With the wide use of power electronic equipment in power systems, harmonic problems have become one of the more prominent ones in the power grid. In recent years, there are many HVDC 50Hz protection mal-operation events in the field caused by converter transformer unloaded energizing. The phenomena of harmonic transfer and amplification in HVDC has an undeniable impact on overall power system operation. Currently, the study about this has relied mainly on detailed electromagnetic transient simulations. However, simulations normally have large computational burdens and lower efficiency due to the complexity of their models and small simulation time-steps used. So an efficient analysis model of harmonics with HVDC various operating modes is suggested in the paper. This model considers DC smoothing reactors and filters in different operating modes, and the modelling of converters and transmission lines are highlighted. Compared with detailed electromagnetic simulation results confirm the validity and accuracy of the model. Moreover, it also can be easily applied to the HVDC systems for harmonic amplification evaluation. All the results show that the proposed model not only has lower computational requirements but also can represent HVDC harmonic transfer and amplification during different operating modes.
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Lines with common-terminal is an economic and practical erection form in the multi-circuit transmission system, which also presents extra challenges to the protection. The feature of cross-line fault on common-terminal lines and its impact on the protection principle are discussed in this paper. Theoretical analysis shows that sound phase can also exhibit fault feature, under the effect of fault phase in the neighboring line. Furthermore, combined with inter-phase measured impedance (IPMI) expression derived in this paper, the operation characteristic of different distance relays are discussed by their setting range. For the loop only has one fault phase, the risk of maloperation is extremely high when fault is nearing the protection installation. The accuracy of theoretical analysis is verified by the validation tests in PSCAD. © 2018 IEEE.

The transmission corridor is scarcity in many areas. Therefore, tower with multi-circuit transmission lines has a high proportion in the power grid. Moreover, it is common that there are short and ultrashort line, diverse tower structure and different wire parameters in multi-circuit lines. Most of the normal modeling methods for multi-circuit lines on the same tower are based on the distributed parameter model. However, they focus basically in the long lines, single type of tower structure and one kind of wire parameter. The present modeling methods cannot deal with the problem in short lines, diverse tower structure and different wire parameters. Hence, this paper proposes an approach to process this problem is proposed through studying the influence of the change of the wire parameters on the impedance/admittance matrix. The modeling approach combines the distribution parameter characteristics of tower structure and the good adaptability for short lines of the lumped parameter model. It can be applied to short lines with different tower structure and wire parameters. High accuracy of the modeling approach in this paper is verified with the measured parameters.