This paper concentrates on the design, control, and implementation of an LCL-filter-based shunt active power filter (SAPF), which can effectively compensate for harmonic currents produced by nonlinear loads in a three-phase three-wire power system. With an LCL filter added at its output, the proposed SAPF offers superior switching harmonic suppression using much reduced passive filtering elements. Its output currents thus have high slew rate for tracking the targeted reference closely. Smaller inductance of the LCL filter also means smaller harmonic voltage drop across the passive output filter, which in turn minimizes the possibility of overmodulation, particularly for cases where high modulation index is desired. These advantages, together with overall system stability, are guaranteed only through proper consideration of critical design and control issues, like the selection of LCL parameters, interactions between resonance damping and harmonic compensation, bandwidth design of the closed-loop system, and active damping implementation with fewer current sensors. These described design concerns, together with their generalized design procedure, are applied to an analytical example, and eventually verified by both simulation and experimental results.

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This paper investigates the inherent damping characteristic of LCL-filters for three-phase grid-connected voltage source inverters (VSIs). Specifically, it is found that there is an inherent damping term embedded in the feedback loop when converter current is used for implementing closedloop control. This additional damping term can indeed neutralize the resonance introduced by LCL-filters, and thus giving rise to a more stable system than that of grid current feedback control. In this case, only one set of current sensors is required to stabilize the system, doing away with passive damping, active damping or complex state observer. Theoretical analysis is then presented and lead to a general design guideline, which suggests a way of choosing the values of grid-and converter-side inductors, so that optimum damping can be naturally achieved by solely using converter current control. In the case when this design criterion is not fulfilled, a simple compensation method is also proposed to tune the damping factor. Both simulation and experimental results are finally provided to validate the theoretical findings developed in this paper.

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A review is presented for the research development of Stirling cycle engines for recovering low and moderate temperature heat. The Stirling cycle engines are categorized into four types, including kinetic, thermoacoustic, free-piston, and liquid piston types. The working characteristics, features, technological details, and performances of the related Stirling cycle engines are summarized. Upon comparing the available experimental results and the technology potentials, the research directions and the possible applications of different Stirling cycle engines are further discussed and identified. It is concluded that kinetic Stirling engines and thermoacoustic engines have the greatest application prospect in low and moderate temperature heat recoveries in terms of output power scale, conversion efficiency, and costs. In particular, kinetic Stirling engines should be oriented toward two directions for practical applications, including providing low-cost solutions for low temperatures, and moderate efficient solutions with moderate costs for medium temperatures. Thermoacoustic engines for low temperature applications are especially attractive due to their low costs, high efficiencies, superior reliabilities, and simplicities over the other mechanical Stirling engines. This work indicates that a cost effective Stirling cycle engine is practical for recovering small-scale distributed low-grade thermal energy from various sources.