Ride-through Performance Research Articles

Grid-Impedance-Based Transient Current Control for Offshore Wind Turbines under Low-Voltage Fault

In order to enhance the transient stability of offshore wind turbines (OWTs) in marine energy systems, the grid codes stipulate that OWTs should possess the low-voltage ride-through (LVRT) ability of being grid-tied and injecting reactive current during grid fault. However, the grid-side converter (GSC) of OWTs may lose stability under weak grid or severe fault conditions due to inaccurate current references. To address this issue, a novel transient current control method is proposed to improve the transient stability of permanent-magnet-synchronous-generator (PMSG)-based OWTs. The feature of DC-link overvoltage is investigated and is alleviated by utilizing the GSC’s overcurrent capacity and chopper. Additionally, the equivalent circuit of the PMSG-based OWT connected to the onshore grid is derived based on Thevenin’s theorem. The feasible current region (FCR) is then determined, taking into account the GSC capacity, pre-fault power ability, LVRT requirement, and synchronization stability. Furthermore, a grid-impedance-based transient current control method is designed to enhance the fault ride-through performance and mitigate power oscillation of the OWT under various transient grid impedance and fault conditions. Finally, a simulation model is conducted using PSCAD v4.6.3 software to validate the effectiveness of the proposed method.

A New Virtual Oscillator-Based Grid-Forming Controller with Decoupled Control Over Individual Phases and Improved Performance of Unbalanced Fault Ride-Through

Virtual Oscillator (VO) control is the latest and promising control technique for grid-forming inverters. VO Controllers (VOCs) can simultaneously provide time-domain synchronization and steady-state droop functionalities. In recent developments, VOCs have been integrated into system-level control architecture to provide additional functionalities such as capacitor voltage regulation and overcurrent protection. This article has further refined the existing VO-based system-level grid-forming control architecture with two essential and specific research contributions. The first contribution is to improve the decoupling in control over individual phases, which is achieved by combining Symmetrical Component based VOC (S-VOC) with nested voltage and current controls. The proposed phase decoupling control feature improves the grid-forming performance of existing VOCs in the presence of unbalanced grid voltages. The second contribution is to improve the fault ride-through performance under unbalanced faults by modifying the feedback of the existing VOC. Systematic development of the proposed grid-forming control architecture with analytical reasoning is presented. The proposed controller is successfully validated using simulations and experiments.

Fault ride-through performance evaluation method of photovoltaic power station based on accurate model

Fault ride-through performance evaluation method of photovoltaic power station based on accurate model

Comparative Study of Robust RSC Control of Doubly Fed Induction Generator Based Wind Turbine under Grid Frequency and Voltage Variations

The configuration of a grid-connected doubly fed induction generator (DFIG) for wind energy generation systems consists of direct coupling of the generator stator windings with the grid and partial coupling of the rotor through power converters. This structure makes the system sensitive to the fault grid. Thus, robust control techniques are required to deal with the undesirable transient sequences in the network. In this paper, two robust controllers have integrated to regulate the stator active and reactive power of DFIG based wind turbine. Then, they have been tested and compared to verify their performances when abnormal grid networks have occurred. Firstly, we used high order sliding mode (HOSM) based Super Twisting, which keeps the same conventional sliding mode performances, such as fast dynamic response and redundancy to the extern disturbances, with a simple design and chattering reduction. Secondly, we implement the backstepping control approach which is known for its robustness in transient sequences. A number of simulations have carried out to evaluate the ride-through performance of both control strategies in case of low grid voltage dips and grid frequency variations.

Adaptive control of DFIG-based wind turbines for fault ride-through and reactive power support based on stator current transient component

The fault ride-through performance is improved through the control of the transient components of the stator current. The reactive power provided by the DFIG within the margin of the converter can significantly enhance the grid-connected safety of the DFIG. Firstly, the reasons for the fault overcurrent of the DFIG’s rotor are analyzed. Secondly, according to the operating conditions of the wind turbine, the reactive power support limit of the DFIG-based wind turbines based on the stator current transient component control is analyzed, and an adaptive reactive power support control suitable for fault ride-through is proposed to maximize its support potential for grid-connected voltage. Finally, the DFIG can not only suppress the rotor overcurrent through the transient component of the stator current but also have a reactive power support capability that is superior to the grid-connected standard.

Power flow management and LVRT enhancement by using multi-functional capacitive bridge-type fault current limiter in DFIG system

This paper proposes a new concept of the fault current limiter (FCL) named multi-functional capacitive bridge-type fault current limiter (MCBFCL). It acts as a variable capacitor to manage active power flow under normal operating condition and enhances the low-voltage ride-through (LVRT) performance of wind farms (WFs) under fault operating conditions. The MCBFCL have been presented based on the capacitive bridge-type fault current limiter (BFCL) and is modified to compensate the transmission line impedance in order to manage power flow, while it is not bypassed under normal operating conditions unlike the conventional FCLs. In this paper, first, the power circuit and principle performance of the MCBFCL are discussed. Then time domain simulations are conducted in PSCAD/EMTDC environment to check the MCBFCL performance under both normal and fault operating conditions. Also, the MCBFCL performance is compared to the resistive and inductive BFCLs in transmission lines with series capacitive compensation. In this work doubly fed induction generator (DFIG)-based wind turbines have been utilized. Simulation results showed that the MCBFCL can manage the power flow under normal operating condition and enhances the LVRT performance of the WF under both symmetrical and asymmetrical fault operating conditions. Also, it has a superior performance in terms of LVRT indices values compared to the resistive and inductive BFCLs.

Global Optimality of Inverter Dynamic Voltage Support

This paper investigates the dynamic voltage support (DVS) from inverter-based resources (IBRs) under voltage sags to enhance the low-voltage ride-through performance. The DVS problem is formulated as a nonconvex optimization program that maximizes the positive-sequence voltage magnitude at the point of common coupling (PCC) subject to the current, active power, and synchronization stability constraints. Then, we perform the optimality analysis to explore the global optimum analytically. It is found that the unique global optimum has three scenarios/stages (S1–S3), which depends on the specific relationship among grid voltage, grid strength, and physical limits of IBRs. The explicit closed-form solutions in S1 and S3 are derived, and the optimality conditions (an implicit solution) of S2 are provided, which guarantees the global optimality and enables the compatibility with fast real-time control. Finally, we implement the optimum with a grid-connected single-stage photovoltaic (PV) power plant by integrating a DVS controller. Dynamic simulations are carried out under different scenarios to test our proposal. The robustness against model errors is also discussed.

Efficient HVRT Scheme of DFIG Using a SSFCL-Chopper Considering Grid Voltage Recovery

To enhance the dynamic high-voltage ride-through (HVRT) performance of doubly fed induction generator (DFIG) considering both the grid voltage swell and recovery influences, an efficient HVRT scheme is proposed by integrating a stator-side superconducting fault current limiter (SSFCL) with a dc-link chopper. The basic principle, circuit modeling, resistance setting, current-limiting characteristics design and coordinated control strategy of the SSFCL and chopper are presented to form an integrated protection scheme. As compared to a SSFCL or chopper-based HVRT scheme, the results obtained in the proposed SSFCL-chopper-based scheme show that the amplitudes of external and internal electromagnetic transient oscillations of the DFIG are largely reduced with shorter recovery time. This enhances significantly the HVRT behaviors in the DFIG considering grid voltage recovery.

Enhanced Fault Ride-Through of Power Converters Using Hybrid Grid Synchronization

Inaccurate phase-angle jump (PAJ) estimation of the grid voltage during faults is one of the major causes for poor fault ride-through (FRT) performance of converters. To overcome this issue and make the converter’s current controller robust, this article proposes a hybrid grid synchronization transition technique. In this concept, a synchronous reference frame-based phase-locked loop (SRFPLL) grid synchronization method is used during normal grid operation and switched to an arctangent-based phase-angle estimation during grid faults. Simultaneously frequency estimation is switched to the arctangent derived frequency. A common transition method, which depends on the phase-angle error between the two-phase estimation techniques, is proposed to ensure a smooth transition between the hybrid phase-angle and frequencies. The transition technique is implemented using the current control of a three-phase voltage source converter in the SRF. The performance of the converter during both symmetrical and asymmetrical grid faults along with the FRT strategies is tested using real-time experiments. It is observed that the transition-based hybrid grid synchronization technique reduces the loss of synchronism duration. Additionally, it offers a more robust converter current control performance compared to the SRFPLL technique over a wide range of voltage sags and PAJs.

Development of HVRT and LVRT Control Strategy for PMSG-Based Wind Turbine Generators

Various challenges are acknowledged in practical cases with high wind power penetration. Fault ride-through (FRT) capability has become the most dominant grid integration requirements for the wind energy conversion system worldwide. The high voltage ride-through (HVRT) and low voltage ride-through (LVRT) performance play a vital role in the grid-friendly integration into the system. In this paper, a coordinated HVRT and LVRT control strategy is proposed to enhance the FRT capability of the permanent magnet synchronous generator (PMSG)-based wind turbine generators (WTG). A dual-mode chopper protection is developed to avoid DC-link overvoltage, and a deadband protection is proposed to prevent oscillations under edge voltage conditions. The proposed strategy can ride through different levels of voltage sags or swells and provide auxiliary dynamic reactive power support simultaneously. The performance of the proposed control scheme is validated through various comparison case tests in PSCAD/EMTDC.

Design and Implementation of Crowbar Circuits Combined with Chopper Circuits for LVRT in Wind Farms

This paper presents the advantages of LVRT technology where a crowbar circuit functions in combination with a chopper circuit. To enhance the low voltage ride-through (LVRT) performance of double-fed wind turbines, the advantage of the crowbar protection circuit, the DC-link chopper protection circuit, and rotor side control (RSC) are combined based on current LVRT technology. The crowbar circuit is fully utilized to protect the rotor circuit from overcurrent and plays the role of the chopper circuit to stabilize the DC bus voltage. In addition, this paper proposes a cooperative control strategy that is used in the control of the protection circuit during LVRT. Simulation results based on PSCAD/EMTDC demonstrate that the proposed method can significantly reduce crowbar switching times and LVRT time, and the control method is simple and feasible.

Modeling of Converter Synchronization Stability Under Grid Faults: The General Case

The synchronization dynamics of grid-connected power converters are known to have a tremendous impact on transient stability and fault ride-through performance under grid faults. Up till now, the modeling of converter synchronization stability, which has paved the way for numerous enhanced control methods, is developed around the assumption that the grid fault is symmetrical. However, this is rarely the case. Moreover, grid codes require dual-sequence current injection during asymmetrical faults, which implies that the previously developed models are no longer valid during unbalanced conditions. To address these issues, this article identifies the necessary stability conditions during asymmetrical conditions and presents a quasi-static large-signal reduced-order model of a grid-following converter for analyzing its synchronizing interaction with the external network during symmetrical and asymmetrical grid faults. The modeling approach is developed and tested for three different short-circuit faults: A single line-to-ground fault, a double line-to-ground fault, and a line-to-line fault. The accuracy of the proposed model is verified through detailed simulation studies and experimental tests. Thus, this model can be used to assess the transient synchronization stability of grid-following converters during any type of grid fault, and due to its low-order representation, it may be well applicable for large-scale power system studies.

Sequence-Impedance Modeling of Voltage Source Converter Interconnection Under Asymmetrical Grid Fault Conditions

Nonlinear behaviors of voltage source converter (VSC) and the interconnection under grid fault conditions are discussed in the article. Based on the discussion, a sequence-impedance model is developed from the complex space vector analysis. Lumped sequence admittances of the converter interconnection and perturbation propagation considering the coupling between ac and dc circuits are presented in this article. The influence of the phase-locked loop design for improving low-voltage ride-through performance on the sequence-admittance model is also introduced. The sequence admittances are calculated, compared, and verified with point-by-point simulations and experiment. Obtained results indicate that the sequence-admittance model is effective for describing and predicting the behavior of VSC interconnection under asymmetrical grid fault conditions.

Investigation and Enhancement of Stability in Grid-Connected Converter-Based Distributed Generation Units With Dynamic Loads

Medium-voltage distributed generation (DG) units can be subjected to a high penetration level of dynamic loads, such as induction motor (IM) loads. The highly nonlinear IM dynamics that couple active power, reactive power, voltage, and supply frequency dynamics can affect the stability of MV grid-connected converter (GCC)-based DG units. However, detailed dynamic analysis and, more importantly, stabilization approaches of GCC-based DG units with IM loads when subjected to the grid faults, are not reported in the current literature. In addition, the literature lacks a thorough study on the effect of the grid strength on the low-voltage ride-through (LVRT) performance of such practical systems. To fill in this gap, this paper presents comprehensive integrated modeling, stability analysis, and LVRT performance improvement methods for GCC-based DG units in the presence of an IM load considering different grid strengths. A detailed multi-stage small-signal model of the complete system is obtained, and the eigenvalue analysis is conducted considering both static and dynamic load modeling. Furthermore, a sensitivity analysis is performed to investigate the effect of the length of the power line between the DG unit and the IM on the stability and LVRT performance of the entire system. Finally, the LVRT performance of the DG unit under an unbalanced grid fault is investigated using three different reference current generation strategies to determine the best strategy to provide a stable and efficient LVRT performance under strong and weak grid conditions. The time-domain simulation and experimental results are also presented to validate the effectiveness of the proposed methods.

An Asymmetric Short-Circuit Fault Ride-Through Strategy Providing Current Limiting and Continuous Voltage Supply for Three-Phase Three-Wire Stand-Alone Inverters

The three-phase three-wire stand-alone inverter is required to have short-circuit fault ridethrough ability to achieve continuity of power supply. Generally the inverter works in current-controlled mode in symmetric short-circuit conditions, but voltage limiting and harmonic distortion will appear under this method when asymmetric short-circuit fault occurs. Several existing methods for asymmetric fault ridethrough can achieve current limiting without voltage limiting, however, none of them can guarantee that the healthy phase voltage keeps constant before and after fault. This article proposes a fault ride-through strategy for asymmetric short-circuit conditions, which not only keeps the healthy phase voltage constant with different loads, but also makes the fault phase current limited and controllable. This goal is achieved by a simple control structure combining α-axis voltage control and β-axis current control at the same time. Furthermore, the voltage and current characteristics of this method under different load conditions have been analyzed, the analysis results indicate that the impact of different loads on fault ride-through performance is almost negligible. The correctness of theoretical analysis and the effectiveness of proposed strategy is verified by experimental results.

Current Reference Generation Based on Next-Generation Grid Code Requirements of Grid-Tied Converters During Asymmetrical Faults

Increased penetration of converter-based power generation has enforced system operators to require ancillary services from distributed generation in order to support the grid and improve the power system stability and reliability. Recent and next-generation grid codes require asymmetrical current provision during unbalanced faults for optimal voltage support. To address this, based on the highly used flexible positive- and negative-sequence control method for current reference generation, this paper presents a general current reference strategy for asymmetrical fault control, where a direct and explicit method is proposed to calculate power references and controller gains while simultaneously complying with converter current limitation and fulfilling the next-generation grid code requirements. The proposed method is tested for three distinct asymmetrical grid faults considering the requirements for dynamic voltage support of the recently revised German grid code as well as the next-generation grid codes. It is shown that the proposed method can improve the fault ride-through performance during asymmetrical faults compared with conventional solutions and comply with modern grid code requirements in a general and flexible manner.

A Novel Converter Station Structure for Improving Multiterminal HVDC System Resiliency Against AC and DC Faults

In an effort to minimize the power disruption between a dc grid and ac grids that host power converters during ac and dc network faults, this paper proposes a novel converter station structure to improve ac and dc fault ride-through performance of the multiterminal HVdc grid. The proposed structure consists of two independent ac and dc interfacing circuits, which are a half-bridge modular multilevel converter and a cascaded H-bridge (CHB) based energy storage system. Taking the advantages of high controllability and flexibility of the independent CHB converter and ease of integrating energy modules, a decoupled power relationship between the ac and dc sides is achieved, which is important for enhancing ac and dc fault performance. Operation of the proposed converter station under normal conditions and during ac and dc faults is explained, with the control system presented. Simulation validation of the proposed structure on a three-terminal HVdc grid confirms the enhanced performance, including the continuous operation during ac and dc faults with negligible power transfer disruption.

Analysis and Control of Fault Ride-Through Capability Improvement for Wind Turbine Based on a Permanent Magnet Synchronous Generator Using an Interval Type-2 Fuzzy Logic System

Recently, wind energy conversion systems in renewable energy sources have attracted attention due to their effective application. Wind turbine systems have a complex structure; however, traditional control systems are inadequate in answering the demands of complex systems. Therefore, expert control systems are applied to wind turbines, such as type-1 and interval type-2 fuzzy logic control (IT-2 FLC) systems. An IT-2 FLC system is used to solve the complexity of the wind turbine system and increases the efficiency of the wind turbine. This paper proposes a new control approach using the IT-2 FLC method applied to a wind turbine based on a permanent magnet synchronous generator (PMSG) to improve the transient stability during grid faults. An IT-2 FLC was designed to enhance the fault ride-through performance of a wind turbine and was implemented to control the machine side converter and grid side converter of a wind turbine. The proposed algorithm performance of a wind turbine based on a PMSG was investigated for different types of grid fault. The analysis results verify that the interval type-2 fuzzy logic control system is robustly utilized under different operational conditions.

Postfault LVRT Performance Enhancement of DFIG Using a Stage-Controlled SSFCL-RSDR

To enhance the dynamic low-voltage ride-through (LVRT) performance of a doubly fed induction generator (DFIG) based wind turbine during the postfault stage, a novel LVRT scheme is proposed by integrating a stator-side superconducting fault current limiter (SSFCL) with a two-step rotor-side series dynamic resistor (RSDR). The basic principle, circuit modeling, and switching strategy of the SSFCL and RSDR are presented to form the primary and backup protection schemes. As compared to conventional SSFCL-based LVRT scheme, the results obtained in the proposed SSFCL-RSDR-based scheme show that the amplitudes of electromagnetic transient oscillations are largely reduced with shorter recovery time. This enhances significantly the internal and external LVRT behaviors in the DFIG during and after the fault simultaneously.

Artificial neural network–aided technique for low voltage ride-through wind turbines for controlling the dynamic behavior under different load conditions

At the level of the electrical distribution networks with wind generation, the disturbances may influence the voltage stability, particularly during low voltage ride-through wind turbine. This research is concerned with studying the effect of implementing different controllers and load types on the low voltage ride-through dynamic recovery performance during disturbances. A conventional proportional–integral–derivative controller is compared with the artificial neural network–based one. The controller construction and its gain are proposed for each type of controller and the impact of each controller on the dynamic behavior of the low voltage ride-through is investigated thoroughly under various operating conditions. Also, the dynamic performance of wind generators is examined with low voltage ride through and different dynamic load models. Both, dynamic induction motor load and composed static and exponential recovery load models are considered. In case of dynamic induction motor load, the effect of the inertia constant has been studied under two types of controllers. The overall system model is simulated using PSAT/MATLAB software in such a way that it can be suited for modeling of voltage controller and loads configurations. The low voltage ride-through performance has been changed with different controllers and load types.