Local Oscillation Modes Research Articles

A many-body quantum model is proposed as the mechanism responsible for accelerating rates of heat uptake by oceans as anthropogenic heat inputs rise

Abstract A recent many-body quantum approach to thermal radiation (1) reveals that the energy capacitance of heated water is about twice its heat capacitance, due to energy stored as local hybrid pairs of photons coupled resonantly to local matter excitations, with pair density weighted by mass density. Energy exchange between photons and localised dipole excitations inside water at steady temperature T, occurs at sites where a hybridisation potential within the Hamiltonian couples local oscillation modes to “free” photon modes. Photon modes in both directions are phase retarded at exit sides of a hybrid site. Optically sharp variations with frequency in the index of refraction n(f) and photon density of states follow. Exit spectral intensities at equilibrium temperature T carry quantum information on the energy density of matter excitations coupled to photons, while occupied hybrid sites are the source of all “free” photons and internal spectral intensities. The matter excitations coupled locally to photons are distinct from those that define specific heat, but in water both arise from molecular oscillations. The energy capacitance CEX(T) of hybrid pairs depends on mass density and is independent of heat capacitance CHT(T) which sets temperature T. CEX(T) is sensitive to T and doubles as a capacitance of energy and of “quantum information”. Expressions for CEX(T) are derived from first principles and applied to water. The density of occupied hybrids is amplified near surfaces by photons reflected off the surface, which create new hybrid pairs, but not extra heat. Energy capacitance CEX(T)+CHT(T) sets dT(t)/dt prior to a new equilibrium state emerging, after input heating rate dQ/dt is switched. Rising atmospheric intensities then raise T, dQ/dt, CEX(T) and CHT(T). The times available to cool overnight are fixed, so that a rising capacitance, adds to energy stored at an accelerating rate. 


A numerical study on the oscillatory dynamics of tip vortex cavitation

In this paper, we numerically study the mechanism of the oscillatory flow dynamics associated with the tip vortex cavitation (TVC) over an elliptical hydrofoil section. Using our recently developed three-dimensional variational multiphase flow solver, we investigate the TVC phenomenon at Reynolds number $Re = 8.95 \times 10^5$ via dynamic subgrid-scale modelling and the homogeneous mixture theory. To begin, we examine the grid resolution requirements and introduce a length scale considering both the tip vortex strength and the core radius. This length scale is then employed to non-dimensionalize the spatial resolution in the tip vortex region, the results of which serve as a basis for estimation of the required mesh resolution in large eddy simulations of TVC. We next perform simulations to analyse the dynamical modes of tip vortex cavity oscillation at different cavitation numbers, and compare them with the semi-analytical solution. The breathing mode of cavity surface oscillation is extracted from the spatial-temporal evolution of the cavity's effective radius. The temporally averaged effective radius demonstrates that the columnar cavity experiences a growth region followed by decay as it progresses away from the tip. Further examination of the characteristics of local breathing mode oscillations in the growth and decay regions indicates the alteration of the cavity's oscillatory behaviour as it travels from the growth region to the decay region, with the oscillations within the growth region being characterized by lower frequencies. For representative cavitation numbers $\sigma \in [1.2,2.6]$ , we find that pressure fluctuations exhibit a shift of the spectrum towards lower frequencies as the cavitation number decreases, similar to its influence on breathing mode oscillations. The results indicate the existence of correlations between the breathing mode oscillations and the pressure fluctuations. While the low-frequency pressure fluctuations are found to be correlated with the growth region, the breathing mode oscillations within the decay region are related to higher-frequency pressure fluctuations. The proposed mechanism can play an important role in developing mitigation strategies for TVC, which can reduce the underwater radiated noise by marine propellers.

Robust multi-machine power system stabilizer design using bio-inspired optimization techniques and their comparison

This paper reports a comparative study among four bio-inspired meta-heuristic techniques i.e. Sooty-Tern Optimization (STO), Grey Wolf Optimization (GWO), Genetic Algorithm (GA), and Particle Swarm Optimization (PSO) to tune the robust Power System Stabilizer (PSS) parameters of the multi-machine power system. These approaches are successfully tested on two bench-mark systems: sixteen-machine, sixty-eight-bus New England Extended Power Grid (NEEPG) and three-machine, nine-bus Western System Coordinating Council (WSCC). The efficacy of planned PSS via STO and GWO is validated by extensive non-linear simulations, eigenvalue analysis, and performance indices for numerous operating conditions under decisive perturbations, and outcomes are matched with those of GA and PSO techniques. In addition, the robustness is also tested for these algorithms. The results indicate that the PSS design using STO and GWO improves the small-signal stability and damping performance for mitigating inter-area and local area modes of low-frequency oscillations compared to GA and PSO.

Indirect adaptive fuzzy finite time synergetic control for power systems

Introduction. Budget constraints in a world ravenous for electrical power have led utility companies to operate generating stations with full power and sometimes at the limit of stability. In such drastic conditions the occurrence of any contingency or disturbance may lead to a critical situation starting with poorly damped oscillations followed by loss of synchronism and power system instability. In the past decades, the utilization of supplementary excitation control signals for improving power system stability has received much attention. Power system stabilizers (PSS) are used to generate supplementary control signals for the excitation system in order to damp low-frequency oscillations caused by load disturbances or short-circuit faults. Problem. Adaptive power system stabilizers have been proposed to adequately deal with a wide range of operating conditions, but they suffer from the major drawback of requiring parameter model identification, state observation and on-line feedback gain computation. Power systems are nonlinear systems, with configurations and parameters that fluctuate with time that which require a fully nonlinear model and an adaptive control scheme for a practical operating environment. A new nonlinear adaptive fuzzy approach based on synergetic control theory which has been developed for nonlinear power system stabilizers to overcome above mentioned problems. Aim. Synergetic control theory has been successfully applied in the design of power system stabilizers is a most promising robust control technique relying on the same principle of invariance found in sliding mode control, but without its chattering drawback. In most of its applications, synergetic control law was designed based on an asymptotic stability analysis and the system trajectories evolve to a specified attractor reaching the equilibrium in an infinite time. In this paper an indirect finite time adaptive fuzzy synergetic power system stabilizer for damping local and inter-area modes of oscillations for power systems is presented. Methodology. The proposed controller design is based on an adaptive fuzzy control combining a synergetic control theory with a finite-time attractor and Lyapunov synthesis. Enhancing existing adaptive fuzzy synergetic power system stabilizer, where fuzzy systems are used to approximate unknown system dynamics and robust synergetic control for only providing asymptotic stability of the closed-loop system, the proposed technique procures finite time convergence property in the derivation of the continuous synergetic control law. Analytical proofs for finite time convergence are presented confirming that the proposed adaptive scheme can guarantee that system signals are bounded and finite time stability obtained. Results. The performance of the proposed stabilizer is evaluated for a single machine infinite bus system and for a multi machine power system under different type of disturbances. Simulation results are compared to those obtained with a conventional adaptive fuzzy synergetic controller.

Inter-annual oscillations of terrestrial water storage in Qinghai-Tibetan plateau from GRACE data

Abstract Based on multidimensional equivalent water height (EWH) time series in the Qinghai-Tibetan Plateau recovered from GRACE data, rotated multi-channel singular spectrum analysis (RMSSA) was employed to separate and reconstruct its more accurate local mode of inter-annual oscillations of terrestrial water storage (TWS). The results show that RMSSA could effectively suppress the mode mixture of MSSA, and improve the physical interpretation of the inter-annual oscillations of TWS. Three significant inter-annual oscillations with periods of 6.1a, 3.4a, and 2.5a have been found in the multidimensional EWH series in the Qinghai-Tibetan Plateau (QTP), which account for 38.5 %, 23.5 %, and 16.7 % of the total variance, respectively (after the seasonal and long term have been deducted). The spatial patterns and propagation paths of these three inter-annual oscillations are different and exhibit their own independent local characteristics. Based on the analysis of multi-source GRACE GSM data, the results show that the data solution errors have little influence on the extraction of inter-annual oscillations of TWS. The significant 6.4a, 3.5a, and 2.5a inter-annual oscillations are also found in CPC hydrologic model in the QTP using RMSSA, which account for 22.9, 29.9, and 19.3 % of the total variance, respectively. Three inter-annual oscillations separated from GRACE and CPC show similar spatial patterns and significant cross-correlations, respectively. The maximum cross-correlation coefficients are above 0.5 at the 95 % confidence level over 42, 71, and 75 % of the grids in the QTP, respectively. The results indicate that the soil moisture and terrestrial water storage from GRACE have common inter-annual oscillations and corresponding driving factors in the QTP. We conclude that these three inter-annual oscillations of TWS can be explained by the influence of the Arctic oscillation, oceanic Niña, and Indian Ocean dipole.

Intelligent Design of Multi-Machine Power System Stabilizers (PSSs) Using Improved Particle Swarm Optimization

In this paper, an improved version of the particle swarm optimization algorithm is proposed for the online tuning of power system stabilizers in a standard four-machine two-area power system to mitigate local and inter-area mode oscillations. Moreover, an innovative objective function is proposed for performing the optimization, which is a weight function of two functions. The first part of fitness is the function of the angular velocity deviation of the generators, and the other part is a function based on the percentage of undershoot and maximum overshoot, and also the damping time of the power system oscillations. The performance of the proposed stabilization method is compared with the genetic algorithm and bacteria foraging algorithm results. Simulations are made in three different power system operation conditions by changing the system load. The simulation results indicate the superiority of the proposed method over the genetic algorithm and bacteria foraging algorithm. In all the scenarios, power system oscillations are damped faster and with lower amplitude when the power system stabilizers coordinate with the proposed optimization method.

Effective damping of local low frequency oscillations in power systems integrated with bulk PV generation

High penetration of renewable sources into conventional power systems results in reduction of system inertia and noticeable low-frequency oscillations (LFOs) in the rotor speed of synchronous generators. In this paper, we propose effective damping of LFOs by incorporating a supplementary damping controller with a photovoltaic (PV) generating station, where the parameters of this controller are coordinated optimally with those of a power system stabilizer (PSS). The proposed method is applied to damp local electromechanical modes by studying a system comprising a synchronous generator and a PV station connected to an infinite bus. The PV station is modeled following the instructions of the Western Electricity Coordinating Council. The problem is modeled as an optimization problem, where the damping ratio of the electromechanical modes is designed as the objective function. Constraints including upper and lower limits of decision parameters and damping ratio of other modes are considered by imposing penalties on the objective function. Different optimization algorithms are used to pursue the optimal design, such as political, improved gray wolves and equilibrium optimizers. The results validate the effectiveness of the proposed controller with PSS in damping local modes of oscillations.

Dynamic Control of the Interparticle Distance in a Self-Assembled Ag Nanocube Monolayer for Plasmonic Color Modulation

We experimentally demonstrated plasmonic color modulation by dynamically controlling the plasmon resonance excited in an assembled Ag nanocube (AgNC) monolayer. The plasmon resonance wavelength of the hybridization plasmon mode of the local plasmon and collective oscillation plasmon in the Ag nanocube monolayer is sensitive to the interparticle distances. The interparticle distances were continuously controlled with a stretchable and transparent polydimethylsiloxane (PDMS) substrate. Crystalline Ag nanocubes were synthesized through a polyol process. The nanocube shape provides a narrower plasmon resonance band as a result of the small dispersion of electron vibrations. The synthesized nanocubes were densely assembled in a monolayer on the PDMS substrate using the Langmuir–Blodgett (LB) method. The interparticle distance of the Ag nanocube monolayer was controlled from 5.8 to 24.8 nm through biaxial stretching of the substrate from 0 to 20%. The transmitted light color of the Ag nanocube monolayer was actively tuned from magenta to orange to yellow by stretching the substrate up to 20% under random polarization incidence. This color modulation was generated by the plasmon resonance transition from the collective oscillation mode between particles to the local oscillation mode in the isolated nanocubes. Such a plasmonic color modulation is expected to enable a multispectral image sensor with compact and high light utilization efficiency.

Can single photon excitation of two spatially separated modes lead to a violation of Bell inequality via weak-field homodyne measurements?

We reconsider the all-optical weak homodyne-measurement based experimental schemes aimed at revealing Bell nonclassicality (‘nonlocality’) of a single photon. We focus on the schemes put forward by Tan et al (TWC, 1991) and Hardy (1994). In our previous work we show that the TWC experiment can be described by a local hidden variable model, hence the claimed nonclassicality is apparent. The nonclassicality proof proposed by Hardy remains impeccable. We investigate which feature of the Hardy’s approach is crucial to disclose the nonclassicality. There are consequential differences between TWC and Hardy setups: (i) the initial state of Hardy is a superposition of a single photon excitation with vacuum in one of the input modes of a 50–50 beamsplitter. In the TWC case there is no vacuum component. (ii) In the final measurements of Hardy’s proposal the local settings are specified by the presence or absence of a local oscillator field (on/off). In the TWC case the auxiliary fields are constant, only phases are varied. We show that in Hardy’s setup the violation of local realism occurs due to the varying strength of the local oscillators. Still, one does not need to operate in the fully on/off detection scheme. Thus, the nonclassicality in a Hardy-like setup cannot be attributed to the single-photon state alone. It is a consequence of its interference with the photons from auxiliary local fields. Neither can it be attributed to the joint state of the single photon excitation and the local oscillator modes, as this state is measurement setting dependent. Despite giving spurious violations of local realism, the TWC scheme can serve as an entanglement indicator, for the TWC state. Nevertheless an analogue indicator based on intensity rates rather than just intensities overperforms it.

Genetic-TLBO Algorithm for Power System Stabilizer

Background: A large number of nature-based optimization methods have been proposed to use as efficient tools in scientific studies. Genetic Algorithm (GA), which operates based on human genetical evolution, has been an outstanding mostly used solver in a wide range of applications. This algorithm reflects the process of natural selection where the fittest individuals are selected for reproduction in order to produce offspring of the next generation. Initialization, selection, crossover, and mutation are the main parts of the GA population-based method which enables GA to have a prominent explorative feature. On the other hand, the Teaching Learning Based Optimization algorithm (TLBO) is of great performance during searching for the optimum solution among individuals. Therefore, it is expected that the combination of both algorithms in a certain logical way improves the optimization time. Objective: The study intends to determine ways of improving performance of the TLBO algorithm to solve a complex non-linear problem. Power system studies are one of the most complex problems for analysis. Therefore, a powerful heuristic optimization procedure would have a valuable contribution in solving such problems. In addition, the proposed heuristic algorithm will help scientists to apply the technique to their problems. Materials and Methods: According to the aforementioned explanation, a new efficient optimization approach is proposed which optimizes the parameters of multi-machine power system stabilizers (PSSs). The TLBO algorithm includes two different stages in its main structure, which are aptly called teacher and student stages. The student stage of TLBO is replaced by the genetic algorithm in order to improve the explorative feature of the main TLBO. The PSS parameters are obtained for four PSSs which are connected to four generators. Results: The performance of the proposed stabilizer is compared with other formerly designed stabilizers reported in the literature consisting of multi-band PSSs for two areas four-machine power system. Simulation results demonstrate the effectiveness and robustness of the proposed PSS in damping local and inter-area oscillation modes under various disturbances and confirm its superiority in comparison with the other types of PSSs. Conclusion: : A search heuristic method like the genetic algorithm can dramatically improve the performance of meta-heuristic optimization technique. In actuality, the TLBO as a meta-heuristic optimization technique suffers from a direct search of random solutions in its primary stage. Then, the TLBO relinquishes some parts of search space which may restrict the algorithm to find absolute maximums or minimums. In this condition, the GA with a great ability in searching the whole search space effectively improves the TLBO. According to the obtained results, the proposed algorithm, named Genetic-TLBO, obviates the conventional TLBO flaws successfully.

Application of partial eigenvalue assignment techniques to dampen electromechanical oscillations in multi-machine power systems

Abstract In this study, a methodology for partial eigenstructure assignment (PEVA) is applied to dampen electromechanical oscillations in electrical multi-machine power systems. The approach is anchored in allocating a small number of undesirable eigenvalues, for example, which are poorly damped, preserving the other eigenvalues in the system - the so-called no-spillover spectrum. The new position of the selected eigenvalues is carried out based on the partial controllability analysis of the system, in order to minimize the control effort. Simulation examples using a system with 68 buses, 16 generators and five areas showed that the presented methodology is efficient in dampening the local and inter-area oscillation modes when compared to the classic power system stabilizers (PSS). The quality of the solution is illustrated through computer simulations, eigenvalues tables and mode-shapes.

A Unified Online Deep Learning Prediction Model for Small Signal and Transient Stability

This paper proposes a novel unified prediction approach for both small-signal and transient rotor angle stability as opposed to other studies that have only addressed transient rotor angle stability. Deep learning techniques are employed in this paper to train an online prediction model for rotor angle stability (RAS) using the voltage phasor measurements which are collected across the entire system. As a result, the trained model provides a fast yet accurate prediction of the transient stability status when a power system is subjected to a disturbance. Also, if the system is transiently stable, the prediction model updates the power system operator concerning the damping of low-frequency local and inter-area modes of oscillations. Therefore, the presented approach provides information concerning the transient stability and oscillatory dynamic response of the system such that proper control actions are taken. To achieve these objectives, advanced deep learning techniques are employed to train the online prediction model using a dataset which is generated through extensive time domain simulations for wide range of operating conditions. A convolutional neural network (CNN) transient stability classifier is trained to operate on the transient response of the phasor voltages across the entire system and provide a binary stability label. In tandem, a long-short term memory (LSTM) network is trained to learn the oscillatory response of a predicted stable system to capture the step-by-step dynamic evolution of the critical poorly damped low-frequency oscillations. The superior performance of the proposed model is tested using the New-England 10-machine, 39-bus, IEEE 16-machine, 68-bus, 5-area and IEEE 50-machine, 145-bus test systems and is verified with time domain simulation.

Few-mode frequency-modulated LiDAR receivers.

We present a few-mode frequency-modulated receiver for light detection and ranging (LiDAR). We show that using a few-mode local oscillator (LO) with spatial modes at different frequencies at the receiver can significantly improve the performance of the LiDAR detection range. A preferred receiver architecture features LO modes with unequal frequency separations based on optical orthogonal codes (OOC) to allow range detection via cross correlation. The required signal-to-noise ratio (SNR) for the frequency-modulated continuous wave (FMCW) LiDAR decreases with the number of LO modes. This receiver can have a potential impact in the area of automotive LiDARs.

A hybrid modified grey wolf optimization‐sine cosine algorithm‐based power system stabilizer parameter tuning in a multimachine power system

SummaryDamping of low‐frequency oscillations due to the unpredictable perturbations of a power network has always been a challenging task. In an interconnected power network, power system stabilizers (PSSs) are in practice to damp out these low‐frequency oscillations by providing a necessary control signal to the automatic voltage regulator unit based on the deviation in generator speed/power output. This article proposes a novel approach of hybrid modified grey wolf optimization‐sine cosine algorithm for tuning the parameters of PSS of an interconnected multimachine power system. The optimal parameter tuning of PSS with the proposed algorithm is achieved by considering a multiobjective function comprises of improving the damping and eigenvalue characteristics of the consolidated multimachine system. A benchmark model of two area four machine system is adopted to investigate the performance achieved with the proposed algorithm in the simultaneous damping of the local and interarea mode of oscillations in a multimachine power system. The system study has been carried out under a self‐clearing fault condition, and the detailed analysis is presented by analyzing the eigenvalues, and their corresponding natural frequencies, damping ratios. The damping nature achieved for the system states under system uncertainties with the proposed algorithm is also presented. The performance obtained from the proposed hybrid algorithm has been compared with the standalone and state‐of‐the‐art optimization methods.

Engineering NonBinary Rydberg Interactions via Phonons in an Optical Lattice.

Coupling electronic and vibrational degrees of freedom of Rydberg atoms held in optical tweezer arrays offers a flexible mechanism for creating and controlling atom-atom interactions. We find that the state-dependent coupling between Rydberg atoms and local oscillator modes gives rise to two- and three-body interactions which are controllable through the strength of the local confinement. This approach even permits the cancellation of two-body terms such that three-body interactions become dominant. We analyze the structure of these interactions on two-dimensional bipartite lattice geometries and explore the impact of three-body interactions on system ground state on a square lattice. Focusing specifically on a system of ^{87}Rb atoms, we show that the effects of the multibody interactions can be maximized via a tailored dressed potential within a trapping frequency range of the order of a few hundred kilohertz and for temperatures corresponding to a >90% occupation of the atomic vibrational ground state. These parameters, as well as the multibody induced timescales, are compatible with state-of-the-art arrays of optical tweezers. Our work shows a highly versatile handle for engineering multibody interactions of quantum many-body systems in most recent manifestations on Rydberg lattice quantum simulators.

Coordination of PSS and FACTS damping controllers to improve small signal stability of large-scale power systems

This paper presents an approach for designing parameters of power system stabilizer (PSS) and FACTS damping controllers in a large scale practical power system. The objective is maximizing damping ratio of the target mode, and tracking technology (MTT) is used to avoid frequent alternations of target mode in optimization procedures. An improved planted growth simulation algorithm (IPGSA), which has high search efficiency and quick convergence speed, is proposed to optimize controller parameters coordinately. Based on case study of a large-scale power grid, and by using local and interregional low-frequency oscillation modes as target modes, simulation results verify proposed method in this paper. Furthermore, coordination optimization strategy adapted to multi-operating conditions demonstrates that the proposed approach is robust.

Phonon-assisted emission and absorption of individual color centers in hexagonal boron nitride

Defect centers in hexagonal boron nitride represent room-temperature single-photon sources in a layered van der Waals material. These light emitters appear with a wide range of transition energies ranging over the entire visible spectrum, which renders the identification of the underlying atomic structure challenging. In addition to their eminent properties as quantum light emitters, the coupling to phonons is remarkable. Their photoluminescence exhibits significant side band emission well separated from the zero phonon line (ZPL) and an asymmetric broadening of the ZPL itself. In this combined theoretical and experimental study we show that the phonon side bands can be well described in terms of the coupling to bulk longitudinal optical (LO) phonons. To describe the ZPL asymmetry we show that in addition to the coupling to longitudinal acoustic (LA) phonons also the coupling to local mode oscillations of the defect center with respect to the entire host crystal has to be considered. By studying the influence of the emitter’s wave function dimensions on the phonon side bands we find reasonable values for the size of the wave function and the deformation potentials. We perform photoluminescence excitation measurements to demonstrate that the excitation of the emitters is most efficient by LO-phonon assisted absorption.

Study of small signal stability using PSAT including DFIG based wind penetration

In early days of power engineering, the power system consisting of synchronous generators faced different low frequency oscillation problems and they were solved by different researchers by using suitably AVR and PSS. Later, the electricity industry is turning increasingly to renewable sources of energy to generate electricity. Wind is the fastest growing and the most widely utilized emerging renewable energy technology for power generation at present. With the increasing penetration of wind power in the power system, the impact in power system performance should be fully investigated, particularly for doubly fed induction generation (DFIG) wind turbine since this type of renewable source is gaining prominence in the power system industry. Main purpose of this study is to examine the impacts of wind power integration in the low grid from low frequency oscillation perspective. The benchmarked Two Area System is considered for this analysis using Power System Analysis Toolbox (PSAT). The critical modes of oscillation are selected from eigenvalue analysis and their participation factors are studied to identify their sensitivity. Also the time domain analysis is run in every cases to study the transient stability. From TAS, concept of local and global mode oscillation can be understood clearly. Replacing synchronous generators in TAS by DFIG WTG one by one of same rating gave conclusion that low frequency stability depends on the location of DFIG penetration and operating scenario. The results show that there is both beneficial and detrimental effects due to DFIG WTGs. The installation of PSS in the critical generators greatly enhances the system damping.

Generator Coherency and Network Partitioning for Dynamic Equivalencing Using Subtractive Clustering Algorithm

In this paper, a new method called subtractive clustering is presented to partition a power system into areas after a disturbance occurs. Subtractive clustering is basically used as a preprocessing step for other clustering methods to overcome their shortcomings, such as the need to predefine the number of areas and high dependency to random functions. However, due to the special characteristics of power systems, this method itself can be used to find areas. In subtractive method, the degree of coherency between all buses is used to form a density value for each bus. To calculate the degree of coherency between buses, the frequency components existing in the angular velocity variation of voltage phasors, in the range of interarea and local oscillation modes, are extracted. Then, the correlation between the real parts of these frequencies and the correlation between the imaginary parts are calculated individually. This means that the coherency between each pair of buses is assessed in two dimensions, which results in more efficient dynamic coherency identification. The capability of the proposed method is demonstrated for disturbances applied on the 16-machine, 68-bus system. It is observed that subtractive method is highly appropriate to find coherent areas for different disturbances.

Damping Inter-Area Oscillations by Coordination of 3PSS and UPFC Using PSO

Power System Stabilizer (PSS) is one of the most used controllers in the local generations, primarily it aimed to suppress local mode of oscillations. On the other hand, the Unified Power Flow Controllers (UPFC) the most versatile member of flexible alternating current transmission system devices to simultaneously control real and reactive power flows on transmission lines, as well as regulate selected bus voltage. Each of these controllers, on their own, can show satisfactory performance to enhance power system stability. However, when they utilized together, their dynamic performance can degrade due to controller interaction, that should be strategically optimized. In this paper, the coordinated design of pss's and upfc is realized to damp inter-area oscillations in two-area power system using particle swarm optimization (PSO) method. The simulated cases in Matlab environment show that the interaction of pss's and upfc can be optimized, so the inter-area oscillations can be effectively mitigated following after fault, the simulation results of the uncoordinated design are also presented.