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Practical testing of distance relays under power swing conditions is becoming all the more essential. This is in view of the increasing use of power electronic apparatus due to penetration of renewable sources and/or FACTS controllers now-a-days. To perform such practical tests, a hardware platform emulating power system swing occurrences, as caused by electromechanical oscillation of alternators, is necessary. As a first step it is required to model the phenomenon before its implementation. In this paper an analytical small-signal model of the low-frequency electromechanical oscillations in a grid-connected alternator is presented. Off-line simulation-based validation for a Single Machine Infinite Bus (SMIB) system is done. To emulate an open-loop alternator, a prototype of a controlled grid synchronised VSI with AC-side reactor is developed and practically tested. A control scheme is proposed and implemented to emulate the active power swing phenomenon. Effects of machine inertia and initial power angle on the swing characteristics are investigated. In-process estimation of apparent impedances, as seen from the distance relay terminals downstream towards the grid, is done. Principle of estimating the apparent impedance during power swing is presented with corresponding experimental results. Both real-time and off-line simulation results are in close agreement with the experimental results thus establishing the accuracy of the model and the precision of the experiments. [ABSTRACT FROM AUTHOR]

In order to suppress the low frequency oscillation of the neutral-point voltage for three-level inverters, this paper proposes a new discontinuous pulse width modulation (DPWM) control method. The conventional sinusoidal pulse width modulation (SPWM) control has no effect on balancing the neutral-point voltage. Based on the basic control principle of DPWM, the relationship between the reference space voltage vector and the neutral-point current is analyzed. The proposed method suppresses the low frequency oscillation of the neutral-point voltage by keeping the switches of a certain phase no switching in one carrier cycle. So the operating time of the positive and negative small vectors is equal. Comparing with the conventional SPWM control method, the proposed DPWM control method suppresses the low frequency oscillation of the neutral-point voltage, decreases the output waveform harmonics, and increases both the output waveform quality and the system efficiency. An experiment has been realized by a neutral-point clamped (NPC) three-level inverter prototype based on STM32F407-CPLD. The experimental results verify the correctness of the theoretical analysis and the effectiveness of the proposed DPWM method. [ABSTRACT FROM AUTHOR]

Power System Stabilizer (PSS) was proposed during 1960s to solve the low frequency oscillation problem raised by the wide application of the high-gain fast-response exciters. The fundamentals of PSS design lie in the angle compensation to increase the damping torque, which, since then, has become an important principle in designing the various power system dampers, such as SVC, TCSC, UPFC. Although many papers have been dedicated to the application of this principle, it is interesting to note that in the real industry applications PSS parameters have to be carefully tuned on site in spite of its mature design theory. So does the classical PSS design theory really meet the PSS design demand? By combining the frequency domain and the time domain analysis, this paper reinvestigates the basic idea behind the classical PSS design theory. The paper clarifies the concepts of the synchronous torque as well as the damping torque and proves that the classical PSS design principles based on these concepts are not theoretically sound. Then the paper discusses the Linear Optimal Controller Design method and analyzes its relations with the conventional PID design. By doing so the paper reveals the real mechanism of the PSS and proposes to use more systematic and advanced control tools to enhance the controller performance. [ABSTRACT FROM AUTHOR]

In this paper, a novel method that integrates the improved empirical mode decomposition (EMD) and signal energy algorithm is proposed to estimate the dominant oscillation parameters and corresponding mode shape. Firstly, the EMD with symmetrical extrema extension (SEE) is utilized to decompose the measured data from wide area measurement system (WAMS) into a finite set of intrinsic mode functions (IMFs). Then, the signal energy algorithm is used to calculate the approximate oscillation parameters of the IMFs. The nodes involved the dominant oscillation mode are classified based on the calculated frequency and reasonable threshold. Furthermore, for the dominant oscillation mode, the IMF with maximum mean amplitude is defined as the reference. Next, the relative phases (RPs) between the reference IMF and other IMFs are calculated in order to identify the negative and positive oscillation groups. According to the values of RPs, the coherent group and corresponding node contribution factor (NCF) can be identified, and the dominant approximate mode shape (AMS) can also be determined. The efficiency of the proposed approach is tested by applying it to synthetic signal and measured data from the simulation model. [ABSTRACT FROM AUTHOR]

The advance in the wide-area measurement system (WAMS) is driving the power system to the trend of wide-area monitoring and control. The Prony method is usually used for low frequency oscillation online identification. However, the identified amplitude and phase information is not sufficiently used. In this paper, the amplitude is adopted to detect the occurrence of the oscillation and to obtain the mode observability of the sites. The phase is adopted to identify the oscillation generator grouping and to obtain the mode shapes. The time varying characteristics of low frequency oscillations are studied. The behaviors and the characters of low frequency oscillations are displayed by dynamic visual techniques. Demonstrations on the '11.9' low frequency oscillation of the Guizhou Power Grid substantiate the feasibility and the validation of the proposed methods. [ABSTRACT FROM AUTHOR]

The low frequency oscillation in both hemispheres and its possible role in the dust weather storm events over North China in 2002 are analyzed as a case study. Results show that the Aleutian Low is linked with the Circumpolar Vortex in the Southern Hemisphere on a 30–60-day oscillation, with a weak Circumpolar Vortex tending to deepen the Aleutian Low which may be helpful for the generation of dust storm events. The possible mechanism behind this is the inter-hemispheric interaction of the mean meridional circulation, with the major variability over East Asia. The zonal mean westerly wind at high latitudes of the Southern Hemisphere in the upper level troposphere may lead that of the Northern Hemisphere, which then impacts the local circulation in the Northern Hemisphere. Thus, the low frequency oscillation teleconnection is one possible linkage in the coupling between the Southern Hemisphere circulation and dust events over North China. However, the interannual variation of the low frequency oscillation is unclear. [ABSTRACT FROM AUTHOR]

The heat distributions in the upper layers of the ocean have been studied and some important low frequency oscillations (LFOs) are already found and quantified by using various characteristic factors. In this paper, the ‘heat center’ of a sea area is defined with a simple method. Then the temperature data set of the upper layer of the global ocean (from surface down to 400 m, 1955–2003) is analyzed to detect the possible LFOs. Not only some zonal LFOs, which were reported early, but also some strong LFOs of the vertical and meridional heat distribution, which might imply some physical sense, are detected. It should be noted that the similar vertical oscillation pattern can be found in the Pacific Ocean, Atlantic Ocean and Indian Ocean. Results from some preliminary studies show that the vertical LFO might be caused by the solar irradiance anomalies. This study may help reveal some unknown dynamical processes in the global oceans and may also benefit other related studies. [ABSTRACT FROM AUTHOR]

The mid—Holocene climate about 6000 years ago was simulated by using the atmospheric general circulation model. The orbital parameters for 6 ka BP (before present) were prescribed and other forcing factors were set in the modern conditions. Results show that the large—scale climate change in the African—Asian monsoon areas during the summer—time is strongly compared to the present climate, while the changes in other seasons and regions are generally weak. The results also revealed the change of the low frequency oscillation in the atmosphere. [ABSTRACT FROM AUTHOR]

An equatorial Β-plane model which includes realistic non-uniform land-sea contrast and the underlying surface temperature distribution is used to simulate the 30-60 day oscillation (LFO) processes in tropical atmosphere, with emphasis on its longitude-dependent evolution and convective seesaw between Indian and the western Pacific oceans. The model simulated the twice-amplification of the disturbances over Indian and the western Pacific oceans while they are travelling eastward. It reproduced the dipole structure caused by the out-of-phase oscillation of the active centres in these two areas and the periodical transition between the phases of LFO. It is suggested that the convective seesaw is the result of interaction of the internal dynamics of tropical atmosphere with the zonally non-uniform thermal forcing from underlying surface. The convective activities are suppressed over Indonesia maritime continents whilst they are favoured over the Indian Ocean and western Pacific warm waters, so there formed two active oscillation centres. The feedback of convection with large-scale flow slows down the propagation of disturbances when they are intensifying over these two areas, therefore they manifest a kind of quasi-stationary component to favor the 'dipole' structure. Whereas the disturbances weaken and speed up over the eastern Pacific cold water region due to the interaction of sensible heating and evaporation with perturbational wind. Therefore the two major centers just show out-of-phase oscillation during one cycle around the latitudinal belt. By introducing the SST anomalies in El Niño and La Niña years into the surface temperature, we also show that they have significant influence on LFO processes. In an anomalously warm year, the LFO disturbances dissipate more slowly over the central-eastern Pacific region and can travel further eastward; whilst in an anomalously cold year, the opposite is true. [ABSTRACT FROM AUTHOR]

Multichannel singular spectrum analysis (MSSA) has been applied to 5-day mean grid point data of 500 hPa geopotential heights over the Northern Hemisphere, to explore low frequency oscillatory variabilitiesover the region. Two major low-frequency oscillations with periods of 60--70 and 30--40 days were found. These two quasi-periodicities are significantly (at the 99% confidence level) different from those found in the corresponding pure red noise process. Also, they were reproducible when using different window lengths for MSSA and when applying MSSA to two equal-length subsets of the basic data series. In addition, when one subset (first subset) is projected onto the patterns of the other subset, the resulting (projection) spatial-temporal principal components corresponding to the two oscillations are highly correlated with those ST-PCs directly derived from the first subset by MSSA, indicating that the oscillatory components obtained from one subset explain a significant amount of variance in the other subset. Somereliable information about the amplitudes of the oscillations has been also obtained by using composite analysis. [ABSTRACT FROM AUTHOR]