The steady state performance of converter type reactive power compensators

As a source or sink of reactive power, compensators can be made from a voltage sourced inverter circuit with the a.c. terminals of the inverter connected to the system through an inductive link and with a capacitor connected across the d.c. terminals. Theoretical calculations on linearised models of the compensators have shown that the parameters characterising the performance are the reduced firing angle and the resonance ratio. The resonance ratio is the ratio of the natural frequency of oscillation of the energy storage components in the circuit to the system frequency. The reduced firing angle of the inverter divided by the damping coefficient, β, where β is half the R to X ratio of the link between the inverter and the system. The theoretical results have been verified by computer simulation and experiment. There is a narrow range of values for the resonance ratio below which there is no appreciable improvement in performance, despite an increase in the cost of the energy storage components, and above which the performance of the equipment is poor with the current being dominated by harmonics. The harmonic performance of the equipment is improved by using multiple inverters and phase shifting transformers to increase the pulse number. The optimum value of the resonance ratio increases pulse number, indicating a reduction in the energy storage components needed at high pulse numbers. The reactive power output from the compensator varies linearly with the reduced firing angle while the losses vary as the square of it.

Stable dual-wavelength oscillation of an erbium-doped fiber ring laser at room temperature

We propose a simple Er-doped fiber laser configuration for achieving stable dual-wavelength oscillation at room temperature, in which a high birefringence fiber Bragg grating was used as the wavelength-selective component. Stable dual-wavelength oscillation at room temperature with a wavelength spacing of 0.23nm and mutually orthogonal polarisation states was achieved by utilising the polarisation hole burning effect. An amplitude variation of less than 0.7dB over 80s period was obtained for both wavelengths.

Analysis and improvement of performance in LCL filter-based PWM rectifier/inverter application using hybrid damping approach

A hybrid passive-active damping solution with improved system stability margin and enhanced dynamic performance is proposed for high power grid interactive converters. In grid connected active rectifier/inverter application, line side LCL filter improves the high frequency attenuation and makes the converter compatible with the stringent grid power quality regulations. Passive damping though offers a simple and reliable solution but it reduces overall converter efficiency. Active damping solutions do not increase the system losses but can guarantee the stable operation up to a certain speed of dynamic response which is limited by the maximum bandwidth of the current controller. This paper examines this limit and introduces a concept of hybrid passive-active damping solution with improved stability margin and high dynamic performance for line side LCL filter based active rectifier/inverter applications. A detailed design, analysis of the hybrid approach and trade-off between system losses and dynamic performance in grid connected applications are reported. Simulation and experimental results from a 10 kVA prototype demonstrate the effectiveness of the proposed solution. An analytical study on system stability and dynamic response with the variations of various controller and passive filter parameters is presented.

Dynamical interplay between ground and excited states in quantum dot laser

Full text: Semiconductor quantum dot lasers are attractive for multipletechnological applications in biophotonics. Simultaneous two-state lasing ofground state (GS) and excited state (ES) electrons and holes in QD lasers ispossible under a certain parameter range. It has already been investigated in steady-stateoperations and in dynamical regimes and is currently a subject of the intesiveresearch. It has been shown that the relaxation frequency in the two-state lasingregime is not a function of the total intensity [1], as could be traditionallyexpected.In this work we study damping relaxation oscillations in QD lasersimultaneously operating at two transitions, and find that under variouspumping conditions, the frequency of oscillations may decrease, increase orstay without change in time as shown in Fig1.The studied QD laser structure wasgrown on a GaAs substrate by molecular-beam epitaxy. The active region includedfive layers of self-assembled InAs QDs separated with a GaAs spacer from a5.3nm thick covering layer of InGaAs and processed into 4mm-wide mesa stripe devices. The 2.5mm long lasers withhigh-and antireflection coatings on the rear and front facets lasesimultaneously at the GS (around 1265nm) and ES (around 1190nm) in the wholerange of pumping. Pulsed electrical pumping obtained from a high power (up to2A current) pulse source was used to achieve high output power operation. We simultaneously detect the total output and merely ES output using aBragg filter transmitting the short-wavelength and reflecting the long-wavelengthradiation. Typical QD does not demonstrate relaxation oscillations frequencybecause of the strong damping [2]. It is confirmed for the low (I<0.68A) andhigh (I>1.2 A) range of the pump currents in our experiments. The situationis different for a short range of the medium currents (0.68Aoscillations appear after the laser turns on andstarts to operate simultaneously. The frequency of oscillations may either significantlydecrease (from 2 GHz to 1 GHz) or sufficiently increase (from 2 GHz to 4 GHz)within 20 ns of the pulse duration. The oscilations appear simultaneously at GSand ES and are always damped, but can be either inphase or antiphase dependingon the pump current and temperature conditions. We address the excitation of the relaxation oscillations to non-instantaneousturn on of the pumping source which activates with 5ns rise time and discussthe swap of the oscillation's frequency in time to spectral shifts caused by thermaleffects. [1] M.Abusaa, J. Danckaert, E. A. Viktorov, and T. Erneux, Phys. Rev. A 87, 063827(2013). [2] T.Erneux, E. A. Viktorov, and P. Mandel, Phys. Rev. A 76,023819 (2007). 1 © 2014 IEEE.

Permanent magnet synchronous generator based wind power generation system fault protection schemes

The PMSG-based wind power generation system protection is presented in this paper. For large-scale systems, a voltagesource converter rectifier is included. Protection circuits for this topology are studied with simulation results for cable permanent fault conditions. These electrical protection methods are all in terms of dumping redundant energy resulting from disrupted path of power delivery. Pitch control of large-scale wind turbines are considered for effectively reducing rotor shaft overspeed. Detailed analysis and calculation of damping power and resistances are presented. Simulation results including fault overcurrent, DC-link overvoltage and wind turbine overspeed are shown to illustrate the system responses under different protection schemes to compare their application and effectiveness.

Strongly localized moving discrete dissipative breather-solitons in Kerr nonlinear media supported by intrinsic gain

We investigate the mobility of nonlinear localized modes in a generalized discrete Ginzburg-Landau-type model, describing a one-dimensional waveguide array in an active Kerr medium with intrinsic, saturable gain and damping. It is shown that exponentially localized, traveling discrete dissipative breather-solitons may exist as stable attractors supported only by intrinsic properties of the medium, i.e., in the absence of any external field or symmetry-breaking perturbations. Through an interplay by the gain and damping effects, the moving soliton may overcome the Peierls-Nabarro barrier, present in the corresponding conservative system, by self-induced time-periodic oscillations of its power (norm) and energy (Hamiltonian), yielding exponential decays to zero with different rates in the forward and backward directions. In certain parameter windows, bistability appears between fast modes with small oscillations and slower, large-oscillation modes. The velocities and the oscillation periods are typically related by lattice commensurability and exhibit period-doubling bifurcations to chaotically "walking" modes under parameter variations. If the model is augmented by intersite Kerr nonlinearity, thereby reducing the Peierls-Nabarro barrier of the conservative system, the existence regime for moving solitons increases considerably, and a richer scenario appears including Hopf bifurcations to incommensurately moving solutions and phase-locking intervals. Stable moving breathers also survive in the presence of weak disorder. © 2014 American Physical Society.