Assessment of subsynchronous oscillations in AC grid-connected VSC systems with type-4 wind turbines

© 2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Converters are the key for increasing the development of renewable energy generation but their dynamic interaction with the grid has an important impact on stability. Oscillatory instabilities in different grid-connected converter systems at several frequency ranges are reported. In particular, sub and supersynchronous oscillatory instabilities in AC power systems with type-3 and type-4 wind turbine generators (WTGs) were recently registered at several wind farm areas. A number of works based on eigenvalue analysis and frequency domain approaches are carried out to analyse this new stability issue and more research is going on to analyse in detail the electric and control parameters that may affect this phenomenon. This study contributes analysing in detail the influence of system parameters on the subsynchronous oscillations (SSOs) in AC power systems with type-4 WTGs. This study is based on a new approach for subsynchronous stability assessment which could also be used for analysing other types of SSOs (e.g. subsynchronous control interactions between doubly-fed induction generators and series-compensated networks) as well as supersynchronous oscillations. A representative example of weak AC grids with type-4 WTGs is used to illustrate the contributions of the study. Results are validated with PSCAD/EMTDC time-domain simulations.Postprint (author's final draft

Influence of active power output and control parameters of full-converter wind farms on sub-synchronous oscillation characteristics in weak grids

Active power outputs of a wind farm connected to a weak power grid greatly affect the stability of grid-connected voltage source converter (VSC) systems. This paper studies the impact of active power outputs and control parameters on the subsynchronous oscillation characteristics of full-converter wind farms connected weak power grids. Eigenvalue and participation factor analysis was performed to identify the dominant oscillation modes of the system under consideration. The impact of active power output and control parameters on the damping characteristics of subsynchronous oscillation is analysed with the eigenvalue method. The analysis shows that when the phase-locked loop (PLL) proportional gain is high, the subsynchronous oscillation damping characteristics are worsened as the active power output increases. On the contrary, when the PLL proportional gain is small, the subsynchronous oscillation damping characteristics are improved as the active power output increases. By adjusting the control parameters in the PLL and DC link voltage controllers, system stability can be improved. Time-domain results verify the analysis and the finding

Passivity - Based Control and Stability Analysis for Hydro-Solar Power Systems

Los sistemas de energía modernos se están transformando debido a la inclusión de renovables no convencionales fuentes de energía como la generación eólica y fotovoltaica. A pesar de que estas fuentes de energía son buenas alternativas para el aprovechamiento sostenible de la energía, afectan el funcionamiento y la estabilidad del sistema de energía, debido a su naturaleza inherentemente estocástica y dependencia de las condiciones climáticas. Además, los parques solares y eólicos tienen una capacidad de inercia reducida que debe ser compensada por grandes generadores síncronos en sistemas hidro térmicos convencionales, o por almacenamiento de energía dispositivos. En este contexto, la interacción dinámica entre fuentes convencionales y renovables debe ser estudiado en detalle. Para 2030, el Gobierno de Colombia proyecta que el poder colombiano El sistema integrará en su matriz energética al menos 1,2 GW de generación solar fotovoltaica. Por esta razón, es necesario diseñar controladores robustos que mejoren la estabilidad en los sistemas de energía. Con alta penetración de generación fotovoltaica e hidroeléctrica. Esta disertación estudia nuevas alternativas para mejorar el sistema de potencia de respuesta dinámica durante y después de grandes perturbaciones usando pasividad control basado. Esto se debe a que los componentes del sistema de alimentación son inherentemente pasivos y permiten formulaciones hamiltonianas, explotando así las propiedades de pasividad de sistemas eléctricos. Las principales contribuciones de esta disertación son: una pasividad descentralizada basada control de los sistemas de control de turbinas hidráulicas para sistemas de energía de múltiples máquinas para estabilizar el rotor acelerar y regular el voltaje terminal de cada sistema de control de turbinas hidráulicas en el sistema como, así como un control basado en PI pasividad para las plantas solares fotovoltaicas

Subsynchronous Resonance in Doubly Fed Induction Generator based Wind farms

The objective of this thesis is to investigate the risk for instabilities due to subsynchronous resonance (SSR) conditions in large wind farms connected to series-compensated transmission lines. In particular, the focus is on doubly-fed induction generator (DFIG) based wind farms. Analytical models of the system under investigation are derived in order to understand the root causes that can lead to instabilities. A frequency-dependent approach based on the generalized Nyquist criterion (GNC) has been applied to investigate the risk for SSR in DFIG based wind turbines. Through this approach, it is shown that the observed phenomenon is mainly due to an energy exchange between the power converter of the turbine and the series compensated grid. This phenomenon, here referred to as subsynchronous controller interaction (SSCI), is driven by the control system of the turbine’s converter, which results in a non-dissipative behavior of the DFIG system in the subsynchronous frequency range. The different factors that impact the frequency characteristic of the wind turbine, thereby making the system prone to SSCI interaction, are investigated. Through the analysis, it is shown that in a DFIG wind turbine, the current controller that regulates the rotor current plays a major role in the risk for SSCI, where an increased closed-loop bandwidth negatively impacts the system damping in the subsynchronous frequency range. The level of active power output from the wind farm also has an impact on the overall system stability; in particular, it is shown that the power-dissipation properties of the DFIG improves when the latter is operated in supersynchronous speed range (high-power output).Methods for proper aggregation of the wind turbines in the farm are investigated. Time-domain studies are performed on the aggregated model connected to a series compensated transmission line to verify the analytical results obtained through the frequency domain analysis. Based on the theoretical analysis, mitigation strategy is proposed in order to shape the frequency behavior of the wind turbine. The effectiveness of the proposed mitigation strategy is evaluated both theoretically through frequency-domain analysis and using detailed time-domain simulations

Subsynchronous Resonance in DFIG-Based Wind Farms

RÉSUMÉ La résonance sous-synchrone (SSR) se produit lorsqu’un réseau de transmission est compensé et un générateur commence à échanger de l’énergie aux fréquences inférieures à celles du système d’alimentation. Ce phénomène a été observé dans la centrale électrique d’Arizona en 1970, lorsqu’un générateur synchrone a été relié radialement à une ligne compensée. Depuis, des recherches approfondies ont été réalisées pour analyser et atténuer ces oscillations. En octobre 2009, un incident d’interaction de contrôle sous-synchrone (SSCI) s’est produit dans le système d’énergie ERCOT (parc éolien de Zorillo Gulf au Texas), ce qui a révélé la susceptibilité des parcs éoliens à générateur d’induction doublement alimentés (DFIG) à des phénomènes sous-synchrone. Dans cette thèse, le problème de SSCI dans un parc éolien basé sur DFIG relié à la ligne de transmission compensée en série est étudié. Les équations linéarisées qui décrivent le comportement du système sont développées sur la base du modèle réaliste de l’éolienne. Cet ensemble d’équations est utilisé pour obtenir un aperçu du comportement du système et de ses modes dominants. Plusieurs points de repère sont également développés sur la base du système d’alimentation réaliste pour aborder le problème de la simplicité et du système irréaliste dans la littérature existante. La simulation électromagnétique (EMT) a été utilisée pour obtenir le comportement transitoire précis du système et vérifier son exécution avec les exigences du code de réseau. Les points de repère développés ainsi que l’approche de balayage de fréquence, la méthode basée sur l’impédance, l’analyse de valeurs propres ou les simulations EMT permettent de détecter le risque potentiel des oscillations SSCI. Les analyses de valeurs propres et de sensibilité sont ensuite effectuées pour observer l’impact de différents paramètres système sur la stabilité. Plusieurs contrôleurs supplémentaires sont proposés pour résoudre le problème de stabilité et les mauvaises performances résultant du phénomène SSCI. Les contrôleurs proposés sont conçus selon la topologie et les conditions de fonctionnement du système d’alimentation. Pour vérifier la performance des contrôleurs proposés, plusieurs études de simulation sont réalisées dans le logiciel EMTP-RV. Dans les études de simulation, un modèle de parc éolien détaillé comprenant les fonctions de détection de défauts (FRT), le contrôleur de parc éolien (WFC), les non-linéarités du circuit électrique et de contrôle, le réseau de sous-transmission détaillé et la vitesse du vent non homogène sont considérés.---------- ABSTRACT Subsynchronous resonance (SSR) occurs when a compensated transmission network and a generator start to exchange energy at frequencies lower than that of the power system. This phenomenon was observed in an Arizona power station in 1970, when a synchronous generator was radially connected to a compensated line. Since then, extensive research has been conducted to analyze and mitigate such oscillations. In October 2009, a subsynchronous control interaction (SSCI) incident occurred at an ERCOT (Electric Reliability Council of Texas) power system (at Zorillo-Gulf wind farm) which revealed the susceptibility of doublyfed induction generator (DFIG)-based wind farms to the subsynchronous phenomenon. This thesis investigates the SSCI problem in a DFIG-based wind farm connected to a series compensated transmission line. The linearized equations which describe the behavior of the system are developed based on a realistic wind turbine model. This set of equations is utilized to gain insight into the behavior of the system and its dominant modes. Benchmarks are developed based on realistic power systems to tackle the problems caused by the application of oversimple and unrealistic case study systems that exist in the literature. Electromagnetic transient (EMT) simulation is carried out to obtain the precise dynamic response of the system and to verify its compliance with the grid code requirements. The developed benchmarks together with the frequency scan approach, the impedance-based method, eigenvalue analysis, and EMT simulations are used to identify the potential risk of the SSCI oscillations and to obtain guidelines for the safe operation of the system. Eigenvalue and sensitivity analyses are then performed to evaluate the impact of different parameters on the stability of the system. Supplementary controllers are proposed to tackle the stability problem and the poor performance due to the SSCI phenomenon. The proposed controllers are designed according to the topology and the operating conditions of the power system. To examine the performance of the proposed controllers, several simulation studies are carried out using the EMTP-RV software. In the simulation studies, a detailed wind farm model is considered. This detailed model includes the fault-ride-through (FRT) functions, a wind farm controller (WFC), the nonlinearities of the electrical and control circuits, a detailed sub-transmission network, and non-homogeneous wind speed. The problems of local and central supplementary SSCI controllers and the impact of aggregating the wind turbine generators in the design procedure are also discussed. It is shown that delays and sensor failure can adversely affect stability when a supplementary controller is included at the secondary level. To overcome these implementation challenges, some existing approaches such as Smith predictor scheme and residue generation method are used in order to increase the delay margin and to achieve a fault-tolerant control system

Stability analysis of VSCs connected to an AC grid

As a typical renewable energy resource, wind power has been extensively exploited in the past years. With the continuous growth of wind power installed capacity, power electronic devices are widely used due to their good control performances. However, the interaction between power electronic devices and the power grid will lead to power system stability problems. Subsynchronous interaction (SSI) between wind farms and AC grids, as one of the most severe power system stability problems, has aroused great concerns. Different from the SSI between wind turbine generator (WTG) controllers and fixed series compensation or between generators and high voltage direct current (HVDC) controllers, recently, a new type of SSI is detected which is caused by the interactions between power electronic devices of WTGs and weak AC grid. The work in this thesis focuses on the mechanism and characteristics of this new type of SSI. A simplified system model with permanent magnet synchronous motors (PMSGs) connected to weak AC grids is established to investigate the new SSI. The linear impedance model of the studied system is conducted. The correctness of the proposed impedance model is validated by the comparison between the analysis in MATLAB and time-domain simulations in PSCAD/EMTDC. In the model of a single VSC connected to an AC grid, the mechanism of the SSI is investigated. The characteristics of the studied system under various conditions are analysed. The effects on system stability of different factors have been studied including the number of connected WTGs line reactance, Phase-locked loop (PLL), feed-forward voltage low-pass filter, current loop and outer loop of the VSCs. Time-domain simulation results verify the correctness of the analysis. An equivalent model of multiple VSCs connected to an AC grid is presented to investigate the characteristics when VSCs with different control systems and control parameters are connected to an AC grid. A new approach based on the Generalised Nyquist Criterion (GNC) is proposed to analyse the system stability. Compared to the existing traditional criteria, the new criterion has the advantages of better accuracy and simplicity. The new approach is validated in time-domain simulation. The study of this research work contributes to the stability analysis of power systems in the subsynchronous frequenc

Power Converter of Electric Machines, Renewable Energy Systems, and Transportation

Power converters and electric machines represent essential components in all fields of electrical engineering. In fact, we are heading towards a future where energy will be more and more electrical: electrical vehicles, electrical motors, renewables, storage systems are now widespread. The ongoing energy transition poses new challenges for interfacing and integrating different power systems. The constraints of space, weight, reliability, performance, and autonomy for the electric system have increased the attention of scientific research in order to find more and more appropriate technological solutions. In this context, power converters and electric machines assume a key role in enabling higher performance of electrical power conversion. Consequently, the design and control of power converters and electric machines shall be developed accordingly to the requirements of the specific application, thus leading to more specialized solutions, with the aim of enhancing the reliability, fault tolerance, and flexibility of the next generation power systems

Subsynchronous Resonance in Doubly-Fed Induction Generator Based Wind Farms

The objective of this thesis is to investigate the risk for instabilities due to SubSynchronous Resonances (SSR) conditions in large wind farms connected to series-compensated transmission lines. In particular, the focus is on Doubly-Fed Induction Generator (DFIG) based wind farms. Analytical models of the system under investigation are derived in order to understand the root causes that can lead to instabilities. A frequency dependent approach, based on the Nyquist criterion, has been applied in order to investigate the risk for SSR in DFIG based wind turbines. Through this approach, it is shown that the observed phenomenon is mainly due to an energy exchange between the power converter of the turbine and the series compensated grid. This phenomenon, here referred to as SubSynchronous Controller Interaction (SSCI), is driven by the control system of the turbine, which presents a non-passive behavior in the subsynchronous frequency range. The different factors that impact the frequency characteristic of the wind turbine, thereby making the system prone to SSCI interaction, have been investigated. Through this analysis, it is shown that in a DFIG wind turbine, the current controller in the rotor-side converter plays a major role and that the risk for SSR increases when increasing its closed-loop bandwidth. In addition, it is shown that the output power generated from the wind turbine has an impact on the frequency characteristic of the turbine. Time-domain studies are performed on an aggregated wind turbine model connected to a series compensated transmission line with the objective of verifying the analytical results obtained through frequency-domain analysis. Based on the theoretical analysis, mitigation strategies are proposed in order to shape the impedance behavior of the wind turbine in the incident of SSCI. The effectiveness of the proposed mitigation strategies are evaluated both theoretically through frequency domain analysis and using detailed time-domain simulations