Browsing by Author "Gavini, Sree Likhita"

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    Control of Non-minimum Phase Power Converters
    (2012-05) Gavini, Sree Likhita; Izadian, Afshin; Li, Lingxi; Rizkalla, Maher E.
    The inner structural characteristics of non-minimum phase DC-DC converters pose a severe limitation in direct regulation of voltage when addressed from a control perspective. This constraint is reflected by the presence of right half plane zeros or the unstable zero dynamics of the output voltage of these converters. The existing controllers make use of one-to-one correspondence between the voltage and current equilibriums of the non-minimum phase converters and exploit the property that when the average output of these converters is the inductor current, the system dynamics are stable and hence they indirectly regulate the voltage. As a result, the system performance is susceptible to circuit parameter and load variation and require additional controllers, which in turn increase the system complexity. In this thesis, a novel approach to this problem is proposed for second order non-minimum phase converters such as Boost and Buck-Boost Converter. Different solutions have been suggested to the problem based on whether the converter is modeled as a linear system or as a nonlinear system. For the converter modeled as a linear system, the non-minimum phase part of the system is decoupled and its transfer function is converted to minimum phase using a parallel compensator. Then the control action is achieved by using a simple proportional gain controller. This method accelerates the transient response of the converter, reduces the initial undershoot in the response, and considerably reduces the oscillations in the transient response. Simulation results demonstrate the effectiveness of the proposed approach. When the converter is modeled as a bilinear system, it preserves the stabilizing nonlinearities of the system. Hence, a more effective control approach is adopted by using Passivity properties. In this approach, the non-minimum phase converter system is viewed from an energy-based perspective and the property of passivity is used to achieve stable zero dynamics of the output voltage. A system is passive if its rate of energy storage is less than the supply rate i.e. the system dissipates more energy than stores. As a result, the energy storage function of the system is less than the supply rate function. Non-minimum phase systems are not passive, and passivation of non-minimum phase power converters is an attractive solution to the posed problem. Stability of non-minimum phase systems can also be investigated by defining the passivity indices. This research approaches the problem by characterizing the degree of passivity i.e. the amount of damping in the system, from passivity indices. Thus, the problem is viewed from a system level rather than from a circuit level description. This method uses feed-forward passivation to compensate for the shortage of passivity in the non-minimum phase converter and makes use of a parallel interconnection to the open-loop system to attain exponentially stable zero dynamics of the output voltage. Detailed analytical analysis regarding the control structure and passivation process is performed on a buck-boost converter. Simulation and experimental results carried out on the test bed validate the effectiveness of the proposed method.
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    EXCESS PASSIVITY BASED CONTROL OF NONLINEAR BUCK-BOOST CONVERTER
    (Office of the Vice Chancellor for Research, 2012-04-13) Gavini, Sree Likhita; Izadian, Afshin
    Buck-boost converters are nonlinear systems and can either step-up or step-down the output voltage. A number of effective non-linear controllers such as sliding mode control, passivity based control, feedback linearization are in use for regulating the voltage of power converters. Direct regulation of voltage in non-minimum phase power converters such as buck-boost converter is challenging, as the zero dynamics of the output voltage are unstable. Consequently, these controllers make use of one-to-one correspondence between the voltage and current equilibriums and exploit the property that when the average output of the buck-boost converter is the inductor current, the system dynamics are stable. So the existing control strategies indirectly regulate the voltage, but their performance is susceptible to circuit parameter variations like load variation. As a result, adaptive versions of the controllers are incorporated to achieve a satisfactory performance, which in turn increases the system complexity. This problem of regulating the non-minimum phase voltage of the power converters continues to challenge and some solutions to this problem are presented based on different energy shaping approaches. The principal investigation in these approaches focuses on characterizing the energy of the system based on the physical structure of the system and uses this energy function description to draw conclusions about the degree of passivity i.e. damping in the system. This work approaches the problem by characterizing the degree of passivity in the system from passivity indices rather than from the system’s energy function, and thus views the problem from a system level rather from a circuit level description. We claim and support our claim from simulation and experimental results that this approach is complementary to existing approaches and uses a linear control to achieve the same objective as direct regulation of voltage and robustness against load variations.
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    SUPERVISORY CONTROL OF COMBINED WIND/SOLAR ENERGY HARVESTING AND BATTERY MANAGEMENT USING MPPT WITH DSPACE IMPLEMENTATION
    (Office of the Vice Chancellor for Research, 2012-04-13) Izadian, Afshin; Girrens, Nathaniel; Yang, Heng; Gavini, Sree Likhita
    The growing demand for renewable energy and for providing electrical power to remote locations provides exciting opportunities as well as interest-ing problems. Wind and solar energies are individually unreliable as a sole means of power generation. Wind power is intermittent at best and can con-stantly change directions; solar power is only available during the day, is considerably affected by environmental conditions and generating maximum power requires proper directioning and controls. They are, however, com-plementary in nature and when combined together with energy storage can provide reliable power. To properly utilize the power generation capability of the system, Maximum Power Point Tracking (MPPT) will be used when there is solar power available, implemented with a Buck-Boost converter for wind and solar. When solar power is not available the wind power will be fed in directly without control. Supervisory control is used to provide reliable power to the load and determine when to charge and discharge the battery based on voltage being fed in by the wind/solar systems. A small scale system is used with dSPACE hardware to demonstrate the effectiveness of the supervi-sory controller.