Research On Control Methods Of HVDC System Based On MMC

HVDC based on modular multilevel converter(MMC) is a new transmission topology of HVDC based on voltage source converter. With the advances of active and reactive power controlled independently by voltage source converter, adjustment of output level according to the numbers of tandem sub-modules, low harmonic components of high multilevel output, MMC-HVDC has been used both at home and abroad increasingly. In this dissertation, the control strategy of MMC-HVDC is investigated by means of theoretical analysis and simulation validation. The dissertation is organized as follows.1) The topological structure and principle of MMC are analyzed in detail. The main modulations and capacitor voltage balance control methods are compared, the nearest level modulation(NLM) is selected as modulation method of modular high-level converter and is simulated to prove its advantage, with the comparison of the main modulation methods of modular multilevel converter. An improved capacitor voltage sequence method is proposed, which can reduce the frequency of IGBT and limit capacitor voltage fluctuations efficiently.2) According to the equivalent circuit of operation, the mathematical model of MMC is built. Through coordinate transformation, the active and reactive components are controlled independently in dq coordinate system. The hierarchical control system of MMC- HVDC is introduced, the outer power and voltage control loops and the inner current control loops are designed accordingly. The component of circulating current is analyzed, and circulation current suppression controller is proposed. The 21-level MMC-HVDC system is modeled using PSCAD / EMTDC software, all above controllers are used to prove the reliability and accuracy of this simulation system.3) The wiring and main control methods of MTDC are analyzed. Combining the advantages of fixed DC voltage bias control and slope control, a hybrid DC voltage bias control is designed to coordinate the MMC- MTDC systems. A five-terminal shunt connection MMC-HVDC system is established in PSCAD and the hybrid DC voltage bias control is simulated to be proved flexibly and reliably.4) Based on the operation principle of MMC- STATCOM, the double synchronous phase detection is used by MMC- STATCOM to detect the reactive and negative sequence of current. The DC transmission line between the MMC converter stations is cut off in simulation system, the MMC converter station works as STATCOM and achieves reactive power and negative sequence of current compensation for AC systems.