Research On Self-adaptive Integrated Control Of MMC-STATCOM Under Steady-condition

The modular multilevel converter has great potential in high voltage and high power applicationsas a novel conversion technology. Because of its common DC BUS, four-quadrant operation capacity,good scalability and fault redundancy, MMC has currently become an effective means of powerquality mitigation. In this dissertation, the Self-adaptive comprehensive control strategy ofSTATCOM based on MMC topology is researched by means of theoretical analysis and simulationvalidation. The main context and related conclusions are as follows:This thesis analyzes the four-quadrant operation principles of MMC under steady state in detail,then through the DC component and AC harmonic characteristic analysis of MMC sub-modulevoltage, a novel DC capacitance selection method aiming at suppressing50Hz AC ripple is proposed.Selection standards and design methods for leg inductor of MMC are also proposed, consideringabout the dynamic tracing performance, current ripple, and output current capability.This dissertation proposes the MMC-STATCOM Self-adaptive integrated PWM control strategy.The main controller adopts direct current feedback decoupling control, and proper capacitor balancingalgorithm is automatically adopted in both stages of constant frequency start and steady state. Theproposed control strategy realizes independent decoupled control of the active current isdand reactivecurrent isq. Moreover, the sub-module capacitor voltage can rapidly respond to control instructions inconstant frequency start. Good output current harmonics characteristics and outstanding sub-modulecapacitor balancing performance are achieved under steady state.A novel optimized capacitor voltage balancing algorithm based on hierarchical sorting has beenpresented, enabling its application in both stages. The proposed ramp voltage tracking algorithm cansignificantly reduce starting current without precharge resistor, which simplifies the start-up circuit.To analyze the features of MMC-STATCOM capacitor balancing algorithm under steady state,this paper establishes the averaged model of the MMC sub-module. What’s more, the sub-modulecapacitor voltage fluctuation problem is studied and the mathematic expression of voltage fluctuationmagnitude is derived, which illustrates the influence factors of voltage unbalance.A prototype of MMC-STATCOM rated at±3kVar is established. Experimental results furtherperfectly verify the effectiveness of the proposed self-adaptive integrated control strategy in further.