Power System Low Frequency Oscillation Research Articles

Control Inversion: A Clustering-Based Method for Distributed Wide-Area Control of Power Systems

Wide-area control (WAC) has been shown to be an effective tool for damping low-frequency oscillations in power systems. In the current state of art, WAC is challenged by two main factors, namely, scalability of design and complexity of implementation. In this paper, we present a control design called control inversion that bypasses both of these challenges using the idea of clustering. The basic philosophy behind this method is to project the original power system model into a lower-dimensional state space through clustering and aggregation of generator states, and then designing a linear-quadratic regulator (LQR) controller for the lower-dimensional model. This controller is finally projected back to the original coordinates for wide-area implementation. The main problem is, therefore, posed as finding the projection which best matches the closed-loop performance of the WAC controller with that of a reference LQR controller for damping low-frequency oscillations. We verify the effectiveness of the proposed design using the IEEE 48-machine power system model of the Northeastern Power Coordinating Council.

Analysis and Adaptive Mitigation Scheme of Low-Frequency Oscillations in AC Railway Traction Power Systems

The diffusion of modern electric trains equipped with high-power voltage-source converters has introduced new dynamic interactions with the ac traction power system that may create unexpected low-frequency oscillations (LFOs). This paper presents for the first time a detailed analysis of the operating conditions of electric trains and a traction network, revealing that specific changes in the operating conditions of the train-network system are responsible for the oscillations and may lead to instability. The study is carried out with an analytical dynamic model considering the transient direct current control and focuses on evaluating how the mismatch between the parameters of the controller and the controlled object of the train-network system affects the system dynamics and stability. It is shown that the retuning of controller parameters improves the dynamic performance and the stability of the train-network system, and hence an adaptive mitigation method is proposed to mitigate LFOs adopting a new design of the controller with minimal change and cost to the original system, which provides a simple solution to guide engineering practice in railway traffic. Simulations and experimental results are presented to fully validate the proposed method.

Damping Low-Frequency Oscillations Through VSC-HVdc Stations Operated as Virtual Synchronous Machines

With proper control methods, grid-connected power converters can mimic the dynamics of conventional synchronous generators (SGs), i.e., acting as virtual synchronous machines (VSMs), for improving the grid frequency stability and ensuring a smooth transition toward converter-dominated power systems. However, the integration of VSMs will inevitably influence the low-frequency oscillations (LFOs) in power grids which are usually caused by the interactions among SGs. This influence should be of concern especially when the VSM's capacity is similar to that of SG, e.g., operating voltage-source converter (VSC)-HVdc stations as VSMs. This paper investigates how the VSM affects LFOs in power systems by analyzing its equivalent damping torque. We show that with proper control design, VSMs can provide considerable positive damping torque to effectively damp LFOs in power grids. Particularly, using the damping torque analysis, the influences of virtual impedance and grid frequency detector (i.e., phase-locked loop) are elaborately studied so as to provide design guidelines for better damping LFOs. Finally, case studies of a two-area test system with integration of a VSM-based VSC-HVdc station verify the validity of the analysis.

Data‐driven disturbance source identification for power system oscillations using credibility search ensemble learning

Low-frequency oscillations in power system degrade power quality and may trigger blackouts. This study identifies the source location of these oscillations using measurements from phasor measurement unit (PMU), offline credibility estimation and classification models. The performance of these classification models is ranked for each reported feature to use highly ranked models during the online stage. This proposed framework named as credibility search ensemble learning was tested and validated with promising results using western interconnection power system in North America (WECC-179). The reliability and robustness of the proposed framework were checked against measurement errors in PMUs as well as for practical topology change scenarios. Experimental results and performance comparison with average weight-based approach proved that the proposed approach is capable enough to predict the source location of oscillations with good accuracy. An interfacing tool, for MATLAB-WEKA, was developed and employed in this work for validation and testing of the proposed approach.

Householder-based Prony Method for Identification of Low-frequency Oscillation in Power System

In this paper, the Prony analysis tool with the software used in the actual operation has a large error. The improved Prony method is used to identify the low-frequency oscillation mode of the power system. The traditional Prony algorithm has the problem of insensitivity to parameter error estimation in low frequency oscillation, which affects the accuracy and accuracy of the least squares calculation. Therefore, this paper proposes a Prony method power system low frequency oscillation method using Householder transform to make the calculation more Accurate and proven to be more efficient and reliable.

Using Effective Generator Impedance for Forced Oscillation Source Location

Locating the sources of forced low-frequency oscillations in power systems is an important problem. A number of proposed methods demonstrate their practical usefulness, but many of them rely on strong modeling assumptions and provide poor performance in certain cases for reasons still not well understood. This paper proposes a systematic method for locating the source of a forced oscillation by considering a generator's response to fluctuations of its terminal voltages and currents. It is shown that a generator can be represented as an effective admittance matrix with respect to low-frequency oscillations, and an explicit form for this matrix, for various generator models, is derived. Furthermore, it is shown that a source generator, in addition to its effective admittance, is characterized by the presence of an effective current source, thus giving a natural qualitative distinction between source and nonsource generators. Detailed descriptions are given of a source detection procedure based on this developed representation, and the method's effectiveness is confirmed by simulations on the recommended testbeds (e.g., WECC 179-bus system). This method is free of strong modeling assumptions and is also shown to be robust in the presence of measurement noise and generator parameter uncertainty.

Damping Degradation of Power System Low-Frequency Electromechanical Oscillations Caused by Open-Loop Modal Resonance

This paper investigates the damping degradation of power system low-frequency electromechanical oscillations caused by the open-loop modal resonance. The investigation is based on a closed-loop interconnected model of a multimachine power system, wherein, the electromechanical dynamics of a synchronous generator being examined and the remainder of power system are modeled as two separate open-loop subsystems. The open-loop modal resonance is the closeness of an open-loop electromechanical oscillation mode of the synchronous generator to an open-loop electromechanical oscillation mode of the power system on the complex plane. Theoretical analysis indicates that when the open-loop modal resonance happens, the dynamic interactions between the synchronous generator and the remainder of power system are strong and degrade the damping of the low-frequency electromechanical oscillations. Test results in two example power systems show that the synchronous generator geographically far away took part in the local low-frequency electromechanical oscillations when the open-loop modal resonance occurred. The macroscopic effect of the open-loop modal resonance was the expansion of area on the complex plane, where the electromechanical oscillation modes were located, when the scale of the power systems increased.

Power System Stabilizer PSS4B Model for Iraqi National Grid using PSS/E Software

To damp the low-frequency oscillations which occurred due to the disturbances in the electrical power system, the generators are equipped with Power System Stabilizer (PSS) that provide supplementary feedback stabilizing signals. The low-frequency oscillations in power system are classified as local mode oscillations, intra-area mode oscillation, and interarea mode oscillations. Double input multiband Power system stabilizers (PSSs) were used to damp out low-frequency oscillations in power system. Among dual-input PSSs, PSS4B offers superior transient performance. Power system simulator for engineering (PSS/E) software was adopted to test and evaluate the dynamic performance of PSS4B model on Iraqi national grid. The results showed that after installing the PSS in a specific plant the oscillation of rotor angle, bus frequency, speed, power flow is better than without PSS during the disturbances that occurred during the simulations. All the PSS/E simulation and tests were done in the National dispatch center (NDC) laboratory, Ministry of Electricity. 

Implementation of Power System Stabilizer Based on Conventional and Fuzzy Logic Controllers

To damp the low-frequency oscillations which occurred due to the disturbances in the electrical power system, the generators are equipped with Power System Stabilizer (PSS) that provide supplementary feedback stabilizing signals. The low-frequency oscillations in power system are classified as local mode oscillations, intra-area mode oscillation, and interarea mode oscillations. A suitable PSS model was selected considering the low frequencies oscillation in the inter-area mode based on conventional PSS and Fuzzy Logic Controller. Two types of (FIS) Mamdani and suggeno were considered in this paper. The software of the methods was executed using MATLAB R2015a package.
 

Analysis of Power System Low Frequency Oscillation Based on Energy Shift Theory

In this paper, a new method for analyzing low-frequency oscillation between analytic areas based on energy coefficient is proposed. The concept of energy coefficient is proposed by constructing the energy function, and the low-frequency oscillation is analyzed according to the energy coefficient under the current operating conditions; meanwhile, the concept of model energy is proposed to analyze the energy exchange behavior between two generators. Not only does this method provide an explanation of low-frequency oscillation from the energy point of view, but also it helps further reveal the dynamic behavior of complex power systems. The case analysis of four-machine two-area and the power system of Jilin Power Grid proves the correctness and effectiveness of the proposed method in low-frequency oscillation analysis of power system.

Effect of Extended State Observer and Automatic Voltage Regulator on Synchronous Machine Connected to Infinite Bus Power System

This paper presents a robust controller known as Extended State Observer (ESO) in order to improve the stability and voltage regulation of a synchronous machine connected to an infinite bus power system through a transmission line. The ESO-based control scheme is implemented with an automatic voltage regulator in conjunction with an excitation system to enhance the damping of low frequency power system oscillations, as the Power System Stabilizer (PSS) does. The implementation of PSS excitation control techniques however requires reliable information about the entire states, though they are not always directly measureable. To address this issue, the proposed ESO provides the estimate of system states as well as disturbance state together in order to improve not only the damping but also compensates system efficiently in presence of parameter uncertainties and external disturbances. The Closed-Loop Poles (CLPs) of the system have been assigned by the symmetric root locus technique, with the desired level of system damping provided by the dominant CLPs. The performance of the system is analyzed through simulating at different operating conditions. The control method is not only capable of providing zero estimation error in steady-state, but also shows robustness in tracking the reference command under parametric variations and external disturbances. Illustrative examples have been provided to demonstrate the effectiveness of the developed methodology.

A single machine infinite bus power system excitation control design with extended reduced-order observer

This paper presents an extended reduced-order observer (EROO) in order to improve the stability and voltage regulation of a synchronous machine connected to an infinite bus power system through a transmission line. The EROO-based control scheme is implemented with an automatic voltage regulator to enhance the damping of low frequency power system oscillations, so that the deviations in terminal voltage are reduced. The proposed observer reconstructs the unmeasurable states of the system and state of the disturbance model simultaneous to implement full state feedback and using the estimated disturbance state to compensate the system efficiently in presence of external and parameter uncertainties. The closed-loop poles (CLPs) of the system have been assigned by the symmetric root locus technique, with the desired level of system damping provided by the dominant CLPs. The performance of the system is analyzed through simulating at different operating conditions. The control method is not only capable of providing zero estimation error in steady-state, but also shows robustness in tracking the reference command under parametric variations and external disturbances. Illustrative examples have been provided to demonstrate the effectiveness of the developed methodology.

Mode identification of low‐frequency oscillations in power systems based on fourth‐order mixed mean cumulant and improved TLS‐ESPRIT algorithm

Wide area monitoring systems (WAMS) provide effective support for online identification of low-frequency oscillations in power systems. The WAMS signal is sensitive to the surrounding environment, and it contains Gaussian white noise. The Gaussian white noise will produce Gaussian coloured noise through the filter, which will bring some errors to the mode identification. In order to solve the above problem, this study proposes a method to identify the low-frequency oscillation modes of a single-channel measurement of the power system based on the combination of the fourth-order mixed mean cumulant and the improved TLS-ESPRIT (total least square-estimation parameter space rotation invariant technique). The actual signal is replaced by the processed signal using the fourth-order mixed mean cumulant mathematical statistical method, thereby the Gaussian coloured noise is suppressed effectively; then, TLS-ESPRIT algorithm is used to identify the signal, which is used to identify the low-frequency oscillation of each mode. A novel relative change rate of singular value was also introduced in order to avoid the artificially threshold-setting error. The simulation results show that the proposed method has better anti-noise performance and higher accuracy of fitting than other methods reported in literature, besides being easy to be implemented and suitable for performing on-line identification.

Identification method for power system low‐frequency oscillations based on improved VMD and Teager–Kaiser energy operator

An improved variational mode decomposition (VMD) method is introduced into the mode recognition of low-frequency oscillation of power system in this study. First, fast Fourier transform is used to determine the number of VMD components, and the approximate fitting formula of the best balancing parameter α is obtained by a large number of tests. The original signal is decomposed into several modes via improved VMD (IMVD) method. Then, Teager–Kaiser energy operator is applied on the fitting of each component to get the amplitude, frequency, and damping factor of it. By the constructed test signals, the proposed method is compared with the methods of non-parametric VMD, empirical mode decomposition, total least-squares estimation of signal parameters via rotational invariance techniques, and Prony on the performance of mode parameter identification. Results show that the IVMD method effectively overcomes the shortcomings of those methods mentioned above in dealing with mode mixing, noise sequence, and non-stationary signals. Finally, the feasibility of the proposed method in extracting the low-frequency oscillation mode parameters of power system is verified by the simulation signals of the IEEE two-area four-generator power system and the New England 39-bus system.

WAMS measurements pre-processing for detecting low-frequency oscillations in power systems

Processing the data received from measurement systems implies the situation when one or more registered values stand apart from the sample collection. These values are referred to as “outliers”. The processing results may be influenced significantly by the presence of those in the data sample under consideration. In order to ensure the accuracy of low-frequency oscillations detection in power systems the corresponding algorithm has been developed for the outliers detection and elimination. The algorithm is based on the concept of the irregular component of measurement signal. This component comprises measurement errors and is assumed to be Gauss-distributed random. The median filtering is employed to detect the values lying outside the range of the normally distributed measurement error on the basis of a 3σ criterion. The algorithm has been validated involving simulated signals and WAMS data as well.

Synchronization of Low-Frequency Oscillation in Power Systems

This paper presents the well-documented concept of synchronization of low frequency oscillation occurring in power systems and describes the characteristics of sync occurring in basic electrical circuits. The theory of sync, observed in basic circuits, is extended to analyze the dynamic characteristics of low-frequency oscillation in power systems.

The Study of STATCOM-BESS Damping Power System Oscillations based on DTA Method

The method of damping torque analysis (DTA) is applied to STATCOM/BESS (Battery Energy Storage System) of multi-machine power system for damping power system low-frequency oscillation. This paper investigates how the damping torque of energy storage elements produces, delivers and affects modal damping, and it presents the installation and location of ESS, the selection of additional stable control galley and adjusting method of parameter allocation based on DTA. The calculation and emulation results indicate that the DTA method could exactly extend the mechanism of EES damping power system oscillations, test the feasibility of adjusting method in ESS, and finally provide the practical example of large-scale power grid.

Extracting inter-area oscillation modes using local measurements and data-driven stochastic subspace technique

In this paper, a data-driven stochastic subspace identification (SSI-DATA) technique is proposed as an advanced stochastic system identification (SSI) to extract the inter-area oscillation modes of a power system from wide-area measurements. For accurate and robust extraction of the modes’ parameters (frequency, damping and mode shape), SSI has already been verified as an effective identification algorithm for output-only modal analysis. The new feature of the proposed SSI-DATA applied to inter-area oscillation modal identification lies in its ability to select the eigenvalue automatically. The effectiveness of the proposed scheme has been fully studied and verified, first using transient stability data generated from the IEEE 16-generator 5-area test system, and then using recorded data from an actual event using a Chinese wide-area measurement system (WAMS) in 2004. The results from the simulated and recorded measurements have validated the reliability and applicability of the SSI-DATA technique in power system low frequency oscillation analysis.

The design of delay-dependent wide-area DOFC with prescribed degree of stability α for damping inter-area low-frequency oscillations in power system

In this paper, the delay-dependent wide-area dynamic output feedback controller (DOFC) with prescribed degree of stability is proposed for interconnected power system to damp inter-area low-frequency oscillations. Here, the prescribed degree of stability α is used to maintain all the poles on the left of s=−α in the s-plane. Firstly, residue approach is adopted to select input-output control signals and the schur balanced truncation model reduction method is utilized to obtain the reduced power system model. Secondly, based on Lyapunov stability theory and transformation operation in complex plane, the sufficient condition of asymptotic stability for closed-loop power system with prescribed degree of stability α is derived. Then, a novel method based on linear matrix inequalities (LMIs) is presented to obtain the parameters of DOFC and calculate delay margin of the closed-loop system considering the prescribed degree of stability α. Finally, case studies are carried out on the two-area four-machine system, which is controlled by classical wide-area power system stabilizer (WAPSS) in reported reference and our proposed DOFC respectively. The effectiveness and advantages of the proposed method are verified by the simulation results under different operating conditions.

Small‐signal stability of power system integrated with ancillary‐controlled large‐scale DFIG‐based wind farm

In this study, small-signal stability of the power system integrated with ancillary-controlled large-scale doubly fed induction generator (DFIG) based wind farm (WF) is studied. A model which considers grid code requirements and ancillary controllers is presented to indicate important DFIG dynamics for the study. The ancillary control of the WF which aims to improve power system voltage/frequency stability is evaluated from the small-signal stability perspective. It is shown that the ancillary controller deteriorates power system low-frequency oscillations and/or induces new lightly damped oscillation modes especially in a weak grid. The potential risks may restrict the WF to fulfil gird code requirements and threaten small-signal stability of the power system. Meanwhile, this study reveals that wind power penetration level and WF connection impedance are two main factors which affect dynamic interactions between the WF and the power system and thus affect system small-signal stability. The principle of the influence on the small-signal stability with different grid weakness and ancillary control schemes is evaluated in this study by eigenvalue analysis and verified with time-domain simulations.