Lecture 9 & 10.pptx high voltage electrical engineering

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1. EE-407: High Voltage Engineering Lecture # 9 Dr. Gul Rukh

2. Corona Discharge The term corona originally belongs to the field of astronomy and is the rarefied gaseous envelope of the sun and other stars. Steller Corona

3. Corona Discharge In electrical engineering corona is ionization of medium due to concentrated localized electric field in the vicinity of high voltage conductor and is a characteristic of non-uniform electric field.

4. Corona Discharge Thus corona can be defined as a localized partial breakdown of the medium surrounding the highly stressed points when the electric stress exceeds a critical value called disruptive critical gradient (which is 21.1kV/cm RMS for air at STP) and the voltage at which this occurs is called disruptive critical voltage or corona inception voltage.

5. Corona Discharge Corona is seen as a localized faint glow in medium such as air around the high voltage surface. Corona manifests (marks its appearance) itself in the form of: 1. Audible noise 2. Visual appearance 3. Irritating smell of ozone as a by-product of corona

6. Corona Discharge Corona is a pre-breakdown phenomenon and has the capability for degrading insulators, and causing systems to fail. Thus corona can be regarded as an early warning signal for some catastrophic electrical discharge event. Corona in Polymeric insulation

7. Corona Discharge Corona can also occur naturally at tall pointed objects (such as minarets, treetops, cellular phone towers and ship masts) during thunderstorms due to charge concentration on the tip of these objects.

8. Characteristics of Corona Corona is partial localized breakdown of air when the field exceeds a critical value. The audible noise produced by corona may be due to the violent activity of ionization, more likely due to the movement of positive ions as they are suddenly formed in an intense electric field region.

9. Characteristics of Corona The visual glow produced is probably due to the recombination of positive nitrogen ions with free electrons in the vicinity of high stress regions. Corona is also accompanied by the production of ozone , due to splitting of diatomic oxygen molecules, which then recombine to form ozone (O3). Ozone can be detected due to its pungent and irritating smell to which sensitive tissues of nasal track may react, resulting in sneezing.

10. Characteristics of Corona Corona discharge usually forms at sharp regions on electrodes, such as corners, projecting points, edges of metal surfaces, or small diameter wires. The sharp curvature causes a high potential gradient at these locations, so that the air breaks down and forms plasma.

11. Characteristics of Corona In practice if a charged object such as current carrying conductor of a transmission line has a sharp point, such as a broken strand , the air around that point will be at a much higher gradient than elsewhere around the conductor. In order to suppress corona formation, terminals on high-voltage equipment are frequently designed with smooth large diameter having more rounded shapes.

12. Characteristics of Corona Corona is also accompanied with localized high temperatures that may result in hotspots at these sites. Corona discharge in power system and high-voltage equipment generally causes the following undesirable effects: 1. Power loss 2. Audible noise 3. Electromagnetic interference 4. Local heating or hot-spots 5. Ozone production 6. Insulation degradation

13. Power Loss due to Corona Corona is accompanied by power loss, especially in the transmission line system. Though a small percentage of the total losses generally are accountable for corona, however, their significance is increased under foul weather conditions and in the case of long lines. Two expressions are generally used to determine the power loss due to corona in transmission line system, these are; Peek’s formula and Peterson’s formula. The Peek formula is:

14. Power Loss due to Corona The Peterson’s formula (preferably used) for power loss due to corona is: Where f is the frequency, V is the operating voltage in kV, F is the corona factor.

15. Laboratory Study of Corona and Radio Interference Corona occurs only under non-uniform or asymmetrical electric fields (also referred to as divergent electric field). In the laboratory, corona is studied using special electrode arrangements, mainly constituting point-plane symmetry across a high voltage DC source. If the point is connected to the positive DC source with the plane electrode grounded, it will result in positive point corona or simply positive corona.

16. Laboratory Study of Corona and Radio Interference In case when point electrode is connected to the negative DC with plane electrode grounded, the resultant corona is referred to as negative point corona or simply negative corona. The small radius of curvature of the point electrode acts as stress raiser and thus produces a high potential gradient around this electrode.

17. Laboratory Study of Corona and Radio Interference The mathematical relation between electric field, applied voltage and the geometry of the electrode system for uniform field is given by the famous Mason’s formula: Where: V is the applied voltage, d is the separation between the tip of point electrode and the plane electrode’s surface and r is the radius of curvature of point electrode.

18. Laboratory Study of Corona and Radio Interference The maximum electric field E max is experienced at the tip of the point electrode and the field lines are directed towards the plane electrode and are concentrated at the tip of the point electrode, spreading apart towards the plane electrode, thus forming a highly divergent field. Fig (1) shows point-plane electrode arrangement and typical shape of visual corona in a laboratory experiment.

19. Laboratory Study of Corona and Radio Interference

20. Laboratory Study of Corona and Radio Interference Corona is classified as positive or negative, more elaborately as positive point and negative point corona; the terminology is used according to the polarity of the voltage on the point electrode. If the point electrode is positive with respect to the plane electrode, the corona so formed is referred to as a positive point corona. If the point electrode is connected to the negative terminal of the high voltage DC source, then the corona so formed is referred to as negative point corona. In case of AC voltages, it simply referred to as corona.

21. Example: Calculate the maximum electric field in MV/cm experienced at the tip of the point electrode of point-plane electrode geometry in air. The radius of curvature of the point electrode is 10μm and the electrodes are 1.5cm apart when a voltage of 5kV DC is applied. Also calculate the electric field for the same gap separation and applied voltage for uniform field electrodes.

22. EE-407 : High Voltage Engineering Lecture # 10 Dr. Gul Rukh

23. Electrostatic Discharge ESD stands for Electrostatic Discharge. The concept of electricity originates from electric charge. Charges at rest are the static charges and are covered in the subject of static electricity whereas; charges in motion constitute electric current and are the subject of dynamic or current electricity.

24. Electrostatic Discharge Most form of ESD events that are experienced in daily life are in the form of spark that can cause minor discomfort to people, severe damage to electronic equipment, and fires and explosions if the air contains combustible gases or particles. However, many electrostatic discharges are small enough to occur without a visible or audible spark.

25. Electrostatic Discharge One form of ESD is lightning discharge , commonly known as lightning discharge. All physical objects are made up of atoms that are electrically neutral, because they have an equal number of positive and negative charges. Therefore, all things are composed of charges. Opposite charges attract each other and like charges repel each other.

26. Electrostatic Discharge The phenomenon of static electricity therefore requires a separation of positive and negative charges .

27. Electrostatic Discharge When two materials are brought in contact with each other, electrons may move from one material to the other , which leaves an excess of positive charge or deficiency of electrons in one material, and an equal increase of negative charge and therefore excess of electrons on the other. When the materials are separated they will retain this charge imbalance. The various methods by which different materials acquire charge are discussed in following sections.

28. Contact-induced Charge Separation (triboelectricity) Rubbing two materials together produces static electricity through a phenomenon known as triboelectricity (or the triboelectric effect). The triboelectric effect is the main cause of static electricity as observed in everyday life.

29. Contact-induced Charge Separation (triboelectricity) A balloon rubbed against the hair becomes negatively charged and when on a wall, the charged balloon is attracted to positively charged particles on the wall, which can cling thus appearing to be suspended against gravity. Thus if we put two different materials in contact, and one attracts electrons more than the other, it is possible for electrons to be pulled from one of the materials to the other.

30. Contact-induced Charge Separation (triboelectricity) When we separate the materials, the electrons effectively jump to the material that attracts them most strongly. As a result, one of the materials gains extra electrons, becoming negatively charged while the other material loses electrons become positively charged. When we rub things together again and again, we increase the chances that more atoms will take part in this electron-exchange, and a static charge builds up.

31. Pressure-induced Charge Separation (Piezoelectric Effect) Applied mechanical stress produces charge separation in certain types of crystals and ceramics, known as piezoelectric effect, and materials exhibiting this effect are called piezoelectric materials. Piezoelectric effect is the appearance of an electrical potential across the sides of a crystal when subjected to compressive mechanical stress.

32. Pressure-induced Charge Separation (Piezoelectric Effect) Normally, piezoelectric crystals are electrically neutral; the atoms may not be symmetrically arranged, but their electrical charges are perfectly balanced; a positive charge in one place cancels out a negative charge nearby.

33. Pressure-induced Charge Separation (Piezoelectric Effect) However, if a mechanical stress (compressive or tensile) is applied to a piezoelectric crystal, deformation of the structure results in pushing some of the atoms closer together or further apart, thereby upsetting the balance of positive and negative centers, and causing net electrical charges to appear. Some naturally piezoelectric occurring materials include Berlinite (structurally identical to quartz), cane sugar, quartz, Rochelle salt, topaz, tourmaline, and bone.

34. Heat-induced Charge Separation (Pyroelectric Effect) Heating produces a charge separation in the atoms or molecules of certain materials, known as pyroelectric effect or pyroelectricity. Pyroelectric effect is exhibited only in crystallized non-conducting substances having at least one axis of polar symmetry. Development of opposite electrical charges on different parts of a crystal is subject to temperature change. When energy in the form of heat is applied to a pyroelectric material generate an electrical charge in response.

35. Heat-induced Charge Separation (Pyroelectric Effect) Development of opposite electrical charges on different parts of a crystal is subject to temperature change. When energy in the form of heat is applied to a pyroelectric material generate an electrical charge in response. Using Pyroelectric Effect

36. Heat-induced Charge Separation (Pyroelectric Effect) The shifting of charges by temperature difference causes them to separate and forms charge centers. Portions of the crystal with the same symmetry will develop like-charges. Changes in temperature produce opposite charges at the same point that is if a crystal develops a positive charge on one face during heating, it will develop a negative charge there during cooling.

37. Walking on a vinyl floor can cause charge buildup on the body