| When the inverter stations of two or more HVDC projects are set in the same receiving AC power system, the HVDC projects are called MIDC (Multi-infeed direct current) system. MIDC system can enlarge transmission capacity of power system and make power system operate more flexibly. However, at the same time, power system is becoming more and more complicated along with the development of MIDC for the tight electrical coupling of different inverters. If the way of parallel operation is not correct, a mass of harmonic transfer between AC/DC systems will occur, changing the impedance - frequency characteristics of DC systems to other frequencies, which may leads to DC system resonance, even emergency switch off. At present, DC system resonance in MIDC system is studied very little both at home and abroad. In allusion to the characteristics of MIDC system, from two aspects of mathematical and simulation modeling, DC system resonance is deeply studied in this paper.The definition and harm of DC system resonance is introduced, DC system resonance is been stated from tow aspects of internal and external causes. Two methods to DC system resonance which can be used in MIDC fields are raised:The theoretical calculation method and the impedance-frequency scanning method. In the theoretical calculation method, calculation formula of impedance-frequency characteristics of DC system in MIDC is deduced; in the impedance-frequency scanning method, the way to build model of receiving AC system and the way using impedance-frequency scanning in PSCAD/EMTDC with power electronic devices are designed.Based on the ShangHai MIDC system, mathematical model of the theoretical calculation method and simulation model of impedance-frequency scanning method are bulit, resonance points of four HVDCs that feed to ShangHai are analysed, by comparisons, both of the two methods can work out the resonance points of four HVDCs explicitly, and impedance-frequency scanning method is more accurate.With the impedance-frequency scanning method, the impacts of different operation parameters on HVDC project is studied. And then, the suppression measures of DC system resonance are put forward. |
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