A Cut-Out Strategy for Wind Turbines that Ensures Low-Voltage Ride-Through Capability

The wind power plant is one of the fastest growing electrical power sources. The integration of wind turbine into the power grid originates various power quality problems. Low-voltage ride through (LVRT) capability improvement is impregnating exigency in recent power quality issue, which is acquainted with various renewable power generation resources like solar PV and wind power plant. Dynamic voltage restorer (DVR) is a custom power device (CPD), which is connected in series with the electrical system, and it is a contemporary and efficient CPD expended in the distribution system to curb the power quality problems. In this paper, LVRT capability of doubly fed induction generator (DFIG) wind turbine system connected to a power grid is enhanced by means of DVR. LVRT capability is improved by the introduction of neural network controller. This controller fulfils the various grid code requirements. Thus, the performance of the DVR becomes far superior by the robust control technique. The simulation results were compared with conventional PI controller and the proposed artificial neural network (ANN) controller. It is proved that the proposed system raises the reliability of grid-connected DFIG wind turbine system.

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.

Enhancing voltage stability and LVRT capability of a wind-integrated power system using a fuzzy-based SVC

Doubly-fed induction generator (DFIG) wind turbines connected to capacitive series-compensated transmission lines are prone to exhibit oscillatory behavior. The phenomena is called sub-synchronous resonances (SSRs), as these oscillations occur at frequencies below the fundamental component. This paper first develops a modeling methodology for DFIG wind turbines, based on impedance matrices, that is applied to model a real wind farm where SSRs were reported. The stability analysis performed shows how the interaction between the grid-side converter and the rotor-side converter contribute to the instability of DFIG wind energy conversion systems connected to series compensated grids. With this model, we propose a simple sub-synchronous resonance control strategy based on an orthogonal proportional action applied to the rotor currents, and a variable gain in the PI controller adjusted as a function of the DFIG rotational speed. This control strategy depends only on the rotor currents, which are local and already measured variables in any DFIG wind turbine, and is implemented in the rotor side converter, so it does not imply an additional cost at wind farm or wind turbine level and can be applied to any DFIG wind energy conversion system (WECS). Additionally, it proves to be robust for any line impedance series compensation level, and it does not need real-time information concerning the grid at which the wind turbine is connected, or its parameters. A real case study is considered, where the sub-synchronous resonance damping strategy presented in this work is able to stabilize the system for every possible line impedance compensation level.

Analyzing low voltage ride through capability of doubly fed induction generator based wind turbine

The paper helps to understand the performance of the grid with battery and doubly-fed induction generator (DFIG) wind turbines when operating in a weak grid scenario with a low short circuit ratio (SCR). The concept will analyze the performance of the grid with battery connected DFIG both in steady state and transient scenario. The grid can be represented as a Thevenin equivalent circuit and is rated at 12.5 kV. A battery system and a wind system are connected at the point of common coupling (PCC). The battery system being modeled in PSCAD software is rated at 1 MW/1 MWh and the wind system model is rated at 2 MW. The grid has been characterized and analyzed based on two categories: strong grid and weak grid. A grid is considered to be weak when the flow of active and reactive power in the network causes a significant amount of voltage fluctuation at the point of common coupling. A grid is considered to be strong when the grid is stable with allowable nominal deviations with voltage and frequency. The strength of the grid can be measured by taking the ratio of grid's short circuit power with grid nominal power.

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.

The rapid expansion of offshore wind energy and its integration into weak power grids have introduced significant challenges to system stability, particularly under fault conditions that cause voltage sags. Ensuring adequate Low Voltage Ride Through has become essential for maintaining grid reliability and preventing large-scale disconnections of wind farms.To address voltage sags caused by short-circuit faults during the grid integration of large-capacity offshore wind farms via weak networks, this paper establishes a simulation model of a permanent magnet synchronous generator (PMSG)-based offshore wind farm in the PSAT platform. Taking the Critical Clearing Time (CCT) as the core evaluation index, the Low Voltage Ride-Through (LVRT) capability is systematically assessed. By simulating short-circuit disturbances, the transient voltage response at the point of common coupling is analyzed, followed by a comparative evaluation of the voltage support effects of three dynamic reactive power compensation devices: Static Var Compensator (SVC), Static Synchronous Compensator (STATCOM), and Unified Power Flow Controller (UPFC). Results indicate that the rational configuration of dynamic reactive power compensation effectively mitigates voltage drops, significantly extends the CCT, and enhances transient stability margins. The distinct response characteristics and support efficacies of different devices are clarified, providing a theoretical foundation for optimizing LVRT strategies and selecting compensation equipment in offshore wind integration.

The paper analyzes the capability of a wind power plant, based on doubly-fed induction generators (DFIGs) with an improved control system, to fulfill low voltage ride-through (LVRT) capability of the wind turbines providing the reactive power support. According to grid code requirements, tripping of wind generators under grid faults is not allowed. The sensitivity of DFIGs to grid faults requires the use of crowbar protection for not damaging the electronic equipment. In this paper, the model of a wind power plant based on DFIG using active crowbar protection is developed in Matlab/Simulink. Nevertheless, the results indicate that DFIGs equipped with crowbar protection can protect the electronic equipment, but the rotor side converter is blocked due to crowbar activation and the reactive power control is lost. Therefore, for improving the LVRT, a coordinated control system of the grid side converter, active crowbar protection, and rotor side converter is implemented and proposed, The grid side converter of DFIG is controlled similar to a STATCOM, considering also the control parameters of the crowbar protection. The simulation results show that the proposed coordinated control grid side converter'active crowbar-rotor side converter can supply the reactive power in order to answer the grid codes requirement for LVRT. Therefore, this control system can be useful to improve the LVRT capability of wind power plants based on DFIG.

풍력발전시스템은 기존의 발전시스템과 매우 다르다. 그러므로 전력계통에 풍력시스템을 연계하기 위해서는 동적특성에 대한 연구가 필요하다. 풍력발전기의 안정도해석은 전력계통의 운영에 있어서 중요 쟁점이다. 기존의 동기발전기만으로 구성된 전력계통의 위상각 안정도는 풍력발전기가 포함되면 그 결과가 달라진다. 즉, 풍력터빈에 연계된 발전기는 대부분 비동기인 유도발전기이기 때문이다. 위상각의 동기화 여부로 판별하는 위상각 안정도는 임계고장제거시간(CCT)을 계산하여 평가한다. 계통해석용 풍력터빈의 모델은 다양하여 그 해석에 어려움이 있으나 지금은 크게 4가지 타입으로 표준화가 되어있다. 본 논문에서는 PSS/E-32에서 제공하는 풍력터빈의 3번째 표준모델인 DFIG(Doubly-Fed induction Generator)모델을 이용하여 풍력단지가 연계된 전력계통의 CCT를 풍력단지의 위치와 용량을 고려하여 분석한다. Wind generation systems are very different in nature from conventional generation systems. Therefore it is necessary to research dynamic characteristics of wind generation systems connected to a power system. The stability analysis of wind turbine generator is an important issue in the operation of the power system. The result of angular stability of the power system that consists of only synchronous generators is different from that of the power system including wind turbine generators. This is due to the fact that generators connected to wind turbines are generally induction generators. The angular stability assessing synchronization of generators is determined by its corresponding critical clearing time(CCT). Wind turbine models for the analysis of power system are varied and difficult to use, but now these are standardized into four types. In this paper, the analysis of the CCT of the power system connected to wind farm considering the location and capacity is performed by using DFIG(Doubly-Fed induction Generator) wind turbine built-in type3 model in PSS/E-32.

With the increasing penetration of wind power into electric power grids, energy storage devices will be required to dynamically match the intermittency of wind energy. This paper proposes a novel two-layer constant power control scheme for a wind farm equipped with doubly fed induction generator (DFIG) wind turbines. Each DFIG wind turbine is equipped with a supercapacitor energy storage system (ESS) and is controlled by the low-layer wind turbine generator (WTG) controllers and coordinated by a high-layer wind farm supervisory controller (WFSC). The WFSC generates the active power references for the low-layer WTG controllers according to the active power demand from or generation commitment to the grid operator; the low-layer WTG controllers then regulate each DFIG wind turbine to generate the desired amount of active power, where the deviations between the available wind energy input and desired active power output are compensated by the ESS. Simulation studies are carried out in PSCAD/EMTDC on a wind farm equipped with 15 DFIG wind turbines to verify the effectiveness of the proposed control scheme.

This study proposes a hybrid forecast-enabled adaptive crowbar coordination strategy to enhance low-voltage ride-through (LVRT) performance of doubly fed induction generator (DFIG) wind turbines. A unified electro-mechanical model in the αβ/dq frames with dual closed-loop control for rotor- and grid-side converters is built in MATLAB/Simulink (R2018b), and LVRT constraints on current safety and DC-link energy are explicitly formulated, yielding an engineering crowbar-resistance range of 0.4-0.8 p.u. On the forecasting side, a CEEMDAN-based decomposition-modeling-reconstruction pipeline is adopted: high- and mid-frequency components are predicted by a dual-stream Informer-LSTM, while low-frequency components are modeled by XGBoost. Using six months of wind-farm data, the hybrid forecaster achieves best or tied-best MSE, RMSE, MAE, and R2 compared with five representative baselines. Forecasted power, ramp rate, and residual-based uncertainty are mapped to overcurrent and DC-link overvoltage risk indices, which adapt crowbar triggering, holding, and release in coordination with converter control. In a 9 MW three-phase deep-sag scenario, the strategy confines DC-link voltage within ±3% of nominal, shortens re-synchronization from ≈0.35 s to ≈0.15 s, reduces rotor-current peaks by ≈5.1%, and raises the reactive-support peak to 1.7 Mvar, thereby improving LVRT safety margins and grid-friendliness without hardware modification.

Doubly fed induction generator (DFIG) based wind turbines must support the grid during faults and voltage sag/swells. In this regard, Low voltage ride through (LVRT) and High voltage ride through (HVRT) could be used to improve DFIG performance. In this paper, a control strategy, based on fuzzy control, has been proposed to improve DFIG LVRT and HVRT capability. The effectiveness and performance of this method has been validated by MATLAB/SIMULINK simulation for both voltage sag and swell under various fault conditions. In addition, this method has been compared with some conventional methods and proficiencies have been discussed.

This paper investigates the control and operation of permanent magnet synchronous generator (PMSG)-based wind generation systems employing the full-scale frequency converter under network fault conditions. The factors inducing the oscillations of DC-link voltage of the full-scale frequency converter during grid voltage dips are analyzed. A novel control strategy is proposed to suppresses the DC-link voltage oscillations and enhance the low voltage ride-through (LVRT) capability of PMSG-based wind turbine. The generator-side converter is controlled to maintain a stable DC-link voltage, while the grid-side converter is controlled to provide a dynamic coordinated control for the active and reactive power output according to the grid voltage amplitude variations during grid faults. The simulation results implemented in Matlab/Simulink show that the proposed control strategy not only improves the stability of PMSG by means of suppressing the DC-link voltage oscillation, but also provides a dynamic reactive power support to restraint the disturbance of the grid voltage. (5 pages)

Due to the increased renewable power penetration level renewable power plants have to provide low-voltage ride-through (LVRT) capability with simultaneous dynamic voltage support, to ensure the grid stability during grid faults. Concerning doubly fed induction generator (DFIG) based wind energy conversion systems (WECS) large electromotive forces and rotor currents, which may damage the rotor-side converter, or adversely affect the DFIG's controllability are induced into the rotor circuit in case of voltage dips. To handle and limit the rotor currents in case of asymmetrical voltage dips without crowbar triggering, a virtual resistance control approach based on the standard dq-control in the synchronous reference frame is discussed. The theoretical results are compared with those of more demanding virtual inductance control. The LVRT capability is verified with measurement results, recorded during a certification campaign at a 2.1 MW WECS concerning the Indian grid code.