A novel HVDC circuit breaker for HVDC application

Hybrid high voltage direct current circuit breakers (DCCBs) are capable of interrupting fault current within a few milliseconds, but this technology has high capital cost, especially in a meshed HVDC grid. To increase the economic competitiveness of hybrid DCCBs, this paper proposes a capacitor commutated dc circuit breaker (CCCB). The CCCB mainly comprises an auxiliary branch with a fast dis-connector in series with semiconductor devices and the main branch with the series connection of a dc capacitor and diode valves. This paper provides a detailed depiction of the CCCB. The topology and operating principles are discussed. The impact of snubber circuits and stray inductances on the commutation process is analyzed. The general sizing method for the main components in the CCCB is detailed. Reclosing to transmission lines with different operating conditions is studied. Several extended topologies are proposed to further reduce the semiconductor cost and on-state operation power loss. The power loss and cost of CCCB are assessed. Extensive simulations on PSCAD/EMTDC verified the dc fault isolation and reclosing of the CCCB

A single-end protection scheme for hybrid MMC HVDC grids considering the impacts of the active fault current-limiting control

In the hybrid modular multilevel converter (MMC) based high voltage direct current (HVDC) systems, the fault current can be actively suppressed by the converter itself, which endows a smaller requirement for current-limiting reactors (CLR) and a larger time margin for fault detection algorithms, comparing with the half-bridge MMC. But the robustness to fault resistance and noise disturbance of existing boundary protection schemes will be deteriorated with small CLRs. Moreover, the fast response of the fault current-limiting control will change the output DC voltage of hybrid MMC, which affects the fault characteristics and may cause mal-operation of existing protection algorithms. Thus, a single-end protection scheme considering the impacts of the active current-limiting control is proposed for the hybrid MMC based DC grids. The traveling-wave characteristics under different fault stages are analyzed to evaluate the impacts of the fault current-limiting control. In addition, a coordination protection strategy versus different fault conditions is adopted to improve reliability. Various cases in PSCAD/EMTDC are simulated to verify that the proposed method is robust to fault resistance, fault distance, power reversal, AC faults, and immune to noise

An adaptive fault current limiting control for MMC and its application in DC grid

This paper proposes an adaptive fault current limiting control (AFCLC) for modular multilevel converters (MMC). Without introducing extra current limiting devices, this control scheme enables fast fault current suppression during DC faults. The AFCLC will be triggered automatically once DC faults occur. By adaptively reducing the output DC voltages of MMCs, the fault current can be suppressed. Compared with the existing current limiting methods, the proposed AFCLC has a better performance on fault current limiting, since it only depends on the real-time operating condition and no fault detection delay is imposed. Firstly, the principle of the proposed AFCLC together with the mathematical analysis is disclosed. Then, the sensitivity analysis of the impact of key control parameters on the current limiting effect is investigated. Finally, the effectiveness of AFCLC is demonstrated in a four-terminal HVDC grid test model. The simulation results show that the proposed AFCLC can reduce the interrupted current and energy absorption of a DCCB from 10.39 kA and 38.24 MJ to 4.62 kA and 8.32 MJ, respectively. The simulation results also prove that the AFCLC will not affect the accuracy of the DC fault detection algorithms under DC faults

A unidirectional DC-DC autotransformer for DC grid application

Conventional unidirectional DC-DC converters for DC grid application employ DC-AC-DC two-stage conversion technology and suffer from high converter cost and power loss. To solve these issues, a unidirectional step-up DC-DC autotransformer (UUDAT) and a unidirectional step-down DC-DC autotransformer (DUDAT) are studied. The UUDAT and DUDAT are composed of a series connection of diode bridges and voltage source converters. Topologies of UUDAT and DUDAT are detailed. The harmonic and un-controllability issues are discussed. Control and possible application scenarios for UUDAT and DUDAT are depicted. DC fault isolation mechanism and the methods of dimensioning the voltage and power ratings of the components in UUDAT and DUDAT are studied. Extensive simulations on power system level and experiments on a UUDAT and DUDAT prototype verified their technical feasibility

An adaptive reclosing strategy for MMC-HVDC systems with hybrid DC circuit breakers

Modular multilevel converter (MMC) based high voltage direct current transmission (HVDC) is an effective solution for large-scale renewable power integration over long-distance. In the overhead MMC-HVDC systems, the high voltage DC circuit breakers (DCCB) are implemented to interrupt the DC fault current. Subsequent to fault isolation, the DCCBs are required to automatically re-close to restore power transmission quickly. However, when the DCCBs are re-closed to permanent faults, they will be tripped again, resulting in a high requirement of interruption capacity for DCCBs and second overcurrent strikes on the HVDC systems. To overcome the drawbacks of the conventional auto-reclosing scheme, this paper proposes an adaptive reclosing scheme based on the active pulse injection from the converter associated with the coordination control of hybrid DCCBs. The single-end adaptive reclosing scheme as well as two ends adaptive reclosing scheme dedicated to two-terminal HVDC systems and meshed DC grids are presented respectively. By applying this method, the location of faults can also be achieved in the case of permanent faults. In order to verify the effectiveness of the proposed adaptive reclosing schemes, extensive simulations have been conducted under PSCAD/EMTDC

An improved DC fault protection algorithm for MMC HVDC grids based on modal domain analysis

To detect the DC faults for MMC based DC grids using overhead line transmission, many protection methods in phase-domain have been proposed. These existing protection methods suffer from incomplete function, weak theoretical basis and sensitivity to fault resistance and noise disturbance. To overcome these shortcomings, this paper proposes an improved DC fault protection algorithm using the modal-domain approach for the MMC based overhead DC grids, which decouples interaction between positive and negative poles and mitigates the strong frequency-dependency of the characteristic impedance in phase-domain. The DC fault equivalent circuits are established in modal-domain and the fault characteristics during the initial stage are analysed. Based on the modal-domain analysis, the line-mode reactor voltage which combines fault characteristics of negative and positive reactor voltages, is employed to identify the internal faults. The zero-mode reactor voltage which enlarges the differences between faulty and healthy poles, is employed to select the faulted pole. This method is robust to fault resistance and noise with high detection speed. In addition, it is not affected by power reversal, AC faults and DCCB operation, which are validated and evaluated by simulations in PSCAD/EMTDC

A modified clinically relevant post-operative pancreatic fistula risk evaluation model based on ultrasound shear wave elastography: a prospective study

Objective·To modify previous clinically relevant post-operative pancreatic fistula (CR-POPF) risk evaluation models with quantitative evaluation of pancreatic tissue stiffness by ultrasound shear wave elastography (SWE).Methods·In this prospective study, the patients who were diagnosed as having pancreatic tumors and scheduled to undergo pancreatectomy at Zhongshan Hospital, Fudan University were initially enrolled, whose clinical information was collected. Virtual touch tissue imaging and quantification technology (VTIQ) assessment was applied to the patients within one week before the surgery to measure the shear wave velocity (SWV) of pancreatic lesions and the normal parenchyma of pancreatic body in the superficial layer of the portal vein. During the surgery, the surgeons qualitatively evaluated the stiffness of pancreases via direct palpation and divided them into soft pancreases and medium-hard pancreases. During the 3-week follow-up period after pancreatectomy, CR-POPF was diagnosed according to 2016 International Study Group of Pancreatic Fistula (ISGPF) standard. Peri-operative risk factors of CR-POPF were analyzed by univariate and multivariate Logistic regression to build the prediction model. Evaluation and comparison of diagnostic efficacy and clinical benefits among different models were then performed via receiver operating characteristic (ROC) curve and decision curve analysis (DCA).Results·From September 2021 to March 2022, 100 patients were enrolled in this study, including 33 patients (33.0%) who received pancreaticoduodenectomy (PD) and 67 patients (67.0%) who received distal pancreatectomy. CR-POPF was diagnosed in 35 patients (35.0%) during the 3-week post-pancreatectomy follow-up. Multivariate Logistic regression analysis revealed that the SWV value of the body part of pancreatic parenchyma in the superficial layer of the portal vein [lgOR=-2.934 (95%CI -4.387‒-1.479), P=0.000] and the presence of a non-dilated main pancreatic duct (≤3 mm) [lgOR=0.805 (95%CI 0.274‒1.335), P=0.003] were independent risk factors that significantly correlated with the occurrence of CR-POPF after pancreatectomy. The modified model based on the SWE parameter achieved the area under the ROC curve of 0.842, with the sensitivity, the specificity, the positive predictive value, the negative predictive value and the likelihood ratio of 85.7%, 64.6%, 70.5%, 81.8% and 2.422 in predicting CR-POPF. DCA revealed a better clinical benefit of the modified model compared to the previous prediction models [fistula risk score (FRS) and alternative fistula risk score (a-FRS)].Conclusion·The modified model based on the SWE parameter and identified clinical risk factors can make non-invasive, quantitative and objective evaluation of CR-POPF risk before pancreatectomy, and provide sufficient diagnostic efficacy and clinical benefits

Active current-limiting control to handle DC line fault of overhead DC grid

To handle with the DC line faults in a DC grid, this paper proposed an active current-limiting controller for hybrid MMC. With this active current-limiting control strategy, the requirement of interruption current of DCCB will be significantly decreased, and the investment of DC grid will be reduced obviously. Firstly, the control architecture of active current-limiting controller is disclosed. To avoid the overvoltage of submodule capacitors during DC fault, a dynamic limiter for the reference value of the DC current controller is proposed. To decrease the peak of fault current, the feedforward controller of DC voltage is put forward. The decoupling controllability of the AC/DC voltage of hybrid MMC is disclosed. The current-limiting mechanism of the active current-limiting controller is analysis. Then, the validity of the active current-limiting control strategy is verified by RTDS

Performance Enhancement in Borocarburized Low-Carbon Steel by Double Glow Plasma Surface Alloying

In this paper, the performance of low-carbon steel is enhanced after introducing a borocarburized diffusion layer via double glow plasma surface alloying technology. Due to the boron-carbon gradient structure of low-carbon steel, the protective coating exhibits an excellent wear and corrosion resistance. Interestingly, the borocarburized layer consists of a 64 μm carburized layer and a 27 μm boride layer, which plays an effective role in enhancing the microhardness of borocarburized low-carbon steel, exhibiting a 1440 Vickers hardness increase in the surface microhardness of low-carbon steel. The potentiodynamic polarization measurement and impedance measurement results indicate that the boride protective film can effectively prevent aggressive chloride ions from invading the substrate, which indicates an excellent property of corrosion resistance. This systematic study paves a promising way for the future application of hard coatings in severe environments

Fault Protection Based on the Ratio of Zero-sequence Current Projection Components for Low-resistance Grounding System

The electrical characteristics of a single-phase grounding fault in a low-resistance grounding system was aualyzed, and a fault protection scheme was proposed for low-resistance grounding systems according to the ratio of the zero-sequence current projection components. The primary line selection was performed according to the zero-sequence current projection method and then the phase difference value was used for the secondary judgment. The proposed protection scheme was verified by Matlab/Simulink. The simulation results show that the scheme has a certain ability to withstand transition resistance, which effectively improves the sensitivity of the ground fault protection