Search

DC circuit breaker (DCCB) is one of the most important technologies for safe and reliable operation of dc grid. And the hybrid DCCB (HCB) is obviously promising in high-voltage application area, because it combines both the advantages of mechanical DCCB and solid-state DCCB. At present, insulated gate bipolar transistors (IGBTs) are generally used for configuring the main breaker branch in the HCB. Compared with IGBTs, thyristors have the merits of ability to bear higher surge current, higher reliability, and lower cost, thus being more economical and reliable when used to replace IGBTs. Right now, several topologies of thyristor-based HCBs have been proposed, but the economy, reliability, control complexity, as well as the operation performance, can still be improved. In this article, a novel passive thyristor-based HCB is proposed. Compared with existing IGBT-based HCBs, the proposed HCB has significant advantages on economy and reliability, due to the application of thyristors for replacing IGBTs. In addition, in the proposed scheme, the required thyristors count is reduced and the additional power supply for capacitor precharging is saved, so the investment and control complexity are both cut down. The feasibility and superiorities of the proposed HCB are validated with scaled-down experiment test and 500-kV-level simulation case study. [ABSTRACT FROM AUTHOR]

Fast and reliable direction criterion is important for the protection of multi-terminal HVDC grid, not only for direction pilot protection, but also for excluding backward fault in single-ended protection. The direction criteria based on dc current variation (Δi) or dc current change rate (di/dt) have been used in the dc transmission system. However, the acting reliability is influenced by the high-transition resistance, line distributed capacitance, and so on. Although the direction criteria based on the boundary effect of the dc reactor have excellent operation performance in dc grid, they are only feasibility when the dc reactors are installed on both ends of each dc line. In this paper, the traveling-wave (TW) based direction criterion, which is not based on the boundary components, is proposed for use in multi-terminal HVDC grid, and its applicability is analyzed. To improve the acting sensitivity and reliability of the TW based direction criterion, an improved transient TW based direction criterion is proposed. Finally, simulations based on the Real Time Digital Simulator (RTDS) are carried out to verify the feasibility and superiority of the proposed improved direction criterion. [ABSTRACT FROM AUTHOR]

The overhead line will be widely applied in the future flexible high-voltage dc (HVDC) grid. Correspondingly, effective reclosing strategy which can identify the fault property must be configured for the dc circuit breaker (DCCB) to improve the system power supply reliability. However, if the DCCB is reclosed directly, large second overcurrent may occur to damage the vulnerable devices in the system. Therefore, the inherent voltage characteristic difference between different dc faults (nonpermanent or permanent) is analyzed in the paper. And then a novel DCCB reclosing strategy for the flexible HVDC grid is proposed. Compared with the method which recloses DCCB directly, the proposed strategy can avoid the second damage under permanent fault condition, because it can identify the fault property without completely reclosing the DCCB (only the residual dc current breaker is reclosed before fault property identification). Moreover, compared with the typical sequential auto-reclosing strategy, the proposed strategy significantly reduces the additional control and design requirements on the DCCB. Finally, simulation cases based on the PSCAD/EMTDC are carried out to verify the correctness of the theoretical analysis and the superiorities of the proposed reclosing strategy. [ABSTRACT FROM AUTHOR]

In the flexible dc grid, the dc fault propagates extremely fast. It requires the dc protection and the dc circuit breaker (DCCB) can identify and isolate the fault line very quickly, for the healthy network ride-through, especially in the multi-terminal dc grids. However, the existing protection and DCCB techniques for flexible dc grid can still not match the dc fault propagation speed. Therefore, a novel current-commutation-based fault current limiter (FCL) is proposed in this paper. It can limit the dc fault current effectively, to guarantee the converter continuous operation and healthy network ride-through after dc faults, so the requirement on the protection and DCCB acting speed can be decreased. Compared with the existing fault current limiting methods, the proposed FCL has tiny negative influence on the system transient response speed, operation stability, and DCCB isolating speed. The experiment tests and simulation case studies are carried out to verify the working principle and superiorities of the proposed novel FCL.3 [ABSTRACT FROM AUTHOR]

The fast dc line protection is one of the key techniques for the multi-terminal flexible dc grid whose main technical difficulty lies in quick and reliable fault identification. The traditional protection principles for the ac system are not suitable for the flexible dc grid, and those for the conventional dc transmission system can also not be applied directly. Therefore, the frequency-domain traveling-wave boundary characteristics after faults are analyzed first in this paper to find the critical condition for distinguishing the internal and external faults as well as the forward and backward faults. And then, a novel single-ended transient-voltage-based protection strategy for flexible dc grid was proposed, which mainly consists of the fault identification criterion and the direction criterion. The protection scheme used wavelet transform to extract transient voltage energy. In order to eliminate the influences of the wavelet transform boundary effect and converter blocking, the principle to select the proper data window was proposed. Compared with the existing traveling-wave-based dc protection, the proposed method has a higher reliability and stronger ability to endure high transition resistance, which was verified by lots of simulation cases based on the PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

Medium voltage direct current (MVdc) distribution system has numerous advantages over medium voltage alternative current distribution system. However, two shortcomings of the existing hybrid direct current circuit breaker (DCCB) with load current switch, including high construction cost and high operating losses, have constrained the application of existing hybrid DCCBs in MVdc distribution system. To overcome these two shortcomings, a novel multi-port hybrid DCCB with negative voltage source (NVS) has been proposed in this paper. First, the general topology and working principle of the multi-port hybrid DCCB with NVS are presented. Then, focused on the current commutation method and economic performance, comparisons between different hybrid DCCBs have been carried out. The analysis shows that by using multi-port hybrid DCCB with NVS, the construction cost could be reduced dramatically and the operating losses could be reduced to be comparable with alternative current circuit breaker, which are negligible. In the end, to fill the blank of experimental research, based on the topology of multi-port hybrid DCCB with NVS, a prototype with rating voltage of 2 kV has been established, and the test result have verified the multi-port hybrid DCCB with NVS. [ABSTRACT FROM AUTHOR]

DC fault current limitation is important for the low-voltage dc distribution network, because the existing dc selective protection cannot act fast enough. However, the typical dc fault current limiting method—installing dc reactors directly in the dc system—will lead to negative influence on the system normal operation and dc circuit breaker (DCCB) fast isolation. This paper proposed a novel solid-state circuit breaker (SSCB) with self-adapt fault current limiting capability, namely, no negative influence on the dc distribution network normal operation, swift fault current limiting response to the dc fault, and efficient cooperation between the fault current limitation and isolation. The hybrid configuration strategy of the proposed SSCB and mechanical DCCB for multiterminal dc distribution network is proposed to reduce the conducting power loss and investment from the scale of the whole system. Finally, the experiment tests and simulation cases are carried out to verify the working principle and superiorities of the proposed SSCB. [ABSTRACT FROM AUTHOR]

This paper investigates the effects of different passivation layers (PVLs) on the electrical performance and reliability of amorphous indium gallium zinc oxide (a-In–Ga–ZnO) thin film transistors (TFTs). By rational design, the fabricated a-InGaZnO TFT with hafnium oxide and aluminumoxide (HfO2/Al2O3) dual PVLs exhibits a field-effect mobility of 13.5 cm2/Vs, low sub threshold swing of 0.32 V/decade, and especially, small threshold voltage shifts of 0.5 (−0.6) V and 1.1 (−1.2) V under positive (negative) gate bias, and light illumination stress at the relative humidity of 40%. Furthermore, the a-In–Ga–ZnO TFTswith HfO2/Al2O3 dual PVLs maintain reasonable mobility and electrical performance even exposure to ambient condition for up to four months. This enhanced stability is attributed to the presence of high-quality HfO2/Al2O3 dual PVLs, which not only could suppress the photodesorption, reduce the total trap density and subgap photoexcitation behavior, but also protect the channel from environmental effects. Thus, the rational-designed HfO2/Al2O3 dual PVLs passivated a-In–Ga–ZnO TFTs with superior reliability represent a great step toward the achievement of long-term reliable zinc oxide-based oxide TFTs. [ABSTRACT FROM AUTHOR]

There is an urgent need for the fault current limiter to guarantee the stability of the dc system. For this reason, the improved inductive superconducting fault current limiter (SFCL) is designed and its working principle is addressed. By analyzing the transient characteristics of the dc fault in modular multilevel converter based dc system, the current-limiting effects of the proposed SFCL are studied in detail. According to the current-limiting requirements and the coordination among the converter, the fault current limiter and the dc circuit breaker, the desired impedance of the SFCL is discussed in this paper. Finally, the effectiveness and correctness of the analyses have been verified by the simulation results based on the PSCAD/EMTDC simulation [ABSTRACT FROM PUBLISHER]

Penalized iterative algorithm, which outperforms the maximum likelihood reconstruction by utilizing the image's prior, has became a standard reconstruction approach in PET (Positron Emission Tomography). Such an algorithm is usually called PML (Penalized Maximum likelihood) algorithm. However, this kind of method still suffers from noisy propagation and edge blurring. Recently, the MRP-like (Maximum Root Prior) reconstruction which combines with anisotropic diffusion filtering, taking PDEMedian (Partial Differential Equations Median) for example, was proposed for noise removing and it could obtain better results than traditional algorithms. However, due to the shortcoming of P-M diffusion model, step artifacts could still be observed in the reconstructed image. This work aims to improve the quality of PET images using a novel nonlocal fuzzy diffusion model. Compared to the PDEMedian method, the proposed approach can impose an effective edge-preserving and noise-removing diffusion model for PET image reconstruction. Experimental results showed that the proposed method is more effective in improving quality of reconstructed images comparing with other common methods. [ABSTRACT FROM PUBLISHER]