Subsynchronous Oscillations In Power Systems Research Articles

A comprehensive localization and assessment method for SSO in power systems with wind power

Subsynchronous Oscillation (SSO) is a common dynamic stability issue in power systems that integrate wind power. This paper proposes a comprehensive method for assessing the localization of SSO in wind power integrated systems. The localization method applies the transient energy flow approach to identify SSO sources in systems with wind power. Additionally, a relevant SSO index system is established where the value of each index is calculated. By using the Fuzzy Inference System (FIS) to judge the index, the SSO disturbance source contribution rating of each area is obtained. Simulation cases demonstrate that the localization method of the transient energy flow approach can effectively identify the SSO disturbance source. Moreover, the SSO disturbance source contribution rating method based on FIS can determine the SSO contribution degree of each area, thus identifying the dominant oscillation disturbance sources. The proposed method can provide valuable insights for formulating emergency control measures.

Measurement-Driven Diagnostics of Mechanism and Source of Subsynchronous Oscillations in Power Systems with Renewable Power Generation

Measurement-Driven Diagnostics of Mechanism and Source of Subsynchronous Oscillations in Power Systems with Renewable Power Generation

Damping sub-synchronous resonance in DFIG wind farms: An innovative controller utilizing wide-area measurement systems

The use of series shunt compensators along with wind energy in power systems has increased significantly. One of the main disadvantages of wind energy and series compensation is the phenomenon of sub-synchronous amplification. In this paper, a control method with fuzzy system based on wide area measurement systems (WAMS) and considering time delay in order to increase the damping of sub-synchronous oscillations in compensated networks with capacitive compensators and wind turbines. The DFIG type is provided in the GSC converter’s voltage control loop for the system’s stable operation. The simulation results using MATLAB/Simulink software on Modified IEEE First Bench Mark test networks show the effectiveness of the proposed method in damping sub-synchronous oscillations in power systems compensated with series capacitors.

Stability analysis of sub‐synchronous oscillation in power system connected with virtual synchronous DFIGs

Concerning sub-synchronous oscillation in power system connected with virtual synchronous DFIGs, an oscillatory stability analysis method based on the dissipation rate of dynamic energy is proposed. First, the dynamic energy function that reflects the sub-synchronous oscillation characteristics of virtual synchronous DFIG is built, and the expression of dynamic energy considering virtual synchronous control parameters is analysed. Then, according to Lyapunov's stability law, the variation rate of dynamic energy function is defined as the energy dissipation rate. The energy dissipation rate reflects the variation trend of system dynamic energy during oscillation, thus it is used as the stability criterion of sub-synchronous oscillation. On this basis, the effects of different virtual synchronous control parameters on sub-synchronous oscillation are analysed for a comprehensive consideration of system stability. Finally, simulation tests are conducted on RT-LAB platform. The results verify that, when the energy dissipation rate is positive, system oscillation diverges; when the energy dissipation rate is negative, system oscillation converges. Meanwhile, the larger the virtual resistance is, the worse the stability of system is; the larger PI control parameters of reactive power control loop are, the worse the stability of system is.

Research on Control Strategy of STATCOM for Suppressing Subsynchronous Oscillation Based on Nonlinear Method

The dq-axis’ current is flowing into the static synchronous compensator (STATCOM) from the power system and the dq-axis’ current reference value form an error signal. After processing the error signal, a trigger pulse is formed to control the turn-on and turn-off of thyristors in STATCOM. At present, various documents generally adopt PI control to process the error signal. But the coefficient of PI control is difficult to set and the response speed of STATCOM under this control is not very satisfactory. In the paper, a STATCOM control strategy based on nonlinear method is proposed. By introducing feedback controlled quantities u 1(t) and u 2(t) into item piq and pid of the STATCOM mathematical model after dq transformation, the aim is to actively control the AC and DC axis current components of STATCOM input system. Based on the principle of nonlinearity, a set of multi-input and multi-output objective functions y 1 and y 2 are found. The dynamic and static performance of the control target is dynamically tracked and constrained by the secondary performance index, thus a closed-loop control system is formed which can quickly reflect the varying reactive power of the system. Finally, PSCAD/EMTDC electromagnetic transient simulation software is used to verify the effectiveness of the STATCOM nonlinear control method of Suppressing sub-synchronous oscillation in power system.

Evaluation Method for Equivalent Models of PMSG-Based Wind Farms Considering Randomness

Power-system sub-synchronous oscillation (SSO) caused by direct-driven permanent magnetic synchronous generator-based wind farms has attracted significant attention from researchers. An equivalent model (EM) is typically used to describe the wind farm dynamics; however, very few works investigated the suitability of EMs for SSO-based research studies. In this paper, a method for evaluating the applicability of EMs to SSO-based phenomena that takes into account wind-power (WP) randomness is proposed. First, EM parameter calculation is modeled as an optimization problem, in which the average Euclidean distance between the corresponding eigenvalues of the detailed and equivalent systems is considered an objective function. The nonlinear relationship between the average Euclidean distance and the injected WP is described by a quadratic function. Further, a Gaussian mixture model is introduced to obtain the distribution function. It is found that the EMs with smaller expectations and variances of the average Euclidean distance can better preserve the dynamics of the real wind farms at random WP fluctuations. The aggregated and output multiplication EMs are considered to illustrate the application scope of the proposed evaluation method. Finally, the feasibility of further equivalence of multiple wind farms in a WP collection region is discussed.

Synchrophasors-Based Identification for Subsynchronous Oscillations in Power Systems

The wide-area measurement system (WAMS) provides the synchronized dynamic measurement for power systems; however, the effective monitoring of subsynchronous oscillations (SSO) with WAMS has not been achieved. To achieve SSO monitoring based on the synchrophasors provided by WAMS, a synchrophasors-based SSO identification technique was proposed in this paper. On the basis of existed synchrophasors, this technique can obtain the frequencies and amplitudes of both the fundamental and subsynchronous components of currents and voltages in SSOs effectively. In this technique, the impacts of both the fast Fourier transform synchrophasor algorithm and the sampling rate of synchrophasors on the spectrum of the subsynchronous component were taken into account. On one hand, the SSO frequency was obtained according to the spectrum of magnitudes of synchrophasors. On the other hand, a correction method for the spectrum of magnitudes of synchrophasors, in contrast to a constructed standard waveform that proportionally matches with the measured SSO, was applied to obtain the actual amplitudes of both the fundamental and subsynchronous components. Both the numerical simulation and the illustrations of two different actual SSO incidents were presented with comparisons to verify the correctness and feasibility of the proposed technique. This technique is of important practical value for mitigations of SSO in actual power systems.

Wide-area monitoring and early-warning of subsynchronous oscillation in power systems with high-penetration of renewables

With the increasing penetration of renewable power generation (RPG) in modern power systems, subsynchronous oscillation (SSO) induced by the interaction between RPG and the grid is posing a great threat to the system stability and equipment safety. To address this issue, this paper proposes a wide-area monitoring and early-warning system of SSO. It incorporates the locally deployed subsynchronous phasor measurement units (S-PMUs) and a centralized data center & subsynchronous dynamics analyzer (DC&SDA). Several key functions have been developed, including: (1) the subsynchronous state estimation (SSE) to form global view or observability of SSO dynamics, (2) the establishment of subsynchronous power & current flow to locate oscillation source/path, and (3) the real-time evaluation of subsynchronous stability using two complementary criteria, one based on the identification of oscillation damping and the other based on the frequency characteristics of the impedance models. The practicality and performance of the proposed system have been demonstrated with electromagnetic transient (EMT) simulations on a practical system that suffered SSO incidents.

Sub-synchronous oscillations in power systems caused by grid-connected wind farms — A survey of mechanism studies

Sub-synchronous oscillation (SSO) incidents caused by dynamic interaction between wind farms and power system grids can threaten safe and reliable operations. This paper investigates mechanism studies of SSOs in power systems induced by grid-connected wind farms. First, characteristics of several severe SSO incidents encountered by real power systems caused by grid-connected wind farms are introduced. Second, in terms of principles and responses, this paper illustrates the most commonly adopted mechanism investigation methods for SSOs, which include impedance-based analysis (IMA) and the open-loop modal analysis (OMA) methods. Systematic reviews that summarize existing studies using the IMA and OMA methods are discussed in this paper. Furthermore, a discussion of the current research gaps in the investigation of SSOs in power systems caused by grid-connected wind farms is presented.

Control of a SMES for mitigating subsynchronous oscillations in power systems: A PBC-PI approach

Abstract This paper proposes a methodology to control the active and reactive power of a superconducting magnetic energy storage (SMES) system to alleviate subsynchronous oscillations (SSO) in power systems with series compensated transmission lines. Primary frequency and voltage control are employed to calculate the active and reactive power reference values for the SMES system, and these gains are calculated with a particle swarm optimization (PSO) algorithm. The proposed methodology is assessed with a classical PI controller, feedback linearization (FL) controller and a passivity-based PI control (PI-PBC). Operating limits for VSC are also considered, which gives priority to active power over reactive power. The IEEE Second Benchmark model is employed to demonstrate the assessment of the proposed methodology where PI-PBC presents better performance than the classical PI and FL controllers in all the operating conditions considered.

Non‐linear dynamics based sub‐synchronous resonance index by using power system measurement data

This study proposes a time-series analysis approach and a non-linear dynamics originated method to detect sub-synchronous oscillation in power systems. Mathematical expressions of the fundamental instantaneous signal and sample discrete signal of peak values are derived to examine the phenomenon of interaction between power system components. The results of the circulating trajectory are shown in a two-dimensional map of the calculated root-mean-square value and estimated Floquet multiplier when two signals of different modes are mixed. Without applying a digital filter or frequency decomposition, non-linear oscillation detection is possible by monitoring a non-linear oscillatory index based on the maximum Lyapunov exponent.

Concept of Modal Repulsion for Examining the Sub-Synchronous Oscillations in Power Systems

This paper investigates the modal repulsion happened in the sub-synchronous resonance between a synchronous generator and series-compensated transmission line. The investigation is based on the closed-loop model of the series-compensated power system consisted of the torsional and electrical open-loop subsystems. Analytical explanation is presented to show how the SSO modal repulsion is formed as the result of moving close and apart between two open-loop SSO modes. Analysis is carried out to indicate that the peak of the SSO modal repulsion occurs when the two open-loop SSO modes from the torsional and electrical subsystem are closest to each other on the complex plane, which is referred to as the open-loop modal coupling. Estimation on the peak, i.e., the maximum damping degradation of the SSO modes, is derived in the paper. Subsequently, a method of open-loop modal analysis is proposed. Applications of the proposed method of open-loop modal analysis to study the SSOs caused by the DFIGs under the condition of the modal repulsion in the power system are demonstrated and evaluated by example power systems. The concept of modal repulsion explains why the grid connection of the DFIGs may bring about the SSO instability risk in the power system.

Application of Synchrosqueezed Wavelet Transforms for Extraction of the Oscillatory Parameters of Subsynchronous Oscillation in Power Systems

The most classical subsynchronous oscillation (SSO) mode extraction methods have some shortcomings, such as lower mode identification and poor anti-noise properties. Thus, this paper proposes a new time-frequency analysis method, namely, synchrosqueezed wavelet transforms (SWT). SWT combines the advantages of empirical mode decomposition (EMD) and wavelet, which has the adaptability of EMD, and improve the ability of anti-mode mixing on EMD and wavelet. Thus, better anti-noise property and higher mode identification can be achieved. Firstly, the SSO signal is transformed by SWT and its time-frequency spectrum is obtained. Secondly, the attenuation characteristic of each intrinsic mode type (IMT) component in its time-frequency spectrum is analyzed by an automatic identification algorithm, and determine which IMT component needs reconstruction. After that, the selected IMT components with divergent characteristic are reconstructed. Thirdly, high-accuracy detection for mode parameter identification is achieved by the Hilbert transform (HT). Simulation and application examples prove the effectiveness of the proposed method.

Alleviation SSR and Low Frequency Power Oscillations in Series Compensated Transmission Line using SVC Supplementary Controllers

In this work, supplementary sub-synchronous damping controllers (SSDC) are proposed for damping sub-synchronous oscillations in power systems with series compensated transmission lines. Series compensation have extensively been used as effective means of increasing the power transfer capability of a transmission lines and improving transient stability limits of power systems. Series compensation with transmission lines may cause sub-synchronous resonance (SSR). The eigenvalue investigation tool is used to ascertain the existence of SSR. It is shown that the addition of supplementary controller is able to stabilize all unstable modes for T-network model. Eigenvalue investigation and time domain transient simulation of detailed nonlinear system are considered to investigate the performance of the controllers. The efficacies of the suggested supplementary controllers are compared on the IEEE first benchmark model for computer simulations of SSR by means of time domain simulation in Matlab/Simulink environment. Supplementary SSDC are considered in order to compare effectiveness of SSDC during higher loading in alleviating the small signal stability problem.

Application of Turbine Torsional Oscillation Damping Controller to Static Var Compensator

This paper proposes a new auxiliary turbine torsional oscillation damping controller (TTODC) for static var compensator (SVC) to dampen out subsynchronous oscillations in power systems containing series compensated transmission lines. A new TTODC algorithm on the basis of synchronized phasor measurements received from phasor measurement units (PMUs) is presented. The idea of using remote signals obtained from PMU to dampen subsynchronous resonance (SSR) presented, too. An auxiliary TTODC is proposed for a SVC, using the generator rotor speed deviation signal as the stabilizing and remote signal to dampen subsynchronous oscillations. The performance of the controller is verified in a detailed nonlinear system considering eigenvalue analysis and transient simulations. Sturdiness of the controller is examined by applying the disturbances in the system that causes significant changes in generator's operating point. The IEEE second benchmark (SBM) model is used for the analysis and the SVC is simulated using the power system blockset (PSB) in the matlab/simulink environment.

Application of STATCOM in Eliminating Subsynchronous Resonance in Power Systems

In this paper, a STATCOM is used to damp the subsynchronous oscillation in power systems with long transmission lines compensated by series capacitors. By using a new auxiliary subsynchronous damping controller (SSDC), the STATCOM connected at the transmission line is able to damp the subsynchronous resonance (SSR) caused by disturbances. The torsional modes characteristics of the system are investigated in this paper and a new design method is proposed for the SSDC based on a nonlinear optimization to suppress the torsional modes in a wide range of operating conditions. A proper stabilizing signal is chosen for the SSDC input in order to modulate the reactive current reference of STATCOM in the torsional modes. The IEEE second benchmark is used to study the SSR. The eigenvalue and FFT analysis as well as the transient simulation are used to demonstrate the ability of STATCOM with the proposed SSDC in damping the SSR.

Application of subsynchronous damping controller (SSDC) to STATCOM

In this paper a novel supplementary subsynchronous damping controller (SSDC) is proposed for the STATic synchronous COMpensator (STATCOM) which is capable of damping out subsynchronous oscillations in power system with series compensated transmission lines. An auxiliary subsynchronous damping controller (SSDC) for a STATCOM using the generator rotor speed deviation signal as the stabilizing signal has been designed to damp subsynchronous oscillations. Eigenvalue analysis and transient simulations of detailed nonlinear system are considered to investigate the performance of the controller. Robustness of the controller has been analyzed by facing the system with disturbances leading to significant changes in generator operating point. This paper deals with a cascaded multilevel converter model, which is a 48-pulse (three levels) source converter. The IEEE Second Benchmark (SBM) model is considered for the analysis and the complete digital simulation of the STATCOM within the power system is performed in the MATLAB/Simulink environment.

A new control strategy of SMES for mitigating subsynchronous oscillations

This paper proposes a new strategy to mitigate the subsynchronous oscillations in power systems compensated by series capacitors via control of active power of superconducting magnetic energy storage (SMES) unit. The strategy is based on the generator acceleration signal. So, the SMES absorbs or generates the energy imbalance caused by different disturbances in the power system and suppresses the subsynchronous oscillations. The chaotic optimization algorithm (COA) is used to achieve the optimal parameter of the proposed controller. To validate the capability of the SMES in damping oscillations, some simulations with different disturbances are performed on the first model of IEEE second benchmark model. All the simulation results show that the subsynchronous resonance as well as low frequency oscillation (LFO) is satisfactorily mitigated by the SMES controlled by the proposed strategy.

Power System Subsynchronous Oscillations Electromagnetic and Electromechanical Transient Hybrid Real-Time Simulation Based on RTDS

In this paper, power system electromagnetic and electromechanical transient hybrid real-time simulation technology is expounded in details, which has advantages of the electromagnetic transient simulation program and electromechanical ones. The more details needed to analyze the dynamic characteristics of power systems are provided by this hybrid simulation technology, and the scale of power system simulated is not limited in the hybrid simulation program. The hybrid simulation program is applied to analyze the power system subsynchronous oscillation problem occurred in a power plant with 4 turbine generators located in the northwestern China. According to the simulation results, it is clear that the stability of generators is threatened by the subsynchronous oscillations caused by capacitor series compensation in the transmission line connecting the power plant and the load center system. In meantime, the effectiveness of the countermeasure is validated simultaneously by the hybrid simulation results.

Damping of subsynchronous oscillations in power system using static synchronous series compensator

In this study, a static synchronous series compensator (SSSC) is used to damp the subsynchronous oscillation in a power system compensated by the series capacitor. In order to achieve an effective damping, a supplementary subsynchronous damping controller (SSDC) is added to the SSSC. The only input signal for the SSDC is the rotor speed deviation to generate the modulation index for controlling the injected voltage of the voltage-sourced converter (VSC). Also, the chaotic optimisation algorithm is employed to tune the parameter of SSDC. The design objective is to suppress the subsynchronous resonance (SSR) caused by the series capacitor. By using the SSDC, the SSSC connected at the transmission line is able to damp the SSR. The first system of IEEE second benchmark model is used to evaluate the effective of SSDC on the torsional oscillations. The several simulations are used to demonstrate the ability of SSDC in damping the SSR.