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In the field of fault isolation and clearance in flexible DC power grids, hybrid DC circuit breakers (HDCCB) have been widely used because of their advantages. However, their development is limited by high construction costs and inapplicability in some fault cases. In addition, adaptive reclosing capability is required for overhead DC lines with high fault probability. Therefore, this paper proposes a new type of multi-port hybrid DC circuit breaker (MP-HDCCB) with current limiting, breaking, and self-charging capabilities. The capacitor polarity reversal circuit is designed to reduce the influence of capacitor pre-charging on the normal operation of the system and to solve the problem of two consecutive line faults occurring in a short period of time. In addition, the circuit breaker can limit the current and break in the face of a single line fault, multiple line faults at the same time, and bus faults. It also has the ability of adaptive reclosing and normal operation of the line after a bus fault. Based on the proposed topology, a 500 kV four-terminal DC system is designed, and a model is established in PSCAD/EMTDC to verify the capabilities of the circuit breaker. Results show that the circuit breaker has adaptive reclosing capability and can realize fault current limiting and breaking under different fault conditions. [ABSTRACT FROM AUTHOR]

In recent years, with the rapid development of high-voltage flexible direct current (DC) transmission systems, more and more multi-port high-voltage DC systems have been successfully put into operation. Therefore, the number of connecting ports on the DC bus is gradually increasing, which necessitates higher requirements for clearing fault current. To better protect the DC power grid, this paper integrates a DC fault current limiter with a multi-port DC circuit breaker. This article proposes a modular cascaded multi-port hybrid DC circuit breaker topology. It also elaborates on the topology structure, working principle, and control program under three different fault conditions. It completes the theoretical derivation and analysis of the circuit breaker fault isolation process, as well as the parameter design of the core components. Finally, a model is established in PSCAD/EMTDC for simulation verification. The application performance of the circuit breaker in four-terminal DC systems is studied and analyzed, and the current-limiting effect of the multi-port DC circuit breaker proposed in this article is demonstrated. In addition, the disconnection performance and energy consumption of the arrester have been improved to a certain extent, which has good feasibility and economy. [ABSTRACT FROM AUTHOR]

Hybrid circuit breakers are the most promising circuit breakers among DC circuit breakers. Conventional hybrid circuit breakers complete fault current isolation by converting current to IGBT when fault current occurs. However, in this case, the fault current continues to increase, so the IGBT must be large enough to withstand this current. In addition, in the case of a method of detecting and operating a fault, a delay time of up to hundreds of microseconds occurs, and in a hybrid circuit breaker, this delay time is a very large value. To solve this problem, this paper proposes a hybrid Z-source circuit breaker that operates without delay for low-impedance faults. The proposed breaker is designed to reduce stress on IGBTs and mechanical switches by generating a current zero crossing, as well as to enable the breaker to trip quickly in high-risk cases such as low-impedance fault circuits. Due to the limited environment of the laboratory, we performed 600 V fault isolation experiments. As a result of the experiment, it was confirmed that the proposed circuit breaker successfully isolated the fault within 1.6 ms. The current limiting feature of the Z-source appears to reduce stress on the IGBTs and MOVs used, and it allows for faster fault isolation. [ABSTRACT FROM AUTHOR]

Aimed at the optimization design of the parameters of the commutation circuit of a hybrid DC-current-limiting circuit breaker (HDCCLCB), a parameter selection model considering the short-time withstand of the thyristor and the volume of the commutation circuit is proposed by simplifying the object, and the commutation circuit parameters were preliminarily obtained. In order to verify the correctness of the method of selecting the commutation circuit parameters, the circuit simulation model of the HDCCLCB was built. The experimental platform was built, and the breaking experiment was completed under the fault condition with the current rising rate of 20 A/μs. The correctness of the simulation model and parameter design method was verified by comparing the circuit model simulation results with the experimental results. In order to further optimize the parameters of the commutation circuit, a mathematical model for optimization design was established. Taking the maximum critical breakdown voltage under unit capacitance energy as the objective function, the arcing time before commutating and the current-limiting inductance and capacitance are enumerated to obtain the optimized commutation circuit parameters. By comparing this result with the result of the preliminary design, the objective function is improved by 20.7%, laying a solid foundation for further research and the development of current-limiting circuit breakers for medium-voltage DC systems. [ABSTRACT FROM AUTHOR]

Nowadays, traditional DC circuit breakers (DCCBs) are always expensive and lack current-limiting capabilities. Hence, this paper proposes a current limiting and low-cost hybrid DC circuit breaker (HCB). When a fault occurs, the paralleled inductors in the proposed HCB are converted to a series connection due to the cutoff of the converter module, effectively limiting the increase of fault current. Then the cascaded IGBTs undertake the transient interruption voltage. Energy dissipation circuits (EDCs) reduce the fault isolation time (FIT) by bypassing the current-limiting inductor during energy dissipation based on the metal oxide varistors (MOVs). Therefore, the proposed HCB can limit the rate of the fault current increase, reducing the FIT and cost. Finally, the feasibility of the proposed HCB is verified by a single-ended equivalent system and DC grid test systems built in PSCAD/EMTDC. In addition, the proposed HCB FIT is 33.7% quicker, its energy consumption is 87%, and its cost is lower than that of the traditional ABB HCB. [ABSTRACT FROM AUTHOR]

The high‐voltage direct current (HVDC) power transmission short‐circuit fault protection is a big challenge, and it is the main obstacle to promote the HVDC multi‐terminal networks. A natural commutation current topology (NCCT) of hybrid HVDC circuit breaker (DCCB) integrated with limiting fault current is proposed in this paper, which shares several obvious merits, such as lower peak fault current, shorter fault isolation time and low turn‐off loss. The main contributions in this paper are made as follows. (1). A second‐order RLC circuit in novel NCCT circuit breaker is utilized as the current limiting circuit to dramatically reduce the fault current rising slope and its peak value. (2) A asynchronous NCCT circuit breaker has been investigated to broaden the proposed NCCT circuit breaker's optimal working area. (3) By taking China Zhangbei 500 kV four‐terminal DC grid as an example, an optimization design technique of the asynchronous NCCT‐DCCB is developed to demonstrate the process of the theoretical analysis and parameter design. The correctness of the proposed NCCT‐DCCB and the feasibility of the optimization design technique are confirmed by EMT simulation on PSCAD/EMTDC and hardware‐in‐loop experiment on PLECS‐RtBox. [ABSTRACT FROM AUTHOR]

Hybrid DC circuit breakers (HDCCBs) have excellent performance in terms of breaking speed and low losses and have become the mainstream technology for dc fault isolation in dc grids. However, because of the breaking limitation of the solid-state branch in HdcCBs, the existing HdcCBs are still weak in breaking high current. Although the breaking ability can be improved by series and parallel devices, the cost will increase rapidly, and it is very difficult to share current by paralleling a single type of device. This article proposes a novel HdcCB topology that combines two types of power electronic devices in solid-state branches to meet the requirement of a large breaking capacity in dc grids. The structure and breaking principles of this novel topology, which aims to break high current, are introduced. The design methods of the key parameters are presented. Then, the current breaking ability of the novel topology is verified by developing a prototype completing a 10 kV/60 kA breaking test in 3 ms. Finally, this novel topology is compared with other dc circuit breakers at the same breaking level, which shows that this novel topology has obvious advantages in cost and volume with fewer devices. [ABSTRACT FROM AUTHOR]

Energy routers act as an interface between the distribution network and electrical facilities, which meet the requirements of clean energy substitution and achieve the energy sharing and information transmission in the energy network. However, the protection of the dc load side of residential energy routers including interruption and isolation of short-circuit fault currents is vital for discussion. Since the traditional mechanical and hybrid circuit breakers for dc fault protection have the drawback of slow operation, a solid-state circuit breaker (SSCB) is an optimal solution for fast dc fault interruption. In this paper, a dc SSCB is proposed that uses an RCD + MOV snubber circuit, which is considered the best and most complete circuit used in common SSCBs. There are two main contributions in this paper: First, a dc SSCB is designed, which isolates both positive and negative terminals of a circuit and its working principle and operating modes along with the formulas for calculation of crucial time intervals, voltages, and currents along with the design procedure are provided. Second, a soft turn-on auxiliary is designed to prevent a high current surge caused by the capacitance difference between the source and the load. The experimental results demonstrate the proper performance of the topology and the validity of the findings. [ABSTRACT FROM AUTHOR]

The multiterminal dc (MTDC) grid plays an important role in large-scale power transmission and renewable energy integration owing to its excellent performance. However, there are still some problems existing in the MTDC grid, such as difficult fault current interruption and current flow control, which impedes its development. Currently, the mentioned two problems can be separately solved by hybrid dc circuit breaker (HCB) and current flow controller (CFC). However, when configured separately, they are expensive and have large footprints. Therefore, this article proposes a multiport hybrid dc circuit breaker (MHCB) with current flow control capability. The HCBs near the same dc bus are first integrated into a single MHCB and then novel CFCs are embedded into the MHCB to replace the load commutation switches. Compared with the existing MHCBs, the proposed topology shows advantages in the aspects of implementation cost, power loss, and operation speed. The operation principle is analyzed and the basis for parameter design is given in detail. The effectiveness of the proposed topology is verified by extensive simulations and experiments. Besides, the proposed MHCB is compared with the existing solutions. [ABSTRACT FROM AUTHOR]

This paper proposes a thyristor-based hybrid DC circuit breaker (HDCCB), which is composed of a fast mechanical circuit breaker and an LC resonant circuit with thyristor switches. The proposed HDCCB provides a zero-crossing point for fault current by injecting a resonant current in the reverse direction of the fault current. The proposed HDCCB has a low conduction loss in normal operation using a mechanical circuit breaker, and it offers the advantages of low cost, high reliability, and large capacity using thyristor switches instead of IGBT switches. It also has the design flexibility to adjust the magnitude of the injection current depending on the fault current level. A bidirectional HDCCB is also proposed by adding several components to the developed unidirectional HDCCB, which can offer compact system size and low system cost. Design considerations of the proposed structure are discussed. The validity of the proposed HDCCB is verified by simulation and experimental results. [ABSTRACT FROM AUTHOR]

Piezoelectrically actuated fast mechanical switches provide a low-loss conduction path in hybrid circuit breakers for protecting medium-voltage direct-current systems. As the desired actuation speed approaches the driving limit of a piezoelectric actuator, excessive vibration starts to dominate its underdamped travel curves, resulting in insulation failures and delayed switching in the fast mechanical switch. To improve the underdamped travel curves into critically damped travel curves, several switching motion control schemes with active damping filters are proposed in this paper. The switching motion controllers are based on an electromechanical model of a fast mechanical switch with experimentally identified parameters. According to hardware implementation tests, the proposed closed-loop control schemes optimized the switch travel curves with a 56% reduction in the undershoot and a 75% reduction in the settling time. Equipped with the critically damped travel curves, the piezoelectrically actuated fast mechanical switch is better suited for sequential submodule operations to reduce the interruption time, the peak current and the fault energy to be absorbed in hybrid circuit breakers. [ABSTRACT FROM AUTHOR]

The dc circuit breaker plays an important protection role in the dc distribution systems. Hybrid dc circuit breaker (HCB) offers low conduction losses and reasonably fast response times. In this article, a high power density, modular, and scalable power electronic interrupter (PEI) design is introduced for medium-voltage (MV) HCB platforms. Following this concept, the single low-voltage module is first investigated in terms of the high pulse current ratio and the voltage clamping circuit. After the comparison and test among different devices, a compact module with 1.7-kV discrete IGBTs are selected as main switches. While the voltage clamping circuit is carefully designed to guarantee no tail current bump and sufficient turn-off voltage margin. Next the whole system with series modules is optimized on the aspects of the turn-off strategy, power supply structure, and layout of modules. An improved staged turn-off scheme is proposed, which can not only reduce the peak current and total metal oxide varistor (MOV) energy, but also help balance the MOV energy distribution. A 12-kV and 1-kV breaking capable PEI was demonstrated based on the proposed PEI design, achieving a peak power density of 7.4 kW/cm3, much higher than the IGBT module-based solution. The breaking capability and effectiveness of the modular PEI is demonstrated in a 6-kV MVDC HCB device. [ABSTRACT FROM AUTHOR]

High voltage direct current (HVdc) power transmission systems do not encounter any natural zero crossing unlike the ac systems, and hence, the dc circuit breakers employed should be fast-acting and reliable to ensure safe operation. Circuit breakers (CBs) that have fast response time with lower energy loss dissipation are required for subsea HVdc transmission systems. This article investigates the performance of various CB topologies, including ultrafast coupled inductor hybrid CB (CIHCB) topology without and with the integration of resistive-type superconducting fault current limiter (R-SFCL) for a 100-kV, 100-MW HVdc transmission systems. It is found that the addition of R-SFCL with 1.7-ms response time decreases the peak current through the breaker and, hence, the power dissipated in the CB during short-circuit fault conditions. Thus, the rating of CB can be significantly decreased with the proposed topology. Furthermore, a comparison of power loss in CIHCB with the existing topologies, such as mechanical CB, resonant CB, and HVdc-hybrid CB, are also presented in this article. [ABSTRACT FROM AUTHOR]

High-voltage direct current (HVDC) circuit breaker development and deployment strongly depend on the testing process, which ensures that the HVDC circuit breakers will satisfy design requirements. This article presents an HVDC circuit breaker test bench circuit configuration that can provide controllable large output currents to simulate different fault conditions for the current breaking test and high output voltage for the dielectric withstand test. The current breaking test circuit is based on multiple cascaded power converters connected in parallel to provide the necessary output current capability. Each cascaded power converter is composed of multiple cells that are operated by a phase-shifted pulsewidth-modulated signal for greater controllability and higher quality of the output waveform. The dielectric withstand test circuit is a simple high-voltage source with a low power rating that can also be used to charge the test bench and the internal circuitry of the circuit breaker that is to be tested. The proposed test bench ensures that fault conditions can be replicated accurately and offers greater flexibility by being able to test mechanical, semiconductor-based, or hybrid HVDC circuit breakers with different current and voltage ratings on the same hardware without any changes. The idea and the operating principle of the proposed test bench are verified experimentally on a downscaled system that consists of three cascaded power converters connected in parallel with three cells per cascaded power converter and with a total equivalent switching frequency of 92.5 kHz. [ABSTRACT FROM AUTHOR]

The primary function of hybrid direct current circuit breakers (HCBs) is to quickly interrupt fault currents to protect high-voltage direct current (HVDC) systems. To enhance the reliability and stability of HVDC systems, optimal design of HCBs is required to minimize the peak fault current, interruption time, and recovery time. Therefore, this study develops a multi-objective genetic algorithm (MOGA)-based optimization model to identify the optimal parameters for HCBs. The MOGA model consists of three objective functions that provide trade-offs among reductions in the peak fault current, the interruption time, and the recovery time. The proposed algorithm is verified with a novel HCB topology using inverse current injection techniques. The performance of the HCB topology with the optimal parameters is validated in the MATLAB/Simulink environment. In addition, a comparison study between the optimal design of an HCB using the proposed algorithm and a typical HCB model is presented in this study to show the effectiveness of the proposed optimization method. Our simulation results show that the optimal parameter design of HCBs significantly reduces the magnitude of the peak fault current and operating time, thus maintaining the safe and stable operation of the entire system. [ABSTRACT FROM AUTHOR]

It has been widely accepted to apply dc circuit breakers (DCCBs) to protect dc grids. Nevertheless, existing DCCBs have one or more following limitations: insulation recovery scheme of mechanical switch, additional precharge power supply, high conduction loss, unreliable current-breaking process, and unidirectional design. In this letter, a new DCCB with low conduction loss, simple precharging scheme, and reliable breaking ability is proposed. The proposed DCCB could break both fault and operating current actively with the same simple operation sequences. Moreover, bidirectional topology possessing all advantages mentioned previously is also presented. Topology introduction, parameters designing, and scaled-down experiment of the proposed DCCB are all presented in the letter. [ABSTRACT FROM AUTHOR]

To improve the consumption of renewable energy and to meet complex load demands. A model of a multi-terminal flexible DC distribution network with multiple overhead branch lines is constructed. Aiming at frequent non-permanent faults of overhead lines, a fast pilot protection scheme is proposed to cover the whole fault process. Using the hybrid DC circuit breaker (HDCCB) action as the boundary mark, the fast pilot protection can be divided into two phases: post-fault and fault restart. The first phase highlights rapidity, sending a trip command to HDCCB within 1-2 ms after the fault. The second stage emphasizes reliability, and points out that the data of the whole fault process should be fully utilized, and to fully excavate the characteristics of fault current. A multi-time scale and multi-criteria fusion protection scheme is proposed, which can effectively improve the reliability of fault area identification. Finally, a corresponding model was built based on PSCAD, a large number of simulation tests and robustness analysis proved that the proposed fast pilot protection scheme is feasible. [ABSTRACT FROM AUTHOR]

A new series-type hybrid circuit breaker (S-HCB) concept for dc fault protection is reported that is fundamentally different from other prior-art hybrid circuit breakers (HCB) or solid-state circuit breakers (SSCB) in the literature. The S-HCB conducts its load current through metal wires instead of power semiconductor switches and curtails its fault current to near zero throughout the entire opening process of a series mechanical switch. It offers the low on-resistance of the conventional mechanical contacts for normal operation and µs-scale fault response even faster than the fast-acting SSCBs. A proof-of-concept S-HCB prototype experimentally demonstrates the interruption of a fault current of 30 A within 10 μs at a dc voltage of 600 V. The operating principle, simulation, and experimental results are discussed in this letter. [ABSTRACT FROM AUTHOR]

In this article, metal oxide varistor (MOV) design optimization and switching control in the main circuit breaker (MCB) branch of a progressively switched hybrid dc circuit breaker (DCCB) is presented. A progressively switched hybrid DCCB can achieve faster fault isolation with reduced peak fault current magnitude and transient recovery voltage compared with a regular hybrid DCCB due to its modified switching strategy. Consequently, thermal stress on the semiconductor devices in MCB is significantly reduced. Analytical model of the system dynamics during fault isolation with progressive switching is derived to demonstrate the switching scheme’s effect on the energy-absorbing component, MOV, during turn-off process. Derived analytical model in conjunction with the displacement curve of the fast mechanical switch of the hybrid DCCB is utilized to optimize the components of the main circuit breaker branch to reduce MOV degradation through asymmetric energy dissipation. A model of the circuit breaker is built in PSCAD to validate the performance of the proposed optimization method in a 10-kV/250-A system with four stage progressive switching. Additionally, a low voltage system model at 380 V is developed in PLECS for two stage progressive switching that works as the basis of experimental validation. This includes both lookup table-based MOV model and device thermal model for junction temperature estimation. Experimental results are provided for a 380-V system to demonstrate reduced fault current peak in a progressive switching and near uniform energy absorption in optimally selected MOVs. [ABSTRACT FROM AUTHOR]

Modular multilevel converter (MMC) high-voltage direct current (HVdc) transmission is a promising method to integrate renewable energies. However, limiting the dc-link fault current and accelerating the fault-current clearance are critical issues for MMC-HVdc systems. In this article, we propose an air-core-transformer-based fault-current limiter (ACT-FCL) in cooperation with the dc circuit breaker to limit fault-current rising and decrease fault-clearing time. Compared with the conventional dc reactor, hybrid current-limiting circuit, current-commutation-based fault-current limiter (FCL), and magnetic coupling FCL, the ACT-FCL can increase the equivalent inductance during the dc fault stage to reduce the rising rate and peak of fault current. The ACT-FCL can also decrease the equivalent inductance during the fault-current-clearing stage to reduce the fault-clearing time. The simulations and laboratory experiments verify the effectiveness of the proposed ACT-FCL in the bidirectional HVdc systems. The ACT-FCL may be extended to more fields, such as dc microgrid and domestic dc transmission. [ABSTRACT FROM AUTHOR]

The hybrid dc circuit breaker (HDCCB) is one of the most promising solutions for the dc interruption. But the power electronics-based load commutation switch (LCS) in the main current path causes the high on-state loss and cost, which inhibits the application of the HDCCB. In this paper, a novel hybrid dc circuit breaker is proposed, in which the vacuum interrupter with the external transverse magnetic field (ETMF) is designed to improve the arc voltage and thus enhance the current commutation to power electronics. The ETMF-based vacuum interrupter achieves the fast current commutation instead of the LCS, which realizes the low on-state loss and cost while preserving the short commutation and interruption time. The arc voltage characteristic with ETMF is analyzed, and the influence of the different factors on the commutation time and di/dt before current zero are investigated in detail. Based on the calculation results, a 10 kV dc breaker prototype based on the proposed topology is developed with the breaking current of 15.5 kA. Finally, the application of the proposed scheme in HVdc grids is discussed. [ABSTRACT FROM AUTHOR]

The hybrid dc circuit breaker is one of the most promising dc circuit breakers (DCCBs). However, its cost is relatively high, and the completion of its fault isolation is significantly delayed because of the large current limiting inductors configured in the multiterminal dc grid. Besides, due to the utilization of overhead lines, the probability of temporary faults is much increased, which means there is a great possibility that the power supply can be rapidly restored by reclosing DCCBs. However, the temporary faults must be distinguished from the permanent faults before the reclosing operation to avoid damaging the fragile power electronic equipment. Therefore, considering the above issues, a novel multiport hybrid dc circuit breaker (MHCB) is proposed in this article. The MHCB mainly has three advantages: the reduced implementation cost by sharing the expensive main breaker among multiple lines, the reduced fault isolation time by bypassing the current limiting inductor during energy dissipation, and the ability to reclose softly. Extensive simulations and experiments verify the effectiveness of the device. In addition, the comparison of the new device and existed solutions is presented. [ABSTRACT FROM AUTHOR]

Multi‐port hybrid DC circuit breakers (MP‐HCBs) are rapidly developing due to their obvious advantages in economy compared to traditional two‐port hybrid circuit breakers (HCBs). However, most of the existing MP‐HCB topologies do not have the current‐limiting capability, and a large number of insulated‐gate bipolar transistors (IGBTs) are still needed in their main breakers. In response to the problems, a multi‐port current‐limiting hybrid DC circuit breaker (MP‐CLHCB) based on thyristors is proposed in this paper. MP‐CLHCB uses low‐cost thyristors instead of IGBTs, and suppresses the rise of fault current through current‐limiting branch. In an attempt to prevent the MP‐CLHCB from reclosing in a permanent fault and causing damage to the DC grid again, an adaptive reclosing scheme is also proposed. In this paper, the topology, operation stages, theoretical derivation, and parameter design of the MP‐CLHCB are provided. Finally, the performance of the MP‐CLHCB is verified and compared through simulations in PSCAD/EMTDC. © 2021 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC. [ABSTRACT FROM AUTHOR]

Flexible DC grids with dedicated metallic return (DMR) encounter two major challenges when reclosing hybrid DC circuit breakers (DCCBs): (1) identifying permanent and nonpermanent faults to determine whether DCCBs should be reclosed; and (2) identifying pole-to-ground (P-GND) and pole-to-DMR (P-DMR) faults to determine whether all DC lines should be isolated when a permanent fault occurs. Under a permanent P-DMR fault, all DC transmission lines should be isolated instead of maintaining half of the power transmission. In this study, the equivalent circuits and analytical expressions of the fault currents under the P-GND and P-DMR faults are deduced. On this basis, two different criteria are proposed to discriminate these two faults. Permanent and nonpermanent faults are recognized according to the characteristics of the residual voltage of the DC transmission line. Finally, a comprehensive reclosing strategy considering fault type discrimination for a hybrid DCCB is developed. A simulation model of a four-end flexible HVDC grid with DMR is constructed in PSCAD/EMTDC. The validity of the proposed reclosing strategy is verified through different simulation cases. [ABSTRACT FROM AUTHOR]

• Bi-directional power electronic switches enable a rapid and flexible conversion between low reactance in non-current-limited mode and high reactance in current-limited mode, suppressing the development of DC faults while achieving current limitation during mechanical switch opening. • The energy absorbed by the MOV is reduced. • DC opening time is shorter. • The proposed DCCB has economic advantages. The DC circuit breaker (DCCB) is a critical device that ensures the safe and reliable operation of the DC transmission system. As the level of DC voltage and transmission power increases, the flexible DC grid will need more and more opening capacity of the DCCB, as well as the burden of high costs. To solve the problem of excessive voltage and current stresses in existing DCCBs during DC fault current opening, an Improved Hybrid DCCB with the capacity of current-limiting and voltage-limiting is proposed, which crucially adds current-limiting and voltage-limiting branches. The current limiting branch can be activated 2 ms earlier in a fault circuit, significantly reducing the number of diodes and IGBTs required for branch transfer. In addition, overvoltage problems can be greatly reduced by properly designing the parameters of the current limiting branch. These improvements significantly increase the open ability, safety, reliability, and economy of the DCCB. The improved hybrid DCCB is shown to be effective and feasible by a large number of simulation and experimental test results. [ABSTRACT FROM AUTHOR]

Two key challenges in meshed multi-terminal dc (MTdc) grids are line power flow management and fault current interruption, which can be solved by current flow controllers (CFCs) and hybrid dc circuit breakers (HCBs), respectively. However, the power losses and the implementation costs of CFCs and HCBs are relatively high. In order to overcome these drawbacks, a combined HCB (CHCB) is proposed in this article by fully utilizing the core ideas of the interline dual H-bridge CFC and the HCB. The proposed device can substitute multiple HCBs at a dc bus with greatly reduced cost. In addition to the fault current interruption capability, the current flows in the adjacent lines can be controlled by the embedded multi-line CFC in the CHCB expanded from the existing interline dual H-bridge CFC. The system-level behavior of the proposed CHCB is similar to the separate solution of the typical HCB and the multi-line CFC. Extensive simulations and experiments verify the effectiveness of the proposed device. Besides, a comparison of the proposed CHCB and the separate solution is presented. [ABSTRACT FROM AUTHOR]

The diode bridge submodule (SM) is one of the basic units of a hybrid high-voltage dc circuit breaker. This article works on the low inductance design of a single SM and series SMs. For the layout design of a single SM, this article proposes a simple and clear +-shaped SM layout, the low inductance design is simplified to the optimization of two laminated busbars with a simple structure. Based on the +-shaped SM layout, this article further proposes an SM structure with holes in diodes region, the stray inductance is further reduced, and only one integrated laminated busbar is needed in a single SM. For the layout design of series SMs, this article makes full use of the layout of a single SM and the mutual inductances between the SMs, series SMs are alternately connected in reverse. The single SM layout proposed in this article is effective in reducing the stray inductance. The loop stray inductance in stage II reduced from 148.4 to 72.3 nH, a 51.3% reduction. When the two parallel IGBTs turn off a 6 kA fault current in 2 μs, the voltage spike is reduced from 786.1 to 423.4 V, a 46.1% reduction. And the stray inductance difference of the bidirectional current path is controlled to about 10 nH. The layout of series SMs proposed in this article is effective in reducing the length of the series SMs, the length is reduced by 16.7%. The experimental results show that the SMs layout proposed in this article is effective. [ABSTRACT FROM AUTHOR]

Due to the rapid development of DC transmission, research on DC side fault protection has also made significant progress. The bipolar short-circuit fault is one of the most severe types of DC side faults. It can cause a current surge, which affects the safe operation of a system. Therefore, this paper proposes a new type of hybrid DC circuit breaker (DCCB) to realize the rapid suppression of bipolar short-circuit current. The new DCCB consists of three parts: a nominal current branch (NCB), an energy absorption branch (EAB), and a freewheeling branch (FB). Different control strategies can be adopted to achieve line protection functions for different types of faults. For transient faults, the absorption and suppression of the fault current are achieved mainly through the EAB and FB. The DCCB ensures that the amplitude of the current delivered by the DC line is maintained in a safe operating area of the transmission line. The EAB is disconnected after a short-term of operation for permanent faults to achieve complete isolation between the converter side and the fault side. The parameter design of the DCCB is carried out to provide a reference for the practical application of this research. Finally, the effectiveness of the proposed DCCB protection strategy is verified by simulations, and the function of the DCCB is further verified by experiments. [ABSTRACT FROM AUTHOR]

Hybrid circuit breaker (HCB) has shown good performance for interrupting dc fault current. However, a massive number of IGBTs in series is needed. These will lead to high costs and significant difficulty in balancing IGBT dynamic voltage. This paper proposes a bidirectional hybrid circuit breaker based on a cascaded half-bridge submodule structure to reduce the overall costs and implementation difficulty of HCB. The number of IGBTs is reduced by 50%, and the operating speed is fast enough. A model of a four-terminal modular multilevel converter (MMC) system with a proposed hybrid circuit breaker in different stages is developed to describe the fault blocking process more clearly. The performance of the proposed HCB is validated and compared with other HCB structure through both theoretical derivation and time-domain simulation. [ABSTRACT FROM AUTHOR]

Medium voltage direct current (MVDC) power systems are in their early phase of development, but the lack of efficient and reliable DC circuit breakers is currently one of the most significant bottlenecks for widespread adoption of MVDC applications. Compared to protection devices in high voltage direct current (HVDC) systems, MVDC circuit breakers are expected to interrupt fault currents with higher rates of rise in more power-dense and more controllable structures. A hybrid approach with a piezoelectrically actuated fast mechanical switch in parallel to a solid-state circuit breaker is expected to meet the above requirements. A prototype of a 15 kV-class piezoelectrically actuated fast mechanical switch is presented in this paper. The prototype demonstrates an actuator opening time of 1 ms, a breakdown voltage of over 20 kV, and a continuous current rating of 500 A. After integrating the fast mechanical switch into a hybrid circuit breaker testbed, the fast mechanical switch was able to open within 1.5 ms and the fault was cleared within 5 ms. Therefore, piezoelectrically actuated fast mechanical switches can enable fast, reliable and controllable protection for various MVDC systems like electric ships, electric aircraft and microgrids that integrate distributed energy resources. [ABSTRACT FROM AUTHOR]

DC system has the potential of vast and rapid fault current generation due to multiple (line and converters) discharge capacitors and small impedance of DC lines. DC fault current spreads through the system exponentially compared to AC. Such an unexpected huge current causes a voltage drop, impacts the normal operation of system components and exposes the system to a great challenge for fault detection and interruption. For prevention of system destruction during the fault, multiple approaches such as application of Mechanical Circuit Breakers (MCBs), fuses, Solid State Circuit Breaker (SSCB), and Hybrid Solid-State Circuit Breaker (HSSCB) have been proposed and applied. In DC fault applications, fast fault detection and interruption without any interference to the other components are quite important. Therefore, semiconductor breakers have been implemented to meet the DC fault protection requirements with a high-speed operation where traditional MBs have failed. Due to the high conduction loss and low efficiency of semiconductor switches, for fast and efficient DC fault interruption, different Fault Current Limiter (FCL) types are suggested. Although a high impedance FCL can prevent the voltage fluctuations due to the current decline, it can cause operation speed issues, coordination troubles, overheat, and malfunction of protective components in a fault situation. This paper focused on a combination of two-way HSSCB with a self-adapt DC short current limiter, ultra-fast switch, and power electronic switch to overcome the above challenges. It can efficiently and fast fault current limiting response with low conducting loss and appropriate cooperation among protective components in a low voltage DC system. The MATLAB/Simulink is used to analyze the effectiveness and consistency of the proposed FCL-HSSCB in 400 V interconnected standalone DC microgrids. [ABSTRACT FROM AUTHOR]

The emerging interconnections of renewable energy sources that generate DC power have resulted in demands on DC grid systems. In this situation, a low-voltage DC (LVDC) grid system could be a solution for these DC power sources. For the reliable operation of a LVDC grid, DC faults are an issue and DC circuit breakers (DC CBs) are appropriate devices to clear fault current. However, there are some difficulties in terms of breaking DC faults such as the absence of zero crossing of the current and voltage, and the high di/dt of the fault current. This paper proposes a hybrid circuit breaker (HCB) with a new structure for a LVDC grid system. The main switch consists of a vacuum interrupter (VI) and a permanent magnetic actuator (PMA). Some improvements are proposed to enhance the performance of the PMA. A bi-directional commutation loop consisting of a minimum number of power electronic devices is suggested. To absorb fault energy, a capacitor and a diode loop are applied. The principle of the proposed HCB and the design methods for all of the components are analyzed. Through a simulation model, the designed components are selected and verified. Experimental results with a realized prototype (750 V/300A) verify the design method and its performance. [ABSTRACT FROM AUTHOR]

Modular multilevel converter (MMC) based medium voltage DC (MVDC) distribution system has become a promising option in the future smart grid architecture due to many distinguishing features. However, MMC-MVDC distribution system is prone to face the short-term outage problem during bipolar DC faults, so how to implement DC fault ride-through (FRT) capability for MMC-MVDC is still a challenge. This paper intends to change the redundant half-bridge submodules (HBSMs) of the HB-MMC into the redundant full-bridge submodules (FBSMs), thus, a practical DC FRT method is proposed by using the combination of the hybrid MMC and the hybrid DC circuit breakers (CBs) for the MMC-MVDC system. Then, the DC fault current characteristics of the MMC with the proposed FRT method are discussed. Lastly, data analysis and simulations of a three-terminal MMC-MVDC system have been performed in PSCAD/EMTDC to validate the effectiveness of the proposed DC FRT method. The proposed DC FRT method is not only easy to implement, but also reduces the requirements and the investment of the DC CB. [ABSTRACT FROM AUTHOR]

Recently the hybrid multi-port DC circuit breaker (MP-DCCB) is becoming popular in protecting HVDC grids, thanks to their reduction of power electronics devices. In this paper, an enhanced multi-port current-limiting DCCB (MP-CLCB) for multiple line protection is proposed. The integrated fault current limiter (FCL) inside the MP-CLCB can clear the fault faster with slightly increased costs. To reduce the energy dissipation requirement for the surge arresters caused by the newly added current-limiting path, an energy transfer path which provides a loop with the inductors during the current decay stage is designed. The theoretical analysis of the pre-charging, current-limiting, fault interruption and energy dissipation of the MP-CLCB is carried out. Moreover, the design principles of the energy dissipation and the key parameters of the MP-CLCB are provided. The proposed approaches are verified through simulations in PSCAD/EMTDC. The results show that the MP-CLCB can replace multiple DCCBs, accelerate the fault current interruption and reduce the energy dissipation requirement for the surge arresters. [ABSTRACT FROM AUTHOR]

One of the major challenges in half-bridge modular multilevel converter-based high-voltage dc (MMC-HVDC) grids is the rapid detection and isolation of short-circuit faults before converter blocking and the entire grid shutdown. With the advancement of the hybrid dc circuit breakers (HDCCBs) in the ultra-fast isolation of faults, a considerable part of the problem has been solved. In this paper, relying on the performance of these circuit breakers and using the cosine distance criterion, a new pilot protection scheme is designed and presented for detecting short-circuit faults in transmission lines of half-bridge MMC-HVDC grids. While this scheme requires a comparatively low sampling frequency, its fault detection speed is quite independent of fault resistance. Moreover, this protection has the correct performance for any fault location along the line under protection and is completely stable against all external faults. The results of the evaluations on a 4-bus test grid under different fault conditions confirm the correct, ultra-fast, stable, selective, and reliable performance of the proposed line protection scheme. [ABSTRACT FROM AUTHOR]

To meet the ultra-fast, selective, and reliable requirements of the DC fault protection, a protection scheme for modular multilevel converter based HVDC grids is proposed in this paper. The protection scheme is consisted of main protection, directional pilot protection, and faulty pole identification. The new fault directional criterion is achieved by employing the surge arriving time difference of line-mode backward traveling wave and line-mode forward traveling wave. This criterion is valid even when the fault current limiting reactors (FCLRs) are installed at the converter side of DC bus. The main protection without communication and complex computation relies on co-operation of the derivative of line-mode forward traveling wave and the directional criterion. The directional pilot protection by comparing the double end directional criteria can be used as a backup protection. The accumulation of zero-mode component is adopted to discriminate the faulty pole. Simulations have demonstrated that the proposed scheme is capable of identifying various types of faults with different fault resistances at different positions. The robustness analysis shows that the protection scheme presented is not sensitive to the FCLR size, noise environment, sampling frequency, and the operation of hybrid DC circuit breakers. [ABSTRACT FROM AUTHOR]

Hybrid dc circuit breaker (HCB) is currently the most promising dc circuit breaker. However, since it requires hundreds of semiconductor switches to withstand high transient interruption voltage, the implementation cost is high. Besides, in the multiterminal dc grid, three or more HCBs may be required at a dc bus, which worsens the disadvantages of HCBs in economy and volume. In order to solve this problem, this article designs a diode-bridge multiport HCB (DB-MPB) with its control procedures. By integrating the n HCBs at the dc bus to a single n-port HCB, the number of semiconductor switches and the footprint can be reduced, while the good current interruption performance can be still maintained. Besides, for the possibility of ultrafast disconnector (UFD) failure, the proposed DB-MPB has the function of UFD failure protection. The effectiveness of the designed DB-MPB under different scenarios is verified by simulations and experimental tests. In addition, the performance comparisons in economy and functionality of the proposed DB-MPB with existed solutions are presented to highlight the advantages of the proposed solution. [ABSTRACT FROM AUTHOR]

• A CAOC-MPHDCB is proposed by combining with the principle of controllable active oscillation and the MPHDCB based on negative voltage source, which can not only greatly cut down the total cost and operation loss, but also reduce the insulation requirement of commutation circuit. The CAOC only needs an energy storage capacitor to store several kV pre-charge voltage, and can generate a high voltage on a pulse capacitor after several oscillations. Like a NVS, the CAOC can generate a negative voltage on the transfer branch to accelerate the current commutating from the main branch into the transfer branch. In addition, the IGBT full-bridge in the CAOC only needs to withstand the pre-charge voltage with low amplitude at any time. The simulation results verify the effectiveness of CAOC-MPHDCB, and shows its great application prospect in the field of HVDC interruption. The hybrid high voltage DC circuit breaker (HDCB) is widely regarded as the most promising solution to the fault current interruption of VSC-based DC grids. However, because of a large number of power electronic modules connected in series in transfer branch, the cost of existing HDCBs are pretty high, which limiting their large-scale commercial application. In this paper, a novel multi-port hybrid high voltage DC circuit breaker commutated by controllable active oscillation circuit (CAOC-MPHDCB) is proposed. Based on the principle of controllable active oscillation, the proposed CAOC-MPHDCB can realize the reliable commutation and fast interruption of fault current while significantly reducing the total cost and operation loss. In addition, the CAOC-MPHDCB can not only isolate line faults that occur continuously in a short time, but also maintain system ring-network structure after isolating the faulty DC bus. The influencing mechanism of parameters on the performance of CAOC-MPHDCB are analysed in detail, and the selection methods of corresponding parameters are obtained. The simulation results show that the principle of CAOC-MPHDCB is effective and feasible. [ABSTRACT FROM AUTHOR]

The coordination of resistive-type superconducting fault current limiters (R-SFCLs) and DC circuit breakers (DCCBs) have been investigated. However, the use of R-SFCLs with mechanical DCCBs (M-DCCBs) in hybrid transmission systems has not been studied. The objective of this paper is to investigate the parameter requirements of R-SFCLs and M-DCCBs for a detailed system protection strategy in a hybrid AC/DC system considering the influence of AC faults and three types of DC faults. When the AC fault is far from all MMCs, its impact on the DC system is weak. If the AC fault is close to one MMC, it will produce a strong transient response in the DC part. Therefore, a bypass switch in parallel with an R-SFCL is used to remove the R-SFCL from the system if it quenches during the AC fault. Then, the parameters of the protection devices are designed based on pole-to-pole DC fault. System restoration can be achieved within 200 ms after the fault. For pole-to-neutral and pole-to-ground faults, except for the R-SFCLs at the faulty pole, the R-SFCLs at the healthy pole will also quench, but the resistances are very low. The system may be restored within 250 ms after the DC fault. [ABSTRACT FROM AUTHOR]

Hybrid dc circuit breaker (HCB) is an effective device for interrupting dc fault currents in dc grids. Conventional two-port HCB requires too many insulated gate bipolar transistors (IGBTs) in series in its main breaker, resulting in considerable costs. Recently proposed multiport hybrid dc circuit breakers (multiport HCBs, and the number of ports is greater than or equal to 2) interrupt fault currents on multiple lines using only one shared main breaker, significantly reducing the implementation costs but losing some fault handling capabilities. To overcome these obstacles, we propose a novel high-performance economical multiport hybrid dc circuit breaker (MP-HCB) in this article. The topology and its control method are proposed. Due to the designed ring-connection structure, the proposed MP-HCB can avoid dc bus fault, and thus is more reliable. The electrical stresses of the breaker are theoretically analyzed, and the parameters are determined to make the MP-HCB capable of multiple-line faults. Compared with the typical two-port HCB and the existing multiport HCBs, the proposed MP-HCB can achieve full fault current interruption capability with lower costs. To protect a dc node connecting with three lines, the cost of the semiconductors of the proposed MP-HCB is only 1/3 of the typical two-port HCB and 1/2 of the existing multiport HCB. Finally, the performances of the proposed MP-HCB are verified via a four-terminal HVdc system in power systems computer aided design/ electromagnetic transients including DC (PSCAD/EMTDC). [ABSTRACT FROM AUTHOR]

Summary: The key obstacle in the realization of multiterminal high‐voltage direct current (MTDC) grids is the lack of existing commercial protection that a device can withstand the DC fault. Although different HVDC circuit breaker technologies have been investigated and demonstrators tested by manufacturers, the innovation of such devices to ensure a reliable protection system has not been proven yet due to the maximum breaking capability limitation especially in MTDC grids. To this, a combination of superconducting fault current limiter (SFCL) technology with a fast DC circuit breaker could bring a solution to this bottleneck. In this work, a new application of an accurate model of resistive‐type SFCL (rSFCL) considering both the electrical and thermal behaviors is proposed to work together with a proactive hybrid DC circuit breaker (PHCB) to fulfill the whole system protection. The viability of the proposed approach is verified under a worst DC fault case‐scenario that imposed in a five‐terminal MTDC meshed grid. The numerical analysis and simulations are carried out using (EMTP‐rv) software, while the obtained results show how significantly the proposed rSFCL can reduce the fault current and enhance the breaking capability. Also, a positive interaction has been observed between the rSFCL and PHCB in terms of design sizing and operation performance. [ABSTRACT FROM AUTHOR]

The fast raising nature of DC fault currents and sensitivity of high-voltage DC (HVDC) power converters to DC faults is critical protection issues. On the other hand, DC fault current does not meet zero; which causes DC to fault current breaking process become more challenging. Specifically, to protect HVDC grids against DC fault currents, the development of very fast breakers is crucial. This study proposes a DC fault current limiter and breaker, which is able to protect the HVDC system/grid in a very fast and safe manner. The main goal of this study is to enhance the performance of already in the market hybrid DC circuit breakersincluding the magnitude of fault current and breaker operation speed. In the proposed HVDC fault current limiter and breaker, the fault current is limited in two stages by an inductive and resistive current limiter, which drastically decreases the main breaker dissipated energy. The functionality of the proposed HVDC fault current limiter and breaker is mathematically analysed, and then verified by PSCAD and Matlab/Simulink. Finally, simulation results are validated by scaled-down laboratory examinations. [ABSTRACT FROM AUTHOR]

DC fault isolation is one of the main challenges of the HVDC grid. DC fault current can be interrupted within several milliseconds by employing hybrid HVDC circuit breaker. However, this scheme is not cost-effective, especially in meshed HVDC grid. In this paper, a novel integrated HVDC circuit breaker by sharing components is proposed to overcome these drawbacks. Suppose that a converter in a meshed HVDC grid is connected to m DC lines. The integrated HVDC circuit breaker consists of m + 1 ultra-fast disconnectors (UFDs), m + 1 load commutation switches (LCSs), m + 1 mechanical disconnectors (DSs), and only one main breaker (MB) and surge arrester (SA). The behavior of the proposed integrated HVDC circuit breaker is similar to that of the conventional hybrid HVDC circuit breaker. These two kinds of breakers have the same operating time, which means the integrated HVDC circuit breaker is fast enough to clear DC faults. Besides, the IGBT modules in the MB and LCSs of the proposed integrated HVDC circuit breaker are connected in one direction, and the m DC lines share a single MB. Therefore, this scheme is more cost-effective compared to the conventional hybrid HVDC circuit breaker. Moreover, the adjacent lines can still be used to transmit power after DC bus faults. Simulation results on a three-terminal HVDC grid are presented to verify the effectiveness and feasibility of the proposed scheme. [ABSTRACT FROM AUTHOR]

Owing to the increased demand for DC interruption, a prototype current-limiting hybrid DC circuit breaker incorporating a fuse and a semiconductor device was developed, based on a completely novel design concept. Recently, hybrid DC breakers have been attracting attention because of their low conduction loss and high-speed current interruption capability. The superiority of this hybrid DC circuit breaker lies in the simultaneous use of the high-speed current-limiting performance of the fuse and the highly reliable current-interruption performance of the semiconductor device. Furthermore, the hybrid DC circuit breaker performs the current interruption without any mechanical contact, thus preventing any delay in operation induced by the mechanical contact. The feasibility of the proposed concept was demonstrated by the successful application of the circuit breaker in a 1 kA fault current interruption procedure. Hence, the current-limiting hybrid DC circuit breaker is suitable for low voltage and medium voltage DC system protection. As the fuses and semiconductor modules progress, the hybrid DC circuit breaker may also adapt to HVDC systems. [ABSTRACT FROM AUTHOR]

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 the 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. [ABSTRACT FROM AUTHOR]

This paper proposes a time-domain method to calculate the fault response in multiterminal dc (MTdc) grids and the performance of hybrid dc breakers. The proposed method, based on traveling waves: 1) provides a sound representation of fault performance by considering all created traveling waves; 2) introduces a new approach to estimate the reflection coefficients; and 3) provides an approximation of the worst case fault location. Then, based on the analytical results, three parameters of the hybrid dc circuit breaker, i.e., current limiting reactor, arrester rated voltage, and time delay are optimally selected with respect to maximum overcurrent, maximum overvoltage, fault clearance time, and energy absorption in arresters through multiobjective optimization. Accuracy and performance of the proposed method are evaluated and verified by time-domain simulation studies in the power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) environment using frequency-dependent models. The results confirm reasonable accuracy of the proposed fault performance calculation and represent a further step towards optimized design of hybrid dc circuit breakers. [ABSTRACT FROM AUTHOR]

This paper examines two new bidirectional hybrid dc circuit breaker (CB) topologies for application in meshed dc grids. The goal is to retain performance of hybrid dc CB with bidirectional current interruption while reducing semiconductor count, dc CB size, and weight. The fault current is routed to the unidirectional internal valve using multiple additional ultrafast disconnectors. Operation of both topologies is studied using a 320-kV, 16-kA simulation model, as well as demonstrated on a 900-V, 500-A lab prototype. The control systems are presented and discussed in detail. The low-voltage hardware prototypes verify performance of several new technical and operating solutions in laboratory conditions. A comparison is made with the existing dc CB topologies and performance and reliability compromises of each topology are assessed. The conclusion is that it might be possible to halve the dc CB semiconductor count while retaining same 2 ms opening speed and bidirectional operation. [ABSTRACT FROM AUTHOR]

This paper proposes a coupled inductor-based hybrid dc circuit breaker topology with zero current switching for fast fault interruption in dc systems. A series resonant circuit comprising of the secondary winding of a two-winding coupled inductor and a charged capacitor (commutation capacitor) is switched on during fault to inject a counter-current pulse, and, consequently, force a zero crossing of fault current. Proposed solution facilitates arcless breaking for a mechanical circuit breaker due to zero current turn-off. The proposed circuit breaker exhibits fast fault response ($\sim$ 30  $\mu$ s), and the response time is programmable based on the design of the coupled inductor and commutation capacitor. Furthermore, presented solution mitigates the requirement of energy-absorbing elements for demagnetizing the dc network following fault interruption, in contrast to conventional dc breakers. The paper also presents two modified circuit-breaker topologies to achieve unipolar voltage profile on the capacitor, which will enable the use of electrolytic capacitors for commutation so that the capacitor stack size is reduced in high-voltage applications. Detailed analysis and design equations are presented to explain the operation of the proposed topologies. Functionality of the proposed circuit breakers is verified through simulation, and experimental results are based on two laboratory prototypes. [ABSTRACT FROM AUTHOR]

• The equivalent discharging circuit of the hybrid cascaded LCC/MMC inverter is simplified into a first-order RL circuit by assuming the firing-advance angle is constant after the fault. • The mathematical fault current expressions of the hybrid cascaded LCC/MMC high voltage direct current system are analyzed. • By changing the inserted sequence of submodules, an active fault current limiting strategy based on the submodule sorting algorithm is proposed. • The proposed active current limiting strategy can cooperate with other current limiting strategies to further suppress the fault current. The hybrid cascaded LCC/MMC high voltage direct current (HC-HVDC) system is a prospective topology that can avoid the commutation failure problem of LCC-HVDC while ensuring bulk power transmission capability. However, the complex topology results in different DC fault characteristics at the receiving end, which brings challenges to fault detection and current limiting technology. As a result, it is of great theoretical importance to investigate the fault characteristics of the HC-HVDC system and propose the corresponding fault current limiting strategy. Firstly, this paper simplifies the equivalent discharging circuit of the hybrid cascaded LCC/MMC inverter into a first-order RL circuit by assuming the firing-advance angle is constant after fault, and proposes the fault current expressions by Laplace transform. Secondly, by changing the insertion sequence of submodules, an active fault current limiting strategy based on the submodule sorting algorithm (SMS-FCL) is proposed, which can accomplish fault current suppression, reactive power and DC voltage support, and energy loss reduction simultaneously. Moreover, the proposed SMS-FCL can cooperate with other FCLs to further suppress the fault current, and the cooperation prerequisites are analyzed. Finally, an LCC-LCC/MMC HVDC system is built in PSCAD/EMTDC to validate the accuracy of the proposed fault current expressions and the effectiveness of the current limiting strategy. [ABSTRACT FROM AUTHOR]

• We propose a multiport hybrid DC circuit breaker (MHCB) for fault isolation. • The MHCB is highly economical and capable of adaptive reclosing. • We describe the operating principle of the MHCB and design its parameters. • We design the reclosing criterion for the MHCB to avoid blind reclosing. • Simulations and comparisons with other schemes show the superiority and of the MHCB. In a high voltage direct current (HVDC) grid, hybrid DC circuit breakers (HCBs) are one of the effective solutions to interrupt fault currents. The conventional two-port HCB features high economic costs since the expensive insulated gate bipolar transistors (IGBTs) are needed. Each line in the multiterminal HVDC grid must be equipped with the HCB, which is extremely costly to implement. In addition, the likelihood of temporary faults is considerably raised by the fact that the HVDC grid transfers power across overhead lines (OHLs). By reclosing operations after temporary faults, the HVDC grid can resume normal operation. However, it is vital to discern between temporary faults and permanent faults before reclosing operation to prevent secondary shocks to the HVDC grid caused by blindly reclosing on permanent faults. For the problems of high cost and blind reclosing of HCBs, this article proposes a multiport HCB (MHCB) with high economy and adaptive reclosing capability. All ports of the MHCB share a single main breaker (MB) for fault isolation. The MHCB utilizes the capacitor voltage to lower the operating voltage of the metal oxide varistor (MOV), which reduces the number of IGBTs connected together at both ends of the MOV. With the above design, the expense of the MHCB is significantly decreased. Additionally, the MHCB achieves adaptive reclosing by using the difference of current amplitudes in the capacitor branch under permanent and temporary faults. [ABSTRACT FROM AUTHOR]