Line-commutated converter-based-high-voltage direct-current (LCC-HVDC) transmission technology has been developing very rapidly. However, the sensitivity and reliability of HVDC line protection require further improvement. An analysis of different line fault types is conducted to show that the normalized voltage change rate without the influence of wave propagation can more effectively distinguish between internal and external faults with high fault resistances. Owing to the difficulty of obtaining an accurate solution for a traveling wave, a novel traveling-wave propagation calculation method is developed by making use of the step response of a transmission line with consideration for its frequency-dependent characteristics. A novel traveling-wave protection (TWP) scheme for two-terminal LCC-HVDC transmission lines is then proposed based on the developed traveling-wave propagation calculation method and fault analysis results. The proposed protection scheme is tested based on fault simulation data using an EMTDC/PSCAD simulation platform and the fault-recorded data from an actual undesired tripping event of conventional TWP. The results demonstrate that the proposed TWP scheme, compared with conventional TWP, can more reliably and correctly detect faults with high transition resistances under different operation modes.

The deployments of inverter-interfaced distributed generators (IIDGs) in medium voltage distribution networks may cause undesirable trips or tripping failures of conventional two-terminal current differential protective relays. It is economically and technically unfeasible to reconfigure large numbers of time-synchronized sampling clocks and communication channels for each terminal inside the protected zone. In this paper, a more accurate IIDG mathematical equivalent model is established that considers the control strategies of IIDG, and the profile characteristics of the positive-sequence voltage at each point of common coupling are investigated. Then, a real-time estimation algorithm is introduced for the fault current contribution of grid-connected IIDGs. On this basis, this paper proposes a novel virtual multi-terminal current differential protection scheme. This scheme only necessitates exchanging the electrical quantities between the protective relays at both terminals via an existing two-terminal pilot channel, thus reducing the cost and requirements for communications. The feasibility of the proposed protection is demonstrated in both the PSCAD/EMTDC simulation platform and the RTDS hardware experimental laboratory. A comparison of the proposed protection alongside the conventional protection is also presented. Test results indicate that the proposed protection is widely applicable to various fault conditions, thereby improving the sensitivity and selectivity of conventional protection schemes.

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.