The development of voltage-source converter (VSC)-based high voltage direct current (HVDC) sys-tems has increased significantly. However, the imple-mentation of VSC-HVDC systems is limited for lacking fast, low-loss and reliable circuit breakers. Hybrid dc circuit breakers (DCCBs) may help to improve this situ-ation. In this paper, an efficient modular multilevel converter (MMC)-HVDC system model based on Zhangbei-Beijing $\pm$500 kV bipolar MMC-HVDC transmission (overhead line) project is first established. Then, the system transient characteristics are investigated involving permanent line-to-ground faults (LGFs), permanent short-circuit fault (SCFs), temporary LGFs and temporary SCFs with either ac circuit breakers (ACCBs) or DCCBs in service. Finally, the performances, including the fault clearance and recov-ery, comparison of DCCBs with two different breaking methods, and comparison of ACCBs and DCCBs, in the MMC-HVDC system are presented. This work provides a deep understanding of LGF and SCF in the MMC-HVDC systems, operation of DCCB, and comparison of faults clearance and recovery under these faults conditions using ACCB and DCCB, respectively. Results also benefit the application of the DCCB and the operation strategies in high voltage and high power MMC-HVDC systems with overhead lines.

This paper proposes a complete and effective smart over-voltage monitoring and identification system. In recent years, smart grids are of the greatest interest in power system research. One of the main features of smart grid is their self-healing, which can continuously carry out online self-evaluation, discover existing faults, and correct them immediately. The over-voltage smart monitoring-identification-suppression systems play a key role in the construction of self-healing grids. In this paper, eight kinds of common over-voltage are discussed and analyzed. The S-transform algorithm is used to extract features of over-voltage. Aiming at the main features of each kind of over-voltage, six different characteristic quantities are proposed. A well designed fuzzy expert system and a support vector machine are employed as the classifiers to build a two-step identification model. The accuracy of the identification system is verified by field records. Results show that this system is feasible and promising for real applications.

Faults on the dc line threaten the stability and reliability of the modular multilevel converter based HVDC (MMC-HVDC) systems. Fault current limiting is one of the crucial steps to handle the dc line fault. In this paper, a mutual-inductance-type fault current limiter (MFCL) using coupled reactors and few power electronic switches is proposed. Compared with the conventional dc reactors, it can automatically limit the fault current during the current rising stage with an increased equivalent inductance and accelerate the decline of the current by reducing the inductance of the discharge circuit. Consequently, the fault current stress and the total capacity of required arresters are decreased. Compared with the existing power electronic-type fault current limiters, the proposed MFCL provides an inexpensive and potential solution to the suppression of dc fault current using simple fault current limiting topology with fewer power electronic switches. The simulation case study and experimental tests are carried out to verify the performance and practicality of the proposed MFCL.

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.