Different types of very fast transients (VFTs) are classified and their characteristic parameters are summarized based on measurements in a 800 kV SF/sub 6/ gas-insulated pilot installation and 420 kV GIS (gas-insulated switchgear) in service. The electromagnetic field between the enclosure and earth results in a transient enclosure voltage of up to some 10 kV and at the same time in a radiated electromagnetic field from the enclosure with amplitude of some 10 kV/m. The electromagnetic wave guided by the overhead line results in an overvoltage with a multifrequent oscillation of some 10 to 100 kHz, depending on the parameters of the overhead line and the adjacent equipment, e.g. the transformer. The transient enclosure voltage and measures to reduce it are analyzed. >

Very fast transients in GIS

Partial discharges (PDs) generate wideband radio-frequency interference which can be used for noninvasive monitoring of discharges. This paper presents a novel method based on this principle for PD monitoring of substations. The significant advantage of this method lies in the ability to detect PD sources in energized equipment anywhere within a substation compound during normal operating conditions. The results obtained from the prototypes installed in the U.K. and U.S. substations are reported. Results include correlation with apparent charge and daily recordings obtained before, during, and after the failure of a 132-kV current transformer and 69-kV voltage transformer.

RF-Based Partial Discharge Early Warning System for Air-Insulated Substations

Measured electric and magnetic fields of switching transients are characterized in terms of their macroburst (series of transients) and micropulse (individual transient) properties and are classified according to substation voltage and type. Transient fields typical for 115-500 kV air-insulated substations (AISs) and gas-insulated substations (GISs) are reported. Macrobursts of up to 10000 individual transients with varying amplitudes and pulse repetition frequencies of 40 kHz or more can be produced during a single switch. The highest amplitude micropulse field transients are shown to scale with substation voltage. Peak vertical electric fields of 16 kV/m and horizontal magnetic fields of 212 A/m have been measured on the ground under the bus in 500 kV AISs during disconnect switching. Peak field component amplitudes are shown to double at several meters above ground and to scale approximately as 1/r with distance from the bus. Electric field enhancement above a grounded structure to many times the normal free-space value was measured. The dominant frequencies of switching transients are between 0.5 MHz and 120 MHz, vary inversely with substation voltage, and are highest in GISs. >

Switching transient fields in substations

Estimation of very fast transient overvoltage (VFTO) has been carried out using EMTP for various switching conditions in a 420 kV gas-insulated substation (GIS). The variation of the VFTO peak along the GIS bus nodes for disconnector and circuit breaker switching operations, as well as the variation of VFTO peak with different magnitudes of trapped charges, have been studied. The results indicate a distinct pattern of variation of VFTO peak along the nodes of the GIS bus in the case of disconnector switch operation as compared to that of circuit-breaker operation. It has also been noticed that the variation of VFTO peak levels are not in direct proportion to the trapped charge present on the HV bus.

https://core.ac.uk/download/pdf/11602874.pdf

Influence of Switching Conditions on the VETO Magnitudes in a GIS

This paper describes the construction of four probes adjusted for fast transient monitoring in 123 kV SF/sub 6/ insulated substations. The primary requirement was the ability to monitor without distortion very fast transients containing frequencies up to 200 MHz. The measuring system was tested and calibrated with the help of computer modeling techniques. The verified experimental apparatus was then used for testing and calibrating the measuring probes. During the test, various effects on the measuring system characteristics were analyzed with respect to materials used for the components of the system.

Measuring probe for fast transients monitoring in gas insulated substation

The authors provide an indication of the waveshape and magnitude of transients which emerge from the SF/sub 6/ GIS and propagate to externally connected apparatus. Capacitively graded bushings, overhead line connections, etc. tend to reduce the magnitude and cause a slight degradation in the rise-time. Computer simulations and measurements have shown that external apparatus connected to GIS can be subjected to waveforms with rise-times in the range of 10-20 ns and voltage rate-of-rise as high as 40 MV/ mu s. This is comparable to or in excess of the rate of rise during a chopped wave test. >

Characteristics of GIS disconnector-induced short rise-time transients incident on externally connected power system components

Line-to-ground faults in gas-insulated substations (GIS) generate fast nanosecond risetime transients that cause sparkovers across the insulated flange of high-pressure oil-filled cable-GIS interface. The ionized path formed by the sparkovers creates a low-impedance path for power frequency fault current, resulting in flange damage with potentially serious consequences. Various technique for protecting the insulating flange from such damage are investigated and discussed, in terms of both new designs and retrofits for existing installations. To be effective, each protection scheme must adequately deal with the fast transients generated by breakdown in the GIS. As the frequencies of these transients are one to two orders of magnitude higher than for usual power system transients, special considerations are necessary in the protection scheme chosen. >

N. Fujimoto

Papers

Characteristics of GIS disconnector-induced short rise-time transients incident on externally connected power system components

Techniques for the protection of gas-insulated substation to cable interfaces