Extinction Angle Research Articles

Evaluation Approach and Controller Design Guidelines for Subsequent Commutation Failure in Hybrid Multi-Infeed HVDC System

Due to the difference in output characteristics between the line-commutated converter-based high-voltage direct current (LCC-HVDC) and voltage-source converter-based high-voltage direct current (VSC-HVDC), the hybrid multi-infeed high-voltage direct current (HMIDC) presents complex coupling characteristics. As the AC side is disturbed, the commutation failure (CF) occurring on the LCC side is the main factor threatening the safe operation of the system. In this paper, the simplified equivalent network model of HMIDC is established by analyzing the output characteristics of VSC and LCC. Hereafter, based on the derived model and the control system of LCC-HVDC, the dynamic equations of the extinction angle are deduced. Consequently, by applying the phase portrait method, the causes of CF occurring in the HMIDC system as well as the impacts of control parameters on the transient stability are revealed. Furthermore, the stabilization boundaries for the reference value of the DC voltage are obtained via the above analysis. Finally, the theoretical analysis is verified by the simulations in the PSCAD/EMTDC.

Geology and characteristics of petrographic rocks in the region of Trepça, Kosovo

Purpose. The purpose of this paper is to accomplish a thorough identification of rock types occurring within the Mitrovica region by describing in detail all encountered varieties. The authors aim to determine the discontinuity or continuity of all inter-formational boundaries, crucial for accurate delineation on the ground and complete reflection on a 1:25000 scale map. Additionally, the objective is to identify the nature of contact between rock types and provide a detailed their description. Methods. Field exploration in the Mitrovica region was conducted for several months. Rock identification involved detailed sampling and petrographic analysis, including thin section preparation of magmatic rocks. The method included sample preparation, polarized light source interactions with minerals and observation of optical properties. Key properties observed are birefringence, interference colors, extinction angles, identification and analysis. Findings. Based on the study of stratigraphic units and geological descriptions of mineral outcrop areas, various types of rocks through petrographic microscope preparation, as well as chemical and geochemical analyses, have been differentiated. The Mitrovica area encompasses the following lithostratigraphic units: harzburgite, gabbro, diabase, metasandstone, sandstone, quartzite and grainstone with calcium (Aeolisaccus sp.; Bioclastic grainstone with dacycladal algae and small milliolides; Salpin-goporella sp.). Originality. The originality of the research lies in employing an optical microscope for the precise identification of rocks. Through the re-search carried out in the study area, we obtained a comprehensive petrographic description of mineral composition, texture, and mineralization, facilitating the assessment of the area exploitation potential. Practical implications. The petrographic exploration yielded the conclusion that the Pb-Zn mineralization is present in the study area, which is a sig-nificant finding for the advancement of the mining sector and the local community, provided that environmental preservation measures are upheld and responsible methods of area exploitation are implemented.

Stability Analysis via Impedance Modelling of a Real-World Wind Generation System with AC Collector and LCC-Based HVDC Transmission Grid

Stability Analysis via Impedance Modelling of a Real-World Wind Generation System with AC Collector and LCC-Based HVDC Transmission Grid

Overvoltage Mechanism and Suppression Method for LCC-HVDC Rectifier Station Caused by Sending End AC Faults

This letter analyzes the mechanism of the AC bus overvoltage at the LCC-HVDC rectifier station caused by the sending-end AC faults. Firstly, the influence of commutation failure on relevant electrical quantities is analyzed. After the commutation failure disappears, the inverter switches to constant current control. Under the joint action of the constant current control of the rectifier, the DC current and the trigger angle of the rectifier decrease rapidly, which reduces the reactive power absorbed by the rectifier. With the recovery of HVDC transmission capacity, the sending end AC bus voltage sharply rises, which increases the reactive power output of the filter. Reactive power redundancy occurs in the sending end AC bus leading to overvoltage. The suppression method proposed in this letter can effectively increase the reactive power absorbed by the LCC-HVDC rectifier during fault recovery. Besides, the method can also significantly enhance the minimum value of the inverter extinction angle and avoid possible commutation failure. The effectiveness and applicability of the strategy are confirmed through Electromagnetic Transient simulation.

A fast and high‐accurate commutation failure identification method for LCC‐HVDC system

AbstractCommutation failure (CF) is one of the most common issues in line‐commuted converter‐based high voltage direct current systems (LCC‐HVDC), leading to critical power system security and stability problems. Accurate and rapid identification of CF is crucial to prevent subsequent CF in HVDC systems. However, the existing CF identification methods are lack of the required accuracy and speed. The relationship between bridge arm current, commutation voltage, and trigger pulse based on the commutation progress mechanism after the AC fault is analysed in this paper. A novel CF identification method is presented, which utilises the CF identification factor, enabling fast and high‐accurate identification without relying on the value of the extinction angle. The proposed method offers a simpler and more efficient implementation in engineering practice. Finally, the effectiveness of the proposed method is verified in both the standard International Council on Large Electric systems HVDC model and the Hardware in Loop test using practical project parameters. The results demonstrate that the proposed method can accurately and fast identify CF.

GC Insights: The crystal structures behind mineral properties – a case study of using TotBlocks in an undergraduate optical mineralogy lab

Abstract. Spatial thinking represents an ongoing challenge in geoscience education, but concrete manipulatives can bridge this gap by illustrating abstract concepts. In an undergraduate optical mineralogy lab session, TotBlocks were used to illustrate how crystal structures influence properties such as cleavage and pleochroism. More abstracted properties, e.g., extinction angles, were increasingly difficult to illustrate using this tool.

Analysis of Secondary Controller on MTDC Link with Solar PV Integration for Inter-Area Power Oscillation Damping

Integration of renewable energy sources is important in limiting the continuous environmental degradation and emissions caused by energy generation from fossil fuels and thus becoming a better alternative for a large-scale power mix. However, an adequate analysis of the interaction with the alternating current (AC) network during network disturbance, especially during inter-area power (IAP) oscillations is needed. Insufficient damping of oscillations can significantly impact the reliability and effective operation of a whole power system. Therefore, this paper focuses on the stability of the modified Kundur two-area four-machine (MKTAFM) system. A robust secondary controller is proposed and implemented on a line commutated converter (LCC)-based multi-terminal high voltage direct current (MTDC) system. The solution consists of a local generator controller and the LCC MTDC (LMTDC) system, voltage-dependent current order limiter, and extinction angle controller. The proposed robust controller is designed for the LMTDC systems to further dampen the inter-area power oscillations. Three operational scenarios were implemented in this study, which are the local generator controller and double circuits AC line, local generator controller with LMTDC controllers, and local generator controller with LMTDC controllers and secondary controller. The simulation result carried out on PSCAD/EMTDC recorded better damping of the inter-area power oscillation with LMTDC. A considerable improvement of 100% damping of the IAP oscillations was observed when a secondary controller was implemented on the LMTDC.

Harmonics Interaction Mechanism and Impact on Extinction Angles in Multi-Infeed HVDC Systems

Harmonics interaction is an important factor in causing concurrent commutation failures (CCF) of multiple converter stations in multi-infeed HVDC (MI-HVDC) systems, but it was not studied sufficiently in existing literature. In this paper, the mechanism analysis and quantitative calculation of harmonic interaction are investigated. Firstly, the generation and interaction process of harmonic during AC fault events in the MI-HVDC systems with single-terminal connection and hierarchical connection on receiving-end are illustrated, respectively. Then, the common characteristics of harmonic interaction in generalized MI-HVDC systems are concluded. Moreover, to evaluate the impact of harmonic interaction on commutation process quantitatively, an equivalent circuit model of harmonic transfer in MI-HVDC system is established. Based on this equivalent model, the harmonic voltage transfer coefficient is deduced, and the extinction angle of inverters considering harmonic interaction in MI-HVDC systems can be calculated. Finally, the calculation accuracy based on equivalent model is verified via the electromagnetic transient simulation of MI-HVDC system. Simulation results show that harmonic interaction will eventually cause the formation of harmonic source in each converter station of MI-HVDC systems. These harmonic components will change the extinction angle of remote inverters to increase the risk of CCFs.

Study of Cyber Attack’s Impact on LCC-HVDC System With False Data Injection

Against the crippling assault of power grid, the impact of cyber attack on line-commutated converters based high voltage direct current (LCC-HVDC) system is investigated in this paper when the false data injection (FDI) attacks its control loops. Based on the basic control strategy of LCC-HVDC, the impact on each HVDC control mode is investigated when FDI occurs, including the rectifier side constant DC current control and supplementary damping control, the inverter side constant DC voltage control and constant extinction control. Through the theoretical analysis, it is found that the rectifier side FDI attack would limit the power transmission and increase HVDC’s reactive power demand, which will finally lead to instabilities, where the damping ratio could also be decreased if the rectifier supplementary damping control is attacked. Another important fact is that the inverter side attack could have much more severe impact when the false data is injected because the inverter extinction angle may be decreased and causes commutation failures. The above analysis and conclusions are validated by the simulations finally.

Suppression to concurrent commutation failure of UHVDC with hierarchical connection mode based on improved dc current prediction

AbstractFor the ultra high‐voltage dc (UHVDC) transmission with hierarchical connection mode (HCM) at the inverter side, local commutation failure (LCF) at one layer after the ac fault may cause concurrent commutation failure (CCF) at the non‐fault layer. The inter‐layer couplings at the ac and dc sides add the difficulty to suppress the CCF. The fault instant may decide the dc current variation during the commutation, which affects the suppression effect. This paper shows the mechanism of the CCF including the inter‐layer ac and dc coupling, and proposes a control method for the inverter at the fault and non‐fault layers. First, an analytical expression among the dc current, ac voltage, and extinction angle of both layers is newly derived to find the dominant factors to the CCF. Second, to improve the calculation accuracy, a three‐point sampling function using Newton interpolation is newly proposed to predict the dc current. Finally, a coordinated control strategy based on the constant extinction area, the overlap‐arc area, and the dominant factor of the CCF is proposed to adjust the firing angle and suppress the CCF. The simulation results using the PSCAD/EMTDC software are given to verify the control effect of the proposed method against the CCF.

Fault Security Region Modeling and Adaptive Current Control Method for the Inverter Station of DC Transmission System

Line-commuted converter-based high-voltage direct current (LCC-HVDC) has undergone extensive development in power systems. Commutation failure (CF) in LCC-HVDC poses a considerable threat to the safe and stable operation of power systems. The coupling of electrical quantities, including DC current, DC voltage, AC voltage, and extinction angle (EA) of an inverter station, is a critical factor that makes CF difficult to suppress effectively. Existing methods adopt preset fixed control parameters and a single control target. The demand of CF recovery typically cannot be satisfied. Control even becomes the driving force of subsequent CF (SCF). Accordingly, a new concept of a fault security region (FSR) in an inverter station under the effects of multi-electrical quantities is proposed. The security range under the coupling effects of electrical quantities is described by the active power and exchanged reactive power of the inverter station. EA, active power, and exchanged reactive power are coordinated through the adaptive regulation of DC current. Thus, SCF can be prevented under the condition that the effect of varying active power and reactive power of LCC-HVDC on a power system is balanced. First, the coupling characteristics of multi-electrical quantities of the inverter station are analyzed. Second, the modeling method for the FSR in the inverter station is proposed. Third, an adaptive current control method is developed by analyzing the evolution characteristics of the FSR. Moreover, a calculation method for the DC current order value is presented. Finally, the effectiveness of the proposed method is verified using the CIGRE HVDC standard system.

Predictive Commutation Failure Suppression Strategy for High Voltage Direct Current System Considering Harmonic Components of Commutation Voltage

The commutation failure of high voltage direct current (HVDC) systems could lead to unstable operation of the alternating current/direct current (AC/DC) hybrid power grid. The commutation voltage distortion caused by harmonics is a considerable but unclear factor of commutation failure. According to the control switching process of HVDC systems, the commutation voltage-time area method is employed to analyze and reveal the influence mechanism of harmonic components of commutation voltage on first and subsequent commutation failures. Considering the distortion characteristics of AC voltage, a predictive commutation failure suppression strategy considering multiple harmonics of commutation voltage is proposed. In this strategy, the new extinction angle and the zero-crossing offset angle after voltage distortion are calculated considering the harmonic components so as to obtain the compensation margin of the lag trigger angle by combining the correction margin with the voltage change rate. Moreover, the tuning method of parameters of extinction angle and voltage prediction variables are provided. Finally, a case study based on CIGRE standard HVDC system is performed and analyzed by using power systems computer-aided design (PSCAD) software. Compared with the International Council on Large Electric systems (CIGRE) standard test model and traditional commutation failure prevention (CFPREV) control model, the results verify that the proposed strategy can effectively reduce the risk of first and subsequent commutation failures and improve the sensitivity of CFPREV control.

Fast and accurate evaluation method of commutation failure in line‐commutated converter‐based high‐voltage direct current systems considering probability characteristics

Line-commutated converter-based high-voltage direct current (LCC-HVDC) systems suffer from commutation failure (CF) when receiving-end AC grid faults occur, which is a major drawback and threatens the security and stability of power systems. This paper presents a fast and accurate evaluation method of CF for the LCC-HVDC systems, considering the CF probability characteristics. Firstly, based on the analysis of the CF probability characteristics from the view of short-circuit impedance, the CF tolerance index (CFTI) and CF vulnerability index (CFVI) are proposed to quantitatively evaluate the LCC-HVDC systems' risk of the possible- and inevitable-occurrence CF, respectively. Secondly, four factors, that is, commutation voltage, fault occurrence time, DC current and firing angle command, are considered to calculate CF evaluation indices (CFEIs) and identify the CF risk area boundaries based on the theoretical analysis. Moreover, the relationships among short-circuit impedance, fault position, and extinction angle are established based on the AC/DC decoupling method, which helps realize the decoupling of multi-factors affecting CF during fault. Compared to earlier methods, this evaluation method combines the advantages of the accuracy of the electromagnetic transient simulation method and the rapidity of theoretical derivation. Finally, the validity and accuracy of the evaluation method are verified in the CIGRE HVDC benchmark model and IEEE 39-bus system.

A novel method to predict and prevent commutation failures in LCC-HVDC systems

Commutation failures are the main drawbacks of the line-commutated converter-based high-voltage direct-current transmission systems. In this paper, the influences of the fault occurring moment on the commutation process are investigated quantitatively to predict and prevent commutation failures effectively. Then, the critical fault voltage and extinction angle calculation methods are presented based on the voltage–time area theory. The concept of the commutation failure domain and the security domain is determined to estimate the probability of commutation failures. Furthermore, the direct current and RMS voltage calculation methods under asymmetrical faults are presented. Based on them, we eventually propose a novel commutation failure prediction and prevention strategy. Compared with conventional methods, the proposed strategy takes account of the probabilistic characteristic of commutation failures. Moreover, the direct current is predicted rather than real-time measured, and the RMS voltage under asymmetrical faults can be calculated instantaneously. Therefore, the proposed strategy can obtain a rapid response and a stronger commutation failure immunity. The accuracy of the method proposed in this paper is validated by EMT results with PSCAD/EMTDC and the experimental results with RTDS.

Estimation of a surface magnetization direction of thin cylinders by magnetooptical Kerr effect

Magneto-optical Kerr effect (MOKE) imaging is a versatile technique for probing a surface magnetization of magnetic materials. Here, we investigate the influence of the polarizers' rotation on the resulting magnetooptical contrast of thin magnetic wires with cylindrical geometry. Employing MOKE microscopy, we image the magnetooptical contrast change and quantify the contribution of the spatially curved cylindrical surface to the overall magnetic contrast. The calculated magnetooptical contrast difference for various extinction angles between polarizer and analyzer provide an estimation of the surface magnetization orientation at curved surfaces. The magnetooptical analysis scheme can be extended to other non-planar magnetic surfaces.

An optical sensor combining surface plasmon resonance, light extinction, and near-critical angle reflection, for thin liquid film biochemical sensing

We present a novel and low-cost optical sensor to obtain at the same time, fast and label-free optical information such as reflectivity, refractive index and extinction coefficient of biochemical samples. The sensor measures in around one minute optical internal reflectivity as a function of the angle of incidence at the interface between a glass prism covered by a gold nanofilm (≈20 nm) in contact with a liquid sample (≈10 µL). The nanofilm is thin enough that a large fraction of the incident light transmits through it at angles of incidence below the critical angle set by the prism and sample refractive indices (≈ 61°), but thick enough to present a Surface Plasmon Resonance dip for angles of incidence beyond that critical angle. The calculated sensitivity of SPR to the surface refractive index is 168.35 deg/RIU. We demonstrate that when a sample contains live bacteria (E. coli) the whole reflectivity curve including the SPR region is clearly affected, and to adjust a theoretical model to the experimental curves, it is necessary to assume a complex refractive index of the nanofilm appreciably different than that of gold. The optical curve of the sensor provides more information than a conventional SPR measurements offering a more complete strategy for the analysis of biochemical substances and bacterial cell suspensions.

Birefringent properties of aqueous cellulose nanocrystal suspensions

Birefringence measurements of aqueous cellulose nanocrystal (CNC) suspensions are reported. Seven suspensions with concentrations between 0.7 and 1.3% per weight are sheared in a Taylor-Couette type setting and characterized using a birefringent measurement technique based on linear polarized light and acquisition with a polarization camera. Steady state measurements with shear rates up to 31 1/s show extinction angles of 23°–40° and birefringence in the order of 1e–5. The findings demonstrate the utility of CNC suspensions for flow birefringence studies.

Multiple commutation failure suppression method of LCC-HVDC transmission system based on fault timing sequence characteristics

Aiming at the problem of commutation failure and the poor transient response of line commutated converter-based high-voltage direct-current (LCC-HVDC) transmission systems, this paper starts from the harmonic components of an AC system fault on the inverter side. The quantitative relationship between the harmonic wave and the trigger angle of the inverter side is derived in detail, and the effect of the trigger on the commutation of the LCC-HVDC transmission system is qualitatively analyzed with a phase-locked trigger. On this basis, an improvement scheme for an anti-harmonic control system is proposed. First, a virtual separated phase-locked control method is used to determine the fault occurrence time. Additionally, the characteristics of the trigger angle of the rectifier side and extinction angle of the inverter side after the fault are used to determine the fault position and generate the corresponding fault timing sequence. Then, the generated fault timing sequence is used to control the notch filter and modify the input voltage of the voltage-dependent current order limiter to improve the robustness of the control system in terms of the harmonics. Finally, based on CIGRE benchmark LCC-HVDC transmission system model, the advantages of the improved method are verified. The results can provide a reference for actual projects.

Research on Battery Energy Storage STATCOM Suppressing HVDC Commutation Failure

Due to the poor performance of traditional STATCOM in DC engineering, a compensation method using battery energy storage STATCOM (STATCOM/BESS) to suppress commutation failure of the weak receiving-end high voltage direct current transmission system is proposed, and its effect is better than traditional STATCOM. First, the mechanism and influencing factors of commutation failure are analyzed; the extinction angle of the converter valve is taken as the decision index of commutation failure; the relationship between various electrical parameters and extinction angle is studied under the condition of the single-phase grounding fault on the inverter side. Second, according to the differences between the positive and negative sequence reactive current output by STATCOM/BESS when a fault occurs in single-phase grounding, a compensation method is proposed to suppress commutation failure by STATCOM/BESS, which increases the proportion of positive sequence reactive current output by STATCOM/BESS under the condition of constant output so as to reduce the probability of commutation failure. At last, STATCOM/BESS is added into the standard model of LCC-HVDC for the simulation experiment and compared with conventional running characteristics of STATCOM in order to verify the effect of STATCOM/BESS in suppressing commutation failure.

Synthesis and characterization of chemical vapour deposited pyrolytic graphite

Pyrolytic graphite (PyG) is the candidate process crucibles material (containers/liners) in pyro reprocessing applications of spent metallic nuclear fuels. In the present work, the chemical vapor deposition (pyrolysis) temperature effects on the density, texture, degree of preferred orientation, defects, graphitization and growth kinetics of PyG were investigated. The sink and float technique, polarized light microscopy, X-ray diffraction, laser Raman spectroscopy (LRS), X-ray photoelectron spectroscopy (XPS), and electron microscopy techniques were used. The near theoretical density (>2.2 g/cm3) was achieved at pyrolysis above 2100 °C. The preferred orientation by extinction angle and orientation density measurement reveals an increasing degree of texture and anisotropy with increasing pyrolysis temperature. The residual strain and defects concentration inferred from the blue shift of the G band position and integrated ratio ID/IG in LRS analysis shows that intermediate temperature pyrolysis at 1800 °C is more defective. The degree of graphitization defined by sp2:defect ratio in the peak fit analysis of C1s XPS peak show 2400 °C processed PyG is highly graphitized while at 1800 °C show higher fractions for sp3, CO content. The crystallite domain sizes parallel and perpendicular to the c-axis show an increasing ordering with pyrolysis temperature. The activation energy calculated for the surface pyrolysis reaction is ∼60 kJ/mol for the surface kinetics controlled regime between 1400 and 2200 °C, producing a conformal, smooth, and uniform PyG growth as evident by the Volmer-Weber 3D island growth model.