The renewable energy sources play amajor role in modern power market due to increasing rate of fossil fuels,energy demand and awareness about global warming. one of the reliable renewableenergy source is the PV cell in which the power is effectively utilized whenused in Grid connected system. The DC power produced by such PV cell isconverted to AC feeding into the grid system by an Inverter. Multilevel Inverters possess good powerquality, better electromagnetic compatibility, low switching losses and canhandle high voltages.
They can be operated at fundamental frequency as well asat high frequencies. There are three commonly use topologies for MultilevelInverter (MLI). They are Diode clamped, Capacitor clamped, Cascaded H-BridgeMLI (CHB-MLI). Among these CHB-MLI is mostly used, because of its structure andimproved efficiency and hence the cost of the CHB-ML inverter is drasticallyreduced due to the utilization of low variety DC voltage sources. But the useof higher number of bidirectional power switches and insulated gate bipolartransistors (IGBT) are leads to more complex and cost effective circuitry.Hence a modified topology of CHB-MLI with reduced number of power switches is requiredto produce output voltage with increased number of voltage levels since thismodified topology requires less number of switches. The cost of the drivercircuits and the total cost and complexity of the circuit is also reduced.
A ReducedSwitch Cascaded H-Bridge MLI(RSCHB-MLI) is proposed in this Thesis.This proposed topology utilizesunidirectional and bidirectional switches with less number of DC voltagesources. various modulation techniques such as Carrier shift Frequency MultiPulse width Modulation (CSF M PWM), CarrierShift Frequency Multi Optimum Pulse Width Modulation (CSF M OPWM), VariableShift Frequency Multi Pulse Width Modulation (VSF M PWM), Variable ShiftFrequency Multi Optimum Pulse Width Modulation (VSF M OPWM) and Phase Shifted Carrier Switching frequency Optimal Multi Carrier Pulse WidthModulation (PSC SFOPWM) are used for theproposed topology and the Total Harmonic Distortion (THD) performances are analyzed.
The performance ofRSCHB MLI with proper modulation techniques are analysed. The performance of RSCHB MLI such as total harmonic distortion,switching losses, power quality, output voltage is improved due to the propermodulation techniques. Such inverters are most suited of Induction Motor drivesin order to improve the motor drive performance such as dynamic response,robust performance, sensitivity etc. The conventional PI controller is mostlyused for speed and position control of IM drive. Though PI controller hassimple structure, it cannot provide better performance over a wide range ofoperating condition.
Hence PI controller has limited capabilities for adaptivestructure. So, a new control schemes have to be investigated for IM drives.Recently Finite Control Set Model Predictive Control (FCS – MPC) is gainingmore attention because of its ability to optimize multivariable with multi constraints.The control technique can be applied power converters used for induction motordrives.Direct Torque Control (DTC) techniques for RS-CHB MLI usingPredictive Controller has been proposed induction motor drives.
Predictivecontroller used here optimally select the voltage vectors, based on thediscrete time model of the system. Whereas, vector control requires coordinatetransformation, MPC allows controls the torque directly by an appropriateselection of voltage vectors. For adjustable speed induction motor drives,Direct Torque Control is the commonly used control technique.
DTC is easy toimplement and achieves fast dynamic responses and has best performance whencompared to other control techniques. The output of seven level RS-CHB MLIusing FCS – MPC direct torque control strategy is fed to high power inductionmotor drive. The effect of variation in motor parameters and the load disturbancesare drastically due to the perfect implementation of control law.
Theconventional controllers such as PI controller can provide optimal performanceonly at specific operating conditions of load and speed, whereas thefast-dynamic performance is attained at any speed and load conditions. ThroughMPC the actual speed, reference speed and the acceleration / deacceleration ofthe rotor is used by the proposed method, to compute the reference currentwithin the specified prediction horizon, the proposed method microcontroller.