| The electrical energy promote the developm ent of civilization in today’s society, it’s also a measure of an important indicator of a country’s economic development and people’s living. China’s power grid with ultra-long distance transmission, transmission corridors to resource constraints, the power base of the giant and DC multi-placement.These characteristics also determine the transmission system must have a high economic efficiency, missed the load power supply to remote areas and the converter station is less and ect. Therefore, Development of HVDC career is a effective way to overcome these problems of China’s power grid. The traditional HVDC system which based on the two-level or three-level voltage source inverter, making the switching frequency of the switching device is high, the loss also increases,at the same time,meet the high-voltage, high-power grid requirements. In addition, the AC side filter device is bulky and also very heavy, these problems not only increase the converter station space but also reduces the efficiency of the inverter.The design of a new modular multi-level inverter (MMC) as the inverter of HVDC system, which aim at problems exist in traditional light HVDC power transmission. Use MMC’s key technology as the core, PSCAD/EMTDC simulation software as the measure, and MMC application in HVDC system for the purpose,carried out a detailed analysis of this the system modulation strategy and control strategy as well as the ultimate power quality.First, analysis the influencing factors of the MMC module’s capacitor voltage fluctuations neutron and circulation problems.Established a sub-module capacitor voltage and the mathematical model of the circulation.Derivation of the results showed that" the fundamental component of the capacitor voltage of the sub-module and the apparent capacity is relevant to modulation ratio; The MMC contains only even harmonic, phase circulation can make the MMC exchange real power and reactive power between the common DC bus and three-phase power grid."Then, give theory analysis of modulation strategies which supply to several multi-level inverter, according to the selected modulation method, to select modulation schemes based on the characteristics of the MMC. Subsequently proposed a capacitor precharge control strategy to achieve the initialization of the MMC capacitor voltage and design a capacitor voltage balance control policy that based on the sub-module capacitor voltage imbalance.At last,through PSCAD/EMTDC simulation software built a simulation platform. Not only verified the modular multilevel HVDC system level inverter modulation strategy and the stability of the control strategy and correctness,but also verified the feasibility of the inverter used in the HVDC.MMC made up the shortage result in the high switching frequency of traditional multi-level inverter,filter unit heavy ect. Electricity and quality problems in HVDC also has the potential for further study. |
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