Low Frequency Oscillation Modes Research Articles

Considerations Toward Coordinated Control of DFIG-Based Wind Farms

This paper highlights the important considerations toward coordinated control of doubly fed induction generator (DFIG)-based wind farms for power oscillation damping. The effect of replacing one existing synchronous generator with a power system stabilizer (PSS) by a DFIG on the local mode and the mode shapes of the critical interarea modes are analyzed. With almost zero participation from the DFIG mechanical side and very little observability of low-frequency oscillatory modes in signals locally available at the wind farms, the choice of appropriate remote feedback signals is discussed. Relative controllabilities of the DFIG rotor current components are compared to determine the most effective control structure. A coordinated control design approach aimed at damping multiple oscillatory modes through more than one DFIG-based wind farm is demonstrated. Heuristic optimization is used for the design problem which is otherwise difficult to solve using an analytical approach. Modal analysis and nonlinear simulation results are presented to substantiate the findings.

The New Identification Method for Low Frequency Oscillation Mode in Power System Based on Prony Algorithm and Neural Network

This paper presented a new improved Prony algorithm based on neural network to train weights.The algorithm solved some problems that difficulty and low precision during matrix inversion in Prony method. According to real-time transform characteristics of low frequency oscillation in power system, the algorithm used limited data windows in on-line parameter estimation and pattern recognition, and improved pattern recognition precision. The simulation results proved that this proposal algorithm has some features of directly ,effective, high reliability, less calculation amount and minor error when it be used to analysis oscillation characteristics and mode identification. So it is suitable for identification of low frequency oscillation mode in power system.

Optimal location of PWM based series compensator in a power system

This paper proposes a new approach for optimal placement of PWM based Series Compensator (PWMSC) in large power systems. The proposed approach is based on the Selective Modal Analysis (SMA) and dynamics index to damp out the inter-area oscillation modes. For this reason, first, SMA is used to calculate the low frequency modes of oscillation, and then Most Dominant Line (MDL) Table based on the dynamic index is proposed which shows the influence of active power flows of the transmission lines on inter-area modes of the power system. The parameters of the PWMSC damping controller are designed by optimization based approach for the purpose of damping inter-area oscillations in practical system. Optimal PWMSC placement is validated by comparing different candidate placements based on the total damping that they provide for system. The effectiveness of the proposed method is demonstrated on the 16-machine five-area power system for various network conditions.

Design of model predictive controllers for adaptive damping of inter-area oscillations

An adaptive damping controller design method by integrating online recursive closed-loop subspace model identification (SMI) with model predictive control theory is proposed in this paper. A reduced order state-space model which contains dominant low frequency oscillation modes is firstly identified online by using an closed-loop SMI algorithm. Then, an infinite horizon closed-loop optimal control is achieved based on model prediction which uses the current state of power system as the initial state. At each control step, the identified model is updated by using the new coming measurements and the optimal control action is solved again. Periodical online model and control updating identification and optimal control enables the proposed controller to adapt to operating condition variations and system parameter uncertainties. It is more robust than offline identification based damping controllers which could suffer from performance degradation under time varying and uncertain conditions. Simulation results demonstrate the effectiveness and robustness of the proposed controller in damping inter-area low frequency oscillations. The abilities to coordinate with power system stabilizers (PSSs) and other similar controllers in multi-machine power systems are also illustrated.

Comparative Analysis on the Characteristics of Low‐Frequency Energy Released by the Wenchuan Earthquake and Kunlun Mountains Earthquake

AbstractThe Mw7.9 Wenchuan earthquake of 12 May 2008 and Mw7.8 Kunlun Mountain earthquake of 14 November 2001 have equivalent energy magnitudes. However, the characteristics of the fault rupture processes and the geographical positions of these two great earthquakes are very different, and the damage and the public attention of the Kunlun Mountain earthquake are far less than the Wenchuan earthquake. The seismic wave energy information generated by great earthquakes is very rich. High‐frequency energy of a big quake decays quickly within short time in a small scope, while low‐frequency energy especially very low‐frequency energy decays slowly, and can spread many laps around the Earth before consumed. We employ the spectral element method incorporated with large‐scale parallel computing technology to investigate the characteristics of seismic wave propagation excited by the two great earthquakes. The transverse isotropic PREM (Preliminary Reference Earth Model) model is employed as a prototype for the numerical global Earth model. The Crust2.0 and S20RTS models are taken into consideration. These wave propagation processes are simulated by solving three‐dimensional elastic wave governing equations. The visualization of the numerical results displays the profile with three components of the seismic wave propagation. Our calculation displays the thrusting and strike‐slip of the source rupture processes of the Wenchuan earthquake and the left‐lateral strike‐slip of the source rupture process of the Kunlun Mountain earthquake, respectively. Comparison of low‐frequency energy information of synthetic seismograms excited by the two earthquakes shows that the energy at low‐frequency oscillation modes is broadly equivalent, but the energy of the Kunlun Mountain earthquake is slightly smaller than that of the Wenchuan event. The results also demonstrate that the frequency components are almost the same at low‐frequency displacement amplitude spectra recorded at different stations between the two great earthquakes, but the energy is obviously different. These can further reveal that the characteristics of Earth's oscillation triggered by large earthquakes with different source ruptures are different, and the characteristics of Earth's oscillations recorded by different stations are also different.

The Influence of Multiple Low Frequency Oscillations on Power Transmission Capability and Their Solutions

There might be dynamic stability problems with multiple low frequency oscillation modes in power system with long distance and large capacity power stations, especially with clean energy stations, which may cause ” seesaw effect” between every power base, and restrict the power transmission capability. Considering this problem, an appropriate integrated solution is put forward according to the factors and measures for each mode. It is proved effective from the view of oscillation suppression and power transmission capability increasing by time domain simulation and frequency domain simulation.

Estimation of the electromechanical characteristics of power systems based on a revised stochastic subspace method and the stabilization diagram

Estimating low-frequency oscillation modes and the corresponding mode shapes based on ambient data from WAMS measurements has a promising prospect in power system analysis and control. Based on the stochastic subspace method, this paper proposes a revised stochastic subspace method by introducing reference channels, which can estimate the modes and the mode shapes simultaneously with great computational efficiency. Meanwhile, the accuracy of the estimated results is not degraded. To discriminate the real modes from the spurious ones, the stabilization diagram is introduced. A novel algorithm is designed to deal with the stabilization diagram which can detect the real modes automatically. Tests conducted on the IEEE-118 system indicate that the proposed method has good performance in terms of both computational efficiency and accuracy, and has the potential of being used on-line.

Sunspot oscillations as derived from the SOHO/MDI magnetograms

As a result of processing long (up to 144 h) series of sunspot magnetograms obtained on the SOHO (Solar and Heliospheric Observatory) spacecraft with the MDI (Michelson Doppler Imager) instrument it is shown that the mode with a period of 800–1300 min is a limiting low-frequency oscillation mode of the magnetic field of a sunspot as a whole. Its period is essentially and nonlinearly depends on the sunspot magnetic field strength. In addition to this mode, higher harmonics are also revealed in the sunspot oscillation spectra in the bands 40–45, 60–80, 135–170, 220–250, and 480–520 min. The oscillation power in these bands monotonically and rapidly decreases with increasing frequency, which is characteristic for overtones arising due to the nonlinear nature of oscillations. The limiting oscillation mode stably exists in sunspots for 1.5–2 days, which coincides with the average lifetime of a supergranular cell. The mode with the period of 35–48 h observed in the power spectrum is not an eigen mode of sunspots, because its period is independent of its magnetic field strength. Probably, it occurs as a quasiperiod of an external exciting force caused by disturbances from supergranular cells surrounding the sunspot.

Impact of Photo Voltaic generation control on multi machine Small signal stability

Abstract The modeling and analysis of the impact of Solar Photovoltaic (PV) on the small signal stability of power system are explored. The analysis provides explanations of the impact of the closed-loop control on the power system stability. Two system models, Single Solar Infinite Bus (SSIB) system and solar connected multi- machine system are considered. In model of PV, module variable are algebraic that is, they change instantaneously and the modeled dynamics are those of the power converter controller. The discussion considers maximum power point tracking, dc link voltage and current controller. In case of SSIB sensitivity to different weather conditions such as irradiance, temperature is studied. Also sensitivity to different power system conditions such as voltage variation, reactive power support from solar is studied. In multi-machine case impact of different active and reactive power injection levels is analyzed. The observations and conclusions obtained from eigenvalue, participation factor analysis suggest that even though the PV generation system dynamic do not participate into low frequency oscillation modes, its control can influence small signal damping depending based on its location in the system.

Effect of ionization distribution on the low frequency oscillations mode in Hall thrusters

It is found that the discharge parameters have notable effects on the mode of discharge current low frequency oscillation in Hall thrusters, but different discharge parameters might form the similar low frequency oscillation mode. In order to study the mechanism of oscillation mode formation, the ionization distribution in the discharge channel was measured experimentally, and one-dimensional quasi-neutrality hydrodynamic model was used to study the relationship between ionization distribution and the oscillation mode. Researches show that the low frequency oscillation with a narrow and condensed ionization distribution has the mode of lower amplitude and scattered frequency. The low frequency oscillation amplitude would become high and have dominative frequency component with the relative wide ionization distribution. Therefore, it can be concluded that the difference of ionization distribution characteristics is the main reason of the oscillation mode variation, and discharge parameters are only the external control parameters of ionization distribution characteristics.

Power Oscillation Damping using VSC-based Multi-terminal HVDC Grids

This paper presents a Power Oscillation Damping (POD) controller for use with Voltage Source Converter based Multi-terminal Direct Current (VSC-MTDC) grids. The controller has been tested on a large meshed AC network into which a five bus, six line VSC-MTDC network has been incorporated. The controller is designed using a Modal Linear Quadratic Gaussian (MLQG) approach allowing targeted control action on critical low-frequency oscillatory modes. By taking multiple delayed wide area signals and modulating active power injection by multiple VSC-MTDC converter stations, significant improvement in system stability is seen. The effect of blocking the POD controller's active power modulation signals to specific VSC-MTDC converter stations is assessed. Such a blocking feature may be desirable at points of connection with weak AC networks which might be temporarily unable to tolerate modulation in active power injection and instead require a fast return to steady state constant power injection. Even with modulation signals blocked, the post disturbance system performance is significantly enhanced, demonstrating the redundancy in using VSC-MTDC for oscillation damping. The controller performance has been assessed through both small and large disturbance analysis.

Stability Analysis and Improvements for Variable-Speed Multipole Permanent Magnet Synchronous Generator-Based Wind Energy Conversion System

This paper discusses the stability issues of the permanent magnetic synchronous generator (PMSG)-based direct driven wind energy conversion system (WECS) and presents a torque compensation strategy to improve the system stability. The stability analysis shows that the drive train dynamics can significantly affect the system stability by introducing a low-frequency oscillatory mode and the stability margin can be enlarged with the slower generator torque control loop or the reduced power converter response. The torque compensation strategy contains a feed-forward compensator in the torque control loop, which is formulated based on the dc current injected into the dc link capacitor of the converters. With such compensation, the oscillatory mode is effectively suppressed and the stability of the WECS is improved. The simulation results verify the theoretical analysis as well as the small-signal and transient performance of the strategy.

Oscillatory modes of interaction of supersonic overexpanded jets with barriers

The self-oscillatory interaction of supersonic jets with barriers has mainly been studied for under-expanded jets. There are only a few experimental studies examining the case of overexpanded jets, with little computational work done in this direction. To fill this gap, we performed numerical simulations of overexpanded supersonic jets with barriers. The calculations were performed by the Godunov method on fine grids using parallel programming techniques. In the course of numerical simulations, the gasdynamic parameters of the jet and the geometric parameter of the barrier were varied. The barrier had the shape of a cylindrical cavity of depth l = (0 − 18)r a , where r a is the nozzle exit radius (the case l = 0 corresponds to a flat-end barrier). Based on the results of the numerical simulations, the conclusion on whether the self-oscillation process occurs was drawn and the dependence its characteristics (frequency and amplitude) on the governing gasdynamic and geometric parameters were obtained. Good agreement with experimental data on the fundamental tone frequency was demonstrated. A low-frequency oscillation mode was mostly realized. In this case, the jet experienced periodic suctions into and ejections from the cavity, counter the oncoming jet flow, with the formation of a structure consisting of three discontinuity surfaces (two shock waves and a separating surface contact).

On-line Estimation of Power System Low Frequency Oscillatory Modes in Large Power Systems

In order to evaluate the small signal stability of power system, eigenvalue analysis is an important method to implement. However, there are some disadvantages for example it cannot analyze in on-line condition and is effective for calculating in limited operation conditions. This paper aims to investigate the performance of online oscillation mode estimation techniques for large power systems. Autoregressive and moving average (ARMA model) with least-square and kalman filter algorithms are employed and compared. The performance of both algorithms is tested in term of calculation time and mean square error. The results present ARMA model with both algorithms can provide the satisfied performance. However, ARMA model with kalman filter algorithm has more advantages such as faster calculation time and more accurate calculation results. Thus, this algorithm can be utilized in online oscillation mode estimation and can be used in design the control system for enhancing the damping in power system.

Detecting hydroclimatic change using spatio-temporal analysis of time series in Colorado River Basin

summary It is generally accepted that the seasonal cycle of precipitation and temperature in cordillera of the western US exhibits a north–south pattern for annual, interannual and decadal time scales related to largescale climate patterns. In this paper we explore these relationships, with special attention to the role of local and regional physiographic, hydrogeologic and anthropogenic conditions on low-frequency climate and terrestrial response modes. The goal is to try to understand the spatio-temporal structure in historical precipitation, temperature and streamflow records (P–T–Q) in terms of climate, physiography, hydrogeology, and human impacts. Spatial coherence in time series is examined by classification of factor loadings from principal component analysis. Classification pattern of P–T–Q stations indicate that local physiography, the hydrogeology, and anthropogenic factors transform atmospheric forcing and terrestrial response into unique clusters. To study the temporal structure, dominant low-frequency oscillatory modes are identified for a region from historical P–T–Q records using singular spectrum analysis. Noise-free time trajectories are reconstructed from the extracted low-frequency modes (seasonal–decadal) for each contributing watershed area corresponding to streamflow observation stations, and the phase–plane plots are obtained. Together, the spatial classification and phase plane provides a means of detecting how large-scale hydroclimatic patterns relate to major landforms and anthropogenic impacts across the CRB. The main result of this paper is that resolving the relative impact of basin-wide patterns of climate, physiography and anthropogenic factors (irrigation, dams, etc.) on runoff response can be a useful tool for detection and attribution for each source of variability.

Robust Design of a TCSC Oscillation Damping Controller in a Weak 500-kV Interconnection Considering Multiple Power Flow Scenarios and External Disturbances

The power oscillation damping (POD) controllers implemented in the two thyristor controlled series compensators of the Brazilian North-South (NS) interconnection, in the year 1999, were solely intended to damp the low-frequency NS oscillation mode. These controllers are still under operation and are derived from the modulus of the active power flow in the NS line that is phase-lagged at the frequency of the NS mode and may experience relatively large excursions generated by exogenous disturbances. This paper utilizes the same 1999 data to compare the performance of a proposed robust POD controller design with those of two conventional designs. A recent robust control synthesis algorithm used in this work is based on a nonsmooth optimization technique and has the capability to handle various controller structures, including reduced-order, and to deal with time-domain constraints on both controlled and measured outputs. Moreover, the nonsmooth design technique encompasses multiple operating conditions subject to various test signals, hence building a truly time-domain multi-scenarios approach. According to the results discussed hereafter, this is a key advantage in the industrial context of increasing demand for performance and robustness. The described results relate to a large-scale system model used in the feasibility studies for that interconnection.

Thermal conductivity and specific heat of bulk amorphous chalcogenides Ge20Te80−xSex (x=0,1,2,8)

Abstract Bulk amorphous chalcogenide samples of Ge 20 Te 80− x Se x ( x = 0, 1, 2, 8) have been prepared using a melting-quench method, and characterized by the differential scanning calorimetry, X-ray powder diffraction, high-resolution transmission electron microscopy, specific heat and thermal conductivity measurements. The low temperature specific heat measurements identified some localized low-frequency oscillation modes (Einstein modes) in conjunction with a Debye-like behavior. It was found that with increasing Se concentration the characteristic Debye temperature did not change whereas the Einstein temperature slightly decreased. The lattice thermal conductivity of all Ge 20 Te 80− x Se x samples exhibited typical amorphous heat conduction behavior, which has been discussed in connection with the phonon mean free path and in the context of a phenomenological model of heat conduction for highly disordered system.

Impact of Madden-Julian oscillations on the Indian summer monsoon sub-divisional rainfalls

Indian summer monsoon rainfall exhibits inter-seasonal variations in the time scales of 2-7 years which are linked to quasi-biennial oscillations and El nino-Southern Oscillation phenomenon and also intra-seasonal variations in the time-scale of 30-60 days which are linked to activity of MJO which emerged as a dominant mode of intra-seasonal oscillations of Indian summer monsoon rainfall in addition to the other modes of low frequency oscillations. In this scenario, the inter and intra seasonal variability of 29 meteorological sub-divisional rainfalls has been investigated by correlating the MJO indices at 10 different longitudes covering Indian, Pacific and Atlantic Oceans with cumulative sub-divisional summer monsoon rainfall (1979 – 2000). The results were discussed.

Needles in haystacks: how to use contemporaneous data in the search for low-frequency modes of oscillation of the Sun

We show how to take advantage of contemporaneous data from two different instruments in the search for low-frequency modes of oscillation of the Sun. Contemporaneous data allow searches to be made for prominent, sharp concentrations of power which are coincident in frequency. Crucial to determining objective measures of the joint probability of the random occurrence of such features, which are potential candidates for modes, is a good understanding of the characteristics of the background noise. In this paper we show how to make proper allowance, in the calculation of the probability, for noise that is common to data from different instruments. This common noise is solar in origin, and comes from the solar granulation. Its presence makes calculation of the probability a non-trivial problem. We demonstrate application of the technique in searches for low-frequency p modes. The data we searched comprised 3071 d of contemporaneous Sun-as-a-star Doppler velocity observations made by the ground-based Birmingham Solar-Oscillations Network (BiSON), and the GOLF instrument onboard the ESA/NASA SOHO spacecraft.

The role of forcing in the local stability of stationary long waves. Part 1. Linear dynamics

The local linear stability of forced, stationary long waves produced by topography or potential vorticity (PV) sources is examined using a quasi-geostrophic barotropic model. A multiple scale analysis yields coupled equations for the background stationary wave and low-frequency (LF) disturbance field. Forcing structures for which the LF dynamics are Hamiltonian are shown to yield conservation laws that provide necessary conditions for instability and a constraint on the LF structures that can develop. Explicit knowledge of the forcings that produce the stationary waves is shown to be crucial to predicting a unique LF field. Various topographies or external PV sources can be chosen to produce stationary waves that differ by asymptotically small amounts, yet the LF instabilities that develop can have fundamentally different structures and growth rates. If the stationary wave field is forced solely by topography, LF oscillatory modes always emerge. In contrast, if the stationary wave field is forced solely by PV, two LF structures are possible: oscillatory modes or non-oscillatory envelope modes. The development of the envelope modes within the context of a linear LF theory is novel.An analysis of the complex WKB branch points, which yields an analytical expres-sion for the leading-order eigenfrequency, shows that the streamwise distribution of absolute instability and convective growth is central to understanding and predicting the types of LF structures that develop on the forced stationary wave. The location of the absolute instability region with respect to the stationary wave determines whether oscillatory modes or envelope modes develop. In the absence of absolute instability, eastward propagating wavetrains generated in the far field can amplify via local convective growth in the stationary wave region. If the stationary wave region is streamwise symmetric (asymmetric), the local convective growth results in a local change in wave energy that is transient (permanent).