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Abstract A variable coefficient segmented iron loss calculation model is proposed for high‐voltage multi‐pole asynchronous motors, which fully considers the influence of high‐order harmonic magnetic density on such motors. This model introduces additional hysteresis loss, improved eddy current loss coefficients, and rotation magnetisation coefficients to account for changes in losses due to small hysteresis loops and skin effect, as well as rotation magnetisation losses. A 710 kW 10‐pole asynchronous motor is used as the research object. A full‐domain iron loss calculation model considering stator and rotor tooth top surface losses is developed. The model can be used to accurately calculate the overall and local harmonic iron losses in the stator and rotor cores of high‐voltage multi‐pole asynchronous motors. And quantitatively analyse the distribution pattern of any harmonic iron loss at any location in the motor core. It realises the refinement calculation and analysis of iron losses in the whole domain of high‐voltage multi‐pole asynchronous motor. Finally, no‐load and load tests were conducted on the 710 kW 10‐pole asynchronous motor. The iron losses of the 710 kW 10‐pole asynchronous motor at no load and load are calculated using the above mentioned model and the classical trinomial constant factor model. The results are compared with the measured iron loss values. It is proved that the iron loss model is more accurate in calculating the iron loss of high‐voltage multi‐pole asynchronous motor. The result helps researchers to calculate the iron loss distribution of high voltage motors more accurately. Thus, the design and operating parameters of the motor can be optimised to improve the efficiency and performance of the motor. It provides the necessary technical support and key basis for the reduction of loss and energy saving of high‐voltage motors and the optimisation of their core structure.

A variable coefficient segmented iron loss calculation model is proposed for high‐voltage multi‐pole asynchronous motors, which fully considers the influence of high‐order harmonic magnetic density on such motors. This model introduces additional hysteresis loss, improved eddy current loss coefficients, and rotation magnetisation coefficients to account for changes in losses due to small hysteresis loops and skin effect, as well as rotation magnetisation losses. A 710 kW 10‐pole asynchronous motor is used as the research object. A full‐domain iron loss calculation model considering stator and rotor tooth top surface losses is developed. The model can be used to accurately calculate the overall and local harmonic iron losses in the stator and rotor cores of high‐voltage multi‐pole asynchronous motors. And quantitatively analyse the distribution pattern of any harmonic iron loss at any location in the motor core. It realises the refinement calculation and analysis of iron losses in the whole domain of high‐voltage multi‐pole asynchronous motor. Finally, no‐load and load tests were conducted on the 710 kW 10‐pole asynchronous motor. The iron losses of the 710 kW 10‐pole asynchronous motor at no load and load are calculated using the above mentioned model and the classical trinomial constant factor model. The results are compared with the measured iron loss values. It is proved that the iron loss model is more accurate in calculating the iron loss of high‐voltage multi‐pole asynchronous motor. The result helps researchers to calculate the iron loss distribution of high voltage motors more accurately. Thus, the design and operating parameters of the motor can be optimised to improve the efficiency and performance of the motor. It provides the necessary technical support and key basis for the reduction of loss and energy saving of high‐voltage motors and the optimisation of their core structure. [ABSTRACT FROM AUTHOR]

Abstract The article introduces a new model of a Reference Signal Tracking (RST) controller for the fractional order quantities. The controller is applied to an indirect field‐oriented control (IFOC) system used for the speed control of an asynchronous motor powered by a photovoltaic (PV) generator with a maximum power point tracking (MPPT) algorithm. This model utilizes two fractional order controllers: the fractional order proportional‐integral (FOPI) regulator and the fractional‐order integral‐proportional (FOIP) regulator. These controllers are used with the generalized predictive control (GPC) technique. The first step in the approach is to derive the equivalent digital RST controller's model from the FOPI and FOIP controller's transfer functions. The GPC technique converts the continuous‐time FOPI (and FOIP) controller into a discrete‐time version. This conversion ensures a fast response and effective disturbance rejection. Simulation tests are conducted to analyze the rotor speed and stator current ripples to evaluate the performance of the proposed method. The results demonstrate the effectiveness of the introduced scheme in achieving improved control performance in terms of response speed and disturbance rejection. The article presents a modified RST controller model based on the fractional order approach applied to an IFOC system for motor speed control driven by a photovoltaic generator. Using FOPI, FOIP controllers, and GPC contributes to enhanced control performance, as evidenced by the simulation results.

The article introduces a new model of a Reference Signal Tracking (RST) controller for the fractional order quantities. The controller is applied to an indirect field‐oriented control (IFOC) system used for the speed control of an asynchronous motor powered by a photovoltaic (PV) generator with a maximum power point tracking (MPPT) algorithm. This model utilizes two fractional order controllers: the fractional order proportional‐integral (FOPI) regulator and the fractional‐order integral‐proportional (FOIP) regulator. These controllers are used with the generalized predictive control (GPC) technique. The first step in the approach is to derive the equivalent digital RST controller's model from the FOPI and FOIP controller's transfer functions. The GPC technique converts the continuous‐time FOPI (and FOIP) controller into a discrete‐time version. This conversion ensures a fast response and effective disturbance rejection. Simulation tests are conducted to analyze the rotor speed and stator current ripples to evaluate the performance of the proposed method. The results demonstrate the effectiveness of the introduced scheme in achieving improved control performance in terms of response speed and disturbance rejection. The article presents a modified RST controller model based on the fractional order approach applied to an IFOC system for motor speed control driven by a photovoltaic generator. Using FOPI, FOIP controllers, and GPC contributes to enhanced control performance, as evidenced by the simulation results. [ABSTRACT FROM AUTHOR]

Abstract High penetration of renewable rotary generations like wind turbine to utility system causes contribution in power oscillations due to occurrence of different phenomena. Ferroresonance and subsynchronous resonance (SSR) in utility system are able to lead in power oscillation, instability of the system and devastating impact on rotary generators. Hence, investigation on impact of such oscillations on operation of nearby renewable rotary generations, like DFIG (doubly fed induction generator) in Wind Park seems to be beneficial to attain insightful guidelines for future of research in this field. SSR causes increasing the voltage, thus, it brings about saturation of transformer core and results in occurrence of ferroresonance. Ferroresonance causes frequency difference between two points in the network and consequently power oscillation, which is superimposed on torsional oscillation in SSR. Therefore, as SSR and ferroresonance encounter each other, they are investigated under one work to scrutinize impact of such phenomena on behaviour of DFIG. The results show deviation of electrical waveforms and devastating stress on mechanical parts of DFIG rotary components due to huge magnitude of electrical and mechanical quantities in effect of SSR and ferroresonance.

Abstract Concerning sub‐synchronous oscillation in power system connected with virtual synchronous DFIGs, an oscillatory stability analysis method based on the dissipation rate of dynamic energy is proposed. First, the dynamic energy function that reflects the sub‐synchronous oscillation characteristics of virtual synchronous DFIG is built, and the expression of dynamic energy considering virtual synchronous control parameters is analysed. Then, according to Lyapunov's stability law, the variation rate of dynamic energy function is defined as the energy dissipation rate. The energy dissipation rate reflects the variation trend of system dynamic energy during oscillation, thus it is used as the stability criterion of sub‐synchronous oscillation. On this basis, the effects of different virtual synchronous control parameters on sub‐synchronous oscillation are analysed for a comprehensive consideration of system stability. Finally, simulation tests are conducted on RT‐LAB platform. The results verify that, when the energy dissipation rate is positive, system oscillation diverges; when the energy dissipation rate is negative, system oscillation converges. Meanwhile, the larger the virtual resistance is, the worse the stability of system is; the larger PI control parameters of reactive power control loop are, the worse the stability of system is.

Abstract Enlarging the speed range of wind turbines is an effective way to increase power production in low‐frequency band wind which contains more energy and is very challenging for DFIG‐based wind energy conversion system (WECS). The analysis shows that current controllability of the converter is the main factor limiting bidirectional speed range extension (BSRE). This problem can be solved by increasing dc‐bus voltage, but a constant and high dc‐bus voltage will increase the electrical stress and switching losses of the converter. This paper proposed a dynamic minimum dc‐bus voltage (DMDCV) technique in which the dc‐bus voltage can automatically adjust to enable the back‐to‐back voltage source converter to work in linear modulation mode at its minimum value, thus supporting BSRE while also achieving multiple goals including low electrical stress and low switching losses. Two implementation methods of DMDCV with and without modulation depth loop are proposed and discussed. The co‐simulation results verified the feasibility of DMDCV and the advantages of BSRE in improving power generation.

Abstract Due to the negative impact of a highly stochastic wind power fluctuation on the power quality and stability during high penetration of wind power in power systems, there is growing interest in power smoothing and energy redistribution in wind power systems by using large‐scale energy storage technologies. The aim of this work is to use a vanadium redox flow battery as an energy storage system (ESS) to smooth wind power fluctuation with two system configurations and corresponding control strategies. As the first step, a vanadium redox flow battery (VRFB) mathematical model, underlain by electrochemical theories, is built to describe the battery behaviours in charge/discharge cycles. Accordingly, the aforementioned system configuration and power fluctuation smoothing schemes are proposed. At the end of this work, simulations are conducted on a 3.6 MW doubly‐fed induction generator (DFIG) at a diversity of wind speeds for an assessment of feasibility that this work can be applied to a high penetration of wind power in practice. Simulation results reveal that power fluctuations can be improved by power smoothing. The root mean square deviation (RMSD) percentage can be reduced to less than 1%.

Abstract In this paper, neural network‐based data fusion is used to detect fault and isolate stator winding short circuit, outer bearing race, and broken rotor bar defects in an induction motor. In addition, the robustness of the proposed method against the disturbance introduced by the coupled pump's unbalanced power source and dry running is investigated. First, three‐phase current and voltage signals are separated by means of independent component analysis (ICA), then extracted features are combined by adopting neural networks, and finally, the system's health condition is evaluated. Experimental results indicate that data fusion based on neural networks can evaluate with high reliability the system's health condition and provide better robustness in the presence of disturbances.

Abstract An efficient power sharing between the solar PV array and the utility is the core aim of this work. A mechanical sensorless direct torque controlled (DTC) induction motor (IM) is implemented for the water pumping application. The PV array is operated at maximum power point (MPP), ensuring full utilization. Therefore, the integrated system ensures uninterrupted power for water pump operation regardless of the irradiance level. For serving the purpose of grid integration and IM control, two voltage source converters (VSCs) are used. The two VSCs share a common DC link. An artificial neural network (ANN) based direct power control is utilized to maintain the power transfer in both directions between the PV array‐grid integration and an induction motor drive with a water pump. Moreover, an observer based current adaptive approach is used for motor current estimation from a single current sensor used for DC link current sensing. The overall system comprising of the three‐phase grid in conjugation with PV array operating induction motor drive used for water pumping, is simulated and verified through test results. MATLAB/Simulink environment with Sim‐power system toolbox is used to perform the software simulation. In the laboratory, a test bed of grid connected water pumping system is developed.

Abstract Recently, a new type of subsynchronous oscillation (SSO) has occurred in doubly‐fed induction generator (DFIG)‐based wind farms. This oscillation is also called the subsynchronous control interaction (SSCI), which is mainly caused by the interaction between the transmission series compensation and the DFIG controller. Therefore, the oscillation frequency of the SSCI is susceptibly impacted by DFIG operating conditions, which presents major challenges to conventional mitigation strategies. To solve this problem, a grid‐connected series‐compensated DFIG‐based wind farm is established, and the formation process of the SSCI is analysed. The strategy of active disturbance rejection control (ADRC), which can automatically estimate and compensate for the total disturbance of the system in real‐time, is introduced into the rotor‐side converter to mitigate the SSCI in the DFIG. ADRC is designed based on dynamic parameter tuning and chaos optimization. Finally, a detailed model‐based time‐domain simulation is conducted to evaluate the performance of the proposed optimized ADRC (O‐ADRC) compared to conventional virtual impedance and ADRC. The simulation results show that O‐ADRC can effectively mitigate the SSCI under different operating conditions and provide sufficient damping for the DFIG‐based wind farms.

Abstract This paper presents a new protection scheme with low communication capacity requirement for protecting lines connected to a doubly‐fed induction generator (DFIG)‐based wind farm (WF). The proposed relaying scheme addresses the line protection challenges which stem from the non‐synchronous frequency component of the current fed from a DFIG‐based WF during a short‐circuit fault. The fault current frequency of a DFIG‐based WF deviates from the synchronous frequency during a fault, which affects the operation of distance relays. In such a scenario, the distance relay located at the WF terminal may lose its coordination with downstream relays, resulting in unnecessary tripping. The proposed scheme relies on the impedance trajectory captured by the local relay, local fault current characteristics, and the frequency tracking of the fault current measured at the distance relays located at the two ends of the transmission line connected to a DFIG‐based WF. The reliable performance of the proposed scheme is verified on a 4‐bus test system under balanced and unbalanced faults during super‐ and sub‐synchronous operating modes of the DFIG as well as its robustness against power system disturbances.

Abstract This paper proposes a frequency‐control scheme for Doubly Fed Induction Generator (DFIG)‐based (Type‐3) wind turbines to improve the primary‐frequency‐control when the grid power balance is disturbed. The increasing penetration level of renewable energy sources, like wind power plants, reduces the total available inertia of modern grids, which deteriorate the frequency response in case of sudden power‐mismatches. The proposed closed‐loop participation of wind power plant interacts with the thermal units to reduce the frequency nadir and frequency settling‐time, during the inertial and primary stages. The designed disturbance observer decreases the uncertainties in the estimation of grid parameters, which results in robust PI performance in adjusting the ancillary power provided by wind turbines. Certain measures considered within the control loop to limit the rate‐of‐change‐of‐frequency within the permissive range to avoid the protective relays tripping. Comparative simulations studies on modified IEEE 9‐bus and 68‐bus test systems verify the effectiveness and advantages of the proposed method.

Abstract Wave energy emulators provide an option to extend studies of wave energy converters (WECs) in an electric power laboratory facility and to evaluate the performance of controllers in an experimental set‐up. In addition, the integration of control strategies in different steps of the energy conversion process is an important step to improve the WEC overall behaviour that has not yet been widely discussed. This paper proposes a real‐time emulator and a hierarchical control scheme for a hyperbaric WEC consisting of floating bodies, hydropneumatic storage system (HSS), hydraulic turbine, and doubly fed induction generator (DFIG). The proposed emulator is based on a typical electric power laboratory facility, where a numerical model reproduces the dynamics of the wet subsystem and a hardware is the WEC electrical subsystem. Thereby, a squirrel‐cage induction motor (SCIM) is coupled shaft‐to‐shaft with the DFIG. The SCIM reproduces the characteristics of torque and speed that would be observed in the real system, and then, the DFIG is subjected to a primary drive with same dynamic characteristics of the real plant. The hierarchical control integrates the generator and HSS controllers considering the optimal operating pressure for local sea state conditions. Experimental results illustrate the performance of the emulator and controllers.

Abstract Low voltage ride through (LVRT) requires wind generation systems (WGS) to maintain continuous operation and provide reactive power support under grid voltage dips. This paper proposes a novel disturbance attenuation control (DAC) approach based on state‐dependent Riccati equation (SDRE) technique to enhance the LVRT capability of doubly fed induction generator‐based (DFIG‐based) WGS. The DAC problems are formulated with the control objectives for rotor side converter and grid side converter, and the weighing matrices are designed with fully studied principles to balance the control effect and cost. The SDRE technique is adopted to solve the DAC problems, and an alternative feasible state dependent coefficient construction algorithm is applied to improve computational efficiency. An active Crowbar circuit with overcurrent limiting mechanism is applied to ensure the rotor current and DC link voltage within the secure zone. Comparisons with conventional PI controller, exact linearization controller and coordinated control strategy are performed, the results demonstrate the proposed DAC approach has a better transient performance and enhances the LVRT capability of DFIG‐based WGS.

Abstract The work presented here deals with a comparison between integral, classical and multiscalar backstepping controllers applied for a PV water pumping system. The solar pumping system is controlled in its first part by a backstepping maximum power point tracking approach in order to extract a maximum power from solar panels. The second part, composed mainly by induction motor–pump is controlled by backstepping structures. The integral backstepping control structure gives interesting features in terms of stability using recursive Lyapunov design, increases robustness despite parameters variation, and provides good tracking and optimization performance. In order to validate the study with a real solar data, a measured irradiance profile is used to fed the PV system, based on solar measurements in tropical insular context. The measurements are collected at Sonapi site located in Haiti country. The fast variability of the tropical irradiance profile will allow to test the robustness of the used control algorithms and determine their limits. Simulation of the proposed solution is validated under Matlab/Simulink. Results demonstrate clearly that integral backstepping provides the best solution with a good tracking, and optimization performance: fast dynamic response and stable static power output, even when weather conditions (irradiation) are rapidly changing.

Abstract Increasing penetration level of renewable energy results in unprecedented operational challenges of AC grids due to generation uncertainty and reduced inertia. In grid emergencies, Fast active Power output Reduction (FPR) is demanded to drop wind turbine generator (WTG) power outputs faster than usual. However, the power mismatch between WTG mechanical power input and reduced electrical power output resulted from FPR can lead to rotor overspeed, which cannot be timely nullified by slow‐responding pitch control. Therefore, this paper proposes a novel FPR scheme for doubly‐fed induction generator (DFIG)‐based WTG through coordinated control of DC chopper and pitch angle. Specifically, the FPR command is executed by DFIG rotor‐side converter control, while rotor overspeed is restricted by triggering the DC chopper to dissipate the mismatched power. The pitch angle is actuated at a maximal rate to eliminate the mismatched power and switch off the DC chopper. The effectiveness of the proposed FPR scheme is verified through comparative simulation studies under FPR command execution and a grid fault. It shows the proposed scheme can achieve FPR in tens of milliseconds, avoid rotor overspeed and provide low‐voltage ride‐through capability in a fully‐controlled manner.

Abstract This paper presents a single‐ended traveling wave‐based fault protection and location method using game theory for a hybrid multi‐terminal transmission system: an overhead line combined with submarine cables. This technique intends to locate and protect a fault regardless of whether it is on the overhead line or submarine cables. Initially, this study investigates the effects of reflection and refraction coefficients of the terminals on the conventional protections based on traveling wave theory. Furthermore, it presents a single‐end method to detect and locate faults on hybrid multi‐terminal transmission systems connected to doubly‐fed induction generator (DFIG) based wind farms. Following that, the extracted traveling waves and the faulty‐segments were discriminated and identified, respectively, using the game theory. Regarding the extraction techniques of the traveling wave, this study also employs two techniques, including discrete wavelet transform (DWT) and mathematical morphology filter (MMF) to process the signals and investigate the sensitivity of the proposed scheme. The results of the simulation studies that used PSCAD/EMTDC confirmed the reliability and accuracy of the proposed protection scheme in all possible types of faults on the overhead line and submarine cables. The proposed technique also signifies noise immunity.

Abstract The proposed technology deals with the reduction in torque ripple of five‐phase induction motor. The machine is controlled by direct torque control technique and fed by three‐level five‐phase inverter (TL‐FPI). In the fraternity of multiphase induction motor, five phase is the most researched drive, considering complexity of control mechanism and advantages which multiphase machine has offered. The selection of TL‐FPI is based on availability of more voltage vectors (243) than two‐level five‐phase inverter (32). The 41 voltage vectors are employed out of 243 based on simplicity and advantages of multiphase machine. The selected vectors are operated through modified seven‐level torque comparator. An increase in levels of the comparator restrains the ripple in a predefined boundary. Here, modification in the predefined boundary of the comparator is suggested which allows the utilization of optimal voltage vectors. According to modification, zero voltage vectors are replaced with small active vectors when actual torque is lower than its reference value. To substantiate the claim various results of simulation and experimentation are given.

Abstract Induction motors play a major role in the industry nowadays due to their simple construction, uncomplicated maintenance, and cost efficiency. As the motor operates for repeated hours, some faults may occur and, depending on the process sensitivity, can cause significant losses to the industrial production. In this context, an alternative method is proposed to detect and classify bearing faults using acoustic emission signals generated by the machines and simple features obtained from them. A pair of condenser microphones was used to acquire these signals and audio feature extraction is performed to obtain time and frequency patterns to characterize healthy and faulty machines. The major differences of the acoustic signals regarding the fault frequency signatures are discussed by analyzing specific peaks observed in their spectra. Several coupled load values and levels of power supply voltage unbalance were considered in the experimental tests, which emulate common situations encountered in industrial environments, obtaining accuracy rates over 97% of success. Finally, a comparison is presented of machine learning techniques for bearing faults classification under different load values and voltage unbalance levels.

Abstract This study presents a strategy involving a reduction in the stator current sensor count for control of power converters, in a doubly fed induction generator (DFIG)‐based wind energy conversion system (WECS). The reduced sensor vector control scheme is effectively used in lieu of the conventional vector control algorithm during sensor failure without any change in the overall control structure. This keeps the system operational in case of sensor failure. The reduced sensor scheme necessitates precise estimation of stator flux amplitude and an orientation angle, for control of power converters under all operating conditions. A composite generalised integrator phase‐locked loop is presented to estimate accurately the stator flux amplitude and orientation angle, even at operating system irregularities such as unbalanced voltages, lower‐order harmonics in the grid voltage, DC bias in sensing circuitry, and so forth, which otherwise lead to distortions in flux and orientation angle. The grid‐side converter control is based on a twin frequency improved second‐order generalised integrator. It enables fast and accurate power quality improvement in DFIG‐based WECS during load and grid abnormalities. A laboratory prototype is developed to validate the control of the system.

Abstract The paper presents the performance analysis‐based reliability estimation of a self‐excited induction generator (SEIG) using the Monte‐Carlo simulation (MCS) method with data obtained from a self‐excited induction motor operating as a generator. The global acceptance of a SEIG depends on its capability to improve the system's poor voltage regulation and frequency regulation. In the grid‐connected induction generator, the magnetizing current is drawn from the grid, making the grid weak. In contrast, in the SEIG stand‐alone operation, an external capacitor arrangement is implemented to render the reactive power support. This capacitor arrangement is connected across the stator terminals during the stand‐alone configuration of SEIG. The capacitor serves two purposes, which include voltage build‐up and power factor improvement. Therefore, the paper deals with obtaining the minimum capacitor value required for SEIG excitation in isolated mode applications, including stand‐alone wind power generation. The SEIG performance characteristics have been evaluated for different SEIG parameters. The simulation and experimental results are then compared and found satisfactory. Then, SEIG reliability is estimated considering the MCS method utilizing SEIG excitation's failure and success rates during experimental work in the laboratory. Finally, the SEIG reliability evaluation is performed considering different wind speeds.

Abstract Dual‐stator brushless doubly fed generator (DSBDFG) is a novel generator applied for wind power generation, and the electromechanical energy conversion between the stator and rotor is realised by the magnetic field modulation of the special rotor structure. Therefore, the rotor design is critical to improve the performance of DSBDFG. In this study, the different rotor structures are compared and analysed, and the position of the non‐magnetic ring is determined. In order to reduce the computational cost and improve the optimisation efficiency, the surrogate model coupled with the multi‐island geometric algorithm is applied for the optimisation of the magnetic barrier layer of the cage barrier rotor. In addition, in order to reduce the effect of skin effect on the copper loss of cage bars of the rotor, the different simulation models, whose cage bars have different layer number, are simulated and analysed. Finally, the simulation and experimental results verify the correctness of the theoretical analysis and the effectiveness of surrogate model used for the rotor optimisation design of DSBDFG.

Abstract This paper describes the controller design aspects of DFIG‐based wind turbine system (WTS) using gravitational search algorithm (GSA). The appropriate control schemes are required for efficient and reliable functioning of the DFIG‐based wind energy conversion system (WECS). The control algorithms are implemented in converters which are placed in the rotor end and grid side of the WECS. The controller design schemes are optimized for accurate, reliable and stable operations of WECS using GSA. The most commonly used other design techniques are bacterial foraging optimization (BFO), and particle swarm optimization (PSO). Moreover, the transfer function modeling of DFIG is also described in this paper. The results show that the proposed GSA technique with sixths order transfer function model of DFIG improves the transient performance including time of rising the response to 90%, settling time, and amplitude of peak overshoot. The proposed GSA technique is compared with the techniques already implemented in the previous research works including PSO and BFO. The DFIG‐based WTS's output waveforms of voltage at dc‐link, reactive power, and active power are improved using GSA based design technique. Finally, it is concluded that the GSA technique gives better results as compared with the PSO and BFO techniques.

Abstract In this paper, a battery‐supported unified power quality controller (UPQC) for a small hydro‐based isolated power generation is investigated as a voltage and frequency controller. The self‐excited squirrel cage induction generator driven by constant power prime mover (small hydro turbine) has unacceptable voltage, frequency and power quality under non‐linear loads. The battery‐supported UPQC‐based voltage controlled‐voltage source converter is designed, modelled and simulated in MATLAB environment. Here UPQC has series and shunt converters with a battery and it is used to feed the local loads. This small distributed power generation system is capable to feed unbalanced non‐linear loads because a shunt converter is used to take care the compensation of unbalanced loads and it always maintains the source currents balanced and sinusoidal. Moreover, a series converter is used to inject the series voltages in the self‐excited squirrel cage induction generator terminals to regulate the common coupling point voltage during unbalanced non‐linear loads. Performance of a battery‐supported UPQC is also validated experimentally using a laboratory prototype of a 3.7 kW, 230 V, 50 Hz capacitor excited induction generator.

Abstract This paper presents a control strategy of large‐scale wind‐thermal power joint primary frequency regulation. First, an integrated control strategy is established rotor kinetic energy control and pitch angle control for wind turbine generators participation in frequency regulation within a wide range of wind velocity; Second, a fuzzy PI controller is designed for the recovery control of the wind turbine, which can avoid the occurrence of the frequency secondary drop accident during the wind generator exits the frequency regulation. Third, an optimal allocation power strategy of wind generator frequency modulation based on weighting factors is proposed in different operating conditions to make full use of the frequency modulation capability of wind turbine rotor kinetic energy control, and the strategy of orderly exiting the frequency modulation of the wind generators is established. Finally, the control framework for proposing the strategy is constructed to enhance the primary frequency regulation ability of large‐scale wind generators connected to power systems. The feasibility of proposing the strategy is verified by two examples. The results show that the presented strategy is able to heighten the characteristics of the power system frequency response availably and provide a reference for power system scheduling with large‐scale wind power.

Abstract The steady‐state models of electric power system components are crucial for the analysis of system load flow or preliminary study of the system stability. At present, penetration of wind power plant (WPP) in power system continues to increase, and its modelling development and investigation is also still a very active research area. This paper proposes a simple technique for modelling doubly fed induction generator (DFIG)‐based WPP to be used in power system steady‐state (i.e. load flow) studies. In the proposed method, derivation of the WPP mathematical model is carried out based only on power formulations of the WPP, namely, DFIG power output and turbine mechanical power input formulas. Detailed power converter modelling, which is usually quite complicated, is not required in the proposed method. Case studies are also presented in this paper where the proposed model is applied to a simple system as well as to multi‐bus systems. In the studies, the model is tested for various wind speeds ranging from 5 to 12 m/s. With these wind speeds, it is found that power outputs of the WPP vary around 0.1 to 2.1 MW. This result also indicates that the proposed DFIG model is valid both for sub‐synchronous and super‐synchronous conditions.

Abstract As the grid codes of wind turbines note, doubly fed induction generator (DFIG)‐based wind energy conversion system (WECS) should maintain uninterrupted operation during certain voltage faults. To meet the codes, a novel configuration of DFIG‐based WECS is proposed in which a superconducting magnetic energy storage (SMES) and a series transformer are incorporated. The control strategies of the WECS are redesigned to smooth the output power fluctuations and ride through different levels of voltage faults. The grid‐side converter (GSC) is controlled to smooth power fluctuations caused by random wind speeds under normal conditions. The GSC and protection circuit are controlled to switch into the series compensation mode to fully compensate the stator voltage during voltage swells and slight voltage sags. While under severe voltage sags, the protection circuit operates in the current limiting mode, which significantly limits the stator and rotor fault overcurrent, improves the stator voltage and supresses fluctuations of the electromagnetic torque. The different faults are applied, the simulation results demonstrate that the presented scheme can effectively smooth output power fluctuations and strengthen the voltage fault ride‐through capability (FRT) of WECS.

Abstract This study presents a winding open‐circuit fault‐tolerant control scheme for an open‐end induction motor drive with common DC bus for dual inverters. The proposed control scheme also permits balanced three‐phase operation of the same motor under healthy operation. It can suppress common mode current easily, yet permits maximum DC bus utilisation in both operating modes. It is mathematically established here that during fault‐tolerant operation, the magnetising current increases by a factor of √3. As a result, the airgap flux during fault‐tolerant operation is same as that during balanced three‐phase operation. An analytical relation is proposed for maximum torque capability of the machine during fault‐tolerant operation, taking into account the limit of winding current to its rated value and increase of magnetising current during such an operation. Extensive mathematical analysis is supported by simulation and experimental results to validate the performance of the proposed scheme during balanced three‐phase operation as well as winding open‐circuit fault‐tolerant operation.

Abstract This paper presents a micro‐hydro generator driven frequency adaptive sliding mode (FASM) control‐based improved power quality permanent magnet brushless DC motor (PMBLDCM) driven air conditioning system for remote and hilly areas. In such locations, the grid availability is poor or limited. The self‐excited asynchronous generator (SEAG) is highly suitable for micro‐hydro power generation. The voltage and frequency regulations of SEAG systems are very poor. Therefore, an improved power quality FASM control of a new zeta function based modified CSC (ZFM‐CSC) converter scheme is developed to drive the micro‐hydro power driven SEAG fed air conditioning system. The conventional air‐conditioning schemes are suitable for the grid fed system, where the frequency and voltage remain constant. However, these conventional control schemes are not suitable for a standalone SEAG based micro‐hydro generator driven air conditioners since the frequency and voltage of this system vary frequently at different loadings and dynamic operating conditions. This leads to an incorrect estimation of reference source current in a conventional system, which in turn affects the system stability. The sudden change of loads in a conventional air conditioning system may de‐magnetize and de‐stabilize the micro‐hydro driven SEAG. The experimental and simulated results presented validate the robustness of this system.

Abstract The transient characteristics of short‐circuit current (SCC) in doubly‐fed induction generator (DFIG) are quite different from the widely researched conventional synchronous machines. However, the second harmonic characteristics of SCC that impact the reliability of the transformer protection have not been accurately analysed. This paper presents the generation mechanism of second harmonic SCC in DFIG by deducing the dynamic response of rotor‐side converter, with particular attention to the influences of the voltage drop, harmonic component of stator voltage, frequency coupling, and deviation of the phase‐locked loop. When grid fault occurs, the terminal voltage sag leads to fluctuations in the active power, then the response of converter to power fluctuation and the frequency coupling leads to the second harmonic current in the abc reference frame. And the second harmonic current injected into the grid leads to fluctuations in the voltage disturbance term and deviation of the phase‐locked loop, which, in turn, affects the second harmonic current. Based on the analysis, the mitigation measures for the second harmonic current are proposed. Simulations are implemented to verify the theoretical analysis.

Abstract Time varying nature of the wind power is studied previously considering different time intervals from seconds to days. However for power quality problems such as flicker, a model which considers the extremely fast variations is essential. Here by using large number of actual records, a time‐varying model is proposed which considers the extremely short‐time variations of wind active and reactive powers. The wind farm is modelled as a current source with time varying amplitude and phase which change every 0.01 s. Autoregressive moving‐average (ARMA) models are utilized to model the variations and ARMA coefficients are calculated for every record. Same to the actual behaviour, the proposed model is non‐stationary as the ARMA order and coefficients are different at every run of the model. The proposed model is confirmed through utilizing several applications which need the power time series with extremely short sampling intervals. The following studies are performed by using the actual data and then the proposed model: power spectral density of active and reactive power variations, instantaneous flicker, short term flicker (Pst), estimating the Pst using the maximum value of the instantaneous flicker, estimation of cumulative Pst for multiple wind turbines, and the impact of SVC on flicker mitigation.

Abstract This paper presents an accurate framework for incorporating the doubly fed induction generator based wind farms in load flow analysis of distribution systems. The proposed model for doubly fed induction generator is based on an accurate equivalent circuit and also takes into consideration the voltage dependent reactive power limits associated with the doubly fed induction generator. In addition, the developed approach is capable of handling a doubly fed induction generator operating in either maximum power point tracking or non‐maximum power point tracking modes. The proposed PQ model of doubly fed induction generator easily fits into the forward–backward technique that is typically employed for obtaining the load flow solution of distribution networks. Test results on 19‐bus and 133‐bus systems are reported to indicate the effectiveness of the proposed approach.

Abstract Transient simulation model of doubly‐fed induction generator‐based wind turbine has a complex model structure and considerable control parameters. An accurate simulation model is especially important for studying the static and transient characteristics of doubly‐fed induction generator‐based wind turbine and the grid‐connected system. Based on the measured data of low voltage ride‐through testing, the integrated parameter identification method is proposed here. Combined with the time‐domain simulation analysis, curve fitting is used to determine the control strategy of doubly‐fed induction generator‐based wind turbine. The improved genetic algorithm is used to identify the generator parameters, power control parameters, and resistance of Crowbar of doubly‐fed induction generator‐based wind turbine. Combined with the electrical parameters provided by the wind turbine manufacturer, electromagnetic transient model on full operation condition and rapid starting period, corresponding to a measured doubly‐fed induction generator‐based wind turbine in West Inner Mongolia Power Grid, is then established, which can operate in the full wind speed range and quickly enter the stable state within 1 s after start‐up. Comparison of simulation results and measured data shows that the established electromagnetic transient simulation model using parameters identified by the proposed method can accurately reflect the key characteristics of doubly‐fed induction generator‐based wind turbine.

Abstract Here, the analytical model of the 2nd order eigenvalue sensitivity is improved, and applied to optimize the parameters of the power oscillation dampers of the doubly‐fed induction generators. To solve the 2nd order eigenvalue sensitivity, the eigenvector sensitivity is required whose difficulty lies in insufficient constraints, for which two constraints about the magnitude and the angle of eigenvector elements are newly introduced, and the normalization conditions are derived to solve the eigenvector sensitivity. The 2nd order eigenvalue sensitivity is applied to analyse the change of the low‐frequency oscillation modes with the varying of parameters in the power systems with the doubly‐fed induction generators. To improve the low‐frequency oscillation modes, the optimization model of the power oscillation damper parameters is newly formulated based on the 1st and the 2nd order eigenvalue sensitivities and solved by the interior point method. The numerical results are provided to verify the effectiveness of the constraints proposed; the advantages of the 2nd order eigenvalue sensitivity in the eigen‐analysis and parameters optimization, and the robustness against wind fluctuation.

Abstract Doubly‐fed induction generator (DFIG) is among the most attractive technologies in wind farms. As the penetration of wind power into the grid has increased in recent years, protecting of wind farm's generators against grid disturbances has become a major issue for the power system industry. Transient instability (TI) is among the most frequent phenomena due to severe grid disturbances. This phenomenon might cause significant damages to network equipment. In DFIG based wind turbine (WT), when the crowbar circuit is activated, the rotor winding is short‐circuited and the power converter is blocked. At this moment, the DFIG operational mode is temporarily changed to squirrel‐cage induction generator (SCIG). This lead to reactive power absorption and voltage drop, that might cause electromechanical oscillations and unstable condition. This study presents an analytical method for detection TI of a grid‐connected DFIG based on analysis of angular velocity and acceleration data, which are readily obtained from locally measured quantities. This technique works on a setting‐free manner, i.e. revising would not be required even when the network configuration changes, which is a favourable feature in protective relaying. The performance of the proposed method is verified using MATLAB/Simulink. Simulation results prove the method's reliability and simplicity.

Abstract To improve the reliability of electric drive systems during the insulated‐gate bipolar transistor short‐circuit fault, a control method is proposed for Y‐connected 3‐phase induction motors. This method allows the operation of 3‐phase induction motor drives under the insulated‐gate bipolar transistor short‐circuit fault without the requirement of a specific inverter topology and control strategy. At first, a modified inverter topology that can rectify the problem produced by an insulated‐gate bipolar transistor short‐circuit fault is presented. Then, based on the introduced inverter topology, a developed model for Y‐connected 3‐phase induction motors is presented. According to this model, an indirect vector control system for the faulted drive system based on the calculation of forward and backward magneto‐motive force of the motor is proposed. The performance of the control system is validated using experimental tests for a 1HP Y‐connected 3‐phase induction motor drive system. This research also compares the performance of the proposed indirect vector control technique with conventional indirect vector control techniques during the insulated‐gate bipolar transistor short‐circuit fault.

Abstract A STATCOM that employs a nearly constant switching frequency hysteresis current controlled VSI is presented in this paper. Hysteresis controller enhances the dynamic performance of STATCOM. The switching frequency variations associated with the conventional hysteresis current controller is reduced by using a variable hysteresis boundary. Variable hysteresis boundary is predetermined by the current error, based on the sampled magnitudes of the reference voltage vector. This makes the switching frequency nearly constant and switching vector selection optimal. As a result, a voltage FFT profile similar to that of a space vector modulated STATCOM is obtained. Choosing a space vector based reference voltage vector keeps the current error within the pre‐calculated boundaries and improves the current tracking capability of STATCOM. The systematic method of selecting a switching sequence avoids random switching. Effectiveness of the proposed method for the control of grid‐connected STATCOM is experimentally verified and compared with a voltage‐controlled space vector modulated voltage source inverter based STATCOM. The hardware implementation of the controller for grid‐connected STATCOM is carried out in OPAL‐RT real‐time simulator platform and experimentally validated for a three‐phase balanced load of 5 kVA.

Abstract This paper investigates the memory‐based integral sliding mode control (SMC) design for nonlinear doubly fed induction generator model. The proposed nonlinear doubly fed induction generator model is equivalent to linear sub‐systems via fuzzy membership functions by utilising the Takagi–Sugeno fuzzy approach. Then, a memory‐based sliding surface is intended, which is different from the conventional sliding surface. Based on suitable Lyapunov functionals and slack matrices, sufficient conditions are obtained, which guaranty the memory‐based Takagi–Sugeno fuzzy system to be globally asymptotically stable under the designed memory‐based sliding mode control through linear matrix inequality. Further, the desired memory‐based fuzzy SMC control gain is obtained by solving the concerned linear matrix inequality. Also, a memory‐based SMC law and an adaptive SMC law are incorporated to make the system state trajectories onto the pretend sliding switching manifold in a finite‐time. The derived conditions are verified in the numerical section with the nonlinear doubly fed induction generator model, which are formulated for reflecting the characteristics of the proposed wind turbine model.

Abstract The prevalent use of the diode bridge rectifier in three phase wind energy conversion systems decreases the efficiency of the power generation. In standalone wind energy generation, bridgeless converters can address this problem by eliminating the front‐end diode bridge rectifier. This work presents a novel single‐stage three‐phase bridgeless AC to DC c̀uk converter for wind energy conversion systems. The proposed converter is designed to work in discontinuous inductor current mode because of its inherent power factor correction and less harmonic distortion. Using body‐diodes of the MOSFETs per phase component reduction is also done in this converter. A unique maximum power point tracking technique is developed for this converter using three‐phase voltages and currents, instead of the DC link states. A wind energy emulation system using a DC motor and self excited induction generator is used to test the laboratory prototype of the proposed converter.

Abstract In the dynamics of grid‐connected operation of brushless doubly‐fed generators, grid voltage dips can cause the extreme imbalance between the voltage and the back electromotive forces in power winding. When these imbalances are combined with the influence of the inertia link composed of converter and generator, a serious over‐current and current oscillation forms in power winding and control winding. This paper gives a process analysis and proposes an improved control to the system during the low voltage ride through event. According to the mathematical model of the generators, the correlation between the power winding and control winding currents is established and then a fully equivalent simplified model is obtained accordingly. When the voltage suddenly drops to zero, the analytic expressions of power winding and control winding are calculated in the complex frequency domain and the influence factors of overshoot and oscillations are analyzed. The conventional control method relies on limited PI regulation that cannot eliminate the oscillations completely. It is found that after adjusting a proportional coefficient and eliminating the integral link of the control winding current controller, the order of the control system reduces. Therefore, the generation system can quickly recover to a steady state without oscillations. Finally, the analysis and improvement are verified by the simulation model and experimental results.

Abstract In order to meet the higher requirements (i.e. large scale, high reliability and direct drive) of generators used for offshore wind turbine, a novel direct drive module dual stator brushless doubly‐fed generator (DSBDFG) with cage‐barrier rotor having many outstanding merits is proposed in this paper. The fundamental structure, operation principle and power flow are presented. The electromagnetic design methods of the proposed generator including power distribution of inner and outer machines, the relationship of pole number, slot number and module number, and winding design are thoroughly investigated. In order to solve the problems in structure brought by module design and reduce the weight as much as possible, a mechanical construction applied for module DSBDFG is proposed. Finally, the correctness of the proposed electromagnetic design method and the stability of the proposed mechanical construction are verified based on finite element analysis respectively.

Abstract The advantages, such as the mature control method, less volume of the converter and generator, make the doubly fed induction generator system prevalent in the wind power industry. Nevertheless, the doubly fed induction generator is more susceptible to grid faults and disturbances. The instantaneous high voltage fault may occur due to the excessive local reactive power after the doubly fed induction generator system achieving low voltage ride through, and then the wind turbine might be disconnected again. Hence, not only the low voltage ride through but also high voltage ride through capability should be required for the doubly fed induction generator system to meet the grid code requirements. A comprehensive review of the state of the art low voltage ride through and high voltage ride through technologies for the doubly fed induction generator system is presented. Firstly, different types of common low voltage ride through and high voltage ride through techniques are classified according to their features, i.e. auxiliary hardware, linear or nonlinear control strategies etc. The pros and cons of different low voltage ride through and high voltage ride through techniques are given. Furthermore, the latest developments of low voltage ride through and high voltage ride through technologies are introduced. Finally, the future trends of both the low voltage ride through and high voltage ride through technologies are discussed.

Abstract A new modular transformer based 36‐pulse ac–dc converter for a cascaded H‐bridge‐inverter fed medium voltage induction motor drive is proposed in this work. The conventional multi‐pulse ac–dc converter has only 18‐pulses in its line input current with six output dc voltages, whereas, the proposed multi‐pulse ac–dc converter has 36‐pulses in its line current with six output voltages. Due to this, the proposed medium voltage induction motor drive has improved power quality at input end. A 9‐level cascaded H‐bridge‐inverter is applied on drive side to boost the performance of medium voltage induction motor drive. A modified model reference adaptive system based speed sensorless vector control is presented in this paper. A nearest level modulation scheme is used to operate a 9‐level inverter at fundamental frequency switching over the entire speed range of an induction motor drive. The performance of this drive is verified through both simulation and experimental results.

Abstract This paper presents a comparative analysis of the continuous space vector pulse width modulation and the discontinuous space vector pulse width modulation in terms of the switching losses and current ripple reduction for the unbalanced two‐phase four‐leg voltage source inverter fed unsymmetrical two‐phase induction motor. The main function of both space vector pulse width modulation strategies is to produce unbalanced two‐output voltages with constant phase shift of 90° for main and auxiliary windings. The discontinuous space vector pulse width modulation principle is modified from a traditional two‐phase four‐leg voltage source inverter by placement of the space vectors to alternatively eliminate zero space vector in each switching sequence. As a consequence, the proposed unmodulation region or clamping zone of the discontinuous modulation strategy has 180° all the time. The experimental results illustrate the modulation waveforms of both methods, output voltage spectrum, normalized switching losses, output current ripple and inverter efficiency at high modulation index. The evaluation of the normalized average switching losses and the current ripple of the discontinuous space vector pulse width modulation method can be reduced more than that of the continuous space vector pulse width modulation method.

Abstract Smart grid comprises utility system and dispersed renewable generations. It establishes bidirectional power transmission and communication among utility system, distributed generations and consumers. Sub‐synchronous resonance (SSR) is a major concern in transmission line. Furthermore, impedance swing during SSR leads in operation of out of step blocking characteristic of distance relay. SSR brings about a significant rise in the magnitudes of voltage and current. It also raises probability of occurrence of ferroresonance. Here, the impact of SSR in utility system is analysed on operation of doubly‐fed induction generator and distance relay with out of step blocking element is given here. Smart grid urge to establish a self‐healing protection, hence, an adaptive algorithm based on sub harmonic and ferroresonance detection is proposed for distance relay. The algorithm discriminates ferroresonance from other nonlinearities by wavelet transform and neural network and utilises time domain analysis to distinguish between different types of ferroresonance. The algorithm is able to observe smart grid protection strategy. It is capable of receiving the commands according to protection strategy to take a correct decision in such conditions to enhance reliability of power electricity in smart grid. Finally, the proposed algorithm is examined in SSR condition to certify the correct operation.

Abstract Winding function (WF) method is one of the electric machines modelling methods, which is used to simulate electric machines under different conditions. It has less simulation time and memory than other methods and, at the same time, has comparable accuracy over other methods. One of the significant challenges of WF‐based models is how to model saturation. The accuracy of the fault detection method in electric machines depends on the accuracy of the model and the method used for the simulation. A new extended model of the WF is presented for saturation modelling, which considers all parts of the core including teeth, slots, and yokes. In this method, the effect of the iron parts is emulated as a partial value that is added to the length of the actual air gap. The simulation results for normal and faulty conditions are shown in two cases, with and without considering saturation effect. Then the importance of saturation modelling has been demonstrated. Simulation is done in MATLAB software. Moreover, the experimental results are shown to verify the proposed model.