Abstract—Offshore wind farms connected with HVDC transmission line is a promising solution to bring the power to shore and assure the system efficiency. A DC-DC converter with high step ratio is required for such application. The modular multilevel converter (MMC) for transformer-less DC-DC converter application is regarded as an alternative solution to replace the two-stage DC-AC-DC conversion. A hybrid multiport modular multilevel DC-DC converter is proposed in this paper, which have one high voltage port and multiple low voltage ports. The low voltage ports can be connected to the dc output of wind turbines. The bidirectional power flow is realized by controlling the arm voltages. The proposed converter has a lower circulating currents as compared to the single-port MMC DC-DC converter. By using the full bridge submodules (FBSMs), the converter also has DC fault blocking capability. 

Abstract—Modular Multilevel Converter (MMC) is composed by hundreds of sub-modules (SMs). Such a large scale power electronic switches and power nodes will become very difficult to solve in the process of real-time simulation of the electromagnetic transient. These switches will occupy a very large amount of calculation, in this case, still use a large step (microsecond) simulation has been unable to meet the sub-module operation during dynamic  response, especially when a short circuit fault occurs in the bridge arm, the electromagnetic transient process of the system cannot be accurately reflected. In order to solve the problem, this paper presents a simplified sub-module model and an equivalent valve model based on state-space node solver implemented in FPGA for the real-time simulation. This method can effectively reduce the power nodes and decrease the computation time, and improves the accuracy of the simulation. In addition, the multi-rate simulation of RT-LAB and the test method of hardware-in-the-loop (HIL) are explained in this paper. Finally the simulation results show that the effectiveness of the proposed method.

Abstract—Xiamen MMC-HVDC demonstration project is the first real bipolar MMC-HVDC project in the world. Generally, before field commissioning of the project, the performance of control and protection system must be verified using hardware-in-the-loop (HIL) real time digital simulation. This paper presents the configuration and performance of a
HIL test platform based on RT-LAB. Results from the HIL test platform and the field test are presented in this paper. By way of comparison between the results of HIL test and field test, the validity of RT–LAB HIL test platform is confirmed.