Fault Ride-through Capability Enhancement Research Articles

Enhancing FRT capability of DFIG based on RC-crowbar considering the resonance and matching control strategies for different fault degrees

This paper compares the problems existing in the current fault ride-through (FRT) schemes of the doubly-fed induction generator (DFIG), and deeply analyzes the transient characteristics of DFIG during FRT. A comprehensive FRT scheme is proposed to improve the FRT capability by using an RC-crowbar and matching control strategy switching. The main innovation of this scheme lies in the consideration of resonance and other factors in the capacitance design of the RC-crowbar. To ensure that the DFIG still has good FRT capability after the RC-crowbar is cut off in advance in the later stage of FRT, an improved rotor side converter (RSC) control strategy is proposed for this situation. At the same time, to balance the reactive power on the stator side and accelerate the recovery of stator voltage throughout the entire FRT period, the control strategy of the grid side converter (GSC) is also improved. The comprehensive FRT scheme proposed in this paper can dynamically switch based on the degree of fault so that it still has good FRT ability when applied to different degrees of faults. Finally, the correctness and effectiveness of the proposed comprehensive FRT scheme were verified through simulation experiments.

Multi-modular bridge-type FCL for enhancing FRT capability of VSC-HVDC connected offshore wind plants during AC faults

This paper presents a novel concept Multi-Modular Bridge-Type Fault Current Limiter (MMBFCL) for enhancing the fault ride-through (FRT) capability in Voltage Source Converter (VSC)-based High-Voltage Direct-Current (HVDC) systems. The MMBFCL can be directly connected to HVDC system in ac grid side without series connection of semiconductor switches and series injection transformers. Also, the limiting resistor of each module is determined based on binary arrangements to provide a multi-step insertion resistor BFCL to limit the fault current and enhance the FRT capability under different voltage sag levels. The principle operation and control system of the MMBFCL is described. Then, the performance of the MMBFCL for enhancing the FRT is validated through time domain simulations by PSCAD/EMTDC software in a HVDC integrated wind farm system. Also, the performance of the MMBFCL is compared with the single-modular BFCL (SMBFCL). Simulation results reveal that the MMBFCL compensate the voltage sag for each voltage sag levels without any over voltage at the HVDC terminal. Also, it has superior performance than SMBFCL.

Enhancement of fault ride-through capability of three-level NPC converter-based HVDC system through robust nonlinear control strategy

Voltage source converter-based high-voltage direct current (VSC-HVDC) transmission systems are becoming increasingly popular for long-distance power transmission due to their numerous advantages. However, the fault ride-through (FRT) capabil-ity of VSC-HVDC systems during AC faults remains a key challenge that needs to be addressed to ensure their uninterrupted operation under challenging grid conditions. This paper presents and analyzes a novel nonlinear control scheme based on a robust sliding mode control approach to improve the FRT capability of VSC-HVDC systems. The proposed control scheme based on the modification of the converter’s active power to make it more sensitive to any changes in voltage magnitude at the points of common coupling (PCCs) on the AC side. This helps to maintain the DC voltage sta-bility and prevent the converter from tripping during AC faults. The performance of the proposed control scheme is evaluated using a series of ex-perimental investigations using a digital signal processor (DSP) within Processor-in-the-Loop (PIL) quasi-real-time simulation. The PIL simulations closely examine the system’s response to AC faults with varying remaining voltages and durations. The results demonstrate that the proposed control scheme can effectively improve the FRT capability of VSC-HVDC systems and ensure their stable operation during AC faults.

Fault ride-through capability enhancement of hydrogen energy-based distributed generators by using STATCOM with an intelligent control strategy

This paper presents an intelligent adaptive neuro-fuzzy inference (ANFIS)-based control method for increasing the Fault Ride-Through (FRT) capability of hydrogen energy-based distributed generators. The static synchronous compensator (STATCOM) system is integrated into the modeled power system with the Solid Oxide Fuel Cell system. To investigate the FRT capability of the proposed method, fault scenarios under different grid conditions are generated. The proposed ANFIS-based method is compared with the conventional PI-based STATCOM model and the system without any flexible AC transmission system devices. When the obtained results are analyzed, with the developed intelligent control method, an improvement of at least 8% and a maximum of 12.6% is achieved in the voltage value, depending on the type of failure that occurs. Besides, there is an improvement of at least 10% and at most 16.6% in the settling time values. The voltage fluctuations and sudden peaks in the system with the proposed method are less than in the other systems and it provides voltage support to the system successfully. The transient response of the Solid Oxide Fuel Cell system provides sustainable and stable reactive power support to the grid. Besides, the proposed method not only contributes to the FRT capability of system but also minimizes voltage changes that may reduce the life of the distributed generator or cause it to malfunction.

A new frequency control method to enhance fault ride-through capability of VSC-HVDC systems supplying industrial plants

• Voltage sags is the most important issue of power quality, which can disrupt industrial process and impose huge costs on industrial plants. • A new frequency control method is proposed for the VSC-HVDC in order to improve the voltage stability in industrial plants during voltage sag. • The performance of the proposed method is investigated under 3LGF, DLGF, and SLGF conditions. • The proposed method maintains the minimum AC voltage at an acceptable value for very sensitive loads, when voltage drop occurs under severe faults. Voltage sags is one of the most important issue of power quality, which can disrupt industrial process and impose huge costs on industrial plants. For improving disturbance ride-through capability of voltage source converter based on high voltage direct current (VSC-HVDC) supplying industrial plant, an auxiliary frequency controller is proposed to control output voltage frequency at inverter station. Since industrial loads are more sensitive to voltage drops compared to frequency deviations, output voltage frequency is slightly deceased based on DC-link voltage changes during disturbances in proposed control scheme in order to prevent voltage collapse and provide desirable voltage quality for industrial plants; hence, the continuity of industrial process will be ensured. The case study is a part of a real industrial plant and variations of DC-link voltage, AC buses voltages, induction motors speed, and performance of adjustable speed drive are investigated when the proposed method is used during faults. In proposed method, voltage drop across industrial plant during a severe fault is less than other methods and dynamic performance of system is improved. The case study is simulated under balanced three-phase fault, double line-to-ground fault, and single line-to-ground fault in PSCAD/EMTDC, and results verify the validity of the proposed control scheme.

Battery-energy-storage-based triple-active-bridge DC unified power quality conditioner for energy management and power quality enhancement of DC renewable sources

As the renewable-based DC power systems are expected to prevail in the near future, stochastic power and weakness of fault ride-through capability are essential issues in renewable-based DC power systems that urgently need to be addressed. This paper proposes a novel triple-port active bridge (TAB)-based DC unified power quality conditioner (DC-UPQC) for the performance enhancement of DC doubly-fed induction generator (DC-DFIG)-integrated DC power systems. The developed TAB-based DC-UPQC has the advantages of bidirectional voltage and current compensation ability, concise circuit structure, straightforward control system, and swift compensation response. The mathematical model, control strategy, and parameter selection of the TAB-based DC-UPQC are systematically analyzed. A scaled-down prototype for experimental verification is established to verify the theoretical analysis and the technical feasibility. Further, a 1.5-MW full-scale simulation of DC-DFIG is carried out to verify the effectiveness of the TAB-DC-UPQC in actual DC power systems. Simulation and experimental results show that the TAB-DC-UPQC has the integrated functions of smoothing output power of the DC-DFIG, limiting DC fault current and enhancing fault ride-through capability. Therefore, the TAB-DC-UPQC can be well expected to enhance the comprehensive energy management of the DC-DFIG wind energy conversion system and other renewable-based DC power systems.

Observer Based Generalized Active Damping for Voltage Source Converters With LCL Filters

An observer-based active damping (AD) controller is proposed along with a unified design and implementation framework for LCL-equipped voltage source converters. The AD controller uses feedback of either of the converter-side current or the grid-side current along with that of the grid voltage. The state of the arts offer observer-based AD only for current-mode control and are limited by their inflexibility to be used in conjunction with established supplementary control methods or by the lack of damping efficacy for all configurations of LCL resonance frequency and the measured current (grid-side vs. converter-side). The proposed AD method is identically applicable for current-mode control and virtual oscillator control (VOC) where explicit voltage/current tracking loops are not used. The proposed AD controller thus overcomes a major limitation of VOC which otherwise offers robust synchronization in reduced/zero-inertia networks and enhanced fault ride-through capability. Simplified design guidelines are presented through comprehensive small-signal analysis including the observer dynamics. The proposed method is shown to be effective for both converter-side and grid-side current measurements irrespective of the LCL resonance frequency relative to the critical frequency. The analysis and design methods are validated through laboratory hardware experiments.

An Anti-Fluctuation Compensator Design and Its Control Strategy for Wind Farm System

Large-scale wind farms in commercial operations have demonstrated growing influence on the stability of an electricity network and the power quality thereof. Variations in the output power of large-scale wind farms cause voltage fluctuations in the corresponding electrical networks. To achieve low-voltage ride-through capability in a doubly fed induction generator (DFIG) during a fault event, this study proposes a real-time reactive power control strategy for effective DFIG application and a static synchronous compensator (STATCOM) for reactive power compensation. Mathematic models were developed for the DFIG and STATCOM, followed by the development of an indirect control scheme for the STATCOM based on decoupling dual-loop current control. Moreover, a real-world case study on a commercial wind farm comprising 23 DFIGs was conducted. The voltage regulation performance of the proposed reactive power control scheme against a fault event was also simulated. The simulation results revealed that enhanced fault ride-through capability and prompt recovery of the output voltage provided by a wind turbine generator could be achieved using the DFIG along with the STATCOM in the event of a three-phase short-circuit fault.

Real-time implementation of ring based saturated core fault current limiter to improve fault ride through capability of DFIG system

Enhancement of fault ride-through (FRT) capability in doubly-fed induction generator (DFIG) system is presented in this paper. The saturated core fault current limiter (SCFCL) works on the behaviour of magnetic field induction theory. It is composed of the ferromagnetic core with three limb construction, where both side limbs are surrounded with AC coil and middle limb with DC coil. For the application of restrictive fault, the ideal SCFCL can work for only low-power rating machines. But for the machine higher power rating, the ideal SCFCL has a drawback of developing heat across the limbs, which can damage the insulation of the coils. To overcome this glitch, the embedding of the copper ring across center limb and using forced-air cooling in SCFCL is used. This ring-based SCFCL (RSCFCL) is placed across the stator of DFIG system, which helps in minimizing fault current during any grid disturbances. Having low impedance during normal condition, it can be located permanently in the system without disturbing its stability. The feasibility of ideal SCFCL and RSCFCL is validated by studying the characteristics using thermal radiation under normal and fault conditions. The use of infrared imaging helps to monitor the thermographic patterns of the core on several conditions. The performance of RSCFCL is analyzed with the help of experimental setup and real-time simulator OPAL-RT 4510. Also, to check its effectiveness, it is used for FRT capability in the DFIG system application.

Enhancement of fault ride-through capability of grid-connected wind farm

The distributed power generation (DPG) at low and medium voltage demands that the renewable generation system is always grid connected during fault condition to ensure the stability of wind power system. The DPG consist of wind turbines (WT) along with fixed speed induction generator (FSIG) does not provide accurate reactive power control and hence, there is need for dedicated compensation. Due to fault condition, the negative sequence component is affected and there is direct impact on DPG with reduction in life expectancy. This paper proposes the application of distributed static compensator (DSTATCOM) for fault ride through (FRT) and reactive power compensation. It has been observed that the compensation of negative sequence component improves the performance of FSIG-based WT. Also, the compensation of positive sequence component avoids the collapsing of voltage and improved the stability of WT. The simulation is done in MATLAB and various tests are considered under fault condition in which results are presented. The FRT enhancement of grid connected WT by using DSTATCOM is 30% and hence, 30% additional wind power is penetrated to the grid.

Enhancement of fault ride-through capability of grid-connected wind farm

Enhancement of fault ride-through capability of grid-connected wind farm

Design and implementation of FLC system for fault ride-through capability enhancement in PMSG-wind systems

Fault ride-through (FRT) capability enhancement for the growth of renewable energy generators has become a crucial issue for their incorporation into the electricity grid to provide secure, reliable, and efficient electricity. This paper presents a new FRT capability scheme for a permanent magnet synchronous generator (PMSG)-based wind energy generation system using a hybrid solution. The hybrid solution is a combination of a braking chopper (BC) and a fuzzy logic controller (FLC). All proportional-integral (PI) controllers which control the generator and grid side converters are replaced with FLC. Moreover, a BC system is connected to the dc link to improve the dynamic response of the PMSG during fault conditions. The PMSG was evaluated on a three-phase fault that occurs on an electrical network in three scenarios. In the first two scenarios, a BC is used with a PI controller and FLC respectively. While the third scenario uses only FLC without a BC. The obtained results showed that the suggested solution can not only enhance the FRT capability of the PMSG but also can diminish the occurrence of hardware systems and reduce their impact on the PMSG system. The simulation tests are performed using MATLAB/SIMULINK software.

Autonomous group particle swarm optimisation tuned dynamic voltage restorers for improved fault-ride-through capability of DFIGs in wind energy conversion system

In this paper, a multi-objective tuning algorithm is proposed for the voltage/current controllers of the dynamic voltage restorer (DVR) in an optimal manner in order to improve the fault-ride-through (FRT) capability of the grid-connected doubly-fed induction generators. An active crowbar protection (ACB_P) coupled with the rotor of the DFIG, provides a very little compensation, on blocking the high rotor current during fault condition. Control of DVR is achieved through use of multiple PI controllers. This paper presents a novel multi-objective tuning known as Autonomous Group Particle Swarm Optimisation (AGPSO) to optimally tune the PI controllers. The optimisation algorithm uses diverse autonomous groups confined within a common target to achieve more directed and randomised search results for any population. Voltage sag compensation, total harmonic distortion (THD), RMS value of signal are the three major indices for comparison study during fault. Finally, the performance improvement in FRT of the DFIG with optimised controller was studied through DFIG responses on comparing with normal controller. Efficacy of the proposed controller proposed is verified by a simulation model of DFIG System with 1.5 MW rating in MATLAB. The results are analysed and comparative data are placed to ensure the enhanced FRT capability of the system.

Stepwise Voltage Drop and Transient Current Control Strategies to Enhance Fault Ride-Through Capability of MMC-HVDC Connected DFIG Wind Farms

This paper proposes advanced control strategies to enhance fault ride-through capability of modular multilevel converter-based high voltage direct current (MMC-HVDC) connected doubly-fed induction generator (DFIG)-based wind farms. A stepwise voltage drop control is proposed for the sending-end MMC to suppress dc line voltage fluctuations by demagnetizing DFIGs. Meanwhile, a modified transient current control is proposed for the DFIG, where a voltage-dependent active current control is integrated into the rotor-side converter to improve synchronization stability of the converter, and a feedforward transient stator current control structure is designed to eliminate dc component in the stator current. The proposed control strategies are validated by comparing with three typical methods through time-domain simulations.

Modified STATCOM control strategy for fault ride-through capability enhancement of grid-connected PV/wind hybrid power system during voltage sag

This paper proposes the employment of Static Synchronous Compensator (STATCOM) in reactive power compensation to enhance the Fault Ride-Through (FRT) capability and improve the dynamic performance of a grid-connected PV/wind hybrid power system during the transient grid disturbances. The hybrid power system consisting of 9 MW Doubly Fed Induction Generator (DFIG)-based wind farm and 1 MW PV station is integrated with 100 MVAR STATCOM at the Point of Common Coupling (PCC) bus. The dynamic performance of the PV/wind hybrid power system with the proposed STATCOM controller is analyzed and compared with another FRT control strategy during a grid voltage sag. The FRT control strategy is based on the injection of reactive power from the hybrid system to enhance the FRT capability during the grid faults, and also activation of the outer crowbar protection system to protect the DFIG. On the other hand, the proposed STATCOM controller adjusts the PCC bus voltage during the grid disturbances by dynamically controlling the amount of reactive power injected to or absorbed from the electrical grid. Modeling and simulation of the proposed hybrid power system have been implemented using MATLAB/SIMULINK software. The effectiveness of both the proposed STATCOM controller and the FRT control strategy is evaluated during a 50% grid voltage sag. The simulation results illustrate that the STATCOM controller decreases significantly the level of voltage drop during the voltage sag, maintains the injected active power from the PV station at its rated value, and protects effectively the PV DC-link voltage from overvoltage. Moreover, when the STATCOM controller is employed, the injected active power from the wind farm is improved considerably and the oscillations of the DFIG rotor speed are reduced efficiently during the fault. Furthermore, the comparison confirms the superior dynamic performance of the STATCOM controller in enhancement the FRT capability as compared with the FRT control strategy.

Salp swarm algorithm-based TS-FLCs for MPPT and fault ride-through capability enhancement of wind generators

This article presents an optimum design of Takagi–Sugeno fuzzy logic controllers (TS-FLCs) to enhance capability of fault ride-through (FRT) and the maximal power point tracking (MPPT) of the grid-tied wind farms. To obtain the optimum TS-FLCs, salp-swarm-algorithm (SSA) applied to minimize the sum of integral squared-error (ISE) function, where the variables of the cost function are the factors of Gaussian membership functions and the control rules of eight TS-FLCs. After that, the optimized TS-FLCs are applied to control the grid-tied variable-speed wind generator system, which is simulated using PSCAD/EMTDC. Thus, the transient and dynamic responses revealed that the optimum TS-FLCs have better stability margins than the optimum proportional–integral (PI) controllers. Furthermore, a realistic variable wind speed measured data are implemented to test the robustness of the MPPT based on the optimal TS-FLCs to extract more power than the MPPT based on optimal PI controllers.

Low voltage ride through capability enhancement in a grid‐connected wind/fuel cell hybrid system via combined feed‐forward and fuzzy logic control

Fault ride through (FRT) capability is an essential practice as per the present grid code demands for grid-connected renewable energy-based distributed energy resources. Studies on FRT capability for grid-connected hybrid systems are rarely found. This study considers a wind energy conversion system and a fuel cell system interconnected at a common dc bus. It proposes a new feed-forward-based FRT control scheme for the inverter control where new current references in dq -axis frame are derived by tracking the positive sequence power. The newly derived references are fed forward to the input of the current regulator of the voltage source inverter. Second, fuzzy logic-based current controllers are suggested to improve the tracking capability of the current references in the inverter control scheme so as to enhance the FRT capability of the hybrid system as a whole. The proposed feed-forward-fuzzy control scheme for achieving an enhanced FRT capability is compared with the conventional dq current control and feed-forward FRT control for various grid voltage sag tests, where the performance of the combined feed-forward-fuzzy control is found better. The validation of the proposed FRT control scheme is performed in MATLAB-Simulink environment.

Improved hybrid modular multilevel converter with enhanced fault ride‐through capability and fast pre‐charging strategies

This study proposes an improved hybrid modular multilevel converter (MMC) topology with enhanced fault ride-through capability and fast pre-charging strategies. This improved MMC topology mainly consists of the half-bridge sub-modules (HBSMs), the modified series connected double sub-modules (MSDSMs), and the adjustable damping sub-module (ADSM). In the improved topology, in addition to being configured on the MMC AC-side, resistors are also configured in MSDSMs and ADSM, respectively, in the MMC. This type of resistance configuration based on the improved MMC topology benefits adjusting the value of the total equivalent current limiting resistance during the pre-charging process, leading to a higher pre-charging speed and a lower SM voltage drop risk. Three different pre-charging strategies can cope with changes in resistance configuration caused by the increasing number of SMs. Furthermore, the resistances in MSDSM and ADSM also help dissipate the energy caused by the DC short-circuit fault so that the fault ride-through capacity is enhanced. The simulation and experimental results validate the effectiveness of the proposed topology and the corresponding pre-charging strategies.

DC-Link Voltage Control of a Grid-Connected Solar Photovoltaic System for Fault Ride-Through Capability Enhancement

The high penetration level of solar photovoltaic (SPV) generation systems imposes a major challenge to the secure operation of power systems. SPV generation systems are connected to the power grid via power converters. During a fault on the grid side; overvoltage can occur at the direct current link (DCL) due to the power imbalance between the SPV and the grid sides. Subsequently; the SPV inverter is disconnected; which reduces the grid reliability. DC-link voltage control is an important task during low voltage ride-through (LVRT) for SPV generation systems. By properly controlling the power converters; we can enhance the LVRT capability of a grid-connected SPV system according to the grid code (GC) requirements. This study proposes a novel DCL voltage control scheme for a DC–DC converter to enhance the LVRT capability of the two-stage grid-connected SPV system. The control scheme includes a “control without maximum power point tracking (MPPT)” controller; which is activated when the DCL voltage exceeds its nominal value; otherwise, the MPPT control is activated. Compared to the existing LVRT schemes the proposed method is economical as it is achieved by connecting the proposed controller to the existing MPPT controller without additional hardware or changes in the software. In this approach, although the SPV system will not operate at the maximum power point and the inverter will not face any over current challenge it can still provide reactive power support in response to a grid fault. A comprehensive simulation was carried out to verify the effectiveness of the proposed control scheme for enhancing the LVRT capability and stability margin of an interconnected SPV generation system under symmetrical and asymmetrical grid faults.

A Modified DC Chopper for Limiting the Fault Current and Controlling the DC-Link Voltage to Enhance Fault Ride-Through Capability of Doubly-Fed Induction-Generator-Based Wind Turbine

A simple conventional dc chopper is employed to protect the doubly-fed induction generator (DFIG) from overvoltage; however, it is not capable to keep the transient overcurrent in an acceptable level in stator and rotor sides. Therefore, an effective current-limiting strategy should be incorporated with the dc chopper to improve the fault ride-through capability of the DFIG. In this paper, a modified DC chopper is proposed not only to keep the dc-link voltage in an acceptable range, but also to limit the high-current level in the stator and the rotor sides in a permissible level without incorporating any extra fault-current-limiting strategy. Unlike the conventional dc chopper, in the proposed dc chopper, it is not required to stop rotor-side converter (RSC) switching and employ high-rated-current antiparallel diodes. The proposed modified dc chopper is placed between the dc-link capacitor and the RSC. In the proposed switching strategy of the modified dc chopper, three extra semiconductor switches are included, which are triggered to insert dc chopper resistance either in parallel or series connections with the dc link regarding the dc-link voltage level and the dc-link current level, respectively. Calculation of the dc chopper resistance is discussed. To prove the effectiveness and robustness of the proposed modified dc chopper in terms of both limiting the fault current and controlling the dc-link voltage of the DFIG, symmetrical and asymmetrical grid faults are applied in a power system including the DFIG-based wind turbine modeled in PSCAD/EMTDC so.