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Existing research on Doubly fed Induction Generator (DFIG) short-circuit current studies are based on hypothesis the rotor current is zero when the Crowbar is input after the fault, which ignores the impact of rotor current attenuation process. It would bring the error to the calculation results. To solve this problem, on the basis of analyzing variation process of DFIG’s transient equivalent potential and the influence of rotor current attenuation process, the flux is calculated accurately. An accurate calculation equivalent model for DFIG short-circuit current is proposed. With the RTDS, an experiment platform with physical controller of converter is founded, the proposed model is validated. The effect of short circuit current to wind farms transmission line is studied, and an improved protection scheme is put forward.

More and more Doubly-fed Induction Generators (DFIG) have a low voltage ride through (LVRT) capability, which needs to provide reactive power for the grid during the low voltage ride-through period. Traditional short-circuit current calculation of DFIG ignore the influence of reactive power support on short-circuit current; it is assumed that the rotor current is zero when the Crowbar is input after the fault. This inaccurate short-circuit current will indirectly affect the protection’s operating characteristics and produce certain errors. Specially, in China’s new grid-connected wind power standards, DFIG need to output reactive current during fault to provide reactive power support for the system. At this time, the rotor-side converter should no longer be locked, and provide continuous reactive current during the fault. In view of the above issues, according to the input–output characteristic of rotor converter, the equivalent model of rotor converter and the transient model of DFIG were provided. The influence mechanism of reactive power support control strategy was analyzed. The short circuit current calculation model of DFIG was established. Based on RTDS the experiment platform with physical controller of converter was founded, the proposed short-circuit current calculation model considering the reactive power support control strategy has been verified. The fault analysis method for the grid with penetration of DFIG is analyzed. Result of this research is available for the study of DFIG influence on the protection characteristics.

A power supply able to generate a pulsed large current (∼26 kA) with a flattop period > 10 ms for a heavily inductive load was designed based on the sequentially fired pulse forming network (SFPFN) scheme with high-energy-transfer efficiency. To overcome the high voltage stress on switching devices due to the SFPFN scheme, a novel electrical circuit topology was developed. In particular, the current changing rate ( di/dt ) on switching devices is limited by new designs of crowbar branch and pulse-forming unit (PFU). The assembled power supply is shown to provide an output current of a flattop of 26 kA and 12 ms on a load of 40 μ H and 4.24 mΩ with the predicted improvement in energy transfer efficiency.

This paper presents a novel approach for determination of the crowbar resistance of the doubly fed induction generator (DFIG) in wind energy conversion system (WECS) that is used to improve the fau...

Crowbar protection and chopper protection (CCP) are commonly used hardware protection measures in the process of low voltage ride through (LVRT) of doubly-fed induction generator (DFIG). The protections operating states during fault have an important influence on the electromagnetic transient (EMT) process of DFIG. Therefore, different from only considering the influence of single protection, the coordinated control of crowbar protection and chopper protection are further considered in this paper, and a novel analysis method of CCP operation criterion is proposed by analyzing the relationship between rotor current, DC-link voltage and post-fault terminal voltage (PFTV) of DFIG. On this basis, the states of CCP under different voltage drop depths are analyzed. And an EMT analysis model of DFIG considering CCP is studied. The proposed method and EMT analysis model for DFIG are later validated through PSCAD/EMTDC-based simulation results. The results show that the method and model proposed in this paper have good accuracy and practicability, which provides more references for further study of the transient characteristics of DFIG.

The significant benefits of brushless doubly fed induction generators (BDFIGs) are higher reliability and lower maintenance cost due to the lack of a brush gear. Recent studies on BDFIGs have shown promising applications in wind power generation. The low-voltage ride-through (LVRT) capability of wind turbines is one of the most important properties specified in the grid codes of many countries. A crowbarless LVRT control strategy based on flux linkage tracking for a BDFIG under symmetrical voltage dips is proposed in this article. A novel and simple controller in static reference frame is presented to implement flux linkage tracking. To both suppress the control winding (CW) current and increase power winding (PW) reactive current, a time-sharing control and CW current-first control solution is proposed. In the most severe case, where the BDFIG rotor speed is 100% of the maximum rotor speed and under 100% symmetrical voltage dips, the experimental results show that the CW current peak can be limited to 2.01 p.u. of the CW current rating and the torque ripple is small during the fault. The experimental results also show the possibility of the BDFIG to ride through the severest voltage dips without extra hardware, such as a crowbar.

In this article, the influence of multistage synchronous induction coil launcher (SICL) coupling on diode advance conduction was analyzed. The coil current and launching performance of the SICL which is based on the crowbar circuit was also investigated. The thyristor was connected before diode in this multistage SICL system circuit. First, the precise mathematical model for this two-stage SICL was theoretically deduced. The model was to analyze the effects of coils coupling under different trigger timings on diode advance conduction and coil current. Second, the derivation of the model was based on applying the precise mathematical methods of coils coupling to the process of SICL launching. From this analysis, the effects of coils coupling under fixed trigger timing on coil current and launching performance can be obtained. By comparing the MAXWELL finite-element simulation with MATLAB numerical calculation, we were able to estimate the correctness of the precise mathematical model. Finally, experiments were carried out and the results were well consistent with the simulation results.

High-voltage power supplies (HVPSs) are widely used to drive vacuum tubes, inherently susceptible to vacuum arc faults. Owing to the considerable cost of vacuum tubes, the HVPS should enjoy a reliable, fast protection mechanism. A common protection mechanism against vacuum arc faults is the shunt crowbar. The crowbar diverts the energy stored in the elements of the HVPS through itself as a parallel path, thereby protecting vacuum tubes from irrecoverable damages. Among all types of the shunt crowbar, the thyristor-based crowbar along with the fault current limiter (FCL) resistors attracts more attention because of its more reliable operation. The absence of the comprehensive design procedure providing an insight into selecting the key components of the crowbar is strongly felt. This article presents a comprehensive design procedure for selecting these components. All constraints which should be satisfied by the protection mechanism and key components are elaborated. By doing so, the upper and lower limits of the FCL resistors, the thyristor specifications, and their series inductance are specified. It is shown that thyristors with high nominal peak current cannot contribute to the operation of the crowbar. The intersection area of the constraints and optimum values are determined for one case study. The simulations and experimental tests are exploited to evaluate the effectiveness of the presented design procedure.

The transient of the DFIG engages a lot of non-smooth elements such as switches of controllers and saturation of limiters, thus, the simplified non-smooth model of DFIG for electromechanical transient analysis is needed. This paper proposes a non-smooth simplified model of DFIG for electromechanical transient analysis, with the viewpoint of the transient response and energy accumulation. First, the general non-smooth dynamic behavior of DFIG during fault-on and post-fault periods are presented and the model is divided into four smooth stages. Then, the detailed model for the transient stage, according to without/with the trigger of the crowbar is presented. Furthermore, the simplified model of DFIG in the steady state is introduced according to without/with the trigger of the crowbar and saturation of limiters, determined by the stator voltage. Moreover, the transient model with/without the crowbar protection are simplified according to the principle of constant energy accumulation. Thus, the overall non-smooth simplified models of DFIG is obtained. Finally, the effectiveness of the proposed non-smooth simplified model is verified with the simulations.

The significant issues which are associated with grid code during grid fault are the fault ride through (FRT) capability of wind farms. This paper presents a detailed investigation of different hybrid fault ride through schemes in DFIG-based wind system under both symmetrical and unsymmetrical fault conditions. This work analyzed the performance of hybrid FRT combinations like crowbar resistance with RL circuit, DC chopper associated with crowbar resistance, DC chopper connected with series dynamic resistor, voltage control scheme with crowbar resistance are developed and studied under symmetrical and unsymmetrical fault conditions. The simulation results are obtained using MATLAB Simulink version 2019b, and the hardware real-time simulation is implemented in OPAL-RT real-time simulator. The proposed hybrid FRT schemes provide promising results improving the system parameters under different fault scenarios in grid-connected wind system.

The performance of doubly-fed induction generators during grid disturbances is of increasing interest due to the growing number of wind turbines employed in this type of technology. Grid codes specify that the generator must have a fault by-pass capability, remaining connected and contributing to grid stability during a disturbance. For doubly-fed induction generator, a crowbar with constant resistance tends to be a simple approach to protect the wind turbine and improve the lower voltage path. However, the responses of the doubly-fed induction generator depend on the degree of voltage sag, since the constant resistance of the crowbar can be over or underestimated, resulting in severe damage. Therefore, the crowbar resistance is an important parameter that should be selected taking into consideration the rotor overcurrent and dc-link overvoltage limits. Thus, this paper proposed an improved method to analyze the low voltage ride through performance of a doubly fed induction generator system with active crowbar rotor. Additionally, an analytical expression for the crowbar resistance variation tied to the control strategy is presented. The effectiveness of the proposed method is validated through theoretical analysis and Matlab/Simulink simulations.

The Fault Ride-Through capacity is increasingly being demanded for wind turbines connected to the distribution networks. For DFIG-based turbines, achieving this requirement is even more difficult since this topology is vulnerable to voltage sags. The most popular solution employed for DFIG to ride through short-duration faults is crowbar protection, however other potential strategies have been presented in the literature. This paper aims to compare the performance between the crowbar and the resistive-type superconducting fault current limiter placed at the DFIG rotor side. For each scheme, different crowbar and R-SFCL resistances and activation currents were investigated to make the comparison fair. The results showed that the R-SFCL performed better in improving the FRT capacity of a distributed DFIG.

High-voltage power supplies (HVPSs) are widely used to supply vacuum tubes. The amount of delivered energy from the HVPS to the tube is an important issue during the vacuum arc in the tube. The conventional protection mechanism consists of a shunt crowbar which diverts the fault current from the tube to itself as a parallel path. The crowbar circuit is usually built of the devices without the turn-off capability. It is a drawback since the output of the power supply is shortened for a long time. Thus, the restoration time of these power supplies is excessive. This demerit can have detrimental effects on mission-critical applications. In this paper, the insulated-gate bipolar transistor (IGBT)-based crowbar structure is studied to overcome this issue. In the proposed protection mechanism, the crowbar can be reopened intentionally after closing. A theoretical analysis is presented to compare power supply performance in the presence of the conventional crowbar with the IGBT-based crowbar. The experimental results are presented to demonstrate the practical considerations of the proposed crowbar structure and its appropriate performance. This paper shows that HVPS has more fault resiliency and less restoration time with the proposed IGBT-based crowbar.

The complexity of interconnection modern power systems has made them more subject to wide area disturbances that have contributed to several failures of system protection that operate as a standalone and with the multi-zones operation. The high penetration of doubly-fed-induction-generator wind farms in power grid has a negative impact on the operation of the modern system protection while the sensing components of the current signals decay rapidly due to the high crowbar resistance during fault events. Such cases can drop the relays that operate in standalone mode and depend on the current magnitude only. Standalone relays are not a proper solution for the future of protecting large-scale interconnected grid. Wide area monitoring has been developed to enhance the protection schemes but unfortunately it is limited to backup protection application and also work in standalone mode. More than 70% of the wide area disturbances are caused by relay mal-operation due to protection settings and miss-coordination. The concept of the idea does not depend on the short-circuit current magnitude and operates as one-zone. The developed idea is based on describing the dynamic operation of the lines before and after the faults using differential-equations. The differential equations are solved through orbits movement in a phase-diagram.

Pulsed magnetic field applied to controlling the dissipative plasma around spacecrafts may alleviate the blackout in communication. To validate it in the laboratory, a pulsed magnetic field system operated by a pulsed current derived from a novel power supply system to a magnetic coil with a special runway shape is constructed. The magnetic field with a flattop 12 ms and a minimum magnitude ~2 T during flattop is generated. The magnet is mounted inside of the cylindrical shell with a cone-shaped head to simulate the spacecraft. To improve the energy efficiency, an innovative power supply with multiple capacitors is proposed to generate a pulsed current with a flattop by discharging capacitors sequentially. Two thyristors are put in series as the main switches and a crowbar module is used to prevent current reversal. The target field was evaluated through a coupled transient analysis of the magnet and the power supply system. The thermal load and structural strength of the magnet were also studied through the coupled field analyses under both the normal operation and one extreme condition.

Low voltage ride-through (LVRT) scheme of doubly-fed induction generator (DFIG) will affect the transient stability of power system since it will change the operation state of wind turbine. The LVRT scheme combining rotor crowbar circuit and Direct Current chopper (DC-chopper) circuit is effective, but the resistances of crowbar and DC-chopper are set separately and the impact on the system transient stability is usually ignored, which makes it difficult to gain optimum control effect. In this paper, the relation between rotor crowbar and DC-chopper in LVRT duration is analyzed in theory, which shows the effect of DC-chopper circuit can be considered as the variation of crowbar in equivalent circuit. Furthermore, by analyzing the effect of crowbar on the LVRT performance and power system transient stability, a coordinated resistance optimization method of crowbar and DC-chopper is proposed. Based on this method, a multi-objective optimization model for crowbar and DC-chopper resistance considering both LVRT performance and system transient stability is established, and the non-dominated sorting genetic algorithms-II (NSGA-II) is applied to solve this multi-objective optimization problem. The simulation results verify that the coordinated optimization scheme will not only achieve better LVRT characteristics, but also improve the transient stability of power system.

Nowadays, most of the doubly fed induction generators (DFIGs) are equipped with rotor crowbar for the requirement of low voltage ride through (LVRT). The crowbar resistance is an important parameter, and it is selected taking rotor overcurrent and dc link overvoltage limits into consideration. However, the impact of grid impedance on the LVRT performance of DFIG and crowbar resistance is not adequately researched. This paper proposed an improved method to analyze the LVRT performance of DFIG system with rotor crowbar taking the influence of the grid impedance to fill this gap. The impedance of the present grid would be decreased in the future due to the installation of more wind farms to the grid. As a consequence, the performance of DFIG under LVRT with rotor crowbar is degraded. So the design rules of the crowbar resistance need to be reconstructed. Additionally, the analytical expression of the crowbar resistance is derived considering the grid impedance. The effectiveness of the proposed method is validated through theoretical analysis and MATLAB simulations.

Many high power RF amplifiers, working at various frequencies are being used in ICRH group. High power RF output is obtained by amplification in stages i.e. 50 W, 2 kW, 20 kW, 200 kW. These stages are connected as chain and act as driver for the final stage amplifier of ∼1.5 MW. The high power tubes used in these amplifiers (e.g. Triodes, Tetrodes), need DC power sources up to few mega-watt working at ∼30 kV. The Triodes and Tetrodes that are in use are vulnerable to internal arc faults. To protect them from permanent damage, DC Voltages need to be removed within few tens of microseconds. The conventional power supplies that are being used are short circuited with a very fast switch known as crowbar switch. Ignitron based crowbar switches are widely in use but are being replaced because of the presence of large quantities of toxic Mercury. Further, above crowbar methods cause enormous stress (during the short circuit) of the power supply and can cause considerable disturbance in the power lines. With the development of solid state series switch such short circuits of conventional supplies can be avoided. It is envisaged that a series IGBT switch rated at 7 kV, 6 A would be suitable for HVDC power supply of 20 kW stage. This development involves complete switch design, selection of IGBTs, static and dynamic voltage balancing circuit design. Further, isolation level requirements of various circuits and components are analyzed and adequate isolation is provided. The 7 kV, 6 A series switch is fabricated and successfully tested.

In recent decades, the installed capacity of wind power is increased greatly, and doubly fed induction generator (DFIG) occupies a dominant position among wind power generators due to its excellent characteristics. However, how to improve the low voltage ride through (LVRT) success rate of doubly fed induction generator during the grid fault especially unsymmetrical grid fault of great value. Based on an improved crowbar control strategy, this paper designs a new control circuit, optimizes the control process, and improves its control speed. Finally, the simulation of MATLAB/Simulink shows that the rationality of this control circuit.

The fundamental challenge in designing high performance input/output circuits stems from interface voltages not scaling at the same rate as the microprocessor core voltages. This typically translates to designing multi-voltage interface circuits with single lower voltage gate oxide components to reduce mask cost. While numerous topologies exist to overcome the associated gate oxide reliability challenges, they invariably run into bottlenecks when targeted for high performance. In this paper, a multi-voltage bidirectional buffer built using just 1.8V transistors in a 65nm process is presented. Proposed solution includes two novel schemes consisting of (1) a replica driver based pre-driver compensation scheme for output buffer, and (2) a new reduced crowbar contention based low skew input buffer. These circuits in conjunction not only help achieve 200MHz operational frequency, but also do so with competitive area while eliminating gate oxide and channel hot carrier associated reliability constraints.

With the enlarging scale of doubly-fed induction generators (DFIGs) connected to power systems, it is important to analyze the influence of a short-circuit current to system relay protection. Due to the correct evaluation of the protection operation characteristics, the DFIG short-circuit current needs to be calculated accurately. But the current research on the short-circuit current of DFIG is based on the following assumption: the rotor excitation current is zero after the rotor crowbar is put, and the influence of its dynamic process is ignored. This will bring errors to the calculation results. This paper takes into account the influence of rotor current dynamics by studying the mechanism of the potential transient change of DFIG. The stator rotor flux linkage of DFIG in the event of a three-phase short-circuit is accurately calculated, and an improved RMS calculation method of doubly-fed wind turbine short circuit current is proposed. A physical experiment platform with an actual controller of a doubly-fed fan is established, based on RTDS. It can be seen from the experiment that the short-circuit current calculation method proposed in this paper is more accurate than those methods that ignore the rotor dynamic process. This study lays a foundation for further study of the influence of DFIG on the protection operation characteristics.

With the enlarging scale of a doubly fed induction generator (DFIG) connected to a power system, the influence of short-circuit current on the system relay protection could not be ignored. Setting and configuring relay protection would be affected by an imprecise short-circuit current calculation. However, some existing studies only consider the condition that the input is the Crowbar and the rotor excitation is blocked. China's new network standard requires the output reactive support current of a DFIG and will change the characteristics of short-circuit current. To solve this problem, on the basis of analyzing the characters of the transient equivalent potential of a DFIG, the transient model of a DFIG with uninterrupted excitation is provided. Based on the characteristics of a non-abrupt change of flux linkage and the requirement of a new grid standard reactive support current, the short-circuit current calculation method of a DFIG with uninterrupted excitation is put forward. Based on the real-time digital simulator (RTDS), a digital-analog experimental platform containing the actual control unit of the DFIG converter is founded, the proposed short-circuit current root mean square (RMS) value calculating method is validated.

Distance relays mal-operate for transmission lines connected to type-3 Wind Farms (WFs). This paper proposes a waveshape property based protection of the intertie zone between wind farm and grid during 3-phase faults. It mitigates the challenges faced by the normally used distance relays and ensures the protection systems' security and dependability. The proposed scheme uses the auto-regressive coefficients of the 3-phase currents obtained from the Current Transformer at one end to distinguish the faults fed by the type-3 WFs and the primary grid and determines the fault zone accurately. PSCAD/EMTDC is used to verify the validity of the technique on three test systems. The results obtained with different wind speeds, crowbar resistance, fault resistance, inception time, and fault locations are encouraging and suggest the possible utilization of feature-based algorithms to improve the power system distance relaying system. In addition, the protection scheme can be utilized for lines compensated with series capacitors and phase shifting transformers.

In view of the complex transient characteristics of wind power plants caused by the coupling of electromagnetic transient of motor and converter regulation process after power grid failure, a calculation method of DFIG short circuit current with or without crowbar is proposed. First, the fault equivalent network which takes the rotor converter control into account is established. On this basis, considering the transient regulation of the grid-side and rotor-side converters, the expression of the stator and rotor short-circuit current under the fault of the power grid is derived. RTDS wind farm simulation results verify the correctness of the derived expressions and related theories.

Subsynchronous oscillation, caused by the interaction between the rotor side converter (RSC) control of the doubly fed induction generator (DFIG) and series-compensated transmission line, is an alleged subsynchronous control interaction (SSCI). SSCI can cause DFIGs to go offline and crowbar circuit breakdown, and then deteriorate power system stability. This paper proposes a novel adaptive super-twisting sliding mode SSCI mitigation method for series-compensated DFIG-based wind power systems. Rotor currents were constrained to track the reference values which are determined by maximum power point tracking (MPPT) and reactive power demand. Super-twisting control laws were designed to generate RSC control signals. True adaptive and non-overestimated control gains were conceived with the aid of barrier function, without need of upper bound of uncertainty derivatives. Stability proof of the studied closed-loop power system was demonstrated in detail with the help of the Lyapunov method. Time-domain simulation for 100 MW aggregated DFIG wind farm was executed on MATLAB/Simulink platform. Some comparative simulation results with conventional PI control, partial feedback linearization control, and first-order sliding mode were also obtained, which verify the validity, robustness, and superiority of the proposed control strategy.

There is proposed a new upper divertor for the A SDEX U p g rade (AUG) tokamak experiment [1] . It is planned to be equipped with internal coils for investigation of advanced magnetic configurations like e.g. „snowflake“ [2] . Due to the close vicinity of the coils to the plasma, high induced and very stiff voltages are expected during disruption events, as shown in [ 3 , 4 ],. This voltage is a high risk for either damaging the connected power supplies and/or for turning into high currents causing damage to the tokamak by Lorentz forces. The proposed device named “ripping crowbar” detects and overtakes the full high voltage of up to 5 kV during a plasma disruption at full load current of up to 3 kA to protect the connected power supply which can deal with voltage not higher than 340 V absolute value. The concept will be given and discussed. Experiments were started at much lower current and voltage by use of commercially available hardware. This is a starting point to enhance the working regime in a time-saving way, because there exist no commercially available solution for the target values. The only known field with comparable power stage solutions can be found in the field of H igh V oltage D irect C urrent (HVDC) circuit breakers [5] . The final solution should be scalable in voltage, current and power to adapt it to other applications in and outside the field of fusion technology. Part of the development are the I nsulated G ate B ipolar T ransistor (IGBT) based powerstage, the drivers, the measurement unit to detect disruptions and the triggering unit. At the end, most important will be reliability of this safety relevant device to protect multi-million investments into power supply and new divertor.

This article studies how possible it is to use the need to protect vulnerable populations, such as children, as a justification to limit freedom of expression in democratic societies. The research was designed and conducted based on the idea that the regulation of speech and access to content is not only a matter of law and legislative interpretations, but also a question of social norms and values. The study is based on two surveys, one implemented with a representative sample of Norwegians aged 15 and older, the other with a sample of journalists. The results show that for the general population sample, 76% of respondents agreed that the protection of weak groups, such as children, is more important than freedom of expression. The data analysis also shows that gender, education, religious affiliation, trust in media and fear of a terrorist attack are all linked to the likelihood of agreeing with this statement. Women are 66% more likely than men to be in favour of limiting freedom of expression to protect weak groups, such as children. While the numbers of those who agree are lower among journalists, up to 50% of journalists still totally or partially agree that protecting weak groups is more important than freedom of expression. We discuss the policy implications of these results for democratic societies.

A fixed speed wind turbine generator system that uses induction generator as a wind generator has the stability problem similar to a synchronous generator. On the other hand, a doubly fed induction generator (DFIG) has the flexibility to control its real and reactive powers independently while being operated in variable speed mode. This chapter focuses on a scheme where an Induction Generator (IG) is stabilized by using DFIG during grid fault. In that case, DFIG will be heavily stressed and a remedy should be found to protect the frequency converter as well as to allow the independent control of real and reactive powers without losing the synchronism. For that purpose, a crowbar protection switch or DC-link protecting device can be considered. This chapter presents a comparative study between two protective schemes, a crowbar circuit connected across the rotor of the DFIG and a protective device connected in the DC-link circuit of the frequency converter. Simulation analysis by using Power System Computer Aided Design and Electromagnetic Transient Including DC (PSCAD/EMTDC) shows that both schemes could effectively protect the DFIG, but the latter scheme is superior to the former, because of less circuitry involved.

The mathematical models of 2MW doubly-fed induction generator (DFIG) wind power generation system under steady and transient state are built in this paper. On the basis theory of control strategy and crowbar circuit parameter optimization, a complete model of DFIG low voltage ride through (LVRT) system is developed by real time digital simulator (RTDS). There is improvement in traditional crowbar protection method during quantitative simulation of grid voltage sag. An improved LVRT strategy of DFIG wind turbine by changing time of switching crowbar circuit is proposed in this paper. Contrastive study between traditional and improved LVRT method is provided to verify the performance of doubly-fed wind power system. Results show that the proposed method can realize uninterrupted operation of DFIG and meet the requirements of LVRT under grid voltage sag.

The energy production future is dominated by renewable energy sources driven by global warming problems and aiming at the reduction of fossil fuel dependence. Wind energy is becoming competitive with fossil fuels considering its less price and less CO2 emission production. Wind turbines consist of different types including Doubly Fed Induction Generator (DFIG) which is a variable speed wind turbine and operates at varying speeds corresponding to the varying wind speeds from the cut-in speed through the rated wind speed to the cut-out speed. In the case of grid failure, the network voltage drops, consequently, the rotor current and DC link voltage increase which leads to damage of the rotor windings and power electronics device. Some protections are applied to the machine in order to help the low voltage ride-through capability of the doubly fed induction generator. In this root, the crowbar protection circuit is used widely in wind power plants. However, crowbar protection should be sized carefully due to its effects on both DC link voltage and rotor currents. In this paper, a doubly fed induction generator with crowbar protection is studied and the optimum value for the crowbar protection is derived; then, a Simulink model of a doubly fed induction generator protected by a crowbar protection is developed and used to analyze the effect of crowbar protection value on the DC link voltage and rotor currents.

In the wind power area, Doubly Fed Induction Generator (DFIG) has many advantages due to its ability to provide power to voltage and constant frequency during rotor speed changes, which provides better wind capture as compared to fixed speed wind turbines (WTs). The high sensitivity of the DFIG towards electrical faults brings up many challenges in terms of compliance with requirements imposed by the operators of electrical networks. Indeed, in case of a fault in the network, wind power stations are switched off automatically to avoid damage in wind turbines, but now the network connection requirements impose stricter regulations on wind farms in particular in terms of Low Voltage Ride through (LVRT), and network support capabilities. In order to comply with these codes, it is crucial for wind turbines to redesign advanced control, for which wind turbines must, when detecting an abnormal voltage, stay connected to provide reactive power ensuring a safe and reliable operation of the network during and after the fault. The objective of this work is to offer solutions that enable wind turbines remain connected generators, after such a significant voltage drop. We managed to make an improvement of classical control, whose effectiveness has been verified for low voltage dips. For voltage descents, we proposed protection devices as the Stator Damping Resistance (SDR) and the CROWBAR. Finally, we developed a strategy of combining the solutions, and depending on the depth of the sag, the choice of the optimal solution is performed.

According to the new technical requirements and standards proposed by the grid operator, wind turbine systems (WTSs) based on doubly-fed induction generator (DFIG) must participate in power system management in the case of grid disturbances such as voltage dip and frequency regulation. Moreover, all WTSs must contribute to voltage and frequency support during and after grid failures. In this paper, the implementation of low voltage ride through (LVRT) and inertial response control based on sliding mode controllers (SMC) to avoid the disconnection of WTS from the grid during faults are investigated. The first strategy consists of keeping the stator flux in a required level to ensure the DFIG’s magnetization, and the insertion of the crowbar resistors in the rotor circuits to limit the rotor and stator currents and the DC bus voltage during voltage dip. The second strategy permits to create an artificial inertial response during frequency drop by means of the kinetic energy stored in the WTS’ rotating masses. The aim of this strategy is to contribute in the network frequency stabilization and, therefore, the need for additional power afforded by conventional generators.

In view of the existing complex analytic calculations of DFIG short-circuit current are hardly applied in engineering projects, a practical calculation of asymmetric short-circuit current of DFIG is proposed. According to the complex sequence and their Thevenin equivalent model of DFIG network, the composition of each sequence component of DFIG short-circuit current is analysed. Moreover, considering the low-voltage ride through strategy of DFIG, the negative sequence periodic components of short circuit current are well analysed during the crowbar activation and deactivation, and the formula of the negative sequence periodic components of short circuit current are derived. On the basis of positive and negative sequence open circuit voltage, calculating impedance and rotor current, the judgement of crowbar activation is established. The pre-calculated surfaces of negative sequence periodic components of short circuit current are proposed, and the procedure for calculating the asymmetric short-circuit current of DFIG is designed. Finally, the proposed method is verified by simulation.

Wide-area protection and control (WAPAC) is a new technology in the smart grid system. The high penetration of wind farms in power systems is likely to have an adverse impact on the relay operation even if the short-circuit current in such systems decays rapidly due to crowbar resistance. By including various renewable sources, and information and communication technology components, the power grid will become more complex. Approaches based on wide-area monitoring system (WAMS) are developed to solve the current challenges of the future of high renewable resources penetration. This study provides the latest main components of the WAMS architecture and the innovation in WAPAC research development using phasor measurement units (PMUs). As the extensive article review in this area is given that show no solution until now for wide-area protection in case of high wind farm penetration. A new wide-area protection based on PMUs is deployed. The main concept is based on describing by differential equations the dynamic operation of the transmission lines. In case of faults, the equations become non-linear and become difficult to be solved. This is the first time to develop such an idea to identify the protected faulted zone and guarantees sustainability for the power flow.

This paper presents a new capacitor bank topology for a pulsed power system to improve the efficiency of multistage synchronous induction coilgun. In the proposed method, a switch, such a gate turn-off thyristor, is inserted in parallel to a crowbar diode and load to generate an attractive force between the next stator and the armature. A mathematical model was developed, and finite-element method simulations were conducted to verify the proposed topology. The results show that the proposed method improves the energy efficiency and launch velocity. The improvement in energy efficiency is confirmed from the simulation results of a three-stage synchronous induction coilgun.

When the capacitor cell is discharged in the short-circuit mode, the current pulse amplitude and duration are maximal. Therefore, this mode is the most severe for discharge switches of capacitive energy storage. The characteristics of the transient process of the discharge capacitive energy storage and the current loads acting in the facility discharge circuit have been defined for this mode. The test bench for definition of the LTT (Light Triggered Thyristors) loading capacity is described. The limiting characteristics have been experimentally obtained for LTT, at which there emerges the thermo-generation peak. The process of OFF-ON switching of LTTs has been investigated, the necessity is shown to use the speed-up R-C circuits to ensure fast and stable transition of LTT into the conducting state. The design of the switch unit for the capacitive energy storage comprising LTTs and crowbar diodes is described, and the transient processes of current switching in crowbar diodes are considered. The tests carried out during switching of pu1se current up to 100 kA at a voltage of 6 kV have confirmed the workability of the switch unit. Keywords: Energy storage, Power semiconductor switches, Photothyristors, Pulse power systems

An active crowbar protective circuit is an effective and common approach for low voltage ride through (LVRT) of a doubly-fed induction generator (DFIG). The crowbar resistance value and its switching scheme both have crucial effects on safety recovery. The effects encountered are correlative dependence and interplay so that analyzing them from a single factor, as most existing LVRT control methods would do, obtains a partial optimal solution. This paper connects these two factors to analyze their coordination effects on the LVRT control, and to also investigate whether the global optimal performance of these factors can be achieved. The principles for resistance selection and the schemes for crowbar switching are discussed first. Next the coupling relationship is analyzed based on statistical sampling simulation data with different resistance values and various switching schemes. The results demonstrate that their coordination has critical influence on rotor peak current, DC-link voltage and reactive power. The optimal coordination will be different according to specific requirements. Hence the global optimal combination could be achieved when all requirements are taken into consideration.

Fault ride through (FRT) is recognized as one of the most exciting topics in inspecting wind turbine performance. In this paper, the behavior of a DFIG offshore wind farm under fault circumstances is reviewed. Three case studies basing on the 3-phase fault location are taken into account. In this situation, the crowbar protection is primordial to prevent any over-currents in the rotor windings and to short the Rotor side converter (RSC) from the system. However, the use of this protection individually is insufficient and causes a lack of rotor speed and reactive power control. Here, the need for the internal model control (IMC) for the grid side converter (GSC) is essential to intensify the fault ride through capabilities of the DFIG. The simulation is performed using MATLAB/SIMULINK. As the results show, the use of the IMC for the grid side converter with the crowbar protection boosts both the Fault ride through and the performance of the DFIG.

In this paper, the fault current limiter (FCL) technique has been revisited in order to deal with the Low Voltage Ride Through capability (LVRT) of the Wind Energy Conversion System (WECS). An inductive type FCL is suggested for a grid-connected Doubly Fed Induction Generator (DFIG) in combination with the Battery Energy Storage System (BESS). The BESS is located across the DC-link, while FCL is situated on rotor side of DFIG. It is observed that placing BESS in DC-link is effective to eliminate ripple and maintain voltage across it. The dynamic behavior of low voltage ride through is studied under a symmetrical fault condition. The proposed protection system performance is validating by comparing it with the conventional crowbar protection scheme. Using the MATLAB / SIMULINK platform, the proposed system is simulated on a 2MW DFIG-WECS.

The connection of wind farms to the grid and their dynamic behavior under different conditions is a real challenging issue which resulted in providing new instructions for the network. One of the important topics related to grid standards is Low-Voltage Ride-Through capability. In recent years, The application of Brushless Doubly Fed Induction Generator (BDFIG) in the wind farms has drawn the attention of researchers. This generator has more advantages than other common generators, including the Doubly Fed Induction Generator (DFIG). In this paper, the performance of the BDFIG under fault conditions in the grid is examined and monitored in order to improve LVRT while considering the dynamic model of the BDFIG connected to a wind turbine. In this method, the reactive power and speed are controlled for stable performance of the generator under various grid conditions. A converter is used to connect the stator control winding to the power grid, which DC link voltage is adjusted using multiple PI controllers under fault conditions. In addition, two controlling systems based on the conventional PI controllers are proposed to control the generator side converter and the wind turbines step angle. The results demonstrate good dynamic performance of the examined generator under different grid conditions achieved by the proposed controlling method without using any additional hardware such as a Crowbar.

A stability analysis model is established for DFIG (doubly-fed induction generator) with power grid fault to study its stability. The control strategy is proposed adding damping controller to the rotor side control of DFIG. It still analyzes the fault ride through ability of DFIG under different crowbar impedance. The simulation results show that the low voltage ride through ability of DFIG can be effectively improved by adding the damping controller and selecting the appropriate crowbar impedance value. The operation of wind turbine is realized safely and steadily during power grid fault.

This work handles the improvement of LVRT competency of double-fed induction generator (DFIG) driven grid-integrated wind energy conversion systems (WECS) using crowbar. In this paper, a series crowbar resistance with an IGBT is recommended to enhance the Fault-Ride-Through ability of the 1.5 MW WECS and also the low voltage ride through testing device is also simulated. The simulation results demonstrate that the crowbar absorbs the high fault current during a fault in the grid, improves the LVRT capability and with the help of LVRT testing device, unsymmetrical faults could be applied at required levels by changing the resistance values. The efficacy of the suggested technique is validated in Matlab Simulink through thorough simulation.

The asynchronous wind farms are based on squirrel cage induction generators (SCIG/WF). The main disadvantage of SCIG/WF is the deeply affected by the faults which may occur in the interconnected grid. These conditions can lead to disconnect SCIG/WF from the grid. The crowbar has been used effectively in protecting the stator or rotor coils of doubly fed induction generators (DFIG) which are used in variable speed wind farms. The large value of crowbar causes an overvoltage after fault removal. In this paper, a developed method is used to enhance the performance of SCIG/WF during the grid faults. The developed method is based on using the crowbar as outer protection to equip save path for the fault current. This method is proposed to investigate the value of crowbar which can prevent SCIG/WF from disconnection, limiting the current at the contact point between SCIG/WF and grid. The impact of adequate crowbar on SCIG/WF is investigated during symmetrical fault (Three phase fault) and unsymmetrical fault (double line) using MATLAB Simulink program.

The electromagnetic transient characteristics (ETC) of doubly-fed induction generator (DFIG) under grid voltage dip have been extensively studied theoretically, and various control strategies have been proposed. However, the ETC of DFIG under grid voltage swell are less studied. In order to study the influence on the ETC of DFIG under grid voltage swell and the corresponding control strategy, this paper compares and analyzes the steadystate and transient components of the stator flux linkage, rotor induced voltage and rotor current of DFIG under grid voltage swell and dip. It is deduced that DFIG is less prone to rotor overcurrent under grid voltage swell so the crowbar protection circuit is not needed, and the grid voltage amplitude changes little, the rotor side inverter is less prone to overmodulation. An experimental setup and simulation model is implemented with high-voltage ride-through of 1.5 MW doubly-fed wind Turbine. Simulation in addition to experimental results verifies the correctness and effectiveness of the theoretical analysis.

Since the accomplishment of the pulsed magnetic field facility at the Wuhan National High Magnetic Field Center in 2013, five 65 T monolithic-coil magnets have been developed and used for scientific research. As the workhorse, the 65 T user magnets show excellent cooling performance and long service life. These user magnets have typical bore size of 21 and 23 mm. For electrical transport measurements, the magnets are energized by a 2 or 3-MJ capacitor bank with crowbar circuit for ∼40–70 ms pulse duration. For magnetization measurements, shorter pulse duration of 11 ms for a field waveform with almost symmetrical rising and falling edges can be obtained with a 1.25-MJ capacitor bank that allows high-voltage reversal. The cooling time between two 60 T pulses ranges from 30 to 50 min depending on the magnets and the field waveform. Up to now, three of these magnets failed after 916, 583, and 947 pulses with the magnetic field above 60 T. The other two have delivered 706 and 56 pulses above 60 T and are still in operation.

This paper proposes an impedance protection circuit configuration to enhance the ride-through of doubly-fed induction generators (DFIGs) based wind turbines (WTs) during faults. The proposed impedance protection circuit configuration comprises of a parallel R-L impedance crowbar and a series R-L impedance circuit. Traditionally, resistive-crowbar (i.e., R-crowbar) circuit is applied as the means of protection for the DFIG WT which efficiently protects the rotor-side converter (RSC) and dc-link capacitor against the effects of severe grid faults. However, the conventional R-crowbar circuit disconnects the RSC leading to the loss of generator excitation control. During that time, the WT generator becomes a conventional squirrel-cage induction generator (SCIG) which obtains its magnetization current from the point of common coupling (PCC) via stator winding terminals. This phenomenon further deepens the voltage-dip at the PCC. Alternatively, integrating the R-crowbar with series R-L branch partially maintains the RSC connection to the rotor winding. Hence the generator excitation control is also partially maintained but oscillations of the rotor currents and dc-link voltage can still affect the ride-through performances of WT driven by the DFIG. With the proposed protection circuit configuration, it compensates the deficiency of the two conventional protection circuits configuration. In order to validate the performance of the proposed scheme, extensive simulation studies are carried out using MATLAB/Simulink software. From the comparative simulation results, improved performances of the DFIG under transient-state are achieved with the proposed protection circuit configuration than the conventional protection circuits.

Renewable Energy Sources (like wind energy) minimize the demand for other types of power. Wind energy generation has become a major power in some countries, covering the essential share in the Energy Balance in Holland, Spain, Brazil, Germany, China and others. The construction of new Wind Power causes new challenges for the transmission distribution infrastructure. However, environmentally friendly Renewable Energy has recently become an important part of generation in industrial applications, worth for distributed lines expansion. A significant part of electric machines used in Wind Industry are doubly fed induction motors (DFIM) used as electric generators. They earned the leading market position over the recent decade and continue their run. The machines of this type offer operation stability along with affordable costs. The out­standing technical benefits include the ability for variable speed operation and independent control of reactive and active power. To protect the rotor side converter (RSC) from transient overcurrent during voltage dips, the crowbar circuit protection is usually used. The paper analyzes and investigates the dynamic behavior of the doubly-fed induction motor, back-to-back converter, and the rotor side converter during symmetrical voltage dips using the crowbar protection system with MATLAB/Simulink simulation. In addition, it studies the crowbar circuit affects the diodes, switches, and resistances at the low voltage ride through (LVRT) capability enhancement.

Doublefed induction generator(DFIG) has shown tremendous success inwind turbines due to its flexibility and ability to regulate the active andreactive power. However, the presence of brushes and slip rings affects itsreliability, stability, and power quality. Furthermore, itdoes not providepromising outcomes in case of faults even in presence of the crowbar circuit.In contrast, thebrushless doubly fed induction generator(BDFIG) is a morereliable option for wind turbines than its mentioned counterpart due to theabsence of the brushes and slip rings. This research work as such attempts toimprove the dynamic performance of thevector control(VC)oriented powerwinding (PW) stator flux-based BDFIG by optimally selecting theproportional-integral(PI) gains throughinternalmodel control(IMC)approach. The proposed control scheme is utilized to regulate the speed,torque, and reactive power of the considered BDFIG independently. Contraryto the previous literature where the “trial and error method” is generallyutilized, the current research work uses the IMC for selecting the mostsuitable PI parameters, thus reduces the complexity, time consumption, anduncertainty in optimal selection. The considered BDFIG based wind turbinewith the proposed control scheme provides a better BDFIG control designwith an enhanced dynamic response as compared to that of the same withDFIG under identical operating conditions and system configurations.

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.

Electromagnetic transients resulting from voltage dips not only present challenges for low-voltage ride-through (LVRT) of doubly fed induction generators (DFIGs) in wind turbines (WTs) but also introduce complications for protective relays in power systems. Analysis of the voltage-dip response is a fundamental requirement for methodologies targeting LVRT performance and is also useful for power system development. To date, it has not been feasible to take into account the interval between voltage dip and crowbar triggering, and, consequently, the accuracy of the analytical results has been compromised. In this paper, a concept of equivalent fault source is presented with which the transient process during voltage dips is decomposed into one response to the equivalent stator fault source and the other response to the equivalent rotor fault source. The former results from dips in the supply voltage, while the latter arises from variations in both the rotor resistance and the control voltage produced by crowbar activation. This makes it feasible to incorporate the interval, i.e., the triggering delay, into the analytical solution. Additionally, this decomposition approach allows for the physical mechanisms behind the individual transient components to be more clearly understood. With the explicit analytical formulas, some intrinsic characteristics of the transients are identified. The comparative accuracy and advantages of this analytical method over the conventional approaches is demonstrated through simulations using a 2-MW DFIG for commercial WTs and further verified experimentally on a laboratory-scale test rig.