Experimental Study on Performance of SF6+N2 Mixed Gas Insulation

This paper designed and constructed a 126kV GIS partial discharge test platform, performed frequency breakdown tests and lighting impulse tests at temperature of -30~20 to SF6/N2 gas mixture. The present experiment studied frequency breakdown tests in different temperatue and slightly inhomogeneous electric field, obtained change in frequency breakdown voltage as a function of temperature; Analysed positive and negative lightning impluse test in gas mixture, in which SF6 had a propotion of 30%~60%, in different temperature. Results indicated 50% probability breakdown voltage variation in positive lightning impluse tests, and in a low temperature environment, the breakdown voltage increased at negative lightning impulse, the polarity effects varied with condition of pure SF6 gas. The experimental results showed implement of SF6/N2 mixed gas with 30% SF6 supersedespure SF6 gas, could effectively reduce the consume of SF6, and remit greenhouse efect.


Introduction
High-voltage gasinsulated switchgear (GIS) is a key equipement in high-voltage transmission system which provides controlling and protectingfuncions under normal operation and fault conditions.It seals high-voltage components, such as circuit breakers, disconnectors, grounding switches, in a metal pipe, uses SF 6 as insulating medium [1] .SF 6 has a strong adsorption ability of electrons, the strong electronegative indicates its excellent properties of insulation and arc extinguishing [2,3] , another feature of the SF 6 gas is having a high thermal conductivity [4] .Till now, SF 6 gas is the best insulation and arc extinguishing medium.Base on its excellent properties of insulation and arc extinguishing, there excists a widespread use of SF 6 in power industry including gas-insulated transformers (GIT), transformers, cables (GIC), line (GIL) and combination units (GIS).According to statistics, 80% of SF 6 gas is used in power industry, but during practical application of SF 6, there are also some problems, mainly in the following aspects [6] : 1)SF 6 gas is very serious greenhouse gas, which is about 25,000 times that of CO 2 ; the degradation of SF 6 under atmosphere condition is very slow, takes about 3200 years [7] ; 2)SF 6 gas is sensitive to non-uniform field, in engineering applications, tiny conductive particles and roughness of electrode surface cause the non-uniformity, result in reduction of SF 6 insulating properties [8] ; 3)In alpine region, SF 6 in GIS has a problem of liquefaction and density reduction; 4)Due to the normal operation of the circuit breaker or an internal short, occurrence of arc causes SF 6 decomposes, the reaction produces toxic gases for instance SF 4 , SOF 2 ; 5)When the SF 6 gas contains water, condensation will produce in the insulation material surface, leads to significantly decrease of insulating properties, SF 4 and other decomposition products hydrolysis into HF and SO 2 and other acidic substance, internal members are corrosived, result in operation failure [9] ; 6) SF 6 gas is in higher cost, there exists no natural SF 6 gas, SF 6 gas in industry is synthetic so far [10] .When reducing the use and emissions of SF 6 gas, the selection of alternative gas of SF 6 , 3 main indicatiors are considered: the electric strength (E/N)tbm, greenhouse index (GWP) and liquefaction temperature .Since no gas could completely replace SF 6 gas, many studies tend to use a mixture of SF 6 and other gases.The SF 6 gas mixture can solve the liquefaction in alpine region, hybrid gas-insulated manner can also spare expensive SF 6 gas, and mixed gas is less sensitive to small defects in the electrode surface.All these properties show a good prospect.Therefore, research of insulating properties of SF Breakdown experiment applies under certain experimental conditions: different electric field distribution and different voltage waveforms to directly obtain experimental electric strength of the insulating gas, but this method cannot access to the discharge mechanism of mixed insulating gas, it is difficult to propose effective method of inhibiting the breakdown.studies of discharge mechanism of electron avalanche stage, serve as methods complement to the experimental studies, could predict and control the gas insulation breakdown, theorically analyse the breakdown process.Simulation gas electron avalanche development process, mainly describe the relationship between collision cross-sectional and electron energy distribution in gas discharge.Boltzmann equation analysis is under the condition of known various cross sections, solving the microscopic parameters of the gas discharge.It obtains various cross sections and other appropriate data by experiment, by solving the Boltzmann equation, describes the gas electron avalanche development process with a series of elastic collision and inelastic collision, eventually derives electron energy distribution ionization coefficient, adhesion coefficient, drift velocity and diffusion coefficient and other parameters.The basic idea of Monte Carlo method is based on the probabilistic model on the basis of physical phenomena, and according to the unknown number of random variation, a random sample test is applied to calculate the approximate probability of occurrence of an event, uses the probabilistic models to obtain its estimated parameters.During the study of the mechanism of gas discharge, Monte Carlo method can simulate the behavior of particle dynamics and transport properties in a gas discharge process, simulate the occurence and development of electronic avalanche.
Currently discharge theories such as Townsend streamer discharge theory are incomplete.The breakdown of specific structure can not be accurately calculated.Engineering design and improvement of insulation structure often rely on experimental methods or variety of typical test data, discharge theory has important reference value of analysis and prediction of the insulating structure, and is helpful for analyse the problems of insulated structural.Combining the method of experimental and theoretical analysis to calculate the variety of insulation in different ratio of the mixed gas, supersede SF 6 gas with gas mixture of high electrical resistance performance, low greenhouse effect, low-temperature liquefied.Solves practical application problems of SF 6 alternative gases.
For SF 6 /N 2 gas mixture with SF 6 gas percentage of x%, x≥10, empirical formula of SF 6 /N 2 gas mixture gap breakdown voltage present as: Where: Ub (SF6/N2) as frequency gap breakdown voltage of SF 6 /N 2 mixed gas,kV; xas the volume fraction of SF 6 gas in the mixed gas required x≥10%.
3Experimental study of power frequency withstand voltage and lightning impulse test of SF 6 /N 2 gas mixture under low temperature environment

Test Platform
Based on the structure and characteristics of switch-type device under condition of low temperature.Established a mixed gas test platform with 126kV GIS equipment carried out SF 6 gas mixture withstand voltage test at different temperatures.tested SF 6 gas mixture variety of mixing ratio, pressure and temperature and other factors along with frequency withstand voltage.126kVGIS simulation test device is shown in Figure 3-1, there is an air chamber, including cables joints, can be set to a variety of types of electrodes, the discharge gap can be adjusted.

Electrode size designand gap arrangement
According to experimental pre-discharge results of gaps in the air and SF 6 gas, set a bar-plate discharge gap in GIS simulation device.Objects are made of copper, plate electrode radius R = 21.5mm, the rod-shaped electrode radius R = 6mm, hemispherical electrode radius 21.5 mm, the discharge gap distance is adjustable between 1.0--15mm.

Voltage Control
Frequency breakdown test used 70% step-up mode, i.e.First constantly increase voltage to 70% of estimated breakdown voltage, then increase voltage by 5% of starting voltage, at a speed of 4kV/s slowly rise until breakdown.Interval between 2 tests was 10 min, to ensure that the gas insulation strenthrecovered [11][12][13][14][15][16][17] .with a digital oscilloscope for recording and storing the voltage waveform, 50% of the breakdown voltage measured by lifting method to calculate the average of 50% probability breakdown voltage values.
Lightning impulse characteristic test, according to the requirements of GB/T16927.1 "High VoltageTestTechnology", test voltage started from 50% of estimated breakdown voltage, increased 5% in amplitude until the breakdown, detected full-wave lightning waves with the hatox of 1.2μs, and wave tail of 50μs.At the breakdown voltage, applied tests of both positive and negative full-wave, each ≥30 times the full-wave.

Impact of mixing ratio of SF 6 gas in the mixed gas to the frequency breakdown voltage
Test results of SF 6 /N 2 mixed gas under different temperature conditions and slightly non-uniform electric field configuration showed that, in the temperature range of -30 ~20 , the frequency breakdown voltage of mixed gas decline linearly as the temprature decreased.When the mixing ratio of SF 6 in the SF 6 /N 2 mixed gas raised, its frequency breakdown voltage significantly increased.In the temperature range of -30 to -10 , the frequency breakdown voltage of mixed gas increased as the the mixing ratio of SF 6 in the SF 6 /N 2 mixed gas raised; in the temperature range of 0 to 20 , frequency breakdown voltage increase as the increase in the mixing ratio of SF 6 is not obvious, there existed a saturation trend.Frequency breakdown voltage and the mixing ratio of SF 6 showed positive correlation with saturation tendency in the normal temperature range, especially when the temperature is above 0 , the saturation trend was particularly evident.

Impact of ambient temperature to the frequency breakdown voltage of mixed gas
The temprature of mixed gas decreased along with the ambient temperature, the frequency breakdown voltage declined.In the temperature range of -30 to -10 , frequency breakdown voltage declined as the temperature decreased significantly; in the temperature range of -10 to 20 , frequency breakdown voltage with decreasing temperature drop is not obvious; A positive correlation existed between frequency breakdown voltage and ambient temperature, in the normal temperature range trend slowed, especially at temperatures above 0 area, its decline is not obvious; In the temperature range of -30 to -10 , mixed gas Power frequency withstand voltage greatly influenced by the ambient temperature decreased significantly.

Impact of mixing ratio of SF 6 gas in the mixed gas to 50% probability of lightning impulse breakdown voltage
The variation of 50% probability of lightning impulse breakdown voltage in mixed gas in different ambient temperature, the mixing ratio of SF 6 gas in the mixed gas varied within 30%~60%, showed in Figures 6 and 7, in the positive and negative lightning impulse, 50% probability of lightning impulse breakdown voltage of SF 6 /N 2 mixed gas shows a downward trend at -30 to -10 range.As the mixing ratio of SF 6 gas in the mixed gas decreased, drop trend of 50% probability of lightning negative impulse breakdown voltage was not obvious, drop trend of 50% probability of lightning positive impulse breakdown voltage had a significant effect.When the mixing ratio of SF 6 gas was within 40%~60%, the impact of the mixing ratio of SF 6 gas to 50% probability of negative lightning impulse breakdown voltage was not clear.When the mixing ratio of SF 6 gas was under 40%, the impact of the mixing ratio of SF 6 gas to 50% probability of negative lightning impulse breakdown voltage was quite obvious.

4.4Impact of ambient temperature to the lightning impulse characteristics in the SF 6 /N 2 gas mixture
Analysed Figures 6 and 7, As Ambient temperature went down, the 50% probability of lightning impulse breakdown voltage showed a decrease trend.The 50% probability of positive lightning impulse breakdown voltage decreased more distinct.In condition of same mixing ratio of SF 6 , in the range of ambient temperature as -30~-10 , positive impulse breakdown voltagewas lower than that in negative; in the range of ambient temperature was 0~20 , negative impulse breakdown voltage was lower than that in positive.Under normal circumstances in pure SF 6 gas, since the test perfomed in a slightly nonuniform electric field, the breakdown voltage of negative polarity lightning impulse was sure to be lower than which of positive polarity lightning impulse at low temperature environment.Experimental results showed that the breakdown voltage of negative polarity lightning impulse was increased, its polar effect was different with pure SF 6 gas, polarity inversion occured at low temperature, the temperature range was about -30~-10 .

Conclusion
This paper designed and constructed a 126kV GIS partial discharge test platform, performed frequency breakdown tests and lighting impulse tests at temperature of -30~20 to SF 6 /N 2 gas mixture.By analysed the experimental results, received following conclusions: In low temperature and slightly nonuniform electric field enviroment, the frequency breakdown voltage had a significant increase along with the augment of mixing ratio of SF 6 , in the temperature range of -30 to -10 ; as in the temperature range of 0 to 20 , the increase trend was unconspicuous.Frequency breakdown voltage and the mixing ratio of SF 6 showed positive correlation with saturation tendency in the normal temperature range, especially when the temperature was above 0 , the saturation trend was particularly evident.
In the temperature range of -30 to -10 , frequency breakdown voltage declined as the temperature decreases significantly; in the temperature range of -10 to 20 , frequency breakdown voltage with decreasing temperature drop was not obvious.Mixed gas Power frequency withstand voltage greatly was influenced by the ambient temperature decreased significantly.
50% probability of breakdown voltage showed a decrease trend as the ambient temperature decreases under the positive or negative impulse.When the mixing ratio of SF 6 was high, the decrease trend was obvious; when the mixing ratio of SF 6 was low, the decrease trend was unconspicuous.
In the temperature range of -30 to -10 , 50% probability of negative lightning impulse breakdown voltage decreased slowly, while probability of positive lightning impulse breakdown voltage decreased observably.In the condition of same mixing ratio of SF 6 , polarity inversion occurs.SF 6 /N 2 mixed gas with 30% SF 6 supersede pure SF 6 gas was feasible in engineering applications.This replacement can effectively reduce the consume of SF 6 , the greenhouse gas emissions and the insulating gas in GIS.

Figure 4 .
Figure 4.Positive and negative lightning impulse test waveform diagram

Figure 5 .
Figure 5.Variation of frequency breakdown voltage in SF 6 /N 2 mixed gas at different temperatures under circumstance of slightly uneven frequency voltage

Figure 6 .Figure 7 .
Figure 6.Variation of 50% probability of negative lightning impulse breakdown voltage in different mixing ratio of SF 6