1552884026060_full Thesis.pdf

CHAPTER 1

INTRODUCTION In recent years many power electronics converters utilizing switching devices have been widely used in industrial as well as in domestic applications. It desires to draw purely sinusoidal currents from the distribution network, but this is no longer the case with this new generation of receivers that take advantage of all the recent advances and improvements in power electronics. These power electronics systems such as high-power diode/thyristor rectifiers, arc furnaces, cycloconverters, and variable speed drives offer highly nonlinear characteristics [1] Some of the small power domestic electrical appliances like TV sets and computers, multiple low-power diode rectifier, and microwave ovens also draw distorted currents. These nonlinear loads lead to generation of current/voltage harmonics and draw reactive power and are becoming troublesome problems in ac power lines. The increase in such nonlinearity causes different undesirable features like low system efficiency and poor power factor. It also causes disturbances to other consumers and interference in nearby communication networks [3]. The effect of this nonlinearity could become sizeable over the next few years. Hence it is very important to overcome these undesirable features Electrical energy is the most efficient and popular form of energy and the modern society is heavily dependent on the electric supply. The life cannot be imagined without the supply of electricity. At the same time the quality of the electric power supplied is also very important for the efficient functioning of the end user equipment .The term power quality became most prominent in the power sector and both the electric power supply company and the end users are concerned about it. The quality of power delivered to the consumers depends on the voltage and frequency ranges of the power. If there is any deviation in the voltage and frequency of the electric power delivered from that of the standard values then the quality of power delivered is affected .Now-a-days with the advancement in technology there is a drastic improvement in the semi-conductor devices. With this development and advantages, the semi-conductor devices got a permanent place in the power sector helping to ease the control

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of overall system [2]. Moreover, most of the loads are also semi-conductor based equipment. But the semi-conductor devices are non-linear in nature and draws non-linear current from the source. And also the semi-conductor devices are involved in power conversion, which is either AC to DC or from DC to AC. This power conversion contains lot of switching operations which may introduce discontinuity in the current [4]. Due to this discontinuity and non-linearity, harmonics are present which affect the quality of power delivered to the end user. In order to maintain the quality of power delivered, the harmonics should be filtered out. Thus, a device named Filter is used which serves this purpose. Application of DC electrified railways as a significant metropolitan means of transportation is increasing greatly. DC Electrified railways play an important role for public transportation because of high efficiency, heavy ridership and fast transportation. However, they result in great

power quality problems for the power

distribution system which feeds the traction system. In DC electrified railways, the rectifiers of the traction substations are a major cause of harmonic distortion in the AC supply High THD of the system current, harmonics and inter harmonics, reactive power consumption, voltage unbalance and flicker and low power factor problems can suffer the power distribution system feeding the traction In anticipation of the proliferation of nonlinear load sand to limit the problems, recommended guidelines like the IEEE Std. 519-1992 specify the allowable harmonic associated in the currents drawn from the utility system. Different methods are utilized for improving the power quality issues of the power distribution system such as dynamic voltage regulators, Statcom and active or hybrid [2]-[5] The consumption of reactive power in industrial and domestic loads presents also an important issue in discussion of power quality problems .the reactive power consumed by non-resistive loads cause higher rms current values in addition to extra heating of power transmissions and distribution system. The use of batteries of capacitors or synchronous machines for local reactive Power production has been proposed for a long time .the accelerated development of power electronics and semiconductor production had encouraged the use of STATIC VAR compensators for the reactive power compensation. However, these solutions looks inefficient and can cause extra problems in power system in the case of high current and voltage harmonic emissions. The fact that these systems are especially designed to compensate the fundamental based reactive power, in addition to high possibilities of interaction between these compensation elements and system harmonics make it unstable solutions in modern technologies. [2]

During the last three decades the researchers were encouraged by the development of power electronics industry the revolution in digital signal processing production and the increasing demand for efficient solutions of power quality problems including harmonics problem .They were encouraged to develop modern flexible, and more efficient solutions for power quality problems .These modern solutions have been given the name of active compensators or active power filter .The objective of dissertation of these active power filter abbreviation mostly APF is to compensate harmonic and reactive power compensation and DC power generation was proposed .The main advantages of the APFs are their flexibility to fit load parameters variations and harmonic frequencies in addition to high compensation performance[3]-[4]. Many types of APF has been proposed and used in harmonic compensation .series APF are used for voltage harmonics compensation shunt APF were proposed for current harmonics and reactive power compensation .the Unified power quality filter or conditioner combines the two types shunt and series APF in one device responsible for simultaneous compensation of voltage current harmonics and reactive power [2]. Although there are different types of APF, the Shunt APF are still the most famous and used type APF. The main function of shunt active power filter is to cancel harmonics current occurring in power grids the principle of SAPF is to generate harmonics current equal in magnitude and opposite in phase to those harmonics that circulate in the grids. The nonlinear loads absorb nonsinusoidal form .In this dissertation work from the static power device SAPF is used with PI controller for the power quality enhancement in distribution system .Here two different loads are considered, nonlinear load and unbalanced nonlinear load to enhance the power quality in distribution

1.2 TYPES OF LOAD Loads can be characterized into many types according to their nature, function etc. The type of load we are interested in are 1. Linear load 2. Non-linear load 1.2.1 LINEAR LOAD Electrical loads whose current wave has a linear relation with the voltage wave are termed as linear loads. These loads do not cause any harmonic in the electrical system. [3]

1.2.2 NON-LINEAR LOAD The nonlinear loads are referred to as the loads that distort the current waveform shape due to the switching action and the current and voltage waveforms are not identical in shape, e.g. fluorescent lamp, PC and TV etc. Figure 1.1 shows how harmonics injected by non-linear loads distort the current waveform

Fig. 1.1 – Distortion in current waveforms due to harmonics

1.3 PROBLEMS CAUSED BY HARMONICS Following are the problems that are caused by the presence of harmonics in power system. 1.3.1 EFFECT ON POWER SYSTEM ITSELF The major effect of power system harmonics is to increase the current in the system. This is particularly the case for the third harmonic, which causes a sharp increase in the zero sequence current, and therefore increases the current in the neutral conductor. 1.3.2 EFFECT ON CONSUMER ITSELF Non-linear loads also causes harmonics/distortions in utility supplied voltages due to which even the linear loads draw non -linear current. Harmonics can also cause thyristor firing errors in converter. The performance of consumer equipment, such as motor drives and computer power supplies, can be adversely affected by harmonics. [4]

1.3.3 EFFECT ON COMMUNICATION SYSTEM Harmonic currents flowing on the utility distribution system or within an end-user facility can create interference in communication circuits sharing a common path. Voltages included in parallel conductors by the common harmonic currents often fall within the bandwidth of neutral voice communications. Harmonic currents on the power system are coupled into communication system by either induction or direct conduction. 1.3.4 EFFECT OF REVENUE BILLING Electrical utility companies usually measure energy consumption in two quantities energy consumed and the maximum power used for given period. Both energy and demand are measured using the so-called watt -hour and demand meters. Harmonic currents from non -linear loads can impact the accuracy of watt-hour and demand meter adversely. Traditional watt -hour meters are based on the induction motor principle. Conventional magnetic disk watt -hour meters tend to have a negative error at harmonic frequencies. That is, they register low for power at harmonic frequencies if they are properly calibrated for fundamental frequency. This error increases with increasing frequency

1.4 Literature Review Singh, Bhim; Al-Haddad, K.; Chandra[1] -The use of active power filters for power quality improvement is discussed in In this paper a review of active filter configuration for power quality improvement is presented along with control strategies. It is found that the active filters are facing some drawbacks when employed for power quality improvement. They are High converter ratings are required, Costlier when compared to its counterpart, passive filter, Huge size, Increased losses.

Rivas, D.; Moran, L.; Dixon, J.W.; Espinoza, J.R[2]-.This paper discusses how a combination of both active and passive filters is an economical solution for power quality improvement To enhance the characteristics of passive filter and also the system, the active filter should be controlled properly. There are different control techniques for this purpose. The main aim of any control technique is to make active filter inject a voltage in to the system that compensates the

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harmonics. To achieve this output voltage of the active filter is controlled such that it is equal to a pre-calculated reference value. Herrera, R.S.; Salmeron, P.[3]-This is presented in and it discusses the different control algorithms from the formulations of instantaneous reactive power theory. Finally it concludes that vector based theory yields better results with sinusoidal currents when compared with other algorithms Salmeron, P.; Litran, S.P.[4]- The control of series active in conjunction with shunt passive filter using dual instantaneous reactive power vector theory is presented in . In this paper the proposed theory is validated by simulating it in MATLAB SIMULINK environment he proposed control strategy is simulated for both balance and unbalanced load M H J Bollen [5]- The quality of power is affected when there is any deviation in the voltage, current or frequency . The common problems that affect the sensitivity of the equipment Leszek S. Czarnecki [6] -This paper investigates how power phenomena and properties of three-phase systems are described and interpreted by the Instantaneous Reactive Power (IRP) p-q Theory This paper shows, moreover, that the IRP p-q Theory is not capable to identify power properties of three-phase loads instantaneously. A pair of instantaneous values of and powers does not allow us to conclude whether the load is resistive, reactive, balanced, or unbalanced. It is known that a load imbalance reduces power factor. However, the IRP p-q Theory does not identify the load imbalance as the cause of power factor degradation. G. Satya Narayana, Ch. Narendra Kumar, Ch. Rambabu [7]- This paper presents a fuzzy logic, PI controlled shunt active power filter used to compensate for harmonic distortion in threephase systems. The Hybrid active power filter employs a simple method for the calculation of the reference compensation current based on Fast Fourier Transform. The presented Hybrid Active Power filter is able to operate in balanced, load conditions. Classic filters may not have satisfactory performance in fast varying conditions. But auto tuned active power filter gives better results for harmonic minimization, and THD improvement. The proposed auto tuned hybrid active power filter maintains the THD well within IEEE-519 standards. The proposed methodology is extensively tested and with improved dynamic behavior of hybrid active power Filter using fuzzy logic, PI controllers. The results are found to be quite satisfactory to mitigate harmonic Distortions, and improve quality.

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Shailendra Kumar Jain & Pramod Agarwal [8] - This paper presents complete design, on a 3phase shunt active power filter to compensate harmonics and the reactive power requirement of nonlinear loads. The paper describes the completed ensign aspects of power circuit elements and control circuit parameters. The compensation process is based on sensing line currents only, an approach different from conventional methods that require the harmonics and reactive voltampere requirement of the load. Various simulation results are presented to study the performance during steady-state and transient conditions to validate the design. A lab oratory prototype has been developed to verify the simulation results. The control scheme is realized on a dedicated micro-controller based system. PWM pattern generation is based on carrier less hysteresis-based current control to obtain the switching signals. Based on simulation and experimental results it can be concluded that the compensation process is simple and easy to implement. The spectral performance shows that the active filter brings the THD of the system well below 5%, the limit imposed by the IEEE-519. Bhim Singh,; Ambrish Chandra and Kainal Al-Haddad[9]-In this paper, a 3-phase active power

filter ('APF) is presented to eliminate harmonics and to compensate the reactive power of an uncontrolled rectifier with active loading taken as non-linear load. APF is realized using 3-phase voltage source inverter (VSI) with dc bus capacitor. Reference source currents are estimate- d using P-I control over dc bus voltage and 3-phase source voltages. Command currents of the APF are obtained with reference source currents and load currents. A hysteresis based carrier less PWM current control over the command currents of the APF is used to derive gating signals to the devices of APF. Modeling and performance characteristics of an I8 kW APF to meet the IEEE-519 standards are presented. W. M. Grady, M. J. Samotyj A. H. Noyola,[10]:- Active power line conditioning (APLC) is a relative new concept that can potentially correct network distortion caused by power electronic loads by injecting equal-but-opposite distortion at carefully selected points in a network. This paper presents the results of an extensive literary survey on the subject of APLCs. Thirty-seven key publications are identified and reviewed. Existing and proposed line conditioning methodologies are compared, and a list of the advantages and limitations of each is presented.

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1.4 ORGANIZATION OF DISSERTATION: The whole dissertation is organized into six chapters including introduction and each chapter is summarized below.  Chapter 1- this chapter describe the introduction of custom power devices to mitigate the power quality issues with literature review, and organization of dissertation.  Chapter 2- this chapter describe the power quality term classification of power quality problems, effect of power quality problem,  Chapter 3-this chapter describe with the types of filters available for harmonic reduction. It explains the merits and demerits of each type of filter with a circuit diagram.  Chapter 4-this chapter describe the control technique of SAPF filter and modeling of filter.  Chapter 5-this chapter describe the simulation and results of dissertation work.  Chapter 6-this chapter describe the conclusion and future scope.

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CHAPTER 2

POWER QUALITY PROBLEMS Both electric utilities and end users of electric power are becoming increasingly concerned about the quality of electric power. The term power quality has become one of the most prolific buzzwords in the power industry since the late 1980s. It is an umbrella concept for a multitude of individual types of power system disturbances. The issues that fall under this umbrella are not necessarily new. What is new is that engineers are now attempting to deal with these issues using a system approach rather than handling them as individual problems. There are four major reasons for the increased concern:  Newer-generation load equipment, with microprocessor-based controls and power electronic devices, is more sensitive to power quality variations than was equipment used in the past.  The increasing emphasis on overall power system efficiency has resulted in continued growth in the application of devices such as high-efficiency, adjustable-speed motor drives and shunt capacitors for power factor correction to reduce losses. This is resulting in increasing harmonic levels on power systems and has many people concerned about the future impact on system capabilities.  End users have an increased awareness of power quality issues. Utility customers are becoming better informed about such issues as interruptions, sags, and transients and are challenging the utilities to improve the quality of power delivered.  Many things are now interconnected in a network. Integrated processes mean that the failure of any component has much more important consequences.

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2.1 WHAT IS POWER QUALITY? There can be completely different definitions for power quality, depending on one’s frame of reference. For example, a utility may define power quality as reliability and show statistics demonstrating that its system is 99.98 percent reliable. Criteria established by regulatory agencies are usually in this vein. A manufacturer of load equipment may define power quality as those characteristics of the power supply that enable the equipment to work properly. These characteristics can be very different for different criteria. Power quality is ultimately a consumerdriven issue, and the end user’s point of reference takes precedence. Therefore, the following definition of a power quality problem is used in this report Any power problem manifested in voltage, current, or frequency deviations that result in failure or mal-operation of customer equipment. Institute of Electrical and Electronic Engineers (IEEE) Standard IEEE1100 defines power quality as “The concept of powering and grounding sensitive electronic equipment in a manner suitable for the equipment.” 2.2 PERCEPTION ABOUT POWER QUALITY PROBLEMS Perception about power quality problem is different when it comes to customer and utility. Fig.2.1 depicts this trend

Fig. 2.1 Results of a survey on the cause of power quality problems 2.3 GENERAL CLASSES OF POWER QUALITY PROBLEMS Classification of power quality problems can be made as follows:

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2.3.1 Transients: The term transient has long been used in the analysis of power system variations to denote an event that is undesirable and momentary in nature. The notion of a damped oscillatory transient due to an RLC network is probably what most power engineers think of when they hear the word transient. Other definitions in common use are broad in scope and simply state that a transient is “that part of the change in a variable that disappears during transition from one steady state operating condition to another.” Unfortunately, this definition could be used to describe just about anything unusual that happens on the power system. Another word in common usage that is often considered synonymous with transient is surge. A utility engineer may think of a surge as the transient resulting from a lightning stroke for which a surge arrester is used for protection. End users frequently use the word indiscriminately to describe anything unusual that might be observed on the power supply ranging from sags to swells to interruptions. Because there are many potential ambiguities with this word in the power quality field, we will generally avoid using it unless we have specifically defined what it refers to. Broadly speaking, transients can be classified into two categories, impulsive and oscillatory. These terms reflect the wave shape of a current or voltage transient. We will describe these two categories in more detail.

2.3.1.1 Impulsive Transient: An impulsive transient is a sudden non–power frequency change in the steady-state condition of voltage, current, or both that is unidirectional in polarity (primarily either positive or negative). Impulsive transients are normally characterized by their rise and decay times, which can also be revealed by their spectral content. The most common cause of impulsive transients is lightning. Fig. 2.2 illustrates a typical current impulsive transient caused by lightning. Because of the high frequencies involved, the shape of impulsive transients can be changed quickly by circuit components and may have significantly different characteristics when viewed from different parts of the power system. They are generally not conducted far from the source of where they enter the power system, although they may, in some cases, be conducted for quite some distance along utility lines. Impulsive transients can excite the natural frequency of power system circuits and produce oscillatory transients.

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Fig. 2.2 Lightning stroke current impulsive transient

2.3.1.2 Oscillatory Transient: An oscillatory transient is a sudden, non–power frequency change in the steady-state condition of voltage, current, or both, that includes both positive and negative polarity values. An oscillatory transient consists of a voltage or current whose instantaneous value changes polarity rapidly. It is described by its spectral content (predominate frequency), duration, and magnitude. The frequency ranges for these classifications are chosen to coincide with common types of power system oscillatory transient phenomena. Oscillatory transients with a primary frequency component greater than 500 kHz and a typical duration measured in microseconds (or several cycles of the principal frequency) are considered high-frequency transients. These transients are often the result of a local system response to an impulsive transient. A transient with a primary frequency component between 5 and 500 kHz with duration measured in the tens of microseconds (or several cycles of the principal frequency) is termed a medium-frequency transient. Back-to-back capacitor energization results in oscillatory transient currents in the tens of kilohertz as illustrated in Fig. 2.3 Cable switching results in oscillatory voltage transients in the same frequency range. Medium-frequency transients can also be the result of a system response to an impulsive transient.

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Fig. 2.3 Oscillatory transient current caused by back-to-back capacitor switching

2.3.2 Long-Duration Voltage Variations: Long duration voltage variations are as follows 2.3.2.1 Overvoltage: An overvoltage is an increase in the rms ac voltage greater than 110 percent at the power frequency for duration longer than 1 min. Over voltages are usually the result of load switching (e.g., switching off a large load or energizing a capacitor bank). The over voltages result because either the system is too weak for the desired voltage regulation or voltage controls are inadequate. Incorrect tap settings on transformers can also result in system overvoltage.

2.3.2.2 Under Voltage: An under voltage is a decrease in the rms ac voltage to less than 90 percent at the power frequency for a duration longer than 1 min. Under voltages are the results of switching events that are the opposite of the events that cause over voltages. A load switching on or a capacitor bank switching off can cause an under voltage until voltage regulation equipment on the system can bring the voltage back to within tolerances. Overloaded circuits can result in under voltages also.

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The term brownout is often used to describe sustained periods of under voltage initiated as a specific utility dispatch strategy to reduce power demand. Because there is no formal definition for brownout and it is not as clear as the term under-voltage when trying to characterize a disturbance, the term brownout should be avoided.

2.3.2.3 Sustained Interruptions: When the supply voltage has been zero for a period of time in excess of 1 min, the long-duration voltage variation is considered a sustained interruption. Voltage interruptions longer than 1 min are often permanent and require human intervention to repair the system for restoration. The term sustained interruption refers to specific power system phenomena and, in general, has no relation to the usage of the term outage. Utilities use outage or interruption to describe phenomena of similar nature for reliability reporting purposes. However, this causes confusion for end users who think of an outage as any interruption of power that shuts down a process. This could be as little as one-half of a cycle. Outage, as defined in IEEE Standard 1008 does not refer to a specific phenomenon, but rather to the state of a component in a system that has failed to function as expected. Also, use of the term interruption in the context of power quality monitoring has no relation to reliability or other continuity of service statistics. Thus, this term has been defined to be more specific regarding the absence of voltage for long periods.

2.3.3 Short-Duration Voltage Variations: Short-duration voltage variations are caused by fault conditions, the energization of large loads which require high starting currents, or intermittent loose connections in power wiring. Depending on the fault location and the system conditions, the fault can cause either temporary voltage drops (sags), voltage rises (swells), or a complete loss of voltage (interruptions).

2.3.3.1 Interruption: An interruption occurs when the supply voltage or load current decreases to less than 0.1 pu for a period of time not exceeding 1 min. Interruptions can be the result of power system faults, equipment failures, and control malfunctions. The interruptions are measured by their duration since the voltage magnitude is always less than 10 percent of nominal. The duration of an interruption due to a fault on the utility system is determined by the operating time of utility [14]

protective devices. Instantaneous reclosing generally will limit the interruption caused by a nonpermanent fault to less than 30 cycles. Delayed reclosing of the protective device may cause a momentary or temporary interruption. The duration of an interruption due to equipment malfunctions or loose connections can be irregular.

2.3.3.2 Sags (dips): Sag is a decrease to between 0.1 and 0.9 pu in rms voltage or current at the power frequency for durations from 0.5 cycle to 1 min.

Fig. 2.4 Three-phase rms voltages for a momentary interruption due to a fault and Sub-sequent recloser operation 2.3.3.3 Swells: A swell is defined as an increase to between 1.1 and 1.8 pu in rms voltage or current at the power frequency for durations from 0.5 cycle to 1 min. As with sags, swells are usually associated with system fault conditions, but they are not as common as voltage sags. One way that a swell can occur is from the temporary voltage rise on the unfaulted phases during an SLG fault.

Fig. 2.5 Instantaneous voltage swell caused by an SLG fault

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2.3.4 Voltage Imbalance: Voltage imbalance (also called voltage unbalance) is sometimes defined as the maximum deviation from the average of the three-phase voltages or currents, divided by the average of the three-phase voltages or currents, expressed in percent.

Fig. 2.6 Voltage imbalance trend for a residential feeder

2.3.5 Waveform Distortion: Waveform distortion is defined as a steady-state deviation from an ideal sine wave of power frequency principally characterized by the spectral content of the deviation. There are five primary types of waveform distortion: 

DC offset



Harmonics



Inter-harmonics



Notching



Noise

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2.3.5.1 Harmonics: Harmonics are sinusoidal voltages or currents having frequencies that are integer multiples of the frequency at which the supply system is designed to operate (termed the fundamental frequency; usually 50 or 60 Hz). Periodically distorted waveforms can be decomposed into a sum of the fundamental frequency and the harmonics. Harmonic distortion originates in the nonlinear characteristics of devices and loads on the power system. Harmonic distortion levels are described by the complete harmonic spectrum with magnitudes and phase angles of each individual harmonic component. It is also common to use a single quantity, the total harmonic distortion (THD), as a measure of the effective value of harmonic distortion. IEEE Standard 519-1992 provides guidelines for harmonic current and voltage distortion levels on distribution and transmission circuits.

Fig. 2.7 A typical fifth harmonic wave

2.3.5.2 Inter-Harmonics: Voltages or currents having frequency components that are not integer multiples of the frequency at which the supply system is designed to operate (e.g., 50 or 60 Hz) are called interharmonics.

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2.3.6 Voltage Fluctuation: Voltage fluctuations are systematic variations of the voltage envelope or a series of random voltage changes, the magnitude of which does not normally exceed the voltage ranges of 0.9 to 1.1 pu.

Fig. 2.8 Voltage fluctuations caused by arc furnace operation

2.4

Conclusions: In this chapter I have given an insight into power quality and effects on power quality by

problems associated with it e.g. waveform distortion, voltage imbalance and fluctuation, various kind of interruptions and voltage variation to name a few. These are problems that we deal on regular basis for betterment of power quality. With advancement of power electronic devices we can reduce these disturbances by a large margin with a whole lot of efficient power electronic devices.

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CHAPTER 3

ACTIVE POWER FILTER 3.1 INTRODUCTION The electric power system is affected by various problems like transients, noise, voltage sag/swell, which leads to the production of harmonics and affect the quality of power delivered to the end user. The harmonics may exist in voltage or current waveforms which are the integral multiples of the fundamental frequency, which does not contribute for the active power delivery. Thus the response at these frequencies should be restricted from affecting the behavior of the system. To achieve this filter is used at the Point of Common Coupling (PCC) where the load is connected to the supply. This filter filters out the harmonics and improves the performance of the system. There are different types of filters available for this purpose. Each of them is explained in detail in this chapter

3.2 FILTER CLASSIFICATION The different filters present in the literature are classified into three basic types. They are Active Filters and Passive Filters and Hybrid filter

Fig.3.1 Classification of Filters [19]

3.2.1 Passive Power Filters: These filters consist of passive elements like- capacitor, inductor and resistor. These are widely used because of their low cost and ease of control. The passive filters also provide reactive power apart from filtering the harmonics [2-[3]. The performance of these filters is heavily dependent on the system impedance. These are again classified into two types- low pass and high pass.

3.2.1.1 Low Pass Filter: The low pass filter is a tuned LC circuit that is tuned to provide low impedance for a particular harmonic current. In addition these filters are also used for power factor correction [2]. In power system network these are generally used to filter 5th and 7th order harmonics. The line diagram of the low pass filter is shown in Fig. 3.2

Fig.3.2 Low Pass Filter

3.2.1.2 High Pass Filter The high pass filters are also made of passive elements like inductor and capacitor but show low impedance for harmonic current above a particular corner frequency. All the harmonics present above that corner frequency are filtered using this filter.

Fig.3.3 High Pass Filter [20]

This filter is again of many types like single-order, two-order, third-order etc., based on the number of passive filters used in it. Among them the two-order filter is widely used. Fig. 2.3 shows the line diagram of a high pass filter .But there are some disadvantages with passive filter, like  The filter characteristics has strong dependence on the system impedance  Possibility of over load in the passive filter because of harmonic current  The change of the load impedance can detune the filter, so it is not suitable for variable loads  The problem of series and/or parallel resonances can be originated which causes of instable operation  Limited operation, that is used to eliminate either a particular order or fewer harmonics  Component aging Because of the above disadvantages the passive filters cannot provide an effective solution to enhance the quality of the power system. Thus, the active power filters are employed to overcome the above drawback. 3.2.2 Active Power Filters (APF): To overcome the drawback of passive filter, active compensation known as Active Power Filter is used recently. The APF is a Voltage Source Inverter (VSI) which injects the compensating current or voltage based on the network configuration. It was proposed around 1970. APF‟s are an up-to-date solution with fast switching devices, low power loss and fast digital processing devices at an affordable price. Depending on the circuit configuration and function, APF‟s are divided into three types and each one is explained in detail.

3.2.2.1 Shunt Active Power Filter: The voltage sourced inverter based Shunt APF is similar to STATCOM. It is connected in shunt at the PCC. It injects the current which is equal and opposite to the harmonic current. It acts as a current source injecting harmonics and is suitable for any type of load. It also helps in improving the load power factor [2] [9]. The circuit diagram of the power system with shunt connected APF is shown in Fig. 3.4. The cost of these filters is relatively higher and so not preferred for large scale systems [21]

Fig.3.4 Circuit Diagram of Shunt active power filter 3.2.2.2 Series Active Power Filter: As the name indicates, these filters are connected in series with the line through a matching transformer. This filter injects the compensating voltage in series with the supply voltage. Thus, it acts as a voltage source which can be controlled to compensate the voltage sag/swell [6][10] These filters have their application mainly where the load contains voltage sensitive devices. The circuit diagram of the power system with series connected APF is shown in Fig. 3.5. These filters are not used practically since they are required to handle high current ratings

Fig.3.5 Circuit Diagram of Series active power filter

Which increase the size of the filter as well as the losses occurring in the filter.

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3.2.2.3. Unified Power Quality Conditioner (UPQC): The UPQC is a combination of series and shunt active power filters. It has the advantage of both series APF and shunt APF. That means, it compensates both the voltage and current harmonics. Therefore, this filter can compensate almost all types of power quality problems faced by a power system network [7]. The circuit diagram of power system with UPQC is shown in Fig. 2.6.

Fig.3.6 Circuit Diagram with UPQC

3.2.3 Hybrid Power Filters: The active power filters are better solution for power quality improvement but they require high converter ratings. So to overcome the above drawback, hybrid power filters are designed. The hybrid power filters are the combination of both active and passive power filters. They have the advantage of both active and passive filters [2][15] .There are different hybrid filters based on the circuit combination and arrangement. They are Shunt Active Power Filter and Series Active Power Filter  Shunt Active Power Filter and Shunt Passive Filter  Active Power Filter in series with Shunt Passive Filter  Series Active Power Filter with Shunt Passive Filter

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3.2.3.1 Shunt APF and Series APF:

Fig.3.7 Shunt APF and Series APF Combination

This filter combination has the advantage of both series connected APF i.e., elimination of voltage harmonics and that of shunt connected APF of eliminating current harmonics. The circuit diagram is shown in Fig. 3.7. This combination finds its application in Flexible AC Transmission Systems (FACTS) [12]. But the control of APF is complex and this combination involves two APF and hence the control of this filter configuration is even more complex. Thus, this filter combination is not used widely. 3.2.3.2 Shunt APF and Shunt Passive Filter: The power rating of the APF depend on the order of frequencies it is filtering out. Thus an APF used for filtering out low order harmonics have low power rating with reduced size and cost. This logic is used in designing this filter combination [15] .The shunt connected APF filters out the low order current harmonics while the shunt connected passive filter is designed to filter out the higher order harmonics. The circuit configuration of this filter topology is shown in Fig. 2.8.

Fig.3.8 Shunt APF and Shunt Passive Filter Combination

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But the main disadvantage of this filter configuration is it cannot be suited for variable loading conditions. Since, the passive filter can be tuned only for a specific predetermined harmonic. 3.2.3.3. APF in Series with Shunt Passive Filter: In this filter configuration, the Active Power Filter is connected in series with a Shunt connected Passive Filter. The circuit diagram of this filter configuration is shown in Fig.2.9. The advantage of this configuration is that the passive filter reduces the stress on the power electronic switches present in the APF. This filter has its application in medium to high voltage ranges

Fig.3.9 APF in series with Shunt Connected Passive Filter 3.2.3.4. Series APF with Shunt Connected Passive Filter: The Series APF and Shunt APF combination seen in Fig. 3.7 has the problem of complex control strategy. To overcome this drawback, the shunt APF is replaced by a shunt connected passive filter [16]. The passive power filter does not require any additional control circuit and the cost is also less. This filter combination is shown in Fig. 3.10

Fig.3.10 Series APF with Shunt Connected Passive Filter [25]

Here the series connected APF provides low impedance (almost zero) for low frequency components whereas the shunt connected APF provides less impedance for high frequency components and filters out all higher order harmonics. So this filter configuration is the most beneficial of all others and has the advantage of reducing both current and voltage harmonics. 3.3 Conclusion: This chapter deals with different filter topologies that are used for the improvement of electric power quality. It explains in detail each filter configuration along with their merits and demerits. From the above discussion, it is clear that the passive filters are low cost solution but are not effective. The active power filters can overcome the draw backs of passive filter but their control is complex and difficult to implement.

[26]

CHAPTER4

CONTROL TECHNIQUE OF SAPF 4.1 The P-Q Theory In Three-Phase, Three-Wire System This concept is very popular and, basically consists of a variable transformation from a, b, c, reference frame of the instantaneous power, voltage, and current signals to the α β reference frame [17],[21]. The transformation equations from the a, b, c, reference frame to the α, β, 0 coordinates can be derived from the phasor diagram shown in Fig.3.1

Fig.4.1. Transformation from the phase reference system (a, b, c) to (α, β, 0) system The instantaneous values of voltages and currents in the α, β coordinates can be obtained from the following equations, the Clarke transformation and inverse Clarke transformation of three phase generic voltage given by, Similarly three phase generic instantaneous line currents ia, ib, ic can be transform on the α-β axis by This transformation is valid if and only if Va(t)+ Vb(t)+ Vc(t) is equal to zero, and also if the voltages are balanced and sinusoidal. The instantaneous active and reactive power in the α-β coordinates are calculated with the following expressions. The instantaneous complex power is possible using the instantaneous vectors of voltage and current. The instantaneous complex power is defined as the product of The voltage V and the conjugate of the current vector i*, given in the form of complex numbers. [27]

4.2 Instantaneous Power Theory

S  V * i *  v  jv * i  ji   v i  v i   j v i  v i 

(4.1)

From this active and reactive power components are

p  v i  v  i

(4.2)

q  v i  v  i

(4.3)

For systems that do not have a neutral connection, the zero sequence does not exist and the mathematical equation will be presented in matrix form

V  V    

1  1   2 2  3 3 0 2

i  i   

1  1   2 2  3 3 0 2

1  Va  2    Vb 3   Vc   2   



 i  a   ib  3   i c   2   



1 2

(4.4)

(4.5)

From this active and reactive components are

p  v i  v  i

(4.6)

q  v i  v  i

(4.7)

The active and reactive powers in matrix form is given below

 p  V  q    V    

V  i  V  i 

(4.8)

Active and reactive powers can be separated into two parts which are AC part and DC part as shown below p p ~ p

(4.9)

q  q  q~

(4.10)

[28]

In order to get the DC part of the active and reactive power, the signals need to be filtered using low pass filter. The low-pass filter will remove the high frequency component and give the fundamental part. Where p DC component of the instantaneous power p is is related to the conventional fundamental active current. ~p is the ac component of the instantaneous power p, it does not have average value, and is related to the harmonic currents caused by the ac component of the instantaneous real power q is the dc component of the imaginary instantaneous power q, and is related to the reactive power generated by the fundamental components of voltages and currents. q~ is the ac component of the instantaneous imaginary power q, and it is related to the harmonic currents caused by the ac component of instantaneous reactive power. In order to compensate reactive power and current harmonics generated by non-linear loads, the reference signal of the shunt active power filter must include the values of ~p , q 𝑎𝑛𝑑 q~ . In this case the reference currents required by the shunt active power filters are calculated with the following expression:

V i *C  1 *  2 2  i C  V  V V

V   p  V    q 

(4.11)

The final compensating currents components in a, b, c reference frame in terms of αβ given as

1 1  *  iCa  1   *  2 2 2  iC  *   *  iCb   3  3 3   iC  * iCc  0     2 2 

(4.12)

These are the compensation current injected by the shunt active filter to reduce harmonics in three phase-three wire systems. 4.3 Hysteresis Current Controller Hysteresis current controller can also be implemented to control the inverter currents. The controller will generate the reference currents with the inverter within a range which is fixed by the width of the band gap. In this controller the desired current of a given phase is summed with the negative of the measured current. The error is fed to a comparator having a hysteresis band. When the error crosses the lower limit of the hysteresis band, the upper switch of the inverter leg is turned on. But when the current attempts to become less than the upper reference band, the [29]

bottom switch is turned on. Figure 4.2 shows the hysteresis band with the actual current and the resulting gate signals [11]. This controller does not have a specific switching frequency and changes continuously but it is related with the band width.

Fig4.2. Hysteresis Current Controller 4.4 Capacitors: Capacitors are discharged through the inverter to generate compensation currents. These Capacitors then become the source of harmonics rather than the main source. The value of the DC capacitor depends upon the rise and fall of the capacitor voltage on the removal and addition of the loads [21]. It formula is basically governed by the energy conversion between capacitor and the system. The value of the DC capacitor may be given

C Where

0.9  I rms 4    f  Vdc   𝑎𝑑 𝑛

=DC voltage f=frequency

[30]

(4.13)

4.5 PI Controller: PI controller is used to remove steady sate error. Here we want it to maintain It with a constant value of signal and if

. If

is greater than

by comparing

is lesser than Vref then it would create a positive p loss it would create negative p loss signal.

4.6 Coupling Inductor: An inductor is used to couple power inverter with point of common coupling (PCC). Its job is to limit

effects. Leakage inductance of a coupling transformer can also be used [22]. The value

of the AC inductance depends upon the value of the switching frequency

f s and the ripple

current I crp . It may be given as, √

Where 4.7 DC Bus Voltage (

4.14

= DC Link Voltage

= supply frequency

= ripple current

):

The DC bus voltage basically depends upon voltage at the PCC. Its value must be higher than the ac mains voltage for proper control of the VSI based SAPF [22]. The relation between PCC and DC bus voltage can be given as

𝑑

Where,

√

= supply voltage

[31]

4.15

CHAPTER 5

MODELLING & SIMULATION RESULTS INTRODUCTION: After having a lot of theoretical background, let’s have some simulation results that helps in better understanding of theory and versatile behavior of Active Power Filter (APF).Following are the cases differ by nature of load connected across the source. All the simulations are performed in the SIMULINK environment; proprietary software from Mathworks Inc.

5.2 System Parameters: The parameter used in the system are shown in table 5.1

S.NO.

Parameters

Value

1

Supply voltage & frequency

100V , 50Hz

2

Source Impendence

R= 0.1Ohm L= .15mH

3

DC Link Capacitor Voltage

300V

4

Coupling Inductance

1.5mH

5

SAPF DC Capacitor

1120e-6

6

Non Linear Balanced Load DC side

R=20 Ohm L=20mH

7

Non-Linear Unbalanced Load DC side

R=20 Ohm L=30mH R=10 Ohm L=100mH R=5 Ohm

[32]

L=0.3mH

5.3 Simulation Model of SAPF:

Fig.5.1 Simulation model with SAPF 5.4 Simulation Model of Control Scheme

Fig5.2 Simulation model of control scheme

[33]

5.5 Non-Linear Balanced Load:

Fig5.3 Non-linear balanced load 5.6 NON-LINEAR UNBALANCED LOAD:

Fig5.4 Nonlinear Unbalanced Load

[34]

5.7 Case-1 Nonlinear Balanced Load 5.7.1 Performance without SAPF: The waveform of supply voltage, supply current and load current without SAPF is shown. In the figure 5.6 for nonlinear balanced load. In this case R=20ohm, L=20mH are taken with 3-phase diode rectifier circuit consider as a nonlinear load balanced load.

Fig.5.5 Waveform of supply voltage, supply current & load current (8A)

[35]

Fig.5.6 FFT analysis of phase A current without SAPF

5.7.2 Performance with SAPF:

Vs

200 0 -200

0

0.05

0.1

0.15 Time(sec)

0.2

0.25

0.3

0

0.05

0.1

0.15 Time(sec)

0.2

0.25

0.3

0

0.05

0.1

0.15 Time(sec)

0.2

0.25

0.3

Is

20 0 -20

Load current

10 0 -10

Fig.5.7 Waveform of supply voltage, supply current & load current [36]

10 8 6 4

APF (current)

2 0 -2 -4 -6 -8 -10

0

0.05

0.1

0.15 Time(sec)

0.2

0.25

0.3

Fig 5.8 SAPF cuurent

500 400

Vdc

300 200 100 0 -100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Time(sec)

Fig 5.9 Capacitor DC link voltage

Fig 5.10 Instantaneous power P and Q

[37]

0.8

0.9

1

Fig 5.11 FFT analysis of Phase (A) current

100

Voltage & Current

50

0

-50

-100 0.05

0.1

0.15

0.2

0.25

0.3

Time(sec)

Fig 5.12 voltage & current without SAPF

100

Voltage & current

50

0

-50

-100 0.05

0.1

0.15

0.2 Time(sec)

Fig 5.13 voltage & current with SAPF

[38]

0.25

0.3

In the fig 5.6 without SAPF it can be seen that the supply current is not in sinusoidal in nature which is due to the non-linear load. Its THD is 21.66% which is not permissible according to IEEE-519 standards and voltage and current crossing zero point is not same as shown in the fig 5.12. In the fig 5.8 , with SAPF Supply current is in proper sinusoidal form in nature and its THD is 1.19% which is permissible according to IEEE-519 standards 5.13. 5.8 CASE-2 Nonlinear Unbalanced Load: In this case single phase rectifier circuit is connected with each phase with different R and L value which is mention in Table no.5.1 5.8.1 Performance without SAPF

Vs

100 0 -100

0

0.05

0.1

0.15 Time(sec)

0.2

0.25

0.3

0

0.05

0.1

0.15 Time(sec)

0.2

0.25

0.3

0

0.05

0.1

0.15 Time(sec)

0.2

0.25

0.3

Is

20 0 -20

load current

20 0 -20

Fig 5.14 Waveform supply voltage,current and load current

[39]

Fig5.15 FFT analysis of phase(A) line current

Fig5.16 FFT analysis of phase(C) line current

Fig5.17 FFT analysis of phase(B) line current

[40]

5.8.2 Performance with SAPF:

Vs

200 0 -200

0

0.05

0.1

0.15 Time (sec)

0.2

0.25

0.3

0

0.05

0.1

0.15 Time(sec)

0.2

0.25

0.3

0

0.05

0.1

0.15 Time(sec)

0.2

0.25

0.3

0

0.05

0.1

0.15 Time(sec)

0.2

0.25

0.3

Is

20 0 -20

Load current

20 0 -20

APF current

20 10 0 -10

Fig 5.18 Waveform of supply voltage, current and load current with SAPF 500 400

Vdc

300 200 100 0 -100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Time(sec)

Fig 5.19 DC link capacitor voltage [41]

0.8

0.9

1

Fig5.20 FFT analysis of phase(A) line current with SAPF

Fig5.21 FFT analysis of phase(B) line current with SAPF

Fig5.22 FFT analysis of phase(C) line current with SAPF

[42]

Fig 5.23 Instantaneous power P and Q

In case of nonlinear unbalance load the supply current is not sinusoidal in nature and unbalanced which is shown in the fig.5.14. THD of phase A 8A line current is 9.77% for phase B 10A line current is 26.11% and for phase C 14A line current is 19.29 % which is not permissible according to the IEEE -519 standards but with SAPF current is sinusoidal in nature and current is balanced in each phases current is 10A which is shown in the fig.5.18 . THD of phase A phase B and phase C are 1.23%, 1.19% and 1.00% respectively which is permissible according to the IEEE-519 standards and SAPF current makes the supply current balanced and makes it in proper sinusoidal form in nature.

[43]

5.9 Case-3 Compensation Of Current With SAPF: This is the unique case, simulated to show that even the load is connected between the single phase and the ground, but after compensation the balanced three phase current is supplied by the source i.e. the APF is acting as a load for the remaining two open phases.

Fig5.24 Load of case compensation of SAPF

Vs

200 0 -200

0

0.05

0.1

0.15 Time(sec)

0.2

0.25

0.3

0

0.05

0.1

0.15 Time(sec)

0.2

0.25

0.3

0

0.05

0.1

0.15 Time(sec)

0.2

0.25

0.3

0

0.05

0.1

0.15 Time(sec)

0.2

0.25

0.3

100 Is

50 0 -50

APF current

50 0 -50

Load current

50 0 -50

Fig.5.25 Waveform of supply voltage, current, filter current & load current waveform [44]

Fig 5.26 Instantaneous power P and Q In this case two phases are kept open, only one phase is connected. It is found that SAPF is working as a load for remaining open phase. which is shown in the Fig.5.24 and instantaneous power P and Q is shown in figure 5.26

5.10 Case-4 Nonlinear load in Dynamic condition In this case we change the load at 0.3 sec all the simulation results shown in below figure. and here consider R=20ohm and L=10mH for first nonlinear load and R=50 ohm and L=20mH for second load

Fig 5.27 Dynamic load condition load change at 0.3 second [45]

Fig 5.28 Waveform of Dynamic load condition load change at 0.3 second Vs (supply voltage), Is (supply current), load current, and If (filter current)

Fig 5.29 Capacitor DC link voltage load change at 0.3 second

[46]

Fig.5.30 Instantaneous power P and Q

Fig.5.31 FFT analysis of load current

[47]

Fig.5.32 FFT Analysis of supply Phase current (A) In this case load is change at 0.3 second, as shown in figure-5.28 load current increase and supply current in sinusoidal in nature due to filter current compensate the harmonic current of supply current .DC link voltage maintain at 300V after dip at 0.3 sec and settle very soon after few second at reference voltage shown in figure (5.29) . The THD of load current at 0.4 second is 20.13% in figure. (5.31) and THD of Supply phase (A) current is 1.04% shown in figure. 5.32 Which is permissible according to IEEE-519 standards. and instantaneous power P and Q is change at 0.3 second shown in figure.5.32.So we can say that Shunt active power filter work in the case of dynamic load condition very effective.

[48]

Comparison of THD for nonlinear balanced, unbalanced load and dynamic load is shown in Table 5.2. Table .5.2 Comparison of THD for nonlinear balanced load, unbalanced load and dynamic load . FOR NON LINEAR BALANCED LOAD PHASE

WITHOUT SAPF

WITH SAPF

A

21.66%

1.19%

B

21.66%

1.15%

C

21.66%

1.17%

FOR NON LINEAR UNBALANCED LOAD PHASE

WITHOUT SAPF

WITH SAPF

A

9.77%

1.23%

B

26.11%

1.19%

C

19.29%

1.00%

FOR NONLINEAR DYANAMIC LOAD PHASE

WITHOUT SAPF

WITH SAPF

A

20.13%

1.04%

B

20.13%

1.02%

C

20.13%

1.05%

Comparison table of Input Power Factor for balanced load, unbalanced load and dynamic load are shown in table 5.3.

[49]

Table .5.3 Comparison of Input Power Factor for nonlinear balanced load, unbalanced load and dynamic load . WITHOUT SHUNT ACTIVE POWER FILTER TYPES OF LOAD

INPUT POWER FACTOR

Nonlinear Balanced Load

.9535

Nonlinear Unbalanced Load

.9541

Nonlinear Dynamic Load

.9547

WITH SHUNT ACTIVE POWER FILTER Nonlinear Balanced Load

.9982

Nonlinear Unbalanced Load

.9989

Nonlinear Dynamic Load

.9997

In Case-1 without SAPF it can be seen that the supply current is not in sinusoidal in nature which is due to the non-linear load. Its THD is 21.66% which is not permissible according to IEEE-519 shown in the table 5.2.but, With SAPF Supply current is in proper sinusoidal form in nature and its THD is 1.19% for phase A and 1.15% for phase B and 1.17% for phase C which is permissible according to IEEE-519. And power factor has been improved to .9982 from .9535. In Case-2 of nonlinear unbalance load the supply current is not sinusoidal in nature and unbalanced which is shown in the table.5.2 THD of phase A 8A line current is 9.77% for phase B 10A line current is 26.11% and for phase C 14A line current is 19.29 % which is not permissible according to the IEEE -519 standards but with SAPF current is sinusoidal in nature and current is balanced in each phases current is 10A which is shown in the fig.5.18 .THD of phase A phase B and phase C are 1.23%, 1.19% and 1.00% respectively which is permissible according to the IEEE-519 standards and SAPF current makes the supply current balanced and makes it in proper sinusoidal form in nature. and power factor has been improved to .9982 from .9541. [50]

In Case-3 two phases are kept open, only one phase is connected. It is found that SAPF is working as a load for remaining open phase. In Case-4 nonlinear dynamic load condition THD has been considered 20.13% and power factor is .9547 without SAPF but with SAPF THD has been improved to1.04% from 20.13% and power factor improved to .9997 from .9547.

[51]

CHAPTER6

CONCLUSION & FUTURE SCOPE The system of Shunt Active Power Filter proposed in this work has following concluding points.  Without SAPF it can be seen that the supply current is not in sinusoidal in nature which is due to the non-linear load. Its THD is 21.66% which is not permissible according to IEEE-519 but, with SAPF Supply current is in proper sinusoidal form in nature and its THD is 1.19% and power factor has been improved to .9982 from .9535  In case of nonlinear unbalance load the supply current is not sinusoidal in nature and unbalanced. THD of phase A 8A line current is 9.77% for phase B 10A line current is 26.11% and for phase C 14A line current is 19.29 % which is not permissible according to the IEEE -519 standards but with SAPF current is sinusoidal in nature and current is balanced in each phases current is 10A.THD of phase A phase B and phase C are 1.23%, 1.19% and 1.00% respectively which is permissible according to the IEEE-519 standards and. power factor has been improved to .9989 from .9541  In case of two phases are kept open, only one phase is connected. It is found that SAPF is working as a load for remaining open phase  In Case-4 nonlinear dynamic load condition THD has been considered 20.13% and power factor is .9547 without SAPF but with SAPF THD has been improved to1.04% from 20.13% and power factor improved to .9997 from .9547 Conventional way of harmonics elimination by using passive filter might suffer from parasitic problem. It has been shown that three phase active filter based on p-q theory can be implemented for harmonic mitigation. Harmonics mitigation carried out by the active filter was showed in this dissertation and we clearly calculate the Total Harmonic Distortion (THD) with active filters and without active filters.

[52]

6.2 Future Scope: The work done in this project can be further extended such new improvements can be found. The feasible options are To implement the control strategy using Artificial Intelligence (AI) techniques  Experimental investigation can be done on shunt active power filters by developing a prototype model in the laboratory to verify this simulation results for controller

[53]

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[56]