Zhihui Dai

High-resistance faults on the DC lines of multi-terminal VSC-HVDC grids lead to insufficient protection reliability, and the introduction of current-limiting strategies alters the system’s intrinsic fault characteristics, degrading protection performance. To address these issues, we propose a DC-line protection scheme that is immune to converter control strategies and highly tolerant to fault resistance. First, based on the grid topology, post-fault current paths are analyzed, and the fault characteristics produced solely by the fault-induced voltage source are identified. A sequential overlapping derivative transformation is then employed to magnify the discrepancy between internal and external faults, forming the core of the fault-identification criterion; the zero-mode component is used for pole selection. Finally, a four-terminal VSC-HVDC model is built in PSCAD/EMTDC version 4.6.2 for validation. Simulation results show that, after applying the current-limiting strategy, the characteristic quantity changes only marginally, and the proposed protection can reliably withstand fault resistances of up to 700 Ω.

For generator stator ground faults, the active arc suppression method typically needs the large-capacity inverter, leading to huge size and high cost. Moreover, the existing compensated parameter calculation method based on coil potentials cannot suppress arc at the non-coil connection points. To address these limitations, a hybrid flexible arc suppression method is proposed in this paper. By combining with the arc suppression coil grounding method, only the third harmonic and partial fundamental currents need to be compensated by the active arc suppression device, which significantly reduces the inverter capacity requirement. Instead of the coil potential analysis unit, slot potential analysis unit is used to characterize the winding potential distribution. Analytical equations are formulated to precisely calculate the fundamental and the third-harmonic fault potentials across the entire windings. The calculation results are used to determine the compensated parameters, enabling reliable arc suppression at any fault position within the entire windings. Effectiveness of the proposed method is validated by simulation and experiment.

The Rotating Asynchronous Machine (RAM) system of a nuclear power plant provides a stable and reliable power supply for the control rod drive mechanism (CRDM) through two parallel generators. At present, the main protection of the generators in the RAM system is only configured with longitudinal differential protection, which cannot reflect the interturn short circuit fault and interbranch short circuit fault of the stator winding. Based on the characteristics of parallel connection of RAM generators in nuclear power plants, this article proposes a dual-generator split phase transverse differential protection and a dual-generator incomplete longitudinal differential protection. Through the internal fault set of the generator and the internal fault potential splitting model of the generator, the quantitative analysis of different main protection configuration schemes is carried out. It is concluded that the existing longitudinal differential protection and the incomplete longitudinal differential protection of two parallel generators can correctly reflect the vast majority of internal short-circuit faults. Finally, the dynamic experiment is carried out to prove the feasibility of the protection method in the actual unit operation.

With the widespread integration of distributed energy resources and novel loads, the DC attributes of distribution networks are becoming increasingly pronounced. Multi-terminal flexible DC distribution networks have emerged as a trend for future distribution grids due to lower line losses, better power quality, etc. However, owing to their low damping and inertia, the multi-terminal flexible DC distribution network is vulnerable to DC faults. Analyzing the fault characteristics and calculating the fault current level is of great significance for the design of relay protection systems and the optimization of associated parameters. Throughout the fault process, the discharge paths of multiple converters are mutually coupled, and the fault characteristics are complex, which poses a great challenge to short-circuit calculations. This paper proposes a method for calculating the characteristic quantities of the whole network throughout the asymmetric short-circuit fault in a multi-terminal flexible DC distribution network. During the capacitor discharge stage, an equivalent model of the fault port is established before the control response. During the fault ride-through stage, a transfer matrix that takes into account the electrical constraints on both the AC and DC sides of the converters is proposed by combining the equivalent circuit of fully controlled converters. Finally, a simulation model of a six-terminal flexible DC distribution network is developed in PSCAD/EMTDC, and the simulation results demonstrate that the proposed method expands the calculation range from faulty branch to network-wide characteristic quantities throughout the process of asymmetric short-circuit faults, with the maximum relative error remaining below 5%.