Fixed-Speed Wind-Generator and Wind-Park Modeling for Transient Stability Studies

In this work, steady state characteristic and transient stability of wind turbine generator system (WTGS) are analyzed in detail with using a power system model, in which two wind farms are connected with multi-machine power system. At each wind farm terminal one three-level PWM based STATCOM is connected. Fuzzy logic controller (FLC) is used as the control methodology of the STATCOM. Since shaft system modeling of wind turbine has a significant effect on the transient stability analysis of WTGS, two-mass shaft model is adopted in this study. The capacitor bank capacity of wind generator is reduced by 15 percentages when STATCOM is installed at wind farm. As wind speed is always fluctuating, the terminal voltage of wind generator also fluctuates randomly, which has an adverse effect on the rest of the power system. It is reported that the STATCOM with reduced capacitor bank can decrease the voltage fluctuations of multi-machine power system as well as wind generator terminals. Moreover, it is shown that STATCOM can also enhance the transient stability of induction and synchronous generators when a network disturbance occurs in the power system. Both of the symmetrical and the unsymmetrical faults are analyzed. Both multi-wind generators wind farm and aggregated single generator wind farm models are used for the sake of detailed analyses. Finally, it is concluded that the STATCOM can enhance both of the steady state characteristic and transient stability of wind farms connected with multi-machine power system.

With the increasing penetration of renewables, the modern power system is facing emerging stability issues. A prime example of such issues is the subsynchronous oscillation (SSO) phenomenon observed in large‐scale wind farms. It has been studied that hydrogen production equipment (HPE) has the potential to smooth the fluctuation in generated power. However, it is still unclear whether the HPE can be used to mitigate dynamic stability problems induced by wind generators. This study is aimed at filling the gap by exploring the capability of HPE to damp the emerging SSO. Firstly, the overall configuration of an MW‐scale HPE is presented. Then its electromagnetic transient model is established for system dynamics studies. Next, a supplementary subsynchronous damping control (SSDC) of HPE is proposed to modulate its power absorption at the subsynchronous frequency, which can improve system damping and stabilise SSO. Finally, the effectiveness of the proposed SSO mitigation strategy is verified on the simulation model of a practical wind power system that suffered actual SSO incidents. The results demonstrated that the HPE with properly designed SSDC, along with suitable capacity and location, can efficiently mitigate the SSO, thus offers a new option to improve the dynamic stability of renewable power systems.

The contribution of wind power in conventional generation is increasing rapidly now-a-days. Hence a better understanding of wind power plants is required. But the wind power plants generally containing induction generators exhibit a rather different behavior than the conventional power plants which use synchronous generators. The purpose of this paper is to analyze and study transient stability of a wind farm based on conventional fixed speed Induction generator when it has been integrated with a weak grid. A comparative analysis of the transient stability improvement of a FSIG wind plant has been done, when it is supported with FACTS like S.V.C and STATCOM of equal ratings. Single case of transient condition i.e. a three phase fault has been studied by simulation. With the increase in demand of power and decrease of fossil fuels, mankind has been forced to search alternative sources for the generation of electricity. Wind power in spite of being stochastic in nature has proved itself as a viable solution to this problem. As the wind turbine technology is developing at a good pace, more and more wind power plants are being integrated with the conventional form of generation. With the increase in the ratio of wind generation to conventional generation, several problems related with integration of wind farms have emerged. These problems are due to distinct properties of the generators used with the conventional form (Thermal & Hydro) of generation and wind based generation (1)-(4). In thermal and hydro power based generation synchronous generators are used while in wind based generation mostly induction generators are used. Induction and synchronous generators do not exhibit similar behavior during power system stresses (transients). Moreover, increase in wind power based generation requires a reliable transmission system for power evacuation to the load centers. As the wind farms are mainly concentrated in far flung and rural areas their interconnection with the grid becomes a problem. This problem is mainly due to the existence weak transmission grids in the rural areas (1)-(4). Earlier when the share of wind power generation as compared with the conventional generation was low, during transient conditions; whenever the power system reached on the verge of instability the wind farm was disconnected from the grid and was reconnected when transients passed away. But as the share of wind power in the conventional generation increased, the practice of disconnecting the wind farms from the rest of the grid led to the drop in frequency of the power system. So now-a- days even if a fault occurs in the power system somewhere near the wind farms, they still remain connected with the grid for a critical time period. This phenomenon is called fault ride through (FRT) in wind farms (5). This paper has studied a transient condition (fault) in a grid connected fixed speed induction generator based wind farm and its fault ride through performance. A comparison of the performance of fact devices like static var compensators (svc) and static synchronous compensators (statcom) in improving the transient stability of wind farms during fault is also done.

In this paper, a model predictive control (MPC)-based coordinated scheduling framework for variable wind generation and battery energy storage systems (BESSs) is presented. On the basis of the short-term forecast of available wind generation and price information, a joint look-ahead optimization is performed by the wind farm and storage system to determine their net power injection to the electric power grid. In conjunction with moderate battery capacity, the excess unpredictable wind generation can be used to charge the battery storage and vice versa. The benefits of the proposed scheduling approach are that (1) the combined profit of wind generation and BESS is increased; (2) the net power injection from the wind farm into the power grid is smoothed out; and (3) the look-ahead optimization updates the price prediction in a moving horizon, which leads to more robust profit for wind farm and BESS against price uncertainties. By formulating the MPC-based coordinated scheduling as a quadratic programming problem, several numerically efficient algorithms to compute the optimal control strategy for wind generation and BESS are proposed. The effectiveness of the proposed algorithm in a modified IEEE 24-bus reliability test system with aggregated plug-in hybrid electric vehicles is demonstrated. It is shown that the proposed algorithm can increase the joint profit of wind farm and BESS while smoothing out the net power injection to the electricity grid. The proposed MPC-based scheduling problem can be solved in approximately 400 ms, which makes the framework implementable in realtime electricity market operations.

Generation and transmission expansion planning with high penetration of wind farms considering spatial distribution of wind speed

This paper investigates the most suitable location to place variable-speed wind generator based wind farms in a power system considering the steady-state voltage stability. The doubly-fed induction generator (DFIG) and the full-converter wind generator (FCWG) based wind generation is considered and the inherent reactive power capability of these generators are accurately represented in the study. Steady-state voltage stability is analysed using QV modal analysis and QV curves. QV modal analysis is used to identify the weak and strong busbars in the test system. Wind power generators with different power outputs are integrated to the power system from weak and strong busbars and the impact of voltage stability is compared using QV stability margins of the busbars. The results demonstrate that adding wind generation from the weak busbars significantly increases the QV stability margin of the weak busbars. However, adding wind generation from strong busbars slightly improve the stability margin of the weak busbars. Therefore, placing wind farms in the weak busbars in a power system is favorable for the overall voltage stability. Furthermore, the results of DFIG and FCWG based wind generation are compared and the higher reactive power capability of the FCWG based wind generation to support the voltage stability is demonstrated.

Wind power has emerged as one of the most popular forms of renewable energy by virtue of its free availability. The larger proliferation of wind generators presents significant challenges to the stable operation of today’s power systems. Due to the advantages of Doubly-Fed Induction Generators (DFIG) over other wind generators, the majority of wind farms are using variable speed wind turbines equipped with DFIG. In this paper, the impacts of grid- connected large DFIG based wind farms on power system Transient Stability (TS) is studied. The well-known Western System Coordinated Council (WSCC) 3-machine, 9-bus system is used as the test system to carry out the said analysis. The test system - with DFIG based wind farm integrated into it, is modeled and analyzed in a simulation environment.

With large wind energy integration into power systems, wind farms begin to influence power systems in a much more significant manner. As wind energy systems utilize different generator technologies from the one utilized in the conventional power plants, the steady-state, transient and small-signal dynamics, as well as, power system stability will thus be significantly affected. The impact of wind energy systems on the power system dynamics and stability is thus of practical importance. As there is a significant increase in installation of wind turbines equipped with doubly-fed induction generator (DFIG) in recent years, a dynamic model of the DFIG wind turbine is firstly developed in this thesis. The model is validated against field measurement data, and the validation gives confidence about the accuracy and applicability of the developed model. DFIG wind farms consist of tens to hundreds of identical DFIG wind turbines increasing the complexity of the wind farm model and simulation time. A novel aggregation technique is developed in this thesis that incorporates a multiplication factor, namely mechanical torque compensation factor (MTCF), to the mechanical torque of the full aggregated wind farm model. The MTCF is initially constructed to approximate a Gaussian function by using fuzzy logic method. By optimizing the MTCF on a trial and error basis, less than 10 percent discrepancy is then achieved between the proposed aggregated model and the complete model. The proposed aggregation technique is then applied to a 120 MVA offshore wind farm comprising of 72 DFIG wind turbines and shows higher accuracy in approximating the wind farm dynamics as it appears at the point of common coupling (PCC) as compared to the full aggregated model. The proposed aggregated model computes faster than the complete wind farm model by 90.3 percent during normal operations and 87 percent during grid disturbances. To overcome the adverse effects due to the fluctuating nature of wind, two wind power smoothing methods are proposed using a fuzzy logic pitch angle controller for a smooth performance with a minimum drop in output power. One method performs partial smoothing with only 4.74 percent drop in the output power while the other method offers complete smoothing with a 8.28 percent drop in output power. The impact of wind energy integration on power system transient stability (PSTS) is studied quantitatively with the transient energy margin (TEM), which is calculated through the evaluation of the transient energy function (TEF). This study is carried out in two ways in the thesis. One is to analyse the impact of transient fault on the DFIG wind turbine as compared to SGs for different factors, like the fault clearing time, the grid coupling, the inertia constant and the voltage sag. The study reveals that transient stability of the DFIG wind turbine is hardly affected by the grid coupling, the inertia constant and the generator terminal voltage sag variations indicating its consistent transient performance within a wide range of these factors. The fault clearing time should be almost 11 percent faster for the system with the DFIG than the synchronous generator (SG). The other is to investigate the impact of the DFIG wind farm on the PSTS with the variation of different factors, which are the voltage sag, the fault clearing time, the load and the wind power penetration level. The study reveals that power systems integrated with DFIG wind farms are sensitive to transient events with high voltage sag, high fault clearing time, low load operation and high wind power penetration level. Machines at different locations individually possess distinct fault response and suffer from a large-scale power imbalance. As a result, reliable operation of DFIG wind farm integrated power systems demands upgraded equipment, such as advanced switchgear, fast breakers/isolators, efficient power reserve systems and advanced reactive power compensating device, etc.

In this paper, a short-term active power curtailment (ST-APC) strategy for doubly-fed induction generator (DFIG) wind farms is proposed to enhance first-swing rotor angle stability under fault disturbances. While wind power is a clean renewable resource that is widely deployed, its large-scale integration heightens concerns about transient stability. After analyzing DFIG operating principles, this study advocates for using short-horizon active power control to mitigate the adverse stability impacts of wind farms. Using the Western System Coordinating Council (WSCC) three-machine nine-bus test system, the effectiveness of the ST-APC strategy across diverse operating conditions was verified. This study is based on the fundamental principle that reducing the output of wind turbines is required for first-swing stability after faults to increase the kinetic energy of synchronous machines. A closed-loop control strategy combining voltage drop, frequency change, and a timer is designed. The correlation laws between various control parameters such as control activation timing, duration, and modulation depth and first-swing stability are analyzed, providing references for parameter selection in engineering applications. The findings indicate that the proposed strategy is practical and adaptable, making it suitable for power systems with high wind power penetration.

In this paper, cross-correlation study between wind generation and load in Texas has been conducted. Through comparison between onshore and potential offshore wind farm sites, offshore wind farm proved to have slightly higher correlation with load than onshore wind farm. The results also show significant seasonal changes, which could be of importance in making operational and planning decisions.

The constant growth of wind farms in the Brazilian electric system increases the concerns regarding power quality, especially related to harmonic distortions. At the same time, wind generation also becomes relevant in the context of power system stability, operating as ancillary services. One of the requirements of the system operator is the injection of reactive power by the wind farms to maintain the voltage levels. Considering that the actual studies only considers the operation of the wind generators with unitary power factor, the present work aims to evaluate the harmonic impact of Wind farms operating with reactive power injection on the electric system buses. For this purpose two monitoring campaigns in a wind farm of the CPFL Renovaveis were performed with different power factors operating modes in the wind generators. These results were utilized in computational studies to evaluate the impact in voltage distortion at the PCC. The results obtained in this work demonstrated a significant impact on the harmonic current generation and in the voltage harmonic distortion due to the reactive power injection by the wind generators of the wind farm.

Current methods to determine the wind farms maximum size do not consider the effect of new wind generation on the Voltage Stability Margins (VSMs). Installing wind power in one area may affect VSMs in other areas of the power system. Buses with high VSMs before wind power injection may be converted into weak buses after wind power injections in other parts of power systems, which may lead to limited future wind farms expansion in other areas. In this paper, two methods are proposed to determine two new wind farms maximum size in order to maximize wind power penetration level. In both methods, the size of any new wind farm is determined using an iterative process which is increased by a constant value. Proposed methods were used in the IEEE 14-bus power system. The results of applying these new methods indicate that the second method results in higher maximum sizes than the first method.

The intermittency of wind generation and the potential need for adequate transmission expansion are the major concerns in wind generation integration to power system. One solution being considered is to build on-site energy storage with the wind farms. The idea of building such a composite system is not only to minimize the real-time variation of the composite system output, but also to optimize the transmission upgrades needed for delivery of the wind generation. A novel probabilistic reliability assessment method is proposed in this paper for determining the adequate size of on-site energy storage and the transmission upgrades needed in connecting wind generation with the power system. The practical applications of the proposed model are illustrated using the IEEE Reliability Test System (IEEE-RTS).

Recently, wind power generation is increasing worldwide. In wind power stations, induction machines are mostly used as generators. Since induction generators have a stability problem similar to the transient stability of synchronous machines, it is important to analyze the transient stability of power systems including wind generators. Although there have been some reports analyzing the transient stability problem, wind turbine and wind generator are, in most cases, modeled as a one‐mass shaft system having total inertia constant. This paper presents simulation analyses of transient stability of power system including induction generator which is expressed by a two‐mass shaft model and analyzes an effect of shaft system modeling on the transient stability characteristics. Simulations are performed by PSCAD/EMTDC in this study. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 162(3): 27–37, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20394

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.