Ultra-low Frequency Oscillation Research Articles

Correlating analysis and optimization between hydropower system parameters and multi-frequency oscillation characteristics

This paper aims to study the relationship between multi-frequency oscillation characteristics and parameters of hydro-turbine governing systems (HTGS), as well as improve the system oscillation quantification accuracy and primary frequency regulation performance. Firstly, an HTGS model with multi-frequency oscillation characteristics is established using an actual hydropower system as the object. Then, system oscillation components and the sources of each oscillating link are identified using eigenvalue and correlation analysis. To address the issue that sub-wave interference with dominant oscillation quantization, we innovatively incorporate correlation and frequency indexes into quantification process, and the dominant oscillations of complex HTGS are accurately quantified. On this basis, the detailed partitioning of control parameters is established to obtain the guide graph quickly reflecting the relationship between control parameters and system oscillations. Finally, an integrated optimization strategy with ultra-low frequency oscillation (ULFO) safety and regulation performance constraints is constructed, and control parameters are further optimized under two typical parameter setting methods. The results indicate over 90 % of the stable parameters enable the system to enter ULFO mode, and optimized control parameters utilizing proposed strategy increase regulation speed by 46 % while effectively suppressing ULFO. This paper provides a powerful tool and novel ideas for complex HTGS oscillation analysis and parameter optimization.

Multiple DC Modulation Coordination Strategies Based on Transient Energy Function and Deep Deterministic Policy Gradient Algorithm

Ultra-low-frequency oscillation (ULFO) is a new problem of frequency stability in asynchronous network back-end power networks. The negative damping provided by hydropower units is the direct cause of ultra-low-frequency oscillation, and DC modulation is an effective means of suppressing system frequency oscillation. Based on the transient energy function and the limitation of DC modulation, this paper analyzes the conditions of suppressing system oscillation by DC modulation from the perspective of energy. The weight of unit energy participation is defined, and the calculation formula of transient oscillation energy in a certain oscillation mode is derived. DC modulation sensitivity is then used to create a DC modulation sequence table based on the Prony identification approach. Ultimately, the DC involved in the modulation is identified using the DC modulation sequence table and the transient oscillation energy, and the Deep Deterministic Policy Gradient (DDPG) algorithm is used to optimize the chosen DC modulation parameters.

A Novel Control Strategy for Hydraulic Turbines to Consider Both Primary Frequency Regulation and Ultra-Low Frequency Oscillation Suppression

In response to the requirements of mitigating ultra-low frequency oscillation (ULFO) and enhancing primary frequency regulation (PFR) performance in hydropower-dominated systems, a novel control strategy, namely the center-frequency-structured governor-side power system stabilizer (CFS_GPSS) is proposed. In this study, the transfer function model of the hydropower system with a proportional-integral-derivative (PID)-type governor is established. Through analysis of damping torque and amplitude-frequency characteristics, the dominant links and key characteristics of ULFO are revealed. Based on these findings, a CFS_GPSS strategy is proposed to compensate for the phase and increase system damping. Finally, the effectiveness of the CFS_GPSS is verified under normal operating conditions of 0.04 Hz, strong network and low hydropower output conditions of 0.034 Hz, and weak grid-connected conditions of 0.054 Hz based on the 3-machine, 9-bus system. Compared to the conventional structured governor-side power system stabilizer (CS_GPSS) control strategy and PID parameter optimization method, the CFS_GPSS demonstrates efficient ULFO suppression across a wide frequency range while significantly enhancing PFR performance. The proposed control strategy exhibited the expected performance under various operating conditions, providing effective technical means to enhance the reliability of hydraulic turbines and guide the safe and stable operation of hydropower-dominated systems.

Research on Ultra-Low-Frequency Oscillation Suppression Method of High-Head, Large-Capacity Hydropower Units

Aiming at the problem of ultra-low-frequency oscillation (ULFO) caused by an irrational setting of governor parameters of high-head hydraulic turbine units, this paper proposes a method to optimize the additional damping control parameters of the governor to suppress ULFO. Firstly, the mechanism of ULFO is elaborated and the effect of system damping torque on ULFO is analyzed. Secondly, Hamilton’s theory is used to rank the magnitude of each unit’s contribution to the ULFO. Then, the parameters of the damping controller are optimized using the variational modal decomposition–Hilbert transform method and the Grey Wolf Optimization algorithm. It realizes the phase compensation of the hydroelectric unit governor and suppresses the ultra-low-frequency oscillation. Finally, the proposed method is validated using the simulation conducted in CloudPSS. The test results indicate that the proposed method can effectively suppress the ULFO caused by the high-head hydroelectric unit.

Suppression method of ultralow-frequency oscillation under ambient excitation in hydro-dominant power systems

Ultralow-frequency oscillations (ULFOs) have frequently occurred in hydro-dominant power systems in recent years. These phenomena seriously threaten the safe and stable operation of power systems. Therefore, measures that can effectively suppress ULFO need to be taken. Currently, the hydropower unit governor can enhance the dynamic stability and dampening oscillations of a system through adjusting the proportional–integral–derivative parameters. However, modifying these parameters can affect the primary frequency regulation performance, which compromises the assurance of grid frequency quality. This study proposes a switching type of hydro generator governor based on the local damping contribution rate to address the aforementioned problems. The approach incorporates the energy structure into the classical governor model, calculates the local damping contribution rate, and enables automatic switching of the hydro generator governor parameters based on positive and negative local damping contributions. This way allows for segmented control with different parameter settings, which enables the system to effectively consider the damping characteristics of ULFO while maintaining primary frequency regulation performance. It can also accurately and efficiently suppress the ULFO. The proposed method is verified through arithmetic analysis conducted on a 16-machine 5-zone system and an actual grid system. Simulation results demonstrate the effectiveness of the method.

Influence Analysis and Control Method of Ultra-Low Frequency Oscillation in a Hydro-Dominant Sending Power System with Wind Power Integration

To make clear the influence of renewable energy on ultra-low-frequency oscillation (ULFO) in a hydro-dominant system via an high voltage direct current (HVDC) transmission system, this paper studies the damping characteristics when wind power is integrated into the sending power system. The damping torque method is applied for the mechanism study and risk evaluation of ULFO. The study of the inner cause and outer performance proves that ULFO belongs to frequency oscillation rather than conventional power oscillation. According to the theoretical analysis, the suggested control mode is identified to reduce the risk of ULFO. Moreover, a robust controller is designed for wind units based on mixed H2/H∞ robust control theory, and the control method makes use of the fast response ability of the converter without compromising the primary frequency regulation ability. Finally, a simulation model of a hydro-dominant sending power system with wind integration is established in PSCAD Version 4.6 software. The simulations verify the proposed control can suppress ULFO effectively.

Stability characteristics analysis and ultra-low frequency oscillation suppression strategy for FSC-VSPSU

The research on control strategies of the variable speed pumped storage unit with full size converter (FSC-VSPSU), mainly focuses on the rapidity of power regulation. But the speed oscillation and slow speed regulation performance caused by fast power regulation are ignored, which is not conducive to the safe and efficient operation of the unit. This article investigates the dynamic stability characteristics of the FSC-VSPSU in detail. In turn, the stability risks are clarified and serve for the design of its control strategy and the improvement of operational efficiency. The small-signal model of FSC-VSPSU considering the variation of water head and load in fast power mode is established. The dominant instability control link and key parameters are revealed based on the eigenvalue analysis. Oscillation mode analysis shows that the unit has a wide-band oscillation characteristic. The dominant oscillation mode is the ultra-low frequency oscillation (ULFO) caused by the governor control system. To suppress the ULFO and improve the optimal speed tracking performance of the unit to the greatest extent, a coordinated control strategy of governor variable parameter control and governor power system stabilizer (GPSS) is proposed. The electromagnetic transient simulation based on PSCAD verifies the effectiveness of the proposed measures.

Generalized Turbine Model Based Analysis and Simulation of Ultra-Low Frequency Oscillation of Hydropower Units

The occurrence of ultra-low frequency oscillation (ULFO) events in the power grid has a significant number of hydropower units in recent years. Considering this aspect, relevant studies have shown that the turbine model has an important impact on the ULFO behavior of hydropower units. Based on the study of the generalized turbine model, the flow factor closely related to the inherent parameters of the turbine and the variation of the generating flow is defined in this paper. The ULFO model of the hydropower unit with full consideration of the flow factor is established. Based on the frequency characteristics of the system and the Routh stability criterion, the mechanism of the influence of the flow factor on the ULFO behavior of the hydropower unit is analyzed. The simulation results of single-machine and multi-machine systems show that, compared with the ideal turbine model, the generalized turbine model considering the flow factor can increase the stability margin of the system. It can provide greater damping torque and restrain the ULFO behavior of hydropower units. The flow factor is larger, and the suppression effect of the ULFO is more obvious.

A Robust Design Method for Ultra-Low-Frequency Oscillation Suppression Control in Hydro–Photovoltaic Complementary Systems

Aiming at the problem of ultra-low-frequency oscillation (ULFO) in power systems, with an oscillation frequency lower than 0.1 Hz, an oscillation suppression control method is proposed. Firstly, the mathematical modeling of the system is carried out, and a cascade correction control method of the governor of hydropower generators is proposed. Secondly, using the ability of photovoltaic (PV) active power to adjust the system frequency, an additional controller for PV active power is proposed. Then, a unified frequency model considering the two controllers is established for multi-hydro and multi-PV systems, and the structural singular value method is used to coordinate the design of the parameters of the hydropower and PV generator controllers to achieve robust performance. Finally, based on a time-domain simulation of single-hydro and single-PV systems and an actual demonstration system located in Sichuan Province, China, the results show that the control method proposed in this paper has a good suppression effect on ultra-low-frequency oscillation under different working conditions, and meets the requirements of robust performance.

Ultralow Frequency Resonators: on the 80th Anniversary of the Discovery of Alfvén Waves

The concept of Alfvén waves, introduced into science 80 years ago, played an important role in the formation and development of cosmic electrodynamics. Alfvén waves differ in that at each point in space the group velocity vector and the vector of the external magnetic field are collinear to each other, due to which the waves can carry momentum, energy, and information over long distances. In memory of the outstanding event, we briefly describe two Alfvén resonators, one of which is located high above the Earth, in the radiation belt, and the second, in the ionospheric layers. Both resonators have a discrete spectrum in the upper part of the range of ultralow frequency oscillations of natural origin (approximately from 0.2 Hz to 7 Hz). The close connection between the concept of Alfvén waves and the current problems of the electrodynamics of geophysical media is especially emphasized.

Analysis of Ultra-Low Frequency Oscillation in Hydro-Dominant Power System and Suppression Strategy by GPSS

After asynchronous operation, the Southwest Power Grid in China has the risk of ultra-low frequency oscillation, which seriously threatens the stable operation of the power grid. Based on the hydropower single-unit with load system, the extend Philips-Heffron model is built considering PSS/GPSS (Governor Power System Stabilizer). Then, the damping torque analysis is carried out in both prime mover and excitation side. The negative damping in the prime mover side is prominent in the ultra-low frequency band and the effect of damping ratio increase by PSS and GPSS is compared. After that, the small signal analysis of the whole system is conducted, the participation factors of the ULFO mode shows the high participation and controllability by GPSS control. At last, the proper GPSS parameters configured by small signal analysis are equipped in the hydro unit of asynchronous four-machine two-area system which reveals an obvious ULFO suppression effect. Furthermore, the practical ULFO identification algorithm based on feature points extraction and analysis which crossing the PFC (Primary Frequency Control) dead zone (49.95 - 50.05 Hz) is proposed, the GPSS control cubicle which embedded the practical ULFO identification algorithm is developed and the semi-physical simulation is conducted to verify the real ULFO suppression effect by GPSS control.

Study on stability margin of ultra-low frequency oscillation in hydropower system

In recent years, the ultra-low frequency oscillation of power system caused by hydropower units has occurred frequently. It is very important to study the ultra-low frequency oscillation characteristics of hydropower stations for the stable operation of power system. It is worth further research on how to determine the system stability margin in engineering practice. Based on the established model of single machine system with load, the analysis of the characteristic equation of closed-loop transfer function of single machine system shows that the ultra-low frequency oscillation will occur when the ratio of proportional parameter to integral parameter is too small. Based on the single machine with load system model and the equivalent single machine with load model of multi machine system, the ultra-low frequency oscillation mode of the system is analyzed by eigenvalue analysis method. According to the requirements of the damping ratio index, the security domain of the system ultra-low frequency oscillation is constructed, and the risk and stability margin of the system ultra-low frequency oscillation are evaluated by the characteristic values of the system ultra-low frequency oscillation mode.

Research on integrated micro-energy storage technology based on multi-type electrochemistry

Electrochemical energy storage is a process that utilizes chemical reactions to store and release electrical energy in the form of chemical energy. These include lead-acid, lithium-ion, flow, sodium-sulfur batteries, etc., while electrochemical energy storage materials and technologies are the keys to solving the utilization, conversion, and storage of clean energy. In order to solve the problem that the current energy storage technology has an extensive range of energy storage frequency fluctuations in the application process, this paper conducts research on a comprehensive micro-energy storage technology based on multi-type electrochemistry, which improves the stability of energy storage. Based on the introduction of multi-type electrochemical technology, MXene (Ti3C2) was selected as the dispersion liquid, and PVA was used as the gel electrolyte to complete both preparations. The device’s fabrication and assembly are completed based on calculating the rated power, rated capacity, and other parameters of the micro energy storage capacitor. Aiming at the ultra-low frequency oscillation problem that may exist in the energy storage process, this paper develops a micro energy storage control method that participates in primary frequency modulation. The comparison experiment proves that the frequency fluctuation of the new energy storage technology is within the controllable range in practical application, which can effectively improve the stability of energy storage.

Performance Assessment for Primary Frequency Regulation of Variable-Speed Pumped Storage Plant in Isolated Power Systems

Primary frequency regulation (PFR) is a crucial operating condition for PSPs to realise frequency modulation, and the effectiveness of PFR is significant to the stability of power system frequency. Several challenges and risks have been presented in the PFR process for conventional PSPs, especially for those which run in the isolated grid, such as water inertia, negative damping of speed governor and ultra-low frequency oscillation (ULFO). Variable-speed pumped storage plants (VSPSPs) have the potential to overcome the negative impacts on regulation performance caused by hydraulic factors, due to the advantages of rapid power regulation and independent active power control from turbine output. In this paper, the primary task is to conduct a comprehensive assessment for PFR performance of VSPSPs in isolated power systems. Initially, the hydraulic–mechanical–electrical numerical models are established. Secondly, the rotational speed stability of the pump-turbine is quantified and the advantages of VSUs in suppressing ULFO are assessed. Relevant results reveal that the performance of VSUs is better than that of FSUs in the regulation process. Finally, assessments of frequency regulation performance under various scenarios are conducted with four indicators (standard deviation of power differences, power regulation time delay, settling time and overshoot).

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.