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With the rapid increase of wind power penetration, the frequency indicators of power system, encompassing frequency deviation and rate of change of frequency (RoCoF), are prone to exceed the pertinent relay thresholds, thus leading to serious power outages. Wind farms can participate in primary frequency regulation (PFR) to alleviate the above problem. However, wind farms reserve capacity (WFRC) presented by overspeed control and pitch angle control is the important factor for frequency safety especially when large disturbance occurs, and it varies obviously in different conditions. Thus, WFRC should be utilized optimally to avoid insufficient or excessive response of wind farms in PFR. To overcome the challenge, this paper proposes frequency‐trajectory‐oriented control (FTOC) for wind farms based on a two‐fold optimization framework, seeking to obtain the best frequency control performances in PFR through realizing the optimal compromise between frequency results and the consumption of WFRC. First, when the frequency indicators deviate significantly from the normal values, the optimal frequency trajectory is planned periodically according to the local standards and operation conditions, thereby minimizing the total consumption of WFRC on the premise of frequency safety. Second, based on model predictive control and a real‐time power system parameters estimator, the total reference power of wind farms, which helps the frequency track the latest planned trajectory, is optimally distributed among wind generators according to their reserve capacity. In this case, through the optimal frequency trajectory planning and tracking, the new method in this paper not only effectively prevents the frequency results from exceeding the local standards under diverse conditions, but also finds the optimal balance among frequency indicators safety, frequency trajectory smoothness, and the reserve capacity consumption of wind power generators. Finally, the simulation results prove the effectiveness and advancement of FTOC. [ABSTRACT FROM AUTHOR]

High energy consumption around the world and concerns over climate change and high greenhouse gas emissions have led to a significant increase in using renewable energy resources, particularly wind power plants. In spite of using small wind farms in distribution networks as distributed generation, the interest in employing large‐scale wind farms in transmission levels has grown significantly in recent years. The presence of large‐scale wind farms with a synchronous generator to supply the energy demand can affect the power swing parameters. These effects are in a way that can mislead power swing detection methods and cause undesirable tripping of transmission line due to mal‐operation of distance relay against power swing. Therefore, in this paper, a method has been presented based on modified empirical mode decomposition (EMD) method to differentiate fault from power swing in the networks connected to the large‐scale wind farms. The results prove the efficiency of the presented method in detecting the power swing in large‐scale wind farm networks. Finally, a comparison is conducted between the proposed method and similar methods which proves the efficiency and superiority of the proposed method in terms of noise tolerance, detection fault detection speed, and multi‐mode power swing detection compared to the current methods. [ABSTRACT FROM AUTHOR]

This paper offers non‐linear control for enhancing the transient stability in power systems containing a photovoltaic (PV) and doubly fed induction generator (DFIG) by transient energy function (TEF) method and finite‐time observers. At first, the TEF method is utilized to enhance the transient stability in a power system containing a PV, DFIG, and synchronous generator (SG) by constructing a hybrid Lyapunov function. Then, the derivative terms in control are estimated with finite‐time observers. The main contribution of the research is the application of the TEF technique in the transient stability evaluation of a power system consisting of PV, SG, and DFIG. Another innovation of the research is the simultaneous control of DFIG and PV, whose uncertain parameters are estimated with the use of finite‐time observer. The proposed non‐linear control scheme is robust against changes in the configuration of the power system. In order to show the capability of this control scheme, time domain simulations are performed on the IEEE 9‐bus, and the NEW ENGLAND Standard 39‐Bus power system. Using the obtained result, some comparisons are made with the back‐stepping control scheme to clarify the superiority of the proposed method. The comparisons show that the proposed non‐linear control scheme can properly provide extra damping to reduce the first‐swing fluctuations in less time. The manuscript contains the new original results of the authors' research. In this article, to maintain and improve the transient stability, a new controller with simultaneous management of photovoltaic (PV) and doubly fed induction generator (DFIG) has been developed. The contributions of the paper are as follows. (1) The transient energy function (TEF) is obtained for the power system containing synchronous generator (SG) and DFIG. (2) The control laws are acquired with four PV and DFIG control variables. (3) The derivative parameters of the PV and DFIG controls by a finite‐time observer are estimated. [ABSTRACT FROM AUTHOR]

This paper proposes a virtual inertia control strategy for wind farms based on auto disturbance rejection controller with artificial bee colony (ABC) algorithm. First, based on the system frequency dynamic response equation, an active disturbance rejection controller is designed according to the non‐linear feedback control law and the extended state observer to improve the anti‐interference capability of the virtual inertia control system. Second, in order to solve the problem of difficulty in tuning the parameters of the active disturbance rejection control (ADRC), an ABC algorithm based on nectar collection behaviour was proposed to iteratively optimize the parameters of ADRC. Finally, the proposed control method is effectively verified based on Matlab/Simulink. The simulation results show that compared with traditional algorithms, the proposed ABC‐ADRC can effectively control the rotor speed to adjust the frequency of the power grid system, achieve power sharing and adaptive noise elimination, reduce the complexity of parameter settings, not only improve anti‐interference ability, but also enhance the robustness of the system. [ABSTRACT FROM AUTHOR]

The grid‐following control‐based hybrid cascaded high‐voltage direct current (HC‐HVDC) links may contribute to the worsening of inertia decline and instability issues, especially in situations characterized by a weak grid. To address the challenge, this paper proposes a grid‐forming (GFM) based optimal frequency support scheme (OFSS) for HC‐HVDC integrated wind farms. Firstly, a GFM control for hybrid converter is proposed and its stability under weak grid conditions is evaluated through a small‐signal model. Then, DC current–voltage droop control is introduced to the line commutated converter (LCC). This control approach ensures the consistency of reactive power in the LCC by regulating the DC voltage and current in the same direction, thereby mitigating AC voltage fluctuations. It also facilitates active power control in the LCC in response to grid frequency variations. Furthermore, the OFSS can convey the frequency of receiving‐end grid to the rectifier and wind farms without communication for precise power control. Finally, to validate the effectiveness of OFSS, a point‐to‐point HC‐HVDC and a 4‐machine test model are constructed on PSCAD/EMTDC. The OFSS significantly enhances the frequency support capability of HC‐HVDC links and improves the robustness in weak grid. [ABSTRACT FROM AUTHOR]

Conventional inverter based resource (IBR) plants generally have control loops with high bandwidth which enable them to maintain tight control of the active and reactive power injected by the IBR device into the grid. These control loops, if not tuned appropriately, can suffer from instability under reduced system stiffness. To bring about an increase in stability of a power network with large percentage of IBRs, recent research done around the world has resulted in the emergence of multiple different types of so‐called grid forming inverter control architectures. But, in order for inverter manufacturers to break the circular (or chicken‐and‐egg) problem, exact performance requirements must be known, which can only be specified either through standards and/or interconnection requirements. Through the use of both electromagnetic transient (EMT) time domain and linearized state space small signal analysis, this paper will provide a way to define interconnection requirements that are agnostic to the specific type of grid forming or grid following IBR control. Further, the benefit of defining grid forming interconnection requirements even when short circuit ratio is not low will be showcased. [ABSTRACT FROM AUTHOR]

The phenomenon of wind abandoned caused by fault of transmission lines has occurred frequently. Since fault current output by wind turbine after fault may induce the frequency variation of the fundamental frequency, fault location based on the fundamental resistance is easily invalid. In order to locate fault accurately and rapidly after wind farm transmission line faults, a time‐domain‐based novel fault location method for transmission line of wind farm is introduced in this paper. The main idea of the proposed scheme is to discretise the distributed‐parameter model of transmission line that is given in the form of partial differential equations by using the finite‐difference time‐domain. In addition, the phase current and voltage at both ends of lines are regarded as boundary conditions. Then, the variational iteration method is utilised to calculate transmission line equations to obtain line length function. At last, fault location function is constructed by a minimised objective function. This paper builds wind power transmission line system simulation model and carries out simulation experiments on PSCAD/EMTDC. To verify the performance of the algorithm, several factors are taken into account for simulation including different fault distances, different fault types, different fault resistances, and different fault inception angles. Simulation results reveal the performance of the proposed method to locate fault for transmission lines connected to wind farm with high accuracy. [ABSTRACT FROM AUTHOR]

Increasing the intermittent outputs of renewable energy sources (RESs) has forced planners to define a new concept named flexibility. In this regard, some short‐ and long‐term solutions, such as transmission expansion planning (TEP) and energy storage systems (ESSs) have been suggested to improve the flexibility amount. A proper optimization procedure is required to choose an optimal solution to improve flexibility. Therefore, a mixed‐integer linear programming (MILP) direct‐optimization TEP versus ESSs co‐planning model is presented in this paper to enhance power system flexibility. In doing so, a novel RES‐BESS‐based grid‐scale system flexibility metric is proposed to investigate the improvement of flexibility amount via ESSs modules in the numerical structure. In this paper, a novel repetitive fast offline method has been proposed to quickly reach the desired amount of flexibility by defining an engineering price/benefit trade‐off to finally find the best investment plan. Also, multiple uncertainties associated with wind farms and demanded loads and a practical module‐type battery energy storage system (BESS) structure for each node are defined. The proposed model is applied to the modified IEEE 73‐bus test system including wind farms, where the numerical results prove the model efficiency as BESS impacts on flexibility, investment plans and power system economics. [ABSTRACT FROM AUTHOR]

This paper suggests a new control algorithm for On‐Load Tap Changer (OLTC) of wind farm main transformer that is responsible for voltage regulation of collector network. The conventional control scheme adjusts the voltage of LV side of transformer without considering the wind turbine voltages. The voltage of end‐string wind turbines may reach the upper or lower limits of Over or Under‐Excited operation regions. Voltages close to upper or lower limits bounds the reactive power capability (generation or consumption) of wind farm due to dependency of wind turbine reactive power capability to voltage. Operation of wind turbine at extreme voltages may stress its power electronic devices. Furthermore, minor disturbances at either grid or power plant may trigger the protection system and interrupt generation due to vicinity of operation point to protection thresholds. The suggested technique integrates the OLTC control into the power plant controller and considers both LV side voltage of transformer and wind turbine voltages. By monitoring the wind turbine voltages, the voltage setpoint of OLTC is adjusted to provide voltages closer to middle of range for wind turbines that prevents the wind turbines from reaching the voltage limits. Dynamical simulations conducted in DigSILENT PowerFactory software for both Under‐Excited and Over‐Excited operation regions demonstrate desired performance for the suggested technique. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editor discusses various reports within the issue on topics including dynamic synchrophasor estimation, voltage controllability and renewable energy.

With large‐scale wind farms connected to AC/DC network, the transient stability assessment (TSA) of the power system becomes more and more difficult. Among them, estimating the region of attraction (ROA) of the equilibrium point is a traditional but still challenging problem. Based on the Lyapunov stability theory, this paper proposes a new approach to obtain the enlarged estimation of the ROA. The optimization and updating strategy of shape function in the sum of squares (SOS) optimization problem are studied to reduce the conservatism of estimation result. In the proposed method, the gravitational search algorithm (GSA) is employed to optimize the coefficients of initial shape function to improve estimation performance. Based on the time‐domain simulation (TDS) of expected faults, the fitness value for shape function optimization is calculated using the values of state variables at the fault clearing time and the system stability information. Furthermore, the optimal Lyapunov function is computed by introducing the update condition of shape function and adjusting the iteration strategy of the ROA estimation algorithm. Finally, the proposed method is applied to a two‐machine‐infinite‐bus system and a more complex nine‐bus AC/DC system with wind power. And the effectiveness of the proposed method is verified by comparing with the existing ROA estimation methods. [ABSTRACT FROM AUTHOR]

Partial discharge (PD) diagnosis is a crucial tool to assess the insulation condition of wind farm cables. Among PD diagnosis techniques, PD localisation is promising as it can provide target maintenance indicators on the insulation weak points of the cables. Accordingly, this paper developed a portable PD detection and location system for wind farm inter‐array cables. The system consists of two non‐invasive and lightweight testing units, which can be conveniently deployed on an energised cable, enabling highly efficient online PD diagnosis of the widely distributed inter‐array cables. The system achieves accurate PD localisation of the energised cable via an improved double‐sided travelling wave method. The method exhibits two superior features: the double‐sided testing units are accurately synchronised via the joint application of Global Position Systems and a pulse‐based interaction process, and a windowed phase difference method is proposed and integrated into the system to robustly estimate the time‐of‐arrival difference in low signal‐to‐noise ratio environment. Validation experiments were conducted on both a 10‐kV cable in the laboratory and a real 35‐kV cable in an on‐shore wind farm. [ABSTRACT FROM AUTHOR]

With an increased focus on renewable power generation, dynamic thermal ratings of various power equipment are being investigated to connect the new intermittent generation to the grid without needing to replace existing infrastructure. Previous research is mainly focused on a dynamic thermal rating for overhead lines. End‐of‐line equipment are also restricted by a thermal limit. This paper combines the thermal model for overhead lines with the transformer thermal model, to investigate the impact of environmental conditions on both. A wind farm case study is used to determine the potential increase in capacity using a combined thermal model. The impact of a higher thermal limit on the loss of life of the transformer is compared to the amount of curtailed wind and the potential revenue from the added wind farm. This analysis serves to provide a method to analyse the risks of using a higher transformer thermal limit, compared to the benefits of increased wind penetration and additional revenue. [ABSTRACT FROM AUTHOR]

This paper presents an open‐access Matlab/Simulink‐based power system simulation toolbox (MatPSST) for research and education. In MatPSST, dynamic modeling is implemented by Simulink. Only the initialization process is coded in Matlab. This structure makes full use of Simulink, enabling users to fully use the rich solvers and built‐in toolboxes in Matlab/Simulink, such as small‐signal analysis, without additional programming. The interaction with real‐time platforms like RTLAB and dSPACE in MatPSST is convenient through the Simulink interface. In addition, MatPSST provides a uniform interface and flexible framework for user‐defined functions. Users do not need to consider the interaction between models and dynamic solvers when implementing state‐of‐the‐art user‐defined models. To avoid the heavy work (replicate units and change parameters) in modeling detailed wind farm or photovoltaic station with hundreds of renewable generation units, the idea of the vector‐variable‐based model is proposed with MatPSST. Case studies introduce some selected application examples of MatPSST in research and education and demonstrate its accuracy by the comparisons with Simscape and PSAT. The cases reveal the advantages of MatPSST in promoting power system research and education. [ABSTRACT FROM AUTHOR]

Nowadays, due to the simplicity of operation and proper coordination characteristics, distance relays are being extensively used in the protection of power transmission lines. Despite many advantages, the distance relay is exposed to a maloperation when a power swing phenomenon occurs. Power swings may occur due to major disturbances, such as switching lines, generator interruptions, and connecting or disconnecting a large part of the loads, causing outages in healthy lines. Detecting power swing and preventing distance relay maloperation has always been a challenge. In addition, different parameters can have a great impact on the performance of power swing detection methods. In this paper, these parameters and their effects are examined along with various methods for detecting power swings. Accordingly, this comprehensive review covers all aspects of power swing phenomenon and its detection, including causes of power swings, power swing detection methods, determining threshold values in power swing methods, effecting of compensated lines and wind farms in power swing detection methods, and testing power swing function in the distance relays. It is hoped that this study will be helpful for researchers in power system protection to find relevant references and develop new methods for power swing function. [ABSTRACT FROM AUTHOR]

Existing fault localization methods for power lines are not applicable to the collector line in onshore wind farms due to the dense access of complex power supplies. Therefore, a fault localization method based on equivalent zero‐sequence injection current (ZIC) is proposed for asymmetric phase‐to‐ground faults on collector lines. Firstly, the idea of load shifting is adopted to generalize the theory of equivalent injection current (EIC) to multi‐terminal and meshed lines. Then, considering features of the zero‐sequence network of wind farms, the EIC theory on two‐terminal and multi‐terminal lines is applied to zero‐sequence networks, and thus faulty areas, faulty sub‐lines, and fault distance can be determined. Attractively, the element correlation in node impedance matrices of zero‐sequence networks is derived from the network branch‐addition process, and the numerical characteristic of matrix elements is further revealed from the particularity of wind farms, based on which root‐mean‐square (RMS) calculation of ZIC can be improved to enable the localization without phasor measurement. Verification results prove that this proposed method can accurately realize the fault localization independent of wind turbine generators with a few measurement points, and the applicability of collector lines with line branches is also considered; additionally, only voltage RMS needs to be measured. [ABSTRACT FROM AUTHOR]

The invention of doubly‐fed induction generator (DFIG) brings the wind energy one step ahead as renewable power generation. But the performance of the grid‐connected DFIGs are greatly affected by grid disturbances as their stator windings are interfaced to the grid directly. Different fault current limiters are capable of improving fault ride through capability during short circuit faults. Nonlinear controller based fault current limiters (FCLs) are superior to deal with the nonlinearity of the power systems. This paper proposes a nonlinear adaptive backstepping controller (ABSC) based capacitive bridge‐type FCL (CBFCL) to enhance the fault ride through capability of a DFIG‐based wind farm connected to a multi‐machine power system. At first, a complete modelling of the CBFCL is derived to understand its behaviour during the normal and fault period more accurately. Then, the ABSC is designed based on that dynamic model, along with the backstepping controller (BSC) and sliding mode controller (SMC) for comparison purpose. Finally, the performance of the ABSC to control the CBFCL has been analysed and verified by comparing with that of the BSC and the sliding mode controller. All the graphical and mathematical analyses favour the ABSC based CBFCL under symmetrical and asymmetrical fault (both temporary and permanent) scenarios. [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]

Inertial and droop control are common methods for improving grid frequency control. Using the first‐order derivative of the frequency in the inertial control loop only investigates the rate of frequency variation, while other useful information is not taken into consideration. Accordingly, in this study, the application of a fractional‐order derivative (between 1 and 2) in the inertial control loop is proposed to have more useful frequency‐based information. To achieve this goal, firstly continued fraction approximations and secondly, the discretization is adopted by using the Tustin Method with Prewarping. Inertia loop and droop loop gain have a significant impact on the operation of the wind turbines and the power system frequency control. Therefore, in this paper, a novel method for the adaptive adjusting of droop and inertia control loops gains in DFIG is proposed. To utilize more frequency error information, fractional‐order adaptive law is proposed to update the loop gains and the inertia control loop. In order to have a robust system, which is permanently stable for every fractional order (FO), is used parameter space method. Simulation results demonstrate the proper performance of the FO‐based adaptive approach to increase the frequency nadir (FN) and decrease the frequency variations. [ABSTRACT FROM AUTHOR]

With the fast development of wind power generation, many countries actively encourage wind power to trade in the medium‐long term market. However, the traditional forward contracts from medium‐long term markets are only suitable to conventional power generation enterprises, instead of wind farms. To facilitate the accommodation of wind power in the medium‐long term market, this paper focuses on two problems: The framework of the block‐based forward contract trading method and the procurement strategy of retailers with block‐based contracts. First, a novel block‐based forward contract is proposed, where the time attributes, that is, starting and ending time, and power during each period are stipulated. Aggregating all the block‐based forward contracts will naturally form the power supply curve, which can provide boundary information for wind farms when they take part in other markets. Secondly, a chance‐constrained procurement strategy model is proposed for retailers, where the uncertainty of wind power generation, constraints of block‐based contracts, and quota obligations of retailers are considered. Furthermore, a bilinear Benders decomposition algorithm with a variant of Jensen's inequalities is applied to solve the proposed model. Finally, experimental results demonstrate the effectiveness of the proposed model and method, which is computationally efficient in solving the chance‐constrained procurement strategy problem. [ABSTRACT FROM AUTHOR]

This paper studies the hybrid energy storage system to provide frequency support for the interconnected AC grid through MTDC systems interfacing renewable resources. A hybrid energy storage structure was created using a supercapacitor (SC) and battery storage systems (BESs). To coordinate the sharing of power between SC and BESS, an improved droop controller based on virtual inductance, capacitance, and resistance gain (VICRC) is suggested. Meanwhile, the droop controller's steady‐state deviation of the DC link voltage is immediately suppressed. The SC will be in charge of compensating the high‐frequency demand, whereas the BESS will be in charge of compensating the low‐frequency power demand, thanks to a decentralized energy management process. In addition, the suggested method aids in the rehabilitation of the SC's SOC. Finally, the simulation results with detailed models of the wind farms and AC–DC grid converters are performed on IEEE tests systems in Simulink/SPS and the results extensively discussed to evaluate the proposed structure. [ABSTRACT FROM AUTHOR]

Evaluation of available transfer capability (ATC) is a complicated process involving the determination of the total transfer capability (TTC) and the existing transfer commitments (ETC). Considering the uncertain renewable generation such as wind power will bring more challenge to this process. Previously the uncertainty of wind power is considered either in the ED model or the following TTC calculation, but not included in two process simultaneously. Therefore, the ATC output may not be accurate since the uncertainty impact two problem simultaneously. To consider the uncertainty and correlation of wind power in both ISO's ED and ATC evaluation, this paper proposes a bi‐level optimization framework in which the ATC evaluation is formulated as the upper level problem and the ED is the lower level. The bi‐level model is first converted to a mathematic program with equilibrium constraints (MPEC) by recasting the lower level problem as its Karush‐Kuhn‐Tucker (KKT) optimality conditions, and then transformed to a mixed‐integer linear programming (MILP) problem which can be solved by existing optimization tools. Case studies on the PJM 5‐bus and IEEE 118‐bus systems are presented to verify the proposed methodology and the impacts of correlation of wind power on ATC are analysed. [ABSTRACT FROM AUTHOR]

The fault classification of medium-voltage transmission lines consisting of wind farms connected to the high-voltage (HV) transmission networks is carried out using the power-spectrum-based hyperbolic S-transform (HST) powerful technique for analysing non-linear and non-stationary fault signals through the non-intrusive monitoring systems. The HST technique extracts the useful features in the time-frequency domain from measuring fault current waveforms of the HV utility side to discriminate the fault types. Parseval's theorem is applied to each HST coefficient to quantify the energy distribution of various fault types for reducing the size of inputs for recognition algorithms. Next, multiclass support vector machines achieve identification. The results have proved that the proposed classification technique is independent of fault resistance, source impedance, and fault inception angles. Extensive simulations are conducted using the electromagnetic transients program to show that the recognition accuracy of the fault classification for all types is up to 96.84%. [ABSTRACT FROM AUTHOR]

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]

In this paper, a comprehensive review of recent researches carried out in the area of optimal reactive power dispatch (ORPD) is presented. The literature review consists of a classification of ORPD problem, along with a survey on the strengths and weaknesses of different models proposed previously. Also, a stochastic multi‐objective ORPD (SMO‐ORPD) model is studied under load and wind power generation uncertainties. A two‐stage stochastic model is employed for dealing with the uncertainties. The considered objective functions are real power losses (PLs) and operation & maintenance (O&M) cost of wind farms (WFs). The proposed SMO‐ORPD problem is solved by considering voltage stability index (i.e. Lmax index). Besides, ɛ‐constraint technique and fuzzy satisfying approach are employed to pick up the best compromise solution. Two cases are considered as follows: (i) voltage stability relaxed SMO‐ORPD, and (ii) voltage stability constrained (VSC) SMO‐ORPD. In this study, reactive power compensation devices are modelled as discrete control variables. Hence, the proposed SMO‐ORPD problem is modelled as a mixed‐integer non‐linear programming optimisation problem. The SMO‐ORPD problem is implemented in GAMS environment, and it is examined on the IEEE 30 and 118‐bus test systems. The numerical results substantiate that in the case of VSC‐SMO‐ORPD, active PLs are higher than the other case, whereas O&M cost of WFs is not different considerably. [ABSTRACT FROM AUTHOR]

With the increasing proportion of wind power connected to grid, power system dispatching is facing more and more challenges from uncertainty. To cope with this uncertainty, robust optimization has been applied in unit commitment (UC) problem. In this paper, a multi‐band uncertainty set considering the temporal correlation (MBUSCTC) of wind/load prediction error is proposed firstly, which has two characteristics: (1) The MBUSCTC rigorously and realistically reflect the distribution characteristics of uncertainties in uncertainty intervals, thereby effectively reducing the conservatism of the traditional singe‐band uncertainty set; (2) the temporal correlation constraints of wind power/load prediction errors in MBUSCTC could limit the realization of uncertainties fluctuating frequently in uncertain intervals, thereby eliminating scenarios with lower probability in uncertainty sets. Then the proposed MBUSCTC is applied to UC problem, leading a robust UC model based on MBUSCTC, which is solved by Benders decomposition method and C&CG method. Finally, case studies based on the modified IEEE‐118 bus system and an actual power system of China demonstrate that the proposed method can effectively reduce the conservativeness of the robust UC model and ensure the robustness of the unit commitment solution. [ABSTRACT FROM AUTHOR]

In this study, an advanced control strategy is proposed for hybrid energy storage systems (HESS) to smooth wind power generation fluctuations. Compared with the limited performance of solo energy storage system, the HESS, composing of lithium‐ion battery (LiB) and a flywheel energy storage system (FESS), can comparatively show improved flexibility and adaptivity. A novel sequence control scheme for the HESS is proposed in this study to improve the overall economic and smoothing performances. Specifically, based on variable‐interval reference outputs, the control horizon of the HESS is statistically determined. Afterwards, a sequence control scheme for the HESS is presented to improve the internal collaboration of HESS media. Especially when the LiB and FESS are synchronously charging/discharging to smooth wind power, the charge/discharge power are optimally distributed between the LiB and FESS via an optimiser to minimise the equivalent cost during each control cycle. Case studies are conducted to demonstrate the performance of the proposed method. [ABSTRACT FROM AUTHOR]

This study presents a bi‐level optimal sizing approach for hybrid energy storage system (HESS) in distribution network with high share of renewable energy. Differently from the traditional planning methods, the proposed two‐level approach can evaluate the investment risks caused by wind power uncertainty. In particular, the upper level describes the financial composition associated with HESS devices investment cost. The lower level searches optimal operation strategies for the minimal cost of each scenario set. The upper‐level and lower level models are bridged by CVaR, which measures the conditional tail expectation of lower level costs for the entire scenarios and are involved as the additional item of the upper objective. Moreover, to improve solution efficiency, some of the mathematics transformations including second‐order cone programming are adapted to transform the holistic non‐linear model into the more effective pattern that can be computed by the commercially available solver. The proposed method can compute the corresponding energy storage capacity based on the assessment of the risk tolerance of investors. Finally, a test system is used to validate the effectiveness of the proposed approaches. [ABSTRACT FROM AUTHOR]

In this study, simultaneous generation and transmission expansion planning is presented in the presence of wind farms. Short-circuit current constraint is included in the problem, and the impact of wind farms on short-circuit current is evaluated. This study presents a novel approach for reducing short-circuit current in composite generation and transmission expansion planning. In this way, new units and lines are determined such that the short-circuit currents of substations are less than the defined limit. In addition, an optimal level of short circuit is determined and the total cost is reduced. The proposed method is applied to a real case study, and its performance is demonstrated by comparison with other methods. [ABSTRACT FROM AUTHOR]

One of the effective methods to reduce the risk of load shedding and its subsequent blackout is to evaluate the risk of cascading outages and detection of key links in a power system. This risk assessment is getting more complicated considering the plans for integration of high wind energy. Therefore, in this study, a new sequential time approach has been proposed to evaluate risk of cascading outages considering the high penetration of correlated wind farms. The cascading processes are investigated using the extended OPA model using the graph traversal algorithm. Therefore, the problem of non‐convergence situations caused by the island formation is resolved. To deal with the future wind farms integration, a correlation is defined between them by a proposed method based on the Copula theory. The numerical results confirm that in evaluation of risk, the limited scenarios cannot always be reliable. The extended OPA model is then compared with a simple OPA to confirm the importance of islanding detection in the cascade simulation. Different aspects of cascading outages, i.e. fractional load shedding, cascade propagation level and number of line interruptions have been investigated. Finally, the impact of wind penetration and correlation of wind farms on results have been presented. [ABSTRACT FROM AUTHOR]

The growth of utilisation of wind power plants (WPP) in power systems placed at long distance of the transmission power system has shown that the best connecting alternatives are by means of high‐voltage direct current lines (HVDC) using voltage source converter (VSC) stations. Generally, the bus closest to the WPP and with the highest level of short circuit is selected as the best option to connect the VSC‐HVDC link, mainly due to economic criteria. This paper proposes a methodology to select the best connection point in the AC system, taking advantage of the potential increase of transmission capability due to the reactive power controlled at the VSC converter stations; measured as the increment of the loadability margin computed by means of PV curves. Nevertheless, a trade‐off between this benefit and the cost of overhead HVDC transmission line as consequence of the length differences among alternatives is required. This trade‐off is computed as the cost of the equivalent STATCOM required to get the same loadability margin. Thus, a technical‐economical comparison of connection alternatives allows the selection of the best option for connecting the VSC‐HVDC line. The methodology has been applied to connect a WPP of 500 MW to the NETS. [ABSTRACT FROM AUTHOR]

The real‐time (RT) energy management of virtual power plants (VPPs) is a complex problem due to the coordinated operation of diverse energy resources and their associated uncertainties. This article presents a comparative analysis on alternative energy management models for a VPP that includes a wind power unit, an energy storage unit, and some flexible demands, which are interconnected within a small size electric energy system. The smart grid technology enables RT operation of the VPP by taking advantage of bidirectional communication infrastructure. To accomplish this task, the RT energy management is implemented through alternative decision‐making tools, namely, (i) a robust model, (ii) a stochastic programming model, and (iii) a stochastic robust model. To handle uncertainties, these optimisation models use prediction intervals, scenarios, and a combination of both, respectively. A realistic case study provides a comparative out‐of‐sample analysis considering the impact of the different parameters used to manage risk in these three models. [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 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 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]

A subsynchronous oscillation (SSO) phenomenon in a wind farm integrated with a modular multilevel converter (MMC)‐based high‐voltage direct current (HVDC) transmission system has been recently observed in the real world. An attempt is made in this paper to contribute to the understanding of the root cause of the SSO in the MMC‐HVDC connected wind farms. For that, the small‐signal impedance model of the MMC is first developed based on the harmonic state‐space (HSS) modelling method. An inherent low‐frequency resonance peak in the MMC excluding any control influence is identified by its terminal impedance. Arguably, this could be the reason why the SSO occurs in the MMC‐HVDC connected wind farms. In addition to that, the influence factors, such as main circuit parameters, controller parameters, and power level, on the stability of the interconnected system are examined, which can provide guidelines for the system design in order to guarantee the stability of the interconnected system. Based on the mechanism analysis, a stabilization control for suppressing the SSO in the MMC‐HVDC based wind farms is also proposed. Finally, the theoretical analysis and stabilization control are validated by both time‐domain simulations and field measurements in a real MMC‐HVDC connected wind farm in China. [ABSTRACT FROM AUTHOR]

Converting the surplus power into rotor kinetic energy by changing the control functions of machine side converter (MSC) and grid side converter (GSC) is considered to be an efficient way for permanent magnet synchronous generator (PMSG)‐based wind turbine generator system (WTGS) to accomplish low‐voltage ride through (LVRT) without additional hardware components. However, because of the time delay of grid fault detection and the control system inertia, the dc‐link capacitor may still be subjected to overvoltage in the early tens of milliseconds after a grid fault happens. This study proposes a compositive control method of LVRT for PMSG‐based WTGS, in which the surplus power during LVRT is converted into rotor kinetic energy by controlling the electrical power of the PMSG according to the grid‐connected power of the GSC, and a crowbar circuit is reserved to consume the surplus power before MSC reacts to the grid fault. The proposed method does not change the control functions of MSC and GSC, and the related control parameters resetting can be avoid. Besides, it can also provide reactive power support to assist the grid voltage recovery. Simulations and experiments proved the proposed method to be feasible and effective. [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]

Large‐scale integration of renewable energy sources (RESs) such as wind power can impact on the existing power system's stability. For example, there is a possibility that the wind farm (WF) cannot be maintained stable and then it is disconnected from the grid when the instantaneous voltage sag caused by a grid disturbance occurs. Moreover, the increasing integration of RESs in the power system can reduce the number of conventional synchronous generators (SGs) connected to the grid, and lead to a reduction of system inertia in the grid. The system inertia reduction can cause larger and faster frequency fluctuations during the transient period. Hence, this paper proposes a new control strategy composed of the voltage control for suppressing the voltage sag and the virtual synchronous generator (VSG) control based on the proportional‐integral‐differential (PID) fuzzy logic control (FLC) for maintaining the frequency stability which is performed by high voltage direct current (HVDC) interconnection line. The aim of the new control strategy is to improve the stability of the entire power system with large‐scale integration of RESs. The effectiveness of the proposed strategy is confirmed through the simulation analyses by PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

Two important and upcoming technologies, wind resources and microgrids, are increasingly being combined. Various control strategies can be implemented, and droop control provides a simple option without requiring communication between microgrid components. However, traditional droop control does not allow the microgrid to maximise the power available from the wind. This study proposes a novel droop control strategy, which implements a droop surface in higher dimension than the traditional strategy. Simulation results show that power from the wind can be maximised, while maintaining the system's bus voltage around a nominal value using a distributed multidimensional droop approach. Selection of the optimal droop relationship is discussed and simulation results are presented. [ABSTRACT FROM AUTHOR]

In this study, a new short‐term wind power prediction model based on a temporal convolutional network (TCN) and the Informer model is proposed to solve the problem of low prediction accuracy caused by large wind speed fluctuations in short‐term prediction. First, an input feature selection method based on the maximum information coefficient is proposed after considering the problem of information interference caused by excessively large input features. A dynamic time planning method is used to select the optimal input step of historical power. Then, the combined forecasting model composed of TCN and the Informer is constructed in accordance with the numerical weather forecast and historical power data. Lastly, the pinball loss function is used to expand the prediction model into a quantile regression model, measure the effect of volatility, quantify the volatility range of prediction, and finally, obtain a deterministic prediction result. The actual measured data of wind farms in the Bohai Sea area are selected for analysis and calculation. The results show that the prediction model proposed in this study achieves better accuracy in deterministic prediction and interval prediction than the traditional model. [ABSTRACT FROM AUTHOR]

This study focuses on the design issue of battery energy storage system (BESS) for a wind–diesel off‐grid power system located in the Whapmagoostui community in Quebec, Canada. The local range of wind speed is from 0 to 24.8417 m/s, and the total yearly load demand in 2013 was 11,176 MWh. An optimal planning model is proposed in this study with the objectives of maximising the economic, environmental benefits, and reliability of the system. The battery energy capacity and the rated capacity of converter are selected as the optimal variables. In order to consider the impacts of renewable energy randomness, the uncertainty of component failures, and the power flow constraints on planning results, quasi‐steady state simulation is adopted to calculate the indices for each design scheme of BESS. The proposed optimal planning model of BESS is implemented and verified in the Whapmagoostui community. Also, a detailed analysis of several scenarios is presented. A base scenario with three diesel generators and four wind turbines is investigated, and its optimal BESS integration reduces fuel consumption by 4% and improves the average annual profit by 19%. The optimal designing of BESS enhances the economic, environmental benefits, and reliability of the wind–diesel system with high fuel prices in the Whapmagoostui community. [ABSTRACT FROM AUTHOR]

Variability of wind power is one of the main concerns of power system operation with significant wind power. Energy storage can be employed in conjunction with wind power to reduce the uncertainty associated with wind power. This study assumes the storage facility being operated by the wind farm operator for the explicit purpose of minimising the risk of wind power commitment. A time‐dependent wind power model is developed in conjunction with the energy storage model for short operating lead times that are conditional on the initial wind and storage conditions. This study presents a probabilistic method to assess the reliability contribution of energy storage using two probabilistic indices: the wind power commitment risk and the unit commitment risk. [ABSTRACT FROM AUTHOR]

With the real-time changes of wind speed and operating conditions, it is a challenge to fully tap the active power regulation ability and improve the control performance of automatic generation control (AGC) in a wind farm (WF). The essence of tapping the active power regulation ability is to realise the coordination and complementarity of each wind turbine's (WT's) dynamic adjustment performance (DAP). To address this, a novel data mining method is developed to derive the internal relations between WTs' output power and pitch angle, impeller speed and pitch angle during the power adjustment process, and a unified mechanism model is established to describe DAP of WTs. Based on the discovered relationship between WTs' DAP and its operating states, an active power distribution algorithm and a dynamic interval control method are proposed. Then, an active power dynamic interval control strategy that has been implemented using Java script in MyEclipse for WFs is further developed. The control strategy has been tested and applied in a 50 MW WF in northwest China. The preliminary results showed that the control strategy has improved the rapidity and accuracy of AGC in the WF. [ABSTRACT FROM AUTHOR]

This work assesses the financial viability of a wind farm where turbines are equipped to provide frequency services. Two control strategies are compared: the former enables the frequency response provision by operating the turbines part loaded, the latter employs the kinetic energy stored in the turbines' rotating masses. The analysis accounts for typical factors affecting the wind farm operation, the wake effect and the forecast errors on wind availability. A two-stage optimisation model determines the day-ahead and (close to) real-time operation of the turbines, which profit from selling energy at the wholesale market (including subsidies), receiving availability payments for frequency services, while facing imbalance costs due to mismatches between forecasted and available wind. The two control strategies are applied to the Teesside Offshore Farm and the economic and financial performance are evaluated against a base case where the turbines do not provide frequency response service. Under current settings, the added value from frequency response capability is marginal. Changes to the market value of frequency services would favour the strategy based on part-load operation of the turbines. The scheme based on the deployment of kinetic energy becomes profitable when the duration of time window for frequency response reduces. [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]

High‐level wind power penetration has significantly changed the power system's static and dynamic characteristics, which tends to decrease the overall system transient instability. This study proposes a multi‐objective preventive‐emergency coordinated control method against transient instability in the presence of uncertain wind power generation. This method optimally coordinates preventive control (PC) and emergency control (EC) to simultaneously minimise the total control cost and the expected stability margin. PC includes synchronous generator dispatch and wind farm power curtailment to reduce the overall percentage of renewable power to a safe level. EC adopts an emergency demand response scheme, in which the demand is immediately reduced for stabilising post‐contingency system and avoiding wide‐spread blackout. Wind power uncertainty is modelled through Taguchi's testing scenarios. The proposed method was verified on both New England 39‐bus system and Nordic‐32 system using industry‐grade dynamic models and simulation software. Simulation results verified that the proposed method can optimally balancing system control cost and system expected stability margin. [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 paper presents a decision tree (DT) induced fuzzy rule‐based intelligent differential relaying scheme for transmission lines including unified power flow controller (UPFC) and wind‐farms. The conventional distance relaying scheme fails miserably in protecting transmission lines including flexible AC transmission systems controllers such as UPFC and further, the protection issues become more challenging when wind‐farm is integrated. The proposed protection scheme extracts features from the instantaneous voltage and current signals from both ends of the transmission line using discrete Fourier transform based pre‐processor and computes corresponding differential features. The differential features are used to build the fault classification tree using a data‐mining algorithm known as DT. Further, the fuzzy membership functions are drawn using the DT thresholds and the corresponding fuzzy rule‐base is developed for final relaying decision. The proposed technique has been extensively tested (in MATLAB environment) for single circuit and double circuit transmission lines including UPFC and wind‐farms with wide variations in operating conditions. The test results indicate that the proposed DT‐induced fuzzy rule‐based relaying scheme is highly reliable and robust for protection of complex transmission line including UPFC and wind‐farm. [ABSTRACT FROM AUTHOR]