Low-frequency Oscillation Phenomenon Research Articles

Pulse Train Controlled Single-Input Dual-Output Buck Converter With Coupled Inductors

The pulse train (PT) controlled switching dc/dc converters possess the benefits of simple structure, fast dynamic response, and good robustness. However, PT controlled converters also give the drawbacks of undesirable low-frequency oscillation phenomenon as operated in continuous conduction mode. In this paper, a single-input dual-output buck converter based on the coupled inductor is newly proposed with the ability to suppress the low-frequency oscillation. The inductor of the conventional buck converter is replaced by the primary side of the coupled inductor, and the secondary side of the coupled inductor connected in series with a diode is paralleled to a capacitor as an equivalent power source for the extra load. By rational allocation of inductance coupling coefficient and the equivalent power supply voltage, low-frequency oscillation of output voltage can be suppressed and the fast response speed is achieved. A hardware prototype of 48 V/16 V converter is built and tested to validate the practicability of the proposed converter.

Pulse train controlled buck converter with coupled inductors

Pulse train (PT) controlled switching DC/DC converters possess advantages of simple structure, fast dynamic response, good robustness etc. but also accompanied by undesirable low-frequency oscillation phenomenon when the converters are operated in continuous conduction mode (CCM). To mitigate the low-frequency oscillations of PT controlled buck converters, a novel buck converter with coupled inductor is proposed in this paper. Firstly, the mechanism of the low-frequency oscillation is analysed, the corresponding oscillation suppression method is discussed and the discrete-time model of the proposed converter is constructed. Furthermore, the bifurcation and time-domain diagrams of the output voltage and the switching peak current in terms of coupling coefficient and load variation are reported, and the influence from the coupling coefficient and load on low-frequency oscillation behaviours is revealed. The experimental results verify that the output voltage of the PT controlled proposed buck converter can be timely adjusted based on proper configuration of the coupling coefficient, and hence the low-frequency oscillation can be eliminated and the maximum switching peak current can be decreased.

High-fidelity numerical simulation of the flow field around a NACA-0012 aerofoil from the laminar separation bubble to a full stall

ABSTRACTIn the present work, large eddy simulations of the flow field around a NACA-0012 aerofoil near stall conditions are performed at a Reynolds number of 5 × 104, Mach number of 0.4, and at various angles of attack. The results show the following: at relatively low angles of attack, the bubble is present and intact; at moderate angles of attack, the laminar separation bubble bursts and generates a global low-frequency flow oscillation; and at relatively high angles of attack, the laminar separation bubble becomes an open bubble that leads the aerofoil into a full stall. Time histories of the aerodynamic coefficients showed that the low-frequency oscillation phenomenon and its associated physics are indeed captured in the simulations. The aerodynamic coefficients compared to previous and recent experimental data with acceptable accuracy. Spectral analysis identified a dominant low-frequency mode featuring the periodic separation and reattachment of the flow field. At angles of attack α ≤ 9.3°, the low-frequency mode featured bubble shedding rather than bubble bursting and reformation. The underlying mechanism behind the quasi-periodic self-sustained low-frequency flow oscillation is discussed in detail.

Distributed Electromechanical Oscillation Mode Online Estimation

For the existence of low frequency oscillation phenomena in the interconnection power systems, a new style of distributed algorithm for detecting the low frequency oscillation modes was proposed to avoid the disadvantages of previous methods that require large amount of concentrated computation and communication. The whole structure of oscillation detection contains two levels, namely the substation level and control center level. In the substation level, damped sinusoid model based on atomic decomposition is employed to represent atomic library. Using the power angle trajectory, the modal parameters were identified from the atomic library by atomic decomposition method and atomic decomposition energy entropy was calculated during the identification process. In the control center level, the oscillation detection result of the substations are uploaded and combined to generate overall oscillation frequency and damping ratio by a weighted linear function. The results of a simulation case show that this algorithm can quickly detect the oscillation mode and the simulation results are identical to that of the normal form theory, and the accuracy and effectiveness of the proposed method were validated.

Numerical Investigation of the Low-Frequency Flow Oscillation over NACA-0012 Aerofoil

The present study investigates the low-frequency flow oscillation phenomenon near stall conditions for NACA0012 aerofoil at Reynolds number of 9 × 104, Mach number of and angle of attack of 11.0°. The phenomenon is clearly captured in the time histories of aerodynamic coefficients. Statistical and spectral analysis are carried out for time histories of aerodynamic coefficients. Qualitative study is performed to investigate the unsteady behaviour of laminar separation bubble over one low-frequency cycle.

Stability Research of High-Speed Railway EMUs and Traction Network Cascade System Considering Impedance Matching

Due to the low-frequency oscillation (LFO) between traction network and electric multiple units (EMUs) of high-speed railway, many traction blockades occur in different EMU depots. Aiming at this problem, the mathematical model and small-signal analysis model of EMUs-traction network cascade system (ETNCS) are built in this paper. The system stability mainly affected by the electrical coupling between source and load module is analyzed and discussed in detail. Considering the autotransformer leakage reactance, traction network impedance, and distributed capacitance, the output source module resistance of ETNCS is calculated based on the generalized symmetrical component method. Using the generalized Nyquist criterion and Bode diagram, the ETNCS's stability is analyzed and the quantitative indicators of system impedance matching are given. Then, the simulation model of full ETNCS is built in this paper, which can reproduce the LFO phenomenon. In addition, the theoretical analysis results and the feasibility of inhibition for LFO are verified based on MATLAB/simulink simulation. Finally, the LFO signal modal analysis is implemented by using Prony and estimating signal parameters via rotational invariance techniques (ESPRIT) methods. The conclusions show that the proposed stability and modal analysis methods could be used not only to analyze the LFO more clearly, but also to provide the technical reference for the stable operation of EMUs.

Discrete iterative-map modeling and dynamical analysis of digital voltage-mode controlled buck converter with dual-edge modulation

The operation principle of digital voltage-mode controlled buck converter with dual-edge modulation is analyzed in this paper. Based on the state equation of buck converter and six possible evolutions in one switching cycle, the discrete iterative-map model of digital voltage-mode controlled buck converter with dual-edge modulation is established. Ignoring the quantization error of analog-digital converter and on the basis of its discrete iterative-map model, the nonlinear dynamical behavior of digital voltage-mode controlled buck converter with dual-edge modulation is investigated in detail. Taking the input voltage and the load resistance as bifurcation parameters, the output voltage bifurcation diagram and the inductor current bifurcation diagram are plotted. Through analyzing the bifurcation diagrams, it is indicated that there are two kinds of similar but different Hopf bifurcation phenomena. By use of Poincar section, time-domain simulation waveforms and phase portraits, two different Hopf bifurcations and low-frequency oscillation phenomena are compared and studied. Observing the inductor current and capacitor voltage waveforms respectively, it is obviously found that their oscillation frequencies and amplitudes are different, the shapes of two Poincar$ sections and phase portraits are also different. In order to verify the correctness of the simulation and theoretical analysis, the eigenvalues of Jacobian matrix of the discrete iterative map model are introduced and solved in two kinds of stable evolutions. Through analyzing variation of eigenvalues of Jacobi matrix with input voltage, the existence and difference of two kinds of Hopf bifurcation phenomena are proved theoretically. Moreover, it is observed in this paper that the odd period-doubling bifurcation phenomenon exists in digital voltage-mode controlled buck converter with dual-edge modulation for the first time, where the operation state of the buck converter turns from period-one into period-three. Its authenticity is verified by using the time-domain simulation waveforms and phase portraits. In order to approach to the actual circuit, the equivalent series resistances of capacitor and inductor are considered. The actual circuit is simulated by using the software Psim. A comparison shows that there are little differences between the theoretical simulation and the actual circuit simulation. So the theoretical simulation can be used to analyze the performances of the actual circuit. The research results in this paper have guiding significance and practical value for designing the digital voltage-mode controlled buck converter with dual-edge modulation.

Capacitor current feedback pulse train control technique for switching DC–DC converters

A capacitor current feedback pulse train (CCF-PT) control technique for switching DC–DC converters is proposed and applied to the control of a buck converter operating in continuous conduction mode (CCM). The analysis and experimental results show that the CCF-PT control technique can effectively eliminate the low-frequency oscillation phenomenon that exists in conventional pulse train (PT)-controlled CCM switching DC–DC converters.

Simulation of Grid Low Frequency Oscillation Based on MATLAB

This paper describes the generation of low frequency oscillations and suppression methods briefly. Observing a phenomenon of low frequency oscillation of Gansu Grid Power and analysis its actual situation. Therefore, we need establish simplified simulation model in MATLAB environment, then do the simulation with the dual power system simulation model which add to power system stabilizer. Simulation results show that: PSS can solve the problem of low frequency oscillations in a region of Gansu steel caused rapid and effective.

Valley current mode pulse train control technique for switching DC–DC converters

To eliminate the low-frequency oscillation phenomenon in pulse train controlled switching DC–DC converters operating in continuous conduction mode (CCM), a valley current mode pulse train control technique is proposed. The experimental results show that by using the proposed control technique, the low-frequency oscillation in the pulse train (PT) controlled CCM switching DC–DC converter can be eliminated, while the great robustness and excellent transient performance of the traditional PT control technique can be retained.

Valley current mode pulse train control switching converter and its energy model analysis

Pulse train (PT) control technique is a novel discrete control technique for switching converter operating in discontinuous conduction mode (DCM). When the inductive energy storage is not zero, the low-frequency oscillation phenomenon may occur in PT controlled switching converters operating in continuous conduction mode (CCM). The low-frequency oscillation phenomenon will seriously affect the steady and transient performances of switching converters. In order to solve this problem, valley current mode pulse train (VCM-PT) control technique, which extends the application range from DCM to CCM, is proposed in this paper. The energy model of VCM-PT controlled switching converter is derived and compared with the energy model of PT controlled switching converter. Result indicates that the VCM-PT controlled CCM switching converter has the same energy transfer mode as the traditional PT controlled DCM switching converter and can eliminate fundamentally the low-frequency oscillation phenomenon.

Multi-period analysis of current-mode pulse-train controlled continuous conduction mode converter

Pulse train (PT) control technique is a discrete, nonlinear control technique for switching converter which benefits from simple design and ultra fast transient response. As an output-power-control-based control technique, a low-frequency oscillation phenomenon occurs in voltage-mode PT controlled switching converter operating in continuous conduction mode. This phenomenon will seriously affect the steady and transient performances of switching converter. In this paper, a current-mode PT (CM-PT) controlled switching converter is studied. The normally working region is studied and the parameter conditions are estimated. Targeting the problem of CM-PT control technique, a modified control technique is proposed.

Low-frequency oscillation of continuous conduction mode buck converter with pulse skipped modulation

Low-frequency oscillation phenomenon in pulse skipped modulation (PSM) buck converter operating in continuous conduction mode (CCM) is reported in this paper. Energy transfer difference between the PSM-controlled buck converter operating in discontinuous conduction mode (DCM) and that in CCM is studied. The mechanism of low-frequency phenomenon in PSM-controlled CCM buck converter is revealed, and a discrete-time model of PSM-controlled buck converter operating in CCM is established. Based on this model, the border collision bifurcation with variations of the ESR is studied; meanwhile, the inhibition effect of output capacitor equivalent series resistance (ESR) on low-frequency oscillation is presented. Based on the above analysis, a capacitor current pulse skipped modulation (CC-PSM) technology is proposed; the energy transfer principle of CC-PSM controlled buck converter is analyzed. The control pulse of CC-PSM is generated by the comparison between the sampled capacitor current and preset peak reference current, and thus the range of inductor current ripple is limited. Results show that CC-PSM effectively eliminates the low-frequency oscillation in CCM buck converter under classical PSM when the ESR is low. Finally, the experimental results validate the correctness of theory and simulation analysis

Pulse-Train-Controlled CCM Buck Converter With Small ESR Output-Capacitor

The pulse-train (PT) control technique for switching dc-dc converters is simple to design and benefits from excellent control performance. Up to now, almost all of the works on PT control are focused on switching dc-dc converters operating in discontinuous conduction mode, with few works reported on PT control of switching dc-dc converters operating in continuous conduction mode (CCM). In this paper, a PT-controlled CCM buck converter is studied and a unique low-frequency oscillation phenomenon is revealed, which results in large undesired inductor-current and output-voltage variations. The effect of equivalent series resistance (ESR) output-capacitor on the low-frequency oscillation is examined. The results indicate that such oscillation occurs when the ESR output-capacitor is relatively small and disappears when it is relatively large. However, a larger ESR will result in a large output-voltage ripple. In order to avoid the low-frequency oscillation and, at the same time, to ensure a small output-voltage ripple, the ICRIF circuit is applied. In this way, a small ESR output-capacitor can be used to decrease the output-voltage ripple. Simulation and experimental results are provided to verify the theoretical analysis.

Large-Eddy Simulation of Natural Low-Frequency Oscillations of Separating–Reattaching Flow Near Stall Conditions

The present work investigates the natural low-frequency flow oscillation phenomenon using large-eddy simulation. Four simulations were carried out for flow around a NACA 0012 airfoil with Reynolds number of 50,000 and Mach number at angles of attack of 9.25, 9.29, 9.4, and 9.6 deg. The results clearly indicate that natural low-frequency flow oscillation is taking place. The oscillations are self-sustained and caused by periodic bubble forming and bubble bursting on the suction side. The low-frequency oscillation phenomenon was found to exist over a range of angles of attack near stall. Computed maximum reverse flow inside the bubble indicates an absolute instability mechanism. The time- and span-averaged flowfield as well as the instantaneous turbulent flowfield illustrated the dynamics of low-frequency oscillation near stall. The location of transition is shown to move downstream during bubble bursting. Spectral analysis of the surrounding acoustic field reveals that the observed phenomenon affects the circulation around the entire airfoil. Analysis of the flowfield led to the proposition of a mechanism for low-frequency flow oscillations near stall in which the separated shear layers from the leading edge and the trailing edge play a major role in driving the separated flow to reattachment.

The Impact of Hydro-Turbine and its Governor System on Power System Low Frequency Oscillation

This paper studies the effect of the soft feedback in hydro-turbine and its governor system on power system transient stability. Low frequency oscillation phenomenon in which the hydraulic turbine sets participate occurred on power system many times this year, which with no mechanism discovered. In this paper, we first study the effect of mechanical load moment output of the prime mover system on the mechanism of low frequency oscillations, then by analysis of small-signal stability on hydro-turbine and its governor system and simulation experiments on an two-generator power system using PSASP, we come to the conclusion that: If the soft feedback output values of hydro-turbine governor systems are set small in the power system, it will come to the unstable oscillation condition.

The Monitor of Inter-area Oscillation Based on Wide Area Measurement System

With the expansion of interconnected power grid, the phenomenon of low frequency oscillation appears unavoidably. In this paper, the existing oscillation mode identifying methods are analyzed from the viewpoint of calculation speed, criterion, calculation accuracy, etc. The applicability of two methods, Prony and ESPRIT are compared mainly. Based on the above research, we put forward a new idea of monitoring power system low frequency oscillation based on WAMS.

Identification the Low-Frequency Oscillation Stakeout of Power System by Wide-Area Measurement System

With the rapid development of interconnected power grid, the phenomenon of power system low-frequency oscillation appears unavoidably. Then, the appearance of wide-area measurement system provided a supporting technology for better scout low-frequency oscillation system and better recognize the oscillation mode . In this paper, the existing oscillation mode identifying methods are analyzed from the viewpoint of calculation speed, criterion, calculation accuracy, etc. Mainly compared the applicability of two methods, Prony and ESPRIT . Based on the above research, a new idea of monitoring power system low frequency oscillation based on WAMS is put forward in this paper.

Analysis of low-frequency oscillation phenomenon in the pulsetrain controlled buck converter

In this paper, the low-frequency oscillation phenomenon in a pulse train (PT) controlled buck converter operating in continuous conduction mode (CCM) is found and studied. The mechanism of this low-frequency oscillation phenomenon and the way to suppress it are studied. The time-domain simulation results and their corresponding phase portraits indicate that the equivalent series resistance (ESR) of the output filter capacitor has a critical effect on this low-frequency oscillation phenomenon. When the ESR is zero or small enough, such a low-frequency oscillation phenomenon appears in the PT controlled CCM buck converter because the output voltage can not be regulated timely. With the increase of ESR, the PT controlled CCM buck converter operates chaotically. The output voltage can be regulated timely and such a low-frequency oscillation phenomenon can be suppressed effectively. Experimental results are provided to verify the theoretical analyses and the simulations.

Study of a low-frequency oscillation phenomena associated with a diffusion process via a hollow-fiber membrane

Experimental data related to low-frequency oscillations of substance concentration are under consideration. The phenomenon has been mathematically modeled for stationary external conditions. The model developed here considers the dependence of local membrane conductivity on solution concentration. The calculation results fit the experimental data well. A theoretical explanation of the phenomenon is presented. It has been demonstrated that a positive feedback between changes in flows and concentrations in a membrane occurs in the process that determines the appearance of oscillations in the process of diffusion of the substance.