Study On Core Saturation Harmonic Instability In An HVDC Transmission System

Harmonic instability in an HVDC system means that harmonic amplifies seriously when a disturbance happens in the convertor station. As HVDC engineering develops rapidly in China, it is necessary to research the harmonic instability deeply in order to decrease the probability of harmonic instability and guarantee the safety and stability of HVDC systems.Interactions between 2nd-harmonic on the AC side of the convertor and fundamental frequency on the DC side of the convertor lead to a type of harmonic instability. The instability can be reinforced by saturation convertor transformers.This instability is called core saturation harmonic instability. When direct current flowing into the winding of convertor transformer, a 2nd-harmonic is induced in the HVDC system. The direct current leads to DC bias magnetization, making the excitation current contain ample low-order harmonics, among which the 2nd-harmonic plays an important role in the harmonic magnification. In this thesis, the harmonic components are theoretically analysed and simulated under DC bias magnetization, and a small capacity transformer is designed for experiment. The results show that 2nd-harmonic holds a big proportion in the harmonics of excitation current under DC bias magnetization. It is shown that a linear relationship exists between the direct current and resultant 2nd-harmonic. As the direct current grows, the 2nd-harmonic grows quicker than the others. The ratio of 2nd-harmonic and direct current depends on the AC excitation and core structure.According to the relationship between the direct current and resultant 2nd-harmonic, and convertion of low-order harmonics in AC-DC system, a coefficient matrix based on core saturation can be established. By use of this matrix, both DC-side fundamental-frequency impedance and AC-side 2nd-harmonic impedance can be calculated, the condition of complementary resonance can also be determined. Finally, the probability of core saturation harmonic instability is analysed in PSCAD/EMTDC.