Subsynchronous Torsional Interaction Research Articles

The Impact of Integration of the VSC-HVDC Connected Offshore Wind Farm on Torsional Vibrations of Steam Turbine Generators

For remote offshore wind farms, transmitting power to the main onshore grid via a Voltage Source Converter High Voltage Direct Current (VSC-HVDC) system is the mainstream of power transmission. It is not only cost-effective in long-distance transmission, but also can fully meet the grid side requirements such as black start, voltage support, fault ride through and frequency support. However, it still has some problems, such as the possible impact on the power grid needing to be paid attention to. In this paper, its impact on the torsional responses of turbine generator units neighboring to the onshore side of AC bus is studied by using the DIgSILENT PowerFactory software. It is found that the effects of the Sub-Synchronous Torsional Interaction (SSTI) with onshore controls and the generator de-rating operations can significantly affect the damping ratio of turbine torsional modes, whereas the effects of the machine configurations and the amount of wind farm power integrated can affect the electrical torque disturbance. The most noteworthy is that their effects can be superimposed on each other if these factors act simultaneously, which would lead to increased vibrations and reduce the turbine shaft’ s life. The findings will be helpful for avoiding accidents caused by torsional vibrations when it is going to integrate a VSC-HVDC connected wind farm into a power grid.

Mitigating Subsynchronous Torsional Interaction Using Geometric Feature Extraction Method

This paper proposes a method to mitigate subsynchronous torsional interaction detected during power system operation. This innovative method employs the delay reconstruction of the damping controller of a thyristor-controlled series compensator. This addresses the need to detect and manage stability and electromagnetic transients in power systems caused by the increasing use of fast-response power electronics. Previously, severe oscillation conditions could be avoided via analysis of the subsynchronous torsional interaction scenarios during the planning stage, enabling the suppression of oscillations. However, planning, modeling, and analysis for various scenarios becomes more difficult as the complexity of the power system increases, owing to the use of renewable energy and the incorporation of topology changes. Therefore, interest in measurement data-based real-time oscillation analysis has increased. The first step of the mitigation strategy proposed herein reconstructs nonlinear time-series data to detect subsynchronous torsional interaction in real time and generate alert signals. The second step of the strategy is that the controller mitigates oscillations by controlling the firing angle using the geometric feature extraction method. In this paper, the relaxation of the frequency oscillation in the subsynchronous region of about 22 Hz and about 18 Hz was verified through two simulation cases.

Phase-Controlled Thyristor Sub-Synchronous Damper Converter for a Liquefied Natural Gas Plant

In electrified liquefied natural gas (LNG) plants variable frequency drives (VFDs) interact with turbine-generator (TG) units creating torsional vibrations known as sub-synchronous torsional interactions (SSTIs). Torsional vibrations can be dangerous for an LNG plant when they involve torsional instability. The stability of an LNG plant depends on the plant configuration and on the number of TG units and of VFDs. In such peculiar configurations stability cannot be achieved acting of the VFDs control system. Alternatively, dedicated equipment is needed to damp the torsional vibrations. In this paper, a sub-synchronous damper (SSD) converter is used to mitigate the SSTI phenomena. The SSD converter consists of a thyristor H-bridge regulating the phase of the additional torque provided at the TG unit air-gap. A phase control system is proposed and is based on the torsional torque oscillations measurement. An adaptive reference signal is employed, also guaranteeing high performance in island-mode operation. The proposed solution increases the damping of the LNG plant in all the considered configurations. The LNG plant successful operation is validated by comprehensive results.

Modal Analysis on Control Impact of Fully Rated Converters-Based PMSG-WECS Connected to Turbine-Generator

The power converter-based wind energy conversion systems (WECS) connected to weak AC networks may cause subsynchronous interactions that can potentially lead to instability. The interaction between the WECS converters and a nearby turbine-generator (TG) may lead to severe torsional vibrations. This article investigates the impact of a fully rated converters-based permanent magnet synchronous generator WECS (PMSG-WECS) on the subsynchronous torsional interactions (SSTI) of a nearby multimass TG connected to series compensated transmission line. A simple but effective systematic approach of modal analysis using open-loop systems is proposed. The proposed approach is used to evaluate the impact of PMSG-WECS on SSTI. Further, the impact of PMSG-WECS is analyzed through the damping torque, eigenvalue analysis and transient simulation under various control modes of PMSG-WECS converters. The results demonstrate that the inclusion of PMSG-WECS increases the subsynchronous resonance frequency of the network, thereby reduces the damping of the torsional modes in the high-frequency range of subsynchronous resonance frequency. The negative damping effect of PMSG-WECS significantly reduces the damping of subsynchronous network mode, thereby can destabilize the system. Also, the negative damping impact of PMSG-WECS significantly reduces the damping of high-frequency torsional modes. As a result, the torsional interactions aggravate at lower compensation levels.

Electrical Damping Assessment and Sensitivity Analysis of a Liquefied Natural Gas Plant: Experimental Validation

Liquefied Natural Gas (LNG) plants are commonly island-operated weak grids where the interaction of high-power Variable Frequency Drives (VFDs) with the Turbine-Generator (TG) units might cause Sub-Synchronous Torsional Interaction (SSTI) phenomena. SSTI phenomena can lead the LNG plant to instability conditions. Each LNG plant configuration is characterized by a risk level, which is considered high when the electrical damping at the TG Torsional Natural Frequencies (TNFs) is negative. Starting from a real case study, a detailed electromechanical model of an LNG plant is presented. The model is comprehensive of the control system of the power conversion stage and of the TG unit. Sensitivity analysis, performed on control system parameters, allows one to detect the parameters that impact the electrical damping and the stability of the overall LNG plant. A complete simulation platform is developed. Experimental results are carried out on a real LNG plant considering four different configurations. The theoretical model and the simulation platform allow one to estimate the electrical damping and the results are confirmed by the experimental validation. It is demonstrated that fine tuning of the power conversion stage control parameters can reduce the risk related to torsional instability.

Analysis of potential low frequency resonance between a 1GW MMC HVDC and a nearby nuclear generator

This paper presents a comprehensive analytical eigenvalue stability study of subsynchronous torsional interactions between the 1 GW Eleclink MMC HVDC and the 1.12 GW Gravelines turbine-generator in north France. It revalidates the conclusions of previous studies on VSC-based HVDC systems by showing that the risk of SSTI may be of significance when the HVDC is based on the MMC and not the two level VSC technology. The study demonstrates that in the case of the weak French grid and the rated HVDC power flow from France to England, the HVDC can destabilize the dominant low frequency, 6.3 Hz, torsional mode. The analysis also expands the work of previous studies on MMC and reveals that the case of the HVDC power flow from the English to the French grid may also introduce the risk of destabilizing the lower frequency mode. Furthermore, it also reveals a noticeable impact of the PLL and that increasing the gains has an opposite effect on the two HVDC power flow directions. Moreover, participation factors are analyzed for the 6.3 Hz mode indicating that the main cause of the torsional interactions is the HVDC power control loop. The main findings are verified on the detailed EMTP model.

Application of the First Replica Controller in Korean Power Systems

The purpose of this paper is to introduce, examine, and evaluate the industrial experiences and effectiveness of a Thyristor Controlled Series Compensator (TCSC) replica controller installed in Korea in 2019 through a review of its configuration, test platform, and practical application, and further to propose operational guidelines for replica controllers. Four representative practical cases were conducted: a Dynamic Performance Test (DPT) under a sufficiently large-scale power system prior to the Site Acceptance Test (SAT), pre-verification for on-site controller modification during operation stage, parameter tuning to mitigate the control interaction, and time domain simulation for Sub-Synchronous Torsional Interaction (SSTI). None of these four cases can be performed in a Factory Acceptance Test (FAT) or on-site. Therefore, TCSC control performance was accurately verified under the entire Korean power system based on a large-scale real-time simulator, which demonstrated its effectiveness as a powerful tool for operations including multiple power electronics devices. Our review herein of these four practical cases is expected to show the usefulness of replica controllers, to demonstrate their strength to deal with practical field events, and to contribute to the further expansion of the application area from a perspective of electric utility.

Electrical Damping Assessment and Stability Considerations for a Highly Electrified Liquefied Natural Gas Plant

In recent years, the Oil & Gas industry has been subjected to a progressive electrification process aiming to comply with global environmental requirements on CO2 emissions reduction. High-power electric motors fed by Variable Frequency Drives (VFDs) have replaced gas turbines as drivers for gas compression applications. In Liquefied Natural Gas (LNG) plants, unexpected downturns could be experienced in case of high torsional vibrations of power generations units. These torsional vibrations derive from the interaction among turbine-generator (TG) units and VFDs and are known as Sub-Synchronous Torsional Interactions (SSTIs). SSTIs can lead to instability when the overall electromechanical system lacks sufficient damping. In this scenario, electrical damping assessment is fundamental in order to ensure stability and reliable operation of an LNG plant. Negative electrical damping is strictly related to the negative incremental resistance behavior of the power converters and it is influenced by the converter’s control system. In this paper, a real case study based on Thyristor Variable Frequency Drives (TVFDs) is considered. Ad hoc dynamic models of the power converters and of the TG unit are developed and combined in order to provide an accurate estimation of the electrical damping. It is demonstrated that the electrical damping is affected by variations of the main control system parameters and how the use of a simplified model instead of an ad hoc model can impact the stability evaluation.

Stability Analysis on Torsional Interactions of Turbine-Generator Connected With DFIG-WECS Using Admittance Model

To evaluate the stability of Subsynchronous Torsional Interactions (SSTI), we propose a simple and effective approach using admittance model. The proposed approach gives the subsynchronous conductance based criterion to find the stability of torsional interactions. The subsynchronous conductance based approach is utilized to evaluate the impact of Doubly-Fed Induction Generator based Wind Energy Conversion System (DFIG-WECS) on the torsional vibrations of nearby turbine-generator. The damping of Induction Generator Effect (IGE) and SSTI is analyzed through admittance model at different wind speed with various compensation level. The results of admittance analysis are validated through eigenvalue analysis and transient simulation. The results show that, the DFIG-WECS reduces the damping of torsional modes in the low frequency region of subsynchronous network mode. The damping of SSTI reduces with increase in wind speed. Hence, the DFIG-WECS significantly destabilizes the low frequency torsional modes at high wind speed with high compensation level. Further, it is found that, the network conductance at all critical torsional modes marginally reduces with Subsynchronous Damping Controller (SSDC) at the inner loop of Grid Side Converter (GSC) controller compared to other locations in DFIG converter controllers.

Mechanism Analysis of Subsynchronous Torsional Interaction With PMSG-Based WTs and LCC-HVDC

In China, the development of wind-thermal-bundled power transmission by line-commutated converter-based high-voltage direct current (LCC-HVdc) is in progress. The dynamics in such a new scenario with three types of devices (three-device scenario), viz., synchronous generators (SGs), wind turbines (WTs), and LCC-HVdc, becomes salient and may cause subsynchronous torsional interaction (SSTI) issue. Such an SSTI issue is still but should not be underestimated. In this article, a path decomposition method is proposed to examine this SSTI issue from a mechanism perspective. With this method, the external electrical system is modeled as four explicit paths viewed from the SG rotor. This gives insight into the dominant paths and contributing devices of the SSTI damping and builds bridges among single-/two-/three-device equivalent scenarios that are used for SSTI studies. With these explicit paths, the impact of the interaction between WT and LCC-HVdc on the SSTI damping of SG is quantified. It is exposed from a specific case that the SSTI instability phenomenon in a three-device scenario does not appear in single-/two-device scenarios. The findings not only reveal that the interaction between WT and LCC-HVdc imposes a significant damping effect on the SSTI issue but also demonstrate that it is inadvisable to study the practical SSTI issue by simplifying such a three-device scenario to single-/two-device scenarios. The path decomposition method can be extended to power systems with more than three devices.

Impact of DFIG‐based wind farm integration on sub‐synchronous torsional interaction between HVDC and thermal generators

In western China, large-scale wind power is generally bundled with thermal power, and transmitted to eastern China by high-voltage direct current (HVDC) systems. This constitutes the wind–thermal bundled system transmitted by HVDC (WTBH). In this study, a typical studied system of WTBH is presented, and its models for eigenvalue analysis are introduced. To improve the efficiency of eigenvalue analysis, an improved block modelling method is proposed for building the state-space model of WTBH. By eigenvalue analysis and electromagnetic transient (EMT) simulation, the impact of doubly-fed induction generator (DFIG)-based wind farm integration on sub-synchronous torsional interaction (SSTI) between thermal generators (TGs) and HVDC was investigated. The sensitivity of the operating parameters of DFIG-based wind farms was analysed, including wind speed, wind farm scale, and distance between the wind farm and HVDC rectifier station. The obtained results show that the integration of DFIG-based wind farms can mitigate the SSTI between the TG and HVDC. The system becomes more stable as the wind speed and wind farm scale increase or the distance between the wind farm and HVDC rectifier station decreases. The conclusions of this study are validated through EMT simulations in PSCAD/EMTDC, and provide theoretical reference for practical WTBH projects.

Studies of Sub-Synchronous Oscillations in Large-Scale Wind Farm Integrated System

With the rapid development and construction of large-scale wind farms and grid-connected operation, the series compensation wind power AC transmission is gradually becoming the main way of power usage and improvement of wind power availability and grid stability, but the integration of wind farm will change the SSO (Sub-Synchronous oscillation) damping characteristics of synchronous generator system. Regarding the above SSO problem caused by integration of large-scale wind farms, this paper focusing on doubly fed induction generator (DFIG) based wind farms, aim to summarize the SSO mechanism in large-scale wind power integrated system with series compensation, which can be classified as three types: sub-synchronous control interaction (SSCI), sub-synchronous torsional interaction (SSTI), sub-synchronous resonance (SSR). Then, SSO modelling and analysis methods are categorized and compared by its applicable areas. Furthermore, this paper summarizes the suppression measures of actual SSO projects based on different control objectives. Finally, the research prospect on this field is explored.

Subsynchronous Torsional Interaction of Wind Farms with FSIG Wind Turbines Connected to LCC-HVDC Lines

High-voltage direct current (HVDC) lines with line-commutated converter (LCC) are being increasingly employed to transmit bulk wind power over long distance. However, this may cause the sub-synchronous torsional interaction (SSTI) between the wind farms and the LCC-HVDC system. The SSTI characteristics of wind farms with fixed-speed induction generator (FSIG) wind turbines connected to LCC-HVDC are investigated in this paper. To simplify the calculations, a modular modeling method is proposed for building the small-signal mathematical model of the investigated system. Small-signal analysis, participation factor analysis, and impact of dominant factors analysis are then applied to investigate the SSTI characteristics under different operating conditions. Three oscillation modes associated with the SSTI are identified in the entire system through small-signal and participation factor analysis, comprising two torsional modes and an electromechanical mode. Impact of dominant factors analysis shows that the system becomes less stable as the wind farm capacity grows and the distance between FSIG wind farm and the rectifier station increases. Moreover, the above analysis suggests that wind farms with FSIG connected to LCC-HVDC lines may not cause unstable SSTI. The electromagnetic transient simulations based on PSCAD/EMTDC (Power Systems Computer-Aided Design/Electromagnetic Transients including DC) verify these results.

SSTI and its mitigation in wind farms connected with an HVDC line

Frequent oscillation occurs at the wind farms of Santanghu District, which are connected to a ±800 kV high voltage direct current line (HVDC). After analysis of the oscillation frequency and its characteristics, it is concluded that the oscillation occurs because of subsynchronous torsional interaction (SSTI) caused by the interaction between the wind turbine generators and the HVDC link. To suppress the SSTI phenomenon, an additional SSTI stabilizer is proposed and installed in Wind Farm 1 of Santanghu. The additional SSTI stabilizer exhibits good performance in mitigating oscillation, with the advantage of low cost and a low additional area footprint.

PLL effect on subsynchronous torsional interaction with VSC‐HVDC

PLL effect on subsynchronous torsional interaction with VSC‐HVDC

Investigation of SSTI Between Practical MMC-based VSC-HVDC and Adjacent Turbogenerators through Modal Signal Injection Test

With increasing applications of voltage source converter-based HVDC (VSC-HVDC), the potential risk of subsynchronous torsional interaction (SSTI) between turbogenerator and VSC-HVDC has been extensively studied in previous work. However, contradictory results were reported by different researchers. Unlike previous studies, wherein the oversimplified models of VSC and generator were used, this paper uses the full-scale model of a practical modular multilevel converter (MMC)-based VSC-HVDC and the multi-mass shaft model of a turbogenerator to analyze the characteristics of SSTI. Moreover, a different approach, namely, the modal signal injection test, is proposed to identify the damping of torsional modes and the impact from decisive factors such as the loading levels, control strategies of converters, and the coupling strength between VSC-HVDC and ac networks. Comparative analyses between the full-scale and the equivalent models have revealed how system modeling affects SSTI characteristics and helped to explain the inconsistencies among the results of previous studies.

Study of HVDC System and Subsynchronous Oscillation

With the application and development of Power Electronics, HVDC is applied more widely China. However, HVDC system has the possibilities to cause subsynchronous torsional vibration interaction with turbine generator shaft mechanical system. This paper simply introduces the mechanism, analytical methods and suppression measures of subsynchronous oscillation. Then it establishes a power plant model in islanding model using PSCAD, and analyzes the effects of the number and output of generators to SSO, and verifies the effect of SEDC and SSDC using time-domain simulation method. Simulation results show that the more number and output of generators is detrimental to the stable convergence of subsynchronous oscillation, and SEDC、SSDC can restrain unstable SSO, avoid divergence of SSO, ensure the generators and system operate safely and stably

電力信号フィードバック直列補償器を用いた発電機軸ねじれ共振現象の抑制

Suppression of subsynchronous torsional interaction(SSTI) in electrical power system achieved with insertion of an inverter to transmission line in series. Active power of the transmission line is sensed and filtered by three sets of bandpass filters to get the torsional signal for feedback. The output signal of controller is converted to voltage reference signals for each phase by dq inverse transformation and the voltage reference signals are input to PWM control circuit to generate the gate firing signals for the inserted inverter. Simulation results show the control is effective to suppress the SSTI

Analysis of Subsynchronous Torsional Interaction of HVDC System Integrated Hydro Units With Small Generator-to-Turbine Inertia Ratios

The constant current controllers at high voltage direct current (HVDC) rectifier stations and the outer loops which set the reference to the current controller have potential to introduce negative damping on the nearby generating units. It is therefore vital to investigate the torsional interaction behavior between turbine-generator (TG) units and the HVDC systems over a frequency range of interest. In this paper, subsynchronous torsional interaction (SSTI) phenomenon for hydroelectric TG units with small generator-to-turbine inertia (GTI) ratios interconnected to an HVDC system has been investigated. The sensitivity of different system parameters, such as generating capacity and loading, DC power flow, AC network strength and current controller gains, towards electrical damping contribution is also examined. Hydro units with different shaft systems are examined for SSTI proneness by conducting perturbation analysis. The impact of hydroelectric TG units with different GTI ratios has also been examined by applying a three-phase to ground fault at the rectifier station. Simulation studies have been carried out in PSCAD <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">®</sup> /EMTDC <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">©</sup> environment. Due to lack of modal damping with relatively low decrement factor and logarithmic decrement, sustained and growing TG torque oscillations are noted for certain operating conditions of hydro units with small GTI ratios interconnected with an HVDC system.

Normal Form Analysis of the Subsynchronous Torsional Interaction with SVC in the Complex Power Systems

Due to the importance of nonlinear interactions in the complex power systems, in this paper the subsynchronous torsional interactions (SSTI) between turbine-generator shafts and flexible AC transmission system (FACTS) are studied using the nonlinear modal analysis method. Normal Form method is applied to study the nonlinear effects on the SSTI phenomenon in the presence of Static VAr Compensator (SVC) as a commonly used FACTS device. The results show that under stressed conditions the SVC voltage controller gains have an adverse effect on the nonlinear interactions of torsional modes. These effects can lead to severe excitation of torsional oscillations. The study also illustrates that the type of SVC control system model will play an important role in the nonlinear analysis of SSTI, and using a simple PI model is not suitable for nonlinear analysis of SSTI phenomenon. Moreover, in a multi-area power system, the location of SVC and its capacity have significant effects on the severity of nonlinear interactions of torsional modes.