Positive Switching Impulse Research Articles

Pre-Breakdown Discharges in Rod-Plane Gaps in SF6 under Positive Switchihg Impulses

Pre-breakdown discharge currents in SF6 rod-plane gaps are measured under positive switching impulses and at different gas pressures. Four types of discharge currents could be distinguished, namely, single pulse discharges, multiple-pulse discharges, incomplete breakdown pulse, and corona-free direct breakdown. The effect of impulse magnitude, gas pressure, and gap geometry on the relative frequencies of occurrence of the four types of discharges are examined. The random dispersion in discharge pulse delays is related to the rate of production of initiatory electrons near the anode. A relationship between this rate of production and the electric field in the gap is derived, and its dependence on gas pressure and impulse front duration is investigated.

Switching-Impulse Corona Inception and Breakdown of Large High-Voltage Electordes in Air

It has been shown in the past that irregularities on the surface of an electrode reduce the corona-inception voltage and, under certain conditions, the breakdown voltage of the corresponding gaps. Recent work indicates that rain may reduce the breakdown voltage of large sphere-plane gaps to that of rod-plane gaps of the same clearance. In the present study, laws are established according to which surface irregularities of various shapes and sizes reduce the corona-inception surface gradient of large electrodes stressed by positive switching impulses. The conditions are-also determined under which acceleration of corona inception may reduce the breakdown voltage of a gap formed by a large electrode and a plane, to the level of the breakdown voltage of a rod-plane gap of the same clearance.

Influence of rain on switching impulse sparkover voltage of large-electrode air gaps

The paper deals with a laboratory investigation into the influence of rain on the switching impulse sparkover characteristics of large-sphere and toroid plane air gaps. Tests were also carried out on coneterminated rod-plane gaps for reference purposes. The factors investigated include the high voltage electrode geometry, nature of the surface, rain intensity as well as the impulse form and polarity. Attention is also paid to the influence of rain on the statistical dispersion of the sparkover voltage and on the nature of the cumulative probability curve. A general correlation, valid for a wide variety of electrodes, has been established between the mean spark-over voltage gradient for positive switching impulses under dry conditions, and the corresponding influence of rain on the breakdown voltage.

The switching impulse strength of a long airgap with a plastic sheet barrier

Insertion of a thin plastic sheet into the midgap region of a 3-m rod-plane gap and an EHV tower window can increase the positive switching impulse strength by 15 percent.

General expression for positive switching impulse strength valid up to extra long air gaps

A new algebraic expression has been established for the critical sparkover voltage (CSOV) afair insulation submitted to positive switching impulses. By using the concept of the gap factor, the expression is found to be valid for all practical gap configurations in the distance range 1 to at least 30 m. Based on these considerations, an absolute limit of 2.4 MV is foreseen for the rated voltage of AC systems. Sparkover voltages under A.C. or lightning impulses are also predictable. Complementary investigations concerning the physics of discharge are needed for further improvements.

Breakdown process of long air gaps with positive switching impulses

This study reports on measurements with multiple techniques delineating the complete sequence of events from the corona streamer to the arc phase in the spark transition for large rod-to-plane air gaps applied by positive switching impulses. Observations with a image converter camera, photomultipliers, a Boys camera, and still cameras as well as measurements of voltages and currents have revealed that starting with the corona streamer the breakdown proceeds through the development of the leader and the intermittent appearance of the light flash to the final spark formation. The detailed analyses were made to the corona streamer, intermittent flashes, and the final transition to the arc phase with extremely high time-resolution techniques.

Influence of the delay of the first corona pulse on the switching impulse breakdown probability

Tests with a rod-plane gap with a rounded termination of the rod show that the delay of the first corona pulse affects the positive switching impulse breakdown probability. The inverted S form or local inconsistencies of the breakdown probability distribution are connected with particular statistical features of the delay of the first corona pulse. The influence of the rod termination or the impulse front on the breakdown probability distribution is explained in terms of the delay of the first corona pulse.

Influence of Switching Impulse Wave Shape on Flashover Voltages of Air Gaps

Flashover tests were conducted by using three kinds of positive polarity switching impulses, exponential, damped oscillatory and linearly rising impulses. The influence of switching impulse wave shape on the flashover characteristics of air gaps was investigated and the basic data were obtained concerning the minimum 50-percent flashover oltage of air gaps and the time to crest of these switching impulses.

Switching Impulse Flashover Characteristics of thin Wire Caps

This paper reports positive switching impulse flashover characteristics of thin wire-plane gaps. Flashover tests were made under both conditions that switching impulses were added with or without ac or dc corona occurring previously. Flashover distances were up to 4 m, and ac or dc bias voltage was 400 kV crest.