HIGH VOLTAGE ENGINEERING (HVE)

1. HIGH VOLTAGE ENGINEERING(HVE)

2. Dr. Alexey V. Mytnikov Associate Professor of Electrical Power Systems Department Institute of Power Engineering

3. COURSE STRUCTURE • High Voltage Engineering course is formed form five main parts: • 1. Electro-physical processes in gases. • 2. Electro-physical processes in condensed dielectric materials. • 3. Generation and measurement of high voltages. • 4. Over-voltages and protection. • Advanced high voltage technologies.

4. CHAPTER 1. ELECTROPHYSICAL PROCESSES IN GASES

5. Basic fundamentals • All gases are good dielectric materials. Basic electro–physical processes of charges appearing are considered in gases. • Before proceeding to discuss breakdown in gases a brief review of the fundamental principles of kinetic theory of gases, which are pertinent to the study of gaseous ionization and breakdown, will be presented. • The review will include the classical gas laws, followed by the ionization and decay processes which lead to conduction of current through a gas and ultimately to a complete breakdown or spark formation.

6. The fundamental principals for the kinetic theory of gas is derived with the following assumed conditions: • Gas consists of molecules of the same mass which are assumed spheres. • 2. Molecules are in continuous random motion. • 3. Collisions are elastic – simple mechanical. • 4. Mean distance between molecules is much greater than their diameter. • 5. Forces between molecules and the walls of the container are negligible

7. Distribution of velocities:

8. Collision–energy transfer • The collisions between gas particles are of two types: • a) elastic or simple mechanical collisions in which the energy exchange is always kinetic, and • b) inelastic in which some of the kinetic energy of the colliding particles is transferred into potential energy of the struck particle or vice versa

9. Ionization processes • Breakdown channel in gases is formed as result of different ionization processes in volume of anode–cathode gap • (volume ionization) and metal electrode surface • (surface ionization).

10. Main volume ionization processes are: impact ionization, step ionization, photoionization, ionization by interaction of metastabes with atoms, thermal ionization, field ionization

11. Schematic representation of electron multiplication:

12. Recombination processes • Whenever there are positively and negatively charged particles present, recombination takes place. The potential energy and the relative kinetic energy of the recombining electron – ion is released as quantum of radiation. At high pressures, ion – ion recombination takes place. The rate of recombination in either case is directly proportional to the concentration of both positive ions and negative ions.

13. Emission processes • Electrodes, in particular the cathode, play a very important role in gas discharges by supplying electrons for the initiation, for sustaining and for the completion of a discharge. Under normal conditions electrons are prevented from leaving the solid electrode by the electrostatic forces between the electrons and the ions in the lattice.

14. Changing of the potential barrier by external electric field.

15. Forms of electrical discharge • As the voltage between electrodes in a gas with small or negligible electron attachment increases, the electrode current at the anode increases in accordance with equation:

17. Taking into account this equation and all above discussed ionization and emission processes, we can write main condition of self–sustaining discharge:

18. Growth of electron is arisen very rapidly, due to exponential law. Such mechanism of discharge processing is called electron avalanche. So, this phenomenon is named “avalanche discharge” or Townsend mechanism of discharge. Townsend is English physicist, who described this discharge form for the first time.

19. In the Townsend avalanche mechanism the gap current grows as a result of ionization by electron impact in the gas and electron emission at the cathode by positive ion impact. According to this theory, formative time lag of the avalanche should be at best equal to the electron transit time.

20. STREAMER FORM • When common quantity of electron in avalanche reaches 108 (), physical mechanism of discharge is critically changed. Self electrical field of avalanche is too large that begins to screen external field. Discharge can’t exist so as no possibility to take energy for electrons and continue charge generation process.

21. In this conditions discharge changes the form. Avalanche transforms to another form which is called “streamer” or “kanal” mechanism of electrical discharge. In this mechanism for discharge formation the secondary mechanism results from photoionization of gas molecules and is independent of the electrodes.

22. SCHEME OF STREAMER FROMATION

23. LEADER FORM • Besides avalanche and streamer form of discharge, one more one exists. This is leader form. Main conditions of this discharge form existence are non–uniform electric field and long enough gap length. Criteria of streamer-leader transition is high temperature in discharge channel area (3 000 – 5 000 K).

24. So, every kind of electrical discharge can exist in one of three form described above. Discharge channel formation means that gap looses electrical strength totally. This phenomenon has other name – breakdown of gaseous gap

25. Paschen’s law • Equation • is known as Paschen’s law, and was established experimentally in 1889 by famous German physicist Friedrich Paschen. Equation (1.77) does not imply that the discharge voltage increases linearly with the product pd, although it is found in practice to be nearly linear over certain regions.

27. Discharge in non–uniform electric fields. Main discharge processes. • In uniform fields at the normal atmosphere conditions breakdown voltage is determined by Paschen’s law. Electrical strength of atmosphere air is constant value in this case and equal approximately 30 kV/cm.

28. In non–uniform fields like such geometrical configurations as point – plate, sharp end – plate, cylindrical surface with sharp edge – plate and others, breakdown voltage is much lower that in uniform field. Below reasons of that are explained. Figure below illustrates the case of a strongly divergent field in a positive point – plane gap.

30. In uniform field and quasi–uniform field gaps the onset of measurable ionization usually leads to complete breakdown of the gap. In non-uniform fields various manifestations of luminous and audible discharges are observed long before the complete breakdown occurs. These discharges may be transient or steady state and are known as “corona”.

31. POLARITY EFFECT • At the DC Voltage in the strictly non-uniform electric field takes place unique phenomena which is called “POLARITY EFFECT”

35. So, all electrophysical processes begin with applying of electrical field, which should be intensive enough. In uniform electric fields breakdown voltage is determined by Paschen’s law and depends on kind of gas, gas pressure and gap length. In non–uniform fields all electrophysical processes begin at the active electrode area. Active electrode has small radius of curve and rod or sharp form.

36. Corona discharge is first step at the non–uniform fields. Depending on polarity in anode–cathode gap in case of d.c. regime, breakdown voltages can be quiet different. If applied voltage is increased, corona passes to spark discharge. In case when external field is growth more, spark transforms to arc discharge. Arc discharge is high current (1 – 100 kA), low voltage (1 – 10 eV). Arcs have unique physical mechanism of existence which is based on cathode spots and explosive emission.