INVESTIGATIONS ON MULTILEVEL INVERTER FOR INDUCTION MOTOR DRIVE

Abstract

Electric motors provide driving power for a large part of modern prime movers. Induction motors are the most numerous in electric motors as they are simple, rugged and cheap. Adjustable speed induction motor drives have become preferred choice for the energy efficiency in many industrial applications. Voltage source inverters (VSIs) are well proven as a standard configuration for PWM inverter drives owning to availability of fast power semiconductor devices, high dynamic response and good spectral performance. To minimize harmonics in two-level VSIs, the general trend is to use high frequency PWM (pulse width modulation) techniques. Inappropriately applied high frequency PWM techniques cause problems like increased switching losses, higher common-mode voltages, high dv/dt, more EMI/EMC problems, etc. In recent years, industries have begun to use higher power equipments, which now reach upto megawatt levels. Above problems become more pronounced at high power levels. Controlled AC drives in megawatt range are usually connected to medium-voltage network due to restriction on current rating at low-voltage. Possible solutions for the inverter at high power medium-voltage range are multilevel inverter (MLI) and series connection of switching devices in traditional twolevel inverter. The later solution is complex even with fast switching devices due to requirement of correct static and dynamic voltage sharing of series devices. Use of several levels of DC-voltage enables MLI to achieve better quality output voltages at low switching frequency. MLI topologies require more device count but they are modular in structure and compact in size as they require reduced sized output filter and transformer or none of them. Reduction in voltage stress on switching devices, common-mode voltages in load, dv/dt and electromagnetic interference can be achieved in MLIs as compared to two-level inverters. Above features and various problems associated with two-level inverter topologies necessitate multilevel inverters as an attractive solution for high power as well as low power induction motor drives. In order to improve the performance of MLI for induction motor drive with simplified modeling, control and implementation, present work is carried out. MLI for induction motor drive is designed, developed and investigated. Problems faced by conventional two-level inverter fed motor drives are addressed first. Their possible solutions and recent trends are discussed. Features, technical challenges, applications, historical developments and research gaps of MLIs are given. A comprehensive literature survey on MLI including various MLI topologies, various modulation schemes and various control schemes for induction motor drive is given in detail. Three-level diode clamped inverter, also known as neutral point clamped (NPC) inverter has found wide applications in induction motor drives. From available techniques, space vector PWM (SVPWM) technique has attractive features for digital control of MLIs. It gives flexibility to optimize switching patterns to reduce harmonic distortions, switching losses and DC-link voltage unbalance. It offers extended DC-link utilization in linear PWM mode. The three-phase three-level NPC inverter for induction motor drive using SVPWM technique is taken as the first investigative object. To analyze the performance of the NPC inverter in steady-state and dynamic conditions, mathematical models of SVPWM for MLI using switching functions and induction motor drive using open-loop constant V/f control are developed. Symmetrical and synchronized SVPWM technique is employed using nearest vector selection and its implementation for MLI is presented. With the aim to increase maximum obtainable output voltage by properly utilizing the DC-link capacity, SVPWM algorithm is extended in overmodulation. For a smooth transition from linear PWM mode to six-step mode of operation, two zones overmodulation algorithm with simplified implementation for MLI and reduced computational burden is developed and analyzed. Simulation analysis is carried out in MATLAB/Simulink environment and verified experimentally on three-level NPC lab prototype inverter connected with 1.5 kW induction motor. Control algorithms are implemented using dSPACE 1103 R&D controller board with fixed-point calculations. Various simulation results are presented to validate the scheme and verified experimentally under different operating conditions like two-level PWM mode, three-level linear PWM mode, overmodulation mode-l, overmodulation mode-ll including six-step operation. MLI output voltages, currents, voltages of DC-link capacitors and induction motor speed are recorded and analyzed for performance evaluation. With the aim to extend SVPWM for higher level inverters, the proposed feed forward generalized n-level SVPWM scheme is developed and analyzed computationally using a five-level inverter. To model the non-linearity of overmodulation look-up tables and solution of complex equations are not required for the scheme. Hence, it can be used for any n-level (n > 3) inverter without increase in computations. Owning to simplicity of control algorithm and presented step by step procedure, the algorithm can be easily implemented using commercially available DSPs or microcontrollers with minor modifications. Resultant space vector locations generated from dual two-level inverters for open-end winding induction motor are similar to a single three-level NPC inverter. The topology eliminates the clamping diodes and tapped DC-link capacitors. Two isolated DC supplies are required in this topology but DC-link voltages and switching device voltage rating are reduced by half as compared to conventional single inverter scheme. More number of redundant voltage space vectors in the scheme gives more degree of freedom for SVPWM control. Dual two-level inverters for open-end induction motor drive are designed and developed using synchronized SVPWM. The mathematical model of dual inverters for open-end induction motor drive is developed and the performance of the proposed scheme is evaluated using different space-vector pulse pattern arrangements. The complete elimination of common-mode voltage is achieved in one of the implementations. Viability of model predicted results are ascertained by implementing algorithms on lab prototype of dual two-level inverters using intelligent power modules (IPMs). DC-link capacitors in three-level NPC inverter carry the load current, causing DClink voltage unbalance. This unbalance is higher at higher load current, poor load power factor and lower capacitance of DC-link capacitors. This may cause increased harmonics, torque pulsations, voltage stress and nuisance overvoltage/undervoltage trip. To overcome these problems, the modified SVPWM for DC-link voltage balancing control is presented for three-level NPC inverter. The mathematical model of DC-link capacitor voltage deviation on inverter parameters is established using switching function and dirac delta function. The proposed SVPWM scheme effectively utilizes in redundant space vectors, their sequencing and splitting of their duty cycles for the neutral point voltage control. The scheme uses nearest vector SVPWM and gives the perfect volt-second balance without significant increase in switching losses. The scheme maintains DC-link voltage variation nearer to zero with good overall performance. The detailed analysis investigates the DC-link voltage control behavior of proposed scheme and other popular SVPWM scheme. The scheme requires neutral point voltage as a feedback; no extra hardware is required. It offers use of reduced rating and longer life of DC-link capacitors. Simulation analysis is carried out and results are verified experimentally on lab prototype under different operating conditions over entire range of modulation index. Measured harmonic distortions are in close confirmative with those simulated and computed analytically. Both steady-state and dynamic performance of proposed scheme appears to be attractive. To summarize, laboratory prototype of the three-phase three-level NPC inverter for induction motor drive and three-phase dual two-level inverters for open-end winding induction motor drive are designed and developed. The performance of three-level NPC inverter topology is thoroughly investigated using popular SVPWM technique and modified SVPWM technique for DC-link capacitor voltage balancing control. The proposed scheme maintains DC-link voltage variation nearer to zero. The performance of dual two-level inverters for open-ended induction motor drive is investigated using different SVPWM schemes. The generalized n-level SVPWM algorithm is developed and extended in overmodulation. Viability of developed control algorithms are ascertained by implementing them on lab prototype inverter circuits and constant V/f controlled induction motor drive. Extensive simulation and experimental results are presented under different operating conditions to investigate the steady-state, transient and dynamic performance of MLIs for induction motor drive. The experimental results confirm that the proposed algorithm for MLI control gives desired high performance of the system.