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The push-pull DC autotransformer (PPDAT) is a potential power equipment to interconnect the MMC and LCC HVDC systems. However, the harmonic stability of the PPDAT remains an unsolved challenge. In this article, considering the dual closed-loop control and bidirectional DC voltage operation of the PPDAT, its DC impedance models are proposed using the harmonic transfer function (HTF) method. Both the DC impedance models at the high/low voltage side considering the harmonic coupling and internal dynamics of the hybrid MMC/LCC HVDC system are deduced. The accuracy of the detailed DC impedance models is validated in PSCAD/EMTDC simulations. The impedance analysis under bidirectional DC voltage operation scenarios reveals that the DC voltage polarity affects the low frequency region. Besides, the main impact of PPDAT's parameters on the wideband impedance characteristics is disclosed. It shows that unreasonable parameter design will bring negative damping characteristics near the resonance point (RP). Moreover, stability analysis considering the interaction of PPDAT and LCC-HVDC at the high-voltage side is conducted. The results indicate that the potential resonances can be readily predicted through the Nyquist diagrams and the negative damping of the PPDAT may cause instability at RP.

Fault management is one of the crucial functions of Smart Grid to bring system back to normal operation from fault events. In this paper, we propose a multi-period cooperative solution for the jointly repair and restoration problem in distribution systems (DSs) to minimize the de-energized loads. In the proposed scheme, the novel fully-controlled power-electronic device SOP (soft open point) and fault isolation are included to accelerate system restoration. As the repair and restoration process goes forward, system topology changes frequently. How to determine the control mode of SOPs considering their characteristics during the entire restoration process is rarely studied in the literature. To this end, we present a new virtual power flow based automatic solution to adaptively perform control mode switching for the SOPs in faulted DS. Furthermore, a co-optimization model is formulated that coordinates SOP control, repair scheduling and DS restoration to obtain optimal repair and switch action sequences matching SOP control mode for DS resilience enhancement. The fault isolation is also conducted, since it has great impacts on repair and switch operations and can guarantee the safety of repair crews in practice. Numerical simulations on a modified IEEE 123-node test system and a large-scale actual distribution system verify the effectiveness of the proposed model.

To detect the dc faults for the modular multilevel converter (MMC)-based dc grids using overhead line transmission, many protection methods in phase-domain have been proposed. These existing protection methods suffer from incomplete function, weak theoretical basis, and sensitivity to fault resistance and noise disturbance. To overcome these shortcomings, this article proposes an improved dc fault protection algorithm using the modal-domain approach for the MMC-based overhead dc grids, which decouples the interaction between positive and negative poles and mitigates the strong frequency dependence of the characteristic impedance in phase-domain. The dc fault equivalent circuits are established in modal-domain and the fault characteristics during the initial stage are analyzed. Based on the modal-domain analysis, the line-mode reactor voltage which combines the fault characteristics of negative and positive reactor voltages is used to identify the internal faults. The zero-mode reactor voltage which enlarges the differences between faulty and healthy poles is used to select the faulted pole. This method is robust to fault resistance and noise with high detection speed. In addition, it is not affected by power reversal, ac faults, and dc circuit breaker (DCCB) operation, which are validated and evaluated by the simulations in PSCAD/EMTDC.

This paper proposed a novel type of DC-DC converter, termed as un-interrupted DC Autotransformer (un-interrupted DC AUTO), which has the capability to ride through DC fault on either side of the converter without blocking any devices. Besides that, the adequately designed converter retains the advantages of DC AUTO technology, namely reduced investment cost and transmission loss. To begin with, the topology and basic attributes of the un-interrupted DC AUTO are illustrated. Then the design goals and the corresponding design procedures are analyzed. Furthermore, an effective control system is proposed to enable the converter steady operation during both normal and faulted conditions. Finally, a 1000 MW ±320 kV/±640 kV test system is built on PSCAD/EMTDC platform to verify the technical feasibility of the proposed converter and the effectiveness of the control strategy.