EE8701 – High Voltage Engineering Department of EEE 2021-2022 As the spacing is increased, the positive characteristics display the distinct high corona breakdown up to a pressure of about 7 bars, followed by a sudden drop in breakdown strengths. Under the negative polarity, the corona stabilized region extends to much higher pressures. The above graph shows the corona inception and breakdown voltages of the sphere-plane arrangement. (i) For small spacing's (Zone–I), the field is uniform and the breakdown voltage depends mainly on the gap spacing. (ii) In zone–II, where the spacing is relatively larger, the electric field is non-uniform and the breakdown voltage depends on both the sphere diameter and the spacing. (iii) For still larger spacings (Zone-III) the field is non-uniform and the breakdown is preceded by corona and is controlled only by the spacing. The corona inception voltage mainly depends on the sphere diameter. 13. Derive the Townsend’s current growth equation in uniform gaseous dielectric field. (or) Derive the expression of current growth equation in a uniform field due to Townsend’s first and second ionization process and thereby deduce the condition for condition for gaseous dielectrics. (Apr 2019, Nov 2019) Townsend's second ionization coefficient The current growth equation in the presence of secondary process is derived by St. Joseph’s College of Engineering 51
EE8701 – High Voltage Engineering Department of EEE 2021-2022 = + + where is the second ionization coefficient and it is function of pressure,E/p let we take −secondary electrons - total number of electron released from cathode and reached anode -no of electrons released by ultra violet radiation =+ An electrons released from gas = −( + ) therefore each positive ion releases effective electrons from the cathode that is = ( −( + )) = − − (1+ ) = ( − ) w.k.t (− ) = (1 + ) substituting we get =( + ) in the above expression (− ) = ( + (1 + ) ) simplify we get St. Joseph’s College of Engineering = 1 − ( − 1) 52
EE8701 – High Voltage Engineering Department of EEE 2021-2022 It is written in terms of current = 1 − ( − 1) The current becomes infinite if 1 − ( − 1) = 0 ( − 1) = 1 =1 normally >> 1 the current in the anode equals the current in the external circuit. Theoretically the current becomes infinitely large under the above mentioned condition but practically it is limited by the resistance of the external circuit and partially by the voltage drop in the arc. The condition = 1 defines the condition for beginning of spark and is known as the Townsend criterion for spark formation or Townsend breakdown criterion. Using the above equations, the following three conditions are possible. (1) =1 The number of ion pairs produced in the gap by the passage of arc electron avalanche is sufficiently large and the resulting positive ions on bombarding the cathode are able to release one secondary electron and so cause a repetition of the avalanche process. The discharge is then said to be self-sustained as the discharge will sustain itself even if the source producing I0 is removed. Therefore, the condition =1 defines the threshold sparking condition. (2) > 1 Here ionization produced by successive avalanche is cumulative. The spark discharge grows more rapidly the more exceeds unity. (3) <1 Here the current I is not self-sustained i.e., on removal of the source the current I0 ceases to flow. St. Joseph’s College of Engineering 53
Search
