Characteristics study and performance optimization of hybrid power supply systems
Date of Issue2016-05-16
School of Electrical and Electronic Engineering
The utilization of renewable energy sources (RESs) and the development of power electronics systems for capitalizing such energy sources have received renewed interest in the past decade. Common forms of RESs including photovoltaic (PV) energy, hydro energy, wind energy, hydrogen energy, etc., and many of such sources are mutually complementary in the sense that they can be utilized to maintain continuous delivery of power to the load. Basically, a pulse-width-modulation (PWM) converter is used to draw power from an energy source with a certain required amount of voltage, current, and power. In order to combine more than one energy source, such as those aforementioned RESs, to get the regulated output voltage or other multivariable control targets, different circuit topologies have been proposed in recent years to construct a hybrid power supply (HPS) in lower-power-level applications or a hybrid distributed generation system (HDGS) in higher-power-level applications. The conventional way to build such a HPS/HDGS is to parallel/series connect several single-input single-output (SISO) converters to a dc-bus before connecting to the load, differing from which however, the multiple-input converters (MICs) addressed in this thesis possess more centralized and simplified circuit topologies and controller configurations. By applying the specified time-sharing switching (TSS) functions, every input-to-output port pair in a MIC can be deemed as a SISO PWM converter at a given subinterval, through which all of the input sources can deliver power to the load either individually or simultaneously. This thesis attempts to cover all aspects of the MIC-relevant topics — classification, circuitry, switching strategies, mathematical steady-state and dynamic modeling, control approaches, and system design — for applications of RESs in both low-power and high-power levels, based upon which some achievements have been made. A systematic analysis on operation principle of multiple-input buck converter (MIbC) topology and its derivation of the expressions in general case, considering both ideal and practical-parametric circuits, has been firstly presented. Input capacitors and equivalent series resistance (ESR) in input sources have been involved. A step-by-step generalization procedure for any kind of MIC topology has been come up with. The thesis has studied the differences in power delivery and operating stability between MIbC topologies with and without input capacitors as well. The criterion for selecting input capacitors for a MIC has then been formulated. TSS strategy is the fundamental pulsating signals applied to MICs’ operation and control. Trailing-edge modulation (TEM) and interleaved dual-edge modulation (IDEM) are the two typical TSS functions. A more specific steady-state analysis on MIbC applied with TEM and IDEM TSS functions, considering both continuous conduction mode (CCM) and discontinuous conduction mode (DCM), has been put forward. The generalization procedure, analogous to the aforementioned one, has been brought forward in order to obtain the generalized mathematical expressions for any kind of MIC topology with any kind of TSS function and at either CCM or DCM. Dynamic characterization of a power electronic converter is crucial for its controller design and stability assessment. It could not be counted as a full-scale circuitry for a power converter without discussing its dynamic characterization. As was done in the steady-state analysis, generalized dynamic equations corresponding to diverse TSS functions and CCM/DCM have been derived based upon state-space averaging theory and equivalent small-signal modeling. Input-to-output transfer functions of the relevant MIC topologies have also been yielded, being applied in the upcoming process of the controller design. Although they are most-commonly-used in practical applications, TEM and IDEM TSS strategies have their own shortcomings. Modified-TSS (MTSS) strategy has been proposed to overcome these demerits at the price of losing full control degree of freedom (DOF). Based on MTSS concept, advanced-TSS (ATSS) strategy has then been put forward to reclaim MIC’s power management capability. Detailed mathematical modeling and analysis for TEM, IDEM, MTSS, and ATSS-controlled MIbC topologies have been figured out, followed by which the closed-loop control system design has been carried out and simulated. The results showed that ATSS-based MIbC closed-loop control system is able to achieve output-voltage-regulation (OVR) and input-current-regulation (ICR) functions, simultaneously, which could not be realized through MTSS-based one. Having been referred and compared to the Bode diagrams of the circuit systems before and after compensation, the entire procedures for designing OVR and ICR controllers have been proposed. Last but not least, three practical illustrative examples developed for HPS/HDGS have been elaborated to exhibit the multivariable-control capability of the MICs and their control approaches to applications of RESs and energy storage devices including fuel cells, PV, battery, supercapacitor, etc. The foregoing MIC topologies, switching functions, and advanced control approaches have been involved in the examples. Some potential research topics around MICs — extension of generalization and synthesis of isolated MIC family; extension of generalization and synthesis of multiple-input multiple-output dc-dc converter family; canonical circuit modeling of MIC family; advanced control approaches to MIC; modularization and productization— have been recommended at the end of the thesis as the orientations we are directing to in the future.
DRNTU::Engineering::Electrical and electronic engineering::Power electronics