Subsynchronous Resonance Studies Research Articles

A Review on Sub-Synchronous Resonance Damping with Thyristor Controlled Series Compensators

Over many years, the power industry has used series compensation with fixed series capacitors for long-distance ac power transmission. With the availability of Thyristor Controlled Series Compensators (TCSC), utilities have the option of using them instead of fixed series capacitors to exploit the advantage of their flexibility and controllability. The use of TCSCs for damping electro-mechanical oscillations and Sub-Synchronous Resonance (SSR) has been investigated and reported over the years. This paper presents a review of those techniques to damp SSR problems associated with conventional multi-mass turbine generator systems in series compensated networks. This paper also demonstrates that SSR can be damped out with the proper choice of TCSC parameters and/or control strategies even without an auxiliary damping controller. Sub-synchronous behavior of the TCSC is simulated in both open loop and closed loop controls in the time domain through Electro-Magnetic Transient (EMT) simulations. IEEE 1st Benchmark model for SSR studies is used in this paper to demonstrate the effect of TCSC parameters and control methodology in damping SSR. KEYWORDS: Thyristor Controlled Series Compensators (TCSC), Sub-Synchronous Resonance (SSR), time-domain analysis, IEEE 1st Benchmark model for SSR studies.

Evaluation of Inherent Damping Introduced by Thyristor Controlled Series Compensators

Series compensation of transmission lines increase the risk of Sub-Synchronous Resonance (SSR) problems. Use of Thyristor Controlled Series Compensators (TCSC) in place of Fixed Series Capacitors (FSC) has benefited in terms of damping SSR. SSR Damping with TCSC can be achieved with supplementary controls or through its inherent damping capability. Use of discrete time models for small signal stability analysis of TCSC is well established. This paper uses a simplified Dynamic Phasor (DP) model of the TCSC for an in-depth evaluation of the inherent SSR damping capability through small signal stability analysis. Source of inherent damping in open loop operation and the representation of it in the DP model is elaborated and the small signal model is validated against Electro Magnetic Transient (EMT) simulations. Effect of inductive-resistive nature of the TCSC on Induction Generator Effect (IGE) and Torsional Interactions (TI) is shown through small signal stability assessment. It is shown that the network resonant frequency exists only in a narrow range of firing angles of the TCSC and thus IGE and TI are avoided in most part of its operating range. IEEE first benchmark system for SSR studies is used to demonstrate the inherent damping capability of a TCSC.

Analytical model building for Type-3 wind farm subsynchronous oscillation analysis

Many real-world scenarios confirmed insights developed based on the dq-frame nonlinear analytical model for doubly-fed induction generator (DFIG)-based type-3 wind subsynchronous resonance (SSR) study by the authors in 2010. A few real-world observations do not align with the simulation results from that model. In real-world scenarios, when a type-3 wind farm is radially connected to a series compensated line, even at a low compensation level, SSR occurs. This phenomenon has not been replicated by the nonlinear analytical model. In this paper, we revisited the analytical model by including phase-locked-loop (PLL). A modular modeling approach is presented in this paper. The inputs and outputs of each subsystem and the interconnections among subsystems are identified first. The overall system is then developed with all blocks integrated. The analytical model is capable of dealing with various transmission topologies, providing both large-signal time-domain simulation results and small-signal analysis. The new analytical model can successfully replicate the aforementioned phenomena and provide insights of type-3 wind SSR phenomena. The accompanying software package will be posted in the public domain.

Optimized Frequency Scanning of Nonlinear Devices Applied to Subsynchronous Resonance Screening

Frequency scanning is a powerful and versatile approach for subsynchronous resonance (SSR) screening in power grids. Among different numerical and analytical frequency scanning techniques, the harmonic injection method is well suited for the scan of black-box models with active elements, such as in wind farms and solar plants. The results can be used for SSR risk assessment, in which accuracy performs a decisive role. The ideal situation for this method, which leads to the most accurate results, is to perform one simulation per single-frequency injection. However, for SSR studies with a wide range of frequencies and simulations with very small time steps (scale of μs), this comes at the expense of increased simulation time. This article proposes a technique to improve the accuracy of the harmonic injection method through the optimization of the crest factor while injecting all frequencies at one shot, therefore also reducing simulation time. The proposed technique was tested on a wind farm connected to a radial test case and on a portion of a Texas synthetic grid with multiple active elements, including two wind farms and a VSC-based STATCOM. The frequency scan results were benchmarked with both time-domain transient simulations and the ideal multiple single-frequency injection case and compared with other techniques. The results show that the proposed method is superior in accuracy when compared to the other techniques and is 11.71 times faster when compared to multiple single-frequency injections.

Dynamic bifurcation analysis and mitigation of SSR in SMIB system

This paper presents a detailed study on Sub-synchronous resonance in a Single machine connected to an infinite bus system by employing Bifurcation Theory. The synchronous machine model considered is a two-axis model, the turbine system is a six mass system. A comparative study of Sub-synchronous resonance for the two-axis synchronous machine models 1.1, 2.1 and 2.2 under constant excitation is presented in this paper. The effect of adding an exciter, power system stabilizer and Static Synchronous Series Compensator to the SMIB system, incorporated with a 2.2 synchronous machine model on the bifurcations of SSR is also investigated. The results obtained on replacing the fixed series compensation of the line by Static Synchronous Series Compensator resulted in the mitigation of Sub-synchronous Resonance. The results obtained for all the above-considered cases are compared with eigenvalue analysis and validated using transient analysis<strong>.</strong>

Replicating Real-World Wind Farm SSR Events

In 2017, three sub-synchronous resonance (SSR) events were reported in the transmission system of Electric Reliability Council of Texas (ERCOT). These three events with different consequences are due to the same cause, i.e., Type-3 wind farms radially connected to a series compensated transmission line. The objectives of this paper are: 1) to build a testbed to replicate these real-world events and investigate what causes the different consequences; 2) to find out the impact of wind penetration level and wind speed on SSR; and 3) to investigate how to mitigate SSR. The replication testbed is built in MATLAB/Simpowersystems with each wind farm represented by a single aggregated wind turbine with full dynamics. The challenge of replication is the limited information about those three events, the ERCOT system, and the wind farms. Nevertheless, relying on simplification and assumptions, we are able to replicate the three events. Our contributions include the configured testbed for Type-3 wind farm SSR studies, diagnosis of impacting factors on SSR, and the confirmation of an SSR mitigation strategy through grid-side converter ac voltage command signal modulation.

Subsynchronous resonance oscillations mitigation via fuzzy controlled novel braking resistor model

Subsynchronous resonance (SSR) torsional torque oscillations is a problem of a great concern in the power engineering community. SSR causes torsional torque oscillations with ever-increasing magnitudes occurring in the machine shaft sections causing a premature fatigue life expenditure of the shaft metal. In this paper, dynamic braking switching strategy designed through fuzzy logic control theory and implemented via novel braking resistor model, namely chopper rectifier controlled braking resistor for tempering SSR torsional torque oscillations of a large turbo-generator. The proposed mitigation scheme has been tested on the IEEE second benchmark model for SSR studies. Comparative simulation study via MATLAB/Simulink-based modeling and simulation environment of the test model with and without the suggested mitigation regime should demonstrate its effectuality for mitigation of SSR torsional torque oscillations.

Variable Speed Pump Storage for the Mitigation of SSR in Power System with Wind Generation

Subsynchronous resonance (SSR) can bring significant negative effects on the grid system like stability, security, and even generator shaft damage. This work presents a new method of using a doubly‐fed induction machine (DFIM) based system of variable speed pumped storage plant (VSPS) to mitigate SSR in the power system with high penetration of wind generation. The mitigation is reached based on the principle of balanced shaft mechanical input power and electromagnetic power. The fundamental concepts and phenomena of SSR with wind farm are discussed in brief in this work. For the sake of analysis and verification of performance of the proposed system, the network model including wind farm, VSPS, HVDC, and synchronous machine is built based on Ningxia grid for the SSR study using PSCAD platform. The result shows that VSPS brings significant effect to dampen the SSR in the power system with high penetration of wind generation.

Coordinated optimization and control of SFCL and SMES for mitigation of SSR using HBB-BC algorithm in a fuzzy framework

Abstract Increasing demand for electrical power and expensive expansion of power systems to meet this demand leads the planners to find techno-economical solutions to overcome these challenges. Using series capacitors as reactive power compensation devices in a long transmission lines is a well-known and suitable way to face these challenges in power systems. Transmitted electrical power and the stability margin of power systems are economically improved by the proposed series devices. However, employing the series capacitors in long transmission lines causes a well-known problem in power systems as Sub Synchronous Resonance (SSR). In the present work, the mitigation of SSR is formulated as a Multi Objective Problem (MOP) in a fuzzy framework by using optimal coordinated control of Superconducting Fault Current Limiter (SFCL) and Superconducting Magnetic Energy Storage (SMES). Objective functions of MOP includes the minimization of the initial energy stored in the SMES unit, the energy loss of SFCL, the kinetic energy in generator rotor, the active power deviations in faulted bus, and the rotor speed deviations. These five objective functions are scaled by a fuzzy operator, then the scaled objective functions are aggregated by the “max-geometric mean” operator to generate the multi objective function. To solve the proposed multi-objective problem, the Hybrid Big Bang-Big Crunch (HBB-BC) as a meta-heuristic optimization algorithm is used. In order to evaluate the efficiency of the proposed algorithm, it is implemented on the IEEE First Benchmark Model (FBM) for SSR studies. According to the simulation results of the present study, the proposed algorithm is more effective in reducing the SSR problem in comparison with other algorithms.

A new method based on artificial neural network, Wavelet Transform and Short Time Fourier Transform for Subsynchronous Resonance detection

Subsynchronous Resonance (SSR) is a big concern for generator protection in power systems, especially with the wide deployments of series compensated lines. However, detecting SSR in an accurate and fast manner is a very challenging task. Various detection schemes were proposed over the years and they are summarized in this paper. Existing detection signal input options and signal processing techniques are reviewed. Furthermore, this paper proposes a new real time scheme based on Artificial Neural Network (ANN), Wavelet Transform (WT) and Short Time Fourier Transform (STFT) for accurate and comprehensive SSR detection. In the proposed method, signatures are extracted from the electrical and mechanical signals in the first step. Then an algorithm is designed to rank all the signatures and pick up the best performing subset of signatures. ANN is built from signatures contained in the best performing subset. IEEE second benchmark model for SSR study is used to simulate the signals for study in EMTP-type simulation environment. Test results with the generated signals show that the new SSR detection scheme achieves desirable speed, dependability and security for SSR detection. Further research will be done to apply the selected techniques and signatures in microprocessor relays.

Identification of Modeling Boundaries for SSR Studies in Series Compensated Power Networks

Given large-scale power networks with series compensation, the study of subsynchronous resonance (SSR) is a significant challenge, in practice, due to the model complexity and computation burden. To tackle these difficulties, this paper proposes a method to reduce a large-scale system to an equivalent model using a metric called damping sensitivity index. The proposed metric can quantitatively identify the boundaries of the equivalent model that ensure the accurate studies of SSR and reduce the computation burdens greatly. The effectiveness of the method is verified by its application to the SSR investigation of the modified IEEE New England 10-machine 39-bus system and 16-machine 68-bus system. The equivalent models obtained by the proposed method are compared with the original systems in terms of driving point impedances, electrical damping, and transient torques. These results obtained show that the equivalent models from the proposed method can reproduce the SSR dynamics of the original systems with a much shorter computation time.

Modeling and control of Type-2 wind turbines for sub-synchronous resonance damping

The rapid increase of wind power penetration into power systems around the world has led transmission system operators to enforce stringent grid codes requiring novel functionalities from renewable energy-based power generation. For this reason, there exists a need to asses whether wind turbines (WTs) will comply with such functionalities to ensure power system stability. This paper demonstrates that Type-2 WTs may induce sub-synchronous resonance (SSR) events when connected to a series-compensated transmission line, and with proper control, they may also suppress such events. The paper presents a complete dynamic model tailored to study, via eigenanalysis, SSR events in the presence of Type-2 WTs, and a systematic procedure to design a power system stabilizer using only local and measurable signals. Results are validated through a case study based on the IEEE first benchmark model for SSR studies, as well as with transient computer simulations.

Mitigation of SSR Oscillations in Series Compensated Line using LCAP Subsynchronous Damping Controller

Subsynchronous Resonance (SSR) is a growing problem in power systems having series compensated transmission lines. Subsynchronous resonance with low frequency that surpasses aggregate fatigue threshold of the generator shaft frequently could significantly reduce the shaft service life, which is a new problem that emerges in recent years. Flexible AC transmission systems (FACTS) controllers are widely applied to alleviate subsynchronous resonance. A line current and active power (LCAP) supplementary subsynchronous damping controller (SSDC) is proposed to damp subsynchronous resonance caused by series capacitors. Both eigenvalue investigation and time-domain simulation results verify that the proposed control strategy can effectively damping power system oscillations of the power system with SVS and SSDC. Time domain simulations using the nonlinear system model are also carried out to demonstrate the effectiveness of the proposed damping controller. The recommended control approach has been accumulated with the IEEE first benchmark model for SSR study. The analysis indicates that SVS using the proposed control strategy has better alleviation effect and output characteristics. All the simulations are validated by using MATLAB/Simulink environment.

Fuzzy logic control of SVS for damping SSR in series compensated power system

SUMMARY In this paper, a fuzzy logic-based supplementary controller has been developed for the Static VAR System (SVS) to damp torsional oscillations due to subsynchronous resonance (SSR) in a series compensated power system. The performance of the proposed controller has been tested on the IEEE second bench mark model for SSR studies. Time domain simulation has been carried out in a Matlab/Simulink environment. The results show that the conventional proportional integral voltage controller of SVS is not sufficient to damp SSR torsional oscillations, and significant damping of torsional oscillations has been achieved with the proposed fuzzy logic supplementary controller. Copyright © 2014 John Wiley & Sons, Ltd.

Performance of Synchronous Machine Models in a Series-Capacitor Compensated System

This paper compares the IEEE-specified generator models such as the equivalent circuit (EC) models and the operational inductance (OI) models to understand their damping performances in the IEEE first-benchmark system for subsynchronous resonance study. These models are found to differ from one another only to an extent they accurately represent the standard transfer functions. By carrying out different case studies an effort is made to analyze the influence of these differences among the models on the damping performance of swing-mode and torsional modes. Through a detailed eigenvalue analysis and time-domain simulations, it is demonstrated that these models offer differing results with respect to swing-mode, thereby strongly influencing the power system stabilizer (PSS) performances, especially when the system is series compensated. Even the torsional mode interactions are found to exhibit dependency on the models employed for the generators. Such a study is expected to provide better insight into the behavior of generator models and controller design/tuning in system analysis.

Modeling, Analysis, and Suppression of the Impact of Full-Scale Wind-Power Converters on Subsynchronous Damping

This paper presents modeling, analysis, and suppression of the impact of a voltage-source converter (VSC)-based full-scale wind turbine (FSWT) on subsynchronous damping in power systems. First, a detailed output impedance model of the grid-interfacing VSC is presented and analyzed under different converter operating modes, control functions, and variations in operating point. Second, the output impedance model is used to analyze the impact of the grid-interfacing VSC on the electrical damping characteristics in the subsynchronous frequency range and interactions with an electrically nearby synchronous generator. The IEEE first benchmark for subsynchronous resonance studies is used for theoretical analysis and simulation results. It is found that the grid-interfacing VSC of the FSWT can have a negative impact on overall electrical damping and there is a risk of subsynchronous torsional oscillation especially at lower frequency modes. Third, to mitigate such interactions, an active damping controller is developed to reshape the output impedance of the interfacing VSC and yield positive electrical damping at torsional modes. Time-domain simulation results are presented to validate the theoretical analysis and show the effectiveness of the proposed active damping controller.

A practical method to construct network state equations in multi-machine system SSR study

Eigenvalue analysis is extensively used in power system subsynchronous resonance (SSR) study. For SSR analysis, dynamic representation of the network is required, whereas both the state variables selection and the state equations formation for the network are difficult to process especially when the system configuration is subjected to variations. Hence, it is difficult to use eigenvalue analysis for multi-machine system SSR study. This paper proposed a new method to deal with this problem. The method proposed provides a way to select network state variables without using the network proper tree, thus complex topology analysis is avoided. Meanwhile, the network state equations can be constructed and modified easily by using the defined generalized capacitor-branch admittance matrix. Therefore, this method is well suited to construct the network state equations of multi-machine systems for eigenvalue analysis of the SSR problem. The proposed method has been used in the eigenvalue analysis of a multi-machine power system in China confronting with SSR problems, and the effectiveness of the proposed method has been verified by time domain simulations.

Analysis of subsynchronous resonance with generalized unified power flow controller

The Generalized Unified Power Flow Controller (GUPFC) is one of the most versatile flexible ac transmission system (FACTS) controllers which controls the active and reactive power flows in multiple transmission lines originating from a substation while controlling the sending end bus voltage. The sending end bus voltage is regulated by control of shunt reactive current while the active and reactive power flows in the transmission line are regulated by series injected voltages. This paper reports the analysis and study of Subsynchronous Resonance (SSR) characteristics of hybrid compensated system with GUPFC. The various operating mode combinations of series and shunt converters are considered to investigate their effect on SSR characteristics. The methods of analysis of SSR with GUPFC is based on the evaluation of damping torque, eigenvalues of the system and transient simulation. The computation of damping torque considers D–Q model of GUPFC to determine the torsional mode stability. The study is performed on a system adapted from IEEE Second Benchmark Model (SBM). The results demonstrate the effectiveness of series real injected voltage in mitigating the SSR.

An analysis of subsynchronous resonance in hybrid marine current and wind farms with squirrel cage induction generator

Subsynchronous resonance (SSR) is a well-known phenomenon in series-compensated systems with synchronous generators. With the rapid growth of renewable energy systems, it is likely that with its integration to series-compensated system for the transmission of bulk power may lead to the problem of SSR. This paper conducts an analysis of SSR phenomena in the squirrel cage induction generator-based hybrid wind farm and marine current farm connected to series-compensated system with power variation due to the addition or removal of small turbine units. A dynamic model has been developed to analyse the induction generator effect and torsional interaction of SSR on the IEEE first bench mark model for SSR studies. The eigenvalue analysis was performed on the developed model with MATLAB and the time domain electro magnetic transient simulation performed on DiGSILENT Power Factory confirms the predicted results by the eigenvalue analysis.

Damping of SSR Using Subsynchronous Current Suppressor With SSSC

Hybrid series compensation using static synchronous series compensator (SSSC) and passive series capacitor can improve the stability of the system, increases the power transfer capability and is useful for the fast control of power flow. This paper analyzes the subsynchronous resonance (SSR) characteristics of the hybrid series compensated power system in detail and proposes a simple method for the extraction of subsynchronous components of line current using filter. The extracted subsynchronous frequency component of line current is used to inject a proportional subsynchronous voltage in series with the transmission line which suppresses subsynchronous current in the transmission network. This novel technique is termed as subsynchronous current suppressor. The design of subsynchronous current suppressor is based on damping torque analysis and using genetic algorithm. A novel graphical representation of series resonance condition when SSSC is incorporated in the system is presented. The detailed study of SSR is carried out based on eigenvalue analysis, transient simulation and damping torque analysis. The results of the case study on a system adapted from IEEE First Benchmark Model demonstrates the effectiveness and robust performance of subsynchronous current suppressor in damping of SSR under various system operating conditions.