[attachment=10199]
Power Factor Correction of Linear and Non-linear Loads Employing a Single Phase Active Power Filter Based on a Full-Bridge Current Source Inverter Controlled Through the Sensor of the AC Mains Current
Abstract:
In the last years the number of non-linear loads has been increasing rapidly. This non-linear load had drawn a current form the AC Mains with harmonic components, leading to low power factor, low efficiency, interference by the EMI, among others. A classic solution is the use of passive filters, which have resonance problems, large size and fixed compensation characteristics. The most common single phase non-linear load is the uncontrolled rectifier followed by a capacitive filter. For this specific non-linear load the use of a Boost pre-regulator provides a reduction in the harmonic contents and an improvement in the power factor.
However, the Boost pre-regulator cannot be used in equipment already in service, and it is applied only to one kind of non-linear load. The active power filter connected in parallel to the non-linear loads IS a more interesting solution because it compensates the reactive power of any load and it may be installed in equipment already in service. The single phase active power filter more widely used is the full-bridge voltage source inverter. Many techniques to control the voltage source inverter have been proposed.
Some calculating the reactive power of the load or even the load harmonic contents, and more lately through the sensor of the AC Mains current. However, the voltage source inverter needs a large capacitive bank and in order to be connected in parallel to the loads an inductance is necessary, and this inductance limits the active filter performance. The full-bridge current source inverter may be connected directly to the load and it is naturally a current amplifier, so a better performance may be expected. Some works have been made involving the full-bridge current source inverter that is usually controlled through the calculation of the reactive power of the load or even the load current harmonic components.
The load current phase displacement. In this work a full bridge current Source inverter controlled through the Sensor of the input current is proposed, allowing the compensation for current harmonics and phase of any linear, non-linear and multiple loads. Source inverter. The active power filter is controlled through the Sensor of the input current, allowing the for current harmonics and phase displacement of any linear, non-linear and multiple loads. Theoretical analysis, design procedure, simulation and experimental results are provided
1. INTRODUCTION
The use of solid state switching devices is increasing in many applications such as furnaces, adjustable speed drives, energy efficient lighting and switched mode power supply. The solid state switching devices behave as non-linear loads causing harmonic current injection which results in degraded power quality of the distribution and transmission system.
As more of these loads enter the power system, this phenomenon is predicted to worsen causing increased power system losses and interference in the communication circuits and sensitive loads. Conventionally, Passive filters have been used to eliminate current harmonics in power systems, but not without their disadvantages. The system impedance strongly affects the filtering characteristic of the passive filter that may create series or parallel resonance, which causes amplification of harmonic current or voltage at a specific frequency. The variability of non-linear load operating conditions also affects the filter performance. The ageing of the filter capacitors will also affect the performance of passive filters. Also, since both the harmonic and the fundamental current components flow into the filter, the passive filters are required to be designed with a high current rating.
To improve the power quality without the disadvantages of passive filters, shunt active power filters (APF) are developed. Shunt Active power filters consists of power switching devices and passive energy storage elements such as capacitors and inductors. A specific control strategy is used to drive the power switching devices in order to produce current that is able to compensate for the harmonics injected by the load.
The active filter used to compensate for the two nonlinear loads, namely uncontrolled rectifier and ac controller is a single-phase inverter. A Neural Network based controller is used to control the shunt active filter. The Neural Network controller forces the line current to the same shape and in phase with the supply voltage.
The active filter used to compensate for these two nonlinear loads is a single-phase inverter. The shunt active filter controlled by a Neural Network (NN) controller forces the line current to be in phase and of the same shape as the supply voltage. A single phase shunt active filter configuration is simulated in this project. The effect of shunt active filter on harmonic reduction is also presented.
1.1. Organization of the Thesis
The work presented in this thesis is organized as follows.
Chapter- 1 gives the introduction
Chapter- 2 explains the basic concepts of harmonics with figures. At the end of the chapter series, parallel resonance and equipments required for analysis of non linear voltage and currents are given.
Chapter-3 gives the introduction of filters and explains the classification of active filters.
Chapter-4 gives the description of Neural Networks that plays a prominent role as a controller in shunt active filter which includes basic neurons, different neuron models, architecture of Neural Networks and back propagation algorithm.
Chapter-5 describes about the design model, working principle and operation of single phase neural controlled shunt active filter with multiple non linear loads. Also the design procedure of the Neural Network block in Mat lab is presented.
Chapter-6 gives the concepts of simulation. In this chapter simulink block diagrams of single phase system having non linear loads without and with Neural Network controlled shunt active filter are given and the simulation results are analyzed.
Finally the conclusion references are furnished.
1.2. Overview of the project
In this project a single phase system consisting of multiple non linear loads such as uncontrolled rectifier and AC controller is considered. Two different cases are simulated and the results are analyzed to study the harmonics injected in the system.
1. Single phase system without neuro controlled shunt active filter
2. Single phase system with neuro controlled shunt active filter
3. Simulink block diagrams are shown in fig 6.1 & 6.6. The results of the two cases a summarized and tabulated in table-6.2. The relevant diagrams and waveforms are also shown for both the cases.
2. HARMONIC DISTRIBUTION
2.1. Introduction
Harmonics are sinusoidal currents and voltages with frequencies that are integral multiplies of the fundamental power line frequencies which is 50Hz in India. Harmonics distorts the supplied 50Hz voltage and current waveforms from their normal sinusoidal shapes.
.Each harmonic is expressed in terms of its order. For example, the second, third and fourth order harmonics have frequencies of 100Hz, 150Hz, and 200Hz, respectively. As the order, and the frequency, of the harmonics increases, the magnitude normally decreases. Hence lower order harmonics usually the fifth and seventh, have the most effect on the power system.