Study On Withstand Performance And Altitude Correction Of Air Gaps Under Impulse Voltage In 500kV Substation In High Altitudes

In order to resolve the imbalance of energy source and power demand in our country, The West-East power transmission project has been the emphasis of in our country's power system development strategy to meet the huge power demand of eastern region. However, the altitude in our country reduced gradually from east to west, so the transmission and transformation project is bound to go through the high-altitude areas in the west, while the external insulation performance of transmission and transformation project in high-altitude areas is differernt from that in low-altitude areas:the breakdown voltage of air gap is closely related to the atmospheric parameters. With the increasing of elevation, air density, temperature and humidity will be changed. The combined effect of the three main atmospheric parameters will reduce the air dielectric strength to varying degrees. Therefore, it is necessary to study altitude correction of gas discharge and impulse withstand voltage of the air gap, in transmission device design at high elevation.At present, many scholars have studied the external insulation performance of the 500kV,800kV, 1000kV transmission lines and transmission lines with other voltage levels, but research on impulse withstand property of typical air gaps in 500kV transformer substation in high-altitude areas is nearly blank, therefore the discharege voltage correction method of typical air gaps of in 500kV transformer substation in high-altitude areas is also lacking, and the common methods are only be suitable for the areas under 2000m and lack of the support from test data which is gotten in high-altitude areas.Given the current research status, this paper has carried out gas discharge tests of typical air gaps in 500kV transformer substation at three locations, including Wuhan (23 m) and Xining (2254 m), Wu (3742 m). Firstly,8m two-split conductor choosen for the test based on the electrostatic field simulation and analysis, and according to the simulation of switching overvoltage of typical 500kV transformer substation, the coefficient of voltage a is set to 0.4, then lightning impulse voltage and switching impulse voltage are applied to the two-split conductor—antry framework gap and the grading ring—antry framework gap, meanwhile, switching impulse voltage is applied to two-split conductor—two-split conductor gap and grading ring—rading ring gap, the length of these gaps is between 2m and 9m. The tests are aimed to study the impulse withstand performance and altitude correction of typical air gaps in 500kVsubstation in high altitudes to supply essential data and technical reference for the external insulation design of 500kV transformer substation in high-altitude areas.By analyzing tests and results, the main conclusions are as follows:(1) The 50% discharge voltage of phase-ground clearance will decrease with the increased elevation and increase with the increased clearance distance under the impact of switching impulse and lightning impulse. However, in this research gap length range, switching impulse 50% discharge voltage has obvious trend of saturation, while the lightning impulse discharge voltage of 50% has a substantially linear relationship with the variation in the gap distance.(2) In 500 kV substation at high altitudes, phase-to-phase switching impulse 50% discharge voltage of grading ring and flexible busbar will increase with the increased voltage distribution coefficient. But the trend becomes weaker with the increasing gap distance.(3) For 500 kv substation, the typical air gap coefficient is as followed. The gap coefficient K from two split conductor to frame is 1.25. The one from grading ring to frame gable is 1.14.The one from two split conductor to cross arm is 1.37.The one from ring to ring and wire to wire is 1.71 and 1.56.(4) The corrected results of the typical phase-ground air gap lightning impulse discharge voltage by using the recommended method in DL/T620-1997 and IEC60071-2 have deviation in the range 3%~9%,when compared to the standard reference set up in this paper. But within the 4 m ~ 5m gap distance, the deviation of calibration results is small at an altitude of 3742m area test voltage with IEC60071-2 recommended method is small. The paper suggests the altitude correction methods recommended by IEC60071-2 when correcting lightning impulse discharge voltage of 500kV substation within the scope of researched gap distance.(5) The paper recommends correction factor to correct the typical phase-phase and phase-ground air gap switching impulse discharge voltage of 500kV substation. The correction factor is Ka=1/(1.0-mHx10-4).The expression of phase-ground switching impulse factor is m=-0.0003U+0.9324. The expression for the factor of two split conductor-two split conductor air gap is m=-0.0003U+1.2384.For grading ring-grading ring air gap, the expression is m—0.0002U+1.164.