Subsynchronous Resonance Damping Research Articles

Event-Triggered robust control scheme for sub-synchronous resonance mitigation in DFIG-based WECS

Series compensation is an economically feasible solution for facilitating the integration of high levels of wind energy sources into the grid. However, high levels of series compensation in transmission lines typically results in sub-synchronous resonance (SSR) in doubly-fed-induction-generator (DFIG)-based wind energy conversion systems (WECS). The introduction of negative equivalent resistance due to negative SSR slip results in self-sustaining oscillating instability and potentially loss of stability of the whole system. This paper proposes an event-triggered sliding mode control (ETSMC)-based control approach for the effective damping of sub-synchronous resonance. The ETSMC control is designed for the rotor side converter of the DFIG, and a phase compensator unit is augmented with the ETSMC control to enhance its efficacy in mitigating specific unstable mode. The proposed control approach is validated using a DFIG-based WECS in IEEE first benchmark model. The results prove the high efficacy of the control approach in mitigating SSR and ensuring system’s fast recovery. In addition to the attributes of the SMC approach, reliance on an event triggering mechanism leads to only seldom control updates, thereby solving the problem of requirement of high communication bandwidth and computational power.

Adaptive Fuzzy Supplementary Controller for SSR Damping in a Series-Compensated DFIG-Based Wind Farm

Although using a series compensation technique in a long transmission line effectively increases the transmittable power; it may cause a sub-synchronous resonance (SSR) phenomenon. Gate-controlled series capacitor (GCSC) is an effective method for SSR damping by controlling the turn-off angle. In the previous studies, a constant supplementary damping controller (SDC) was used for controlling the turn-off angle, which can mitigate the SSR phenomenon. However, these methods can not capture the maximum transmittable power at different operating points. In this paper, a fuzzy logic controller (FLC) is proposed to compute the gain of SDC based on the wind speed and the error between the measured and reference line currents for transferring as much power as possible and damping the SSR phenomenon simultaneously. Using the MATLAB/SIMULINK program, the proposed method is tested at different operating points to validate its effectiveness and robustness. Compared to the traditional method (constant SDC), the maximum transmittable power, as well as SSR damping, is achieved in all studied cases by the proposed method (variable SDC).

SSR Stable Wind Speed Range Quantification for DFIG-Based Wind Power Conversion System Considering Frequency Coupling

The subsynchronous resonance (SSR) characteristics of doubly-fed induction generation-based wind power conversion system (DFIG-WPCS) varies with the wind speed. Obtaining the small signal SSR stable wind speed range is urgently important for wind farm dispatch planning. However, there are few methods to quantify the small signal SSR stable wind speed range of DFIG-WPCS. Moreover, the inherent frequency coupling when DFIG-WPCS is integrated to modular multilevel converter-high voltage direct current (MMC-HVDC) may lead to inaccurate stability assessment results. In this article, the frequency coupling mechanism between WPCS and MMC is revealed by analyzing the law of harmonic interaction in the interconnected system. The wind speed-frequency two-variable equivalent admittance model of DFIG-WPCS is established by taking the frequency coupling account. Relying on the two-variable open-loop transfer function, the multiple phase margin contours plot approach is proposed to quantify the small signal SSR stable wind speed range and damping properties. The effectiveness of two-variable admittance and analysis method are validated against electromagnetic transient (EMT) testbed with MATLAB /SimPowerSystems. The analysis results can provide a reference for the early planning and stable operation of DFIG-WPCS.

Damping of Subsynchronous Resonance in Utility DFIG-Based Wind Farms Using Wide-Area Fuzzy Control Approach

This paper presents a novel fuzzy control scheme for damping the subsynchronous resonance (SSR) according to the wide-area measurement system (WAMS) in power systems including doubly fed induction generator (DFIG)-based wind farms connected to series capacitive compensated transmission networks. The SSR damping is attained by adding the fuzzy controller as a supplementary signal at the stator voltage loop of the grid-side converter (GSC) of DFIG wind farms. Additionally, time delays due to communication signals are important when using WAMSs. If the time delays are ignored, it causes system instability. In this paper, the time delays are modeled with a separate fuzzy input to the controller. The new fuzzy control approach is executed by using the angular velocity of synchronous generators (w) and its variation in the angular velocity (dw/dt). The effectiveness and success of the WAMS-based fuzzy controller is demonstrated by comparison with the particle swarm optimization (PSO) and imperialist competitive algorithm (ICA) optimization methods. The efficacy and validity of the planned auxiliary damping control are verified on a modified version of the IEEE second benchmark model including DFIG-based wind farms via time simulations using the MATLAB/Simulink toolbox.

Damping of Subsynchronous Resonance in a Hybrid System With a Steam-Turbine Generator and an Offshore Wind Farm Using a Unified Power-Flow Controller

This article proposes a unified power-flow controller (UPFC) to suppress subsynchronous resonance occurred in a hybrid power-generation system connected to an infinite bus through a series-capacitor compensated line. The hybrid power-generation system contains a steam-turbine generator (STG) and an offshore wind farm (OWF) based on doubly-fed induction generator (DFIG). The d – q axis equivalent-circuit model under three-phase balanced loading conditions is derived to establish the complete system model including the STG set, the DFIG-based OWF, the series-capacitor compensated line, the UPFC, etc. A damping controller of the proposed UPFC is designed by using pole-assignment approach based on modal control theory to effectively damp out unstable STG modes of the studied system. Both small-signal stability and transient simulation results of the studied system are systematically performed. The simulation results show that the proposed UPFC joined with the designed damping controller can effectively suppress unstable STG modes of the studied power system.

Robust finite-time controller for damping of subsynchronous resonance in doubly fed induction generator wind farm

To alleviate subsynchronous resonance in doubly fed induction generator–based wind farm, this study devises a robust nonlinear control for a rotor-side converter based on a fractional-order sliding mode controller. The designed fractional-order sliding mode controller is able to realize finite-time control and reduce the control time compared with terminal sliding mode control, which contributes to the faster mitigation of subsynchronous resonance. Impedance-based analysis and transient simulation are carried out to evaluate the performance of the fractional-order sliding mode controller compared with terminal sliding mode control and subsynchronous damping control. Simulation results verify that the fractional-order sliding mode controller is able to damp SSR within shorter time and effectively reduce the fluctuation range of a system’s transient responses under various working conditions of compensation degrees and wind speeds. Furthermore, the fractional-order sliding mode controller enhances the robustness under external disturbance and parametric uncertainty, ensuring safe operation of the practical wind farm.

An Effective Control Scheme for Multimodal SSR Suppression via VSC-Based Controller

Multimodal sub-synchronous resonance (SSR) induced by the interactions between the generator shafts and the fixed series capacitors has become a serious threat for the power system. To overcome this, a voltage source converter (VSC) based controller for multimodal SSR suppression, named sub-synchronous resonance dynamic suppressor (SSRDS), is presented and analyzed in this paper. The controller is placed at the point of common coupling (PCC) of the power plant for multi-generator SSR suppression purpose so that the investment costs can be minimized while making sure the requirements of the SSR damping. To quantify the contribution of SSRDS to the SSR damping, the damping torque analysis is carried out, which can reveal the influences of the operating conditions and the correlated control parameters on the damping performance. Furthermore, to avoid the negative affects between the different TMs in close range as well as maximize the damping ability of SSRDS, the parameters tuning method of the mode filters and the gain and phase shifters are proposed. A detailed nonlinear simulation model is established in PSCAD/EMTDC based on the Jinjie series compensated transmission system located in China. The damping torque analysis, the eigenvalues analysis and the simulation results have fully demonstrated the effectiveness and the robustness of the proposed SSRDS for multi-generator and multimodal SSR suppression under both small and large disturbances in various operating conditions.

Waveform-Based Method for Fast and Accurate Identification of Subsynchronous Resonance Events

An increasing number of unstable subsynchronous resonance (SSR) has occurred for wind farms recently. These events can lead to significant loss of generation, threatening the safe operation of the system, and even damaging the generators. Accurate knowledge of SSR characteristics (SSR frequency, current magnitude, and damping) at an early stage of the event is critical for its effective mitigation. This paper proposes a method for fast and accurate identification of an SSR event and its characteristics. It takes advantage of time-stamped waveform measurements that are typically available on the terminals of series compensated transmission lines. It uses the known model of the line as a “filter” to extract the SSR components. Case studies demonstrate that the proposed algorithm can accurately compute the SSR frequency, SSR current magnitude, and damping in around one SSR cycle or less. In addition to line parameters uncertainties, the method has also proven to remain robust in the presence of harmonic distortions and reduced waveform sampling rates.

Real-Time Estimation and Damping of SSR in a VSC-HVDC Connected Series-Compensated System

Infrastructure reinforcement using high-voltage direct-current (HVDC) links and series compensation has been proposed to boost the power transmission capacity of existing ac grids. However, deployment of series capacitors may lead to subsynchronous resonance (SSR). Besides providing bulk power transfer, voltage source converter (VSC)-based HVDC links can be effectively used to damp SSR. To this end, this paper presents a method for the real-time estimation of the subsynchronous frequency component present in series-compensated transmission lines—key information required for the optimal design of damping controllers. A state-space representation has been formulated and an eigenvalue analysis has been performed to evaluate the impact of a VSC-HVDC link on the torsional modes of nearby connected thermal generation plants. Furthermore, the series-compensated system has been implemented in a real-time digital simulator and connected to a VSC-HVDC scaled-down test-rig to perform hardware-in-the-loop tests. The efficacy and operational performance of the ac/dc network while providing SSR damping is tested through a series of experiments. The proposed estimation and damping method shows a good performance both in time-domain simulations and laboratory experiments.

Research on the Impact on Sub-synchronous Resonance Risk from DFIG Number and Series-compensated Degree

Sub-synchronous resonance (SSR) occurred in the power system with both Doubly Fed Induction Generators (DFIGs) and series-compensated lines. Existing paper discovered the nonlinear relationship of SSR damping and the number of DFIGs. In this paper, the equivalent model of wind farms and series-compensated power system in Guyuan is built in RT-LAB. Based on that model, the SSR characteristics of wind farms in Guyuan area under different DFIG numbers and line parameters are investigated through impedance method. The results show that: 1) The relationship between SSR characteristics and DFIG number is nonlinear in Guyuan area. Under the condition of a given wind speed, there is certain number of DFIGs which will induce worst damping effect. 2) A higher series-compensated degree results in a less stable system.

Application of an Additive Self-tuning Controller for Static Synchronous Series Compensator for Damping of Sub-synchronous Resonance Oscillations

In this paper, an additive self-tuning (ST) control scheme is presented for a static synchronous series compensator (SSSC) to improve performance of conventional PI control system for damping sub-synchronous resonance (SSR) oscillations. The active and reactve series compensation are provided by a three-level 24-pulse SSSC and fixed capacitor. The proposed ST controller consists of a pole shift (PS) controller and a recursive least squares (RLS). The RLS identifier algorithm is used to estimate parameters of auto regressive moving average exogenous (ARMAX) model. With this scheme, there is no any necessity for retuning the PI controller parameters under different operational circumstances of the system. The SSR analysis are performed using eigenvalue analysis, transient simulation and FFT analysis. The considered test system is adapted from IEEE First Benchmark Model. As shown in the results, the proposed additive ST controller has an effective and acceptable performance for damping of SSR under different disturbances and operating conditions. It should be noted here that all digital simulations have been done by using MATLAB.

An STATCOM-based Hybrid Shunt Compensation Scheme Capable of Damping Subsynchronous Resonance

The paper presents the potential use of supplemental control of a new economical phase imbalanced shunt compensation concept for damping sub synchronous resonance (SSR) oscillations. In this scheme, the shunt capacitive compensation in one phase is created by using a Single-Phase Static Synchronous Compensator (STATCOM) in parallel with a fixed capacitor (Cc), and the other two phases are compensated by fixed shunt capacitor (C). The proposed arrangement would, certainly, be economically attractive when compared with a full three-phase STATCOM which have been used/proposed for power swings and SSR damping. SSR mitigation is achieved by introducing a supplemental signal into the control loops of single phase STATCOM. The validity and effectiveness of the proposed structure and supplemental control are demonstrated on a modified version of the IEEE second benchmark model for computer simulation of sub synchronous resonance by means of time domain simulation analysis using the Matlab program.

An STATCOM-Based Hybrid Shunt Compensation Scheme Capable of Damping Subsynchronous Resonance

The paper presents the potential use of supplemental control of a new economical phase imbalanced shunt compensation concept for damping Sub Synchronous Resonance (SSR) oscillations. In this scheme, the shunt capacitive compensation in one phase is created by using a single-phase Static Synchronous Compensator (STATCOM) in parallel with a fixed capacitor ( ), and the other two phases are compensated by fixed shunt capacitor (C). The proposed arrangement would, certainly, be economically attractive when compared with a full three-phase STATCOM which have been used/proposed for power swings and SSR damping. SSR mitigation is achieved by introducing a supplemental signal into the control loops of single phase STATCOM. The validity and effectiveness of the proposed structure and supplemental control are demonstrated on a modified version of the IEEE second benchmark model for computer simulation of sub synchronous resonance by means of time domain simulation analysis using the Matlab program.

SSR Damping in Fixed-Speed Wind Farms Using Series FACTS Controllers

Subsynchronous resonance (SSR) damping in fixed-speed wind turbine generator systems (FSWTGS) by using two series flexible ac transmission system (FACTS) devices, the thyristor-controlled series capacitor (TCSC), and gate-controlled series capacitor (GCSC) are studied in this paper. The former is a commercially available series FACTS device, and the latter is the second generation of series FACTS devices using gate turnoff (GTO) or other gate-commuted switches. The GCSC is characterized by a fixed capacitor in parallel with a pair of antiparallel gate-commuted switches enabling rapid control of series impedance of a transmission line. It is shown that the SSR damping with a GCSC is limited to changing the resonance frequency, in comparison with a fixed capacitor, which may not be adequate to damp out the SSR. Therefore, a supplementary SSR damping controller (SSRDC) is designed for the GCSC. Moreover, it is proven that the GCSC equipped with a well-designed SSRDC can effectively damp the SSR in FSWTGS. In order to verify the effectiveness of the GCSC in SSR damping, its performance is compared with the TCSC, which is an existing series FACTS device. In addition, time-frequency analysis (TFA) is employed in order to evaluate and compare the SSR time-varying frequency characteristics of the GCSC and TCSC. The IEEE first benchmark model on SSR is adapted with an integrated FSWTGS to perform studies, and extensive simulations are carried out using PSCAD/EMTDC to validate the result.

New findings on bypass damping filter in increasing subsynchronous resonance damping of series compensated system

Subsynchronous resonance (SSR) has three categories, namely induction generator effect, torsional interaction and torque amplification. Bypass damping filter (BDF) was generally considered particularly effective for the induction generator effect. In this study, it is shown that BDF is also an effective countermeasure for torsional interaction and transient torque amplification. In addition, it can help reduce transient overvoltage of the series capacitor in a series compensated system. The parameter tuning method for BDF is proposed, based on electrical damping analysis and criterion for torsional stability. The SSR damping effectiveness of BDF in a series compensated system is verified by electrical damping analysis, torsional mode analysis and time-domain simulation.

Investigation of SSR in Practical DFIG-Based Wind Farms Connected to a Series-Compensated Power System

Subsynchronous resonance (SSR) was observed in wind farms located in North China. These wind farms prevailingly consist of doubly-fed induction generators (DFIGs) and are connected to series-compensated transmission lines. The observed resonant frequency is about 6 ~ 8 Hz, which is much lower than that of the reported SSR occurred in Texas. The frequency varies during the occurrence and this phenomenon is observed for the first time. The output power is usually within a certain range, when SSR occurs. Based on the practical system, an equivalent simulation system has been established, in which wind farms are modeled as many identical low rating DFIGs. Then, the SSR event is reproduced by simulations. Analysis results indicate that SSR happens even when the equivalent transmission system compensation level seen from wind farms is only 6.67%. Eigenvalue analysis shows that this phenomenon is an electrical resonance, and could be affected considerably by wind speed, number and control of DFIGs. The number of in-service DFIGs has a nonlinear impact on the damping of SSR. An equivalent electric circuit is deduced to intuitively explain why SSR happens and how the above factors affect it. Considering its features, this phenomenon is recognized as DFIG control participated induction generator effect.

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.

An active power control strategy for a DFIG-based wind farm to depress the subsynchronous resonance of a power system

Abstract This study presents a novel auxiliary damping control strategy to depress subsynchronous resonance (SSR) oscillations in nearby turbine generators. In the proposed control strategy, SSR damping is achieved by adding turbine generator speed as a supplementary signal at the active power loop of the rotor-side converter (RSC) of doubly-fed induction generator (DFIG)-based wind farms. To design the SSR auxiliary damping controller, a transfer function between turbine generator speed and the output active power of the wind farms was introduced to derive the analytical expression of the damping coefficient. Then the damping effect of the active power of the DFIG-based wind farms was analyzed, and the phase range to obtain positive damping was determined. Next, the PID phase compensation parameters of the auxiliary damping controller were optimized by genetic algorithm to obtain the optimum damping in the entire subsynchronous frequency band. The last, the validity and effectiveness of the proposed auxiliary damping control were demonstrated on a modified version of the IEEE first benchmark model by time domain simulation analysis with the use of DigSILENT/PowerFactory.

Mitigation of SSR by Subsynchronous Current Injection with VSC HVDC

The Voltage Source Converter based HVDC provides asynchronous interconnection between two AC systems. The use of VSC HVDC enables independent control of real and reactive power. Unlike conventional HVDC, the VSC HVDC does not cause Subsynchronous Resonance (SSR) when it is close to turbo-generators. The system strength of long ac transmission line can be enhanced by providing series compensation. However, there is a possibility of SSR interaction with turbine-generator due to the presence of the series capacitor in transmission line and may lead to shaft failure of the turbine-generator. The objective of this paper is to analyze the damping of subsynchronous resonance when the series compensated long AC line and VSC HVDC are originated from the same substation. A simple method for the extraction of subsynchronous component of line current using filter is proposed. The extracted subsynchronous frequency current is injected by the VSC HVDC converter close to the turbine-generator. This suppresses the subsynchronous current flowing through the generator and increases the damping of the system at critical torsional frequencies. This technique is termed as Subsynchronous Current Injection (SSCI). Subsynchronous current injector is designed using D–Q model of VSC HVDC. Genetic Algorithm (GA) is used for tuning the converter controllers and filter parameters. The results show the effectiveness of the proposed subsynchronous current injector in damping of SSR.

Sub-Synchronous Resonance damping via Doubly Fed Induction Generator

Interests of low frequency oscillation damping with the Doubly Fed Induction Generator (DFIG) has been increased recently. In addition, it is widely accepted that, the Sub-Synchronous Resonance (SSR) is another unfavorable dynamical phenomenon in power systems. In this study, the implementation of the DFIG in SSR mitigation will be investigated. The IEEE second benchmark model aggregated with a DFIG based wind farm is employed as the case study. Two different control methodologies are proposed: a Fuzzy Logic Damping Controller (FLDC) and a conventional damping controller (CDC) that are added to the main control loop of the DFIG in order to damp the SSR. To validate the results of SSR suppression via DFIG, several case studies are introduced based on changing the operation point of the system and duration of the fault. It has been shown that the DFIG can damp the SSR through both proposed methodologies, but when the system operating point or fault duration is changed, the FLDC can present a cost-effective solution for SSR damping. The Fast Fourier Transform (FFT), simulation results, and a method based on performance index (PI) are conducted to compare two controllers through various cases in Matlab.