BREAKDOWN IN GASES

BREAKDOWN IN GASES

• Electrons released at cathode = n₀
• _{Electrons at distance x = n(x)}
• _{No. of electrons at distance dx = n(x)adx}
• _{dn(x) = dn+ = dn- = n(x)adx}
• _{dn(x) / n(x)= adx}
• _{Integrate both sides, n(x) = n0exp(ax)}
• _{At anode, n(d) = n0exp(ad)}
•  _{At steady state, I = I0exp(ad)} 
• _{I – average current in gap}
• _{Io – initial current at cathode}
• _{a - 1^st Townsend   Ionisation coef.}
• _{d – gap distance}

CURRENT-VOLTAGE CHARACTERISTIC

Plot of ln(I/I₀) as a function of d will give the value of a

I/I₀ = exp(ad)

TOWNSEND’S SECONDARY COEFFICIENT( g )

• Discharge is not self-sustaining with a alone
• Current falls to zero if initial electrons source is removed
• Additional current produced by secondary emission processes
• g is the no. of secondary electrons produced at cathode per electron produced in the gap
• Causes of secondary electrons;
• +ve ions, g_I - release electrons on colliding with cathode surface
• Photons, g_p - gas molecules excited through collision and release electrons by photoemission
• Metastables, g_m – diffuse to cathode & release electrons
• g = g_I  + g_p + g_m

• Effect of g-process on current;
  
  
  
  
  

• Breakdown Criteria;
            1 - g[exp(ad) – 1] = 0
                          gexp(ad) = 1

• Effect of attachment coefficient, h on current;
• Without secondary effects, g = 0

• With secondary effect, g ¹ 0

• Thus, breakdown criteria given by;

EVALUATION OF h

• Region A1 – effect of h/(a - h)
• Region A2 – straight line slope
• Region A3 – secondary processes are significant

As pd increases, E_s/p decreases

PASCHEN'S LAW

Vs = y(pd)

• Neglecting attachment, breakdown criterion is g(e^ad – 1) = 1
• Since a/p = f(E/p) and g = g (E/p)
• Then at breakdown;
• ad_s = pd_s.f(E_s/p) = pd_s. f(V_s/pd)
• \ g(V_s/pd_s)[exp{pd_s.f(V_s/pd_s)} – 1] = 1
• pd_s is constant, V_s is fixed

TEMPORAL GROWTH STUDIES

• Statistical time lag, t_s – prior to the appearance of electron to initiate primary avalancHe
• Formative time lag, t_f – current build up by secondary process

STREAMER BREAKDOWN

• To explain the observation of formative time lags of ≤ 50 ns
• Depends on primary avalanche reaching a critical size
• Local fields high enough to generate rapidly-moving regions or ‘streamers’ which propagate toward electrodes
• Avalanche developed at point x
• Cloud of electrons at tip and positive ions at tail
• Space charge field E_s cause field enhancement
• Electron produced behind ‘head’ generate avalanches feeding the main avalanche and caused increased ionization
• Thus, breakdown criteria given by;

BREAKDOWN IN NON-UNIFORM FIELDS

• Sphere-sphere gaps, coaxial cylinders, point-plane gaps
• Field strength is a function of position
• Breakdown criteria given by;

BREAKDOWN IN HIGHLY NON-UNIFORM FIELDS

• Strongly divergent fields such as point-plane gaps
• Streamer discharges do not lead to breakdown
• This incomplete breakdown phenomena is called ‘corona’ discharges

VOLTAGE MEASUREMENT BY SPARK GAP

Uniform-Field Gaps

• Specially profiled electrodes with flat central portion and curved outer portion
• Use at spacing up to the flat diameter
• Field does not vary more than 1% in the central uniform field region
• Electrodes overall diameter of three times the flat diameter
• Breakdown voltage is extremely consistent
• Corrected to STP with less than 1% error
• Breakdown voltage follows Paschen’s curve;  V = A(pd) + B(Öpd)
• Calibration of spark gaps in room air;

VOLTAGE MEASUREMENT BY SPHERE GAP

• Not as consistent as uniform-field gap
• Much easier to set uo and maintain in laboratories
• Most frequently used arrangement for voltage measurement
• Reference standard – BS 358 with 3% error

VOLTAGE MEASUREMENT BY ROD-ROD GAPS

• Wide scatter in breakdown voltage
• Strong polarity & proximity effects due to presence of earthed objects in laboratory
• Voltage calibration to within 8%
• Strong humidity effect
• Cheap and easy to set up
• Reference standard IEC 32 (1962)

BREAKDOWN OF HIGHLY NON-UNIFORM FIELD GAPS

Field strength in one or both electrodes is high compared to average stress in gap

Non-attaching gases: dc and ac stress - argon

Voltage-spacing curve

Effect of pressure: dc or ac stress

Surge Breakdown: effect of wave shape

Probability of breakdown voltage

Electric discharge in gases into Crookes, Geissler and Cathode Rays Tubes shows several effects:lightning, fluorescent, deflected rays by magnetic fields and so on.

Old video of a 500,000 volt high tension line switch being opened up.