Ultra-low Frequency Oscillation Research Articles

Ultra-low frequency oscillation analysis considering thermal-hydro power proportion

The Ultra-low Frequency Oscillation (ULFO) has attracted much attention due to its field test capture in China, and it is believed that the ULFO is related to the hydraulic generator. To clarify the cause and innate essence of ULFO, the develop of the ULFO under different thermal-hydro power proportions is investigated. A small-signal model of two-machine system considering different thermal-hydro power proportions is established. Based on the small-signal model, the influence of hydropower proportion on the oscillation characteristics is analyzed in different conditions, and it is found that a higher proportion of hydro power results in a lower oscillation frequency, though the hydropower proportion is not the crucial cause in ULFO. Then Based on the eigenvalue analysis, it is observed that the essential cause of ULFO is the weak grid condition rather than the high hydropower proportion. The system connection strength determines whether the system is prone to ULFO. In addition, the participating factors of the generators’ significant state variables are obtained, which indicates that the governor plays a more vital role in ULFO in the case of weak grid connection. To verify the theoretical analysis, the simulations of the two-machine system and six-machine nine-node system are implemented, respectively. The results reveal that system with all thermal power is still prone to ULFO when the connection is weak. Conversely, when the connection is strong, the ULFO does not occur even if all the generators are hydraulic. Furthermore, in a weak grid condition, the oscillation frequency and damping ratio decrease with increasing hydropower proportion, which indicates that a high hydropower proportion deteriorates the ULFO problem.

A Multiagent Deep Reinforcement Learning-Enabled Dual-Branch Damping Controller for Multimode Oscillation

This study develops a multiagent deep reinforcement learning (MADRL)-enabled framework for the decentralized cooperative control of a novel dual-branch (DB) damping controller for both low-frequency oscillation (LFO) and ultralow-frequency oscillation (ULFO). It has two branches, each of which consists of a proportional resonance (PR) and a second-order polynomial that is designed to handle target oscillation modes. To improve the robustness of the controller to system uncertainties, MADRL is developed, where multiagents are centrally trained to obtain the coordinated adaptive control policy while being executed in a decentralized manner to provide the optimal parameter setting for each controller with only local states. Comparisons with the IEEE 10-machine 39-bus system demonstrate that the proposed method achieves better robustness to uncertainties, lower communication delay, and single-point failure, as well as damping control performances for both LFO and ULFO.

Analysis and control for ultralow frequency oscillation damping caused by asynchronous networking mode

Analysis and control for ultralow frequency oscillation damping caused by asynchronous networking mode

Analysis and control for ultralow frequency oscillation damping caused by asynchronous networking mode

Analysis and control for ultralow frequency oscillation damping caused by asynchronous networking mode

Deep Reinforcement Learning Enabled Bi-level Robust Parameter Optimization of Hydropower-dominated Systems for Damping Ultra-low Frequency Oscillation

Deep Reinforcement Learning Enabled Bi-level Robust Parameter Optimization of Hydropower-dominated Systems for Damping Ultra-low Frequency Oscillation

Non‐Adiabatic Acceleration of Injected Electrons in the Inner Magnetosphere: Joint Observations by the Van Allen Probe and the BeiDa Imaging Electron Spectrometer

AbstractWe present joint observations of a substorm injection event on 5 February 2016. The enhancements of energetic electron (∼30 to ∼300 keV) fluxes, the drift echoes of the injected electrons, and the quasi‐monochromatic ultralow frequency oscillations embedded in the flux variations were simultaneously observed by the BeiDa Imaging Electron Spectrometer and the Van Allen Probe B. These vivid features of substorm injection were concretely captured by multiple spacecraft within the geosynchronous distance for the first time. The radial separation of the spacecraft (∼0.3 RE) allowed for a quantitative comparison of the electron flux and phase space density (PSD) across different drift shells. It is found that a more intense increase of equatorial PSD occurred at the lower L‐shell (L* ∼5.5), which indicated a non‐adiabatic acceleration of the injected electrons in the inner magnetosphere.

Nonlinear dynamic characteristics analysis and adaptive avoid vortex-coordinated optimal control of hydropower units under grid connection

Hydropower units operate as the first option for renewable energy supply. The coupling characteristics of hydro-turbine governing system (HTGS), misaligned shafting (MSH), and power grid (PG) (HTGS-MSH-PG) have always been omitted. Meanwhile, the safety hazard from the draft tube vortex zone (DTVZ) and the difficulty of coordinating the ultra-low frequency oscillation (ULFO) and primary frequency regulation (PFR) performance of hydropower unit grid-connected operation have not been resolved. In this study, we aim to investigate the interaction mechanism and nonlinear dynamic characteristics of the HTGS-MSH-PG coupling system and determine the optimal control strategy for considering the DTVZ, ULFO, and PFR of hydropower units. First, a novel coupling model of HTGS-MSH-PG is established. Thereafter, the stability of the HTGS-PG coupling system is examined by employing the Hopf bifurcation theory, and the damping characteristics are obtained using the hydraulic damping model. The nonlinear dynamic behavior of misaligned distance, nonlinear vibration characteristics of MSH, and influence of MSH vibrations on HTGS and PG are revealed based on this model. Finally, an adaptive avoid vortex-coordinated optimize control strategy which considers the means to prevent a strong DTVZ and coordinate the ULFO and PFR performance of HTGS, MSH, and PG is proposed. NSGA-II is introduced to obtain the Pareto optimal solution set, and the optimal scheme is determined by the fuzzy satisfaction method. Moreover, the nonlinear dynamic characteristics of hydropower units under optimal control are revealed. The results demonstrate that the nonlinear dynamic characteristics of the HTGS-MSH-PG coupling system mainly include the quasi-periodic multi-frequency vibration of MSH and multi-frequency dynamic performance of HTGS and PG. The adaptive avoid vortex-coordinated optimal control strategy can effectively prevent a strong DTVZ, coordinate the ULFO and PFR performance of HTGS, MSH, and PG, and increase the satisfaction rate by 72.2%. These results lay a theoretical and technical foundation for the safe, stable, and optimal operation of hydropower units.

Noise analysis of optoelectronic oscillators in the presence of the relative intensity noise of the laser

Optoelectronic oscillators (OEOs) have recently been considered very good candidates for producing ultra-low phase noise radio frequency (RF) oscillations. One of the most important noise sources of any OEO is the relative intensity noise (RIN) of the optical field in the optical part of this system. It is the dominant noise source in some OEO cases. The initial source of the RIN is the laser; however, it may be largely enhanced in the fiber by some phenomena such as the guided entropy mode Rayleigh scattering. Here a frequency domain analysis approach is introduced to separately analyze the effect of both the low-frequency RIN (LFRIN) and the high-frequency RIN (HFRIN), i.e., the RIN around the RF harmonics, on both the phase and amplitude noises of the delay-based single-loop/dual-loop OEOs. The presented approach can take into account the RIN at both the input and the output of the optical fibers. The measured or analytical nonlinear gain function of the photodetector and the RF amplifier can be taken into account to accurately characterize the important effect of the amplitude noise to phase noise (AN-PN) conversion. It is shown that the AN-PN conversion can largely enhance the LFRIN-induced phase noise. Also, to a much lesser degree, it can enhance the HFRIN-induced phase noise. Furthermore, it is shown that the AN-PN conversion reduces the HFRIN- and LFRIN-induced amplitude noise. It is also shown that the fibers’ dispersion has a small effect on the phase/amplitude noise power induced by either LFRIN or the HFRIN, especially for smaller fiber lengths. The validity of the new analysis approach is verified by comparing its results with those of the previously published works in the literature.

Multiple DC coordinated suppression method for ultra-low frequency oscillations

Systems with a high proportion of hydropower are prone to induce ultra-low oscillations when the local load or the delivered power of the DC system changes. For systems where multiple DC circuits exist, system damping can be greatly improved by coordinating supplementary control of the multiple DC circuits to suppress the ultra-low frequency oscillations. However, solving this problem based on the conventional output feedback design approach requires dealing with difficult bilinear matrix inequalities. In this paper, a controller-design approach based on a differential evolution algorithm is proposed. First, a centralized or decentralized structure of multiple DC coordinated controller of a specific order and the corresponding state space equations of the closed-loop system are derived. Next, the random method is used to generate the initial controllers, then the minimum damping ratios of the closed-loop system are calculated, and finally the minimum damping ratios of the closed-loop system are optimized using the differential evolution algorithm. This method can be used to solve for the DC supplementary controller based on the lead-lag structure, which is widely applied in engineering practice, by converting it to the state space form. Therefore, the proposed method can be used to solve the multiple DC coordinated controllers of three structures (i.e., centralized, decentralized, and decentralized based on the lead-lag link) in a unified framework. The proposed method is applied to two case studies, Cigre Nordic and China Southern Power Grid. Results indicate that the designed controllers of all three structures improved the system damping significantly and eliminated the risk of ultra-low frequency oscillations to the system operation. The proposed method is highly practical as it can be applied to DC supplementary controllers based on the lead-lag module.

Modelling and Analysis of Frequency-Responsive Wind Turbine Involved in Power System Ultra-Low Frequency Oscillation

This paper presents a dynamic analysis for ultra-low frequency oscillations (ultra-LFOs) observed in the wind-hydropower hybrid systems. In such systems, DFIG wind turbines (WTs) are required to be frequency-responsive with providing the inertia and frequency support. First, this paper establishes an analytical model to capture the dynamics of frequency-responsive DFIG WTs at the electromechanical timescale. A systematic analysis is then conducted on a 2-machine system (include a hydraulic generator (HG) and an aggregated DFIG WT) to reveal WTs’ dynamic behaviors and their interference mechanism with the HG, accounting for the system ultra-LFO mode. The result shows that WTs’ frequency control, speed control, MPPT control and pitch control would be involved in the system ultra-LFOs, but they have different effects under different operating modes. Cases studies are carried out on the 2-machine system and a modified 10 machine 39-bus New-England power system. Based on the modelling effort and simulation studies, some recommendations are made for using DFIG WTs to help damp the ultra-LFO in wind-hydropower hybrid systems.

Study on AGC strategy involving the frequency control of HVDC power regulation

The asynchronous interconnection between the Southwest China power grid (SCPG) and Central China power grid (CCPG) reduces the inertia of rotation of SCPG to 1/5, which leads to a prominent risk of frequency stability. Moreover, in order to suppress the ultra-low frequency oscillations, the parameters of hydropower governors were optimized, which further worsen the frequency regulation capability of the system. Therefore, the frequency control (FC) of high voltage direct current (HVDC) line along with automatic generation control (AGC) are designed to improve the frequency restoration after a large power disturbance. However, the characteristic of FC regulation is contradictory to the subsequent AGC regulation. In order to improve the post-contingency frequency response of power system, an improved AGC strategy is developed. The proposed strategy involves the FC regulation power and distribute the control power according to the characteristics of different generator units. Case study on the actual model of SCPG proves the effectiveness of the strategy.

Stochastic Ultralow-Frequency Oscillations of the Luminescence Intensity from the Surface of a Polymer Membrane Swelling in Aqueous Salt Solutions.

Photoluminescence from the surface of a Nafion polymer membrane upon swelling in isotonic aqueous solutions and Milli-Q water has been studied. Liquid samples were preliminarily processed by electric pulses with a duration of 1 μs and an amplitude of 0.1 V using an antenna in the form of a flat capacitor; experiments on photoluminescent spectroscopy were carried out 20 min after this treatment. A typical dependence of the luminescence intensity, I, on the swelling time, t, obeys an exponentially decaying function. The characteristic decay time of these functions and the stationary level of luminescence intensity depend on the repetition rate of electrical pulses, and the obtained dependences are well reproduced. It transpired that, at certain pulse repetition rates, the dependence, I(t), is a random function, and there is no reproducibility. Stochastic effects are associated with a random external force of an electromagnetic nature that acts on a polymer membrane during swelling. The source of this random force, in our opinion, is low-frequency pulsations of neutron stars or white dwarfs.

Analysis of Ultra-Low Frequency Oscillation Characteristics of Wind Power System Considering Dead Zone of Governor

Analysis of Ultra-Low Frequency Oscillation Characteristics of Wind Power System Considering Dead Zone of Governor

Discrete Spectrum of Ultralow-Frequency Oscillations of the Ionosphere

This work relates to the field of ultralow-frequency (ULF) spectroscopy of near-earth plasma. Our attention is focused on the Alfvén ionospheric resonator, the spectrum of which is a series of discrete lines. The aim of the study is to experimentally verify a well-defined theoretical prediction. Within the framework of an idealized resonator model, a hypothesis about the relationship between the frequencies of spectral lines has been formulated. To test the hypothesis, observations of ULF oscillations at the mid-latitude Mondy observatory were used. The result of the analysis confirmed the hypothesis that odd harmonics of the resonator discrete spectrum are observed on the earth’s surface. In conclusion, the possibilities of ULF spectroscopy for remote sensing of the ionosphere are briefly discussed.

Influence of water diversion system topologies and operation scenarios on the damping characteristics of hydropower units under ultra-low frequency oscillations

Ultra-low frequency oscillations are the typical dynamic stability problem of hydropower-dominated power systems. Hydraulic factors can directly affect hydropower unit damping, and related studies focus on the influence of hydraulic parameters on unit damping under a fixed operation situation, while ignoring the characteristics of water diversion system topologies (WDST) and diverse hydropower operation scenarios. In this paper, some simulation models of the hydro-turbine governing system (HTGS) with a variety of WDST are first established. Then, a damping calculation method based on phase diagrams is used to calculate the damping of overall system under different WDST and operation scenarios. The results show that the increase of the number of branch pipes in WDST will deteriorate the damping characteristics and setting a surge tank can effectively increase the damping. In addition, if there is a hydraulic connection between the unit integrated into a small power grid (SPG) and the unit integrated into a large power grid (LPG), the strong stability of the LPG will increase the damping of the unit integrated into SPG. Closing some units in multi-unit HTGS will improve damping characteristics of the running unit. Some measures to suppress ULFO are proposed from the perspective of design and stable operation.

Research on Governor Parameter Optimization to Suppress Ultra-Low Frequency Oscillation of Power System Caused by Hydropower Unit

In the actual power system with hydropower, long-time and ultra-low frequency oscillation events occur many times. It is found that the unreasonable setting of governor parameters is an important reason for the oscillation. Firstly, the single machine on load system model is used to analyse the relationship between the PID parameters of the governor and the system stability, then the relationship between oscillation mode and PID parameters of governor is analyzed by eigenvalue analysis method, and the negative damping provided by speed regulation system is analyzed by damping torque method, and then the particle swarm optimization algorithm is used to optimize the PID parameters. Through the analysis of the step response of the single machine system before and after the optimization and the damping torque coefficient provided by the speed regulation system, it shows the effectiveness of the optimization algorithm. Finally, in the simulation platform MATLAB/SIMULINK, a single machine load system model which is closer to the actual power grid is built. The governor parameters of the generator are simulated and verified, and the PID parameters are adjusted by using the parameters obtained by the optimization algorithm. The results show that the optimized parameters have a good suppression for the ultra-low frequency oscillation.

Equivalent circuit modelling of large hydropower plants with complex tailrace system for ultra-low frequency oscillation analysis

Focused on the hydraulic factor-induced ultra-low frequency oscillation phenomenon occurring in a large hydropower plant, this paper investigates the precise modelling of hydraulic transient characteristics in its complex tailrace system and reveals the feasible oscillation suppression schemes from the source side. Combining the telegraphist's equations with the Saint-Venant equations, the mathematical expression of equivalent circuit model (ECM) for open channels is deduced, thus to expand the utilization of ECM to hydraulic transients description in pressurized pipe and open channel combination systems and to establish the high-order equivalent circuit topology of the overall tailrace system in a real hydropower plant. Based on the mathematical model, the feasibility and effectiveness of two different oscillation suppression approaches involving increasing the hydraulic losses on the branch tailrace channels or the main tailrace channel, respectively, are discussed in detail via numerical simulation. Furthermore, the superiority of the oscillation suppression scheme involving increasing hydraulic losses in branch channels in reducing the extra power generation loss is demonstrated. The application of equivalent circuit theory in complex hydraulic systems can contribute to the mechanism-related research on ultra-low frequency oscillations in hydro-dominant power systems.

Operational characteristics and parameter sensitivity analysis of hydropower unit damping under ultra-low frequency oscillations

Hydropower is probably the largest sustainable energy source in use and plays an important role in maintaining power and frequency stability of the power system. However, in hydropower-dominated power systems (HDPS), hydropower units often exhibit negative damping, increasing the risk of ultra-low frequency oscillations (ULFO). In this paper, the operating characteristics and parameter sensitivities of hydropower unit (HPU) damping in an HDPS are investigated, taking a large hydropower plant in southwest China as an example. To this end, a refined mathematical model of a cluster of HPUs is established, and an improved damping quantification method and parameter sensitivity analysis indices are proposed for the complex energy system. On this basis, the damping of the HPU with different control modes under a ULFO is investigated with the change of operating conditions represented by guide vane opening, water head and the number of parallel-running units. Meanwhile, the sensitivity of the damping to the system parameters is analyzed to reveal the influence mechanism of the parameters on the ULFO. Also, measures to suppress the ULFO are formulated from two levels of operation and control combined with theoretical analysis.

Impact of Governor Non-Linearities Representation on Power System Ultra-Low Frequency Oscillations

Power system dynamic models are essential for the stability analysis of a perturbed electrical system. In recent years, hydro-dominant power systems have experienced the occurrence of ultra-low frequency oscillations (ULFO). While power system dynamic simulation stands as an essential tool for the research community to analyze and understand this phenomenon, such systems involve non-linearities in their dynamic model affecting the ULFO analysis’s validity. This present work investigates the impact of governor non-linearities representation on the validity of the ULFO analysis. Sinusoidal-input describing function (SIDF) is borrowed from control theory to appreciate existing governor non-linearities models’ capacity to respond accurately to ULFOs. Through rigorous test-cases, results prove the imperativeness of prior accurate measurement of those governor non-linearities before their numerical implementation to ensure the power system dynamic simulation model’s validity for ULFO analysis.

Study on the Strategy of Improving the Primary Frequency Regulation Capability by Adaptive Parameters of Turbine Governor

Many studies have shown the inverse regulation characteristics of water hammer effect inherent in the hydropower system and the unreasonable parameter configuration of the governor result in the generation of the ultra-low frequency oscillation. The main research work of this paper is above two points. Based on the detailed analysis of the influence of each parameter of the speed regulating system on the frequency characteristics of primary frequency regulation, combining with the improved particle swarm optimization algorithm, a self-adaptive parameter configuration technology on PID governor of hydraulic turbine regulation system is presented, the PID parameters can be adjusted continuously according to the rotation speed deviation. And the simulation analysis is carried out. The effectiveness of the adaptive PID parameter method to improve the primary frequency regulation ability is verified.