Transfer capacities of urban distribution networks need to be increased to fulfill the increasing load demands and to accommodate distributed generation (DG). However, there are limited spaces to build new substations and lines, and curtailment of DGs may happen due to voltage violation during DG outputs fluctuation. This paper proposes and analyzes various methods to convert some existing AC MV lines to DC lines in order to form a hybrid AC/DC distribution network, based on which transfer capacities of lines can be increased, and flexible power shift can be achieved through a voltage source converter (VSC) between two lines. The increases of transfer capacities are quantified. Optimal operation to fully utilize the increased capacities is achieved, in which losses are minimized in day-ahead scheduling, and node voltages are regulated real-time within security ranges based on limited measurement. Not only reactive power but also real power optimization are designed to maximize load supply and DG accommodation. A sensitivity method is proposed considering relatively large r/x ratio of a MV distribution network, which is effective for the real-time voltage regulation. Simulations are performed to verify the proposed method.

Recent analyses have shown that the grid-integration of offshore wind farms through MTDC systems has brought low inertia and small-signal stability issues, in which the dynamics of phase-locked-loop (PLL) play a crucial role. To address this issue, this paper proposes a control strategy for the multi-terminal VSCs aiming at PLL-less synchronization and autonomous frequency response of the MTDC system. One of the significant features of the proposed control is that the deviation of the grid frequency can be instantaneously reflected on the deviation of the DC voltage without ancillary control. Based on this feature, a fast inertia response and primary frequency regulation among wind farms and AC systems interconnected by the MTDC system can be achieved. A small-signal model is established to evaluate the overall system stability using the proposed control. Finally, comparative studies of this proposed control with the conventional PLL-based vector control are conducted in PSCAD/EMTDC based on a practical MTDC system in China, the Zhangbei four-terminal HVDC transmission system. The analysis shows that the proposed control exhibits advantages in weak grid operation and autonomous frequency response.

The virtual synchronous generator (VSG) control is regarded as an effective solution for operating converters in weak grid conditions due to its excellent grid support functions. However, successful implementation of the VSG control of an AC/DC converter relies on the existence of a steady DC voltage at the DC side, while this is not easy to achieve for the cascaded converter system of this work, i.e., the Modular Multilevel Converter-based High-Voltage Direct-Current (MMC-HVDC) transmission systems with offshore wind farm integration. To address this issue, a novel grid-forming control strategy with real-time inertia support and direct DC-link voltage regulation is proposed for the Receiving End Converter (REC) of the MMC-HVDC. The intrinsic power balancing regime of the internal energy stored in MMC's submodule (SM) capacitors is utilized for grid synchronization rather than emulating the swing equation as requested by the VSG control. For being more robust to sudden power variations, proper insertion strategies of the REC are developed to decouple the DC-link voltage of MMC-HVDC from the voltage of SM capacitors. Furthermore, in terms of the grid fault-ride-through issue of the REC, an associated FRT control strategy is proposed aiming for power angle stability and the stability of SM capacitor voltage. Finally, simulation results in PSCAD/EMTDC show that the proposed control can provide fast inertia support with satisfactory control of DC-link voltage and FRT capability, etc.

Although the probability of occurrence of station internal ac grounding faults in modular multilevel converter (MMC) based high-voltage direct-current systems is low, they may lead to severe consequences that should be considered when designing protection systems. This paper analyzes the characteristics of valve-side single-phase-to-ground (SPG) faults in three configurations of MMC systems. Fault responses for symmetrical monopolar MMCs are first studied. Upper arm overvoltages and ac side non-zero-crossing currents arising from SPG faults in asymmetrical and bipolar configurations are then investigated. DC grounding using an LR parallel circuit is employed to create current zero-crossings which will enable the operation of grid side ac circuit breakers. The theoretical analysis is verified through simulations performed in PSCAD/EMTDC, with simulation results and the theoretical analysis showing a good agreement. The studies in this paper will be valuable for the design of protection systems for station internal ac grounding faults.