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Title:  Generation Of 12Sided And 18Sided Polygonal Voltage Space Vectors For Inverter Fed Induction Motor Drives By Cascading Conventional TwoLevel Inverters 
Authors:  Lakshminarayanan, Sanjay 
Advisors:  Gopakumar, K 
Keywords:  Induction Motors Inverters Multilevel Inverters Pulse Widh Modulation Space Vector Induction Motor Drive PWM Controller Space Phasor 
Submitted Date:  Jun2007 
Series/Report no.:  G21508 
Abstract:  Multilevel inverters play a significant role in high power drive systems for induction motors. Interest in multilevel inverters started with the threelevel, neutral point clamped (NPC) inverter. Now there are many topologies for higher number of levels such as the, flying capacitor and cascaded Hbridge etc. The advantage of multilevel inverters is the reduced voltage stress on the switching devices, lower dv/dt and lower harmonic content. The voltage space vector structure in a multilevel inverter has a hexagonal periphery similar to that in a twolevel inverter. In the overmodulation region in multilevel inverters, there is the presence of lower order harmonics such as 5th and 7th in the output voltage, and this can be avoided by using a voltage space vector scheme with more than six polygonal voltage space vectors such as 12, 18, 24 etc. These polygonal voltage space vectors can be generated by using multilevel inverter topologies, by cascading twolevel inverter structures with asymmetric DClinks. This thesis deals with the development of 12sided and 18sided polygonal voltage space vector schemes for induction motor drives. With the 12sided polygonal structure, all the 5th and 7th harmonic orders and 6n±1, n=1, 3, 5.. are absent throughout the modulation range, and in the 18sided voltage space vector scheme, 5th, 7th, 11th and 13th harmonics are absent throughout the modulation range. With the absence of the low order frequencies in the proposed polygonal space vector structures, high frequency PWM schemes are not needed for voltage control. This is an advantage over conventional schemes. Also, due to the absence of lower order harmonics throughout the modulation range, special compensated synchronous reference frame PI controllers are not needed in current controlled vector control schemes in overmodulation.
In this thesis a method is proposed for generating 12sided polygonal voltage space vectors for an induction motor fed from one side. A cascaded combination of three twolevel inverters is used with asymmetrical DClinks. A simple space vector PWM scheme based only on the sampled reference phase amplitudes are used for the inverter output voltage control. The reference space vector is sampled at different sampling rates depending on the frequency of operation. The number of samples in a sector is chosen to keep the overall switching frequency around 1kHz, in order to minimize switching losses. The voltage space vectors that make up the two sides of the sector in which the reference vector lies, are time averaged using voltsec balance, to result in the reference vector. In the proposed 12sided PWM scheme all the harmonics of the order 6n±1, n=1, 3, 5... are eliminated from the phase voltage, throughout the modulation range.
In multilevel inverters steps are taken to eliminate commonmode voltage. Commonmode voltage is defined as one third of the sum of the three pole voltages of the inverter for a three phase system. Bearings are found to fail prematurely in drives with fast rising voltage pulses and high frequency switching. The alternating commonmode voltage generated by the PWM inverter contributes to capacitive couplings from stator body to rotor body. This generates motor shaft voltages causing bearing currents to flow from rotor to stator body and then to the ground. There can be a flashover between the bearing races. Also a phenomenon termed EDM (Electrodischarge machining) effect occurs and may damage the bearings. Commonmode voltage has to be eliminated in order to overcome these effects. In multilevel inverters redundancy of space vector locations is used to eliminate commonmode voltages. In the present thesis a 12sided polygonal voltage space vector based inverter with an openend winding induction motor is proposed, in which the commonmode voltage variation at the poles of the inverter is eliminated. In this scheme, there is a threelevel inverter on each side of the openend winding of the induction motor. The threelevel inverter is made by cascading two, twolevel inverters with unequal DClink voltages. Appropriate space vectors are selected from opposite sides such that the sum of the pole voltages on each side is a constant. Also during the PWM operation when the zero vector is applied, identical voltage levels are used on both sides of the openend windings, in order to make the phase voltages zero, while the commonmode voltage is kept constant. This way, commonmode voltage variations are eliminated throughout the modulation range by appropriately selecting the voltage vectors from opposite ends. In this method all the harmonics of 6n±1, n=1, 3, 5.. and triplen orders are eliminated.
In the 12sided polygonal voltage space vector methods, the 11th and 13th harmonics though attenuated are not eliminated. In the 18sided polygonal voltage space vector method the 11th and 13th harmonics are eliminated along with the 5th and 7th harmonics. This scheme consists of an openend winding induction motor fed from one side by a twolevel inverter and the other side by a threelevel inverter comprising of two cascaded twolevel inverters. Asymmetric DClinks of a particular ratio are present.
The 12sided and 18sided polygonal voltage space vector methods have been first simulated using SIMULINK and then verified experimentally on a 1.5kW induction motor drive. In the simulation as well as the experimental setup the starting point is the generation of the three reference voltages v, vB and vC . A method for determining the sector in which the reference vector lies by comparing the values of the scaled sampled instantaneous reference voltages is proposed. For the reference vector lying in a sector between the two active vectors, the first vector is to be kept on for T1 duration and the second vector for T2 duration. These timing durations can be found from the derived formula, using the sampled instantaneous values of the reference voltages and the sector information. From the pulse widths and the sector number, the voltage level at which a phase in the inverter has to be maintained is uniquely determined from lookup tables. Thus, once the pole voltages are determined the phase voltages can be easily determined for simulation studies. By using a suitable induction motor model in the simulation, the effect of the PWM scheme on the motor current can be easily obtained. The simulation studies are experimentally verified on a 1.5kW openend winding induction motor drive. A V/f control scheme is used for the study of the drive scheme for different speeds of operation. A DSP (TMS320LF2407A) is used for generating the PWM signals for variable speed operation.
The 12sided polygonal voltage space vector scheme with the motor fed from a single side has a simple power bus structure and it is also observed that the pole voltage is clamped to zero for 30% of the time duration of one cycle of operation. This will increase the overall efficiency. The proposed scheme eliminates all harmonics of the order 6n±1, n=1, 3, 5…for the complete modulation range. The 12sided polygonal voltage space vector scheme with commonmode elimination requires the openend winding configuration of the induction motor. Two asymmetrical DClinks are required which are common to both sides. The leg of the high voltage inverter is seen to be switched only for 50% duration in a cycle of operation. This will also reduce switching losses considerably. The proposed scheme not only eliminates all harmonics of the order 6n±1, n=1, 3, 5…for the complete modulation range, but also maintains the commonmode voltage on both sides constant. The commonmode voltage variation is eliminated. This eliminates bearing currents and shaft voltages which can damage the motor bearings.
In the 18sided polygonal voltage space vector based inverter, the 11th and 13th harmonics are eliminated along with the 5th and 7th. Here also an openend winding induction motor is used, with a twolevel inverter on one side and a threelevel inverter on the other side. A pole of the twolevel inverter is at clamped to zero voltage for 50% of the time and a pole of the threelevel inverter is clamped to zero for 30% of the time for one cycle of operation. The 18sided polygonal voltage space vectors show the highest maximum peak fundamental voltage in the 18step mode of 0.663Vdc compared to 0.658Vdc in the 12step mode of the 12sided polygonal voltage space vector scheme and 0.637Vdc in the sixstep mode of a twolevel inverter or conventional multilevel inverter (where Vdc is the radius of the space vector polygon).
Though the schemes proposed are verified on a low power laboratory prototype, the principle and the control algorithm development are general in nature and can be easily extended to induction motor drives for high power applications. 
URI:  http://etd.iisc.ernet.in/handle/2005/693 
Appears in Collections:  Department of Electronic Systems Engineering (dese)

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