Low Voltage Ride Through Methods Research Articles

Fault ride through enhancement of large‐scale solar plants using distributed converters

AbstractThe grid‐connected photovoltaic plants (GCPVPs) encounter various types of anomalies during fault occurrence in the main grid. During a fault occurrence on the grid side, the voltage of the point of common coupling (PCC) drops, causing the DC connection voltage to become too high for power balancing. This high DC voltage may damage the inverter and DC link equipment, which necessitates a reliable control scheme with low voltage passage requirements. These requirements, which obligate the GCPVP to remain connected to the main grid during abnormal conditions are generally known as low voltage ride‐through (LVRT). Herein, a developed LVRT method in the control approach that injects maximum rated current during a sag is proposed to rectify the shortcomings in single and multi‐inverter grid topologies. The proposed approach utilizes a current limiter combined with a DC braking chopper to guarantee the GCPVP's continuous participation in the power supply. Different scenarios including low and high power generations as well as two‐phase and three‐phase severe faults to the ground are considered. The results show that the proposed approach not only improves the performance in the single‐inverter system but also remedies the defects aggravated in the case of a GCPVP with several inverters.

Enhancing Low Voltage Ride-Through Resilience using Empowering Solar Energy Strategies

The rapid advancement of Low Voltage Ride-Through (LVRT) technology has been highlighted by the exponential rise of photovoltaic (PV) generation systems. An extensive analysis of LVRT methods specifically designed for PV generating systems is presented in this research. Important topics like main causes, current control schemes, power balance and overall solutions are the main focus of the review. Moreover, it provides comprehensive insights and elaborates on integrated solutions intended for large-scale PV systems. The objective of this research is to improve the resilience and reliability of renewable energy sources, especially solar photovoltaic systems, against grid disruptions and voltage changes by investigating LVRT approaches in solar power systems. Enhancing the integration of solar power systems into current electrical grids, reducing downtime, and optimizing system performance are the objectives in order to accelerate the shift to a more resilient and sustainable energy infrastructure.

R-SFCL and RRCOCS-TVC Based Cooperative Control for Enhancing LVRT Capability of DFIG System

For the extraction of wind energy through a doubly-fed induction generator (DFIG), low voltage ride through (LVRT) is an essential technical requirement specified by the transmission system operator (TSO). Under a grid fault condition, DFIG should remain in connection with the grid for a certain minimum period and offer reactive power support as required by the TSO. A cooperative control scheme consisting of hardware solution through a superconducting resistance type fault current limiter (R-SFCL) and software solution constructed on the rotor reference current orientation control strategy (RRCOCS) with transient voltage control (TVC), is proposed in this paper to address the LVRT requirement. In the proposed control strategy, RRCOCS will limit the rotor current directly during a fault condition. The reactive power needs to be generated during fault to maintain grid code which is achieved through TVC. At the same time, improvement of stator terminal voltage, as well as suppression of stator current, is achieved by R-SFCL. The suppression of stator current by R-SFCL is also transformed to the rotor side aiding the rotor current limiting. The proposed cooperative scheme’s performance is simulated and tested on a 9 MW grid-connected DFIG wind system. The results obtained by the proposed strategy are compared with RRCOCS and RRCOCS-TVC.
 HIGHLIGHTS
 
 Independent software and hardware LVRT methods fail under severe faults and strict grid codes
 Software methods are challenged by current suppression and voltage support limits while the hardware methods involve high costs and undergo high thermal stress
 A cooperative LVRT can overcome the challenges and limitations associated with these independently deployed methods. The proposed R-SFCL device, in cooperation with RRCOCS-TVC, successfully meets the grid codes, and the transient behavior of the system's electrical and mechanical variables gets improved
 The proposed scheme helps reduce R-SFCL resistance, reduces its thermal stress, brings cost reduction in R-SFCL design, and improves its recovery characteristics. The listed improvement in R-SFCL can prevent the system from overcompensation and the adverse effects of recurring faults
 
 GRAPHICAL ABSTRACT

Adaptive super-twisting terminal sliding mode control and LVRT capability for switched reluctance generator based wind energy conversion system

Wind Energy Conversion systems (WECS) based on the Switched Reluctance Generator (SRG) devoid of rotor windings and permanent magnets are becoming more and more an attractive generator in wind energy market. Currently, the applicability of this machine in the WECS is now possible thanks to the mitigation of its drawbacks, mainly its high torque fluctuations and its low power factor. In this paper, an Adaptive Super Twisting Terminal Sliding Mode (AST-TSM) control is proposed for SRG-based WECS interfaced to the grid via a full-scale back-to-back converter. The main purpose of the proposed scheme is controlling the generator speed in order to achieve a Maximum Power Point Tracking (MPPT) with better performances. The second issue addressed consists of exploring the Low Voltage Ride Through (LVRT) capability of SRG-based WECS for compliance with recent grid code requirements. In modern grid codes, wind farms are only allowed to be connected to the grid if they have the capacity to remain in service during a short grid disturbance. As a matter of fact, under grid faults, the main tasks of the LVRT method consist in feeding the required reactive power into the grid in order to facilitate the recovery of the grid voltage. In this work, the investigated LVRT scheme, classified as a software method, is responsible for swapping the back-to back converter duties without external devices under grid voltage sags. Thereby, the generated power is stored as kinetic form, preventing strong overvoltage on the DC-bus. The proposed AST-TSM control scheme and the adopted LVRT method are substantiated by simulation results. All simulation case studies show that the AST-TSM control achieves good performance and extreme robustness as compared with the Proportional Integral (PI) and Integral Backstepping (IBC) controllers. Furthermore, the proposed controller is able to improve the suppression of chattering effect when its parameters are adjusted carefully. Also, simulation under grid faults conditions has proven that the LVRT method is effective for SRG-based WECS.

Topology and Control Strategy of PV MVDC Grid-Connected Converter with LVRT Capability

This paper proposes an isolated buck-boost topology and control strategy for the photovoltaic (PV) medium-voltage DC (MVDC) converter with low-voltage ride through (LVRT) capability. The proposed isolated buck-boost topology operates on either boost or buck mode by only controlling the active semiconductors on the low-voltage side. Based on this topology, medium-voltage (MV) dc–dc module is able to be developed to reduce the number of modules and increase the power density in the converter, which corresponds to the first contribution. As another contribution, a LVRT method based on an LC filter for MVDC converter is proposed without additional circuit and a feedback capacitor current control method for the isolated buck-boost converter is proposed to solve the instability problem caused by the resonance spike of the LC filter. Five kV/50 kW SiC-based dc–dc modules and ±10 kV/200 kW PV MVDC converters were developed. Experiments of the converter for MVDC system in the normal and LVRT conditions are presented. The experimental results verify the effectiveness of the proposed topology and control strategy.

Low voltage ride-through control strategy for virtual synchronous generators based on virtual self-inductive flux linkage

Virtual synchronous generators (VSGs), with the operational characteristics of synchronous generators (SGs), have been employed in renewable energy generation grid-connected systems to solve the problem of insufficient equivalent inertia, which is caused by the high permeability of distributed generation systems in the grid. However, a VSG does not possess low voltage ride-through (LVRT) capability. A novel LVRT control strategy for a VSG based on virtual self-inductive flux linkage is proposed in this paper. First, the electromagnetic transient response mechanism of a SG under grid faults is analyzed in detail. Then, a memory and retention strategy are proposed to simulate the effect of the switch law. Furthermore, the virtual q-axis armature self-inductance is introduced into the VSG and a virtual self-inductive magnetic chain is utilized to block the change of the transient fault current. This helps the inverter adjust the output voltage quickly. In addition, under the premise of meeting the reactive power margin, the reactive power compensation strategy is optimized to achieve a quick response in terms of the reactive power compensation of grid faults, which is helpful for realizing the LVRT of a VSG. Finally, the feasibility and effectiveness of the proposed LVRT method are verified by thorough simulation results.

Low-Voltage Ride-Through Coordinated Control for PMSG Wind Turbines Using De-Loaded Operation

Low-voltage ride-through (LVRT) is important for the grid integration of wind power systems (WPSs). Each grid operator defines a grid code, including the LVRT requirement, considering their power system characteristics. During grid faults, LVRT is needed for the protection of WPS because the over current can induce damage to the power converter system. An important issue in the LVRT method of WPS is the energy capacity because sufficient reserve energy capacities of wind turbine inertia and an energy storage system (ESS) are needed for a successful LVRT operation. Although ESS can store excess energy of the WPS during a grid fault, it is physically limited considering its energy capacity. Moreover, WPS has a low reserve energy capacity, especially when the WPS operates in high wind speed conditions. Thus, we propose the deloaded operation of WPS for the preparation of LVRT by reducing the power generation of WPS. The effectiveness of the proposed method is validated by modeling a WT and an ESS topologically and performing simulations using the MATLAB/Simulink SimPowerSystems toolbox.

Mechanical Stress Comparison of PMSG Wind Turbine LVRT Methods

The grid code for renewable energy is increasingly strict to circumvent issues with grid stability and reliability. One grid code standard that is enforced for most modern variable speed wind turbines (WTs) is the low-voltage ride-through (LVRT) criteria, where WTs are to be grid-connected during voltage dips. Traditionally, for permanent magnet synchronous generator (PMSG) WTs, LVRT is achieved by using a DC crowbar or DC chopper to dissipate the power difference between the grid and the generator. Alternatively, a popular LVRT strategy proposed by the research community for PMSG-based wind energy conversion system (WECS) is the stored energy in rotor inertia (SEIRI) strategy, which is done by altering the control of the machine-side converter (MSC) with potential cost savings. However, there are some concerns regarding additional mechanical stress to the drivetrain that may pertain to this method. A hybrid LVRT method has been suggested to combine the crowbar and the SEIRI methods to incorporate the benefits from both methods. In this article, we are studying and comparing the electrical and mechanical performance of PMSG WTs operating with the traditional crowbar, the SEIRI, and the hybrid LVRT method. To do so, the electrical and mechanical dynamics of these strategies are simulated using a two-mass drive train model, which is necessary for analyzing WTs under mechanical transient. Finally, the performance of wind farms with power reserves while using an inertia based LVRT method will be investigated to show the impact of the power reserve on the WT's LVRT mechanical dynamics.

Coordinated LVRT Control for a Permanent Magnet Synchronous Generator Wind Turbine with Energy Storage System

This study introduces a coordinated low-voltage ride through (LVRT) control method for permanent magnet synchronous generator (PMSG) wind turbines (WT) interconnected with an energy storage system (ESS). In the proposed method, both the WT pitch and power converters are controlled to enhance the LVRT response. Moreover, the ESS helps in regulating the dc link voltage during a grid fault. Previous LVRT methods can be categorized into strategies with or without an additional device for the LVRT. The latter scheme is advantageous from the perspective of no additional installation cost; in this case, pitch and converter controllers are used. Meanwhile, the former method uses an additional device for LVRT operation and hence, involves additional expense. However, it can effectively enhance the LVRT response by reducing the LVRT burden on the WT. Moreover, the additional device can be used for various WT power control applications and it is common that the ESS is interconnected to the WT for multiple objectives. Previous studies focused on these two aspects separately; hence, a method of coordinated control for an ESS and a WT is needed as more ESSs are required to connect to WTs for flexible wind power operation. The proposed method introduces a control method with different LVRT modes considering the ESS state of charge (SoC). When the WT does not have a sufficient inertial response operation range, the ESS reserve energy capacity is required for LVRT operation. This coordinated LVRT method employs both the WT and ESS controls when it is hard to handle the LVRT using the WT control alone at high wind speeds. In this case, power curve analysis is used to obtain the appropriate power reference during the fault period. In addition, a power reference is also used to ensure a safe operation. Using the proposed method, an ESS can be operated in a manner that is appropriate for WT operation, especially at high wind speeds. To validate the effectiveness of the proposed method, we considered two case studies. One study compares the LVRT response between the WT itself and the proposed method. The other research work compares the response of the conventional LVRT method that uses a WT and an ESS and with that of the proposed method. From these case studies, we concluded that the proposed method achieved a better performance while operating within the constraints of the WT rotor speed and ESS SoC limits.

Coordinated Fuzzy-Based Low-Voltage Ride-Through Control for PMSG Wind Turbines and Energy Storage Systems

Coordinated control methods involving a wind turbine (WT) and an energy storage system (ESS) have been proposed to meet several objectives, such as smoothing wind power (WP) fluctuations, shaving peaks, enabling power scheduling, and allowing low-voltage ride-through (LVRT). LVRT requirement is defined by grid operators, and it should be satisfied whenever grid faults occur. Several methodologies have been proposed for the LVRT both with or without the use of an ESS. Furthermore, using an ESS is more advantageous for several WP applications. By using an ESS, WTs can be operated in a more economic and reliable way. However, the installation cost of an ESS is high and it has operation range constraints for charging and discharging. Moreover, the WT operation condition and ESS state-of-charge (SoC) can be different when a grid fault occurs. Therefore, it is necessary to coordinate both units, WT and ESS, for reliable and economic operation during a grid fault. Thus, we propose a coordinated fuzzy-based LVRT method that considers the different operation conditions of a WT and an ESS. From the proposed method, the effective reference powers of a WT and an ESS are evaluated by considering the rotor speed and SoC in the fuzzy control algorithm. The effectiveness of the proposed method is validated by considering two case studies on ESS SoC and WT rotor speed violations.

Comprehensive study on fault‐ride through and voltage support by wind power generation in AC and DC transmission systems

This study exploits the impact of different low voltage ride-through (LVRT) methods and equipment on both the wind energy elements and the grid including wind turbine/farm ability to provide reactive compensation and maintain controllability during faults. The potential of using SFCL as an alternative LVRT equipment is preliminary studied. The study also exploits some severe scenarios that could face a multi-terminal high-voltage DC (HVDC) network. The influences of AC faults and control errors are examined. Results show limited deviations between the adopted LVRT methods. The wind turbine has to contribute to the stability of the AC collection grid of the wind farm, but it does not influence the grid, as both are decoupled through the multi-terminal HVDC grid. The implemented test systems and the examined events are developed in Matlab/Simulink and DIgSILENT.

An adaptive control strategy of crowbar for the low voltage ride-through capability enhancement of DFIG

Abstract Doubly-fed induction generator (DFIG) is sensitive to the disturbance of grid voltage, and its low voltage ride-through (LVRT) technology is still a hotspot issue. Rotor side crowbar is widely used to realize LVRT. However, traditional fixed crowbar can’t strike a balance between the limitation of rotor over current and DC-bus over voltage, and the crowbar is usually activated repeatedly due to the inappropriate deactivation strategy. Based on the analysis of rotor current considering crowbar operations, an improved LVRT method that dynamically adjusting crowbar resistance and adaptively deactivating the crowbar circuit is proposed. The corresponding adaptive control strategy and the resistance setting method are given. With the proposed scheme, both the rotor current and DC-bus voltage can be controlled within the restriction. Besides, the crowbar can be deactivated in the least time under the precondition of avoiding being reactivated, which will effectively reduce the absorbed reactive power from the system and benefit the service life of switching devices. The effectiveness of proposed LVRT scheme is verified by MATLAB/Simulink simulation.

Comprehensive Overview of Low Voltage Ride Through Methods of Grid Integrated Wind Generator

The wind power generation is a rapidly growing grid integrated renewable energy (RE) technology with an installed capacity of 539.291 GW. The capability of the wind energy conversion system (WECS) to remain integrated into the utility network in the case of low voltage events is called low-voltage ride-through (LVRT) capability. This paper offers a comprehensive overview of improvement techniques of the LVRT capability in WECS to increase the wind energy penetration level in the utility grid. Exhibited portrait manifests a broad spectrum of 1) wind turbines, 2) electrical generators used for wind power applications, 3) international grid codes applicable for grid integration of WECS, 4) LVRT fundamentals in WECS, 5) wind turbines LVRT methods by doubly fed induction generator (DFIG), 6) wind turbines LVRT methods by permanent magnet synchronous generators (PMSG), and 7) LVRT methods of wind turbines using squirrel cage induction generator (SCIG). This ready-reckoner paper critically reviews and classifies more than 190 research papers on LVRT issues, practices, and available technologies for grid integration in wind energy systems, and it aims to be a quick reference for the researchers, designers, manufacturers, and engineers working in the same field.

A new LVRT strategy for DGs with different droop gains in islanded microgrid with various loads

ABSTRACTThe Low Voltage Ride Through (LVRT) is an important requirement in fault conditions for any power system specially microgrids. When the fault occurs, the overcurrent and unbalanced voltage are yielded and these result in serious damages in inverter-based Distributed Generators (DGs) in the microgrid. To prevent this phenomenon, many types of research have been guided to limit the current in faulty conditions and to improve and balance the PCC voltage to be without any distortions. In this paper in islanded mode with reactive power injection during the fault occurrence, the LVRT method is proposed. This reactive power is injected to compensate the voltage dip and simultaneously limit the overcurrents in faulty phases. Also using this method, the faultless phases are not degraded and have no overvoltages. It is noticeable that two inverters in this paper assumed with varying droop gains and so the current and power of them must be divided relative to their droop gains both in faulty and faultless conditions. The simulation results are based on MATLAB Simulink which shows the system improvement in faulty conditions using this method.

Low‐Voltage‐Ride‐Through Performance of an HVDC Transmission System Using Two Modular Multilevel Double‐Star Chopper‐Cells Converters

SUMMARYThis paper presents an intensive discussion on a long‐distance high‐voltage direct‐current (HVDC) transmission system that combines two modular multilevel cascade converters based on double‐star chopper cells (MMCC‐DSCC) with DC power cables. Hereinafter, a single MMCC‐DSCC is referred to as a DSCC converter or just as a DSCC for the sake of simplicity. The HVDC transmission system is required to provide low‐voltage‐ride‐through (LVRT) capability to enhance transmission system availability. This paper proposes a new LVRT method without any direct information exchange between the two DSCC converters. The validity of the method is verified, using simulated waveforms from the software package of “PSCAD/EMTDC” and experimental waveforms from a three‐phase 200‐V, 400‐Vdc, 10‐kW, 50‐Hz downscaled HVDC system with a set of 300‐meter‐long DC power cables.

Controllable powers range and control method of DFIG for transient stability of power system

Wind generation is increasing rapidly in the grid, which has greater impacts on the power system than ever. The impacts are more serious under grid fault. The low-voltage ride through (LVRT) controls of doubly fed induction generators (DFIGs) may change characteristics of synchronous generators (SGs) and threaten the transient stability of power system. However, the influence on the transient stability and even the controllable powers of a DFIG has not been considered in the existing LVRT method. The contributions of the reactive power supplied by a DFIG in the oscillation process of power angle of SG were analysed. The controllable range of powers of a DFIG was researched with consideration of the rotor speed limit. An LVRT control method of a DFIG based on the controllable range was proposed to improve the transient stability of power system. Simulations show that the proposed method could enhance the power angle stability of power system under the condition of avoiding overspeed and out of service of DFIG.

モジュラー・マルチレベルDSCC変換器を用いたHVDCシステムの瞬時電圧低下時の運転継続性能

This paper presents an intensive discussion on a long-distance high-voltage direct-current (HVDC) transmission system that combines two modular multilevel cascade converters based on double-star chopper cells (MMCC-DSCC) with dc power cables. Hereinafter, a single MMCC-DSCC is referred to as a DSCC converter or just as a DSCC for the sake of simplicity. The HVDC transmission system is required to provide low-voltage-ride-through (LVRT) capability to enhance transmission system availability. This paper proposes a new LVRT method without any direct information exchange between the two DSCC converters. The validity of the method is verified, using simulated waveforms from the software package of “PSCAD/EMTDC” and experimental waveforms from a three-phase 200-V, 400-Vdc, 10-kW, 50-Hz downscaled HVDC system with a set of 300-meter-long dc power cables.

A control scheme to fulfill the grid-code under various fault conditions in the grid-connected wind turbines

This paper presents systems and algorithms for the low-voltage ride-through control of wind power systems. The low-voltage ride-through (LVRT) requirement which is included in the grid-code regulates wind power systems to maintain their operation and supply reactive power during grid fault. When the grid fault arises some problems occur in wind power systems, such as over voltage and over current by transient conditions or surplus power. This paper proposes the LVRT strategy for a stable operation of wind power systems under the grid-fault condition. Proposed LVRT methods include a fault detection method, a design of dynamic braking resistor, and a flexible power control under various fault conditions. The simulation results show a validity of the proposed LVRT strategy with a 2-MW wind power system model.

An LVRT Control Strategy Based on Flux Linkage Tracking for DFIG-Based WECS

For doubly fed induction generator (DFIG)-based wind energy conversion systems (WECSs), large electromotive force will be induced in the rotor circuit during grid faults. Without proper protection scheme, the rotor side of DFIG will suffer from overcurrents, which may even destroy the rotor-side converter (RSC). To mitigate this problem, a new flux-linkage-tracking-based low-voltage ride-through (LVRT) control strategy is proposed to suppress the short-circuit rotor current. Under the proposed control strategy, the rotor flux linkage is controlled to track a reduced fraction of the changing stator flux linkage by switching the control algorithm of RSC during grid faults. To validate the proposed control strategy, a case study of a typical 1.5-MW DFIG-based WECS is carried out by simulation using the full-order model in SIMULINK/SimPowerSystems. In the case study, a comparison with a typical LVRT method based on RSC control is given, and the effect of the control parameter on the control performance is also investigated. Finally, the validity of the proposed method is further verified by means of laboratory experiments with a scaled-size DFIG system.