Modular multilevel converter based high voltage direct current (MMC-HVDC) transmission systems may increase the short-circuit current and affect the AC fault clearing of circuit breakers. To address this issue, this paper analyzes the influence of the MMC on the short-circuit current when a symmetrical or asymmetrical fault occurs in the AC system. Firstly, the influence mechanism of the MMC on the short-circuit current under symmetrical AC faults is studied. It is found that the short-circuit current magnitude at the fault point is mainly affected by the q-axis output current of the MMC. For the single-phase line-ground fault, the three-sequence equivalent circuit of the MMC is analyzed, and the calculation method of the three-sequence short-circuit current is presented. It is observed that the negative-and zero-sequence currents at the valve side of the MMC are both zero, and the positive-sequence current is determined by MMC operating conditions. Besides, the short-circuit current magnitude under the single-phase line-ground fault is also mainly related to the q-axis output current of the MMC. Based on these results, an additional controller is proposed to reduce the short-circuit current under the symmetrical and asymmetrical faults. Finally, the effectiveness of the proposed mechanism and suppression control is verified by PSCAD/EMTDC simulations.

The unified power flow controller (UPFC) is recognized as one of the most promising flexible ac transmission system (FACTS) devices for modern power systems, due to its versatile functionalities. Based on typical operation conditions, this paper investigates the global and local power flow operation rules of the UPFC embedded transmission system theoretically. First, the detailed global power flow model (GPFM) and the local power flow model (LPFM) are established concerning several selected critical points (SCPs) in the system. The theoretical power flow operation rules for the ranges, the movements and the trajectories of GPFM and LPFM are analyzed, respectively. Then, the case study is performed based on the proposed simplified model, and the results are analyzed from GPFM and LPFM perspectives respectively. The operation rules and the power flow ranges is studied. Further case studies based on the proposed original mathematical model are also carried out, and the results are listed from different viewpoints of two-dimensional planes and three-dimensional spaces. All types of results are also analyzed from GPFM and LPFM perspectives, which verify that the UPFC is adaptive to various operation conditions for power flow regulation.

This paper determines the minimum short circuit ratio (SCR) requirement for a modular multilevel converter based high-voltage direct current (MMC-HVDC) transmission systems. Firstly, a simplified model of MMC is introduced; the MMC is represented by its AC and DC side equivalent circuit. Next, by linearizing the MMC subsystem and the DC network subsystem, the deduction of the small-signal models of MMC subsystem, the small-signal model of the DC network and MMC-HVDC are carried out successively. Thirdly, the procedure for determining the minimum SCR requirement of MMC-HVDC is described. Finally, case studies are performed on a two-terminal MMC-HVDC system under four typical control schemes. The results show that the restraint factors for the rectifier MMC is predominantly the voltage safety limit constraint, and the restraint factors for the inverter MMC are mainly the phase locked loop (PLL) or the outer reactive power controller. It is suggested that the minimum SCR requirement for the sending and the receiving systems should be 2.0 and 1.5 in the planning stage.

The implementation of a 500 kV unified power flow controller (UPFC) project in Suzhou southern power grid is reported in this study. The UPFC project has been put into operation at the end of 2017, and has become the largest capacity and highest voltage‐level UPFC in the world. The overview of the Suzhou southern power grid is first introduced. The challenges of the grid, such as the limited transmission capability and the insufficient reactive power supply, are also demonstrated. Great technical and economic difficulties have to be overcome for the implementation of traditional solutions, so an attempt of the UPFC scheme is put forward. The UPFC scheme is introduced in detail, including the topology structure, the main equipment selection, the expected effect, the implementation difficulties and the investment. It is shown that the 500 kV UPFC project is more economical and feasible than the traditional solutions. The operation problems of the Suzhou southern power grid will be alleviated effectively by the UPFC project. Besides, this project is of much demonstration value to the application of high‐voltage‐level and large rating UPFC projects.

Latest unified power flow controller (UPFC) projects adopt novel device structures to meet the requirements of practical applications. Developing power flow calculation methods for these new UPFC structures is of great significance for the design and operation of related projects. To address this issue, this paper deduces an equivalent model of the general structure UPFC, and presents the modified power mismatch equations and Jacobian matrix of the system. Then, three power flow calculation methods suitable for the novel structure UPFC are proposed based on the Newton–Raphson algorithm. The main difference between the three methods is the processing method of UPFC equivalent power injections and the Jacobian matrix. The characteristics of these methods are compared by case studies. Results demonstrate the effectiveness of the three methods for different UPFC structures, and it is found that the improved method, which considers the impact of the Jacobian matrix modifications in UPFC equivalent power injections, can realize power flow calculation in existing calculation programs, and improve the convergence characteristics.

As the latest generation of flexible AC transmission system (FACTS) devices, the unified power flow controller (UPFC) has comprehensive control capability and has played an important role in modern power systems. Research on UPFC steady-state power flow regulation capability is valuable for the design and operation of UPFC projects. To analyze the UPFC regulation capability, in this paper, the interaction between the UPFC and the external system is studied and the key factors of UPFC regulation capability are analyzed. It is demonstrated that the changes in the voltage magnitude and phase angle difference of UPFC-embedded line terminals will hinder the UPFC from regulating the power flow and decrease the UPFC regulation capability. Therefore, the UPFC power flow regulation capability is not only related to the parameters of the UPFC and its installation line, but also related to grid parameters. To analyze the UPFC active power flow regulation capability in practical power grids, the relationship between the UPFC regulation capability and grid parameters is deduced, and an estimation method is proposed to calculate the regulation range. Then, the estimation method is applied in the Chuxiong Power Grid and compared with power flow calculations. The results verify the UPFC power flow regulation characteristics given by the analysis and the effectiveness of the estimation method. Besides, it is shown that the estimation method can lower the requirements for the power flow calculation program, and reduce computation load.