| As the disconnector in gas insulated switchgear (GIS) is operated under energized, the repeat breakdown between contacts appears and very fast transient overvoltage (VFTO) is generated which threats the safe operation of GIS, primary and secondary equipment connected with or near GIS in extra high voltage (EHV) and ultra high voltage (UHV) systems. It is important to investigate VFTO characteristics for the insulation coordination of UHV and EHV GIS substations. The simulation is an important method for studying VFTO characteristics. At present, the models of the static components in GIS are relatively mature, the characteristics and model of repeat breakdown in GIS disconnector needs to be further improved. But the repeat breakdown in GIS disconnector is important to the magnitude and static characteristics of VFTO. In order to improve scientificity and accuracy of VFTO simulation, it is necessary to investigate the characteristics of the repeat breakdown in GIS disconnector and establish the simulation model. The method combined experimental investigation with theoretical analysis was adopted to further investigate the repeat breakdown model, including the breakdown voltage model, the resistance model of high frequency arc and the simulation method for repeat breakdown progress.A test circuit to operate 252kV GIS disconnector was established. The optical system to measure the clearance between open contacts of GIS disconnector based on position sensitive detector (PSD) was developed and its measurement error is lower than 1%. The tests of switching disconnector were carried out. The repeat breakdown waveforms of VFTO and the scatter diagram of repeat breakdown voltages between open contacts were obtained. The repeat breakdown voltages between open contacts in GIS disconnector were statically computed, and the repeat breakdown voltage curve was obtained with statistics methods of Nadaraya-Waston regression and the linear regression. The curve is linear with the polarity effect. The probability density function of random item for the repeat breakdown voltage was obtained with the assessment method of the smoothing kernel probability density function. The breakdown voltage model for GIS disconnector was set up with the curve and random item of the repeat breakdown voltage. The breakdown voltage tests combined with AC and DC voltages were conducted for GIS disconnector. AC power frequency voltage was applied at the source side and DC voltage of different proportion was applied at the load side of GIS disconnector. The single breakdown voltage curve between open contacts was obtained. The test results have shown that the single breakdown voltage between open contacts is independent of DC voltage components and it can be obtained by the DC or AC breakdown voltage tests. The slope of the repeat breakdown voltage curve is a little lower than that of the single breakdown voltage curve, the positive and negative lower about 8.4% and 11.6% respectively. So the repeat breakdown voltage curve can be replaced by the single breakdown voltage curve with a lower slope.The test circuit of high frequency arc in millimeter length SF6 gap was established, the maximum distance was 5mm and the maximum gas pressure was 0.1 MPa. The pulse capacitors were used to discharge to generate the high frequency arc with the current magnitude of 10kA and the frequency of 1.1 MHz. The coil to measure arc current differential was developed, the optical imaging and high speed synchronous trigger system for the spectrometer and framing camera were developed, achieving the rapid joint simultaneous observations of arc voltage, current, differential of current, emission spectrums and images with the electrical and optical measurement systems. The method to compute arc resistance by solving the circuit formula with the electrical measurement results was studied. The optical analysis method was proposed, by analyzing arc emission spectrums, the electron temperature calculated by the relative intensity ratio method, the electron density calculated by Saha equation, the arc conductivity calculated by the modified Spitzer formula, the arc radius determined by the radial intensity distribution of the framing camera and images of the spectrometer, the arc resistance calculated by combining the arc conductivity with the arc radius. A large number of tests on high frequency arc characteristics were carried out. The test results have verified the valid of optical analysis method for arc resistance. The time-variable trends of arc resistance were obtained, divided with three stages of the initial breakdown, steady arcing and insulation recovery. According to the principle of deposited energy equation, the time-variable arc resistance is equivalent to a constant resistance that is the arc steady equivalent resistance, which is close to the arc resistance at the time of arc current peak and the steady arcing resistance. The arc steady equivalent resistance decreases as the arc current peak increases. When the current frequency increases, the time for arc resistance to reach the steady-state shortens, but the steady equivalent resistance value is almost constant. The arc steady equivalent resistance increases with the increase of gas pressure. The arc steady equivalent resistance is approximately linear with the arc length. According to the relation of the arc resistance and the balance between the energy injection and energy loss of the arc, the mathematical model for time-variable arc resistance in the whole process was established, which presents the influence of gas pressure, arc length and current waveform, achieving the simulation of arc resistance only by the arc current.The test circuit of high frequency arc in centimeter length SF6 gap was established, the maximum distance was 33mm and the maximum gas pressure was 0.2MPa. The VFTO generator was adopted to generate the high frequency arc with the current magnitude of 20kA, the frequency of 14MHz and the duration of 1 μ.s, which is close to the high frequency arc in GIS disconnector in size. VFTO measurement system, very fast transient current (VFTC) measurement system, the spectrometer and framing camera were configured in the test circuit, achieving the rapid joint simultaneous observations of breakdown voltage, VFTC, differential of VFTC, spectrum and images with the electrical and optical measurement systems. The optical analysis method was proposed, by analyzing arc emission spectrometry, the arc temperature and electron density calculated by the blackbody radiation model, the arc conductivity calculated by the modified Spitzer formula, the arc resistance calculated by combining the arc conductivity with the arc radius. A large number of tests on high frequency arc characteristics were carried out. Based on the investigation result of high frequency arc in millimeter length SF6 gap, the arc resistance at the arc current peak is set as the arc steady equivalent resistance. The arc steady equivalent resistance increases with the increase of gas pressure. The arc steady equivalent resistance increases and tends to saturate as the arc length increases. Based on the test results, the relation between the arc resistance and the current peak, gas pressure, arc length was fitted to establish the model of arc steady equivalent resistance.The VFTC measurement system with Rogowsiki coil at the high potential was developed which frequency width is 70MHz. The VFTC measurement system was installed in the test circuit to operate 252kV GIS disconnector. A large number of tests on high frequency arc in GIS disconnector were carried out. The waveform parameters of VFTC were statistically computed. With the time-variable rule of arc resistance obtained from the millimeter length SF6 gap and the model of arc steady equivalent resistance from the centimeter length SF6 gap, the mathematical model for the time-variable arc resistance in GIS disconnector during the whole process was proposed to exhibit the influence of current peak I, arc length l and gas pressure p, which is described as R=Rie--τ/τ1+Rceτ/τ. The model parameters were determined, herein Ri is the insulation resistance set as 1012 Ω, τ2 is the discharge time constant set as 2ns, Rc is the arc steady resistance set as Rc=34I-0.86p0.24l0.63,τ2 is the recovery time constant set as about 1μs.Based on the breakdown voltage model and the high frequency arc resistance model of GIS disconnector, the randomness of the initial operating phase and the breakdown voltage of disconnector were simulated with Monte Carlo method. The simulation efficiency was improved by using the step-variable alternative simulation method in the power frequency process and the breakdown process. The simulation method for repeat breakdown process in GIS disconnector was proposed. The waveforms of VFTO and VFTC generated by the repeat breakdown during switching of 252kV GIS disconnector were computed and verified by tests. The waveforms of repeat breakdown and single breakdown of VFTO and VFTC were coincided, the accuracy of simulation model and method were confirmed. The statics characteristics of VFTO and VFTC, including the breakdown number of single operation, the maximum peak of VFTO and VFTC, the oscillation coefficients of VFTO were coincided basically. The valid of simulation model and method were further confirmed. |
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