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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]

Flexible DC and AC transmission lines can be arranged across vertically or sharing transmission corridor. Under this condition, the possibility of intersystem fault increases significantly, which endangers the safety of both DC and AC systems. Here, the intersystem fault between bipolar half‐bridge MMC‐HVDC system and AC system is analysed. The reliability of DC wavefront protection after intersystem fault is analysed based on initial traveling wave, while the reliability of DC intersystem fault protection and AC protection is analysed based on 50 Hz component characteristics. Since wavefront protection cannot be guaranteed to operate reliably, intersystem fault protection based on 50 Hz components is applied to DC system to reliably detect intersystem faults and isolate the fault by commanding DC circuit breaker to trip. Although current differential relaying and zero‐sequence overcurrent relaying in AC system can reliably detect intersystem faults, a large DC offset prevents effective fault isolation of AC circuit breaker. This highlights the necessity of MMC‐HVDC system to reliably isolate intersystem faults. [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]

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]

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]

• The Bergeron model error resulting from parameter deviation is discussed, followed by a line parameter adjustment strategy to mitigate its impact. • The analysis of the coupling mechanism between the differential current and the fault current is followed by a proposed modified method to eliminate the 0-mode line parameter error. • A novel time-domain current differential protection scheme with high sensitivity is proposed in this paper. The influence of line parameters is addressed by adjusting α - and β -mode parameters and eliminating the 0 -mode parameter from the criterion. Integrating large-scale renewable energy sources in modern power systems has altered fault characteristics, resulting in the performance degradation of conventional current differential protection. This paper proposes a novel high-sensitivity time-domain current differential protection scheme. The proposed scheme utilizes the Bergeron model to eliminate the impact of the distributed capacitive current. Furthermore, the Bergeron model error is analyzed theoretically during normal operation, and an algorithm is designed to identify and adjust the α- and β-mode line parameters. Additionally, novel internal fault criteria are presented to mitigate the negative impact of the 0-mode line parameter error. Compared to traditional frequency-domain approaches, the proposed method can more accurately and rapidly distinguish internal and external faults, while reducing the dependency on precise line parameters. Eventually, a VSC-HVDC sending-end grid model consisting of 100% renewable power generations is established using PSCAD/EMTDC software. Simulation results confirm that the proposed protection scheme exhibits exceptional performance in terms of operating speed, sensitivity, and tolerance to fault resistance and noise. [ABSTRACT FROM AUTHOR]

• The influence of DFIG-fed fault current characteristic on measured impedance the frequency estimation of DFIG-fed fault current by MUSIC algorithm are analyzed. • The improved distance protection method based on dynamic frequency estimation is proposed. • Compared with the existing methods, the improved distance protection can prevent the influence of the frequency deviation of the fault current on the traditional distance protection. In addition, the delay time of the protection can be significantly shorted compared with the fixed-delay method due to the fast estimation of the transient frequency. The frequency of the fault current fed by the wind farm may deviate from the fundamental frequency after the line fault. This characteristic seriously affects the performance of the traditional distance protection. This paper proposes an improved distance protection method based on dynamic frequency estimation. After the protection is started, the dynamic frequency is estimated by multiple signal classification algorithm firstly. The distance criterion is not activated until the estimated frequency is close to the fundamental frequency. Accordingly, the influence of the frequency deviation of the fault current on the traditional distance protection is prevented. The delay time of the protection can be significantly shorted compared with the fixed-delay method due to the fast estimation of the transient frequency. Finally, the RTDS simulation model is built to validate the correctness of theoretical analysis and superiority of the proposed method. The fault recording data of practical engineering is also discussed to further verify the feasibility of the proposed method. [ABSTRACT FROM AUTHOR]

• The paper proposed a dc fault transient analysis method for hybrid HVDC transmission system considering traveling wave propagation and converter control response. • The traveling wave propagation process is divided into several periods according to the times of reflection and refraction at fault point, then the multiple traveling waves can be easily calculated by building propagation coefficient matrices. • The converter control response under traveling wave impact is calculated by the closed-loop transfer function formed by the electrical equivalent circuit and converter control system. By introducing the control response as a compensation component into traveling wave, the calculation of traveling wave and control response is decoupled. • Compared with existing methods, the proposed method can achieve accurate analysis during the whole fault transient process, because the traveling wave propagation is simplified and the fault control strategies to limit rising current are considered. • By decoupling the interactive effect between traveling wave and control response, the effects of different control strategies on fault transients and protection performance can be analyzed independently. For example, the impact of FBSM-based fault-current-limiting control on traveling-wave-based directional protection is evaluated by the proposed method, which proves that the fault-current-limiting control does not change the valid time of the protection while guaranteeing the sensitivity to forward and backward faults. Accurate fault analysis is essential for protection and control design of hybrid HVDC transmission system. This paper proposes a dc fault transient analysis method, which considers both the traveling wave propagation process and converter control response. Firstly, the multiple reflected and refracted traveling waves at line terminals as well as fault point are calculated by building propagation coefficient matrices. Secondly, the transfer function between the converter equivalent source and the traveling wave transients is built, then the fault current and voltage are calculated by compensating multiple traveling waves with corresponding control response. Compared with existing methods, the proposed method can calculate long-period traveling waves in a simplified way, and implement more accurate analysis for the converter response caused by the basic control strategy and the fault control strategy. An LCC-MMC HVDC system is built based on PSCAD/EMTDC, and the simulation cases are carried out to verify the correctness and superiorities of the proposed method. Finally, the impact of control response on the performance of traveling-wave-based directional protection is evaluated by the proposed fault analysis method. [ABSTRACT FROM AUTHOR]

• A lateral coordination scheme for zero-sequence overcurrent relaying is proposed. • The mechanism and influence factors of unbalanced current are researched. • The impact of ground faults on unbalanced current of adjacent circuit is analyzed. With features of large transmission capability and high utilization efficiency of transmission corridor, multi-circuit transmission lines sharing tower or corridor are widely applied in extremely high voltage (EHV) systems. However, mal-operation of zero-sequence overcurrent relaying for multi-circuit transmission lines could occur, and this paper is focused to solve this problem. Overview of typical multi-circuits transmission lines, including phase arrangement and erection form, are introduced first. Then, simulation model of a typical 500 kV triple-circuit transmission line in real project is established in Power Systems Computer Aided Design (PSCAD), and it is verified by real record data provided by power utility in China. By using this simulation model, factors on current unbalances are discussed in detail, and the simulation results have shown that mal-operation of zero-sequence overcurrent relaying is caused by large unbalanced current in normal operation and under faults. Consequently, a lateral coordination scheme for zero-sequence overcurrent relayings among multi-circuit lines is proposed to avoid the mal-operation caused by unbalanced current to the greatest extent without receiving samples from remote end or adjacent circuits, and the reliability of protective relaying is improved significantly. [ABSTRACT FROM AUTHOR]