Research On Some Problems In Over-voltage Of UHV AC Transmission System

From the time when the first UHVAC single-circuit transmission line was put into operation,China has accumulated a lot of experience in UHVAC transmission system.But,there still exists some problems about overvoltages of UHVAC transmission system,especially for UHVAC long-distance transmission lines.This paper mainly discuss about single-phase grounding overvoltage and very fast transient overvoltage of UHVAC transmission system.With the increasingly rapid growth of power load, the present main network of our country has been unable to cope with the multiplied transmission capacity needs. The establishment of UHV power grid has been the inevitable developing direction of the electric power industry. UHV transmission’s large capacity, rechargeable power, and long distance can cause high power frequency over-voltage, which seriously affecting the reliability of transmission. Therefore, the study of UHV transmission line power frequency over-voltage has significance on the design and construction of UHV transmissions.As to UHVAC long-distance transmission lines,single-phase overvoltage cannot be ignored.Based on the rule of the effect of source’s impdedance on single-phase grounding overvoltage,a method to get the maximum value of the overvoltage is put forward.On this basis,the restriction methods of this overvoltage is studied.Results show that allocation of the compensation capacity of high voltage(HV) shunt reactors and MOA at the same time can limit overvoltage effectively. Besides, with this limitation method,this paper discusses the maximum length of UHVAC lines when the amplitude of this overvoltage does not exceed1.7p.u under different types of line-segment. Results show that as to the lines that are divided into three segments,the maximum length of these lines can reach1300km when the amplitude of this overvoltage does not exceed1.7p.u;the maximum length can reach1400km and1600km respectively when the lines are divided into four segments and five segments. In1000kV GIS substations or power plants, VFTO poses a big threat to the longitudinal insulation of power transformers because of its steep wavefront. Through experiment and simulation,we discussed the effect of the overhead line between the GIS bushing and the transformer on the wavefront steepness of VFTO at the transformer’s port.With the help of ATP-EMTP, we also analyzed the quantitative relation between the length of the overhead line and the steepness of the VFTO at the transformer’s port. The result shows that the overhead line with the length of10-25m could effectively protect the longitudinal insulation of transformers; when the length of the overhead line is about10m,the steepness of VFTO drops dramatically at the transformer’s port; for usual1000kV GIS substations and power plants,when the length is about25m, the steepness of VFTO at the transformer port wouldn’t exceed the restriction level and wouldn’t threat the longitudinal insulation of the transformer.In addition this paper makes a comparison of VFTO characteristics between1000kV GIS substations and plants which can give reference for designing substations and plants.