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The Qinghai-Henan ±800kV UHV DC power transmission project is an UHV channel specially constructed for clean energy delivery, ±800kV converter transformers of the project are assembled and tested in Xining at an altitude of 2500 m, full-wave lightning impulse test is one of the main means to evaluate converter transformer insulation performance, its valve side lightning impulse voltage waveform parameters often fail to meet IEC60076-3 standards because of the large winding to the ground capacitance. In order to effectively assess the lightning tolerance of converter transformer, a simulation model of impulse test circuit of ±800kV converter transformer was built based on the existing 6000kV/560kJ impulse voltage generator and ATP-EMTP simulation software, three kinds of optimization methods are proposed, which include increasing the wavetail resistance, increasing load capacitor, and connecting the wave head in parallel with different inductors, the results show that the single optimization method can not meet the waveform requirements. Therefore, a combinatorial optimization test method of increasing the wavetail resistance and connecting the wave head in parallel with different inductors is proposed, which is completed and verified in the full-wave lightning impulse test on the valve side of Qinghai ±800kV high-end converter transformer, the results show that the optimized test waveform parameters meet the standard requirements.

During the impulse voltage comparison test, it is important to keep the waveforms smooth for obtaining consistent experiment results, especially for the front time parameter. However, steps or oscillations are often found on the front of the waveforms. The authors propose a theory that these oscillations can be caused by the resistors in the damped capacitive voltage divider, which often serves as the load capacitor in the impulse voltage generation circuit. The theoretical analysis, numerical simulation, and experimental results are performed, and a good agreement has been found in support of the proposed theory. Furthermore, the waveform parameters measured by two resistive reference standard systems were found to be influenced by the shape of the waveform and the oscillation on the front. It is recommended to avoid using a damped capacitive divider as the load capacitor but use an impulse capacitor with low equivalent series resistance instead. This would help to eliminate this common cause of oscillation on the front of the lightning impulse voltage waveform. Waveform uniformity and uncertainty of the result could then be improved during the comparison of reference measuring systems. [ABSTRACT FROM AUTHOR]

• Discharge experiments for 5–8 m long transmission line-tower gaps show the critical wave front time increases with air gap distance. • An approximate U-shaped curve was found between the front time and the average velocity of the continuous leader propagation. • Choosing standard switching impulses (250 μs/2500 μs) to conduct the discharge test in a ±800 kV transmission line-tower gap is both suitable and effective, however when the operation voltage further increases, the front time should be increased appropriately when conducting switching impulse tests. The front time of switching impulses has a significant influence on long air gap discharge performance, but most testing experiments have been conducted using standard impulses. Here, we carried out experiments investigating the discharge characteristics of a ±800 kV transmission line-tower gap (5–8 m) under positive switching impulses with different front times and obtained important physical parameters, such as the time and voltage of the leader inception, the leader propagation velocity, and the 50% breakdown voltage (U 50). The results show that (1) the critical wave front time increases with the air gap distance; (2) for the tested gaps, the time and voltage of the leader inception show minimum dispersion when the front time is 250 μs; (3) an approximately U-shaped curve occurs between the front time and the average velocity of the continuous leader propagation, and the velocity is the slowest with a front time of 500 μs. The results illustrated that choosing standard switching impulses (250 μs/2500 μs) to conduct a discharge test in a ±800 kV transmission line-tower gap is both suitable and effective; however, when the operation voltage further increases, the front time should be increased appropriately when conducting switching impulse tests. [ABSTRACT FROM AUTHOR]

The standard waveform of switching impulse is important for insulation tests of air gaps as well as external insulation and inner insulation of power apparatus in the power system, which should be consistent with the waveforms in actual power systems. Different definitions of waveform parameters of standard switching impulse are discussed in this paper, and a definition based on breakdown characteristics of air gaps is suggested to obtain the front times. Waveform parameters of switching overvoltages in two 1000-kV ultra-high voltage systems in China are studied and their statistical regularities under different operation conditions are analyzed. Based on the breakdown characteristics of air gaps and the concept of equivalent insulation performance, the typical front times in the system are obtained by a weighted average method. [ABSTRACT FROM PUBLISHER]

In evaluating the uncertainty of the standard measuring system for lightning-impulse high voltages, which is composed of a standard voltage divider, a digital recorder, and calibrators, step-response tests of the standard voltage divider may be useful. In this paper, a convolution algorithm is employed to calculate the output impulse voltage waveforms from measured step-response waveforms. The uncertainties of peak-value measurement due to the influence of the nominal epoch, uncertainty of the peak-value measurement due to dispersion of the AC scale factor, and uncertainty of the virtual front-time measurement due to long-term stability are evaluated. Furthermore, the error of the virtual front time of the output waveforms is discussed. The front part of the step-response waveform, t≤ T30%, does not influence the error of the virtual front time. Therefore, for the standard voltage divider, the step-response parameters, that is, the experimental response time, partial response time, settling time, and overshoot, have almost nothing to do with the error of the virtual front time. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 180(2): 24-32, 2012; Published online in Wiley Online Library (). DOI 10.1002/eej.21279 [ABSTRACT FROM AUTHOR]

In lightning impulse withstand voltage tests for substation equipment, waveforms regulated in the standard are sometimes difficult to generate on large equipment of the UHV class due to the level of capacitance of the test equipment "Ct" and the level of the residual inductance of the test circuit including the power supply "Ls". In the present study, the relationships between the capacitance of the test equipment, residual inductance of the circuit, the front time "Tf" of generated waveforms, and the overshoot rate "β" were obtained using a simple equivalent circuit by assigning parameters systematically. Consequently, it has been found that, while Ct and Ls are small, there is a series resistance "Rs" range in which both the Tf and β conditions are met and that, as Ct and Ls grow larger, there is an Rs range in which only one of the conditions is met but not both the conditions, and in the end even the Tf condition alone is not achieved. An increase either in β or Tf increases the testable Ct upper limit or permissible Ls limit value, and an increase in Tf is found to be more effective. [ABSTRACT FROM AUTHOR]

Presently, due to several factors, metal–oxide arresters find very limited applications against very-fast transient overvoltages (VFTOs). To enhance the utilization of these arresters and to meet the needs of the dynamic performance of arrester under VFTOs, an improved model for the prorated block is proposed and its performance is analyzed by applying current surges of different front times (5 ns to 8 \mus), tail times, and peak values. There is a time lag between the residual voltage and discharge current due to block capacitance. The delay thus observed in the initial peak of residual voltage is experimentally verified. [ABSTRACT FROM AUTHOR]