The Stability Analysis of a Multi-Port Single-Phase Solid-State Transformer in the Electromagnetic Timescale

This paper proposes an overall practical stability assessment for a multi-port single-phase solid-state transformer (MS3T) in the electromagnetic timescale. When multiple stable subsystems are combined into one MS3T, the newly formed MS3T has a certain possibility to be unstable. Thus, this paper discusses the stability assessment of the MS3T in detail. First and foremost, the structure of the MS3T and its three stage control strategies are proposed. Furthermore, the stability analysis of each of the MS3T’s subsystems is achieved through the closed loop transfer function of each subsystem, respectively, including an AC-DC front-end side converter, dual active bridge (DAB) with a high-frequency (HF) or medium-frequency (MF) transformer, and back-end side incorporating DC-AC and dc-dc converters. Furthermore, the practical impedance stability criterion in the electromagnetic timescale, which only requires two current sensors and one external high-bandwidth small-signal sinusoidal perturbation current source, is proposed by the Gershgorin theorem and Kirchhoff laws. Finally, the overall stability assessment, based on a modified impedance criterion for the MS3T is investigated. The overall practical stability assessment of the MS3T can be validated through extensive simulation and hardware results.

REVERSE RECOVERY OF POWER pn JUNCTION DIODES

A quasi-steady state approximation is used to derive expressions for the waveforms of minority carrier charge stored in p+nn+ power junction diodes driven by large-signal sinusoidal and ramp voltages. These waveforms are derived by solving a diode charge-control differential equation. Using the charge waveforms, the storage time is determined and the diode current and voltage waveforms are predicted. It is shown that three frequency ranges can be distinguished: (1) low-frequency range in which the reverse recovery is negligible, (2) mid-frequency range in which the reverse recovery is detrimental, but the diode is still of practical value, and (3) high-frequency range where the diode does not exhibit its rectifying properties. A simple method for measuring the minority carrier lifetime is proposed. Experimental results are given for an MR826 fast-recovery pn junction diode and a 31DQ06 Schottky diode for operating frequencies of up to 10 MHz. The theoretical and experimental results were in good agreement.

Effects of Temperature, End-Conditions, Flow, and Branching on the Frequency Response of Pneumatic Lines

Experimental results are presented to show the effects of temperature, flow, end-conditions, and branching on the small-signal sinusoidal amplitude frequency response of pneumatic lines. Distributed parameter transmission line theory for uniform lines is extended to include varying conditions of pressure and temperature along the line as well as series-connected lines of different diameters and parallel branching. The experimental results are compared with the formulated theory for several test configurations. Agreement between experiment and computer solutions based on the theory is relatively good.