Circulating-current states and ring-exchange interactions in cuprates

We consider the consequences for circulating-current states of a cyclic, four-spin, ``ring-exchange'' interaction of the type shown recently to be significant in cuprate systems. The real-space Hartree-Fock approach is used to establish the existence of charge-current and spin-current phases in a generalized Hubbard model for the CuO_2 planes in cuprates. We compare the results of the Hartree-Fock approximation with the correlated states renormalized by Gutzwiller projection factors which allows us to gauge the qualitative effects of projection to no double site occupancy. We find that charge flux states may be competitive in cuprates, whereas spin flux states are suppressed in the strongly correlated regime. We then include the ring-exchange interaction and demonstrate its effect on current-carrying states both at and away from half-filling.Comment: 14 pages, 11 figure

Observable NMR signal from circulating current order in YBCO

Assuming, as suggested by recent neutron scattering experiments, that a broken symmetry state with orbital current order occurs in the pseudo-gap phase of the cuprate superconductors, we show that there must be associated equilibrium magnetic fields at various atomic sites in the unit cell, which should be detectable by NMR experiments

Reduction of the Circulating Current among Parallel NPC Inverters

In medium/high power applications including smart transformers, active power filters and wind turbines, 3-level Neutral-Point-Clamped (NPC) inverters proved to be a reliable solution, providing high efficiency and low harmonic distortion. In practice, several NPCs are parallel connected and operated in interleaved to further increase the power handling and reduce the line filters size. However, if such configuration has a common dc-link, High-frequency Zero-Sequence Circulating-Current (HF-ZSCC) arises among the inverters, increasing power losses of the switching devices and propagating the stress on the dc-capacitors. Moreover, the amplitude of the HF-ZSCC is inversely proportional to the filter inductance size, therefore in real applications it can reach hundreds of Amperes even with relatively low output currents. The research on the HF-ZSCC is mostly concentrated on 2-level inverters for low voltage grids and traction applications, where the inductance size is relatively big and the HF-ZSCC does not affect the system efficiency. Differently, NPCs provide higher switching degree of freedom and more sophisticated methods can be applied to reduce the HF-ZSCC. This paper investigates a Double-Reference Pulse-Width Modulation (DRPWM) as solution for diminishing the HF-ZSCC in paralleled NPCs. The performance of DRPWM method is confirmed by both simulation and experiments, performed on a 1.6MVA system

Carrier shifting algorithms for the mitigation of circulating current in diode clamped MLI fed induction motor drive

Abstract: Reduction of circulating current is one of the major considerations in inverter fed electrical drives. Diode clamped MLI enables higher output current per phase, thereby rating of the drive gets increased effectively. Various methods of triggering in the inverter legs creates better voltage profile and leads to the enabling of circulating current in the drive system. The induced circulating current flows through the apparatus neutral (N) and supply ground (G) is caused by the existence of parasitic capacitance. This circulating current may cause potential danger especially when parasitic capacitance poses large. In the past, different modulation techniques and conversion topologies have been introduced to minimize the flow of circulating current. However, these techniques lead to complexity, high cost, low voltage profile and efficiency due to lower modulation parameters. This paper proposes PS, POD, PD carrier shifting PWM algorithms for diode clamped MLI to tumbling the circulating current within the each phase of inverter legs. The performances of proposed algorithm, in terms of circulating current, THD, losses and efficiencies are analyzed theoreticallyand are validated via simulation and experimental results

Persistent activation of transducin by bleached rhodopsin in salamander rods.

The hydrolysis-resistant GTP analogue GTP-gamma-S was introduced into rods isolated from the retina of the salamander Ambystoma tigrinum to study the origin of the persistent excitation induced by intense bleaching illumination. Dialysis of a dark-adapted rod with a whole-cell patch pipette containing 2 mM GTP-gamma-S resulted in a gradual decrease in circulating current. If the rod was first bleached and its sensitivity allowed to stabilize for at least 30 min, then dialysis with GTP-gamma-S produced a much faster current decay. The circulating current could be restored by superfusion with the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine, suggesting that the decay in current originated from persistent excitation of the phosphodiesterase by transducin bound to GTP-gamma-S. We conclude that the persistent excitation which follows bleaching is likely to involve the GTP-binding protein transducin, which mediates the normal photoresponse. This observation suggests that a form of rhodopsin which persists long after bleaching can activate transducin much as does photoisomerized rhodopsin, although with considerably lower gain

A Novel Circulating Current Suppression for Paralleled Current Source Converter Based on Virtual Impedance Concept

The circulating current is one of the important issues for parallel converters. It affects the system stable operation and degrades the power quality. In order to reduce the circulating current of the parallel converter and reduce the harmonic pollution to the power grid, a new circulating current suppression strategy is proposed for the parallel current source converter without any communication line. This strategy is able to realize the current sharing between parallel modules by changing the external characteristics of the parallel modules to thus suppress the circulating current among the parallel current source converters. The proposed control strategy adopts DC-side droop control and AC-side virtual impedance control. The DC-side droop control is used to generate the reference voltage of each parallel module, while the AC-side virtual impedance is used to the circulating current suppression. We performed a time domain test of the parallel converter, and the results show that the proposed control strategy reduced the RMS circulating current of the parallel converter by 50% and effectively reduced the grid-side current THD while ensuring the stable operation of the converter. The effectiveness of the proposed control strategy was, therefore, verified

Analysis and Suppression of Zero-Sequence Circulating Current in Multi-Parallel Converters

The use of a multi-parallel converter system has many advantages in increased scalability, better maintenance, scheduling, and improved output current quality. However, a periodic zero-sequence circulating current (ZSCC) may occur due to the asymmetry of parallel-connected converters. ZSCC produces additional losses and possible instability of the system. Therefore, proper control must be applied to suppress this harmful ZSCC. In order to design an effective controller for suppressing ZSCC, it is necessary to analyze the cause of the circulating current generation. However, most of the existing studies have applied the controller without detailed analysis. Therefore, this paper mathematically analyzes the ZSCC spectrum using the Fourier series to identify which harmonics are included in ZSCC. From the analysis results, the necessity of multi-resonant controllers to suppress the ZSCC at specific harmonics is demonstrated. Simulation and experiments are conducted to validate the analysis results and the necessity of multi-resonant controllers

Improvement of Single-Switch Bridgeless PFC Cuk Converter for Circulating Current Elimination and Components Maximum Current Stress Reduction

This paper presents the improvement of a single-switch bridgeless PFC Cuk converter for circulating current elimination and maximum current stress reduction on components. Circulating current is eliminated by rearranging the position of input diodes thus the input diodes can block the returning path of current through the input inductors. The principle of circulating current elimination is also discussed in detail in this paper. A 100 W converter with an output voltage of -48 V has been tested to verify the principle. The results of the experimental hardware show the removal of the circulating current, the maximum current stress in input diodes is reduced from 8 A to 2.8 A and the maximum current stress in input capacitors also reduced from 11 A to 9.8 A

Control of atomic currents using a quantum stirring device

We propose a BEC stirring device which can be regarded as the incorporation of a quantum pump into a closed circuit: it produces a DC circulating current in response to a cyclic adiabatic change of two control parameters of an optical trap. We demonstrate the feasibility of this concept and point out that such device can be utilized in order to probe the interatomic interactions.Comment: 5 pages, 4 figures, uses epl2.cls, revised versio