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.

Despite being discovered more than 20 years ago, nanofluids still cannot be used in the power industry. The fundamental reason is that nano-insulating oil has poor stability, and its electrical performance decreases under negative impulse voltage. We found that C₆₀ nanoparticles can maintain long-term stability in insulating oil without surface modification. C₆₀ has strong electronegativity and photon absorption ability, which can comprehensively improve the electrical performance of insulating oil. This finding has great significance for the industrial application of nano-insulating oil. In this study, six concentrations of nano-C₆₀ modified insulating oil (CMIO) were prepared, and their breakdown strength and dielectric properties were tested. The thermally stimulated current (TSC) curves of fresh oil (FO) and CMIO were experimentally determined. The test results indicate that C₆₀ nanoparticles can simultaneously improve the positive and negative lightning impulse and power frequency breakdown voltage of insulating oil, while hardly increasing dielectric loss. At 150 mg/L, the positive and negative lightning impulse breakdown voltages of CMIO increased by 7.51% and 8.33%, respectively, compared with those of FO. The AC average breakdown voltage reached its peak (18.0% higher compared with FO) at a CMIO concentration of 200 mg/L. Based on the test results and the special properties of C₆₀, we believe that changes in the trap parameters, the strong electron capture ability of C₆₀, and the absorption capacity of C₆₀ for photons enhanced the breakdown performance of insulating oil by C₆₀ nanoparticles.

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.