| On-load tap-changer(OLTC)is an irreplaceable component in converter transformer.Compared with the ordinary power transformer,the converter transformer OLTC has the characteristics of high voltage regulation level,frequent action and high operating temperature.When the load fluctuates,the maximum switching times per day can reach about 20.Frequent switch action will cause the insulation performance of transformer oil to decline,and the discharge on the bare metal surface of OLTC under long-term operating conditions may lead to serious accidents.However,the factors that affect the discharge characteristics of bare metal in vacuum OLTC are not clear in.Therefore,this paper takes the equal size model of the main contact of UHV transformer vacuum tap changer as the research object.(1)Study of the electrical characteristics of the on-load tap-changer during long-term operation.The breakdown voltage in the insulating oil decreases significantly with the increase of particles in the oil,indicating that the particles in the switching oil are a direct factor affecting the breakdown voltage of the insulating oil,and the size of the particles generated in the oil is mainly about 20 μm.The characteristic gases of the OLTC diverter switch during normal operation are mainly CH4 and C2H6,with the average growth of the CH4 gas content being 3.85 μL/L and the C2H6 growth content being 0.96 μL/L.(2)In Simulink to establish the main contact switching when the current conversion simulation model.The single switching contact arc current energy is calculated to be around 0.2 J.And study the effect of current arc ablation on the contact oil gap breakdown characteristics,according to the measured current waveform to calculate the total arc energy corresponding to multiple ablation.It is found that when the total arc energy is 2 J,the ablation produces 9.83 mg of particles,and these particles can reduce the oil gap breakdown field strength by about 21.64%.(3)The oil gap breakdown voltage characteristics and partial discharge characteristics of the bare metal contacts of the switch under the action of water,air bubbles,fibers and metal particles in the oil were measured quantitatively according to the operating conditions of the on-load tap-changer.The oil gap breakdown voltage of the bare metal contacts under the action of a single factor is much higher than the maximum voltage between the on-load tap-changers and does not easily cause oil gap breakdown accidents.In the extreme case of simultaneous action of multiple factors,the breakdown voltage of the contact oil gap is significantly reduced by 77%to 29.52 kV,which is close to the maximum voltage between the switching stages,and the oil gap is at risk of discharge.Under this condition,both oil gap discharges and suspension discharges are likely to exist in the switch,and the signal transformation of the discharges in the positive half-axis is more significant.(4)The existing tap changer structure was analyzed and a simulation model of the bare metal electrodes of the tap changer was established to study the electric field distribution in the oil gap of the contacts and the effect of different electrode structures on the insulation characteristics of the oil gap.It was found that the shorter the mutual distance between the minimum radius of curvature points of the on-load tap-changer movable and static contacts at different spatial locations and the higher the average radius of curvature of the sharp corners,the higher the maximum spatial electric field.(5)To study the effect of metal particles on the breakdown voltage of different curvature electrodes in the inhomogeneous field,the joint particle motion simulation results,the inhomogeneous field,metal particles in the high curvature electrode oil gap on the breakdown voltage is more significant.Simulation calculation of particle aggregation in oil,metal particles by interstage voltage will be aggregated at high field strength to cause electric field distortion.The research results provide data support for the optimization of the insulation structure and operation and maintenance of UHV on-load tap-changers in China. |
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