Double-circuit transmission lines, with bipolar transmission lines of two different line-commutated converter-based high-voltage direct-current (LCC-HVDC) systems constructed on the same tower, are a new technology emerging in China to increase power transmission capacity. Double-circuit HVDC transmission lines sharing the same tower will lead to an electromagnetic coupling effect of the two circuits. According to the geometry configuration of same-tower double-circuit transmission lines in HVDC trans-mission projects, double-circuit transmission lines are unbal-anced without transposition. Therefore, the coupling effects on the healthy circuit cannot be completely eliminated. These cou-pling components will affect the sensitivity of conventional travel-ing-wave-based protection criteria. This work first develops a fault analysis for double-circuit HVDC transmission lines with various internal fault conditions, and then the linear combina-tions of four independent modal components based only on each single circuit are achieved and investigated. Finally, an integrated traveling wave-based protection scheme for double-circuit HVDC transmission lines is presented. Numerous simulation results verify that the proposed protection scheme can operate correctly in double-circuit HVDC transmission lines even for a fault with high resistance up to the maximum of 300 Ω.

DC circuit breakers (DCCBs) are vital for the safe and continuous operation of large high-voltage DC grids. Insulated gate bipolar transistor (IGBT)-based hybrid DCCBs (IHCBs) can interrupt currents quickly and have good operational characteristics. Thyristor-based hybrid DCCBs (THCBs) are superior to IHCBs in terms of interruption capacity because of the large surge current capacity of thyristors. Nevertheless, existing THCBs have limitations regarding one or more of the following characteristics: pre-activation, fast reclosing, quick interruption of small current and no requirement for an additional pre-charge source. This paper proposes a thyristor full-bridge-based DC circuit breaker (TFCB) with all favorable characteristics mentioned above. The TFCB contains a main conductor, a main breaker (MB), an auxiliary branch and an energy absorber. After an interruption, the capacitor in the TFCB can restore its initial voltage quickly to guarantee that the TFCB is available for the next interruption. Quick interruption of small currents is achieved using the additional charge current provided by the auxiliary branch. Pre-activation is achieved using the conduction path in the MB. The parameter designs are analyzed to ensure that the TFCB reliably interrupts currents without damaging itself. The proposed TFCB is validated using prototype experiments and PSCAD/EMTDC simulations.

A novel fault location scheme using feeder terminal units (FTUs) for high-impedance grounding faults in resonant grounding systems (RGSs) is presented. The parallel resonance for a high-impedance fault (HIF) in an RGS with inverter-interfaced distributed generators (IIDGs) is analyzed. The process exhibits unique distribution features for transient zero-sequence components involving underdamped and overdamped states. We conclude that the transient zero-sequence admittances (TZSAs) from FTUs upstream of the fault point have the maximum value, while the TZSAs obtained from FTUs downstream of the fault point are small for HIFs. Therefore, the proposed location scheme uses FTUs to calculate the maximum values of the TZSA. By comparing with action criteria, the distribution automation (DA) master station utilizes the above results of each FTU to locate the faulty feeder section. The accuracy of the fault characteristics and the proposed method are validated by applying a real-time digital simulator (RTDS) and an FTU-based monitoring system. Additionally, the proposed scheme demonstrates an excellent and reliable performance, considering various faulty feeder sections, fault resistances, arcing faults, fault inception angles, imbalances and the presence of IIDGs and noise.

The Conventional zero-sequence (ZS) overcurrent protection method exhibits poor performance for single-phase to ground faults (SPGFs) with high impedance in low-resistance grounded systems (LRGSs), a type of medium-voltage (MV) distribution system. Hence, in this study, a novel SPGF detection methodology, designed specifically for LRGSs, is proposed. Considering the effects of system imbalance, a crucial limitation to the method based on ZS components, the concept of the delta-circuit is first utilized to suppress such effects, maintaining the independence of the ZS network. Based on this, the proposed method computes the ZS power factor of a line to discriminate the line state, with the assistance of single-phase instantaneous reactive power theory (SIP) and the 4th-order components of the instantaneous active power. Not only the arcing features, but also the inherent characteristics brought by the unique grounding mode are subtly involved. Multiple simulations conducted in PSCAD software verify the good performance of the proposed detection method for various SPGFs, including those with low or high impedance and those with linear fault impedance or nonlinear arcs.

With the trend towards achieving high efficiency, high power density and high operating frequency in power converters, a new circuit architecture is proposed in this paper for high step-up photovoltaic applications. This converter consists of a current-fed resonant push-pull converter (PP) as a DC transformer (DCX) dealing with most of power with high efficiency and an active clamp flyback converter (ACF) as a regulator within a large conversion range. As a single stage converter, the ACF regulator is paralleled with PP converter to share the total power and regulates the high output voltage. An experimental prototype with an input voltage range of 24 V—32 V and output 400 V/400 W is built under 1 MHz switching frequency. A peak efficiency of 97.1% and a power density of 210 W/in3 (or 13 W/cm3) are achieved. Experimental result validates the correctness of the analysis and proves the feasibility of the proposed converter for MHz high step-up DC-DC conversion.

Because the traditional zero-sequence overcurrent protection method is not effective in detecting single-phase-to-ground faults (SPGF) in a low-resistance grounded system (LRGS), this paper analyzes the fault characteristics of a 10-kV LRGS in detail. From the perspective of the time domain, the relationship between zero-sequence current and zero-sequence voltage is deduced, and the characteristics corresponding to faulty lines and nonfaulty lines are analyzed. The analysis reveals that the ratio fluctuation coefficients of the zero-sequence current to the differential zero-sequence voltage corresponding to faulty lines and nonfaulty lines have notably different characteristics; consequently, a high-sensitivity SPGF detection method is proposed. This method considers the existence of unbalanced loads and asymmetric parameters in the distribution network, can effectively identify high-impedance faults as high as 5000 Ω and nonlinear arc grounding faults, and can resist noise interference with a signal-to-noise ratio of 20 dB. Finally, many simulations and comparisons based on PSCAD/EMTDC verify that the proposed detection method has better applicability than the existing methods in detecting high-impedance SPGFs.