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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]

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]

To improve the reliability of power supply, reclosing schemes are required after transient faults, which commonly occur in overhead line based high voltage DC (HVDC) systems. However, in the event of permanent faults, the auto-reclosing scheme may cause a severe strike. To avoid the severe impacts caused by permanent faults, the fault type should be discriminated before activating the reclosing scheme. Therefore, an adaptive reclosing scheme based on phase characteristics is proposed in this paper. Firstly, the modulation of a periodic voltage by actively controlling the hybrid DC circuit breaker (DCCB) is introduced. Then, a cascaded π equivalent model and its decoupling algorithm are presented to analyze the frequency-domain characteristics of the measured impedance of the coupled overhead lines. From the frequency-domain characteristics, the frequency of the periodic detecting voltage is determined to analyze the phase features of the measured impedance at primary frequency. The permanent or transient faults can thus be accurately identified by using these different phase characteristics, with negligible influence on the healthy lines. In addition, the proposed scheme is robust to various fault resistances, leading to improved reliability. The effectiveness of the proposed scheme is verified in PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

The reliability of meshed DC grids mainly relies on their ability in DC fault protection and line power flow managing. Multiple DC circuit breakers (DCCB) and current flow controller (CFC) need to be installed, which significantly increases the system cost. A novel current flow controller integrated with hybrid dc circuit breaker (CFC-IHCB) is proposed to significantly reduce the loss and cost of the whole system. Only one unidirectional IGBT string based main breaker branch is needed for n lines, and the CFC function is integrated into the load communication switches (LCSs). The basic working principle of proposed topology is presented and analyzed. The functionality is verified and a comparison of the proposed solution and other DCCB/CFC solutions is given based on a multi-terminal HVDC system case study. [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]

Hybrid dc circuit breakers (HCBs) are recognized as suitable devices for protecting high-voltage direct-current (HVDC) systems, along with other dc circuit breakers (DCCB). However, compared to mechanical circuit breakers, HCBs exhibit higher conduction losses. Such losses are inevitable under no-fault conditions as current may flow through some of the semiconductor switches. An integrated circuit breaker (ICB) minimizing these losses is presented in this paper, and this is achieved by replacing semiconductor switches by mechanical components in the current path. For completeness, the topology design, operating sequence and a mathematical analysis for component sizing of the device are provided. In addition, an estimation of the conduction losses is quantified. It is estimated that the power losses of an ICB are 2 to 30% of an HCB only. The ICB has been implemented in PSCAD/EMTDC to demonstrate its effectiveness for isolating dc faults, with simulations conducted on a three-terminal HVDC grid. [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]

A mechanical switch can be used instead of a power electronic load commutation switch in orderto reduce the losses during normal power flow in standard hybrid DC circuit breakers. The success of current commutation in such hybrid breakers depends on the arc voltage formed across the contacts in the mechanical switch. The behaviour of rapidly elongated, wall constricted arcsgenerated during the opening of an ultra-fast mechanical switch in the air is studied here. The voltage-current characteristics of the generated arcs are obtained for steady state contact opening velocities of 6, 11 and 21 m/s. It is found that the arc voltage at a given current and contact separation increases with the contact opening velocity. It is shown that stationary,zero-contact-velocity characteristics can not be used to accurately quantify the voltage build-up in fast elongating arcs in hybrid breakers. A best-fitting black-box equation for the dynamic arc characteristics is presented for the tested velocities. The obtained voltage-current characteristics are used as input to simulate the current commutation process in a 12 kV hybridDC circuit breaker case-study. Different scenarios of current commutation failure related to the arc voltage build up are identified. It is shown that these failures scenarios can be avoided by increasing the contact opening velocity. [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]

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]

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 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]

The paper presents a new family of low voltage direct current, hybrid circuit breakers for urban traction vehicles (DCH-T) that also can be used in related industrial applications. A hyper-speed current breaking operation is performed by the means of a serial hybrid system, additionally protected by a fuse and a surge arrester. This process is arcless under any conditions. DCH-T circuit breakers are characterized by unique constructional and operational features and superior short-circuit parameters that are unreachable for classical magnetic blow-out devices. They effectively reduce short-circuit currents and the thermal and electrodynamic consequences of this phenomenon. They can protect all types of semiconductor converters used in electric traction vehicle drives. Moreover, they operate selectively in relation to the substation circuit breakers. Their contact life (even under short-circuit conditions) is equal to their mechanical life. Despite all the breaker standard functions, they offer special functions, such as cooperation with an on-board vehicle computer or a traffic control system. They are environmentally neutral and they can meaningfully improve passenger safety. Chosen thermal problems of DCH-T breakers has been modeled with the aid of simulation software and analyzed. DCH-T short-circuit parameters have been verified experimentally and the results are presented herein. [ABSTRACT FROM AUTHOR]

To open electromechanical switches within the range of tens of microseconds, we exploit repulsive electromagnetic forces between current-carrying conductor loops. For this we study the power conversion efficiency from electromagnetic to kinetic form. These studies suggest connecting moving coils in series rather than employing inductive coupling. These techniques are put into practice in two prototype switches: the first one is based on induction and the other on series connected coils. Measurements with a high-speed camera verify that both switches achieve accelerations in the magnitude of 140000 g and breaking distances of 1 mm within 50 \mus. The switch with series connected coils obtains 44% greater force than the one based on induction. [ABSTRACT FROM AUTHOR]