Severe Voltage Sags Research Articles

A detailed study on Transient Stability Enhancement of Grid-Forming Inverters

The integration of the weak grids along with the clean energy that comes from the natural resources requires stable interconnection of Voltage-Source Inverters (VSIs) with weak grids. During severe voltage sags, weak grid integrated VSIs may fail synchronization with the grid.These inverters' transient stability under severe grid disruptions is weak and significantly different from that of synchronous machines. To prevent the side band oscillations in the weak grids, the emerging inverters based on grid-forming are preferred over the grid following inverters in the weak grids. These inverters synchronise with the grid using a power-based synchronisation technique in order to prevent the instability brought on by a typical Phase Locked Loop in weak AC grids. This study presents a comprehensive review of the transient stability improvement of Grid-Forming VSIs.

Low‐voltage ride‐through handling in wind farm with doubly fed induction generators based on variable‐step model predictive control

AbstractA severe voltage sag on the wind‐farm grid side with doubly fed induction generators (DFIGs) can induce a peak inrush current in the rotors and damage converters, resulting in wind‐turbine disconnection from the grid. To prevent this from happening, a variable‐step model predictive control (VS‐MPC) strategy is proposed for improving the wind turbines’ ability to operate without disconnecting themselves from the grid when a fault occurs. First, the predictive‐control state‐space model of a doubly fed wind farm is established according to its working principle. Second, model predictive control (MPC) is applied on the rotor side of the DFIG to realize the rapid tracking of the rotor current to the reference value during low‐voltage ride‐through (LVRT) of the DFIG. Finally, a variable‐step size algorithm is introduced into the MPC controller to change the step size continuously during the LVRT period. This increases the control accuracy, realizing rapid attenuation of each transient component, whereby the wind‐farm LVRT capability is enhanced. The proposed control strategy was simulated and verified using MATLAB/Simulink. The simulation results indicated that VS‐MPC can effectively handle LVRT, allowing the recovery of a wind farm that uses DFIGs and improving the performance of wind‐farm.

The transient instability mechanism and stability‐enhanced LVRT control for VSC riding‐through severe grid voltage sag

AbstractAs the grid‐connected interface device of wind and photovoltaic power generation, the voltage source converter (VSC) must pass the low voltage ride‐through (LVRT) test. However, existing literature demonstrates that under weak grid conditions, there is a possibility of severe fluctuations in the terminal voltage of VSC during transiting from the pre‐sag normal operation mode to the LVRT mode. Unfortunately, it will make VSC frequently enter and exit LVRT mode. In order to address the tricky problem of transient instability of VSC riding‐through severe grid voltage sag under the weak grid, in this paper, the full‐order large‐signal model of VSC is established first. Then, based on the established model, the analysis indicates VSC will face the risk of losing stability under the weak grid during LVRT due to current transients. By studying the impacts of low short circuit ratio (SCR) on the basin of attraction of the post‐sag equilibrium point, the intrinsic mechanism of VSC losing stability under weak grid conditions is revealed. Moreover, considering that the transient stability is related to the current control, the stability‐enhanced LVRT control ensuring both small‐signal stability and transient stability of VSC during LVRT is proposed. Finally, the correctness of the above theoretical analysis is verified by real‐time simulation

Hybrid UPQC for Power Quality Management of Renewable Energy Penetrated System

Severe voltage sag and radiation pollution is an obstacle for low-voltage intermediate-requirement requiring power facilities, like optical semi conduct, data storage, and precision equipment, like pharmaceuticals and pharmaceuticals, where radiation is involved. As more voltage and economic as well as becoming slower and more difficult to maintain, current equipment is now needed to have higher energy capability. Achieving bi-links for bi-side operation in a medium-voltage condition power-supply application. The first step, followed by the study of a three-phase equivalent circuit and mathematical model, reveals the underlying mechanism of harmonic suppression and voltage control combined with sequence-shunt act phasing and shunt HAPF for continuous phase synthesis (CPS) and the different discrete control and CPS (p-p/Discrete Fourier Transform and Differential Mode Digital Filtration are considered for series, which uses the sliding fourier Phase Lock Loop and its sychronous shunt capability are implemented. The L-C series-filter and passive double-resonant circuits (PDR) are then discussed, along with the key circuit components of the design, the series inductor (LL) circuit, the PDR passive low-pass, and the passive double resonant circuit design for applying the induct filtering. After all, the proof of results in the high voltage delivery validation in MATLAB/Simulink, IHUPC has been demonstrated, 10kV distribution now is supported. With these effects, it being effective in regulating voltage and current and dynamic power generation, the simulation results show the performance of dynamic grid voltage & current regulation and dynamic load-harmonic regulation.

Effect of Series/Shunt Capacitors on the Performance of a Distribution Network with Motor Driven Loads

Improving the power-quality is one of the important measures for operating the electrical-distribution networks. Motor driven-loads supplied from distribution-networks can cause severe voltage-sag during motors’ acceleration. In this research, the application of series- and shunt-compensation for improving the voltage-profile during steady-state and motor-startup is studied by simulation and checked experimentally. The obtained results indicate that series-capacitors provide an adaptive improvement to the voltage-profile being related to the load reactive-power demand while the shunt-capacitors provide a nearly constant-voltage improvement irrespective of the load-level. With motor acceleration, the series-capacitor reduces the voltage-sag during motor-starting and the acceleration-time by up to 19.37% and 54.7%, respectively when compared with motor-starting without capacitors. To the authors’ knowledge, the present-paper is the first to successfully reduce the voltage-sag to its acceptable levels during the motor's starting period using series-capacitor. On the other hand, shunt-capacitors show no significant improvement of the voltage during the starting-period of the motor. Finally, a case study of a real distribution-network (Karot line) in Egypt loaded by several water-pumps driven by induction-motors is investigated using ETAP-package. Performance of the line under the influence of series- and shunt-compensation is studied and the obtained simulation results conform to those obtained experimentally in the laboratory.

Transient Synchronization Stability Analysis and Enhancement of Paralleled Converters Considering Different Current Injection Strategies

Renewable energy sources (RESs) are highly penetrated in power systems through phase-locked loop (PLL) synchronized converters. Under severe grid voltage sags, PLL-synchronized RES system is susceptive to transient instability and tripping from the grid due to inappropriate current injection. The existing literatures mainly focus on synchronization instability issues of a single grid-connected RES system, and the effect of the interaction between them is rarely studied. To fill this gap, this paper aims to clarify the interaction mechanism and characterize stability boundary of paralleled PLL-synchronized RES system with different converter parameters. More specifically, the effect of current injection angle and magnitude on transient synchronization stability is investigated for guiding current injection reference design during low voltage ride-through (LVRT) and rated capacity option of RESs. First, the transient interaction model of the paralleled converter system is developed, and interaction mechanism under different current injection angles is revealed. Next, stability region analysis is performed to evaluate the impact of current injection magnitude, wherein current injection boundary is obtained. Then, a current injection design method is proposed for improving both transient stability and voltage support. The theoretical analysis and the proposed method are validated by simulation results.

Enhanced Deadbeat Control Approach for Grid-Tied Multilevel Flying Capacitors Inverter

This paper proposes an enhanced Deadbeat Controller (DBC) for a grid-tied Flying Capacitors Inverter (FCI). The proposed DBC guarantees the balancing of the capacitors’ voltages while injecting current to the grid with lower Total Harmonics Distortion (THD). The proposed controller has the following advantages: 1) Improved current tracking quality even at zero crossing instants by using a weighted state-space model, 2) Superior steady-state performance (lower current THD) compared to other prediction-based control techniques such as Finite-Control-Set Model Predictive Control, 3) The generated duty cycles are normalized to the common base when the desired state is out of reach within the sampling time, 4) Voltage Ride-Through (VRT) capability, and 5) Robustness to parameters variation. Theoretical analysis, simulation, and experimental results are presented to show the effectiveness of the proposed control technique in ensuring uninterruptible and smooth transfer of energy to the grid during normal/abnormal operating conditions (severe voltage sags, parameters variation, etc.).

Large-Signal Grid-Synchronization Stability Analysis of PLL-Based VSCs Using Lyapunov's Direct Method

Grid-synchronization stability (GSS) is an emerging stability issue of grid-tied voltage source converters (VSCs), which can be provoked by severe grid voltage sags. Although a qualitative understanding of the mechanism behind the loss of synchronization has been acquired in recent studies, an analytical method for quantitative assessment of GSS of grid-tied VSCs is still missing. To bridge this gap, a dedicated Lyapunov function is analytically proposed, and its corresponding stability criterion for GSS analysis of grid-tied VSCs is rigorously constructed. Both theoretical analysis and simulation result demonstrate that the proposed method can provide a credible GSS evaluation compared to the previous EAC/EF-based method. Therefore, it can be applied for fast GSS evaluation of the grid-tied VSCs as is exemplified in this letter, as well as an analytical tool for some GSS-related issues, e.g., GSS-oriented parameter design and stabilization control.

An Advanced Proportional Multiresonant Controller for Enhanced Harmonic Compensation With Power Ripple Mitigation of Grid-Integrated PV Systems Under Distorted Grid Voltage Conditions

This article proposes an advanced integrated control strategy for enhanced grid current harmonic compensation with power ripple mitigation in a grid-integrated photovoltaic systems (GIPVS) under distorted grid voltage conditions. The proposed control strategy consists of an advanced proportional multiresonant (APMR) current controller and a compensator for grid voltage sag (CGVS), which is implemented with a phase-locked loop block. The APMR is designed in the α-β reference frame and minimizes the harmonics of the grid injected current. The CGVS, along with the APMR, eliminates the active and reactive power ripples under unbalanced voltage sags conditions. Further, an adaptive step size incremental conductance (ASINC) maximum power point tracking controller in integration with the proposed control strategy is implemented. Along with current harmonic reduction by APMR, the ASINC method ensures effective and faster tracking of maximum power point and negligible oscillations in the steady-state under different environmental conditions. By utilizing the proposed APMR control strategy, the frequency adaptability of the GIPVS is improved compared to the conventional proportional multiresonant (PMR) control strategy under grid frequency deviations. It successfully keeps the grid current total harmonics distortion below the acceptable limits for the grid voltage polluted by several odd harmonics, up to the nineteenth harmonics. The proposed control strategy of APMR with CGVS almost completely eliminates the power ripples in even severe unbalanced voltage sag conditions. Experimental studies are carried out using a real-time digital simulator to verify the effectiveness of the proposed control strategy in comparison with the conventional PMR controller.

The LVRT Control Scheme for PMSG-Based Wind Turbine Generator Based on the Coordinated Control of Rotor Overspeed and Supercapacitor Energy Storage

With the increasing penetration level of wind turbine generators (WTGs) integrated into the power system, the WTGs are enforced to aid network and fulfill the low voltage ride through (LVRT) requirements during faults. To enhance LVRT capability of permanent magnet synchronous generator (PMSG)-based WTG connected to the grid, this paper presents a novel coordinated control scheme named overspeed-while-storing control for PMSG-based WTG. The proposed control scheme purely regulates the rotor speed to reduce the input power of the machine-side converter (MSC) during slight voltage sags. Contrarily, when the severe voltage sag occurs, the coordinated control scheme sets the rotor speed at the upper-limit to decrease the input power of the MSC at the greatest extent, while the surplus power is absorbed by the supercapacitor energy storage (SCES) so as to reduce its maximum capacity. Moreover, the specific capacity configuration scheme of SCES is detailed in this paper. The effectiveness of the overspeed-while-storing control in enhancing the LVRT capability is validated under different levels of voltage sags and different fault types in MATLAB/Simulink.

Investigation of Voltage Sags Effects on ASD and Mitigation Using ESRF Theory-Based DVR

Voltage sag is a frequently occurring power quality disturbance in the industries equipped with adjustable speed drives (ASD). A detailed investigation of voltage sag effects on ASD performance with the novel mathematical analyses to estimate the ASD parameters during different types of voltage sag is discussed in this article. The effects of voltage sags are mitigated using an enhanced synchronous reference frame (ESRF) theory-based dynamic voltage restorer (DVR). The working principle of the ESRF theory-based DVR during sag is also described. The investigation of the effects of type A, type B and type F voltage sags on ASD parameters are verified using the simulation and the experimental studies. Further, these effects are mitigated by the ESRF theory-based DVR using the developed simulation and experimental models. The ESRF controller of DVR is working effectively during voltage sags by improving its transient response, which tightly regulates the DC-link voltage of ASD around its reference value. Also, the steady-state response of DVR is enhanced during severe voltage sag, which further validates the ability of the ESRF theory-based DVR. This type of improved performance of ASD during voltage sags cannot be obtained using other existing SRF theories of the DVR.

A Voltage Support Control Strategy for Grid Integrated Solar PV System During Abnormal Grid Conditions Utilizing Interweaved GI

The prime facets for the control of grid integrated voltage source converters (VSC) during abnormal grid variations are the control of voltage as well as power quality. In this article, a unique control strategy is presented for the control of solar photovoltaic (PV) system interfaced to the grid utilizing an interweaved generalized integrator. A single-stage three-phase topology is considered. The primary purpose of control is to deliver the PV power to the grid even during various abnormal grid variations. During normal operation, the system delivers power at unity power factor. However, during variations in the grid voltage, the profile of the PCC voltage is maintained within prescribed limits by reactive power injection. Moreover, LVRT operation is undertaken during severe voltage sags. The utilization of the system is increased in the absence of PV generation during night, the VSC and dc link capacitor act as a distribution static compensator. Contrary to traditional control techniques, power quality of the system is not compromised. The achievements of the control are demonstrated through simulation as well as with hardware implementation. Furthermore, a comparative analysis, with the state-of-the-art techniques, is highlighted, which shows the efficacy of the presented control.

Enhancement of the Dynamic Behavior of Grid-Connected DFIG Based Wind Turbines by using Fuzzy Logic Controller under LVRT Conditions

In recent years, due to the interconnection of large capacity wind turbines to the power grid lead, there are serious issues in the stability of Grid and generation of electrical power. Also, it is showing effect on the dynamic performance of the electrical power systems. To maintain stability during sudden changes in the grid, the LVRT (Low Voltage Ride Through) capability of the Wind Turbines is one of the prime requirements. Wind turbines attached to DFIG (Doubly Fed Induction Generators) are advantageous which have LVRT capability at limited extent. In this paper, the elaborated discussion of the LVRT of Wind turbines shafted to DFIG's in the Grid. It also presents the complete description of the sudden changes in the systems like transient characteristics and the Doubly Fed Induction Generators dynamic response at the time of grid voltage faults (Symmetrical and Asymmetrical). The latest rotor side control technology is displayed in this paper for DFIG and wind turbines with improved capacity of low voltage ride through at the time of severe grid voltage sags. A Fuzzy Logic controller-based control technology is introduced in this paper which performs the balancing the rotor-side voltage and short circuits during the disturbances in the Grid. The advantage in this proposed control scheme is that it reduces the additional cost and reliability issues. So, the DFIG is efficient and usability company norms are satisfied with the proposed Fuzzy logic controller compared to regular controller like PI controller. The performance of the proposed system is simulated and verified in the computer. The results are displayed and it conclude that the control strategy of LVRT capability for Grid connected DFIG based wind turbine systems with Fuzzy Logic Controller are more effective than the conventional control Methods.

Providing Fault Ride-Through Capability of Turbo-Expander in a Thermal Power Plant

This paper aims to make possible the operation of a turbo-expander (TE) as a renewable resource at the Neka power plant in fault condition in the auxiliary service system (ASS), which is considered one of the fundamental problems in network operation. In this paper, the effect of the failure on the performance of the TE is analyzed whilst the performance of a dynamic voltage restorer (DVR) and static synchronous compensator (STATCOM) to compensate the fault in the ASS network is investigated. To improve the performance of DVR, a novel topology is developed; additionally, the compensatory strategies are assessed, simulated, and validated. In order to optimize the performance of the compensators, their possible presence situations on the ASS in various scenarios under the conditions of severe disturbance, synchronization of fault conditions, and starting of TE are tested. The results of PSCAD/EMTDC software simulation demonstrate that by applying the improved topology and selected compensation strategy of DVR, severe voltage sags are compensated, and the fault ride-through (FRT) capability for the TE is provided. Eventually, it is evident that the proposed solution is technically and economically feasible and the TE can supply the total ASS power consumption in all disturbances.

Control Strategy of Intergrated Photovoltaic-UPQC System for DC-Bus Voltage Stability and Voltage Sags Compensation

Power quality problem, because of its various forms and occurrence frequency, has become one of the most critical challenges confronted by a power system. Meanwhile, the development of renewable energy has led to more demands for an integrated system that combines both merits of sustainable energy generation and power quality improvement. In this context, this paper discusses an integrated photovoltaic-unified power quality conditioner (PV-UPQC) and its control strategy. The system is composed of a series compensator, shunt compensator, dc-bus, and photovoltaic array, which conducts an integration of photovoltaic generation and power quality mitigation. The fuzzy adaptive PI controller and the improved Maximum Power Point Tracking (MPPT) technique are proposed to enhance the stability of dc-bus voltage, which is aimed at the power balance and steady operation of the whole system. Additionally, the coordinate control strategy is studied in order to ensure the normal operation and compensation performance of the system under severe voltage sag condition. In comparison to the existing PV-UPQC system, the proposed control method could improve the performance of dc-bus stability and the compensation ability. The dynamic behavior of the integrated system were verified by simulation in MATLAB and PLECS. Selected results are reported to show that the dc-bus voltage was stable and increased under severe situations, which validates the effectiveness of the proposed integrated PV-UPQC system and its control strategy.

Design and implementation of interval type-2 fuzzy logic-PI based adaptive controller for DFIG based wind energy system

In this paper, an advanced interval type-2 fuzzy logic-proportional integral (PI) controller has been proposed for torque and voltage control loops of rotor side converter of doubly fed induction generator (DFIG). The gains of PI controller are determined and tuned by interval type-2 fuzzy logic method according to system operating condition. Thus, the adaptive nature of type-2 fuzzy logic and robust nature of PI controller are combined eventually, which exhibits good steady state and dynamic responses. The performance of the proposed controller has been evaluated for different operating conditions of DFIG such as severe fault and voltage sag with reference to varying wind speed. Real time simulations are developed for DFIG based wind energy system (WES) to validate the proposed controller using the OPAL-RT digital simulator. The performance of the proposed controller is examined through a comparative analysis with its traditional fuzzy logic-PI counterpart. The transient analysis of DFIG with interval type-2 fuzzy-PI shows the improved results subjected to three-phase fault and voltage sag as desired by the grid codes.

Operating performance analysis on wind turbines with the speed regulating differential mechanism

In this paper, a hybrid transmission system is presented for variable-speed constant-frequency wind turbines (WTs). This approach employs an electrically controlled speed regulating differential mechanism (SRDM) so that the synchronous generator is able to rotate at a stable speed and, thus, generate electricity in constant frequency regardless of the changing wind rotor speed. In order to find out the operating performance of the proposed SRDM-based WT, a detailed 1.5 MW simulation model is established by which the speed-regulating accuracy, the electrical harmonic pollutions, and the system low-voltage ride-through (LVRT) capability are all studied in depth and then compared with those of the existing direct-drive or double-fed wind power systems. Simulation results indicate that, under various wind conditions, SRDM can efficiently regulate the output power frequency with satisfactory deviations by using a low-power (less than 262 kW) permanent magnet synchronous motor. Compared to the existing WTs, the 5th, 7th, 11th, and 13th current harmonics of the proposed WT are all ideally reduced to 0.4% of the fundamental current. Moreover, around 0.45 p.u., reactive power can be provided for the rapid recovery of the grid during severe voltage sags. The output power quality, system transient stability, and LVRT capability are all improved. Theoretical analysis and case studies give good verifications of the feasibility and superiority of the proposed SRDM-based WT.

Detailed Investigation and Performance Improvement of the Dynamic Behavior of Grid-Connected DFIG-Based Wind Turbines Under LVRT Conditions

Power generation and grid stability have become key issues in the last decade. The high penetration of large capacity wind generation into the electric power grid has led to serious concerns about its influence on the dynamic behavior of power systems. The low-voltage ride-through (LVRT) capability of wind turbines during grid faults is one of the core requirements to ensure stability in the power grid during transient conditions. The doubly-fed induction generators (DFIGs) offer several advantages when utilized in wind turbines, but discussions about their LVRT capabilities are limited. This paper presents a comprehensive study of the LVRT of grid-connected DFIG-based wind turbines. It provides a detailed investigation of the transient characteristics and the dynamic behavior of DFIGs during symmetrical and asymmetrical grid voltage sags. A detailed theoretical study supported by computer simulations is provided. This paper also provides a new rotor-side control scheme for DFIG-based wind turbines to enhance its LVRT capability during severe grid voltage sags. The proposed control strategy focuses on mitigating the rotor-side voltage and current transients during abnormal grid conditions, without any additional cost or reliability issues. As a result, the DFIG performance is improved and utility company standards are fulfilled. Computer simulations are used to verify the expanded ride-through capability of the novel strategy and its effective performance compared to the conventional control schemes.

Enhanced low‐voltage ride‐through capability of flying capacitors inverter using model predictive control

Summary This paper proposes an enhancement of the low-voltage ride-through (LVRT) capability of multilevel flying capacitors inverter using an improved model predictive control (MPC). The proposed solution is a multiobjective control strategy ensuring simultaneous capacitors' voltages balancing and grid current injection with reduced total harmonics distortion (THD) and unity power factor even under severe voltage sags. The proposed MPC technique makes use of a multioptimization solution for the common problem of weight dissimilarity of the control objectives. The first optimization is achieved by the inclusion of the state variables normalization within the cost function computation. This will provide to the proposed MPC technique more robustness with the ability to take into consideration external disturbances (grid voltage sag) while ensuring the meeting of the state variables constraints. The second one is achieved through accurately tuning the weighting factor based on minimizing the grid current total harmonics distortion and the maximal errors on the capacitors' voltages. Theoretical analysis and simulation results are presented to prove that the presented solution guarantees continuous and smooth power transfer to the grid even under severe voltage sags. The obtained results are validated on a 1-kW experimental prototype.

Simulation Research on LVRT Capability of DFIG-based WECS During Severe Grid Voltage Sags

The low voltage ride through (LVRT) means that wind turbines must connect to grids under grid-side faults. This paper provides a simulation study on the LVRT capability of wind energy conversation systems (WECS) under doubly-fed induction generator (DFIG) during severe grid side voltage sags. Flexible AC Transmission (FACTS) devices such as static synchronous compensator (STATCOM) is an option available to provide controlled dynamic reactive power to improve the voltage sags. Moreover, this system is modeled and simulated in MATLAB. The simulation result shows that STATCOM is more effective to enable the DFIG to ride through the severe faults.