Power Quality Standards Force To Limit 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. The Most of pollution issues create in power system is due to the non-linear characteristics and fast switching of power electronic equipment where Power quality issue are becoming stronger because sensitive equipment go to 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 filter has 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 increase the nonlinear load drawing non sinusoidal currents. Active filters used widely for harmonic mitigations as well as reactive power compensation, load balancing, voltage regulation, and voltage flicker compensation. 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 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 are the shunt active power filter, this technique include phase multiplication, passive filters, active power filters (APFs), and harmonic injection mainly a current source, connected in parallel with the non-linear loads. Shunt APFs are controlled in such a way as to inject harmonic and reactive compensation currents based on calculated reference currents. The injected currents meant to cancel the harmonic and reactive currents drawn by the nonlinear loads. Recently, fuzzy logic controller has been 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 variables are a member of one or more sets, with a specified degree of membership. Fuzzy logic have been allowed 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 controllers require that each input/output variable meant as the control surface be expressed in fuzzy set notations using linguistic levels. The linguistic value of input and output variable divide its universe of discourse into adjacent intervals to form the membership functions.

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

B. Inference

The behavior of control surface which relates to input and output variable of the system is governed by a set of rules. A typical rule will 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 been 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.