Rajni Kant Burra

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

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

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.

DV/DT related spurious gate turn-on of bidirectional switches in a high-frequency cycloconverter

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.

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 identified a failure mode in a two stage dc/ac converter, comprising a high-frequency dc/ac inverter followed by an ac/ac cycloconverter, both operating at the same switching frequency. The failure-mode is a short-circuit condition, which is a combined effect of the reverse recovery of the MOSFET body diode and simultaneous spurious turn-on of the bidirectional switches of the cycloconverter, owing to a significantly high dv/dt (>2/spl times/10/sup 8/V/ns). A high dv/dt causes appreciable current to flow through the gate-to-drain (Miller) capacitance, thereby producing a significant amount of voltage drop across the external gate resistance. Consequently, the gate-to-source voltage of the power MOSFET may exceed the threshold voltage of the device, which turns the device on. We explain the mechanism for the dv/dt-related gate turn-on and present experimental results to validate the explanation. We also demonstrate, how a two-fold increase in the value of external gate resistance of the inverter switches (to reduce the dv/dt applied to the cycloconverter) reduces the periodicity of the short-circuit condition.

Utility energy storage life degradation estimation method

SOFC stacks respond quickly to changes in load while the balance of plant subsystem (BOPS) responds in times several orders of magnitude higher. This dichotomy diminishes the reliability and performance of SOFC electrodes with increasing load as do current and voltage ripples which result from particular power electronics subsystem (PES) topologies and operation. These ripples and the difference in transient response between the electrical-electrochemical components for the PES and stack subsystem (SS) and those for the chemical-thermal-mechanical components of the BOPS must be approached in a way which makes operation of the entire system not only feasible but ensures that efficiency and power density, fuel utilization, fuel conversion, and system response is optimal at all load conditions. Thus, a need exists for the development of transient component- and system-level models of SOFC-power conditioning systems (i.e. coupled BOPS, SS, and PES) and the development of methodologies for optimizing subsystem responses and for investigating system-interaction issues, which reduce the lifetime and efficiency of a SOFC. A preliminary set of chemical, thermal, electrochemical, electrical, and mechanical models based on the first principles and validated with experimental data were developed and implemented using a number of different platforms. These models were then integrated in such a way as to permit component, subsystem, and system analyses; the development of control strategies; and the synthesis/design and operational optimization of a SOFC based auxiliary power unit (APU) and its components both for steady state and transient operation in transportation and stationary applications. Some pertinent results of these efforts are presented below.

Fuel Cell Power Conditioner for Stationary Power System: Towards Optimal Design From Reliability, Efficiency, and Cost Standpoint

Energy Storage installations have been used in electric utilities for a few decades now. The primary applications of energy storage in utilities include grid stabilization, back-up power and peak shaving, while other potential applications include arbitrage, reduction in renewable variability and frequency regulation. Selection of the appropriate energy storage system for each of the above applications depends on the power/energy ratio requirement, along with the cost, lifetime and other technical challenges. This paper presents a matrix that evaluates the state-of-art energy storage technologies for various utility applications. The characteristics of energy storage systems required for these applications and challenges inherent in the design of such systems are also summarized in this paper. A specific methodology used for analyzing and optimizing the size of the battery energy storage system for frequency regulation is discussed in detail. The same methodology can be extended to the selection of the most optimum technology for other utility applications.

Single-stage low-cost and energy-efficient isolated phase-shifted high-frequency inverter followed by a forced cycloconverter for universal residential fuel cell power system

We describe an energy-efficient, fuel cell (FC) powerconditioning system (PCS) for stationary application, which draws practically zero switching-ripple current from the FC and can potentially meet the

Development of a Comprehensive Simulation Platform to Investigate System Interactions Among Solid-Oxide Fuel Cell, Power-Conditioning Systems, and Application Loads

40/kW cost target. The PCS consists of a zero-ripple boost converter (ZRBC) followed by a soft-switched and multi-level high-frequency (HF) inverter and a single-phase cycloconverter. The zero-ripple input inductor significantly reduces the input current ripple which may be necessary to enhance the long-term durability of the fuel cell. A new phase-shifted sine-wave modulation of the multi-level high frequency inverter is proposed which results in the zero voltage turn-on (ZVS) of all four switches (without the use of any auxiliary circuit components). For such a sine-wave modulation technique a > 90 % ZVS range is obtained from 25% of the full load to full load. Further, the line-frequency switching of the cycloconverter (at close to unity power factor) results in extremely low switching losses. The intermediate high voltage DC (HVDC) bus facilitates the inclusion of power systems based on other forms of alternative-energy techniques. A cost effective 1 kW prototype of the proposed PCS is built, which achieved a high overall efficiency. We present a detailed description of the operation of the PCS along with its key features and advantages. Finally, experimental results showing the performance and operation of the PCS are demonstrated.Copyright

GE Brilliant wind farms

We describe a single-stage low-cost and efficient fuel-cell (FC) power-electronics system (PES), which comprises a front end phase-shifted high-frequency (HF) full-bridge inverter followed by a step-up high-frequency transformer and a forced cycloconverter for universal residential FC power system (i.e., 110 V/240 V and 60/50 Hz). This single-stage dc/ac converter approach meets the higher energy efficiency requirement without exceeding the U.S. Department of Energy (DOE) cost constraint of

Bearing Wear Due to Mechanical Stresses and Electrical Currents

40/kW. The described topology achieves a direct power conversion and does not require any intermediate energy storage components. The FC PES increases the reliability by precluding the need for front-end dc/dc converter and also provides a low-cost viable alternative to high power density.