REVIEW ON SUB-SYNCHRONOUS OSCILLATIONS IN WIND FARMS: ANALYSIS METHOD, STUDY SYSTEM, AND DAMPING CONTROL

More and more attention on wind farm sub-synchronous oscillation (SSO) has been paid as many SSO incidents in wind farms have occurred. This paper presents an overview of recent SSO issues in wind farm from the perspective of control, including the analysis methods, the study system, and the SSO mitigation by damping control. Three major analysis methods, as well as different study systems for wind farm SSO study, are comprehensively reviewed. The adaptability and complexity of the methods and study systems are analysed, and an overall survey of recent SSO analysis is given. Among the wind farm SSO mitigation methods, the sub-synchronous damping controller (SSDC) is one of the most commonly used methods in practice. Its configuration and signal selection are introduced in this paper

Dynamics of power systems with wind power generation and the fractional frequency transmission system

Under pressure for low carbon emissions and environmental protection, large scale wind farms are constructed and integrated into power systems to meet energy demands. On the other hand, the long distance transmission for large scale wind power and conventional power plants is another technical issue in modern power systems. These two challenges stimulate the research and development of wind energy and the fractional frequency transmission system (FFTS). Thus, the research of this thesis focuses on the dynamics of power systems with wind power generation and the FFTS. This thesis can be divided into the following three parts: Firstly, the influence of wind farms on the subsynchronous resonance (SSR) of conventional power systems is systematically examined. Both eigenvalue analysis and time domain simulations are conducted to examine the influence of wind farms. Secondly, the FFTS is proposed to deliver the energy from large scale offshore wind farms. The small signal stability of the FFTS with wind farms is studied. To improve the damping performance, a proper controller for the FFTS is also proposed. Thirdly, the FFTS is also applied in grid interconnections. The power flow controller for the FFTS is also proposed in this application

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

Power system dynamic enhancement using phase imbalance series capacitive compensation and doubly fed induction generator-based wind farms

ABSTRACT Wind energy is among the fastest growing renewable energy technologies in the world that has been increasing by about 30% a year globally. Wind energy has proven to be a clean, abundant and completely renewable source of energy. Owing to the rapidly increasing use of wind power, the aspect of integrating high level of penetrations wind power into the grid is becoming more and more of reality. Examples of large wind farms in the United States are the 781.5 MW Roscoe wind farm in Texas, the 735.5 MW Horse Hollow Wind Energy Center in Taylor and Nolan County, Texas, the 845 MW Shepherds Flat wind farm in Oregon and the 1550 MW Alta wind farm being developed in California. As most large wind farms in North America employ Doubly-fed Induction Generator (DFIG) wind turbines, their voltage-sourced converter-based back-to-backs offer independent control of the real and reactive power. The use of these control capabilities have been recently proposed for damping power swings, inter-area oscillations as well as subsynchronous resonance. There is, however, a question that is always associated with the use of voltage-sourced converter -based back-to-back wind farms for damping power system oscillations: what happens when there is no wind? The keyword to the answer is “combined”. The potential benefit of using these types of wind farms for damping power system oscillations should always be combined with conventional damping devices (power system stabilizers, thyristor controlled series capacitor, static synchronous series compensator, high voltage dc systems, etc.). This thesis reports the results of digital time-domain simulation studies that are carried out to investigate the potential use of supplemental controls of DFIG-based wind farms combined with a phase imbalanced hybrid series capacitive compensation scheme for damping power system oscillations. The thesis also addresses the recent concern over the case of large share of wind power generation which results in reducing the total inertia of the synchronous generators and degrading the system transient stability. In this regards, the results of the investigations have shown that in such a case; properly designed supplemental controllers for the wind farm converters could be an asset in improving the system transient stability rather than degrading it. Time-domain simulations are conducted on a benchmark model using the ElectroMagnetic Transients program (EMTP-RV)

Predicted dynamic performance of a possible AC link between SaskPower north and south systems

SaskPower has two separate systems, namely the North and the South systems. The South system contains SaskPower major generation and system load. The North system load is located relatively far from its generation (200 to 300 km). The North system is considered, therefore, to be electrically weaker than the South system. Recently there has been an interest in connecting the two systems to improve the security, stability and reliability of the integrated system. Grid interconnections, however, especially between weak and strong systems, often result in the arising of low-frequency oscillations between the newly connected areas. These oscillations that are termed “inter-area oscillations” exhibit, generally poor damping and can severely restrict system operations by requiring the curtailment of electric power transfers level as an operational measure. There are two options for SaskPower North and South systems interconnection, namely HVAC and HVDC interconnections (tie-lines). This thesis reports the results of digital timedomain simulation studies carried out to investigate the dynamic performance of a proposed 260 km, 138 kV double-circuit HVAC tie-line incorporating a hybrid three-phase Thyristor- Controlled Series Capacitor (TCSC) compensation scheme that would connect the SaskPower North and South systems. The potential problems that might arise due to such an interconnection, namely power flow control and low-frequency oscillations are studied and quantified and a feasible solution is proposed. In this context, the effectiveness of the TCSC compensation scheme in damping power system oscillations in the tie-line is investigated. Time-domain simulations were conducted on the benchmark model using the ElectroMagnetic Transients Program (EMTP-RV). The results of the investigations demonstrate that the proposed HVAC link that incorporates a TCSC compensation scheme is effective in mitigating the low-frequency oscillations between the North and South systems for different system contingencies and operating conditions

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