CHAPTER 1 INTRODUCTION

Transcription

1 1 CHAPTER 1 INTRODUCTION 1.1 MOTIVATION OF THE RESEARCH Electrical Machinery is more than 100 years old. While new types of machines have emerged recently (for example stepper motor, switched reluctance motor, servo motor) the bulk of the electrical energy conversion systems is still dominated by classical induction machines (motors and transformers), synchronous machines and D.C. machines. While designs of these have been perfected (Finite element methods, optimization, computational, electromagnetic, new materials, etc.) their operation is still a challenge. This is more due to poor power quality that almost haunts the entire developing world. The productivity of the industrial systems based on these drives is very critical. Thus reliability of this operation is of paramount importance. The reliability of this operation is frequently impacted by faults which affect this operation. In particular, in a country like India (Majority of the third world) the quality and reliability of supplied power (grid power) is so poor, initiating numerous faults. Thus the study of performance of these machines under fault conditions gives better idea of their reliable operation perhaps even indicating better choices while designing them. Three phase induction motors are the work horses of industry and the most widely used electrical machines due to their reliability, low cost and high performance. In an industrialized nation, 40% to 50% of the industrial drives are three phase

2 2 induction motors in various capacities. Thus the primary focus of this work is to investigate by modeling, simulating and related case studies. This popular AC motor performance is generally affected by the following types of faults (IEEE Survey 1985 and 1987): i) Electrical related faults (33%) : The faults come under this classification are unbalanced supply voltage, single phasing, over load, over and under voltages and earth fault. ii) iii) Mechanical related faults (32%): The stator and rotor winding failure and bearing faults are predominant mechanical faults in three phase induction motor. Environmental related faults (15%): The external moisture contamination and ambient temperature affects the performance during operational conditions. Electrical related faults are frequently occurring faults in three phase induction motor which will produce more heat on both stator and rotor windings. This leads to reduce the life time of induction motor. To develop the protection strategy, the performance of the induction motor during electrical faults is analyzed by both simulation and experimental methods. This study was done at Electrical and Electronics department of Kumaraguru College of Technology, Coimbatore, Tamilnadu, (India). Based on the results and analysis, an on-line monitoring and protection scheme was formulated with allowable tolerant limit values for the three phase induction motor. A prototype model is designed and implemented with the association of M/s. Lakshmi Electrical and Control system Ltd., Coimbatore, Tamilnadu, (India).

3 3 In the existing protection scheme, each and every individual faults require separate protective relay (i.e., over current, earth fault, unbalance voltage, etc.).this will occupy more space and also expensive. To overcome all the practical difficulties, a newly integrated, low cost, reliable and compact protection scheme for three phase induction motor was developed to protect all incipient faults using PIC 16F877 micro controller. 1.2 OBJECTIVES Electrical faults may exist in any component of the system. Some faults, which may cause catastrophic results should be cleared out immediately to protect the other parts of the system from further damage. For example, the failure of switching devices in the inverter. While other faults may undergo a long time of development before break down. For example, the motor insulation failure. Usually the fault diagnosis of the drive system can be a manual one by monitoring the voltage, current or other variables of the components. The accuracy of such diagnosis is much more subjective, subject to the experience of the engineers. To accumulate the knowledge of the faulty conditions by simulation is a very efficient approach and the simulated data can be used as bench marks for empirical diagnosis. Condition Monitoring of electrical machines is becoming increasingly essential from both practical and theoretical points of view. It plays very important role in the safe operation of industrial plants and hence heavy production losses can be avoided. However, the choice of adequate monitoring methods is a challenging task. Most of the indicators used for monitoring electrical machines are currents, temperatures, voltages and

4 4 vibrations depending on their varying accessibility, reliability and sensitivity. This thesis deals with the diagnosis of electrical faults by mean of current and voltage measurements only. Fault diagnosis of AC induction machines (ACIM) has been widely researched. However, an integrated algorithm for motor control with automated fault detection, prevention and condition monitoring is still missing. Condition monitoring can reduce the downtime of the processes and increase the maximum interval between failures. Thus the number and cost of unscheduled maintenances or minimized, which is highly beneficial. Embedding a system s condition analysis into the motor control algorithm results an integrated approach. This can reduce the cost of the system and enhance its integrity for the control of ACIM with embedded system fault prediction and diagnosis. The diagnosis algorithm can shutdown the drive system on an imminent catastrophic fault. The algorithm employs various techniques to detect and predict different faults of the AC drive system. The following are the main objectives of the thesis: i) To analyze the behavior of the induction motor under faulty conditions - Unbalanced supply voltage, single phasing, over and under voltages, phase reversal, over load and earth fault, etc. ii) iii) To perform the fault analysis in the MAT LAB / Simulink environment - The induction motor is modeled using arbitrary reference frame theory, the fault conditions are simulated and the performance of three phase induction motor during fault conditions is observed. To perform the fault analysis experimentally - The induction motor is loaded with brake drum arrangement. All the three

5 5 phases of the induction motor are connected separately through single phase auto transformers. The faults are created by varying the auto transformers and loading arrangements. The behavior of induction motor during fault conditions is observed and compared with simulation results. iv) Based on the analysis, a control algorithm is developed and embedded in a PIC16F877 micro controller chip. The proposed system combines control, monitoring, fault prediction and protection functions in one assembly. v) Majority of the microcontroller based motor protection relays are based on thermal modeling and current measurements. The heating of induction motor is very much depends upon the current flow. For a particular percentage voltage unbalance (PVU), different percentage current unbalances (PCU) are produced based upon the combination of phase voltages. Therefore, only current measurements will not give sufficient protection for three phase induction motor. Hence, to improve the performance of the embedded relay both current and voltage measurements are taken into consideration while formulating the control algorithm. vi) The protection system works with any motor design up to 200 A at full load condition, on-line with the name plate data input and provides a high degree of accuracy. By providing suitable current transformers and potential transformers, the range of the relay can be extended for high capacity motors. The method is very sensitive, fast and detects faults while running and before start.

6 6 1.3 METHODOLOGY This thesis examines the effect of electrical faults upon the operation of induction motors. Both healthy and faulty operations of the machine under different load conditions were simulated using MATLAB / Simulink program. Electrical faults (Unbalanced supply voltage, single phasing, over and under voltages, phase reversal, over load, power frequency variations and earth fault) under various load conditions (no load, 25%, 50%, 75%, 100% and 125% of the rated load) are considered for analysis. The above results are compared with the experimental results to determine the maximum tolerable limit values of induction motor operation. Based on the analysis, a protection scheme is developed using embedded micro controller. Figure 1.1 shows the design, development and implementation of the embedded motor protection relay. 1.4 OVERVIEW The overall objective of the thesis is to analyze the performance of induction motor during electrical fault conditions and based on the fault analysis, to provide a micro controller based on- line, multi function motor protection scheme. Chapter 2 summarizes the review of the literature surveyed towards this thesis. Chapter 3 describes the causes and effects of electrical faults like unbalance, single phasing, under and over voltages, phase reversal, overload, earth fault and power frequency variations on three phase squirrel cage induction motor. Chapter 4 discusses the dynamic modeling of three phase induction motor using reference frame theory. Chapter 5 describes the simulation and test set up of fault analysis.

7 7 Review of Electrical Fault analysis of 3-Phase Squirrel Cage Induction Motor Simulation of Electrical fault analysis of 3 Phase Induction Motor using MATLAB / Simulink Electrical Fault analysis of 3-Phase Induction Motor Using Test Set up Modeling of 3-Phase Induction Motor using reference frame theory Simulink implementation of D Q Model Observe the performance of 3 -Phase Induction Motor during electrical faults from experimental data Simulate Electrical faults and observe the performance of three Phase Induction Motor Compare Simulated and Experimental results and determine maximum tolerable limit values under faulty conditions of Induction Motor Development of Multifunction, integrated protection scheme for Induction Motor by developing software algorithm using embedded programming and MPLAB/IDE tool and Hard ware Implementation using PIC 16F877 Micro Controller Testing of Prototype embedded relay Figure 1.1 Flowchart for development of embedded motor protection relay

8 8 Chapter 6 analyzes the performance of three phase squirrel cage induction motor during i) Balanced supply with normal (rated voltage), under voltage and over voltage conditions, ii) Unbalanced supply with seven types of unbalance conditions like single phase, two phase, three phase under voltage unbalances, single phase, two phase, three phase over voltage unbalances and single phasing, iii) Over load condition, iv) Ground fault with single phase, two phase and three phase line to ground fault conditions, v) Phase reversal with balanced rated voltage, unbalanced over and under voltage conditions, vi) Power frequency variations with five various power frequencies of (25Hz, 40Hz, 50Hz, 60Hz and 100Hz). Chapter 7 describes the design, development and implementation of micro controller based multi function motor protection system based on fault analysis. A prototype model is developed and tested on a low voltage induction motor and the requirements are verified. Chapter 8 describes the consolidation of research findings and future extension of this thesis.