Control of MMC-HVDC System and Its Application to the Jeju Island Power System

Abstract

This dissertation presents an analysis and control of a MMC-HVDC system under various conditions and its application to Jeju Island power system. The MMC is a promising converter topology for the HVDC applications. Thus, the structure and operating principle of the MMC should be understood well. The suitable selection of the modulation methods will reduce losses in the converter. To design the MMC, the principles to calculate two of main components, namely arm inductance and submodule capacitance, are also important. After getting the parameters, the control methods of the MMC-HVDC system will be analyzed under various operating conditions. The effectiveness is verified by using PSCAD/EMTDC simulation program. Under normal operating conditions, the aim is to demonstrate that the MMC can operate as a VSC. The operation is tested in three cases:  Independent control of the active and reactive powers  Active power reversal control  AC grid voltage control Under unbalanced voltage conditions, the HVDC is required to operate reliably without any trip. If the PI controller is used, the operation method of the HVDC will depend on the reference values of the negative sequence currents. That is three cases of controlling:  Eliminating negative sequence currents  Eliminating oscillation of the active power at the PCC  Eliminating oscillation of the reactive power at the PCC If the PR controller is used, the control aim will be the elimination of the negative sequence currents. The effectiveness of the PI and PR controllers will be compared by using simulation results. Under nonpermanent DC faults, the control objective is to protect for the IGBTs and diodes in the submodules. In this case, two thyristor switches are used to connect in parallel to the diode D2 of the submodule. A DC fault will become an AC fault, and the DC fault current will be extinguished. Then, the MMC-HVDC system is back to the normal operation. The control method to eliminate the circulating current is also analyzed. Under unbalanced voltage conditions, the controller of the circulating current is the PR controller instead of the PI controller. The application of the MMC-HVDC system in this dissertation is to check its response to a real system such as Jeju Island power system. In this case, the modeling of the Jeju Island power system in PSCAD/EMTDC simulation program is established based on the parameters and measured data from the real Jeju Island power system. The application is performed in two cases:  The application of the MMC-HVDC system for regulating grid voltage based on the Jeju Island power system. In this simulation, the HVDC #1 is replaced by the MMC-HVDC system. The effectiveness of using the MMC-HVDC system will be verified by comparing to the case of using HVDC #1.  The application of the MMC-HVDC system for stabilizing the Jeju Island power system in 2020. The mission of the MMC-HVDC system is to ensure a power balancing in the Jeju Island power system under different situations. It can transfer the active power from the mainland to the Jeju Island power system and vice versa.