| Nowadays it is integrant to achieve the interconnection between large regional power grids by developing long-range and high-capacity HVDC system in order to solve the imbalance problem of energy distribution and load growth in China. Harmonic problems occur attendantly due to the devotion of the high-power electronic devices. This paper analyzes the transmission characteristics and influencing factors of the harmonics in HVDC system.Then the harmonic instability problems caused by transferred harmonics and its solutions are discussed.Firstly, theoretical analysis of harmonic transfer characteristics is proposed by using switching function method.The characteristics contain the transfer law of harmonics from the AC side converter to DC side, from the DC side converter to AC side and between the AC/DC systems. Then the harmonic instability caused potentially by transferred harmonics is discussed by giving resonant frequency complementation formula and the equivalent converter harmonic impedance between AC and DC side.Several harmonic instability suppression measures are put forward.HVDC system harmonic transfer model closely to the practical engineering is built in PSCAD/EMTDC simulation software. In this model, harmonic injection method is used to inject different frequency and magnitude harmonics at the different points.The simulation results verify the harmonic transfer characteristics theory. The factors influence harmonic transfer characteristics including the transmission power, running mode and smoothing reactor are discussed, also their influence degree are contrasted.To figure out the reason of harmonic instability caused by injecting3rd harmonic at the DC side in simulation of the model, this paper uses frequency scanning method combined with theoretical analysis to obtain the harmonic impedance frequency characteristic of AC and DC systems. Harmonic instability is restrained effectively by furnishing non-characteristic harmonics filters at AC and DC side of the converters. |
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