Presentation on theme: "Chapter 3 HV Insulating Materials: Gases Air is the most commonly used insulating material. Gases (incl. air) are normally good as electrical insulating."— Presentation transcript:
Chapter 3 HV Insulating Materials: Gases Air is the most commonly used insulating material. Gases (incl. air) are normally good as electrical insulating material. Under high E-field conditions, gases become ionized, leading to: corona, sparks and flashover. Why?
Discharges on an insulator Why? How are these discharges formed?
Ionisation processes Photo-ionization Bohr model of an atom: electrons in fixed orbits. Photo-ionisation: Planck: W = hf (Joules).
Ionisation processes Photo-ionization (cont.) Energy gained from light raises electrons to higher energy level (orbit) or quantum band. Energy is absorbed when moving to higher orbit. Energy is emitted when falling back. If energy gained exceeds the ionisation energy of the gas the electron leaves the atom.
Ionisation processes By collision Free initiating electrons always present (cosmic rays) Initiating electrons accelerated by Lorentz force due to the E-field. Electron gains kinetic energy. Collide against gas atoms - kinetic energy converted to potential energy. Ionisation occurs if this energy exceeds the ionisation energy of the atom, sets free more electrons and leaves positive charge behind.
Ionisation processes By collision (cont.) Townsend’s primary ionisation coefficient: : No. of ionising collisions for 1 mm length movement by one electron. Exponential growth: avalanche formation n = n 0 exp( x) – number of electrons liberated at point x
Ionisation processes By collision (cont.) Electrons are more mobile than (relatively heavy) positive ions. Not a self-sustaining process (depends on initiating electron) Typical application - Geiger counter
Avalanches really do exist Wilson’s cloud chamber
Ionisation processes By collision (cont.) Townsend’s secondary process An avalanche is not self-sustaining: process stops if initiating electrons not available. Positive feedback thus required. Pos. ions move back to cathode (-) and collide against cathode, releasing more initiating electrons. : new electrons gained at cathode by (+) ion impact New avalanches form, plasma column formed - higher current leads to breakdown Thus a self-sustaining process.
Electronegative gases Some gases are electronegative: have electron affinity. Electrons attach to the molecules. Thus lower mobility and collision ionization probability. This raises the flashover voltage. Attachment process represented by the attachment coefficient . Townsend’s first ionization coefficient ( ) is effectively lowered to ( - ). If > , then ionization stops.
Electronegative gases SF 6 Sulphurhexafluoride (SF 6 ) is an electronegative gas Flashover voltage roughly 4 times higher than air. The following attachment processes occur in SF6: SF6+e SF5+F+2e SF6+e SF6 – SF6+e SF5 – + F
Electronegative gases SF 6 Substations (GIS) Colourless, odourless, non-toxic, chemically inactive. 5 times heavier than air. Also arc quenching medium in circuit breakers.
Streamer discharges A self-sustaining discharge can develop from a single avalanche. Space charge (ions) distort and enhance field. Photons cause further avalanches in high field regions. Streamer discharges occur if n 5.10 8. Occurs for non-uniform long gaps and at high pressures.
Dr WL Vosloo Cathode (-)Anode (+) E - Field Photons Avalanche with x = 20 Flashovers Streamer mechanism – Medium gaps (> 5 Bar.mm)
Paschen’s Law Sustained Townsend discharge leads to spark then arc (flashover). Formulated mathematically by Paschen (see p 52). The flashover voltage is a function of the product of the gas pressure and the gap length for a uniform field. Implications in practice: Altitude effect Compressed gases Vacuum contactors
Asymmetrical, non uniform gaps The polarity Effect Region of high field strength near the sharp point, in both cases. Avalanches are formed in these regions, leaving a positive space charge in this region. In the case of the positive tip the space charge has the same polarity as the electrode and assists in increasing the field. In the case of the negative tip the space charge opposes the polarity of the tip.
Asymmetrical, non uniform gaps: The polarity Effect A lower flashover voltage is thus obtained for the positive tip, compared to the negative one.
Long gaps Leader mechanism For gaps > 1 m a different flashover mechanism exists. Corona at tip merges into thermal leader channel, similar to lightning. Long gaps are vulnerable for switching surges as leader mechanism occurs. Note minimum at pulse front time of 100 s – typical for switching surges
Flashover in gases Townsend vs. Streamer mechanism Dr WL Vosloo
Flashover When do the different mechanisms apply?
Flashover Non-Uniform Gap – Effect of voltage type
Corona Discharges Non-uniform gaps High E-field near electrode with smallest radius: E r =V/(r ln(b/a)) Ionisation threshold ( 30 kV/cm) exceeded in purple region Partial discharge in this region: no flashover Peek’s formula defines inception surface gradient, E > 30 kV/cm: m < 1.0: surface roughness factor
CoronaNo Corona Corona Discharges Corona inception if E max > E peek Critical disruptive voltage: Yield E max > 30 kV/cm Visual critical corona voltage: Yield E max > E peek
Corona Discharges AC Bluish luminous discharge, ozone formed Causes Interference : 0,2 to 10 MHz (pulse corona) Power losses (tens of MW on 500 kV line) Corona increases during rain (water drops) Use bundled conductors (twins and quads) and corona rings to curb corona AC Corona
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