Low-frequency Oscillation Phenomenon Research Articles

Experimental study on the combustion and pressure drop characteristics of a pintle injector for LOX/kerosene rocket engine

Pintle injectors have attracted much attention in recent years for their excellent throttling and combustion performance. However, the combustion characteristics of cryogenic propellant on pintle injectors with varying operating conditions remain unclear. A LOX/kerosene throttleable rocket engine with a fuel-centered liquid-liquid pintle injector was designed and its pressure drop and combustion characteristics were tested. The pintle engine achieved stable combustion with the total flow rate and oxygen-fuel mixture ratio varying from 59 to 559 g/s and from 0.73 to 1.02. A calculation model for the gas ratio in the LOX channel of the pintle injector was established. The deviation of measurements from the calculated values did not exceed ±10%. The vaporization of LOX from the pintle injector in the hot test was calculated and the positive correlation between gas ratio and combustion efficiency was analyzed. For the phenomenon of low-frequency oscillation of pressure in the hot tests, the source and development process of the oscillation were studied by FFT. The primary frequency and amplitude of low-frequency oscillation had been discovered to exhibit a positive correlation with the gas ratio. Low pressure drop of injection was the main cause of low-frequency oscillation. The strong vaporization of LOX under low operating conditions made the combustion efficiency remain in a high range even if low-frequency oscillation was observed. This work can provide a reference for the design of pintle injectors for cryogenic liquid rocket engines that can be deeply throttled.

Rotating gliding arc discharge induced flame oscillation near the lean blowout limit

This work investigates the rotating gliding arc (RGA) assisted swirling methane combustion characteristics near the lean blowout limit by utilizing the high-speed videography. As the flow rate and the global equivalence ratio were varied, four flame modes, namely self-sustained flame, RGA-sustained stable flame, RGA-induced oscillating flame, and RGA anchored flame modes were detected. In particular, the oscillating flame mode with an oscillating frequency of 2∼4 Hz occurs with the air flow rates above 13 L/min and the equivalence ratio between 0.52 and 0.56. It is an RGA-induced low-frequency flame oscillation phenomenon. The RGA discharge characteristics can impact the oscillating behaviors that with the increase of the transformer input voltage, the flame oscillation frequency is enhanced and the averaged flame area becomes larger to result in a higher combustion efficiency. Besides, increasing the wall temperature of the combustion chamber promotes the combustion during the oscillating stage. Based upon the experimental and numerical results this RGA-induced low-frequency oscillating flame are further interpreted by a three-staged mechanism. It implies that the first stage, during which the fuel/oxidant mixture is refreshed in the chamber and the temperature increases through recirculating flow gradually, determines the oscillating frequency. The fast accumulation of released energy from plasma discharge and partial fuel burning by recirculation can accelerate the temperature growth in the recirculation zone to increase the oscillation frequency. In all, this unique oscillating phenomenon and related explanations indeed broaden our knowledge of oscillating combustion behaviors and GA assisted combustion.

Analysis of the Influence of Grid-Connected Photovoltaic Power Stations with Virtual Inertia on Low-Frequency Oscillation of Power System Based on Small Signal and Prony Analysis Methods

With the virtual synchronous generator (VSG) technology gradually becoming an emerging technology for new energy consumption, its introduction has solved the problems of weak support and weak anti-interference of traditional new energy stations. However, the practice shows that the grid-connected characteristics of VSG limit its large-scale utilization, and there is little research on the interaction between VSG and a multi-machine system. In this paper, small signal models and time domain simulation models of each link of a photovoltaic(PV) power station with the PV virtual synchronous generator (PV-VSG) are first conducted, and then the influence of the grid-connected PV power station with the PV-VSG on the low-frequency oscillation of power system and its interaction mechanism with the multi-machine system are qualitatively obtained based on the small signal analysis method and prony analysis method from two dimensions of frequency domain and time domain, respectively. The analysis and simulation show that the PV power station based on VSG technology affects the dynamic characteristics of the system by changing the electromagnetic torque of each synchronous generator. Unreasonable operating conditions and parameter settings will aggravate the phenomenon of low-frequency oscillation of the system. Before the grid connection of PV power station, the operating parameters shall be reasonably set to ensure their safe and stable grid connection.

An Improved Q-Axis Current Control to Mitigate the Low-Frequency Oscillation in a Single-Phase Grid-Connected Converter System

An electric railway system is a typical single-phase grid-connected converter system, and the low-frequency oscillation (LFO) phenomenon in electric railway systems has been widely reported around the world. Previous research has indicated that the LFO is a small-signal instability issue caused by impedance mismatching between the traction network system and electric trains. Therefore, this paper proposes an improved q-axis current control method to reshape the train’s impedance. The proposed method can be implemented easily by relating a reverse q-axis reactive current directly to the reference of the q-axis current under the dq current decoupled control. Moreover, considering the additional q-axis reactive current control, a small-signal impedance model of a train–network system is built. The impedance-based analysis results indicate that the proposed q-axis reactive current feedback control can increase the magnitude of the train’s impedance, which is beneficial to enhancing the system’s stability. Finally, this paper employs experimental results to verify the effectiveness of the proposed method.

Low-Frequency Oscillation Analysis of Grid-Connected VSG System Considering Multi-Parameter Coupling

With the increasing integration of new energy generation into the power system and the massive withdrawal of traditional fossil fuel generation, the power system is faced with a large number of stability problems. The phenomenon of low-frequency oscillation caused by lack of damping and moment of inertia is worth studying. In recent years, virtual synchronous generator (VSG) technique has been developed rapidly because it can provide considerable damping and moment of inertia. While improving the stability of the system, it also inevitably causes the problem of active power oscillation, especially the low mutual damping between the VSG and the power grid will make the oscillation more severe. The traditional time-domain state-space method cannot reflect the interaction among state variables and study the interaction between different nodes and branches of the power grid. In this paper, a frequency-domain method for analyzing low-frequency oscillations considering VSG parameter coupling is proposed. First, based on the rotor motion equation of the synchronous generator (SG), a second-order VSG model and linearized power-frequency control loop model are established. Then, the differences and connections between the coupling of key VSG parameters and low-frequency oscillation characteristics are studied through frequency domain analysis. The path and influence mechanism of a VSG during low-frequency power grid oscillations are illustrated. Finally, the correctness of the theoretical analysis model is verified by simulation.

The Mechanism Research of Low-Frequency Pressure Oscillation in the Feeding Pipe of Cryogenic Rocket Propulsion System

In the propulsion system of cryogenic liquid rockets, low-frequency pressure oscillation is a long-standing issue occurring in its feeding pipe, and is not conducive to the normal operation of the rocket. Its mechanism and excitation process are not very clear due to the limitation of the existing numerical method and the difficulty of the real dynamic experiment. Inspired by the periodic suck-back flow phenomenon of steam condensation, the fluctuation of the two-phase interface might be the crucial factor to initiate the low-frequency pressure oscillation. To simulate this interfacial characteristic of cryogenic propellant, a novel numerical model is proposed to predict the mass transfer rate weighted by the interfacial curvature. Aiming at the oxygen jet condensation simulation, the low-frequency pressure oscillation phenomenon is obtained successfully with the excitation frequency of 10.6 Hz, consistent with the natural frequency of the engine test run. It is conducted so the low-frequency pressure oscillation is caused by the periodic condensation of the continuous oxygen vapour plume, along with an oxygen suck-back flow phenomenon. In addition, the results indicate that both the oxygen and liquid oxygen mass flux promote the rise in the frequency of pressure oscillation. These conclusions provide theoretical instructions for the design and operation of the propulsion system of a cryogenic liquid rocket.

Power-Hardware-in-the-Loop Emulation of the Low-Frequency Oscillation Phenomenon in AC Railway Networks

Dynamic interactions among the AC railway traction network and power electronics converters feeding the trains have been reported to cause low-frequency oscillations (LFO) of the catenary voltage and current. This can result in railway system instability, eventually leading to a power outage and the shutdown of the train traffic. To avoid LFO, control of train power electronic converters must be properly designed and tuned. Experimental verification of control performance regarding the LFO phenomenon in the railway traction network is not easy. Alternatively, the railway traction network can be emulated using a power electronic converter, which would feed the train power converter under test. This paper addresses the design of a network emulator able to reproduce the dynamic behavior of the actual network at low frequencies, including LFO. Three different options will be considered for the network emulator. Their performance will be studied first by means of simulations. Finally, the selected solution will be verified on a downscale test bench.

Virtual Inertia Control Strategy of Traction Converter in High-Speed Railways Based on Feedback Linearization of Sliding Mode Observer

Aiming at the low-frequency oscillation (LFO) phenomenon in high-speed railways, this paper proposes a feedback linearized virtual inertia control (VIC) strategy of the traction converter based on a sliding mode observer (SMO) to improve the robustness of the system and suppress the occurrence of LFO. First, to optimize the total harmonic distortion of the traction grid-side current, the inner loop controller is designed as the feedback linearization control. Then, the voltage outer loop is designed as a combination of the VIC designed in this paper and SMO, which enhances the system inertia and reduces the impact of external interference on the system, while reducing the controller's demand for electrical power on the DC-side. Furthermore, a small-signal model is established to analyze the stability of the single-phase vehicle-grid system under the proposed control. The influence of the control parameters on stability is discussed in detail. Finally, the comparison of software simulation and hardware-in-the-loop experimental results between traditional PI control, feedback linearization control, feedback+SMO, feedback+VIC and the proposed control method shows that the proposed control has better dynamic and static characteristics, and can effectively inhibit the occurrence of LFO.

Low-Frequency Stability Analysis of Vehicle-Grid System With Active Power Filter Based on dq-Frame Impedance

To eliminate the low-frequency oscillation (LFO) phenomenon in the vehicle-grid system of high-speed railways, a single-phase cascaded H-bridge multilevel active power filter (APF) directly accessed to the 27.5-kV traction network is proposed in this article, which can provide the dynamic reactive and interharmonic compensation. To explore the mechanism of APF on LFO suppression, the dq-frame impedance model of vehicle-grid system with APF is established first. The impact of the second-order generalized integrator model and the duty ratio model on the impedance accuracy is discussed in detail. Then, based on the generalized Nyquist stability criterion and system power consumption criterion, the stability of vehicle-grid system and its sensitivity to parameters in different cases are compared. The results indicate that the proposed APF can significantly improve the low-frequency stability of vehicle-grid system and owns much better performance than the STATCOM proposed in previous literatures. Finally, the time-domain simulations in MATLAB and the experiments on the controller hardware in the loop platform are implemented, which further validate the accuracy of theoretical analysis and the superior performance of APF in LFO elimination.

Visual experimental investigation on stable operating process of the plane-radial rotating detonation engine

Using visual observation methods, assisted by synchronous measurement of dynamic pressure and ion tracks, the flow field structures of stable operating modes in plane-radial rotating detonation engine (RDE) are revealed, and the influences of injection conditions on the RDE stable operating range are analyzed. The flow field structures of single-wave mode, symmetric dual-wave mode and triple-wave mode are observed. Typical structures such as rotating detonation wave (RDW), oblique shock wave, combustion products, deflagration surface and contact discontinuity are obtained in the flow fields. Obvious reactants pre-combustion ahead of detonation wave is found in the combustor, and low-frequency oscillation phenomenon is gained under specific conditions. With the increase of wave-head number in combustor, the detonation-wave height and parameters decrease. The RDW propagating velocity and pressure peak are higher in single-wave operating mode, but the flow fields in multi-wave mode are more stable. Both the RDW velocity and pressure peak increase generally with equivalence ratio. Keeping the equivalent ratio (ER) fixed, as the mass flow rate increases, the number of detonation wave in combustor increases, and the operating mode transforms from single-wave mode to multi-wave mode. Keeping the mass flow rate fixed, when the equivalent ratio deviates from stoichiometric ratio, the operating mode varies from multi-wave mode to single-wave mode. Additionally, with the mass flow rate increasing, the ER upper and lower limits of multi-wave mode decrease linearly, and the operating range of dual-wave mode remains unchanged. Mass flow rate has little influence on the ER lower limit of the RDE stable operation, which is approximately ER=0.55 in this structure.

Investigation of the Low-Frequency Oscillations in the Flowfield About an Airfoil

The present investigation shows that a triad of three vortices is behind the quasi-periodic self-sustained low-frequency flow oscillation phenomenon. Large-eddy simulations are carried out at , , and sixteen angles of attack at near-stall conditions. It is shown that the best description of the underlying mechanism is that of a button whirligig. A globally oscillating flow whirls around the airfoil in the clockwise direction and stores energy in the triad of vortices. When the oscillating flow loses momentum and comes to an equilibrium, the energy stored in the triad of vortices reaches a threshold. Thus, the triad of vortices reverses the direction of rotation of the oscillating flow, gives it a pulse of energy to whirl in the anticlockwise direction, and starts a new disequilibrium. When the oscillating flow rotates in the clockwise direction (in the streamwise direction on the suction surface of the airfoil), it adds momentum to the boundary layer and helps it to remain attached, and vice versa. Consequently, the instantaneous flowfield switches between an attached phase and a separated phase in a periodic manner with some disturbed cycles.

Effect of Reactive Power Characteristic of Offshore Wind Power Plant on Low-Frequency Stability

Oscillation phenomena of offshore wind power plant (OWPP) in a wide frequency range can be caused due to impedance interactions between grid-connected inverters (GCIs) and transmission cables. In this article, impedance model of GCI with outer power control loop, inner current control loop and phase-locked loop is first established in dq reference frame. The correctness is validated by frequency scanning method. Then, the effects of active and reactive power/current references on dq impedance characteristics of GCI with/without consideration of power control loop are investigated using complex space vectors and complex transfer functions. Furthermore, RLC circuit model of transmission cable considering frequency-dependent characteristics is also established for dq-domain IBSC. On the basis of them, it's found that low-frequency oscillation phenomena of OWPP under power control mode may occur if active power reference exceeds a certain threshold value, which can be mitigated by injecting a certain amount of negative reactive power. Impacts of PLL parameters, length of transmission cable and number of paralleled GCIs on required negative reactive power for low-frequency stabilization are further investigated. Both Matlab/Simulink-based simulation and OPAL-RT-based real-time verification are implemented in an OWPP with four permanent magnet synchronous generators to validate the correctness of the reactive power characteristic analysis results and the feasibility of mitigating low-frequency oscillation phenomena by negative reactive power injection.

Admittance Modeling and Stability Analysis for Inverter-Based Microgrid With Voltage-Source-Rectifier Load

Instability and low-frequency oscillation phenomenon of inverter-based microgrid (IBM) with voltage-source-rectifier load have obtained increasing concern. However, the influence of instantaneous angle of operation point on IBM is usually neglected during voltage faults, and it may generate the uncertain impact of voltage faults on the stable and safe operation of IBM. Besides, the influence of instantaneous angle of operation point may cause varying degrees of system oscillation and stability at a different angle of system states' change, including fault recovery. To solve this issue, a systematic approach of small-signal admittance modeling for IBM in the αβ frame is proposed in this article. The small-signal admittance model of each submodule (SM) of IBM is established and formed as a multi-input-multi-output (MIMO) system considering the actual closed-loop controls and decoupled strategies in the dq frame. Based on the proposed admittance models of SMs, the complete model of IBM is integrated and linearized, and the influence of both amplitude and instantaneous angle of operation point on the IBM stability is analyzed in small-signal model comprehensively. Besides, using the generalized Nyquist stability criterion (GNC), the influence of instantaneous angle in large-signal operation point on oscillation amplitude and stability of IBM is investigated. The proposed analysis approach can analyze and determine the responded oscillation and stability of the system more accurately during system states' variation, such as voltage sag and sudden changes in load. It also reduces the uncertainty of IBM behaviors during states' variation to ensure that IBM operates smoothly, flexibly, and stably. Finally, experimental results are presented to validate the correctness of the proposed analysis method.

Stability Analysis of Low-Frequency Oscillation in Train-Network System Using RLC Circuit Model

The low-frequency oscillation (LFO) between traction network and electric locomotives is a typical phenomenon in high-speed railway and will result in traction blockade accidents. To analyze the respective contribution of traction network and electric locomotives to LFO and reflect the interaction between them clearly, an RLC circuit model is proposed in this paper. First, the negative incremental resistor and capacitance characteristic of electric locomotives is analyzed. Then, the RLC circuit model is built based on single-input single-output (SISO) supersynchronous and subsynchronous impedance. By analyzing the equivalent impedance parameters and resonance of the RLC circuit, the respective contribution of network and locomotives to LFO and system stability is studied. Furthermore, the influential factors of LFO, both grid parameters and locomotives parameters, are investigated. Finally, the analysis results are verified by simulation and experiments based on RT-LAB. Compared with multiple-input multiple-output (MIMO) impedance model, the proposed RLC circuit model can not only explain the LFO phenomena in high-speed railway intuitively and clearly but also simplify the calculation procedure.

VCT-AOC Comprehensive Method to Suppress High-Frequency Resonance and Low-Frequency Oscillation in Railway Traction Power Supply System

Aiming at the high-frequency resonance (HFR) and low-frequency oscillation (LFO) phenomena in high-speed railways, a comprehensive suppression method named voltage constant transient active oscillation compensation (VCT-AOC) is proposed. This method is optimized on the basis of AOC, and the adjustment time is shortened by the VCT suppression measure. When serious HFR happens, the system can be quickly restored and stabilized by completely removing the network voltage oscillation in the control part, thereby avoiding the transient overvoltage and overcurrent damage to the system. According to the characteristics of AOC and VCT, the VCT-AOC comprehensive suppression method is proposed, and the specific implementation scheme of this method is given. This paper established the train-network impedance model with AOC and VCT suppression methods. The Generalized Nyquist Analysis (GNA) method and the dominant pole analysis method are used to analyze the effect of VCT and AOC on the system. Finally, the effectiveness of suppression methods is verified by simulations and experiments.

Novel sliding valley current mode pulse train control for switching DC-DC converter

ABSTRACTMulti valley current mode pulse train (MVC-PT) control technology can completely eliminate the low-frequency oscillation phenomenon of DC-DC switching converters operated in continuous conduction mode (CCM). However, this method needs to increase the number of preset value to broaden the output power range, which will increase the complexity and cost of the controller, and reduce the robustness of the system. Meanwhile, the output voltage ripple is uncontrollable and varied with the changes of the load. In this paper, a novel sliding valley current mode PT (SVC-PT) control technique is proposed to solve the above-mentioned problem. The SVC-PT operating principle and the deficiency of MVC-PT method are analysed in details. The approximate discrete-time models of SVC-PT and MVC-PT controlled switching DC-DC converters are constructed and compared. The comparison results verify that the proposed control method can thoroughly lessen the restriction of the controller on the output power range. Moreover, the control circuit is simplified and the output voltage ripple can be controlled. Finally, the simulation and experimental results are present to prove the feasibility and scientific of SVC-PT control technology.

A Practical Approach to Mitigate Low-Frequency Oscillation in Railway Electrification Systems

The low-frequency oscillation (LFO) phenomenon has frequently happened in the railway electrification systems (RES), which often caused the traction atresia of electric trains and protection malfunction of traction network. In this letter, a practical mitigation approach is presented based on impedance analysis, which indicates that changing the operation mode of the electric trains to diode-bridge rectification (DBR) circuit can mitigate the LFO because the impedance of the rectifier is increased. Then, combining the Prony technology, an online identification and mitigation approach, is proposed here for mitigating the LFO phenomenon. Experimental results verify the effectiveness of the proposed method in stabilizing voltage fluctuation of the RES. This method can also identify the oscillatory frequency and the corresponding attenuation factor.

Integrated Full-Frequency Impedance Modeling and Stability Analysis of the Train-Network Power Supply System for High-Speed Railways

To investigate the harmonic resonance, harmonic instability and low-frequency oscillation phenomena in high-speed railways, this paper proposes a full-frequency impedance model of the train-traction network system (simplified as ‘train-network’) based on d-q coordinates system. Compared with traditional models which deal with only the grid-side converter, the proposed model also includes its load models—the inverter and the traction motor. It also reflects complete control scheme of grid-side converter, which makes it easier to analyze unstable phenomena mentioned above. Moreover, this paper improves the impedance modeling of the network by taking the network impedance and admittance into detailed consideration. In addition, based on the proposed train-network model, the 3D figure and zero-pole diagram are also presented for the analysis of the stability of the integral system. Simulation and experiment results verify the accuracy of the model.

Train–Network Interactions and Stability Evaluation in High-Speed Railways—Part II: Influential Factors and Verifications

Low-frequency oscillation (LFO), harmonic resonance, and harmonic instability phenomena in high-speed railways (HSRs) result from the interactions between multiple electric trains and the traction network. A train–network interaction system and a unified impedance-based model are therefore presented to reveal the mechanism of these three phenomena in Part I of the two-part paper. Part II of the paper presents and compares the detailed simulation and experiment results to investigate the available possible influential factors, verifications, and discussions of the three phenomena. The influential factors include the following: 1) four-quadrant converter (4QC) parameters involving the control parameters of the current controller (CC), the voltage controller (VC), and the phase-locked loop (PLL); 2) network parameters involving the utility power system, traction transformer, primary/secondary supply lines, catenary lines, and autotransformers (ATs); and 3) different numbers and positions of trains and railway lines will also be considered and discussed. In order to validate the theoretical results, the time-domain simulation and experiment system have been conducted. Finally, the differences and the relations among the three phenomena are discussed.

An Active Oscillation Compensation Method to Mitigate High-Frequency Harmonic Instability and Low-Frequency Oscillation in Railway Traction Power Supply System

This paper investigates the high-frequency harmonic instability (HHI) and low-frequency oscillation (LFO) phenomena in high-speed railways, and proposes an active oscillation compensation method to mitigate these HHI and LFO. Such mitigation method introduces the input voltage oscillating components of grid side to the control loop of the grid-side converter, which helps to mitigate the voltage oscillation variables and restore the steady state of the train and traction network (simplified as “train-network”) system. From the perspective of train-network impedance, this paper proposes the train impedance models with and without the mitigation process. The generalized Nyquist analysis method is used to analyze the stability of the system with and without the mitigation. Finally, the mitigation method is verified by simulated and experimental data, to be capable of suppressing both HHI and LFO effectively.