SS resonance analysis of complex power system incorporating wind power

This paper examines the small-signal oscillation stability of a grid-connected wind farm affected collectively by multiple doubly fed induction generators (DFIGs) with similar dynamics. An analysis of this issue, which is referred to as the collective impact of similar DFIGs, revealed that the interconnection of a standalone DFIG with the rest of the wind farm determines whether the collective impact of similar DFIGs is beneficial or detrimental to the oscillation stability of the wind farm. The network topology and parameters of the wind farm affect the scale of the collective impact in several respects. First, increasing the length of the transmission line connecting the wind farm to the AC grid leads to an increase of the collective impact. Second, when the transmission line is much longer than the cables within the farm, the collective impact is approximately proportional to the number of DFIGs. Accordingly, increasing the transmission line length and/or number of DFIGs may magnify the detrimental collective impact, resulting in growing oscillations in the wind farm in the worst case. The analysis of the collective impact of similar DFIGs in this study revealed a new mechanism through which grid-connected DFIG wind farms can lose oscillation stability.

The wind farm based on Doubly Fed Induction Generator (DFIG) is built. Not only it can be as active power source to provide active power to network, but also as reactive power source to stabilize voltage or supply Var compensation (reactive power absorbed or generated). Therefore, Var coordinated control strategy is proposed, instead of expensive equipments, such as Static Var Compensator(SVC) or Static Synchronous Compensator (STATCOM), in order to provide sufficient reactive power to maintain system stability. DFIG comprehensive model is built based on correlation models in Dig SILENT/Power Factory. The eigen value analysis method is employed to verify its impact on low-frequency oscillation modes and small signal stability of power system integrated with wind farm. The results from eigen value analysis are elucidated the effectiveness of Var coordinated control in DFIG system.

With a growing number of wind farms connected to the traditional grid, it may cause a series of instability because the wind power's randomness. In order to analyze that the small-signal stability of the power system whether affected by the different transmission lines between wind farms and the power grid or not? At first, a PMSG (Permanent Magnet Synchronous Generator) model of direct-driven wind turbine was established, and the WSCC (Western Systems Coordinating Council) 3-generator 9-Bus system was taken as the test system. Then, the distribution of the system's eigenvalues was considered under the different penetration rate of wind power. Lastly, the time domain simulation in PSAT was finished. The results showed that the overhead transmission line and the cable have the distinguishing influence on the small-signal stability of the power system when PMSG-Based wind farm conducted to the existing system, using cable as the transmission line can make the system get better small signal stability.

Nowadays, integration of large-scale wind farms (WFs) into power systems is experiencing rapid growth. As this rapid integration can affect transient stability significantly, employing doubly fed induction generator (DFIG)-based wind turbines, which have shown better behavior regarding system stability, has attracted much attention. This research contributes to the literature by investigating the transient stability of the power system with increasing penetration of DFIG-based WFs. In the proposed framework, the current-balance form is utilized for the network equations, and in this way, transient stability is performed using time-domain simulation. According to the simulation results, when the rate of wind power generation exceeds 0.7 per-unit, the increasing trend of the critical clearing time (CCT) is reversed and the CCT decreases greatly with the increased wind power penetration. In addition, the reactive power compensation by DFIG, the gearbox ratio, the power system strength, and DFIG parameters are comprehensively investigated as effective parameters on transient stability. Since the rated rotor speed of DFIG significantly impacts the electrical torque and machine currents, the reduction of the rated rotor speed due to the change of the gearbox ratio has been investigated as one of the effective factors to improve the transient stability. The simulation results demonstrate the effectiveness of the proposed approach in improving power system transient stability.

This paper presents the stability simulation results of an integration of an offshore wind farm (OWF) based on doubly-fed induction generator (DFIG) and an OWF based on permanent-magnet synchronous generator (PMSG) connected to a large power grid through a high-voltage direct-current (HVDC) link based on voltage-source converter (VSC). These two OWFs are connected to the same AC bus through individual step-up transformers and transmission lines. A damping controller located at the converter station of the proposed VSC-HVDC link is designed by using pole-assignment approach to contribute adequate damping to the studied two OWFs under various operating conditions. A systematic approach using a frequency-domain method and a time-domain scheme is performed to examine the effectiveness of the proposed control scheme. It can be concluded from the simulation results that the proposed VSC-HVDC link joined with the designed damping controller located at the converter station can not only render appropriate damping to the modes of the studied OWFs under various operating conditions but also effectively mitigate inherent fluctuations of the power injected to the power grid.

The ESS is considered as an effective tool for enhancing the flexibility and controllability of a wind farm, and the optimal control scheme of a wind farm with distributed ESSs is vital to the stable operation of wind power generation. In this paper, a coordinated active and reactive power control strategy based on model predictive control (MPC) is proposed for doubly fed induction generator (DFIG)-based wind farm (WF) with distributed energy storage systems (ESSs). The proposed control scheme coordinates the active and reactive power output among DFIG wind turbines (WTs), grid-side converters (GSCs), and distributed ESSs inside the WF, and the aim is to decrease fatigue loads of WTs, make the WT terminal voltage inside the extent practicable, and take the WF economic operation into consideration. Moreover, the best reactive power references of DFIG stator and GSC are produced independently based on their dynamics. At last, the control scheme generates optimal power references for all ESS to make the SOC of each ESS converge to their average state. With the distributed ESSs, the WF controller regulates the WTs inside WF more flexibly. A WF composed of 10 DFIG WTs was utilized to verify the control performance of the proposed coordinated active and reactive power control strategy.

The transient stability analysis incorporating wind power intermittency and volatility is studied in this study. The wind turbines with doubly fed induction generator (DFIG) and direct‐driven permanent magnet synchronous generator (PMSG) are considered, respectively, during analysis. In modelling of DFIG and PMSG, the default GE wind turbine model and the Western Electricity Coordinating Council generic wind turbine model are employed, respectively. Based on the Jensen model, which is applied to describe the wake effect in a wind farm located on flat terrain, an equivalent simplified model of a wind farm is established regarding wind farm layout. On the basis of the built wind farm model, the Monte Carlo simulation technique combined with two evaluation indices, namely angle‐based margin index and critical clearing time (CCT) is applied on the IEEE 10‐generator‐39‐bus test system and a real‐sized China‐Jiangxi power grid as a benchmark to exploit and explore the effects of wind power intermittency and volatility on power system transient stability. The frequency distributions of transient stability evaluation index, CCT, probability of system transient instability and the range of wind speeds causing system transient instability are simulated. In addition, the influence of wake effect on transient stability is discussed as well.

The analysis of reactive power for offshore and onshore wind farms connected to the grid through high-voltage alternating-current transmission systems is considered in this paper. The considered wind farm is made up with doubly fed induction generators (DFIGs). Modeling and improved analysis of the effective reactive power capability of DFIGs are provided. Particularly, the optimal power-tracking constraints and other operational variables are considered in the modeling and analysis of the DFIG reactive power capability. Reactive power requirements for both overhead and cable transmission systems are modeled and compared with each other as well as with the reactive power capability of the wind farms. Possibility of unity power factor operation suggested by the German Electricity Association (VDEW) is investigated for both types of installations. Aggregate reactive power demands on both wind farms are assessed such that the bus voltages remain within an acceptable bandwidth considering various operational limits. The reactive power settings for both types of wind farm installations are determined. In addition, the minimum capacity and reactive power settings for reactive power compensation required for cable-based installations are determined. Several numerical examples are given to illustrate the reactive power characteristics and capability of DFIGs, performance of transmission lines and reactive power analysis for DFIG-based grid-connected wind farms. A summary of the main outcomes of the work presented in this paper is provided in the conclusions section. Copyright © 2012 John Wiley & Sons, Ltd.

In order to sustain the overall competitiveness of the wind power industry, unrelenting focus is required on working toward the advancement of enabling technologies and research studies that are associated with wind farm systems. First, wind farm technologies that include various turbine generator systems coupled with different power transmission configurations have enormous impact in determining the quality of wind power production. In addition, modern wind farms are expected to implement robust power control algorithms to meet more advanced requirements of electricity generation. Accordingly, this study explores the statuses of wind energy harvesting technologies and wind farm control strategies by discussing their recent and future impact on transforming the wind power industry. Doubly fed induction generator (DFIG)-based wind energy harvesting technology is well-matured and has exhibited an excellent track-record in past and recent experiences, but its capability of being further scalable for large-scale power production is limited as it is largely incompatible with high-voltage power transmission networks. On the other hand, permanent magnet synchronous generator (PMSG)-based technology is making significant advancements to attain the maximum possible efficiency level in greatly facilitating larger scale power generation, although the construction of bulky and costly power transmission systems is required. In this regard, future technological advances in the wind farm industry are expected to reasonably optimize the design and cost of high-voltage power transmission systems. Similarly, an increasing number of research studies are introducing a number of power optimization-based control models to create an ideal integration of the aforementioned wind farm technologies so as to ultimately enhance the reliability of electricity production by maintaining the systems’ safety. Yet, additional work is still expected to be undertaken in the future for a more extended evaluation of the performances of many different control models under a similar environment.

This paper investigates the sub-synchronous resonance (SSR) phenomenon in a series compensated doubly-fed induction generator (DFIG) based wind farms. A detailed linear state space model of a fixed-series compensated DFIG wind farm is presented for different operating point conditions. The model of the system includes a wind turbine aerodynamics, a 6 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> order induction generator, a 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> order two-mass shaft system, a 4 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> order series compensated transmission line, an 8 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> order rotor and generator side converter controllers, and 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> order DC link model. The 22 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> order system is modeled in Matlab/Simulink, and modal analysis is performed on the modeled DFIG wind farm, and eigenvalues of the system are calculated. The eigenvalue analysis and time-domain simulation results show that SSR can potentially occur in the fixed-series compensated DFIG-based wind farm even at realistic levels of series compensation. The fixed-series capacitor is replaced with a gate-controlled series capacitor (GCSC), which is a series flexible ac transmission system (FACTS) device composed of a pair of gate-commutated switches in parallel with a capacitor that enables one to control impedance and power flow of transmission lines, for series compensation and SSR damping. The root-locus analysis is employed to design a SSR damping controller (SSRDC) for the GCSC. It is shown than a well-designed SSRDC for the GCSC can alleviate the SSR in DFIG-based wind farms. The IEEE first benchmark model adapted with a DFIG-based wind farm is employed as a case study.

Wind turbine vibrations can result from periodic excitation caused by the wind and by both wind and seas for the case of offshore units. In particular, the turbine drivetrain is subject to torsional vibrations caused by both changes in the wind and grid disturbances. This paper uses a collection of different control schemes to damp the vibrations and seeks the best controller by optimizing each in terms of gain selection. Two 750 kW wind turbine models, one with a DFIG (doubly fed induction generator) and the other with a PMSG (permanent magnet synchronous generator) are used in the investigation. Numerical simulations of the wind turbines using the NREL developed software FAST 8 are the means of conducting the tests. In varying the gains, the work discovers that the best controller for a DFIG differs greatly from the best controller for a PMSG. In addition, the work found that the vibration damping in a DFIG turbine is different when the source of the vibration-causing disturbance is considered. The paper reports both the optimized gains and a set of questions raised by the behavior of the DFIG turbine where the response of the shaft torsion differs according to the source of the disturbance, i.e. either grid side or wind side.

This paper presents a new configuration of doubly fed induction generator (DFIG)-based wind farm with multilevel cascaded H-bridge (CHB) converter and its control system, which is suitable not only for unbalanced grid voltage conditions, but also for small voltage sags. The proposed configuration of DFIG can balance the point of common coupling (PCC) voltage without needing additional DC link capacitor or energy storage unlike other configurations and methods. Consequently, the stator voltage and the other loads voltage connected to the PCC are kept constant at the nominal value. The new configuration improves all the DFIG signals under unbalanced grid voltage and small voltage sag conditions unlike other methods. Some of the DFIG units in wind farm require to connect the CHB converter, therefore, it is cost-effective. Matlab/Simulink is used for simulation of a 13.5-MW wind farm consisting of nine 1.5-MW DFIG units and the results demonstrate the good efficiency of the proposed scheme.

To investigate the new type of sub-synchronous oscillation (SSO) incidents in permanent magnet synchronous generators (PMSG) based wind farm in Xinjiang grids, this paper establishes an equivalent grid model based on practical Xinjiang grids data in PSCAD/EMTDC. To avoid the complexity and limitations of eigenvalue analysis, the Prony method is adopted to obtain system damping information. Then, taking one of those wind farms as a research case, the impacts of main factors on SSO characteristics are discussed in detail. The analysis of results reveal that improper operating conditions would weak the sub-synchronous damping and stimulate SSO in wind farm. Additionaly, a possible safe operation range of one PMSG-based wind farm through extensive simulations is presented in this research work. To further extend the solution of exsiting problems, an additional mitigation strategy based on static synchronous series compensator (SSSC) is introduced to compensate the sub-synchronous voltage and thus to suppress the SSO.

This paper mainly proposes a capacity configuration strategy for a hybrid wind farm with fixed-speed induction generator (FSIG)-based and permanent magnet synchronous generator (PMSG)-based wind farms during grid faults. In this paper, FSIG-based wind farm realizes low voltage ride through (LVRT) with the associated control of nearby PMSG systems as StatComs to provide the required reactive power. By establishing the mathematical model of hybrid wind farm containing network parameters, the operation characteristics of the FSIG-based wind farm during grid faults are analyzed in detail. Based on the characteristics and the critical clearing time of FSIG-based wind farm, a novel capacity configuration method for the hybrid wind farm is proposed to calculate the required reactive power of FSIG-based wind farm for a LVRT operation. Based on the capacity configuration result, the minimum installed capacity of PMSG-based wind farm could be determined. Taking into account the impact of reactive power compensation capacity and grid transmission line parameters, the coordinated LVRT capability of the hybrid wind farm is then analyzed. Finally, simulation and experimental results demonstrate the effectiveness of the proposed capacity configuration method and the coordinated operation performance of the hybrid wind farm respectively.

Fractional frequency transmission system (FFTS) is a relatively new transmission technology that can be used to deliver the energy from remote offshore wind farms, and it can increase the transmission capacity through the reduced transmission frequency. However, the dynamic performance of a FFTS with the wind farm may be different from that of a traditional ac transmission with the wind farm. In this paper, a doubly fed induction generator (DFIG)-based wind farm is connected to the main grid via the FFTS. The detailed dynamic model of the FFTS with wind farm is established first, and then the eigenvalue analysis is carried out to evaluate the system dynamic damping performance. To verify the results of eigenvalue analysis, dynamic simulations are carried out on a single machine infinite bus and a multimachine system. Both eigenvalue analysis and simulations demonstrate that the FFTS will have a negative influence on the damping of the DFIG-based wind farm in comparison to that of the wind farm with the traditional ac transmission system. However, with the control of cycloconverter, this problem can be easily overcome, and the damping performance of wind farm with FFTS can be even better.