The modular multilevel converters (MMCs) have emerged as the most suitable converter technology for HVDC applications. Besides the recognized advantages over conventional voltage source converters, one of the remarkable features of the MMC is its ability to store energy in the distributed submodule capacitors. This important feature can be used to mitigate the fluctuations of the DC voltage, which is inherently volatile against power disturbances compared to the frequency of conventional AC systems. This paper proposes a novel control strategy, called virtual capacitor control, which enables the utilization of the energy storage capability of the MMC to attenuate voltage fluctuations of HVDC systems. With the proposed control, the MMC behaves as if there were a physical capacitor whose size is adjustable and can be even bigger than the physical capacitor embedded in the converter. This control allows the system operator to optionally adjust this virtual capacitor of each MMC station and, thus, it provides an additional degree of freedom to the HVDC system operation. The EMT simulations of a 401-level MMC-based HVDC link system show the effectiveness of the proposed control to improve dc voltage transient dynamics.

Electric power transmission and distribution (T and D) systems are composed of a great deal of aged apparatus, which may cause a decrease in reliability owing to their deterioration. In order to maintain high efficiency and high quality in T and D systems, the authors have proposed an "intelligent grid management system" (IGMS), which determines the optimum maintenance strategy and optimum power flow control based on condition monitoring and diagnostic results of the operating power apparatus. This means that the IGMS essentially includes both concepts of an asset management system and a smart grid. Further, the IGMS optimizes power flow routes and maintenance plans based on the failure risk, T and D loss, overload operation, life estimation of the power apparatus, customer outage, and other metrics. The impact of individual apparatus failure affects the entire T and D system's performance, causing blackouts and secondary failures. Reduction in reliability of whole system is highly dependent on ageing of the materials. The IGMS evaluates all of the events occurring in the T & D system as the cost. Additionally, the risks are evaluated in the cost according to the impact of the failure rate estimated by the condition monitoring diagnosis results of the power apparatus. Insulation system determines the transition in reliability and maintenance cost of the system. In this paper, the IGMS is applied to T and D system models including aged apparatus, such as transformers and circuit breakers, and suitable power flow routes and maintenance strategies are derived. Consequently, by the effective application of the IGMS, the system reliability can achieve an optimum state, and the total cost can be minimized.

This paper proposes a comprehensive methodology for determining the converter droop settings to ensure the voltage security in a multi-terminal direct current (MTDC) grid. The novelty lies in the introduction of the concept of over- and under-voltage containment reserves in analogy to the frequency containment reserves in AC systems. This not only gives a distinction of the contribution between the converters in rectifying and inverting modes, but also significantly facilitates the sizing of the necessary amount of reserves within their available headroom capacity of power. In particular, the constraints of converter outages, converter rating, and DC voltage deviations are all taken into account in the design of reserves. The effectiveness of the proposed approach is demonstrated by a contingency power flow analysis algorithm and EMT simulations of a four-terminal MTDC grid.

Massive off-shore wind integration in the European power system, the necessity to increase the power flows between asynchronous areas and inside continental Europe, call for HVDC transmission development, and multi-terminal-HVDC-based transmission. HVDC technology is highly controllable, but also sensitive to faults and degraded modes. Achieving an efficient and robust AC/DC transmission system requires to develop a vision of HVDC role and functionalities in different situations. In addition to HVDC converter core control functions and DC grid protection strategies, a coordinated DC grid control layer is beneficial to supervise control modes towards overall AC/DC transmission system security and stability, as well as to ensure converter controls interoperability. This paper offers a vision of what a robust AC / DC network can be, based on control and protection strategies that can be implemented via HVDC converter capabilities.

This letter first identifies a new mode of low-frequency harmonic instability when combining grid-forming control with the typical dc voltage droop control necessitated in multiterminal high voltage direct-current transmissions. This mode does not exist when grid-following control is applied and its mechanism is explicitly revealed by the dc-side input admittance modeling of the converter. Then, a phase compensator plus a virtual power system stabilizer are introduced to pacify the input admittance within the critical frequency range and derisk the related instability. Last, it is also demonstrated that the proposed control strategy achieves excellent performance in contingent active power imbalance ride-through in the dc system. All the analyses are verified by electro-magnetic-transient simulations with a modular multilevel converter model comprising 278 submodules per arm.

Purpose: Self-commuted voltage source converter (VSC) can significantly extend the flexibility and operability of an HVDC system and be used to implement the concept of multi-terminal HVDC (MTDC) grid. To take full advantage of MTDC systems, its overall behaviour must be characterized in quasi static and dynamic states. Based on the numerous literatures, a dedicated two-level VSC model and its local controllers and DC grid voltage regulators are developed for this purpose. Furthermore, the requirement of the system to guarantee all the physical constrains must be well assessed and concrete demonstrations must be provided by numerical simulations. Design/methodology/approach: First, a two-level VSC model and its local controllers and DC grid voltage regulators are developed. Then, DC cable models are investigated and their characteristics are assessed in the frequency domain. Those developed models are combined to form a three-terminal HVDC grid system on Matlab/Simulink platform. To analyze the stability of this electrical system, the dynamics of the system against variations of power dispatch are observed. Findings: To analyze the stability of this electrical system, the dynamics of the system against variations of power dispatch are observed. The differences in the DC grid voltage dynamics and the power flow of the converter stations coming from the embedded primary controls are analysed, and the technical requirements for both cases are assessed. Originality/value: In this paper, the dynamic stability of an MTDC system has been analysed and assessed through an adequate simulation model, including its control scheme and the cable models. The interest of the improved PI model for cables is highlighted.