Streaks to Rings to Vortex Grids: Generic Patterns in Transient Convective Spin-Up

We observe the transient formation of a ringed pattern state during spin-up of an evaporating fluid on a time scale of order a few Ekman spin-up times. The ringed state is probed using infrared thermometry and particle image velocimetry and it is demonstrated to be a consequence of the transient balance between Coriolis and viscous forces which dominate inertia, each of which are extracted from the measured velocity field. The breakdown of the ringed state is quantified in terms of the antiphasing of these force components which drives a Kelvin-Helmholtz instability and we show that the resulting vortex grid spacing scales with the ring wavelength. This is the fundamental route to quasi-two dimensional turbulent vortex flow and thus may have implications in astrophysics and geophysics wherein rotating convection is ubiquitous. sics.Comment: 4 pages, 5 figure

Nonlinear Control of Single-Phase PWM Rectifiers with Inherent Current-Limiting Capability

In this paper, a nonlinear controller with a currentlimiting property is proposed to guarantee accurate dc output voltage regulation and unity power factor operation for singlephase PWM rectifiers without the need of a phase-locked-loop (PLL). The proposed current-limiting controller is fully independent of the system parameters and can guarantee asymptotic stability and convergence to a unique solution for the closedloop system using nonlinear control theory. Without requiring the instantaneous measurement of the grid voltage, a PLL, an external limiter or a saturation unit, the proposed strategy guarantees that the input current of the rectifier will always remain below a given value. An analytic framework for selecting the controller parameters is also presented to provide a complete controller design procedure and it is also proven that the currentlimiting property is maintained even when the grid voltage drops. Extensive experimental results are presented to verify the proposed controller when the load changes, the reference dc output voltage changes and the grid voltage drops

Structural Resemblance Between Droop Controllers and Phase-Locked Loops

It is well known that droop control is fundamental to the operation of power systems and now the parallel operation of inverters while phase-locked loops (PLL) are widely adopted in modern electrical engineering. In this paper, it is shown at first that droop control and PLLs structurally resemble each other. This bridges the gap between the two communities working on droop control and PLLs. As a result, droop controllers and PLLs can be improved and further developed via adopting the advancements in the other field. This finding is then applied to operate the conventional droop controller for inverters with inductive output impedance to achieve the function of PLLs, without having a dedicated synchronization unit. Extensive experimental results are provided to validate the theoretical analysis

Current-limiting DC/DC power converters

A new nonlinear control framework that guarantees the desired regulation (voltage, current, or power) with an inherent current-limiting capability for different types of dc/dc power converters is presented in this brief. This framework is based on the idea of applying a virtual resistance in series with the inductor of the converter, which changes according to nonlinear dynamics that depend on the control task. Without requiring any knowledge of the converter inductance, capacitance, or the load, the controller structure is appropriately formulated for each power electronic system based on the nonlinear model of the converter. Using input-to-state stability theory, it is proven that the inductor current remains below a maximum value at all times, even during transients, independently of load and input voltage variations. This offers an inherent current-limiting property of the converter under faults, input voltage sags, and unrealistic power demands without the need of external protection mechanisms, saturation units, or current limiters. Extensive simulation and experimental results validate the effectiveness of the proposed control scheme and its current-limiting property, with comparison to traditional control strategies

Current-Limiting Droop Control of Grid-connected Inverters

A current-limiting droop controller is proposed for single-phase grid-connected inverters with an LCL filter that can operate under both normal and faulty grid conditions. The controller introduces bounded nonlinear dynamics and, by using nonlinear input-to-state stability theory, the current-limiting property of the inverter is analytically proven. The proposed controller can be operated in the set mode to accurately send the desired power to the grid or in the droop mode to take part in the grid regulation, while maintaining the inverter current below a given value at all times. Opposed to the existing current-limiting approaches, the current limitation is achieved without external limiters, additional switches or monitoring devices and the controller remains a continuous-time system guaranteeing system stability. Furthermore, this is achieved independently from grid voltage and frequency variations, maintaining the desired control performance under grid faults as well. Extensive experimental results are presented to verify the droop function of the proposed controller and its current-limiting capability under normal and faulty grid conditions