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At the moment of electric locomotive passing the neutral-section, the voltage and current of traction network may have low-frequency oscillation (LFO), which will seriously affect the safe operation of the railway. Aiming at eliminating the transient LFO, a modeling method of vehicle-grid coupling system based on flux-current theory is presented in this paper. In the model of electric locomotive, the transient current control strategy and magnetic saturation of on-board transformer are considered, and the equivalent model of excitation inductance of on-board transformer is established based on the flux-current theory. Using this model, based on the Middlebrook impedance ratio criterion, the stability region of vehicle-grid coupling system is analyzed under the influence of average saturation of on-board transformer and control parameters of electric locomotive. The simulation results are analyzed to evaluate and verify the proposed modeling method of LFO. This work has the benefits of defining the opening and closing phase angle region of system stability as well as providing guidance to adaptively modify the control parameters to suppress the LFO.

With the increase of photovoltaic penetration, the power-electronic converter (PEC)-interfaced generators with traditional control strategy may cause severe power oscillations after being disturbed due to lack of inertia and damping. The photovoltaic power plant controlled by virtual synchronous generator (VSG) technology, while introducing virtual inertia and virtual damping to improve the stability of the system, will also inherit its oscillation characteristics and cause a new oscillation mode. Aiming at this problem, this paper analyzes the difference between the system low-frequency oscillation (LFO) modes before and after the VSG is connected to the two-area system by the modal analysis. It is pointed out that the PEC-interfaced renewable energy generators based on VSG control technology could change the oscillation mode of the original system. The influence of VSG's main parameters and capacity on system dynamics is analyzed. Finally, a synchrophasor-based inter-area oscillation suppressed comprehensive control strategy is proposed to suppress the LFO of active power on the tie line, and the superiority of the strategy is verified by DIgSILENT /PowerFactory. [ABSTRACT FROM AUTHOR]

The increase in electric power demand pushes the modern power system for more interconnected networks. It leads to a lack of inertia and creates more critical disturbances in the power system. When this oscillation isn’t damped out, it results in cascade tripping. Immediate detection of low-frequency oscillatory modes and their parameters will help the power system operator to act on a particular event without consuming much time. This research paper proposes novel strategies for identifying low-frequency modes using deep learning techniques, and the model can predict the LFO modes in different topologies. This work presents the Long Short-Term Memory Recurrent Neural Network (LSTM-RNN) approach to predict the instantaneous mode oscillatory parameters in the power system. Once the LSTM-RNN model is trained for different power disturbance situations, it can be used for any events associated with the system. Simulation results are verified using two area Kundur systems at various disturbance conditions. The simulations are performed using MATLAB software and python tensor flow library. The results are validated using statistical methods, and it confirms the superior viability and adaptability of the proposed approach in predicting the instantaneous mode parameters.

Virtual inertia and damping control (VIDC) improves the stability of DC microgrid (DC-MG). However, the potential positive feedback aggravates low-frequency oscillation induced by the interaction insides control loops, which is explained and solved in this paper. The multi-timescale impedance modelling framework is established to clarify stability mechanism of VIDC and the low-frequency oscillation of VIDC controlled DC-MG. Control loops of different timescales are visualized as independent loop virtual impedance (LVI) elements to form an impedance circuit considering the constant power load (CPL), rather than an all-in-one impedance as the external dynamic representation of power converters. Concrete impedance analysis is performed on LVI to reveal the impedance-shaping effect of control loops intuitively, the physical impedance nature of control parameters and the interaction among different timescale, which illustrates the stability mechanism of VIDC. The low-frequency oscillation (LC impedance interaction) in voltage- and inertia-loop is elaborated by RLC circuits of LVIs. The potential instability factors, resulting in poor damping against voltage oscillation, are also revealed. Thus, dynamic stability enhancement method is further proposed to compensate for the negative damping caused by positive feedbacks of VIDC and CPL, and super-capacitor is added to alleviate rapid voltage changes. Accordingly, the passivity property of system impedance is strengthened and the stability can be evaluated by Nyquist plot. Finally, the simulation and experiment results have validated the low-frequency oscillation analysis and stability enhancement method. [ABSTRACT FROM AUTHOR]

Low-frequency power oscillations pose serious problems to the operation of Distributed Generations (DGs) equipped with droop control. This paper presents a new hybrid power-sharing control for damping low-frequency power and current oscillations in DC microgrids. The proposed control consists of a decentralized droop control together with a centralized average current sharing control. A small-signal equivalent model is analytically derived for the proposed hybrid control and is incorporated into the DC microgrid model. The parameters of the PI controllers involved in the average current sharing control units are tuned individually to achieve a satisfactory performance in a stand-alone operation of DGs with a local load. The contributions of control signals contained in the hybrid control signal to the damping characteristic are analyzed. The dynamic response of the DC microgrid under a variety of disturbance scenarios is investigated. The power-sharing accuracy of the proposed control is also examined under the investigated scenarios. The method performance is also investigated for the oscillations caused by constant-power loads. Sensitivity analyses are conducted to evaluate the impact of the loading level and line parameters on the damping ratio associated with the dominant low-frequency modes. The effectiveness of the control method is examined in case of losing a number of communication links. The impact of the delay in the communication links on the method performance is also examined. The simulations are conducted on a multi-source DC microgrid, and the results are verified in the OPAL-RT real-time simulator. [ABSTRACT FROM AUTHOR]

The occurrence of low-frequency electromechanical oscillations is a major problem in the effective operation of power systems. The scrutiny of these oscillations provides substantial information about power system stability and security. In this paper, a new method is introduced based on a combination of synchrosqueezed wavelet transform and the stochastic subspace identification (SSI) algorithm to investigate the low-frequency electromechanical oscillations of large-scale power systems. Then, the estimated modes of the power system are used for the design of the power system stabilizer and the flexible alternating current transmission system (FACTS) device. In this optimization problem, the control parameters are set using a hybrid approach composed of the Prony and residual methods and the modified fruit fly optimization algorithm. The proposed mode estimation method and the controller design are simulated in MATLAB using two test case systems, namely IEEE 2-ar-ea 4-generator and New England-New York 68-bus 16-genera-tor systems. The simulation results demonstrate the high performance of the proposed method in estimation of local and interarea modes, and indicate the improvements in oscillation damping and power system stability.

With more renewables integrated into power grids, the systems are being transformed into low inertia power electronic dominated systems. In this situation, the virtual synchronous generator (VSG) control strategy was proposed to deal with insufficient inertia challenge caused by the reduction of synchronous generation. However, as the VSG control method emulates the dynamic behavior of traditional synchronous machines, the interaction between multiple VSG controllers and synchronous generators (SGs) may cause low-frequency oscillation similar to that caused by the interaction between multiple SGs. This paper reveals that the system low-frequency oscillatory modes are affected by multiple VSGs. Then Prony analysis is utilized to extract the system mode information which will be subsequently used for VSG controller design, and a decentralized sequential coordinated method is proposed to design the supplementary damping controller (SDC) for multiple VSGs. The system low-frequency oscillation is first analyzed based on a modified two-area system with a linearized state-space model, and a further case study based on a revised New England 10-machine 39-bus system is used to demonstrate the effectiveness of the proposed coordinated method for multiple VSGs.

Aiming at the low-frequency oscillation (LFO) phenomenon in high-speed railways, this paper proposes a feedback linearized virtual inertia control (VIC) strategy of the traction converter based on a sliding mode observer (SMO) to improve the robustness of the system and suppress the occurrence of LFO. First, to optimize the total harmonic distortion of the traction grid-side current, the inner loop controller is designed as the feedback linearization control. Then, the voltage outer loop is designed as a combination of the VIC designed in this paper and SMO, which enhances the system inertia and reduces the impact of external interference on the system, while reducing the controller's demand for electrical power on the DC-side. Furthermore, a small-signal model is established to analyze the stability of the single-phase vehicle-grid system under the proposed control. The influence of the control parameters on stability is discussed in detail. Finally, the comparison of software simulation and hardware-in-the-loop experimental results between traditional PI control, feedback linearization control, feedback+SMO, feedback+VIC and the proposed control method shows that the proposed control has better dynamic and static characteristics, and can effectively inhibit the occurrence of LFO. [ABSTRACT FROM AUTHOR]

Electric multiple units (EMUs) have been the cause of low frequency oscillation (LFO) incidents in high-speed railway (HSR) network train systems (NTSs). In this paper a norm criterion with the advantages of simpler calculation while being less conservative than other norm criteria is introduced to analyze the stability of the NTS. A control strategy of EMU four-quadrant converters (4QCs) consisting of ac current passivity-based control (PBC) and dc voltage sliding mode variable structure (SMS) control, i.e. PBC-SMS, is proposed for LFO suppression. The NTS stability analysis with PBC-SMS results in superior critical LFO condition. A single EMU simulation validates the improved steady-state and dynamic performance of the PBC-SMS compared to other conventional methods; NTS simulations based on field tested network parameters verify the superior LFO suppression capability of the proposed method. Finally, guidelines for LFO suppression are recommended based on a comprehensive sensitivity analysis of the traction network, 4QC circuit and controller parameters of an NTS.

LCL filters have been widely used as the interface between inverters and power grids, but damping methods must be taken to solve the resonance of LCL filters. Considering overcurrent protection and inherent active damping, this paper focuses on the inverter-side current feedback (ICF) control. Some literatures have proposed that the capacitor voltage feedforward (CVF) is beneficial to the stability of the ICF control. However, in practical applications, it is found that the ICF control with CVF mainly faces two problems. Firstly, the CVF can be seen as a positive feedback loop, and it tends to cause low-frequency oscillation, especially in weak grid. Secondly, low-frequency harmonics in the grid voltage can easily distort the grid-side current of the inverter, even though the inverter-side current has been regulated well. In order to solve these two problems simultaneously, this paper proposes a novel inverter-side current control method, which adds a high-pass filter into the CVF path. In this paper, the design process of the high-pass filter parameters is analyzed, and the specific voltage feedforward structure of the proposed method to avoid the inverter startup inrush current is also discussed in detail. Simulations in MATLAB/SIMULINK and experiments based on a 6.6 kW prototype have been utilized to verify the effectiveness of the proposed method.

To better damp out the multi-mode oscillations in an uncertain environment, a novel multi-band power system stabilizer (MBPSS) is proposed. Compared with other MBPSSs, the proposed controller has a well-balanced structure, and each band is designed to address a target low-frequency oscillation (LFO) mode. A deep reinforcement learning-enabled agent is developed to effectively tune the control parameters that are adaptive to system uncertainties and different operating conditions. Comparative results with other types of PSSs on the IEEE 68-bus system demonstrate that the proposed method has better performance of damping out LFO and robustness against unseen operating conditions and faults. [ABSTRACT FROM AUTHOR]

The low-frequency oscillations (LFOs) inherent in power systems will be impacted by the increasing penetration of renewable energy sources (RESs). This paper investigates the impact of virtual synchronous machine (VSM) based RESs on the LFOs in power systems. A detailed two-machine test-bed has been developed to analyze the LFOs which exists when VSMs replace synchronous generators. The characteristics of the LFO modes, and the dominant states have been comprehensively analyzed. Furthermore, this study analyzes the LFO modes which exists in an all-VSM grid. The role of the power system stabilizers in the all-VSM grid has been comprehensively evaluated. The IEEE benchmark two-area four-machine system has been employed to corroborate the results of the small-signal analysis, and observe the transient performance. The analysis in this paper have been performed in MATLAB/SIMULINK environment. [ABSTRACT FROM AUTHOR]

Aiming at the high-frequency resonance (HFR) and low-frequency oscillation (LFO) phenomena in high-speed railways, a comprehensive suppression method named voltage constant transient active oscillation compensation (VCT-AOC) is proposed. This method is optimized on the basis of AOC, and the adjustment time is shortened by the VCT suppression measure. When serious HFR happens, the system can be quickly restored and stabilized by completely removing the network voltage oscillation in the control part, thereby avoiding the transient overvoltage and overcurrent damage to the system. According to the characteristics of AOC and VCT, the VCT-AOC comprehensive suppression method is proposed, and the specific implementation scheme of this method is given. This paper established the train-network impedance model with AOC and VCT suppression methods. The Generalized Nyquist Analysis (GNA) method and the dominant pole analysis method are used to analyze the effect of VCT and AOC on the system. Finally, the effectiveness of suppression methods is verified by simulations and experiments.

This paper proposes a fuzzy-observer-based control algorithm to improve the efficiency of traction dual rectifiers of high-speed trains and address the low-frequency oscillation issue induced by the interaction of the DC-link voltage controller with the traction dual rectifiers circuit parameters. The proposed observer checks for the errors in the system model by estimating the system premise variables, and the fuzzy controller then accurately compensates the estimated variables. The method is deduced from the Takagi-Sugeno fuzzy logic model, which is described by a class of IF-THEN fuzzy systems. Fast computation with high precision is what makes them superior to proportional-integral-derivatives controllers. Their capability to deal with nonlinearity in time-varying systems makes them a suitable choice for traction dual rectifier control. The simulation model is formed in SIMULINK, and the waveforms are checked with a conventional fuzzy controller, predictive control, and conventional proportional-integral control. The proposed method can achieve better performance, such as fast transient, more damping, and small overshoot. Furthermore, the method can maintain stability when the system is subjected to external disturbance. In the end, hardware-in-the-loop experiments are performed to validate the proposed control algorithm. [ABSTRACT FROM AUTHOR]

The wide adoption of a new type of electric trains results in multiple frequency-scale instability problems, such as low-frequency oscillation (LFO), harmonic resonance, and harmonic instability. Among them, LFO has been commonly reported at electric railway depots with multiple-energized electric trains around the whole world, thus attracting widespread research attention. LFOs cause adverse effects on the system stability and protection infrastructure of both the traction network and electric trains. This article presents a comprehensive review of LFOs that consider both academic theoretical analysis and industrial engineering requirements. A unified mathematical model of the traction network and electric train is derived with a view to revealing the mechanism and factors that induce LFOs when these two systems interact. The article also draws deep insights from results of experimental measurements and industrial mitigation schemes already implemented in Chinese railway systems. A variety of mitigation techniques and recommendations based on theoretical principles and practical constraints are presented. [ABSTRACT FROM AUTHOR]

The pulse train (PT) controlled switching dc/dc converters possess the benefits of simple structure, fast dynamic response, and good robustness. However, PT controlled converters also give the drawbacks of undesirable low-frequency oscillation phenomenon as operated in continuous conduction mode. In this paper, a single-input dual-output buck converter based on the coupled inductor is newly proposed with the ability to suppress the low-frequency oscillation. The inductor of the conventional buck converter is replaced by the primary side of the coupled inductor, and the secondary side of the coupled inductor connected in series with a diode is paralleled to a capacitor as an equivalent power source for the extra load. By rational allocation of inductance coupling coefficient and the equivalent power supply voltage, low-frequency oscillation of output voltage can be suppressed and the fast response speed is achieved. A hardware prototype of 48 V/16 V converter is built and tested to validate the practicability of the proposed converter.

This paper investigates the high-frequency harmonic instability (HHI) and low-frequency oscillation (LFO) phenomena in high-speed railways, and proposes an active oscillation compensation method to mitigate these HHI and LFO. Such mitigation method introduces the input voltage oscillating components of grid side to the control loop of the grid-side converter, which helps to mitigate the voltage oscillation variables and restore the steady state of the train and traction network (simplified as “train-network”) system. From the perspective of train-network impedance, this paper proposes the train impedance models with and without the mitigation process. The generalized Nyquist analysis method is used to analyze the stability of the system with and without the mitigation. Finally, the mitigation method is verified by simulated and experimental data, to be capable of suppressing both HHI and LFO effectively.

Recently, the low-frequency oscillation (LFO) frequently happens in the vehicle-grid system of high-speed railways when electric multiple units (EMUs) are put into operation. As a possible solution, a single-phase cascaded H-bridge static synchronous compensator (CHB-STATCOM) directly connected to the traction network is applied to dynamically compensate the reactive power of vehicle-grid system. This paper focuses on the impedance modeling and stability analysis in the vehicle-grid system with CHB-STATCOM. Firstly, the vehicle-grid system is simplified to a single-phase rectifier with large grid inductance. The accurate dq-frame impedance of the rectifier system with CHB-STATCOM is derived in detail. Secondly, through the generalized Nyquist stability criterion (GNSC), the influence of CHB-STATCOM and its different control parameters on the stability of rectifier system is revealed. Thirdly, the excellent performance of CHB-STATCOM in suppressing the LFO and the accuracy of theoretical analysis are verified via time-domain simulations. Finally, to further verify the effectiveness of CHB-STATCOM in real applications, a dSPACE based controller hardware-in-the-loop (HIL) simulation platform is established, which includes the physical control circuits of eight EMUs. The experimental results show that the CHB-STATCOM has a good dynamic compensation effect and can efficiently suppress the LFO. [ABSTRACT FROM AUTHOR]

Considering the dynamic characteristics of phase-locked loop (PLL), the effect of interaction between doubly fed induction generator (DFIG) and power grid on system low-frequency oscillation is revealed from the perspective of dissipation energy. First, the dynamic energy model of DFIG with PLL is derived, and the component that varies nonperiodically is extracted and defined as the dissipation energy. Second, the dissipation energy is decomposed into the free dissipation energy and DFIG-grid coupled dissipation energy. On this basis, the free dissipation intensity and coupled dissipation intensity are defined, which, respectively, characterize the immanent damping of DFIG and the DFIG-grid interaction degree. Thus, the contribution degree of DFIG to electromechanical oscillation is quantified for evaluation, and the mechanism of interaction between DFIG and power grid is further revealed. Finally, hardware-in-the-loop simulation tests with real-time digital simulator of the IEEE 10-machine 39-bus system verify that when the oscillation frequency of PLL is close to electromechanical oscillation frequency, strong coupling and resonance exist between DFIG and grid, and negative dissipation effect on the oscillation of DFIG will result. Thus, DFIG, as an oscillator, continuously injects dynamic energy to the grid, causing the system to oscillate to instability. [ABSTRACT FROM AUTHOR]

A reduced-order dynamic phasor model of modular multilevel converter (MMC) in long time scale, which is applied for power system low-frequency oscillation analysis, is investigated. A tenth-order dynamic phasor model is derived in consideration of the internal circulation current dynamic of MMC. The model is verified via electromagnetic transient simulation. Then, an application of the singular perturbation method to MMC model reduction is presented. Separation of eigenvalues is used as a characterization of time scales and small time constant states is eliminated in the full-order model of MMC. MMC high-voltage direct current (HVdc) models with various orders are embedded into the four-machine two-area system to compare the low-frequency oscillation analysis results. The frequency domain analysis and time domain simulations show that the second-order model has satisfactory performance in describing the low-frequency characteristics of MMC. Moreover, the influence of MMC-HVdc transmission power and outer loop controller parameters on low-frequency oscillation is investigated using the second-order model. The effectiveness of reduced-order model in supplementary controller parameter design is also validated by comparing the difference in the response of various-order models to supplementary frequency-based control. [ABSTRACT FROM AUTHOR]

A new generator phasor measurement unit can provide a generator internal response during transient periods, thereby enabling online evaluation of generator damping in detail. In this paper, a method for the online detailed evaluation of the generator damping contribution during oscillation periods based on the damping loss factor (DLF) and multi-scale chirplet sparse signal decomposition (MCSSD) is proposed. First, the DLF in a simple elastic oscillation system, which can be used to quantitatively evaluate the damping characteristics of components, is introduced. After analyzing the feasibility of using the DLF during low-frequency oscillation, the generator damping loss factor (GDLF) is established to quantitatively evaluate the generator damping contribution, including the effects of the excitation system, governor system, and generator winding. Next, the modal composition of the generator dissipation energy is studied to extend the application of the GDLF to a multi-machine power system. Finally, a method for the online evaluation of generator damping during low-frequency oscillation based on the GDLF and MCSSD is presented. The proposed method is examined in detail using a single-machine infinite-bus system and then further demonstrated by applying it to a practical power system with multiple electromechanical oscillations. [ABSTRACT FROM AUTHOR]

Instability and oscillation issues have frequently occurred in high-speed railways due to the impedance mismatch versus frequency between the four-quadrant converter (4QC)-based high-speed train and the traction network (hereinafter train–network). However, solely utilizing the mathematical deduction to quantify the impedances seems to be difficult owing to unknown detailed parameters of both traction network and electric train. This paper proposes a method to measure the equivalent impedances of the traction network and the 4QC of the electric train in the stationary frame for stability analysis. A disturbance circuit consisting of antiparallel insulated gate bipolar transistor (IGBT) modules and an excitation load is adopted by means of the pulsewidth modulation (PWM) signal to drive the IGBTs. Consequently, a desired broad spectral excitation is then generated to measure the output impedance of the traction network. When injecting the harmonics twice, which are linearly independent at the same frequency, the input impedance of the 4QC can be calculated at the corresponding frequency considering the frequency-coupled effect. The proposed method shows a good measurement accuracy. Additionally, the stability and oscillation issues of the train–network system can be then identified using measured impedances. Both simulation and experimental results validate the effectiveness of the proposed measurement method. [ABSTRACT FROM AUTHOR]

The low-frequency oscillation (LFO) phenomenon has frequently happened in the railway electrification systems (RES), which often caused the traction atresia of electric trains and protection malfunction of traction network. In this letter, a practical mitigation approach is presented based on impedance analysis, which indicates that changing the operation mode of the electric trains to diode-bridge rectification (DBR) circuit can mitigate the LFO because the impedance of the rectifier is increased. Then, combining the Prony technology, an online identification and mitigation approach, is proposed here for mitigating the LFO phenomenon. Experimental results verify the effectiveness of the proposed method in stabilizing voltage fluctuation of the RES. This method can also identify the oscillatory frequency and the corresponding attenuation factor. [ABSTRACT FROM AUTHOR]

Equivalent inertia of the power system is reducing with the increasing wind power penetration. To alleviate this issue, more and more countries have put forward grid codes that require wind farms (WFs) to provide inertial response as traditional synchronous generation does. It is found in this paper that the synthetic inertial control of the WF in a weak grid deteriorates drive-train torsional oscillation of wind turbine generators (WTGs). Furthermore, the WF emulating inertial response is involved in power system oscillations and induces a new critical low-frequency oscillation. The above oscillations threaten small-signal stability of the WF and the power system, which restricts the WF to fulfill grid code requirements. To overcome this problem, this paper proposes a two-level damping control scheme consisting of a WTG level torsional oscillation damping control and a WF level low-frequency oscillation adaptive damping control to enhance damping characteristic of the WF. Simulation results show that the two-level damping control can simultaneously counteract these concerned multiple oscillations under various operating conditions, which facilitates the WF to meet grid code requirements, and improves dynamic stability of the WF and the power system. [ABSTRACT FROM AUTHOR]

The performance of the Forward and Backward Extended Prony (FBEP) method on power system small-signal stability analysis is investigated in this paper. The effectiveness of the FBEP method in identifying system eigenvalues from the output signal is validated by experiments on a test system model. Using a four-machine-two-area power system model the estimation of the dominant modes contained in oscillatory signals given by the FBEP method is compared with that given by the SVD-TLS method, Trudnowski's algorithm, and the Prony with stepwise regression method. The multi-signal analysis and sliding window analysis using the FBEP method are also studied. [ABSTRACT FROM PUBLISHER]

This paper presents a literature overview of all techniques proposed until the submission of this paper in terms of mitigating power oscillation in single-phase applications. This pulsating energy is the major factor for increasing the size of passive components and power losses in the converter and can be responsible for losses or malfunctioning of the dc sources. Reduction of power ripple at twice the fundamental frequency is one of the key elements to increase power converter density without lack of dc stiffness. Pulsation reduction is achieved by incorporating control techniques or auxiliary circuitries with energy storage capability in reactive elements to avoid this oscillating power to propagate through the converter, creating what is called as single-phase power decoupling. The topologies are divided as: rectifiers, inverters, and bidirectional. Among them, it is possible to classify as isolated and nonisolated converters. The energy storage method may be classify as: capacitive and inductive. For the power decoupling technique, it is convenient to divide as control and topology. The power decoupling technique may be implemented as series or parallel with respect to the ac, dc or link side. This paper represents the best reference on this topic. [ABSTRACT FROM AUTHOR]

This paper investigates a control method to significantly reduce the low-frequency current oscillation for six-step operation of a three-phase inverter. The six-step operation of an ac electric machine drive system under high speeds has a few advantages compared with pulse width modulation (PWM) operation including reduced switching losses, a better utilization of the dc bus voltage, and an enhanced speed capability. In digital implementation of the six-step control, power electronics may not be able to switch on and off exactly at the zero crossing points of reference voltages, which will result in a dc offset in phase voltages and cause a low-frequency oscillation in phase currents. The low-frequency oscillation will create issues such as higher power losses, lower efficiency, and excessive heat that may demagnetize magnets on the rotor for a permanent magnet synchronous machine. This research presents a control algorithm using asymmetric PWM to ensure that power electronics switch on and off exactly at the reference voltage zero crossing points. Both simulation and experimental results demonstrate the algorithm can reduce the low-frequency oscillation in phase currents by more than 90%. [ABSTRACT FROM AUTHOR]

A number of methods have been proposed and implemented to improve system damping against low-frequency electromechanical oscillations in the power system. Among these, flexible ac transmission systems (FACTS) can be used to provide power oscillation damping (POD) function to the power system. The design of the POD algorithms in these devices requires estimation of the power oscillation frequency components and this is mostly achieved through the use of various filter combinations. However, these filter-based solutions are characterized by low bandwidth to extract the required signal components accurately, and this limits the dynamic performance of the FACTS controllers. Moreover, the filters are designed for specific frequencies, and a change in the system would reduce the performance of the methods. Thus, there is a need for a better estimation algorithm with fast and selective estimation of the required signal that is robust against system parameter uncertainties. In this paper, this is achieved by the use of a recursive least square algorithm that uses variable forgetting factor and a frequency adaptation mechanism. The investigated method has fast estimation in transient conditions without compromising its selectivity in steady state. The effectiveness of the proposed method is proven through simulation as well as experimental verification. [ABSTRACT FROM AUTHOR]

Low-frequency inter-area oscillation is a potentially dangerous phenomenon in power systems involving exchange of power among generators in different areas. In this paper, we study the characteristics of inter-area oscillation by analyzing the distribution of the oscillation energy. First, we introduce equations for the kinetic energy of generators and the potential energy of network branches, derived from the linearization of system dynamic equations. Then, we study the mode composition and the distribution of the generators' kinetic energy during low-frequency oscillations. A modal kinetic energy participation factor is proposed for evaluating the participation of each generator in the oscillation. The relationships among electromagnetic power oscillation distribution, rotor speed deviation, and generator kinetic energy are analyzed and compared to their modal parameter left eigenvectors, right eigenvectors, and linear participation factors, respectively. The bus voltage angle deviation, active power oscillation, and branch potential energy are calculated using the right eigenvector from modal analysis. The distribution characteristics of voltage angle oscillation and branch potential energy in inter-area oscillation is developed. The simulation results for a four-generator two-area system and the New England 10-machine 39-bus system illustrate the feasibility and validity of the proposed theory and method for analysis of inter-area oscillations. [ABSTRACT FROM AUTHOR]

The line-side converter, which can represent an ac–dc–ac drive electric vehicle when analyzing the low-frequency oscillation in electric railways, is modeled in dq frame with the proposed single-phase dq-decomposition method. With this model, an underdamped mechanism of the vehicle-grid interaction is confirmed. Besides, a low-frequency stability prediction approach is presented, based on the dominant-pole analysis and verified by the simulation and the testing result. The proposed modeling method and prediction approach benefit in three aspects: It is more convenient to rebuild the model with a change of either the line-side converter controller or the system condition, much quicker to predict the low-frequency instability and more explicit to guide the modification of the controller software to damp the low-frequency oscillations. [ABSTRACT FROM AUTHOR]

This paper deals with the design of an adaptive power oscillation damping (POD) controller for a static synchronous compensator (STATCOM) equipped with energy storage. This is achieved using a signal estimation technique based on a modified recursive least square (RLS) algorithm, which allows a fast, selective, and adaptive estimation of the low-frequency electromechanical oscillations from locally measured signals during power system disturbances. The proposed method is effective in increasing the damping of the system at the frequencies of interest, also in the case of system parameter uncertainties and at various connection points of the compensator. First, the analysis of the impact of active and reactive power injection into the power system will be carried out using a simple two-machine system model. A control strategy that optimizes active and reactive power injection at various connection points of the STATCOM will be derived using the simplified model. Small-signal analysis of the dynamic performance of the proposed control strategy will be carried out. The effectiveness of the proposed control method to provide power oscillation damping irrespective of the connection point of the device and in the presence of system parameter uncertainties will be verified through simulation and experimental results. [ABSTRACT FROM PUBLISHER]