Grid Distortion Research Articles

Comparison of the Impacts of Background Distortion with Even and Odd Orders in the Supraharmonic Range on Grid-Connected Converters

In this paper, a comparison of the emissions of grid-connected converters under grid distortion in the supraharmonic range is performed. The investigated supraharmonic grid distortion is divided into two types, namely odd-integer and even-integer multiples of the grid frequency. It is found that the first distortion type does not affect the emissions of the grid-connected converters, while the second distortion is responsible for the 2N-harmonic emissions that appear in the frequency spectra of the grid-connected converters, which, in turn, interact with the supraharmonic primary emissions of these converters, resulting in additional supraharmonic emissions. To explain this behavior, a mathematical analysis is provided for two types of grid-connected converters with an inverter interface and with a diode bridge rectifier interface. Simulations and experimental studies are performed to verify the main findings of the mathematical analysis.

Numerical method of lateral pipe-soil interaction and sensitivity analysis investigation

The pipe-soil interaction force is a crucial load that ensures the on-bottom stability of submarine pipelines and acts as a vital input parameter for the design of controlled pipeline buckling. It is necessary to precise prediction of soil lateral resistance and sensitivity to environmental parameters. This study developed a numerical simulation technique based on the coupled Euler-Lagrange (CEL) method to systematically analyse the deformation of the pipeline and the soil-pipeline forces on the seabed foundation during service. To validate the numerical method for large lateral displacement of pipe-soil interaction, an experimental approach employing parameter variation was utilized to conduct sensitivity analysis on the environmental parameters. The key parameters affecting soil-pipeline interaction were determined through an analysis of relative variation factors. The research findings demonstrate a satisfactory agreement, with an amplitude difference range of within 15 %, between the numerical simulation technique using the CEL method and the experimental results. This method resolves the issue of non-convergence due to grid distortion under conditions of large displacement, thereby enhancing computational accuracy. Sensitivity analysis shows that the relative variation factors for soil friction angle, initial burial depth, and soil density exceed 50 %, which is notably higher than the elastic modulus, friction coefficient, and Poisson's ratio of the soil. The results of this study enable the prediction of lateral forces in pipe-soil interaction and identification of environmentally sensitive parameters. This study establishes a foundation for calculating pipeline buckling strength in complex environments and holds paramount importance for the design and construction of pipelines to ensure on-bottom stability.

High-performance passive impedance network model for grid-tie inverters in steady state

Nowadays, the high penetration of inverter-interfaced distributed energy resources (DERs) has raised a well-founded concern regarding the interaction of these devices with the upstream grid. The harmonic interaction phenomenon between these players can be described by time-domain simulations, using high computational effort switched and/or average models often impractical for large power systems. This paper proposes an innovative low-computational-burden model named passive impedance network (PIN), capable of accurately representing DERs in electric power systems (EPSs). Through passive RLC components, the PIN model can describe DER dynamic behavior at a certain frequency of interest, making it able to represent harmonic flow within the EPS nodes. The proposed model is compared with the switched model, average model, and an experimental setup under different grid distortions and injected current profiles. A comparison of the total demand distortion (TDD) between the proposed PIN model and the experimental results is carried out. The effect of DER parameter deviations on PIN model accuracy is addressed, in which the absolute error of the PIN model TDD values are 0.65% and 0.06% for the experimental and switched model, respectively. Besides, the PIN model presents an improved computational performance compared to the time-domain-based switching model, requiring 99% less computational burden.

UHV AC/DC power grid geomagnetically induced currents harmonic characteristics analysis

The power grid’s geomagnetically induced currents (GIC) during geomagnetic storms, are associated with a broad and abrupt harmonic impact, with the potential for grid paralysis and extensive power outages in severe scenarios. Therefore, the harmonic characteristics of geomagnetically induced currents in China’s ultra-high voltage (UHV) AC/DC power grid are analyzed in this paper. Firstly, a harmonic equivalent model of transformer GIC is established, and the injection of harmonics into the UHV AC/DC power grid during GIC events is quantified.Secondly, a harmonic state-space model is established by combining the 12-pulse converter switching function theory with the exponential Fourier series. The harmonic transmission and dynamic coupling characteristics of GIC in the UHV AC/DC power grid are analyzed using the harmonic equivalent model of the main transformer GIC and the harmonic equivalent model of the converter transformer GIC. Finally, a harmonic simulation model of the UHV AC/DC power grid under GIC is constructed using PSCAD, and the distribution of harmonics in the UHV AC/DC power grid under GIC is calculated. The calculations indicate that with 30 A injected GIC in the sending end transformers, the 2nd harmonic current on the sending end network side alone is 61.45 A, and the 3rd harmonic voltage on the valve side is 8.57 kV. The characteristics of harmonic transmission in the power grid result in a 2nd harmonic current of 34.87 A on the receiving end network side. Simultaneously, the total harmonic distortion of the AC power grid at the sending end is 2.35 %, surpassing the permissible limit established by the national standard. Consequently, this issue should be prioritized by the relevant power authorities.

Current Controller Design of Grid-Connected Inverter with Incomplete Observation Considering L-/LC-Type Grid Impedance

This paper presents a current control design for stabilizing an inductive-capacitive-inductive (LCL)-filtered grid-connected inverter (GCI) system under uncertain grid impedance and distorted grid environment. To deal with the negative impact of grid impedance, LC-type grid impedance is considered in both the system model derivation and controller design process of an LCL-filtered GCI system. In addition, the integral and resonant control terms are also augmented into the system model in the synchronous reference frame to guarantee the reference tracking of zero steady-state error and good harmonic disturbance compensation of the grid-injected currents from GCI. By considering the effect of grid impedance on the control design process, an incomplete state feedback controller will be designed based on the linear-quadratic regulator (LQR) without damaging the asymptotic stabilization and robustness of the GCI system under uncertain grid impedance. By means of the closed-loop pole map evaluation, the asymptotic stability, robustness, and resonance-damping capability of the proposed current control scheme are confirmed even when all the system states are not available. In order to reduce the number of required sensors for the realization of the controller, a discrete-time current-type full-state observer is employed in this paper to estimate the system state variables with high precision. The feasibility and effectiveness of the proposed control scheme are demonstrated by the PSIM simulations and experiments by using a three-phase GCI prototype system under adverse grid conditions. The comprehensive evaluation results show that the designed control scheme maintains the stability and robustness of the LCL-filtered GCI when connecting to unexpected grids, such as harmonic distortion and L-type and LC-type grid impedances. As a result, the proposed control scheme successfully stabilizes the entire GCI system with high-quality grid-injected currents even when the GCI faces severe grid distortions and an extra grid dynamic caused by the L-type or LC-type grid impedance. Furthermore, low-order distortion harmonics come from the background grid voltages and are maintained as acceptable limits according to the IEEE Std. 1547-2003. Comparative test result with the conventional one also confirms the effectiveness of the proposed control scheme under LC-type grid impedance thanks to the consideration of LC grid impedance in the design process.

Accuracy and Usability of Smartphone-Based Distance Estimation Approaches for Visual Assistive Technology Development.

Goal: Distance information is highly requested in assistive smartphone Apps by people who are blind or low vision (PBLV). However, current techniques have not been evaluated systematically for accuracy and usability. Methods: We tested five smartphone-based distance-estimation approaches in the image center and periphery at 1-3 meters, including machine learning (CoreML), infrared grid distortion (IR_self), light detection and ranging (LiDAR_back), and augmented reality room-tracking on the front (ARKit_self) and back-facing cameras (ARKit_back). Results: For accuracy in the image center, all approaches had <±2.5 cm average error, except CoreML which had ±5.2-6.2 cm average error at 2-3 meters. In the periphery, all approaches were more inaccurate, with CoreML and IR_self having the highest average errors at ±41 cm and ±32 cm respectively. For usability, CoreML fared favorably with the lowest central processing unit usage, second lowest battery usage, highest field-of-view, and no specialized sensor requirements. Conclusions: We provide key information that helps design reliable smartphone-based visual assistive technologies to enhance the functionality of PBLV.

Numerical and Experimental Study on cold rolling process of 5B02 aluminum alloy tubes

Taking the rolling of 5B02 aluminum alloy tubes in Pilger LG50 mill as an example, the finite element analysis of the tube rolling process was carried out by using ABAQUS software. The effects of rolling speed, rotary angle and feeding amount on the inner wall stress of the tubing and the dimensional accuracy and ellipticity of the finishing section in the forming process of Pilger rolling of 5B02 aluminum alloy tubes were investigated through the orthogonal test. The optimal process parameters were preferably selected for the experimental validation. The results show that the stresses in the inner wall of the wall-reduced section of the billet are the highest during the whole rolling process. The average stresses in the inner wall of the reduced wall section of the billet are in the following order: feed>rotary angle>rolling speed. The feed and rolling speed have a greater influence on the dimensional accuracy and ovality of the finishing section. Excessive rotary angle can lead to grid distortion in the reduced-wall section of the billet. The 5B02 aluminum alloy tubes rolled at a rolling speed of 1989mm/s, a rotary angle of 57°, and a feed of 2.5mm meet the requirements of dimensional accuracy, internal surface quality, and metallurgical structure, and improve production efficiency.

A Method for Predicting the Load Interaction between Reinforced Thermoplastic Pipe and Sandy Soil Based on Model Testing

This study aims to investigate the interaction between reinforced thermoplastic pipes (RTPs) and sandy soil. The mechanical properties of sandy soil in the South China Sea region were determined through shear tests to obtain fundamental data. Subsequently, a specialized experimental setup was designed and assembled to study the pipe–soil interaction, specifically measuring the lateral soil resistance of flexible pipes at varying burial depths. Data analysis revealed the relationship between soil resistance, lateral displacement, and initial burial depth. To simulate the mechanical behavior of the pipe–soil interaction, the coupled Eulerian–Lagrangian (CEL) method was employed for numerical simulations. The research findings indicate that the lateral soil resistance is influenced by the uplift height and accumulation width of the soil ahead of the pipe. Within a lateral displacement range of 0.5 times the pipe diameter (0.5D), the lateral soil resistance rapidly increases, resulting in a soil uplift along the circumferential direction of the pipe. This process not only enhances the load-bearing capacity of the pipe but also increases the accumulated soil resistance, consequently expanding the soil failure zone. Furthermore, the ultimate soil resistance exhibits an increasing trend with an increasing burial depth. Once the pipe reaches a certain burial depth, the uplift height of the soil reaches a critical state. To address the grid distortion caused by soil deformation, numerical simulations based on the CEL method effectively modeled the pipe–soil interaction forces under significant lateral displacements, exhibiting good agreement with the experimental results. This study provides a solution for investigating soil resistance in submarine pipelines, thereby contributing significantly to the design and performance prediction of underwater pipelines.

Моделирование процесса прокатки слоистого композита АМг3/Д16/АМг3

Introduction. Over the past decades, laminated composites based on aluminum alloys have been increasingly used in the aerospace and automotive industries. Laminated composites are usually produced by accumulative roll bonding, which results in the metallurgical bonding of initially prepared sheets. Hence, the main task of accumulative roll bonding is to obtain a reliable bond between materials. However, at present, the process of joining similar or dissimilar materials by plastic deformation is still a poorly understood phenomenon. In this regard, in recent years, methods of finite element modeling of the processes of joining materials have begun to develop intensively. The purpose of the work is to establish a relationship between stress-strain state parameters and the formation of a stable bond between aluminum alloys of different compositions. To achieve this goal, the following tasks are formulated: 1. Simulation of the laminated composite “AMg3/D16/AMg3” rolling process using data corresponding to physical experiments carried out at the Institute of Engineering Science of the Ural Branch of the Russian Academy of Sciences; 2. Selection and analysis of the most important stress-strain state parameters of the laminated composite “AMg3/D16/AMg3” rolling process. Research methods. Process simulation system Deform-3D was chosen as the main research tool. Results and Discussion. An analysis of the coordinate grid distortion and velocity vectors of material flow of layers revealed that the deformation is distributed inhomogeneously in the cross section after rolling: the outer layers flow more intensively compared to the middle layer. The maximum scatter of strain intensity ei in the cross section, observed at a maximum reduction ratio of 75%, is 12%. This allows one to accept for analytical calculations in the first approximation the assumption of deformation uniformity. A relationship is established between the beginning of the formation of a bond between composite layers and the threshold expansion of the contact surface and normal pressure at the interlayer boundary. In the final part of the study, future directions for improving the approaches of simulation the laminated composites rolling processes are proposed.

Modular Modeling and Statistical Validation for Grid-Connected FS-MPC-Controlled Matrix Converters

In recent publications statistical model checking (SMC) has been proposed as a method for verifying the performance of finite-set model predictive control (FS-MPC) algorithms applied to power electronics converters. One of the reasons the full potential of the method in the power electronics systems (PES) has not yet been explored is the time consuming modelling process. In this paper we propose a modular method of modelling the power electronics system components by providing simple building blocks, which can be connected to build different PES. The modelling method is here demonstrated on a direct matrix converter, which operates in a stochastic grid with different harmonic distortion levels and voltage sags. By applying the SMC, the performance of the control algorithm in terms of the output current distortion, effects of the weighting factor selection and grid distortions on the device utilization can be evaluated. The obtained results confirm, that high grid distortions and voltage sags will increase the stress of several devices. This information can be of great importance to identify the most stressed components and how the control algorithm can be adapted to extend the lifetime of the components and thereby the system during different grid conditions. The verified FS-MPC algorithm has also been implemented in an experimental set-up.

Material Point Simulation Method for Concrete Medium Fracture and Fragmentation under Blast Loading

The nature of the fracture and fragmentation processes in concrete medium under blast loading is the transformation of the medium from continuum to discontinuity. Coupled with the significant rate correlation of concrete medium, its mechanical behavior presents a high degree of complexity. When tackling this problem, the finite element method (FEM) frequently encounters problems such as grid distortion and even negative volume, whereas the material point method (MPM) can efficiently avoid these problems. Furthermore, the original Holmquist-Johnson-Cook (HJC) model does not take the segmented characteristics of the calculation function for the dynamic increasing factor into consideration. As a result, in this article, first, the calculation function for the dynamic increasing factor in the HJC model was modified by the Split-Hopkinson Pressure Bar (SHPB) experiment, and an improved HJC model was proposed; second, an MPM simulation program was developed, and the improved HJC concrete model was embedded into the simulation program; and finally, the simulation program was verified by numerical examples, and the results show that the developed simulation program can better simulate the fracture and fragmentation process of the concrete medium under blast loading, especially the pulverization characteristics of the medium in the near zone of the load.

Deformation behavior of high-chromium cast iron / low-carbon steel laminates based on hot rolling and finite element simulation

In this study, the hot deformation behavior of high-chromium cast iron (HCCI)/low-carbon steel laminates during hot rolling was studied via finite element simulation and experimentation. The effects of different rolling processes, including rolling reduction (30%, 45%, and 60%), rolling temperature (1000, 1100, and 1200 °C), and rolling speed (0.17, 0.26, and 0.34 r/s), on the forming quality of laminates were discussed. Results show that the shape of the laminates is straight without warping and grid distortion. The middle layer of the low-carbon steel plays a role in stress release. In terms of mechanics, large rolling reduction (60%), high rolling temperature (1200 °C), and low rolling speed (0.17 r/s) are favorable for increasing the interface stress and stress release time. This promotes interface recombination, reduces damage to the brittle HCCI layer, and effectively improves the bending and impact strengths of the laminates. In terms of microstructure, increasing the atomic active energy and diffusion time is beneficial to the formation of the microinterface morphology with high bonding quality and refinement of the HCCI carbide.

Design of a Large Field of View and Low-Distortion Off-Axis Optical System Based on a Free-Form Surface

Free-form surfaces have good aberration correction capability and balance capability for nonrotationally symmetric imaging systems. In this study, we analyzed the quantitative relationship between X–Y polynomial combination and aberration for the efficient design of X–Y free-form optical systems. The purpose of this study was to provide an exhaustive design method for off-axis triple inverse optical systems with X–Y free-form surfaces. Finally, we designed a free-form off-axis optical system with a large field of view (FOV) and low distortion based on the off-axis triple inverse optical system without an intermediate image plane. The primary mirror (PM) of the system adopted an X–Y polynomial free-form surface to correct the aberration of different FOVs of the system and improve the image width and quality. The optical system had a focal length of 1000 mm, an F-value of 9.5, an FOV angle of 23° × 1°, a maximum distortion grid in the FOV less than or equal to −0.05%, and a full-field average wave aberration better than 0.055 λ (λ/18.2, λ = 632.8 nm). The analysis of the design results showed that the system had high-quality imaging and a compact structure. This design method can provide a technical reference for the study of such free-form off-axis systems.

New Modeling and Improved Current Control Strategy to Eliminate the Impact of Synchronization Method and Parks Transformation for Grid-connected Four-leg PWM Inverter

The synchronization method’s ability is one of the fundamental guarantees for the stability and accuracy of injected current control for grid-connected converter-based distribution resources. These synchronization methods, such as phase locked loop (PLL), are generally based on Park transformation and grid voltage regulation, which may affect an unstable phenomenon under distortion and unbalanced grid voltage conditions and result in more computational complexity. In this present paper, analogous to the traditional voltage-oriented control strategy (VOC) in the synchronous rotating frame (dq0-frame) based on PLL and Parks transformation, an improved voltage-oriented control strategy (IVOC) without synchronization methods and Parks transformation is proposed for grid-connected four-leg inverters (GC-FLVSI) to achieve accurate current control with high-quality performance in the dq0-frame. This proposed strategy is not only used for controlling the GC-FLVSI but also to provide the module of GC-FLVSI in the dq0-frame based on the instantaneous active and reactive powers theory (DPC). The proposed IVOC strategy has the same properties and identical performance as the traditional VOC when the grid phase angle is correctly detected by any synchronization method, with the advantages of both traditional DPC and VOC at the same time. In order to validate the superiority and excellent dynamic and steady-state performances of the proposed IVOC strategy in comparison with the traditional VOC strategy, some simulation scenarios using MATLAB/Simulink under different operations and grid conditions have been performed and presented.

Low Computational Burden Model Predictive Control for Single-Phase Cascaded H-Bridge Converters Without Weighting Factor

In this article, a low computational burden model predictive control (MPC) strategy without weighting factor is proposed for the single-phase cascaded H-bridge CHB converters. To reduce the switching state candidates, a hierarchy control algorithm is proposed. The grid current is controlled by selecting a subregion from the designed 2-D control plane, instead of the entire area. Two vectors are chosen in one sampling period for more accurate tracking. Then, the voltage balancing is achieved by selecting the optimal switching state from the subregion candidates to form the above two vectors. The cost function can be constructed of one variable: load voltage. Therefore, the weighting factor can be eliminated. No tuning or retuning processes are required in the proposed method. To reduce the computational time further, the principle of eliminating the switching state candidates operating the same voltage balancing performance is proposed. Conventional and proposed MPC methods are verified by experimental tests via a laboratory setup of a three-cell connected CHB converter. Steady-state and transient operations demonstrate that the proposed method guarantees less distortion grid current and shorter execution time (reduced from 15 to 3 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu s$</tex-math></inline-formula> ). Fast response speed to variations in voltage reference and load resistance can be achieved t.

Structural dynamics of a thermally silent triiron(II) spin crossover defect grid complex.

The structural evolution of spin crossover (SCO) complexes during their spin transition at equilibrium and out-of-equilibrium conditions needs to be understood to enable their successful utilisation in displays, actuators and memory components. In this study, diffraction techniques were employed to study the structural changes accompanying the temperature increase and the light irradiation of a defect [2 × 2] triiron(II) metallogrid of the form [FeII3LH2(HLH)2](BF4)4·4MeCN (FE3), LH = 3,5-bis{6-(2,2'-bipyridyl)}pyrazole. Although a multi-temperature crystallographic investigation on single crystals evidenced that the compound does not exhibit a thermal spin transition, the structural analysis of the defect grid suggests that the flexibility of the grid, provided by a metal-devoid vertex, leads to interesting characteristics that can be used for intermolecular cooperativity in related thermally responsive systems. Time-resolved photocrystallography results reveal that upon excitation with a ps laser pulse, the defect grid shows the first two steps of the out-of-equilibrium process, namely the photoinduced and elastic steps, occurring at the ps and ns time scales, respectively. Similar to a previously reported [2 × 2] tetrairon(II) metallogrid, FE3 exhibits a local distortion of the entire grid during the photoinduced step and a long-range distortion of the lattice during the elastic step. Although the lifetime of the pure photoinduced high spin (HS) state is longer in the tetranuclear grid than in the defect grid, suggesting that the global nuclearity plays a crucial role for the lifetime of the photoinduced species, the influence of the co-crystalising solvent on the lifetime of the photoinduced HS state remains unknown. This study sheds light on the out-of-equilibrium dynamics of a thermally silent defect triiron SCO metallogrid.

ELECTROMAGNETIC AND THERMAL CHARACTERISTICS SIMULATION FOR THREE-PHASE THREE-LIMB TRANSFORMERS UNDER DC MAGNETIC BIAS

Direct current (DC) bias is caused by DC intrusion into the transformer. DC bias will not only cause current distortion and damage electric power grid but also will lead to hot spot overheating and increase in the transformer insulation aging. The effect of DC bias on both electromagnetic and thermal characteristics of transformers should be simultaneously considered for transformer design and operation control. In this study, different levels of DC bias were introduced to a three-phase three-limb transformer. Magnetic flux offset and current distortion were obtained by field-circuit coupling simulation. The winding loss and hot spot for different loads were computed with the analytic method and the electromagnetic-thermal-fluid coupling model. The results indicate that the magnetic flux offset is sensitive to the variation of loads, but insensitive to the increase of DC. As the transformer is overloaded, the current distortion increases, and a peaked wave occurs. The harmonic frequencies of distorted current concentrate on even-order. Compared with the rated sinusoidal load, the hot spot temperature increased by 18.59 K with 10 A DC introduction. The hot spot location is transferred from the upper yoke of the core to the HV coil. As the load increases, the hot spot temperature increases first and then decreases. This study is of great significance to the assessment of DC bias effect.

Study on Burning Surface Regression Algorithm under Erosive Burning Based on CT Images of Solid Rocket Motor Grain

The presence of the erosive burning effect during the operation of a solid rocket motor (SRM) is one of the most important factors affecting the proper operation of the motor. To solve the effects of the operating process of the motor under erosive burning, a synthesis algorithm based on the actual CT images is proposed to combine the Level-set (LS) method with the minimum distance function (MDF) method for the simulation of the burning surface regression of the grain under erosive burning. The Hamilton–Jacobi control equation can be solved exactly for the discrete form of LS. To improve the computational efficiency of the LS method, the minimum distance field is initialized and only the distortion grid is adjusted during the reinitialization. The third-order TVD Runge–Kutta method is used to solve the problem of numerical oscillation and improve the calculation accuracy. The experiments simulate the burning process of the NAWC No. 6 partial grain under erosive burning, which can provide the main basis for the performance design of solid propellant. The experimental results show that the method has good applicability to three-dimensional complex grains. It can realize the simulation of grains under erosive burning and its calculation accuracy is high.

Soft-Switching Smart Transformer Design and Application for Photovoltaic Integrated Smart City Power Distribution

Smart city power distributions have become promising technologies to meet the demand for energy in developed countries. However, increase in smart grids causes several power quality problems on the smart grid, in particular, current and voltage harmonic distortions, sudden voltage sag and swells, fault current, and isolation deterioration. Smart transformers are potential solutions to improve the power quality on the electric grid. They present energy efficiency, ensure grid reliability and power flow control, voltage regulation, bidirectional power flow, fault current limiting, harmonic blocking, and galvanic isolation. Therefore, this paper offers an optimal selection of a three-stage (AC-DC-DC-AC) smart transformer model and power control strategy for solar PV power plant integrated smart grids. The topology of the rectifier, isolated bidirectional converter, and inverter has soft-switching features. This enables low conduction loss, low electromagnetic interference (EMI), high efficiency, achievable zero-voltage switching for converters, and zero-current switching for electrical auxiliary systems. Operation strategies of the proposed ST, PWM control, voltage, and current control between converters, including a medium-voltage (MV) high-frequency transformer to realize a 10 kVA, 450 Vdc to 220 Vdc, or 220 Vac ST, are presented. Significantly, the ST prototype achieves 96.7% conversion efficiency thanks to its control strategy, even under unstable power generation conditions from the solar PV plant. Experimental results obtained on the 344 Vac 10.4 A load current validates the dv/dt rate 6.8 kV/us. The dynamic and experimental results of the proposed bidirectional smart transformer demonstrate the success in preventing power quality problems for photovoltaic integrated smart city power distribution.

Assessment of Sequence Extraction Methods Applied to MMC-SDBC STATCOM Under Distorted Grid Conditions

Under grid distortions, Modular Multilevel Converters (MMC) must adopt proper control strategies to fulfill the power system requirements and ensure a stable operation. An inappropriate control under such conditions may lead to energy unbalances between the MMC legs, inaccurate current injection, and failure in the synchronization process. In this context, sequence extraction methods play a critical role in enhancing the performance of the control, firstly, by aiding the Phase-Locked Loop (PLL) to maintain the synchronization with the AC grid by following the positive sequence fundamental component of the voltage, secondly, by allowing accurate active and reactive currents injection via the decoupled Voltage Oriented Control (dVOC), thirdly, by properly managing the internal energy of the MMC through the circulating current control. In prior researches, some sequence extraction methods have been used for MMC STATCOM. However, the sequence extraction was not the core of the performed studies and their impact on the system behavior has not been highlighted or tested in several grid conditions. This work fills this gap by first assessing the performance of a Single Delta Bridge Cell MMC (SDBC-MMC) STATCOM with four well-known sequence extraction methods (i.e., Decoupled Double Synchronous Reference Frame (DDSRF), Dual Second Order Generalized Integrator (DSOGI), Improved DSOGI, and Fortescue matrix-based (FMB) filter under normal and abnormal grid conditions, then, finding the most suitable one in terms of stability, dynamics, and functionalities.