Kaustuva Acharya

A Ripple-Mitigating and Energy-Efficient Fuel Cell Power-Conditioning System

We describe an energy-efficient, fuel-cell power-conditioning system (PCS) for stationary application, which reduces the variations in the current drawn from the fuel-cell stack and can potentially meet the

Master–Slave Current-Sharing Control of a Parallel DC–DC Converter System Over an RF Communication Interface

40/kW cost target. The PCS consists of a zero-ripple boost converter (ZRBC) followed by a soft-switched and multilevel high-frequency (HF) inverter and a single-phase cycloconverter. The ZRBC comprises a new zero-ripple filter (ZRF), which significantly reduces the input low- and high-frequency current ripples, thereby potentially enhancing the durability of the stack. A new phase-shifted sinewave modulation of the multilevel HF inverter is proposed, which results in the zero-voltage switching (ZVS) of all four switches without the use of any auxiliary circuit components. For such a sine wave modulation technique, >90% ZVS range is obtained per line cycle for about 70% of the rated load. Further, the line-frequency switching of the cycloconverter (at close to unity power factor) results in extremely low switching losses. The intermediate dc bus facilitates the inclusion of power systems based on other forms of alternative-energy techniques (e.g., photovoltaic/high-voltage stack). A 5 kW prototype of the proposed PCS is built, which currently achieves a peak efficiency of 92.4%. We present a detailed description of the operation of the PCS along with its key features and advantages. Finally, experimental results showing the satisfactory performance and the operation of the PCS are demonstrated.

Solid-oxide-fuel-cell performance and durability: resolution of the effects of power-conditioning systems and application loads

Using analog wireless communication, we demonstrate a master-slave load-sharing control of a parallel dc-dc buck converter system, thereby eliminating the need for physical connection to distribute the control signal among the converter modules. The current reference for the slave modules is provided by the master module using radio-frequency (RF) transmission, thereby ensuring even sharing of the load current. The effect of delay due to RF transmission on system stability and performance is analyzed, and regions of operation for a stable as well as satisfactory performance are determined. We experimentally demonstrate a satisfactory performance of the master-slave converter at 20-kHz switching frequency under steady state as well as transient conditions in the presence of a transmission delay. The proposed control concept, which can potentially attain redundancy that is achievable using a droop method, may lead to more robust and reconfigurable control implementation of distributed converters and power systems. It may also be used as a (fault-tolerant) backup for wire-based control of parallel/distributed converters.

A Universal Grid-Connected Fuel-Cell Inverter for Residential Application

We describe methodologies for comprehensive and reduced-order modeling of solid-oxide-fuel-cell (SOFC) power-conditioning system (PCS) at the subsystem/component and system levels to resolve the interactions among SOFC, balance-of-plant subsystem, and power-electronics subsystem (PES) and application loads (ALs). Using these models, we analyze the impacts of electrical-feedback effects (e.g., ripple-current dynamics and load transients) on the performance and reliability of the SOFC. Subsequently, we investigate the effects of harmonics in the current, drawn from the SOFC by a PES, on the temperature and fuel utilization of the SOFC. We explore the impacts of inverter space-vector modulation strategies on the transient response, flow parameters, and current density of the SOFC during load transients and demonstrate how these two traditionally known superior modulation/control methodologies may in fact have a negative effect on the performance and durability of the SOFC unless carefully implemented. Further, we resolve the impacts of the current drawn by the PES from the SOFC, on its microcrack density and electrode/electrolyte degradation. The comprehensive analytical models and interaction-analysis methodologies and the results provided in this paper lead to an improved understanding, and may yield realizations of cost-effective, reliable, and optimal PESs, in particular, and SOFC PCSs, in general.

Multiple Lyapunov Function Based Reaching Condition for Orbital Existence of Switching Power Converters

This paper describes a universal fuel-cell-based grid-connected inverter design with digital-signal-processor-based digital control. The inverter has a direct power conversion mechanism with a high-frequency zero-voltage-switched dc/ac primary-side converter followed by a pair of ac/ac cycloconverters that operates either in parallel or in series to simultaneously address the issues of universal output and high efficiency. The critical design issues focus on the impact and optimization of transformer leakage inductance with regard to effectiveness of zero voltage switching of a primary-side converter, duty-cycle loss, resonance, and voltage spike that has effect on the breakdown voltage rating of the cycloconverter devices. An additional concept of dynamic transformer tapping has been explored to address the impact of varying input voltage on secondary-side voltage spike and inverter efficiency. Finally, detailed grid-parallel and grid-connected results are presented that demonstrate satisfactory inverter performances.

Joint Optimization of Control Performance and Network Resource Utilization in Homogeneous Power Networks

A methodology to analyze the reaching condition of a switching power converter (SPC) using Lyapunovs direct method and a piecewise linear model is outlined. By using a multiple Lyapunov function, the reaching criteria for orbital existence of a SPC is formulated as a linear matrix inequality that is solved using a convex optimization solver. Further, the criterion is modified to distinguish the different modes (i.e., sliding and asymptotic modes and combination of the two fundamental modes) of convergences of the reaching dynamics. The applicability of the reaching criteria for solving practical problems using case illustrations of dc-dc converters and three-phase dc-ac converters operating with different control and modulation techniques is demonstrated. The methodology developed in this paper can be potentially extended to other SPCs and may lead to the development of optimal-sequence control techniques that can dynamically change the mode and the rate of convergence of a SPC.

Reaching Criterion of a Three-Phase Voltage-Source Inverter Operating With Passive and Nonlinear Loads and Its Impact on Global Stability

A framework that jointly optimizes the control and communication networks of network-controlled interactive power electronics networks is described in this paper. The joint optimization framework includes two coupled blocks, one whose focus is to ensure optimal performance of the power network within its stability bounds and the other whose thrust is on optimizing the information flow in a communication network. These two networks have contrasting requirements because, on the one hand, time delays are detrimental to the stability and performance of the control system, while on the other hand, allowing higher time delays leads to efficient utilization of the communication networks resources. The proposed framework leads to an optimal compromise between these two noncooperative networks. Three different implementation approaches for the integrated control-communication framework are investigated, namely, centralized, distributed, and clustered. A case illustration of a homogeneous power network is provided to demonstrate the efficacy of the joint control-communication framework and compare the performance of the three implementation approaches.

Efficient, High-Temperature Bidirectional Dc/Dc Converter for Plug-in-Hybrid Electric Vehicle (PHEV) using SiC Devices

We develop and demonstrate a technique based on composite Lyapunov functions (CLFs) to analyze the impacts of passive (RL and RC) and nonlinear (diode rectifier) loads on the reaching dynamics of a three-phase voltage-source inverter (VSI). The reaching criterion (which ensures convergences of state trajectories to an orbit) is synthesized using piecewise linear models of the VSI and the loads and conditions for switching among the various models (corresponding to the different switching states). Once orbital existence is ensured using the reaching criterion , we extend the CLF-based approach to predict the stability of the nominal (period-1) orbit of the system (comprising the three-phase VSI and the load) and compare these predictions with those obtained using a conventional impedance-criterion technique that is developed based on a linearized averaged model. Overall, we demonstrate the significance of analyzing the reaching condition from the standpoint of orbital existence and why such a criterion is necessary for analyzing global stability. On a broader note, the methodology outlined in this paper is useful for analyzing the global stability of multiphase inverters, potentially leading to advanced control design of VSI for applications including uninterrupted power supplies, telecommunication power supplies, grid-connected inverters, motor drives, and active filters.

Pseudo-decentralized control-communication optimization framework for microgrid: A case illustration

A multiphase all-SiC dc/dc bidirectional converter for plug-in-hybrid electric vehicles (PHEV) is described in this paper that serves as a regulated charger for the intermediate high-voltage energy storage device (e.g. ultracapacitor) in the motoring mode and allows recharging of the batteries during regenerative mode. The primary focus of this paper is to describe the design of the converter and experimentally evaluate its performance (steady-state efficiency and load-sharing and dynamic performance) at high coolant temperatures (¿ 105 °C). Further, the key components of the converter losses at high temperature are identified. Because VJFET switching losses are the key component of the converter loss, the effectiveness of a soft-switching scheme that mitigates these losses is also evaluated.

Investigation of system and component performance and interaction issues for solid-oxide fuel cell based auxiliary power units responding to changes in application load

We describe a joint wireless communication network (WCN) and power network (PN) optimization framework for implementing a pseudo-decentralized control of a distributed generation network. The optimization framework includes two coupled blocks: one whose focus is on ensuring that the PN operates within its stability and performance bounds and the other, whose thrust is on optimization of information flow of a WCN from a dynamic network capacity standpoint. We have outlined a control architecture that can be used to implement the joint optimization scheme in a distributed manner using a global supervisory controller (GSC) and local controllers with some intelligence. We describe mechanisms for sustaining optimal performance in the presence of link disruptions. Extensive simulations are performed to verify the effectiveness of the proposed framework.