The hybrid distribution transformer (HDT) achieves a compromise between the low-frequency transformer (LFT) and the solid-state transformer. It combines the LFT's efficiency and reliability with the added advantage of partial power controllability via an integrated converter. However, the existing research on the HDT fault ride-through does not consider the coordination relationship with its own safe operation constraints, while few studies have considered capacity coordination between multiple converters. Therefore, this article proposes an improved hierarchical coordination control of integrated HDT (IHDT), considering grid fault and normal conditions. First, the IHDT topology is proposed, and the operation mechanism of IHDT to realize the voltage support and current compensation is clarified by analyzing the coupling mechanism between the converter and LFT. Furthermore, under complex conditions, the unified expressions of the IHDT winding voltage, current, and power are derived, the safe operation constraints of IHDT are analyzed, and a hierarchical coordination control strategy is proposed. Finally, the correctness and effectiveness of the IHDT topology and control strategy are demonstrated by simulation analysis and experimental verification.

The communication-dependent inertia supporting of MMC-HVDC systems transmits the ac frequency information to the MMC station via phase measured units (PMUs), and provides frequency support for the connected ac grid. However, the random disturbances including the communication time delays and denial of service (DoS) attacks in the communication networks introduce new threats on the stable operation of the MMC-HVDC and the connected ac system. This article proposes a Markov-based stochastic stabilization control for MMC-HVDC systems with inertia supporting under random disturbances. Firstly, the Markov time-delay jump model of MMC-HVDC systems with inertia supporting is investigated under random disturbances, while the communication delay and DoS attacks are modeled as the stochastic Markov jumps with bounded time-varying delay and state discontinuities. Then the time delay term is regarded as a kind of equivalent disturbance acting on system input and compensated through the second-order communication disturbance observer (CDO). Besides, a Markov-based finite-time stabilization controller is designed for MMC-HVDC to mitigate the influence of random DoS attacks without need of information about the detailed DoS attack states. Simulations of several cases on PSCAD/EMTDC platform show that the inertia supporting control of MMC-HVDC can remain stable and unaffected in realizing fast frequency recovery for the ac grid under the random disturbances in the communication networks.

Recent applications of Modular Multilevel Converter (MMC) have posed various problems such as ac/dc grid faults, asymmetric operation conditions, ac grid voltage excessive harmonics, and circulating currents, which deteriorate the system's performance and also threaten the safe operation. In this paper, an improved comprehensive control architecture of MMC is proposed to maintain a stable voltage control, and to eliminate harmonics in ac-side and circulating currents, and dc-bus voltage ripple under ac/dc grid faults and harmonic operation conditions. The proposed comprehensive control consists of improved hierarchical voltage control and repetitive controller-based arm current control. The improved hierarchical voltage control, including novel negative sequence current injection (NSCI) and dc circulating current injection (DCCI)-based leg-averaging control and their seamlessly switching handling, enables the MMC to operate under various operating conditions, producing an arm current reference for inner loop control. The proposed repetitive controller-based arm current control can recognize the control of multi-frequency components of arm current without separating positive-, negative- and zero-sequence current components of ac-side current and circulating current. Finally, simulations and experimental studies of the three-phase MMC were carried out in detail, and the findings demonstrate the efficiency and merits of the proposed control method.

Passivity-based control (PBC) relies on an accurate mathematical model and thus its performance will be degraded by the weak robustness against to parameters uncertainties, modeling error, and external disturbances. Moreover, it cannot achieve zero tracking error of the steady-state current under parameters uncertainties and modeling error. This paper proposes a novel disturbance observer (DO) based PBC (DO-PBC) for static synchronous compensator (STATCOM) to achieve better stability and dynamic performances against disturbances. A DO which has been introduced into the PBC current loop is used to compensate system disturbances, which can improve the robustness of the control system and eliminate the steady-state tracking error. Moreover, the proposed DO-PBC provides faster responses in handling various kinds of disturbances. Then, the detail design process, stability and robustness analysis, and parameters tuning method are investigated and presented. Also, the proposed method is simple to be implemented by the separation principal. The performance comparisons among the PI, the conventional PBC, and the proposed DO-PBC are carried out to show the effectiveness of the proposed method against disturbances and the precise current tracking, via simulation tests and experimental tests based on a down-scale laboratory prototype experiment of 380 V STATCOM.

The power router (PR) is a promising piece of equipment for realizing multi-voltage level interconnection and flexible power control in the future distribution power grid. In this paper, a hybrid PR (HPR) topology based on power-frequency transformer electromagnetic coupling with converters is proposed for the medium distribution power grid. The power-frequency transformer is used to undertake power transmission, voltage conversion, and other main tasks, while the power electronic converters are combined to achieve active control. Equivalent magnetic and electrical circuit models are established to help discuss the operating principle of the proposed HPR. Additionally, the power flow and control principle of the HPR in different operating conditions are analyzed, with the control system design scheme presented. The theoretical analysis results are verified by MATLAB/Simulink + Plecs simulation and a controller hardware-in-the-loop study, as well as a down-scale experimental test, indicating that the proposed HPR is flexible in active voltage support and current control.