybrid Filter
Considering the merits and demerits of the various configuration of the Active Power filters, the shunt active power filter is technically suitable choice for the proposed work.
The nonlinear loads (Rectifiers with RL Load) generate non-sinusoidal current in AC mains and cause reactive power burden, low power factor, excessive neutral current and distortion in quality of power. They are also responsible for lower efficiency and interference of the system. In this thesis, the SAPF is proposed to mitigate the problems mentioned and various algorithms for SAPF are proposed to improve its performance thereby improving the quality of power. The reactive power requirement by the SAPF is also minimized to enhance its performance. A suitable algorithm is proposed and proved to have better performance of SAPF.
Modern electrical system has sudden increase of single and three phase load. The use of power semiconductor converter causes many problems like harmonics, reactive power burden. This work aims to minimize all the problems using Shunt Active Power Filter. The control strategy which generates compensating current is based on Time domain approach or Frequency domain approach. The frequency domain takes the use of Fourier Transform (FT) which leads to large amount of calculations, making the control method much more complicated. Time Domain approach based reference compensating current is generated to overcome the limitation of Frequency domain approach. In order to keep harmonic measures under limits proposed by standards, it is necessary to include some sort of compensation.
The aim of this research is to investigate the performance of Shunt Active Power Filter for power quality enhancement with various control strategies. The problems of the existing methods of compensating harmonic components in the supply system are resolved by framing the following objectives.
‘ To propose new control strategies for SAPF using I-COS??
and Synchronous Detection (SD) algorithms for reducing
THD and improving power factor.
‘ To analyze and regulate DC link capacitor voltage of SAPF in order
to improve power quality by incorporating Proportional Integral
(PI) and Fuzzy Logic Controllers (FLC).
‘ To formulate the new control approach for improving the
performance of SAPF using instantaneous P-Q theory and self-
adaptive algorithms to enhance power factor and quality of power.
It is hypothesized that different control strategies of time domain approaches like I-COS?? algorithm, Synchronous Detection (SD) algorithm, Synchronous Frame Theory and Instantaneous P-Q theory are adopted to generate the reference compensating current. The reference compensating current is compared with filter current to develop switching pulses for the proposed scheme of Shunt Active Power Filter (SAPF) for enhancement of quality of three phase system. The DC link voltage of SAPF is maintained constant by PI controller and FLC to minimize its reactive power requirements.
To achieve the aim and objectives of the proposed work, a modest attempt has been made to improve the performance of SAPF with different control algorithms and soft computing approach. Reductions of harmonics, improvement in power factor of three phase system and power quality enhancement are the thematic area of the proposed research work.
Fast progress in the field of power electronic devices with active filters has been extensively studied and a large number of works have been published in the past two decades. The following sections describe the related work carried out by different researchers.
Bhuvaneswari & Manjula Nair (2008) had dealt with the
I-COS?? algorithm for Shunt Active Power Filter based on the active portion of the fundamental load current. The implementation of algorithm by making use of OP-AMP was discussed and is found to cause serious problem during initiating and load perturbations.
Parmod Kumar & Alka Maharajan (2009) had discussed the design of non-model based controllers to control the switching of the active power filter and were found to provide much response under varying load and supply conditions. Absence of systematic procedure for designing system and deficiency in completing the procedure is the major drawback of this technique.
Salem Rahmani et al (2010) had discussed computational control delay compensation. The control method provides compensation for reactive, unbalanced and harmonic load current components. The control scheme requires several steps such as load current measurement, harmonic current detection and reference filter current generation and filter current control as the major precincts of the system.
Francisco et al (2012) had investigated the methods for determining the fundamental frequency and harmonic positive and negative sequence component of three-phase signals. The merit includes that current controllers have been developed for Active Power Filters to achieve good control performance. Complex mathematical functions, difficulty to achieve both dynamic response and steady state harmonic tracking performance were the major drawbacks of the system.
Rondineli Rodrigues Pereirala et al (2011) had discussed that the Phase Locked Loop (PLL) provides fast and robust frequency estimation, even for distorted and unbalanced conditions; however in some cases its performance can be affected by a wrong choice of the centre frequency, undesired oscillations due to harmonics and sub-harmonics, transient errors due to a narrow bandwidth chosen to achieve a good noise rejection. The strategy improves the transient time and diminishes the computer burden of the algorithm. The drawback is the use of the Clarke transformation directly into the load current, generating the two orthogonal inputs signals for the notch ‘lters.
Proportional-Integral Controller was developed by Quoc- Nam Trinh and Hong- Hee Lee (2013). The control scheme requires only two current sensors at the supply side and does not need a harmonic detector in order to make the supply currents sinusoidal. It has good steady state performance with nonlinear loads as well as dynamic response against load variations. It has high ripples due to switching action of the inverter.
Pereira et al (2011) had discussed about the Least Mean Square algorithm which was used to adjust the coefficient of adaptive notch filter. An algorithm based on the Clarke Transformation was used to detect the load current amplitude transients and then modify the step size value. Transient detection requires Clarke Transformation which is more tedious to realize.
Mikkili Suresh et al (2011) discussed the Fuzzy Logic Controller based three phase four wire shunt active power filter for mitigation of current harmonics with combined p-q and Id- Iq control strategies. The performance of two control strategies for extracting reference currents of shunt active power filter under balanced, un-balanced and non-sinusoidal conditions by Fuzzy Logic Controller was evaluated and compared. The well-known methods, instantaneous active and reactive power method (p-q) and active and reactive current method (id-iq) are two control methods which are extensively used in active filters.
Peng Xiao et al (2009) had discussed about two or more paralleled semiconductor switching devices which can be used to handle the large compensation currents and provide better thermal management topology. The two active filter inverters are connected with tapped reactors to share the compensation currents and also produce seven voltage levels which significantly reduce the switching current ripple and the size of passive components. Due to more voltage levels, a converter has less harmonics and better power quality. It can also reduce voltage stress across switching device. However the increase in converter complexity and number of switching devices are major concern for multi level converters.
Mooley kutty George & Karthik Prasad Basu (2008) had dealt the performance of three phase Shunt Active Power Filter using Synchronous Detection (SD) algorithm and had been compared with Nonlinear Auto Regressive Moving Average Controller(NARMA)L-2 based Active Power Filter(APF).The NARMA L-2 Controller is used to determine the amplitude of the reference source current required in APF system and also discussed about modeling and control of APF system using Synchronous Detection(SD) algorithm. The performances of the different system have been compared. It is observed that the complicated calculations used in Synchronous Detection (SD) algorithm could be eliminated by the use of NARMA-L2 Controller. The simulink model is necessary to train the NARMA-L2 Controller.
Ahmed A Helal (2009) had discussed the Fuzzy Logic Controller based SAPF used to regulate the filter current and reference compensation current calculation depends on Fast Fourier Transform (FFT). FFT method is simple and reliable for reference compensating current calculation and it is used to extract the magnitude of fundamental component of the load current from which the reference compensating current will be computed.
Hamza Bentria (2011) had dealt about the time domain methods which are used to detect the harmonic current references. Requirement of less calculation and widely followed for computing the reference current are the advantages of these methods.
Mauricio Aredes et al (2009) had discussed the control methods based on P-Q & P-Q-R theory and it includes additional calculations to allow the elimination of energy storage elements in the active filters. P-Q theory is proposed to eliminate the neutral current without the need of energy storage elements with the active filter. The P-Q-R has the advantages of ‘?0 to pqr transformation.
Vasco Soares et al (2000) had discussed about an active filter based on the instantaneous active and reactive current (id-iq) method. A comparison between active and reactive current component had been carried out. Under balanced and sinusoidal voltage conditions the control method proposed is identical to the instantaneous active and reactive power P-Q method. Neither of the harmonics to be accurately compensated under unbalanced and non sinusoidal conditions nevertheless the id-iq method always leads to better results.
Raddek et al (2013) had focused on the implementation of two basic representations of adaptive algorithms. First algorithm is with a stochastic Least Mean Square (LMS) gradient adaptation and then another is the Recursive Least Square (RLS) optimal algorithm. The proposed system reduces distortion and change in supply voltage. With the use of the adaptive algorithms, the Active Harmonic Compensation (AHC) system shows very good dynamics, resulting in a much faster transition during AHC connection and disconnection and also a change in harmonic load on the system. The comparison had been carried out between two algorithms and the adaptive LMS algorithm is simple and mathematically undemanding, unfortunately, it achieves a lower speed and high convergence error during filtering process. On the other hand RLS algorithm is very much involved mathematically. It produces accurate result with low error rate and has an extremely high rate of convergence.
An improved control algorithm of shunt active filter for voltage regulation, harmonic elimination, power factor correction and balancing of nonlinear loads was discussed by Chandra, A. et al (2000).The control algorithm of the active filter uses two closed loop PI controllers. The DC bus voltage of the AF and three phase supply voltages are used as feedback signals in the PI controllers. The control algorithm of the AF provides three phase reference supply currents. A carrier wave pulse width modulation (PWM) current controller is employed over the reference and sensed supply currents to generate gating pulses of IGBTs of the AF.
Simone Buso et al (1998) have dealt that comparative evaluation of the performance of current control techniques for active filters. The linear rotating frame current controller, the fixed frequency hysteresis controller and the dead beat controller are used to control SAPF. The advantage of linear rotating current controller solution is that fundamental harmonic components of voltage and current signals appear constant to the current regulator. Therefore the sinusoidal line voltage is seen by a current regulator as a constant quantity. On the other hand the bandwidth limitation of the PI regulator which remains unchanged, still implies significant errors in the tracking of the higher order harmonic components of the current reference. The advantages of the dead beat controller are that it may not require the line voltage measurement in order to generate current reference. Indeed the dead beat controller’s algorithm implies an estimation of the line voltage instantaneous value which can therefore be used for the current reference generation. On the other hand, the inherent delay due to the calculation is indeed a drawback of this technique. However the result of comparison, hysteresis controller has better performance. The control strategy is almost unaffected by the variation of the firing angle and on the basis of performance of indices like THD, RMS of current error.
Bhim Singh & Jitendr Solanki (2009) had discussed about the control of Shunt Active Filter using an Adaptive linear element (Adaline) based current estimator to maintain sinusoidal and unity power factor source current. Three phase load currents are sensed using Least Mean Square (LMS) algorithm based Adaline and on line calculation of the weights is performed and these weights are multiplied by the unit vector template which gives fundamental frequency real component of load current. The switching of Voltage Source Inverter is performed using hysteresis based pulse width modulation indirect current control scheme which controls the source currents to follow the derived reference source current.
By reviewing all the methods in literature, it is found that each method has its own merits and demerits. To overcome the limitations and to enhance the performance of Shunt Active Power Filters, new control schemes are proposed to improve the quality of power.
The objective of the proposed research work is to analyze the performance of Shunt Active Power Filter for improving the quality of the three phase system. Mainly two steps such as reference compensating current generation and gating signal generation are required for the implementation of Shunt active power filters. Various algorithms and techniques are proposed to improve the performance of SAPF. The frame work of the research work is shown in Figure 1.10 which describes the works proposed and carried out in SAPF with various control techniques for improving the profile of source current.
Figure 1.10 Overview of the Proposed Scheme of SAPF for Power
Quality Enhancement
I-COS?? algorithm is one of the methods proposed to generate reference compensating current to improve the quality of the source current by reducing the harmonics. The mathematical expressions are obtained by proper derivations and this would be incorporated in the simulation. The whole power circuit with necessary control algorithm will be simulated with balanced and unbalanced load in the MATLAB. The simulation results would be analyzed and compared before and after compensation.
Synchronous Detection (SD) algorithm is proposed to generate compensating current from SAPF. The effectiveness of SD algorithm is to be validated under balanced as well as unbalanced load conditions because the compensating currents are calculated taking into account the magnitudes of per phase voltages. The SAPF power circuit will be simulated with the SD algorithm. The simulation results would be obtained before and after compensation. The SD and I-COS?? algorithms based simulation work would be compared to identify better algorithm. The power circuit of SAPF will also be simulated with various multiple carrier disposition PWM techniques. Based on the simulation work, the effectiveness of disposition technique would be established.
PI control algorithm is proposed to regulate the DC link voltage of the SAPF. The three phase model of the SAPF will be simulated with PI and PID controllers and compared to find better controller for SAPF.
The Fuzzy Logic Controller based control scheme of SAPF is also proposed with designed fuzzy inference system. The Fuzzy Logic Controller based three phase SAPF would be simulated. The results are presented and analysed to evaluate the performance of SAPF. This new method combines both the strategies for extracting the three phase reference currents for active power filters and DC link voltage control method. To investigate the performance of SAPF, the study has been accomplished using simulation with MATLAB Simulink.
Recursive Least Square (RLS) algorithm and Least Mean Square (LMS) algorithm are utilized to generate compensating current by the SAPF. The SAPF is simulated and the results are presented. The self adaptive algorithms recently get more and more applications because of its simplicity and fast convergence.
The simulation of SAPF incorporating LMS and RLS algorithms would be carried out and the results are analysed. The DC link capacitor voltage will also be maintained by closed loop control to minimize the reactive power requirement. From the simulation work, the expected results are obtained and it will be concluded that the shunt active power filter with Recursive Least Square algorithm performs well and it maintains capacitor voltage constant. The THD in the source current is also calculated and the performance of SAPF is discussed.
In the chapter I, the background of power quality issues, power quality problems, the different traditional and modern solutions of harmonic problems and the available solutions are discussed briefly. Also certain active power filters topologies have been briefly discussed. Moreover, this chapter includes the details of literature review which have been referred for this thesis work.
In the chapter II, the shunt active power filter is discussed in detail. The basic compensation principle of shunt active power filter, estimation of reference source current, control scheme, design of dc link capacitor have been discussed. The simulation model of SAPF has also been discussed briefly. The I-COS?? and Synchronous Detection algorithms based shunt active power filter is proposed for eliminating current harmonics and compensating reactive power. Simulation results are analysed to show the differences between the performances of SAPF with two algorithms. The results are presented and discussed in this chapter.
The design of PI and Fuzzy Logic Controllers based SAPF is discussed in chapter III. A detailed study of Fuzzy Logic Controller is presented. The simulation details of PI and Fuzzy Logic Controllers have also been discussed. The simulation results are presented and analyzed. The comparison of THD for three phase system is also carried out.
In Chapter IV, The concept of self adaptive algorithms is explained. The LMS and RLS algorithms are derived. The simulation of SAPF with these algorithms is discussed. The detailed discussion on simulation results are presented in this chapter.
Chapter V, presents the conclusion and also summarizes the outcome of the research work. The possible direction for future works is also outlined in this chapter.
An introduction about power quality and solutions to harmonic related problems is discussed. Several types of harmonic filters and their performance are also presented in this chapter. Various types of active power filters used in power system are presented. Motivation, Hypothesis and objectives of the thesis are briefly discussed. The organisation of the thesis is also presented.

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