The Inner Electrical Field Analysis For Converter Transformer And The Testing System Design For Oil-paper Insulation

Converter transformer plays an important role in HVDC (high voltage direct current) transmission system, its reliability directly impact to the safe operation of power system. The operating converter transformer not only withstands AC voltage, lightning impulse and switching overvoltage, but also withstands DC voltage, superimposed AC-DC voltage and polarity reversal voltage. Therefore, there are considerable differences in inner electrical field distributions, breakdown characteristics of insulation material, space charge's accumulation and distribution between a converter transformer and a power transformer, which may cause abnormal discharge and breakdown in the converter transformer.In order to analyze the electrical field distribution, the simulation model of oil-paper insulation is established, and the electric field's distribution of the insulation under different excited voltages such as DC field, AC field, superimposed AC-DC field and a polarity reversal voltage are emulated with FEM (Finite Element Method). The distinguishing features of the transient field are summarized, and the calculation results show that the characteristics of electric field at valve side winding of converter transformer are both regularity and complexity. The electric field's distribution under the DC voltage is depended on the resistivity, which leads to the higher electric-field intensity in the insulating cardboard. And the electrical field distribution under AC voltages is depended on the dielectric constant, which leads to the higher electrical field intensity in the oil. Under the polarity reversal voltage, the oil will undertake a very high voltage caused by the space charge accumulation.For further study of breakdown characteristics of oil-paper insulation, both the polarity reversal testing system and the superimposed AC-DC testing system are designed, and then a preprocessing system is established. At last, a test is made to study the relaxation time of space charge.The results above give some new understandings for converter transformer insulation, which can be referred to the insulation design and the HVDC apparatus testing.