Power Quality Standards Force Computer Science Essay

Published Date: 02 Nov 2017

Abstract

Power quality standards force to limit the total harmonic distortion (THD) within acceptable range caused by rapid usage of power electronic equipment. Therefore the main purpose of this thesis is to widen the investigation of the quality problems in power system. Where the none-linear loads are increased dramatically in recent years and turn out to is cost affected. Most of the pollution issues created in power systems are due to the non-linear characteristics and fast switching of power electronic equipment. Power quality issues are becoming stronger because sensitive equipment will be more sensitive for market competition reasons, equipment will continue polluting the system more and more due to cost increase caused by the built-in compensation and sometimes for the lack of enforced regulations. Efficiency and cost are considered today almost at the same level. Active power filters have been developed over the years to solve these problems to improve power quality. Among which shunt active power filter is used to eliminate and load current harmonics and reactive power compensation.

In this work introduces a new method for the design analysis of shunt Active Power Filter (APF) in p-q instantaneous theory and using hysteresis current band to obtain the gating signals for flying capacitor multilevel inverter (FCMLI) and Fuzzy logic controller to compensate the reactive power. The proposed active power filter is employed to reduce the current Total Harmonic Distortion (THD) drawn by the non- linear load and improve the power factor of the load. Also, this work explores the behavior of the control approaches for real time current compensation harmonics.

The advantage of fuzzy control is that it is based on linguistic description and does not require a mathematical model of the system. The compensation process is based on sensing line currents only, an approach different from conventional methods, which require sensing of harmonics or reactive power components of the load.

A MATLAB/Simulink software program has been developed to check proposed design; the Active power filter system was simulated in Matlab/simulink application. Simulink is a simulation tool based on the Matlab mathematical computing package in the time domain. Under the simulink environment, the user is able to model detailed equations of the system under study by using a wide range of graphical building blocks including control system notations, equations, and state-space representations for various models such as power-electronic devices and control circuit. Since -Matlab/Simulink provides full capabilities required for accurate simulation of all test cases in the study, they are adopted to validate this work.

Keywords

Keywords: Active power filter (APF), Flying capacitor multilevel inverter (FCMLI), Hysteresis band controller, Harmonic compensation, Total Harmonic Distortion (THD), Fuzzy logic controller

Chapter One: Introduction

Background.

Shunt active power filter.

Five level diod clamped inverter.

Instantaneous Real and Reactive Power Method (p-q).

Fuzzy Logic Current Controller.

Hysteresis band current controller.

Problem statement.

Aims and objectives.

Project Outlines.

Chapter One: Introduction

Background

One of the most serious problems in electrical power systems is power quality distortion due to the Increase of nonlinear loads drawing non sinusoidal currents. Active filters which used widely for harmonic mitigations as well as reactive power compensation, load balancing, voltage regulation, and voltage flicker compensation. In three-phase four-wire systems with nonlinear loads a high level of harmonic currents in both the three line conductors and more in the neutral wire has been enrolled. Unbalanced loads also results in further declination of the supply quality. Various harmonic mitigation techniques proposed to reduce harmonics effect.

The most popular APFs is the shunt active power filter, these techniques include phase multiplication, passive filters, active power filters (APFs), and harmonic injection. It is mainly a current source, connected in parallel with the non-linear loads. Conventionally, shunt APF is controlled in such a way as to inject harmonic and reactive compensation currents based on calculated reference currents. The injected currents are meant to cancel the harmonic and reactive currents drawn by the nonlinear loads. Recently, fuzzy logic controller has generated a great deal of Interest in various applications and has been introduced in the power electronics field [1].

According to several control strategies which have been developed but still two control theories methods are always dominant, instantaneous active and reactive currents (id-iq) method and instantaneous active and reactive power (p-q), mainly. The proposed work will concentrate on two control strategies (p-q and Id-Iq) with fuzzy controller, to validate current observations. Extensive simulations are carried out with fuzzy controller for both p-q and Id-Iq methods for different voltage conditions like sinusoidal, non-sinusoidal, and un-balanced conditions to adequate results. On watching the performance of Id-Iq control strategy with fuzzy controller is quite good over p-q control strategy with fuzzy controller [2].

sHunt active power filter

In a development electrical distribution system, there has been a sudden increase of nonlinear loads, such as power supplies, rectifier equipment, domestic appliances, and adjustable speed drives (ASD), etc.

As the number of these loads increased, harmonics currents generated by these loads may become very significant. These harmonics can lead to a variety of different power system problems including the distorted voltage waveforms, equipment overheating, malfunction in system protection, excessive neutral currents, light flicker, inaccurate power flow metering, etc. They also reduce efficiency by drawing reactive current component from the distribution network [3].

Figure (1) illustrates the concept of the harmonic current cancellation so that the current being supplied from the source is sinusoidal, the voltage source inverter used in the active filter makes the harmonic control possible.

Active power filters (APFs) have been developed. The voltage-source inverter (VSI) based shunt active power filter has been used in recent years and recognized as available solution the control scheme, in which the required compensating currents are determined by sensing line currents only, which is simple and easy to implement. [4]

Figure 1: Shunt active filter[3].

Flying capacitor multilevel inverter (FCMLI):

Figure (2) shows a topology for five-level flying capacitor multilevel converter circuit. Multilevel Flying Capacitor Converter (FCC) topology has been recently introduced and it present advantages and disadvantages compared with other multilevel to apologies. FCC topology uses several floating capacitors in each phase that connect several points in the converter to achieve different voltage levels in the output signals.

The flying capacitor multilevel converter is a recently developed converter topology assuring a flexible control and modular design. Flying capacitor multilevel converter requires a balanced DC voltage distribution. This can be realized by using a special control leading to natural balancing or by measuring the voltages and selecting the appropriate switching state.

The balancing is influenced by three factors, namely the harmonic content of the reference waveform, the switching frequency and the load impedance. [5].

In addition to the voltage balancing of the flying capacitor multilevel converter, the output voltage must ensure the control of the load, e.g. a three phase AC machine

Figure 2: Five-level flying capacitor multilevel converter circuit topology [5]

Instantaneous Real and Reactive Power Method (p-q)

The active filter currents are obtained from the instantaneous active and reactive powers p and q of the non-linear load. Transformation of the phase voltages Va, Vb, and Vc and the load currents Ia, Ib, and Ic into the á – â orthogonal coordinates are given in Equation (1 and 2).

The compensation objectives of active power filters are the harmonics present in the input currents. Present architecture represents three phase four wire and it is realized with constant power controls strategy. The power calculation is given in detail form in Equation (3) [6][7].

= (1)

= (2)

= (3)

= (4)

Fuzzy Logic Current Controller

Fuzzy logic uses fuzzy set theory, in which a variable is a member of one or more sets, with a specified degree of membership. Fuzzy logic allow us to emulate the human reasoning process in computers, quantify imprecise information, make decision based on vague and in complete data, yet by applying a "defuzzification" process, arrive at definite conclusions. The block diagram representation of fuzzy logic controller (FLC) is shown in Figure 3 [8].

Figure 3: Block diagram of FLC[8].

The FLC mainly consists of three blocks:

• Fuzzification.

• Inference.

• Defuzzification.

The details of the above processes are given below:

A. Fuzzification

The fuzzy logic controller requires that each input/output variable which define as the control surface be expressed in fuzzy set notations using linguistic levels. The linguistic values of each input and output variables divide its universe of discourse into adjacent intervals to form the membership functions.

The member value denotes the extent to which variable belong to a particular level. The process of converting input/output variable to linguistic levels is termed as fuzzification.

B. Inference

The behavior of the control surface which relates the input and output variables of the system is governed by a set of rules. A typical rule would be

If x is A, Then y is B, when a set of input variables are read each of the rule that has any degree of truth in its premise is fired and contributes to the forming of the control surface by approximately modifying it. When all the rules are fired, the resulting control surface is expressed as a fuzzy set to represent the constraints output. This process is termed as inference.

C. Defuzzification

Defuzzification is the process of conversion of fuzzy quantity into crisp quantity. There are several methods available for defuzzification. The most prevalent one is centroid method, which utilizes the following formula:

∫ (µ (x) x) dx / ∫µ (x) dx, Where ì is the membership degree of output x [9].

hysteresis band current controller

The hysteresis band current control technique has been proven to be most suitable for all the applications of current controlled voltage source inverters in active power filters. And it's implemented to generate the switching pattern in order to get precise and quick response. The hysteresis band current control is characterized by unconditioned stability, very fast response, and good accuracy [10]

Problem statement

The problem of this project is to study the power quality distortion due to the Increase of nonlinear loads drawing non sinusoidal currents problem and find solution for this problem by using shunt active power filter, these techniques include phase multiplication, passive filters, active power filters (APFs), and harmonic injection which is describe by fuzzy logic controller.

Aims and objectives

Power quality standards force to limit the total harmonic distortion (THD) within acceptable range caused by rapid usage of power electronic equipment. Therefore the main purpose of this thesis is to widen the investigation of the quality problems in power system. Where the none-linear loads are increased dramatically in recent years and turn out to is cost affected. Accordingly the proposal of this work will go through following steps to validate our objectives

Design and analysis a five-level flying capacitor multilevel inverter circuit to apply an equal but opposite to the distorted harmonics in to the line of source current to cancel the non-linear load harmonics.

Design a fuzzy logic controller requires controlling the capacitor Dc voltage to improve the time response of the shunt active power system.

Visualize on the simulation of instantaneous active and reactive theory based shunt active filter with MATLAB/ Simulink, as a better solution for reduction of the harmonics

Simulations will carry out with fuzzy controller for both p-q methods.

Analyze the obtained result.

Project Outlines

This research report will be divided to many chapters; five chapters will be combined with each other and organized as follow:

Chapter 1: is an introduction about shunt active filter, this chapter gives an introduction that includes the scene of the project, explain the terminology, and outline the goals that the project will revolve around and cover some of the methodology and techniques that will be used in its production.

Chapter 2: will explore the introduction to shunt active filter and P-Q theory in general, and contains a literature review about this approach.

Chapter 3: will discuss the design and analysis of the system, this chapter will contain system design and program design. This chapter will be based upon the screen shots of the project, all the screen shots of the simulation design.

Chapter 4: explores the results which have been derived with simulation/analysis and measurements.

Chapter 5: contains the conclusion of research work and the suggested future work.

Chapter Two: Literature Review

Shunt active filter based on d-q theory and fuzzy logic.

flying capacitor converter and hysteresis controller.

Chapter Two: Literature Review

Shunt active filter based on d-q theory and fuzzy logic.

Fuzzy logic controller applied and extended to a three level shunt APF is proposed in [11], fuzzy logic control algorithm is proposed for harmonic current and inverter dc voltage control to improve the performances of the three levels active power filters and shows how we can use the three-level inverter as a shunt active power filter.

The original instantaneous reactive power theory or p–q theory has been systematically used in the control of active power filters (APFs). When the APF is connected in parallel to a non-linear and unbalanced load, the p–q theory application has allowed a compensation strategy named constant power to be obtained in [12] shows that any compensation strategy may be developed into the p–q theory frame, besides, on p–q theory reformulation without using mapping matrices.

In [13], shows how shunt Active Power Filter (APF) for power quality improvements in terms of harmonics and reactive power compensation in the distribution network by Integral (PI) or Fuzzy Logic Controller (FLC).

In [14], the electric network behaves as an ‘‘healthy carrier’’ of disturbances, and this disturbance generated by one customer can be distributed to other customers, causing possible damage to their equipment, the measurement of the quality.

In [15], describes the development of a low cost shunt active power filter with digital control, which allows dynamic power factor correction and both harmonics and zero-sequence current compensation. The active filter controller is based on the instantaneous power theory (p-q theory).

In [16], the structure of a fuzzy PID controller is presented. The application of the fuzzy logic controller as a power system stabilizer is investigated by means of simulation studies on a single machine infinite bus system.

In [17] Describe harmonic problem and find solution for this problem by using shunt active power filter with three levels which is controlled by using fuzzy logic controller. The active shunt power filter making important work to solution the quality problem which can do compensation on the harmonic current which is delivered by nonlinear load and also compensate the reactive power. By [17] the active shunt filter which use to solve harmonic problem is using by three phase of voltage and evolved by using the strategy that very important after the P-Q theory is applied. P-Q theory is tool to calculation the amount of current must be applied to compensate.

In [18] Talking about optimal design of fuzzy logic controller about shunt active filter and how could be controlled. This design is very important for harmonic compensation which serves the power quality. The [18] work based by two cases, the first one that this system is design and controlled by fuzzy logic controller and the rules are using in this design is accordingly robust. In the second one the membership function of this system and the normalization of gain are comprehend by optimal Ant Colony method which describe in [18].

As describe in previous section that the non-sinusoidal current is increased in the last year this non-sinusoidal current comes from harmonic that produced by non-linear load. Shunt active filter is used to eliminate the harmonic current that causes. Shunt active filter criteria based in two cases: the first case that the shunt active filters accepted the dc connection voltage, in second case, shunt active filter decide the harmonic which will be eliminating [18].

The idea of Fuzzy Logic (FL) planned Lotfi Zadeh in 1965; at initial as a method of dispensation data by allowing incomplete situate membership quite than crisp membership. Before long after, it was established to be admirable selections for some organize system applications because it mimic individual control logic. Fuzzy controller work as converter from linguistic manage stratagem to automatic manage strategy. The fuzzy rules are constructed by specialist knowledge database. Initially, the error e (t) and the difference error ∆e (t) have been located of the angular velocity. So the output of the fuzzy controller is obtainable controlling the voltage u (t).

IN [19] they use the increasing in power quality problem that cause by large number of nonlinear load in electronic device. And the voltage harmonic problem and the power equipments of power distribution system comes from the harmonic current that cause by nonlinear device. When the harmonic current are produce the nonlinear current flow in the transmission line and enter to the electrical device, so additional distortion on voltage are produced leading that to ensure that any operation in power system grid such that generates , transmission and distribution the distortion current and voltage are increase producing.

Significant problems normally caused in 3-phase 4-wire system. As clear when the harmonic current problem are caused the ground line which called the cold line may be go to overheated and fire. So the needed to perfect compensator are necessary available. More than one technology used to solve and control this huge problem one of that technologies are describe in [19].

The major aim [19] is to build up Fuzzy controller to study and describe the performance of instantaneous (id-iq) control plan for extort reference currents with balanced and unbalanced voltage situation by use shunt active filters, If the voltage of supply is sinusoidal or in balance, the criteria of control join in the similar. In other hand, when the voltages of supply are unbalanced or distorted, the criteria of control go in different criteria of harmonic compensation. The p-q theory which used particularly is not enough to find an optimal solution when the voltage of supply is not perfect. The [19] use fuzzy logic controller of Id-Iq under dissimilar voltage. And the system implemented system contains 3-ph 4-wire with shunt active filter.

2.2 FLYING CAPACITOR CONVERTER AND Hysteresis Controller.

In [20] shows the feature of a flying capacitor converter is the natural voltage balance property. The reported voltage balance dynamics analytical research methods are based on heavy frequency domain transformations (Fourier transform, Bessel functions) and are rather algorithmic and difficult to use in an everyday engineering practice. Suggested time domain approach uses stitching of piece-wise analytical solutions for consecutive switching intervals. The small parameter analysis of a five-level single-leg converter yields physically meaningful, simple, and accurate expressions for average voltage balance dynamics giving an in-depth insight into parameters, carrier frequency, and modulation strategy impact for both DC and AC PWM.

For a DC modulated five-level single-leg FC converter, simple and accurate expressions for voltage balance natural frequency and time constants and capacitor voltage balance dynamics were obtained by applying the time domain analysis technique and utilizing the small parameter naturally arises for practical converters with low current and voltage ripples. The frequency and time constants formulas along with the average voltage balance dynamics expressions clearly reveal the dependences on inductive load parameters, carrier frequency, voltage command, and modulation strategy.

An insight into the voltage balance mechanism gained from

DC PWM consideration is definitely useful for AC PWM as well. Accurate AC PWM frequency and time constants dependences on modulation index are obtained by averaging on AC fundamental period. Practically, AC fundamental frequency has no impact on voltage balance dynamics.

In [21] a three-level inverter based Shunt Active Power Filter (SAPF) using Multi-Level Hysteresis Current Controller (MLHCC) is presented for solution of power quality problems in distribution system. A simulation model of SAPF is prepared in Matlab/Simulink environment. MLHCC is used for control of SAPF currents. In simulation study, it is considered that SAPF executes two tasks covering compensation of harmonics and reactive power. Therefore, control algorithm is formed to execute two tasks. The Synchronous Reference Frame Method (SRFM) is used to extract the reference currents. Dynamic performance of SAPF is evaluated using two nonlinear loads switched at different times. Some simulation results are given to show performance of a three-level inverter based SAPF using MLHCC. Simulation results show that current harmonics has been kept inside specific recommendation of IEEE-519 and ac grid is approximately kept in unity power factor.

In [22] presents a comprehensive design and simulation of three-phase shunt active power filter to compensate the harmonics of nonlinear loads. The paper describes the complete design aspects of power circuit elements and control circuit parameters. The process is based on sensing line currents, line voltages and DC side capacitor voltage to compensate the harmonics in the nonlinear load. In this paper, hysteresis current control band is used in order to obtain switching signals to compensate the harmonics. The graphical outcomes show that the active filter brings the THD of the system well below

15%, while it shows 30% THD without filter

In [23] The Shunt Active Power Filter (SAPF) is one of the key controllers in Flexible Alternating Current Transmission System (FACTS) to control the transmission line voltage and can be used in Power System (PS) to enhance the power quality. This paper compares performance of Cascaded Five-Level Inverter (CFLI) based SAPF in PS with PI, Fuzzy and Neurofuzzy controller. Making use of the CFLI has benefits of low harmonics distortion, reduced number of switches and switching losses. In order to compensate the reactive power, balance the capacitor DC voltage and suppress the total harmonics distortion (THD) drawn from a Non-Linear Diode Rectifier Load (NLDRL) of SAPF, Sub-Harmonics Pulse Width Modulation (SHPWM) technique, and D-Q reference frame theory are proposed in this paper. The SHPWM pattern generation is used as control for the switches of CFLI. The D-Q reference frame theory is used to calculate the reference compensating currents for SAPF. PI controller fuzzy and neurofuzzy controller are used for capacitors dc voltage regulation for SAPF. The results are verified and validated through MatLab/Simulink simulation software with SAPF and without SAPF.

In [24] presents Hybrid Cascaded Seven-Level Inverter (HCSLI) used in SAPF to compensate reactive power, improve the power factor and to suppress the total harmonic distortion (THD) in supply current due to linear load and Non- Linear Diode Rectifier Loads (NLDRLs).In this paper d-q reference frame theory for reference current computation, Constant Switching Frequency Multicarrier Sub-Harmonic Pulse Width Modulation (CSFMSHPWM) technique for controlling the switches of HCSLI, Fuzzy logic controller (FLC) for regulating dc side capacitor voltage are proposed.

Chapter three: System Design

Artefact design.

Execution plan of the artefact.

Overview.

The theory and device uses in the project.

Linear and non linear loads.

Total harmonic disturtions THD.

P-Q theory explanation.

Shunt active filter..

Fuzzy logic

Flying capacitor inverter.

Hysteresis band current controller.

The model of design circuit.

chapter three: system design

Artifact Design

To design the shunt active filter based on instantaneous P-Q theory by using matlab Simulink with fuzzy logic controller with complete and success design we must make Court plan and doing this planning step by step carefully to achieve the objective that we hope to get. To design our system we must ask some essential question that the shunt active filter based on instantaneous P-Q theory system will be answering this question completely. These questions are:

Kind of filter that need to use.

Criteria of compensating harmonic that the system must do it.

By what can the user make control on shunt active filter based on instantaneous P-Q theory system

Type of Simulink that the system will to be used

END

If result mutual with the objective of project?

Start

Making review about the project title

Making description about P-Q theory we used

Project is fail and must make modify in some principle

Our project is success and can be used internationally

Making review about shunt active filter used

Modeling the system on FLC

Getting scope result by running the program

Execution Plan of the Artefact

NO YES

Figure 4: Execution plan of the artifact.

Overview

Recently, by heavy utilization of power system device such as power transformer, using nonlinear load with huge application and increase the number of customer which take power from system grid, the failed system and more of distortion in voltage and current wave form are present. The harmonic problem which occurred in the system is become dangerous case in power system grid which lead to wastage more power in distribution system, interfacing problem between electromagnetic of power system and communication system and fail in protection device and electrical devices. This problem lead to: increase the cost of commercial activities and industry import. Because that the productivity and quality will be decrease.

In recent years the growth of power quality problems increased due to the increasing of power electronics equipment such as adjustable speed drive, programmable logic controller, electronic lightning and other nonlinear loads. These loads cause to change the electrical nature of the current and voltage of the source. This change will affect mainly the electrical equipment which is sensitive to the power quality problems. Therefore, much of research has been performed on active power filters and their practical applications

Reference to this problem, find solution to that problem is being Compulsory. The passive filter is important device that in general is called the conventional solution to solve harmonic current problem. The disadvantages by using this method are: the filter has only the frequencies that previously adjusted, its procedure cannot be operate in definite load or collection of loads, create the resonance that present by communication among of passive filter and the load and may come unforeseen consequences. To deal with these disadvantages innovate recently anew active filter to that.

This project development the shunt active filters which depend on P-Q theory. This method is effectively in harmonic current compensate and keeping to make the power factor in unity as possible and in addition to balance the current of supply in the three phase in the system. Many research talk about the introduce a new concept of p-q instantaneous reactive power theory with the three phase voltages and currents.

The APF controlled on the bases of instantaneous p-q theory have a good dynamic compensation characteristic for load current. It improves the utility supply system power-factor as the ac source provide only the fundamental frequency of current, in addition to that reactive power compensation and harmonic mitigation. At the same time it have some draw- backs such as it is difficult to realize high power PWM inverters with a rapid current response, some resonance at specific frequency occurs between the source impedance and the APF initial cost is high when compared with passive filter.

My thesis will describes the design and analysis of a novel work that uses instantaneous power theory along with Fuzzy logic controller to reduce the THD and improve the power factor. The sensed voltages and currents of the load have been used for instantaneous power calculation to generate reference currents. A hysteresis band current controller generates switching signals for APF to follow the reference current within specified band limits. The THD and the PF of the load have been investigates for the nonlinear load system before and after using the proposed filter at different load conditions.

The system will be studied by using MATLAB/Simulink software, to check proposed design; the Active power filter system was simulated in Matlab/simulink application.

LINEAr and NON LINEAR LOADS:

3.4.1 Linear Loads:

AC electrical loads where the voltage and current waveforms are sinusoidal and the current at any time proportional to voltage are treated as linear loads. If pure sinusoidal voltage is passed through the resistive element, then the shape of the current wave form will be purely sinusoidal without distortion. Voltage and current waveform in a circuit involving inductor make voltage lead current. On the other hand for a circuit involving capacitor, current leads voltage [25].

3.4.2 Non-Linear Loads:

AC loads where the current is not proportional to the voltage are considered as nonlinear loads. Non-linear loads generate harmonics in the current waveform that leads to distortion of the voltage waveform; under these conditions the voltage is no longer proportional to the current. Table 1 shows the examples of linear and non-linear loads and comparison between them [25][26].

Table 1: Comparisons between Linear and Non-Linear Loads:

NO.

Linear loads

Nonlinear loads

1

Examples: power factor improvement capacitor, heaters, incandescent lamps

Examples: computer,laser printer,SMPS, rectifier, refrigerator

2

Ohms law is valid

Ohms law is not valid

3

Load current does not contain harmonics

Load current contains all odd harmonics

4

Could be inductive or capacitive

Can’t be categorized as leading or lagging loads.

5

Zero neutral current, if I-phase loads are equally balanced on 3-phase mains (vector sum of line current)

Even if single phase loads are equally balanced on 3-phase neutral current could be 2.7 times the line current

6

May not demand high inrush currents while starting.

Essentially very high inrush current (20 time of normal) is drawn while starting for approximant on cycle.

Total Harmonic Distortion (THD):

The total harmonic distortion (THD), of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. Harmonic distortion is caused by the introduction of waveforms at frequencies in multiplies of the fundamental i.e. 3rd harmonic is three multiplied by the fundamental frequency (150Hz). THD is a measurement of the sum value of the waveform that is distorted

THD = (5)

The THD is a very useful quantity for many applications. It is the most commonly used harmonic index. However, it has the limitation that, it is not a good indicator of voltage stress within a capacitor because that is related to the peak value of voltage waveform [27].

3.5.1 SOURCES OF CURRENT HARMONICS:

Among the sources of harmonic voltages and currents in power systems three groups of equipment can be distinguished:

• Magnetic core equipment, like transformers, electric motors, generators, etc.

• Arc furnaces, arc welders, high-pressure discharge lamps, etc.

• Electronic and power electronic equipment.

Transformers:

The first source of harmonics of power systems is transformer. The relationship between the primary voltage and current is shown in Figure (5) as a magnetization curve is strongly nonlinear and hence its location within the saturation region causes distortion of the magnetizing current. Transformers are designed so that the magnetizing current will not exceed 1–2% of the nominal current. The nominal operating point is located below the knee of the magnetizing curve, within its linear region. As a result, even if a large number of transformers are operated in the power system, they are not a significant source of harmonics under normal operating conditions. This condition may change because of a slight voltage increase for a short interval of time. Within the saturation region even a small voltage increase above the nominal value results in a large increase in the magnetizing current. Also the harmonic content rises significantly [28].

For instance, at the voltage above the nominal point (UN), the magnetizing current third harmonic value may increase up to 50 %. This may occur under low-load conditions in the cable networks or as a consequence of switching (on or off) large reactive power loads, e.g. switching off shunt reactors or switching on a capacitor bank. The effects of switching are transients which propagate in the system and can cause transformer saturation and sometimes over a large area.

Figure 5: (a) Schematics diagram, (b) transformer magnetization curve [28]

Motors and Generators:

Motors can also generate harmonic currents in order to produce a magnetic field but less compare to transformer due to very small magnetizing characteristics. The magnetizing characteristic of motors is much more linear compared to the transformer due to the presence of air gap. The pitch of motor winding can also be a root of harmonic currents. Typical motor windings have 5–7 slots per pole, which results in the generation of the fifth or seventh harmonic. In spite of the fact they are incomparably smaller than high harmonics in converter equipment; their presence is noticeable in the case of very large motors. Harmonics (of very small magnitude) also occur in generator voltage, since for both practical and economic reasons a spatial distribution of the stator windings which could guarantee a purely sinusoidal voltage waveform is neither advisable nor possible. The induced voltages are therefore slightly distorted, and usually the third harmonic is the dominant one. It causes the third-harmonic current flow under generator load conditions.

Arc Furnaces:

Distortion of arc furnace is an important issue because of their common use and large in comparison to the short-circuit capacity at the point of connection with individual powers. Moreover, for technological reasons, arc furnaces are presently operated at a lower power factor than in the past. The form of the furnace voltage and current waveforms implies that representation of their distortion employing a discrete spectrum. These waveforms, having the nature of stochastic variables, are non-periodic functions of time as Figure (6). A continuous spectrum between the dominant harmonics has the nature of white noise of significant value [28].

Figure 6: Example time graph of a furnace current during the starting phase of melting[28]

Switched Mode Power Supplies (SMPS):

The major part of modern electronic devices is fed by switched mode power supplies (SMPS) with single-phase rectifiers. The main difference from older units is in the lack of the traditional step-down transformer and rectifier: they are replaced by direct controlled rectification of the supply to charge a reservoir capacitor from which the direct current for the load is derived in order to obtain the output voltage and current required. With this approach, the main advantage is that size, cost and weight have been reduced and the power unit can be made with practically any form factor. The disadvantage introduced is that now, instead of drawing continuous current from the supply, the unit draws pulses of current which contain large amounts of third- and higher-order harmonic components.

EFFECTS OF HARMONICS:

The voltage or current distortion limit is determined by the sensitivity of loads (also of power sources), which are influenced by the distorted quantities. The least sensitive is heating equipment of any kind. The most sensitive kind of equipment are those electronic devices which have been designed assuming an ideal (almost) sinusoidal fundamental frequency voltage or current waveforms. Electric motors are the most popular loads which are situated between these two categories [29]. An example classification of the effects of the presence of high harmonics is given in Table 2.

Table 2: Effects of current and Voltage Harmonics:

Classification of criterion

Type of effect

Comments

Period

Very short-term effects

These effects are associated with a failure, malfunction or inoperative state of equipment exposed to high harmonics such as control and instrumentation equipment , IT equipment

Long-term effects

Mainly of thermal nature. The thermal effect (causing accelerated ageing of insulation) is function of many variable of which the most important are the values and orders of harmonics

Physical nature of the distorted quantity

Current effect

Related to the instantaneous or time-averaged current value (overheating of electric machines. Capacitor fuses blowing increased losses in transmission lines, unwanted operation of relay. Harmonics in power supply system are the main cause of temperature rise in the equipment and shortening of in—service time.

Voltage effects

Related with the peak, average or RMS value of distorted voltage.

the Theory and device uses in the project

P-Q theory explanation

It is also called instantaneous theory that is expansion at 1983 by AKAGI to control active power filter. In the beginning, the theory urbane to use with three phase without the earth wire. But recently the theory innovates to be used with three phase- four wires.

The P-Q theory depend on the time domain that leading to able using it in steady-state or transitory cases. The main advantage of this theory is: it is very simple to use it in calculation that is algebra calculation just. The strategy of this theory based on transformation the stationary value a-b-c to reference coordinates α-β-0 which is also stationary. The section of P-Q theory in 3-phase power system is obvious in figure (7) below. [30]

Figure 7: the component of P-Q theory in 3-phase power system [30]

The synchronous reference frame theory or d-q theory is based on time-domain reference signal estimation techniques. It performs the operation in steady-state or transient state as well as for generic voltage and current waveforms. It allows controlling the active power filters in real time system. Another important characteristic of this theory is the simplicity of the calculations, which involves only algebraic calculation. The basic structure of SRF controller consists of direct (d-q) and inverse (d-q)-1 park transformations, see figure (10) which shows the calculation of P-Q theory. These can useful for the evaluation of a specific harmonic component of the input signal.[30]

This theory is used to calculate the instantaneous active current (ip) and instantaneous reactive current (iq) of three phase system based on instantaneous reactive power theory. This theory is based on the Clark transformation which applied to the voltage and current vectors Where:

= (6)

= (7)

To find the real power and reactive power in instantaneous form as below:

= (8)

P=Ph+Pc (9)

q= qh+qc (10)

Finally the reference current can be calculated as below and where the first control strategy used in this work to compensate harmonic currents is based on the synchronous reference frame detection method. The three phase currents ia , ib and ic are transformed from three phase (abc) reference frame to two phase’s (α −β ) stationary reference frame currents iα and iβ using, :

= (10)

So:

= (12)

Appropriate design of PLL allows proper operation under distorted and unbalanced voltage conditions. Controller includes small changes in positive sequence detector as harmonic compensation is mainly concentrated on three phase four wire. In three- phase three wire, va′, vb′, vc′ are used in transformations given by (12). In three phase four wire are modified as vα′, vβ′ and given by (13)

= (13)

3.6.2 Shunt active filter

To cancel the harmonics and compensate the reactive power APF is the suitable solution. The APF concept is to use an inverter to inject currents or voltages harmonic components to cancel the load harmonic components. The more usual configuration is a shunt APF to inject current harmonics into the point of common coupling (PCC). The APF can be installed in a low voltage power system to compensate one or more loads; thus, it avoids the propagation of current harmonics in the system. The developments of different control strategies give APF to a new location. As APF compensate the reactive power and cancel the harmonics, it is also called as active power line conditioners (APLC). The concept of shunt APLC was first introduced by Gyugyi and strycula in 1976 [31].

The three main aspects of an active power conditioner are:

The configuration of power converter (the scheme and the topology of converter and the electronics device used)

The control strategy (the calculation of APLC control reference signals)

The control method used (how the power inverter follows the control reference)

APF’s can be classified based on converter type, topology, and the number of phases.

The converter type is mainly two types.

Voltage source inverter (VSI)

Current source inverter (CSI)

The topology of active power filter is classified in to three types.

Series active power filters.

Shunt active power filters.

Hybrid active power filters.

The main component used in our project is the shunt active power filter. To insure that the energy is transfer effectively to the load, only the mean of instantaneous real power and instantaneous zero sequence power. The operation of active power filter is to allow the real power transfer to the load and phases in power system without need to neutral wire and also doing to compensate the all other values. See figure (8) which shows the P-Q theory component with shunt active filter [29][31].

Figure 8: the component of P-Q theory with shunt active filter [29]

Shunt active power filter compensate current harmonics by injecting equal-but-opposite harmonic compensating current. In this case the shunt active power filter operates as a current source injecting the harmonic components generated by the load but phase shifted by 180o. This principle is applicable to any type of load considered a harmonic source. Moreover, with an appropriate control scheme, the active power filter can also compensate the load power factor. In this way, the power distribution system sees the non linear load and the active power filter as an ideal resistor. The current compensation characteristic of the shunt active power filter [32].

In figure (9) below, seeing that shunt active filter is composed by the block of controller which called inverter block, the DC bus, and the coupling to the power system block.

Figure 9: block diagram of shunt active filter [32]

The calculation of P-Q theory is complete by using the shunt active filter block. This controller block is allowing in systematic criteria and compute the amount of compensation current the system need by the filter. See figure (10) which shows the inverter block in power system [32]

Untitled.png

Figure 10: the calculation of P-Q theory [32]

3.6.3 FUZZY LOGIC

During the recent year, the fuzzy logic controller has emerged as one of the most important and active areas for research in fuzzy set theory.

Fuzzy logic controller is based on human thinking and the natural language than the traditional logical system. It represents a powerful way to Analyze and Control Complex Systems. It connects with reasoning that is fixed or approximate rather than fixed and exact. In contrast with "crisp logic", where binary sets have two-valued logic: true or false, fuzzy logic variables may have a truth value that ranges in degree between 0 and 1. Fuzzy logic has been extended to handle the concept of partial truth, where the truth value may range between completely true and completely false. Furthermore, when linguistic variables are used, these degrees may be managed by specific functions [33].

Fuzzy Logic provides a simple way to arrive at a definite conclusion based upon vague, ambiguous, imprecise, noisy, or missing input information. Fuzzy Logic's approach to control problems mimics how a person would make decisions, only much faster.

In the present work fuzzy controller is employed to compensate for the reactive power. The goal of the proposed controller is to decrease the THD and improve the power factor. Figure (11) shows the block diagram of the FLC on the proposed circuit.

Figure 11: The FLC on the proposed circuit [33]

The FLC proposed in the present work involves three stages, the Fuzzification inference, the rule base, and the defuzzification. The two input variables, the error signal and the error deviation that all come from the reactive power calculations are entered to the controller as a crisp input. The maximum values of the input variables determine the scale of mapping that transfers the input variables to the corresponding universe of discourse. Each one of the inputs has its own linguistic variables. [34]

The three linguistic variables used for the error and error deviation are, Large negative (LN), Zero (Z), and Large positive (LP).

The fuzzification process converts input data into suitable linguistic rules using a suitable method. Some of If then rules is used in the proposed controller:

1. If the error is large negative and the error deviation is negative, then the reactive power is large negative.

2. If the error is large negative and the error deviation is zero, then the reactive power is large negative.

3. If the error is large negative and the error deviation is large positive, then the reactive power is zero.

4. If the error is zero and the error deviation is zero, then the reactive power is large zero.

5. If the error is zero and the error deviation is large positive, then the reactive power is large positive.

The Fuzzification process generates the suitable output depending on the linguistic rules between the input and output membership functions. The output from the proposed FLC which is the corrected reactive power signal is sent to the defuzzification process that converts the output from the fuzzy inference to the crisp output again.[33][34].

3.6.4 FLYING capacitor Multi-level Inverter

A multilevel inverter can switch either its input or output nodes (or both) between multiple (more than two) levels of voltage or current. The multilevel voltage source inverter is recently applied in many industrial applications such as ac power supplies, static VAR compensators, drive systems, etc. One of the significant advantages of multilevel configuration is the harmonic reduction in the output waveform without increasing switching frequency or decreasing the inverter power output.

The output voltage waveform of a multilevel inverter is composed of the number of levels of voltages, typically obtained from capacitor voltage sources. They called multilevel starts from three levels. As the number of levels reach infinity, the output THD approaches zero. The number of the achievable voltage levels, however, is limited by voltage unbalance problems, voltage clamping requirement, circuit layout, and packaging constraints [35].

Mostly we have three inverter used voltage synthesis-based multilevel inverters are introduced:

Diode-Clamped Multilevel Inverter.

The diode-clamped multilevel inverter uses capacitors in series to divide up the dc bus voltage into a set of voltage levels. To produce m levels of the phase voltage, an m-level diode-clamp inverter needs m-1 capacitors on the dc bus. A single-phase five-level diode-clamped inverter, which can produce a nine-level phase to phase voltage waveform, is shown in Figure (12).

Figure 12: single-phase five-level diode-clamped inverter [35]

2) Cascaded-Inverters with Separated DC Sources.

The second structure introduced here is a multilevel inverter, which uses cascaded inverters with separate dc sources (SDCSs). The general function of this multilevel inverter is the same as that of the other inverters. The multilevel inverter using cascaded-inverter with SDCSs synthesizes a desired voltage from several independent sources of dc voltages, which may be obtained from either batteries, fuel cells, or solar cells. This configuration recently becomes very popular in ac power supply and adjustable speed drive applications. This new inverter can avoid extra clamping diodes or voltage balancing capacitors [36].

3) Flying-Capacitor Multilevel Inverter (FCMI)

Probably the most important multilevel topology to appear recently is the flying capacitor inverter, or imbricated cells multilevel inverter, proposed by Meynard and Foch.

A FCMI shown in Figure (13) uses a ladder structure of dc side capacitors where the voltage on each capacitor differs from that of the next capacitor. To generate m-level staircase output voltage, m-1 capacitors in the dc bus are needed. Each phase-leg has an identical structure. The size of the voltage increment between two capacitors determines the size of the voltage levels in the output waveform.

Figure 13: single-phase five-level flying-capacitor inverter [35].

The switch pair-capacitor ‘cell’ is isolated and inserted within a similar cell.

This inner pair of switches and their associated capacitor now ‘flies’ as the outer pair of devices switch. The combination of conducting switches and capacitors ensures that the voltage across any blocking switch is always well defined. Table 3 shows the switch combination of the voltage levels and their corresponding switch states. In fact, there is more than one combination to produce output voltages V2, V3, and V4. That makes the FCMI more flexibility than DCMI.

Table 3: Switch combination of the voltage levels and their corresponding switch states.

Output Va0

S1

S2

S3

S4

S5

S6

S7

S8

V5=Vdc

1

1

1

1

0

0

0

0

V4=3Vdc/4

1

1

1

0

0

0

0

1

1

1

0

1

0

0

1

0

1

0

1

1

0

1

0

0

0

1

1

1

1

0

0

0

V3=Vdc/2

1

1

0

0

0

0

1

1

1

0

1

0

0

1

0

1

1

0

0

1

0

1

1

0

0

1

1

0

1

0

0

1

0

1

0

1

1

0

1

0

0

0

1

1

1

1

0

0

V2=Vdc/2

1

0

0

0

0

1

1

1

0

1

0

0

1

0

1

1

0

0

1

0

1

1

0

1

0

0

0

1

1

1

1

0

V1=0

0

0

0

0

1

1

1

1

The flying capacitor family of converters appears very attractive:

1) The flying capacitor concept can be applied to a number of different converter types - current or voltage source, DC-DC, DC-AC or AC-AC.

2) Any switch combination is valid and ensures voltage sharing, so long as switch pairs receive complementary drive signals. Most modulation strategies are easily applied to this topology simply by phase shifting the drive signals.

3) The voltages of the capacitors are automatically balanced by this conventional modulation strategy. If desired, the capacitor voltages can be actively controlled by an appropriate modification of the control signals.

4) The load is by default equally shared among the switches.

5) The topology is modular and not reliant on a transformer

There are some significant disadvantages, which are not at first apparent:

1) The topology requires a lot of high voltage capacitors — many more than other topologies. These capacitors need to conduct the full load current for at least part of the switching cycle. Fortunately, if the switch frequency is high, these capacitors can usually be relatively small in capacitance value.

2) Since these capacitors initially have zero voltage across them, starting the converter safely may be a non-trivial task.

3) The topology is not inherently fault tolerant.

Figure (14) shows a single-phase, full-bridge, five-level converter based on a flying-capacitors multilevel inverter (FCMLI). The numbering is immaterial as long as the switches are turned on and off in the right sequence to produce the desired output waveform. Each phase leg has an identical structure. Assuming that each capacitor has the same voltage rating, the series connection of the capacitors indicates the voltage level between the clamping points. Three inner-loop balancing capacitors (Ca1, Ca2, and Ca3) for phase-leg a are independent from those for phase-leg b. All phase legs share the same dc-link capacitors, C1 through C4. The voltage level for the flying-capacitors converter is similar to that of the diode-clamped type of converter. That is, the phase voltage vao of an m-level converter has m levels (including the reference level), and the line voltage Vab has (2m - 1) levels [36][37].

Figure 14: Circuit diagram of a five level, flying capacitors, single phase inverter[36].

The advantages of the flying-capacitor implementing on a large amounts of storage capacitors can provide capabilities during power outages and provide switch combination redundancy for balancing different voltage levels, however an excessive number of storage capacitors is required when the number of levels is high. High-level inverters are more difficult to package with the bulky power capacitors and are more expensive. The FCMLI requirements to clamp the device (switch) voltage to one capacitor voltage level. Provided all the capacitors are of equal value, an m-level inverter will require a total of (m-1)×(m-2)/2 clamping capacitors per phase leg in addition to (m-1)main dc bus capacitors [37].

3.6.5 Hysteresis Band Current Controller

Hysteresis current control is one of the simplest techniques used to control the magnitude and phase angle of three phase shunt active filter injection currents for high speed compensation systems, primarily because of its simplicity of implementation, fast current control response, and inherent peak current limiting capability. However conventional fixed hysteresis band control has a variable switching frequency throughout the fundamental period, and consequently the load current harmonic ripple is not optimum. Among the various adaptive hysteresis band techniques, analytical method is a regular and simple for solving misrules of fixed hysteresis band. But it requires good knowledge of the load parameters. This research describes the application of fuzzy logic theory to the three-phase shunt active power filter for the power-quality improvement and reactive power compensation required by a nonlinear load under constant switching frequency. The advantage of fuzzy logic control is that it does not require a mathematical model of the system. Fuzzy hysteresis band techniques are employed to derive the switching signals [38].

The hysteresis band current control technique has been proven to be most suitable for all the applications of current controlled voltage source inverters in active power filters. And it's implemented to generate the switching pattern in order to get precise and quick response. The hysteresis band current control is characterized by unconditioned stability, very fast response, and good accuracy.

The conventional hysteresis band current control scheme used for the control of active power filter line current is shown in figure (15), composed of active a hysteresis around the reference line current (IL*) and actual line current of the active power filter is referred to as (IL). The hysteresis band current controller decides the switching pattern of active power filter. The switching logic is formulated as follows:

- If IL< (IL*- HB) upper switch of the leg is OFF and lower switch of the leg is ON.

- If IL> (IL*+ HB) upper switch of the leg is ON and lower switch of the leg is OFF

The switching functions SB and SC for phases B and C are determined similarly, using corresponding reference and measured currents and hysteresis bandwidth (HB).[38]

Figure 15: Current and voltage waves with hysteresis band current control (for APF)[38].

The switching frequency of the hysteresis band current control method described above depends on how fast the current changes from the upper limit of the hysteresis band to the lower limit of the hysteresis band, or vice versa. The rate of change of the actual active power filter line currents vary the switching frequency, therefore the switching frequency does not remain constant throughout the switching operation, but varies along with the current waveform. Furthermore, the line inductance value of the active power filter and the dc link capacitor voltage are the main parameters determining the rate of change of active power filter line currents. The switching frequency of the active power filter system also depends on the capacitor voltage and the line inductances of the active power filter configuration. The bandwidth of the hysteresis current controller determines the allowable current shaping error. By changing the bandwidth the user can control the average switching frequency of the active power filter and evaluate the performance for different values of hysteresis bandwidth. In principle, increasing the inverter operating frequency helps to get a better compensating current waveform. However, there are device limitations and increasing the switching frequency causes increased switching losses, and EMI related problems. The range of switching frequencies used is based on a compromise between these factors [38][39][40]

Figure 16: Simulation of hysteresis current control [39]