An improved approach based on sequential method for the AC-DC power flow calculation is proposed in this paper. This approach solves the convergence problem caused by voltage violations at AC buses during the power flow calculation for the DC subsystems. The convergence property can be significantly improved by adjusting the converter transformer tap position flexibly. In order to adjust the tap position of the converter transformer flexibly, three mainly modifications are proposed. Firstly, the equations for whole DC systems are decoupled into individual DC systems so as to easily figure out which DC system's tap position needs adjustment. Secondly, the tap ratio of a converter transformer is selected as an alternative state variable to replace the cosine of the control angle when necessary. Thirdly, the Newton-Raphson method is utilized to solve DC subsystems instead of the method using the linear equations. Furthermore, a theoretical analysis of the advantages of the proposed approach is also presented. Numerical simulations and practical applications show that the proposed approach meet the requirement of different system operating conditions and has advantages in terms of convergence and speed. The proposed approach has been successfully integrated into the Energy Management System (EMS) for China Southern Power Grid. Index Terms-AC-DC, HVDC transmission, multi-infeed DC systems, Newton-Raphson method, power flow analysis. I. INTRODUCTION C HINA has made remarkable strides in high voltage direct current (HVDC) transmission implementation. At present, there are more than ten HVDC lines in operation in China. By 2020, the power grid of China will become a robust AC-DC system with ultra high voltage links. China will have 33 HVDC lines by then. North China Grid and Central China Grid will become AC-DC hybrid power systems with multi-infeed DC systems. Furthermore, the complexity of East China Grid,

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 Abstract-This paper introduces an enhanced droop-based DC voltage control method, including dead-band, for applications to the high-voltage direct-current (HVDC) grid that utilizes the voltage-sourced converter (VSC) technology. The proposed droop-control structure also autonomously imposes energy balance between the HVDC grid and its host AC system. The droop-control method (i) divides the VSC stations into four groups, (ii) activates the droop-control of each group based on a pre-specified voltage margin and (iii) introduces an improved power-voltage characteristic for desirable VSC station dynamic performance. Feasibility and performance of the proposed control method are evaluated based on time-domain simulation studies in the PSCAD platform, using the IEEE-39-Bus system which imbeds a five-terminal VSC-HVDC grid. Each VSC station is a monopolar modular multilevel converter (MMC). The study results show that the proposed droop-control method enables the HVDC-AC system to reach a new steady-state after transient events.

For decades high voltage alternating current (HVAC) was considered as most economical solution to transmit and deliver electric power. With the recent developments in power electronic devices, high voltage direct current (HVDC) system becomes most prominent technology. Multi-terminal direct current (MTDC) based system as a promising technology for future power system is the major focus area for researchers and industries these days. A number of MTDC systems have been implemented physically. The major motivation to construct such MTDC systems is the integration of large-scale offshore power sources such as wind turbines and solar systems. This paper discusses the most critical challenges and issues related to operation, control and protection schemes for integration of modular multi-level converter (MMC) based MTDC systems. At first detailed literature survey has been presented to show the challenges for MMC based MTDC systems, then an analysis related to those challenges for operation, control and protection schemes for existing MMC based MTDC systems has been provided. Finally, a road map to tackle such challenges has been suggested.