Research On Cophase Power Supply Device Based On Modular Multilevel Structure

The collaborative development of science technology and power system promote the rapid development of national electrified railway evolution’s shift towards the innovation of fast speed and heavy load, which makes the electrified railway’s three biggest power quality problems of negative sequence, harmonics, reactive power and the electrical sectioning extremely obvious. To meet the development requirements of the high-speed railway preferably, traction power supply system is now facing the traction reform, and the cophase traction power supply system is an effective approach to solve those problems essentially.The structure and topology of the existing cophase traction power supply system are comprehensively analyzed, and it concludes that the existing devices not only demand the bulky, costly transformers, but also have those drawbacks of being high cost and low utilization, being confronted with the realistic requirements that electric railway system needs high-voltage and high-capacity requirement. In order to improve the existing system, a new cophase traction power supply unit which based on the three-arm three-phase modular multilevel converter and V/V connection transform is established in this dissertation.The structural schematic diagrams of the new cophase traction power supply unit and the characteristic of negative-sequence current, together with the capacity utilization, its merits and demerits, are proposed and analyzed, plus with the introduction of the MMC converters’ working principle. By using the vector analysis respectively, the compensation principle of new cophase traction power supply unit under two different operating circumstances based on the fully compensation and half compensation is intensively studied, also the principle that reveals the devices’ active transfer and negative-sequence compensation, along with the relationship between compensation current, compensation ratio and the load current are illustrated. To lower the initial expense for the devices, there has been a further discussion about the devices’ compensation capacity which compares and analyzes the compensation capacity, active power transfer and compensation result in four compensation methods, and therefore the compensation method in the basis of minimum compensation capacity is stated.A real-time detection for synthesis compensation current in view of the instantaneous power method is raised, and the approach using the carrier phase-shifting PWM is also adopted in allusion to the generation of compensation current. The compound control strategy based on the PI control and repetition control is introduced for the purpose of enhancing the stability accuracy and response rate for the compensation current traction. Meanwhile, a loop-current control strategy on account of proportional-resonance control is presented with the aim to solve the problem resulting from the inadequacy that traditional negative-sequence double frequency coordinate transformation is not available for cophase traction power supply system, and the choosing principle for the new cophase traction power supply unit devices’ main-circuit parameters is designed as well.Finally, cophase traction power supply device is modeled and simulated by the electromagnetic transient simulation software PSCAD. Simulation results demonstrate that the new cophase traction power supply device proposed above is of high static and dynamic performance, and able to improve the power quality of electric railway system effectively by making use of the lowest compensation capacity, thus the correctness and feasibility of the topology structure, compensation scheme and the control strategy of the proposed cophase traction power supply system are verified, providing a valid means to give the electrified railway impetus to the development of fast speed and heavy load and possessing a vital significance both in the field of engineering and economy.