INVESTIGATION ON MULTI-INVERTER FED INDUCTION MOTOR DRIVE

Abstract

Electrical drives play an important role in electromechanical energy conversion in transportation and industrial applications for precise and continuous control of speed, torque, position, good transient response and high efficiency. From the early stages of the development of industrial adjustable speed drives, dc motors (especially separately excited) have been considered the most suitable choice for high level dynamic performance. However, dc motor is not an ideal solution to the problem of adjustable speed motor operation. Its mechanical commutator not only requires regular maintenance but is also responsible for restricting the voltage rating of the motor. The induction motor, on the other hand, has robust rotor construction, which permits reliable and maintenance-free operation at high speed. However, its speed, being closely tied to the supply mains frequency, is more orless constant. The introduction of thyristors has heralded the advent of solid state power electronics and revolutionized the traditional methods of electric power conversion and control. The rapid advances in power semiconductor devices, integrated electronics and microprocessors have dramatically reduced the size and cost of power electronics converters. With these developments, the picture ofinduction motor, as a machine with near constant speed characteristics underwent a major change. The application of induction motors in adjustable speed drives has since progressively increased. In adjustable speed applications, the induction motor is supplied from a power electronic modulator, which converts fixed voltage and fixed frequency of supply mains into adjustable voltage and adjustable frequency supply. During early nineties in USA and Japan, Gate Turn Off thyristors (GTO's) were being used successfully in static power controllers in the range of 80 MVA. The switching rate of GTO's was only one pulse per 60 Hz periods in order to have minimum switching losses and optimum efficiency. The inherent large on-state losses and large commutation losses due to the existence of tail current were the reasons for such alow rate of switching although the design turn-on and turn-off times set by the manufacturers were quite low i.e. of the order of tens to hundred of micro-second. The survey reveals that the high power converters rated tens of kilo-volt to hundreds of kilo-volt and mega-volt amperes are beyond the capacity of a single solid-state switch of maximum possible kilo-volt and kilo-ampere ratings. The high voltage, large rating PWM inverters, employing power switching devices in series and /or parallel configuration, suffers from the limitations such as an unequal voltage and current sharing, existence of high common mode voltages, and corona discharge, voltage surges /dielectric stresses and the resulting motor winding insulation breakdown as well as motor bearing failure, mainly due to the excessive dvIdt. These limitations have been overcome to a great extent through a basic concept of using the devices ingeniously in such away that ultimately lower dvIdt exists in the converter. Based upon the converter topologies used in high current-high voltage power applications, the power converters can be categorized into two major groups: multilevel inverters (MLI's) and multi-converter /linear power amplifier with a view to provide a perfect output to input linearity, best efficiency and power quality. The concept of utilizing multiple small voltage levels to perform power conversion was patented by an MIT researcher over twenty four years ago. Advantages of this multilevel approach include good power quality, good electromagnetic compatibility (EMC), low switching losses, and high voltage capability. The main disadvantages of this technique are that a large number of switching semiconductors are required for lower-voltage systems and the small voltage steps must be supplied on the dc side either by a capacitor bank or isolated voltage sources. The first topology introduced was the series H-bridge design. This was followed by the diode-clamped converter, which utilized a bank of series capacitors. A later invention was the flying capacitor design in which the capacitors were floating rather than series connected. Another multilevel design involves parallel connection of inverter phases through inter-phase reactors. The multilevel PWM inverter combines the multilevel waveforms and PWM concept to produce high power output with low harmonic contents. In MLI's, the switching is in between several voltage levels that results in low dv Idt. Hence, the dv Idt related problems are reduced. In order to improve the quality of the output waveform, the most popular sub-harmonic PWM with one modulating reference and several phase shifted triangular carriers is used. Although the MLI's ii have reduced total harmonic distortion (THD), dv Idt stresses and EMI related problems, the inverters of this group have several limitations such as increased overall KVA rating of the converter module for a given load due to the large device count, a problem with voltage balancing of DC bus capacitors in absence of explicit control strategy, complex power converter configurations, reduced power capacity utilization at low modulations indices unless an explicit carrier phase rotation technique is adopted that results in a very complicated PWM control signal generation. In multi-converter topology, several voltage source PWM inverters using common sinusoidal modulating signal and phase shifted triangular carriers operate as linear power amplifier with their outputs coupled through current sharing inductors. To obtain an ultimate high voltage and high current rating in overall converter system, the 'parallel connected series paired modules' (PCSP) power circuit structure is used. The high voltage rating is obtained through series connection of low voltage converter module (LVCM) structures, whereas high current rating is obtained by parallel connection of the multiple LVCM The multi-converter system exhibits direct control of magnitude and /or frequency; output to input linearity, and power quality with the possibility ofimplementation ofsophisticated modern control theory. Such converter have intensive application in FACTS as shunt or series STATCOM's and UPFC's as well as in high rating poly-phase ac drives, which have demanding applications in rolling mills, electric tractions; gearless mill drives for cement industry, centrifugal pumps and compressors. As high performance converters, they also have applications in power supplies for super conducting magnets, as linear power amplifiers in large capacity electrodynamics' vibrations and UPS. The first part of this dissertation reviews the design philosophy of the two well-known topologies of the converters used for high voltage and high current applications to facilitate the design and development of near future mega-volt ampere rating power converters /conditioners having a fast, efficient and reliable power control with the use of recently developed higher switching speed GTO or IGBT's and the advanced power circuit topologies or converter configurations operating with improved pulse width modulation strategies. Transient behavior of three-phase iii induction machine fed through single three-phase inverter, and multi-inverter system (two and four) is also included in this part. An open-loop speed control scheme is developed to analyze the performance of the induction machine. In order to study the system dynamics, the complete drive system is described by means of a mathematical model in terms of a set of first order differential equations. The differential equations are developed using the d-q variables in general reference frame. The SIMULINK block of three-phase inverter bridge and triangular carrier wave is used to model the PWMinverter. The complete analytical model is developed in Matlab/SIMULINK to analyze the transient and dynamic behavior of the drive. Total losses incurred per devices in single inverter and multi-inverter system is also calculated. In later part of the dissertation, a laboratory prototype of multi-inverter fed induction motor drive is indigenously developed. A simple indirect field-oriented control scheme is derived and adopted for operation and control of induction motor fed through multi-converter system. The control strategy is based on the two-axis {d-q) model of the machine. The practical implementation of the scheme is simple. The necessary experimental test results are recorded to verify the validity of the proposed scheme. In the study, on-line analysis has been performed using C++, whileMATLAB /SIMULINK have been used to perform the off-line analysis. Results of theoretical and experimental analyses clearly demonstrate several advantageous features of the multi-inverter fed induction machine such as - The devices are operated at reduced switching frequency resulting in better efficiency due to the reduced switching loss, - Staggering of switching instants in the individual converters due to the phase shift in triangle carrier creates an overall high frequency switching effect at the ac side of the paralleled module structure of the multi-converter system, - The triangle carrier frequency for the individual converter is selected judiciously so that each module behaves as a 'linear power amplifier', IV - Due to the relatively higher switching frequency (N times the carrier frequency, N is number of converter modules), the switching noise of the carrier frequency and the lower side band frequencies do not spread over the spectrum of modulation signal (reference sine) and the resulting current unbalances in ac side are avoided, - Voltage stresses on the devices are ensured to remain within the range of DC link capacitor voltage, which is set through negative feedback loop around each converter, Low total harmonic distortion in current and voltage waveform Based on this research work, one paper has been published in IEEE Transaction on Industrial Electronics. One paper on scalar closed-loop speed control scheme is under review. In addition, one paper in International conference has already been presentedand published.