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When investigating voltage and current stresses in critical main circuit components during faults of the converter, a detailed equivalent model capable of representing the balancing control strategies of the capacitor voltages on the submodule level, along with blocking and delocking, is always necessary. Among previously proposed equivalent models of the modular multilevel converter (MMC), only submodule averaged models (SAMs) can capture interested inner dynamics inside each arm. However, the simulation accuracy of SAMs is not always satisfactory, especially when the time step is larger than 10 $\mu$s. In order to further improve the simulation accuracy with guaranteed simulation efficiency, the shifted frequency modeling of the half- and full-bridge hybrid MMC is proposed in this paper. Therein, each submodule is represented by Thévenin equivalents derived by submodule dynamic phasors. The arm of the MMC is represented by Norton equivalents to guarantee the efficiency, considering both normal and dc-blocking conditions. The effectiveness of the proposed model in terms of accuracy and efficiency is validated by simulating an MMC-based HVdc transmission. [ABSTRACT FROM AUTHOR]

The emerging subsynchronous resonance (SSR) caused by the interaction of wind turbine generators (WTGs) with series compensation has aroused great concerns. For this particular issue, this paper is aimed to fill the gap between theoretical studies and actual observations. By analyzing the field data of 58 SSR events captured in a practical wind power system and examining the observed dynamics with previous theoretical results, the mechanism and characteristics of SSR are revealed in a more explicit and substantial way. The necessary conditions and dominant influential factors are identified and the underlying reasons are discovered. Theoretically derived as well as practically measured impedance models have demonstrated that the converter control of doubly fed induction generator (DFIG) produces negative resistance at the slip frequency and thus causes unstable SSR; while permanent magnet synchronous generators and self-excited induction generators are just passively engaged in those SSR incidents. The distribution of the oscillation frequency has also been examined with field measurements. It is discovered that WTGs at different locations participate into the same SSR mode and their frequencies are not fixed but keep changing with the time, the variation of grid topology, and the number of online generators. [ABSTRACT FROM PUBLISHER]

Blocking filters (BFs) were designed to solve subsynchronous resonance (SSR) problems induced by fixed series capacitors (FSCs) of the Tuoketuo transmission system. However, during the commission of BF, unstable self excitation (SE) occurred unexpectedly, resulting in suspension of the project. This paper first analyzes the mechanism of how BF causes SE. Then, the parameters' sensitivity is investigated and the quality factor (Q) of BF reactors is discovered to have opposite impact on SE and torsional interaction (TI). Thus, by appropriately lowering Q of BF reactors with supplemental resistors, an improved BF scheme is put forward to simultaneously mitigate the different types of SSR problems. As a key issue, the determination of supplemental resistors is formulated into a multi-objective optimization problem, which is then effectively solved with our proposed searching method. Finally, the effectiveness of improved BFs is verified with field tests. For the first time, actual measurements of filed tests on BFs are offered. What is more, their comparison with model studies demonstrates a good agreement. This work paves the way for putting both BFs and FSCs into service, thereby ensuring safe operation and transfer-capability enhancement of the system. [ABSTRACT FROM PUBLISHER]

DC short-circuit fault is a crucial issue in DC grids because the DC fault current increases rapidly with accompanying high fault energy. Limiting the DC fault current is a promising solution to simplify the design of the DC breaker and DC protection. The majority of existing DC fault current limiting (FCL) approaches relies on extra devices, such as DC line inductors. However, the control ability of modular multilevel converters (MMCs) is not optimized. In this study, the submodule (SM) capacitor dynamics during the short DC fault procedure was considered a new control objective, and its effect on the limiting DC fault current was analyzed and revealed. The complex time-varying DC fault transient circuit was comprehensively represented by introducing two base circuit structures and one duty cycle index D. The averaged circuit model was proposed on the basis of the state–space averaging method, which can intuitively show the relation between SM capacitor and DC fault current. The analytical expressions of the DC fault current was given, which can guide the design of the DC FCL control of MMC. The duty cycle index D can represent the discharge of SM capacitors, and the increasing speed of the DC fault current declines with the decrease in D. Accordingly, a feasible FCL control strategy was proposed by choosing D as a tool for affecting and controlling the DC fault current. The proposed FCL control strategy can evidently reduce dc fault current, and avoid the drawbacks of the conventional block or bypass approaches. The simulation results validated the effectiveness of the proposed analysis method and corresponding conclusions. [ABSTRACT FROM AUTHOR]