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Considering the negative resistance effect of power electronic equipment, unstable resonance problems may occur between wind farms and converter stations. This paper focuses on the resonance stability problem of a wind power base coupling with a multi-level converter−high-voltage direct-current transmission (MMC–HVDC) system. Firstly, the s-domain impedance models of the doubly-fed induction wind generator, the permanent magnet synchronous wind generator, and the modular MMC are built, through the theoretical derivation and simulation test. Secondly, based on the s-domain nodal admittance matrix method, the resonance structure of the wind power base coupling with an MMC–HVDC system is analysed, including the resonance mode frequencies, the resonance mode damping factors, the nodal participation factors, and the nodal voltage mode shapes. Thirdly, the main factors influencing the resonance structure are studied, including the topology of the wind power base, the capacity of the wind turbines, and the operation mode of the MMC station. Finally, a brief conclusion ends the paper. [ABSTRACT FROM AUTHOR]

Detection of sub‐synchronous oscillation (SSO) accurately within a short time poses a big challenge to avoiding the potential risks of the SSO phenomenon. This paper proposes a novel technique for detecting the SSO phenomenon based on identifying the SSO frequency and magnitude rather than applying fast Fourier transform. The voltage magnitude signal is used as an input signal. The SSO magnitude is identified based on estimating the max value of the voltage magnitude signal, whereas the time corresponding to the zero crossings is defined for calculating the SSO frequency. In order to confirm its effectiveness, the proposed technique is tested with different SSO types (induction generator effect, torsional interaction, and sub‐synchronous control interaction). Moreover, the impact of the series compensation level change on the proposed method performance is investigated. The results prove the effectiveness and the speed of the proposed method in all studied cases. Compared to the traditional methods within a wide range of compensation levels, the proposed technique is fast and accurate. The IEEE first benchmark model is adapted with a doubly‐fed induction generator in the MATLAB program to analyze the performance of the proposed method for detecting the SSO phenomenon. [ABSTRACT FROM AUTHOR]

Types I, III and IV wind distributed generators show different and wide range of current sharing capacity during the occurrence of faults in the distribution feeders. In a hybrid microgrid consisting of any of the above mentioned types of wind distributed generators and photovoltaic distributed generators, the fault current in a feeder shows different behaviour which changes as per the type of distributed generators, grid/islanded connection of microgrid operation, distance of the fault from the point of common coupling and nature of the loads in the microgrid system. The novel contribution of this paper is the implementation of a fuzzy logic based adaptive protection scheme through analysis of the q0 components of fault current which are used to detect the low X/R ratio of distributed generators, modes of operation, transient reactance during the series and shunt faults in types I, III and IV wind turbines and PV integrated hybrid microgrid. Considering q0 components and transient reactance, a new relation between the relay current settings and modes of operation has been identified for adaptive relaying. The effectiveness of the proposed adaptive protection scheme for the hybrid microgrid is verified through a simulation case study using Matlab -Simulink software. [ABSTRACT FROM AUTHOR]

This paper proposes a WECS based on a back to back diode‐clamped multilevel inverter systems (DCMLI) fired comparatively by sinusoidal pulse width modulation (SPWM) and third harmonic injection pulse width modulation (THIPWM) techniques. DFIG performance is compared using these technologies for different wind speed under normal operation condition. The proposed approach shows that the DCMLI systems generate a nearly sinusoidal voltage with reduced total harmonic distortion (THD) thus upgrading the power quality of that produced by DFIG. As SPWM and THIPWM MLI are most common usage techniques in research, we applied them in the system to discuss a comparison of using those techniques to determine the number of levels that achieve the IEEE 519 criteria of THD which is less than 5% for both techniques (5th level for SPWM and 4th level for THIPWM) in all operation wind speed. Compared with simple PWM, and THIPWM have many advantages as lower THD, minimal number of switching to decrease switching losses and the output fundamental voltage is increased. Lastly, the paper investigates the variation of the frequency of induced rotor voltage and the active power flow due to the wind speed change when the rotor speed changes from super‐synchronous to sub‐synchronous speeds. [ABSTRACT FROM AUTHOR]

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Voltage stability in general is of paramount importance with increasing penetration levels of variable‐speed wind power generators (VSWGs) in power systems. Limited knowledge exists considering the impact of VSWGs on long‐term voltage stability (LTVS) of power systems, focusing on doubly fed induction generators and full‐converter wind generators. This paper presents the results of a comprehensive study on the impact of VSWGs on LTVS. Integration of wind generators while operating the existing synchronous generators improves the LTVS. However, it is unlikely that power systems will retain all existing synchronous generators operating, with high wind penetration levels. Therefore, this study compares the capability of the synchronous generators and VSWGs on LTVS by considering the dynamic reactive power capabilities. The significance of overload capability of synchronous generators on LTVS is highlighted. It is also illustrated that integrating wind farms at remote and local locations from load centres has distinct impacts on LTVS. Replacement of synchronous generators located in close proximity to load centres has a detrimental effect on LTVS. Furthermore, this study demonstrates the impact of different wind generator loading levels on LTVS. The study outcomes highlight several distinct factors which influence the LTVS of power systems with high wind penetration levels. [ABSTRACT FROM AUTHOR]

This paper presents a control strategy to improve the low-voltage ride-through capability of a doubly fed induction generator (DFIG); since the stator of a DFIG is directly connected to a grid, this sort of machine is very sensitive to grid disturbance. Grid voltage sag causes overcurrents and overvoltages in rotor windings, which can damage the rotor-side converter (RSC). In order to protect the RSC, a classical solution based on installation of the so-called crowbar is adopted; however, as the DFIG absorbs reactive power from the grid, this type of solution deteriorates grid voltage sags and cannot meet the requirements of a new grid code. An improved control strategy which uses virtual resistance to limit rotor side overcurrents is proposed in this paper, which can make a crowbar inactive and supply reactive power to fulfill the latest grid code requirement during voltage sags. In order to validate the proposed strategy, simulations and experiments have been carried out, and the results demonstrate the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

This paper presents the voltage‐controlled oscillator (VCO) less phase‐locked loop (PLL)‐based control of voltage source converter. It is used in the distributed power generation system for improved power quality. This control extracts the fundamental component from the sensed load current using three phases to alpha–beta transformation without using VCO, unlike synchronous reference frame–PLL, where fundamental component extraction needs two transformations with the use of VCO. Owing to the absence of VCO in the proposed method, the computational burden of trigonometric operations is very less that results in a faster dynamic response under variable conditions. This characteristic makes it suitable for the distributed power generation system, where fast changes occur frequently. In this study, wind‐based distributed generation system has been taken for study, which comprises of self‐excited induction generator feeding non‐linear load with varying electrical conditions. Using the proposed control, the reactive power compensation, harmonics reduction and load balancing are carried out in the system. In addition to it, neutral current compensation is also accomplished with the help of star–delta transformer connected at the load end. The system performances are investigated under steady state as well as in transient state and results are found satisfactory. [ABSTRACT FROM AUTHOR]

This paper proposes a new hybrid frequency control strategy for doubly fed induction generator (DFIG)‐based wind turbines (WTs) to simultaneously provide short‐term and long‐term frequency regulation support to the connected power system. The kinetic energy stored in the rotating mass and the mechanical power reserve of the WT are coordinately configured to participate in frequency recovery at both primary and secondary stages. A frequency response model (FRM) is also derived to analyse the mutual interactions between WTs and the power system. Based on the FRM, the impacts of relevant parameter variations on the overall system stability can be investigated through Nyquist criterion and modal analyses and the stable operation range of proposed strategy could be easily identified. Theoretical and numerical results have both verified the effectiveness of the proposed control strategy and its advantages over existing methods. [ABSTRACT FROM AUTHOR]

The widespread use of renewable energies around the world has generated the need for new tools and resources to allow them to be properly integrated into current power systems. Power system operators need new dynamic generic models of wind turbines and wind farms adaptable to any vendor topology and which permit transient stability analysis of their networks with the required accuracy. Under this framework, the International Electrotechnical Commission (IEC) and the Western Electricity Coordinating Council (WECC) have developed their own generic dynamic models of wind turbines for stability analysis. Although these entities work in conjunction, the focus of each is slightly different. The WECC models attempt to minimise the complexity and number of parameters needed, while the IEC approach aims to optimise comparison with real turbine measurements. This study presents a detailed comparison between these two different approaches for modeling a Type 3 (i.e., DFIG) wind turbine in MATLAB/Simulink. Finally, several simulations are conducted, with which the consequences of the different approaches are evaluated. The results of this paper are of interest to power system operators as well as wind turbine manufacturers who require further assistance in adapting their specific models to the simplified versions provided by the International Committees. [ABSTRACT FROM AUTHOR]

This paper proposes an identification method of multiple harmonic sources in the presence of different inverter‐based distributed generations (DG) such as photovoltaic (PV), wind turbine with doubly fed induction generator (DFIG), and microturbine (MT) at the point of common coupling (PCC). To classify the linear loads, non‐linear loads (NL) and inverter‐based DGs, K‐nearest neighbours (KNN) classifier was employed. The features were extracted using empirical mode decomposition (EMD), just from voltage waveforms. To reduce the redundant data, dimension of features vector, and time, the Relief‐F method was implemented on the extracted features. Utilising only the voltage waveform for extraction of features increases the speed of the process and decreases the number of measurement equipment. To verify the effectiveness of the proposed method, a number of scenarios such as different harmonic sources with various harmonic orders, load and DG levels were simulated. The results on two test systems showed that the proposed method has a high accuracy even in the presence of different inverter‐based DGs. It can be used as an added tool for power‐quality engineers and can be integrated into monitor instruments. Also, the speed and precision of this method make it suitable for real‐time applications in the power‐quality issues. [ABSTRACT FROM AUTHOR]

This study firstly derives the relationship between the grid voltage and the rotor current of doubly-fed induction generator (DFIG) during the grid voltage swell. Then, an effective control strategy aiming to restrain rotor over-current is proposed and the control strategy of the grid-side converter is also improved. The proposed rotor current suppression and grid voltage suppression method, cooperating with the DC chopper, can not only solve the problem of short circuit of the rotor side converter resulting from the frequent switching of crowbar device, but also maximally improve the capability of wind turbine in dynamic reactive power support without adding any other power electronic devices. With the help of this control strategy, the wind turbine could maintain its normal operation even during the fault and the high voltage ride through of wind turbine can be achieved. As a result, the stability of the wind power integration is significantly improved. Finally, the simulations and analysis in the power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) verify the effectiveness and feasibility of the proposed control scheme. [ABSTRACT FROM AUTHOR]

With the increasing proportion of wind power installed capacity, the influences of transient characteristics of wind turbines generator on safety and stability of power grid is becoming more and more important. The rotor side of doubly fed induction generator (DFIG) is connected to the grid through a back-to-back inverter, and vector control based on phase-locking synchronisation is used generally. Phase-locked loop (PLL) drive internal potential phase changes to achieve synchronous operation with the grid. Therefore, PLL has great influence on transient characteristics of DFIG. A dynamic model of PLL is built in this study. Phase-locking performance of PLL under voltage phase jump is studied by changing its parameters, and its impact mechanism on transient characteristics of DFIG is proposed. Simulation models are built with DIgSILENT/PowerFactory. The influence of parameters of PLL on phase-locking performance of DFIG under voltage phase jump is simulated. [ABSTRACT FROM AUTHOR]

The complex torque coefficients (CTC) method has been widely adopted to investigate the conventional sub-synchronous oscillation (SSO) issues. However, the application of the CTC method is limited when large-scale wind power generations are integrated into power systems. Therefore, in order to overcome the disadvantages of the CTC method on the aspects of frequency scan and stability determination, this paper proposes an improved CTC method based on the shaft system modal decoupling and small-signal self-excitation theory. The improved method is demonstrated and validated through the IEEE first benchmark power system (FBMPS). By establishing the small-signal model of the series-compensated DFIG-based transmission system, the proposed method is applied to examine the SSO in a grid-connected DFIG system. Moreover, the impacts of different parameters on the electrical damping characteristics of the system are deeply analysed. In addition, an effective VSC control strategy for suppression of the SSO in series-compensated DFIG-based transmission system is proposed. [ABSTRACT FROM AUTHOR]

As large‐scale integration of wind systems into the power grid is on the rise, advanced control techniques for wind power generators are highly desired. This paper proposes a simple but effective control technique for doubly fed induction generators (DFIGs) based on the multi‐objective model predictive control (MOMPC) scheme. The future behaviors of the DFIGs are predicted by using the system model and the possible converter switching states. The most appropriate vector is then determined by a cost function. By properly modifying the cost function with active and reactive powers as the control objectives, fast grid synchronisation, smooth grid connection, flexible power regulation and maximum power point tracking (MPPT) can be achieved, respectively. In order to reduce the switching frequency for switching loss reduction, a nonlinear constraint is integrated into the cost function. The controller is simple without using any Proportion Integration (PI) regulators, current loops, and switching tables. A numerical simulation of a 2MW system based on MATLAB/Simulink is built to verify the effectiveness of the proposed method. The results show that the proposed method can achieve quicker transient response, better steady‐state performance, and lower switching frequency compared to the conventional switching table based direct power control (DPC). [ABSTRACT FROM AUTHOR]

This paper provides insight into the testing of three‐phase equipment exposed to voltage sags caused by faults. The voltage sag recovers at the fault‐current zeros, leading to a 'discrete' voltage recovery, that is, the fault is cleared in different steps. In the literature, the most widespread classification divides 'discrete' sags into 14 types. The authors study shows that it is generally sufficient to consider only five sag types for three‐phase equipment, here called 'time‐invariant (TI)' equipment. As the remaining nine sag types cause identical equipment behaviour in Park or Ku variables, the number of laboratory tests (or of extensive simulations) on equipment under sags is reduced by a ratio of 14/5. The study is validated by simulation of a three‐phase induction generator and a three‐phase inverter, which are 'TI', and a three‐phase diode bridge rectifier, which is not 'TI'. Both analytical study and simulation results are validated by testing a three‐phase induction motor and a three‐phase diode bridge rectifier. [ABSTRACT FROM AUTHOR]

This paper first presents a step‐by‐step tutorial on modal analysis of a doubly‐fed induction generator (DFIG)‐based series compensated wind farm. The model of the system includes a wind turbine aerodynamics, a sixth‐order order induction generator, a second‐order two‐mass shaft system, a fourth‐order order series compensated transmission line, an eighth‐order rotor‐side converter (RSC) and grid‐side converter (GSC) controllers, and a first‐order DC‐link model. Then, using modal analysis and time‐domain simulations, it is shown that a fixed‐series compensated DFIG is highly unstable due to the sub‐synchronous resonance (SSR) mode. In order to damp the SSR mode, the wind farm is interfaced with the gate‐controlled series capacitor (GCSC) ‐ which is a new series flexible AC transmission system (FACTS) device. A SSR damping controller (SSRDC) is designed for the GCSC using residue‐based analysis and root locus diagrams, and an effective input control signal (ICS) to the SSRDC is identified in order to simultaneously increase damping of both the SSR and super‐synchronous (SupSR) modes. The IEEE first benchmark model on SSR is adapted with an integrated DFIG‐based wind farm to perform studies. Matlab/Simulink is used as a tool for designing process, and PSCAD/EMTDC is used for time‐domain simulations. [ABSTRACT FROM AUTHOR]

This paper proposes a simple but very effective method to achieve the predictive direct power control (PDPC) for doubly fed induction generator (DFIG)-based wind energy conversion systems. The novel approach is able to operate at low switching frequency and provides excellent steady-state and dynamic performances, which are useful for high-power wind energy applications. Three vectors are selected and applied during one control period to reduce both active and reactive power ripples. Compared to prior three-vectors-based art using two switching tables, the novel approach only needs one unified switching table to obtain the three vectors. Furthermore, the duration of each vector is obtained in a much simpler and straightforward way. The switching frequency can be significantly reduced by appropriately arranging the switching sequence of the three vectors. The influence of one step delay caused by digital implementation is also investigated. The possibility of operating the proposed PDPC under unbalanced grid voltage is also briefly discussed. The novel PDPC is compared with prior three-vectors-based art and its effectiveness is confirmed by the simulation results from a 2-MW-DFIG system and the experimental results from a scaled-down laboratory setup. [ABSTRACT FROM AUTHOR]

This study proposes an optimal voltage control scheme to deal with long‐term voltage stability of power systems. The control of voltage is accomplished by model predictive control (MPC) scheme. The control objective function considers the control efforts and the difference between the predicted and reference voltages. Also, this study considers the detailed non‐linear dynamic model of the system including doubly‐fed induction generator wind turbines, over excitation limiter and under‐load tap changer, which are important elements, should be considered in voltage stability evaluation of power systems. The proposed approach composed of the following two major stages at each time instance: first, the power system non‐linear dynamic equations are linearized and optimal control laws are obtained by MPC technique; in the second stage, the system dynamic behavior is investigated via time‐domain simulations by applying the attained optimal control signals at the first step. The proposed MPC‐based voltage control scheme is implemented on a well‐known test system, under variable wind speed and fault conditions. Also, this method's performance is compared with state feedback control technique. The obtained numerical results validate the capability of the proposed control scheme to preserve voltage stability at the presence of stochastic wind speed variations and severe disturbances. [ABSTRACT FROM AUTHOR]

This paper describes an intelligent sensor fault detection and compensation (FDC) scheme for a hybrid grid involving renewable energy (RE) sources with power electronic interfaces. To evaluate the sensor FDC scheme, a wound rotor induction generator (WRIG)‐based wind energy system connected to the hybrid grid is examined. The system employs a WRIG with excitation from DC RE grid via a single, rotor side converter. To analyse the dynamic performance, a decoupled voltage vector control is employed. The sensors involved in the control algorithm may not be ideal and if fault occurs might lead to system collapse. To overcome electrical sensor failure, a robust, intelligent sensor fault control scheme is proposed with ANFIS. A hardware‐free solution with PLL block is proposed for mechanical sensor fault. The ANFIS‐based FDC scheme involves a bank of observers that computes residuals, detects and isolates faults. When fault occurs, the observer output is utilised for the execution of control algorithm thereby avoiding system collapse. The focused system is simulated in MATLAB and experimentally tested with a dSPACE controller for various fault scenarios. The results show the effectiveness of the FDC scheme. [ABSTRACT FROM AUTHOR]

Voltage harmonics in the grid can introduce stator current harmonics in a doubly fed induction generator (DFIG) wind turbine system, which may potentially impact the generated power quality. Therefore, wind turbine current controllers need to be designed to eliminate the impact of grid voltage harmonics, especially low-order harmonics. This paper proposes a stator current harmonic suppression method using a sixth-order resonant controller to eliminate negative sequence fifth- and positive sequence seventh-order current harmonics. A stator current harmonic control loop is added to the conventional rotor current control loop for harmonic suppression. The overall control scheme is implemented in dq frame. Based on a mathematical model of the DFIG control system, the effects on system stability using the resonant controller, an analysis of the steady-state error, and the dynamic performance, are discussed in this paper. Taking these effects into account, the parameters of the resonant controller can be designed and effectively damp the influence from the grid voltage harmonics. As a result, the impacts of the negative sequence fifth- and positive sequence seventh-order voltage harmonics on the stator current as well as the electromagnetic torque are effectively removed. Simulations and experiments are presented to validate the theoretical analysis. [ABSTRACT FROM AUTHOR]

Australian Grid Code has recently enforced stringent regulations on the transient response of large wind power plants (WPPs). The new grid code requires wind generators to ride-through severe low- and high-voltage conditions, provides reactive power support during the fault period, and exhibits fast power recovery after the supply voltage restoration. This paper proposes a new control scheme for doubly fed induction generator (DFIG)-based WPPs to fulfill these requirements in one inclusive approach. New design strategies for the outer power control loops of DFIG are suggested and their corresponding P-Q capability curves are rigorously studied. It is shown that safely overloaded converters can enhance the reactive power capability of DFIGs during the fault periods. Moreover, for the inner current control loops, the conventional PI current regulators are replaced with enhanced hysteresis-based current regulators. This current regulator, with very fast transient response, increases low- and high-voltage ride-through capabilities of the DFIG, as requested by the Australian Grid Code. Finally, time-domain simulation studies are conducted to evaluate the capability of the proposed control scheme to fulfill the Australian regulations and examine its positive impacts on the transient response of the adjacent fixed-speed WPP. [ABSTRACT FROM AUTHOR]

Due to increasing wind power penetration into utility grids, system operators have instituted stringent standards for ride-through compliance of grid-connected wind turbines. Series voltage compensation can be an effective solution to meet the new standards, especially if a low-cost system can be developed. This paper first presents the characteristics of ride-through behavior of doubly fed induction generator (DFIG) using phasor analysis. The analysis is extended to include the effects of generic series voltage compensation. A new control scheme is then described and analyzed, which allows relatively low-energy ratings in the series compensator, thereby significantly lowering the overall system cost. The new controller enables the DFIG with the compensator to provide the reactive power required for system voltage recovery during grid fault, in compliance with the new grid codes. Simulation and experimental results have demonstrated the enhanced performances resulting from the application of the control scheme. [ABSTRACT FROM AUTHOR]

Wide application of wind energy in the world towards reduction of global warming, have made the low cost maintenance of these turbines very important. Basically, mechanical faults such as gear‐box faults in these turbines lead to a long down time and consequently heavy financial loss. Generally, these faults cause eccentricity in the generator air gap, so by its early detection the fault can be largely prevented. In this paper, eccentricity fault detection is studied in doubly‐fed induction generator (DFIG) which is the most established generator in wind turbines and also is more prone to mechanical faults compared to other technologies. One of frequency components of reactive power is introduced as eccentricity fault index. As the reactive power is always evaluated in the control system, there is no need to include a new hardware for fault detection. Finite elements method is used to model the wound rotor induction machine (WRIM) which is coupled with closed‐loop control system. Following the validation of the model in the healthy case experimentally, eccentricity under different operating conditions is included in the modeling. Finally, a simple procedure based on simple signal processing methods is introduced for eccentricity fault diagnosis considering the operating conditions of the wind turbine. [ABSTRACT FROM AUTHOR]

The doubly‐fed induction generator (DFIG) system currently occupies close to 50% of the wind energy market. The vector control is the proven and state‐of‐the‐art solution for its back‐to‐back power converters by using the dual‐loop controller design: the inner current and the outer voltage/power. This paper focuses on the modelling of power converters and the parameters design of proportional‐integral controller. According to the Bode plots, the relationship among the switching frequency, inner loop bandwidth, and outer loop bandwidth can be found. At least one‐tenth difference between them is necessary for the sake of either the switching harmonic mitigation or the fully decouple of the dual loops. The procedure to design bandwidth for the grid‐side converter and the rotor‐side converter is thoroughly addressed and explained on a real‐scale 2 MW and a down‐scaled 7.5 kW DFIG systems. On the basis of the relationship between the controller bandwidth and the rise time, the theoretically designed bandwidth is able to be verified in both the simulation and the experiment. [ABSTRACT FROM AUTHOR]

In this paper, a complex vector model is provided for an integrated three‐phase shell‐type rotating transformer. The equivalent magnetic circuit is used to obtain the transformer parameters. Furthermore, using vector representation of the dq transformation, the voltage equations of the rotating transformer in double‐reference synchronous frame are obtained. Thanks to the developed model, the transformer design parameters are selected such that the voltage equations in double‐reference frames are decoupled. For the first time, the machine complex vector representation is exploited to design a symmetrical rotating three‐phase transformer. For this purpose, two separated equivalent circuits in double‐reference frames or two decoupled sequences networks are developed. The decoupled circuits simplify the analysis of the rotating transformer in the unbalanced conditions as well as balanced circuits. Being symmetrical is an important issue for the rotating transformer connected to the rotor winding of a doubly‐fed induction generator. Moreover, the steady‐state performance and sequence analysis of the rotating transformer are discussed. Finally, the validity of the obtained model and accuracy of the presented analysis are verified by finite element analysis and experimental tests. [ABSTRACT FROM AUTHOR]

All the sources are required to supply the scheduled power to the system as directed by the system operator, within a time frame of operation. The fluctuating nature of wind makes supplying the scheduled power a challenging task. To address this, gas turbines are proposed to augment the wind generator to work as buffer owing to their high ramping capability. However, the cost of gas turbine increases with increased ramp rates, whereas, operating efficiency and the life time decreases. Hence, to relieve the buffering gas turbines from higher ramp rate requirements, emulation of inertia of the wind generator is proposed in this paper. The controller is designed to operate in inertia emulation mode whenever there is a retardation of rotor of the wind turbine and the proposed ramp controller will be in action only for first few seconds after the fall in wind speed. While emulating the inertia, the performance of the proposed control philosophy with different ramp rates is investigated in this paper. Guide lines for the selection of the ramp duration and ramp rate for seamless transition from inertia emulation mode to maximum power extraction mode are also suggested in this paper. [ABSTRACT FROM AUTHOR]

This paper presents a robust reference current generation technique for rotor side converter (RSC) control of a grid connected doubly‐fed induction generator (DFIG) to eliminate the natural flux revolving in air gap implemented in the positive d–q reference frame. The objective of the scheme is to enhance the performance of the generator under grid unbalance by eliminating the double frequency content in the d–q axes stator flux without using double vector control. RSC is employed to minimise the oscillations in stator flux and consequently in power and torque while grid side converter minimises the dc‐link voltage ripples and maintains zero grid reactive power. It is revealed that, minimising the stator flux pulsations as a single control target enables minimisation of unbalance in stator and grid currents and pulsations in torque, power and dc‐link voltage. Further, the simplicity in implementation and computation of reference currents and rotor flux, and robustness under parameter variations render the scheme attractive. The performance of DFIG is investigated through numerical simulations in PSCAD/EMTDC for a 250 kW DFIG system and further, through experimental results for a 2.3 kW DFIG test setup. The results show the enhanced performance of DFIG with the proposed scheme. [ABSTRACT FROM AUTHOR]

As a new highly penetrated power source of modern power systems, doubly fed induction generator (DFIG)‐based wind turbine (WT) has a different fault current contribution when compared with a synchronous generator. DFIG‐based WT's fault current has significant impacts on distance relay's performances and may result in maloperation. In this study, the impacts of DFIG‐based WT's fault current on the dynamic performances of two typical distance protection elements, mho and polarisation, are studied in terms of EMTP simulations and fault current expressions in symmetrical fault scenarios. It is discovered that the zone‐1 distance relays of the lines connecting to DFIG‐based wind power plants tend to overreach. Also, the following analysis indicates that the transient components of DFIG‐based WT's fault current and the phasor estimation unit are the underlying reasons. With those understanding, this study is expected to help design new setting methods and for distance protection in power systems integrating wind power plants to prevent this kind of possible overreach. [ABSTRACT FROM AUTHOR]

Offshore wind plant will play an increasingly important role in future decades if ever‐stringent requirements of energy security and low carbon emissions are to be met. Although some analyses of the DFIG impact on system stability have previously been reported, none are based on a large network representing a real system, or the large network is simply not publicly available. This paper describes one such suitable equivalent dynamic model for stability studies based on the UK transmission system. The methodology for appropriate control system design and adjustment of the parameters under different dispatch conditions is presented. Two important aspects contributing to future system stability are studied in detail, namely maximum value of the rate of change of frequency and transient stability. Following National Grid future energy scenarios, a number of detailed case studies are presented to quantify the impact of key influencing factors including the size of the largest generating unit for n−1 contingency, amount of primary system response, and frequency dependency of load. The study concludes that none of the individual factors can provide a complete solution and that careful cost benefit analysis is needed to determine the proper mix of services and reinforcements needed in the future. [ABSTRACT FROM AUTHOR]

The introduction of distributed generation (DG) into low voltage (LV) systems demands that the generation system remain grid connected during voltage sags to ensure the operational stability. The DG consisting of fixed speed squirrel cage induction generator (SCIG)-based wind turbines is unable to provide reactive power control and needs a dedicated compensating device. Under asymmetrical grid faults the negative sequence flux circulation in the airgap introduces the torque oscillations that lead to the reduction of lifetime of the generation system. This paper proposes the use of distributed constant power loads (CPLs) for asymmetrical fault ride through (FRT) instead of using a centralized STATCOM. It has also been observed that the compensation of negative sequence voltage improves the performance of SCIG by eliminating the torque ripples. The compensation of positive sequence voltage avoids a possible voltage collapse at the LV distribution level and improves the reliability and stability of the wind farm. Centralized compensation of the asymmetrical grid fault by a STATCOM is compared with the distributed compensation by CPLs. The results suggest that each individual CPL injects lower current for maximum FRT enhancement compared to a dedicated STATCOM. [ABSTRACT FROM AUTHOR]

This paper deals with a new conversion topology for doubly-fed induction generators (DFIGs) suitable for wind energy conversion systems integrated in microgrids. It consists of a DFIG which is fed by a pulse width modulation converter on the rotor and with the stator connected to a dc grid through a diode rectifier. In this configuration, the stator diode rectifier and the rotor-side inverter share the same dc bus, so that the conventional grid-side inverter is avoided. Since only a diode rectifier designed for the full power and a reduced power inverter are required, this layout allows a cheap and effective integration of the DFIG with other generating and storage systems connected to the same dc bus. A simple control technique suitable to regulate the power delivered to the dc grid is proposed. The scheme is based on the regulation of the amplitude of a suited fraction of the rotor flux linkage: the optimal value of this fraction is theoretically deduced in order to minimize the DFIG derating due to the current harmonics. The effectiveness of the proposed control is proven by simulations and experimental tests. [ABSTRACT FROM AUTHOR]

This paper presents a wide-speed-range-operation dual stator-winding induction generator (DWIG) dc generating system for wind power applications. In this system, the dc bus on the control-winding side and the dc bus on the power-winding side are connected in parallel to further widen the speed range of the constant dc voltage output, especially at low speeds. Based on the control mechanism of the DWIG and the instantaneous power theory, a control-winding-flux-orientation control strategy is employed. In order to improve the wind energy conversion efficiency at low wind speeds, the generator efficiency optimization method is investigated. The experimental results from a 20 kW/600 V prototype show that the proposed generating system can output the constant dc voltage in a speed range of 1:4, and the generator efficiency in the low speed range is improved, which both bring the advantages for wind power applications, such as expanding the scope of the wind energy utilization and capturing much more wind energy. [ABSTRACT FROM AUTHOR]

Low-voltage ride-through (LVRT) compliance of wind energy conversion systems is one of the significant concerns for both the wind industry and the system operator. In this study, power electronic-assisted on load tap changer (OLTC) is proposed as a pragmatic solution to enhance LVRT performance of wind turbine generator (WTG). Furthermore, a coordinated control strategy between OLTC and WTG controller is proposed to mitigate post-fault over-voltage issues that are encountered due to the relatively slow dynamics of hybrid OLTC. The results suggest that employing hybrid OLTC does not only improve LVRT compliance of WTG but also significantly improve voltage dip-induced delayed active power recovery of WTG. [ABSTRACT FROM AUTHOR]

When a hybrid wind farm based on doubly fed induction generator (DFIG) and fixed speed induction generator (FSIG) is connected to an unbalanced network, most of the negative sequence current caused by the unbalanced grid will be centralised at the FSIG for its inherent small negative sequence output impedance (NSOI). The centralised negative sequence current can cause significant torque pulsation in the drive train of FSIG, which may go beyond the pressure endurance of the drive train and cause some damages. Meanwhile, losses and risks of overcurrent and grid code violation can also be increased. This study presents a control strategy for DFIG in hybrid wind farm during network unbalance, which can disperse the negative sequence current and the consequent pressure among the three parallel power sources without communication, i.e. among the DFIG stator, the grid side converter (GSC), and the FSIG, rather than centralising the pressure at any of them. In order to achieve this target, a novel hybrid virtual impedance method is proposed to flexibly control the NSOI of the DFIG stator and the GSC from zero to infinity. Theoretical analysis and simulation results are provided to verify the operation performance of the proposed strategy. [ABSTRACT FROM AUTHOR]

With the increasing penetration of distributed energy resources (DERs) in the active distribution network (ADN), how to enable joint planning of DERs under the uncertainty of distributed generations (DGs) has become a challenging problem. This study establishes a two‐stage joint planning model considering doubly‐fed induction generator, photovoltaics (PVs) with the ancillary services of PV inverter, distributed energy storage systems and different types of controllable loads in the ADN. To address the uncertainties of DGs, a two‐stage data‐driven distributionally robust planning model is constructed. The proposed model is solved in a 'master and sub‐problem' framework by column‐and‐constraint generation algorithm, where the master problem is to minimise the total cost and find the optimal planning decision under the worst probability distributions, and the sub‐problem is to find the worst probability distribution of given uncertain scenarios. Besides, the original mixed‐integer non‐linear planning problem is converted into a mixed‐integer second‐order cone programming problem through second‐order cone relaxation, Big‐M and piecewise linearisation method. The numerical results based on 33‐bus system verify the effectiveness of the proposed model. [ABSTRACT FROM AUTHOR]

Voltage sag frequency estimation is necessary for understanding the voltage sag severity in power system and offering full information for the interested parties to mitigate voltage sag. The high penetration of wind power in the power system and the uncertainty of the fault distribution raise new challenges to accurate voltage sag frequency estimation. This study presents a systematic voltage sag frequency estimation method, considering the fault distribution density, fault ride‐through (FRT) process of wind turbines during voltage sag and the interval characteristic of voltage sag frequency. First, this study proposes a fault distribution estimation model based on adaptive kernel density. Second, this study proposes a method for calculating the residual voltage and duration of voltage sag during FRT and combines the common distance protection action to analyse the effect on voltage sag by FRT process of wind turbines. Lastly, this study proposes an interval‐valued voltage sag estimation method considering the interval characteristics of fault rate in the power system. IEEE 30‐bus test system is used to verify the proposed method, the estimation results show better performance of the proposed method compared with the typical estimation methods. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper presents the control strategy and dynamic performance of the dual stator-winding induction generator variable frequency ac (VFAC) generating system with inductive and capacitive loads. In this generating system, the power winding produces the VFAC electricity to feed loads, and the control winding is connected to a static excitation converter to keep the output voltage stable. Based on the change law of the reactive power provided by the control winding, an improved instantaneous slip frequency control (IISFC) strategy both with the load active and reactive power feedforward is proposed to improve the system dynamic performance. The information of the load active and reactive power is employed through two feedforward loops to rapidly change the reactive power provided by the control winding, so as to quicken the regulation of the slip frequency and excitation reactive power. The simulations and experimental results show that the system using the IISFC strategy has a good dynamic performance with inductive and capacitive loads, and demonstrate the correctness and validity of the proposed control strategy. [ABSTRACT FROM AUTHOR]

Abstract Detection of sub‐synchronous oscillation (SSO) accurately within a short time poses a big challenge to avoiding the potential risks of the SSO phenomenon. This paper proposes a novel technique for detecting the SSO phenomenon based on identifying the SSO frequency and magnitude rather than applying fast Fourier transform. The voltage magnitude signal is used as an input signal. The SSO magnitude is identified based on estimating the max value of the voltage magnitude signal, whereas the time corresponding to the zero crossings is defined for calculating the SSO frequency. In order to confirm its effectiveness, the proposed technique is tested with different SSO types (induction generator effect, torsional interaction, and sub‐synchronous control interaction). Moreover, the impact of the series compensation level change on the proposed method performance is investigated. The results prove the effectiveness and the speed of the proposed method in all studied cases. Compared to the traditional methods within a wide range of compensation levels, the proposed technique is fast and accurate. The IEEE first benchmark model is adapted with a doubly‐fed induction generator in the MATLAB program to analyze the performance of the proposed method for detecting the SSO phenomenon.

Considering the negative resistance effect of power electronic equipment, unstable resonance problems may occur between wind farms and converter stations. This paper focuses on the resonance stability problem of a wind power base coupling with a multi-level converter−high-voltage direct-current transmission (MMC–HVDC) system. Firstly, the s-domain impedance models of the doubly-fed induction wind generator, the permanent magnet synchronous wind generator, and the modular MMC are built, through the theoretical derivation and simulation test. Secondly, based on the s-domain nodal admittance matrix method, the resonance structure of the wind power base coupling with an MMC–HVDC system is analysed, including the resonance mode frequencies, the resonance mode damping factors, the nodal participation factors, and the nodal voltage mode shapes. Thirdly, the main factors influencing the resonance structure are studied, including the topology of the wind power base, the capacity of the wind turbines, and the operation mode of the MMC station. Finally, a brief conclusion ends the paper.

Brushless Doubly‐Fed Induction Generator has attractive features to be the first choice in next generation of wind generators. However, its efficiency and power‐to‐weight ratio are slightly lower in comparison to induction machine with the same rating. Considerable part of these imperfections arises from the rotor design, which produces magnetic field with considerable undesirable spatial harmonics. This paper proposes a novel rotor configuration to reduce spatial harmonic distortion of air‐gap magnetic field as well as improving some drawbacks of the conventional structure, including unequal magnitudes of rotor bar currents, teeth saturation at low average air gap magnetic fields, high core loss and inefficient magnetic material utilization. The rotor loops are connected in series in the new scheme rather than nested arrangement of the conventional design. Furthermore, the Imperialist Competitive Algorithm is used for optimising the conductor distributions in order to improve spatial distribution of the rotor magneto‐motive force. The rotor current is evaluated in each iterative step using electric equivalent circuit. The analytical procedure of determining the circuit parameters is modified for the case of series loops. Effectiveness of the novel configuration is verified by comparing the results of optimised and conventional designs in several experimental and simulation studies. [ABSTRACT FROM AUTHOR]

This paper presents enhanced control strategies for doubly fed induction generator (DFIG)-based wind power generation systems under distorted grid voltage conditions. The mathematical model of DFIG, in view of the fifth- and seventh-order components of grid voltage harmonics, is proposed and analyzed in detail. Based on the analytical model, further studies are conducted on the distortions of stator/rotor currents and the oscillations in the stator active/reactive powers as well as the electromagnetic torque, where the impact of DFIG's load conditions is considered. Meanwhile, alternative rotor current references are calculated to enhance the uninterruptable operation capability of the wind-turbine-driven DFIG systems under distorted grid conditions. An improved software PLL is designed, which is capable of accurately and rapidly tracking the frequency and phase angle of the fundamental grid voltage under distorted grid conditions. A proportional integral plus resonant (PI-R) current controller in the synchronously rotating (dq) reference frame is employed to simultaneously regulate the fundamental and harmonic components of rotor currents without any sequential component decomposition. Experiment results on a 3-kW DFIG prototype demonstrate the correctness of the analytical results and the effectiveness of the software PLL and PI-R current controller when the grid voltage is distorted. [ABSTRACT FROM AUTHOR]

A novel shaft-position sensorless algorithm for decoupled control of torque and reactive power (TRPC) of doubly fed machines, such as the classical wound-rotor induction machine (DFIM) and the emerging brushless reluctance machine (BDFRM), has been discussed and experimentally verified in this paper. The underlying control concept is derived from first principles of magnetization and torque production in the machines. For control purposes, only the grid-connected winding measurements and rough knowledge of its resistance value are required. Such a weak parameter dependence makes the TRPC inherently robust, structurally simple, and fast to execute even on low-cost DSPs. A variety of applications are possible including drive and generator systems with limited variable speed ranges (e.g., large pumps and wind turbines), where cost savings of using partially rated power electronics are significant. Two custom-designed and built BDFRM prototypes have served as case studies to evaluate the controller performance by computer simulations and through laboratory experiments. [ABSTRACT FROM AUTHOR]

This article presents a harmonic compensation capability‐based coordinated control strategy for a hybrid wind farm including doubly fed induction generator (DFIG)‐based and direct‐driven permanent‐magnet synchronous generator (PMSG)‐based wind turbines operation under distorted grid voltage conditions. First, the maximum harmonic current compensation capabilities of the DFIG and PMSG systems are analysed in detail when the grid voltage is harmonically distorted. Combining the maximum harmonic current compensation capabilities with the harmonic current requirements of different control targets, the controllable operating regions of each control target of the DFIG and PMSG systems during network voltage distortions are presented. Furthermore, the effects of the voltage total harmonic distortion (THD) and the system operating conditions on the controllable operating regions are analysed. Based on the analysis, the coordination strategy is proposed for a hybrid wind farm without communication equipment between the different wind power generators. The proposed strategy can improve the interoperability and the output power quality of the entire hybrid wind farm during grid‐voltage harmonic distortions. Finally, the effectiveness of the proposed coordination strategy for the hybrid wind farm is validated by simulation and experimental results. [ABSTRACT FROM AUTHOR]

Series capacitor compensation technology has been widely used in long‐distance wind power transmission, which can improve the power transmission capacity and enhance the static stability of the power system. However, adding a series capacitor in the line may present a potential risk of subsynchronous resonance (SSR) to the system. This study focuses on the SSR caused by induction generator effect of the doubly‐fed induction generator‐based wind farms. The SSR phenomenon of wind power system is researched by the impedance analysis method. A mitigation strategy of adding a virtual inductance based on integral control is proposed for enhancing the system stability and alleviating the SSR. It has clear and intuitive physical concept, which is beneficial to system integration and provides a simpler and more economic method for mitigating SSR. Moreover, the principle and process of controller design are elaborated. Moreover, the time‐domain simulation is carried out to prove the effectiveness of the proposed control strategy under different operating conditions considering various compensation levels and wind speeds. Finally, the suppression effects of the virtual inductance in rotor‐side converter and grid‐side converter are compared. [ABSTRACT FROM AUTHOR]

As China's AC transmission system with series capacitor and HVDC transmission technology are widely used in long-distance and large-scale power transmission network, sub-synchronous resonance (SSR) problem becomes more prominent. This paper studied induction generator effect (IGE) and torsional interaction (TI) of synchronous generator by focusing on amplitude and frequency modulation of synchronous generator's flux linkage. IGE involves only electric system dynamics, which will only cause amplitude modulation of flux linkage, resulting in one or two frequency component added in stator current of non-salient pole synchronous generator (NPSG) or salient pole synchronous generator (SPSG) respectively. TI involves both electric and mechanical system dynamics, which will cause both amplitude and frequency modulation of flux linkage, resulting in multiple sub and super-synchronous frequency components added in stator current by means of Bessel function. This result will provide a new reference to understand the SSR phenomena containing multiple frequency components in practical power system. The IEEE first benchmark model is used to demonstrate the accuracy of the analysis results. [ABSTRACT FROM AUTHOR]

Power system voltage stability can be significantly affected by load dynamic characteristics. This study examines the impacts of dynamic interaction between doubly fed induction generator (DFIG)-based wind farms and induction motor (IM) loads on system dynamic voltage stability. It is revealed that the IM load model exhibits a recovery response which is accompanied by an IM oscillation mode (IMOM). The IMOM is first demonstrated to be closely related to system voltage stability. Moreover, a closed-loop interconnected model is established, which consists of the following two open-loop subsystems: DFIG; the rest of power system with IM loads. Generally, due to the fast response of the voltage-source converter control loop, the dynamic interaction between the two open-loop subsystems is weak. However, it is discovered that under a special condition named as open-loop modal coupling, the aforementioned dynamic interaction will become tremendous. As a consequence, the damping of IMOM may be degraded substantially which can cause system voltage instability. Therefore, the mechanism of dynamic voltage instability issues is investigated from the perspective of modal coupling. A modified Western Systems Coordinating Council (WSCC) 9-bus system integrated with a DFIG-based wind farm is adopted to demonstrate and validate the above conclusions. [ABSTRACT FROM AUTHOR]