UNIVERSITY OF MINES AND TECHNOLOGY, TARKWA

SECOND SEMESTER EXAMINATIONS, MAY/JUNE 2019

COURSE NO: EL 380

Name: __________________________________________ Index Number: _______________

INSTRUCTION: ATTEMPT ALL QUESTIONS IN SECTION A AND ANY ONE(1) IN

SECTION A (80 marks)

1) Modern techniques of high voltage testing are designed and standardized to assess the electrical
health of an entire electric motor system and other electrical equipment. TRUE

2) Modern high voltage testers use modern high-speed electronic evaluation of changes to resistance,
leakage current, leakage current versus time, voltage, frequency response, wave shape, and more to
detect faults at or under the levels of energy exposed to the electrical equipment like the motor during
operation. TRUE

3) Power systems equipment must withstand only the rated voltage (Vm), which corresponds to the
highest voltage of a particular system. FALSE

4) Transient overvoltages accompanying sudden changes in the state of power systems, e.g. switching
operations or faults, are known as capacitor impulse voltages. FALSE

5) For GIS (gas-insulated switchgear) on-site testing, oscillating switching impulse voltages are
recommended for obtaining lower efficiency of the impulse voltage generator. FALSE

6) Direct measurement of high voltages is possible up to about 20 kV, and several forms of voltmeters
have been devised which can be connected directly across the test circuit. FALSE

7) If in two–electrode system both electrodes (spheres) are not grounded, then such a system is said to
be asymmetrical, whereas when one of the spheres is grounded, then such a system is said to be
symmetrical. FALSE

8) The shapes of the electrodes for testing solid dielectric could be cylindrical, semispherical and strip–
types. TRUE

9) One of the direct methods of measuring high voltages is by means of electro-magnetic voltmeters.
FALSE

10) The breakdown voltage of sphere gap is dependent of humidity of the atmosphere, but the presence
of dew on the surface lowers the breakdown voltage and hence validates the calibrations. FALSE

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11) In sphere gaps used in measurement, to obtain high accuracy, the minimum clearance to be
maintained between the spheres and the neighbouring bodies and the diameter of shafts are also
specified, since these also affect the accuracy. TRUE

12) An impulse voltage is a unidirectional voltage, which without appreciable oscillations, rises rapidly
to a maximum (peak) value and falls more or less rapidly to zero. TRUE

13) The flashover voltage of insulation in the air, usually varies directly as the temperature and varies
inversely with the atmospheric pressure. FALSE

14) The energy dissipated in a lightning flash is of the order of 10 to 100 MJ. FALSE
15)During an average lightning storm, a total of the order of kilo-coulombs of charge would be generated,
between the 0 oC and the -40 oC levels, in a volume of about 50 km3. TRUE

16)In a rod gap arrester, the distance between the rod end and the surface of the insulator should be at
least two-third of the main gap between the rods to avoid any arc being blown over the bushing
insulators. FALSE

17) During the propagation of lightning channel, the return stroke may carry currents as high as 200 A,
although the average current is about 10 kA. FALSE

18) The average number of strokes in a multiple stroke is four, but as many as 40 have been reported.
TRUE

19) The standard voltage waveform used in high voltage testing has a 1.2/50 ms wave to take into account
the most severe conditions. FALSE

20) Due to skin effect and resistance of transmission lines, a surge wave propagating along a transmission
line is rapidly damped, thereby making the front end of the surge wave flattened, the tail end
lengthened and the amplitude decreasing rapidly with time and distance from the lightning strike.
TRUE

21) The steepness of the wave front of a lightning stroke indicates the rate of fall of voltage (current) at
any point in the system. FALSE

22) Generally, in a direct stroke, the lightning current path is directly from the cloud to the line, causing
the voltage, to fall rapidly at the contact point. FALSE

23) A lightening discharge may have currents in the range of 100 kA to 9000 kA and the duration of the
flash may be of the order of one millionth of a second. FALSE

24) The negative charges at the bottom of cloud induces charges of opposite polarity on the transmission
line. TRUE

25) The design of a transmission line against lightning for a desired performance is not dependent upon
the operating voltage. FALSE

26) Lightning induced surges cannot alter the contents of the ROM of programmable controller. FALSE

27) Power cables, telephone lines and even underground plumbing have the potential to transfer lightning
surges into a building. TRUE

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28) The location of places which are vulnerable to lightning strikes when a storm develops in the area is
determined by the nature of the soil, and subsoil. TRUE

29) Buildings are protected from lightning by metallic lightning rods extending to the ground from a
point above the highest part of the roof. TRUE

30) Power lines and radio sets with external aerials are protected against lightning by lightning arresters
that consist of a small gas-filled gap between the line and ground wire. TRUE

31) The soil is enriched with oxygen that is released from the atmosphere by lightning and carried to the
ground by raindrops. FALSE

32) The angle of protection is defined as the angle formed between the normal passing through the ground
wire and the line joining the supported centre points of the outer conductor and the ground wire.
TRUE

33) Practically, the presence of two earth wires reduce the induced voltage on the line to half. FALSE

34) The area over which a lightning stroke will be attracted to and will terminate on a lightning conductor
in preference to earth is termed the protective zone. TRUE

35) For the protection of high voltage systems i.e. 33 kV and above, protector tubes are not employed
for the protection of the station insulation since their use is only restricted to transmission line
flashovers. TRUE

36)Station class arrestors are typically used in electrical power stations or substations and other high
voltage structures and areas. TRUE

37)Distribution class arrestors are most commonly found on transformers, both dry-type and liquidfilled.
TRUE

38) One of the limitations of the rod gap arrester is that, the polarity of the surge does not affect the
arrestor. FALSE

39) The rod gap should be so adjusted that the break down should occur at 80% of spark over voltage in
order to avoid cascading of very steep wave fronts across the insulators. TRUE

40) On the occurrence of over voltage, the air insulation across the gap of a surge diverter breaks down
and an arc is formed providing a high resistance path for the surge to the ground. FALSE

41) Portable partial discharge locators have been developed to assist in the early detection of partial
discharges. TRUE

42) For power levels up to 240 MVA with a maximum voltage up to 166 kV, besides paper-oil insulated
transformers, dry-type transformers usually insulated by epoxy-resin are also used. TRUE

43) Partial discharges can simply be defined as localized breakdown in an insulator, example, the
breakdown of a void in a solid insulator. TRUE

44) Unlike the off-line treatment methods, the transformer oil treatment on-line are carried out while the
transformer is energized. TRUE

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45) The off-line oil treatment often fails in removing residual moisture from the paper insulation. TRUE

46) Retrofills are defined as the complete replacement of the insulating liquid in a liquid filled piece of
electrical equipment. TRUE

47) The off-line treatment of transformer oil is strictly a corrective measure for restoring the oil to near
original quality, while the on-line treatment of the transformer oil can be approached correctively or
preventively. TRUE

48) The Interfacial Tension Test (IFT) is employed as an indication of the sludging characteristics of
power transformer insulating oil. TRUE

49) It has been established that an IFT of less than 0.015 kN/m almost invariably shows sludging.
FALSE

50) Used oil power factor in excess of 2.0 percent may be an operational hazard. TRUE

51) The power factor of new oil should not exceed 0.5 percent at 55 oC. FALSE

52) The breakdown voltage is indicative of the amount of contaminants (usually moisture) in the oil.
TRUE

53) Oxidation is the formation of acid in oil due to the reaction of unstable hydrocarbons with oxygen.
TRUE

54) One of the sources of moisture in transformer oil is residual moisture in insulation. TRUE

55) The two main sources of solid contamination are dissolved metals and dust during transformer
manufacture. TRUE

56) Oil- filled transformers are essentially of the same construction with the dry type, except that the
core and coil assembly is placed in a tank and the tank is filled with low dielectric cooling oil. TRUE

57) For measurements on liquid dielectrics, where test cells are small, electrode preparation is much
more critical than it is for measurements on gases or solids. TRUE

58) The highest breakdown strength obtained under carefully controlled conditions is known as the
“extrinsic strength” of the dielectric. FALSE

59) If an insulating liquid contains in suspension a globule of another liquid, then breakdown can result
from instability of the globule in the electric field. TRUE
60)The critical electric field stress of a globule depends on surface tension of the globule, relative
permittivity of the insulating liquid and initial radius of globule. TRUE

61)The electric stress to which an insulating material is subjected to is numerically equal to the voltage
gradient and is equal to the electric field intensity. TRUE

62) The dielectric strength of an insulating material can be defined as the maximum dielectric stress
which the material can withstand. TRUE

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63) In the manufacture and repair of electrical equipment like transformers, the quality of insulation is
practically evaluated for the entire equipment or for its individual parts, by applying a high voltage.
TRUE

64) The dielectric loss in a dielectric is a function of applied voltage, angular frequency, capacitance of
the dielectric and loss factor. TRUE

65) The phenomenon where the elastic displacement of electric charges under the influence of electric
field forces is called dielectric ionisation. FALSE

66) The breakdown stress of a dielectric is referred to as the dielectric strength of the material. TRUE

67) The average electric field strength of the insulating materials used in transformers range from 5 to
90 kV/mm at 20 0C. TRUE

68) If a dielectric of thickness 5 mm is subject to a breakdown voltage of 100 kV, the breakdown stress
on it is approximately equal to 20 000 V/m. FALSE

69) A 5 μF dielectric is subjected to a 2 kV, 50 Hz voltage. The dielectric loss if the loss factor of the
dielectric is 0.01 is equal to 62.84 kW. TRUE

70) It is a well-established fact that for most insulating materials, the service life can be related to
temperature as an approximately linear plot of log of life versus reciprocal of absolute temperature.
TRUE

71) Insulation breakdown in high-capacity transformers results in heavy destructions accompanied with
arcing and fusion of metal structural components. TRUE

72) With an improper choice of the permittivities and thickness of insulation components, the electric
field strength may exceed the electric strength of the insulation, and the insulation will fail or
breakdown. TRUE

73) At a temperature of 20 oC and frequency of 50 Hz, the relative permittivity of insulating materials
used in transformers range from 20 to 80. FALSE

74) At a temperature of 20 oC and frequency of 50 Hz, the loss factor of the insulating materials used in
transformers range from 0.05 to 0.2. FALSE

75) For long and reliable operation, insulating materials must possess high thermal stability. TRUE

76) The electric power (P) transmitted on an overhead a.c. line increases approximately with the surge
impedance loading or the square of the system’s operating voltage. TRUE

77) Conductors and insulators are the basic materials used in high voltage apparatus, while the
conductors carry the current and the insulators prevent the flow of currents in undesired paths. TRUE

78) The most common cause of insulation failure is the presence of discharges either within the voids in
the insulation or over the surface of the insulation. TRUE

79) Uniform field is normally established using the following electrodes: plane to sphere or sphere to
plane electrodes. FALSE

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80) In the absence of space charges, the average field E in a non-uniform field gap is maximum at the
surface of the conductor which has the smallest radius of curvature. TRUE

SECTION B (25 marks)

QUESTION 1

(a) Define the following terms: i. factor of earthing ii. critical flashover voltage iii. impulse
ratio
iv. basic impulse insulation level (4 marks)

(b) Mention THREE (3) types of impurities which have the potential of initiating complete breakdown
of commercial liquids below their intrinsic strength. (6 mark)

(c) Explain FOUR (4) major important factors that need to be taken into consideration when designing
a transmission line for protection against lightning. (10 marks)

(d) Briefly describe with the aid of a suitable diagram the Abraham electrostatic voltmeter used in high
voltage testing. (5 marks)

QUESTION 2

(a) Explain with the aid of diagrams, the basic concept of lightning phenomena indicating the charge
distribution at various stages of lightning discharge. (10 marks)

(b) Describe each of the following classes of arresters:

(c) Mention THREE (3) common characteristics of electrodes used for testing dielectrics. (3 marks)

(d) Give TWO (2) limitations of the rod gap arrester. (2 marks)

(e) Briefly explain with the aid of a suitable diagram the process of electro mechanical breakdown in
solid insulating materials. (6 marks)

“MAY GOOD SUCCESS BE YOUR PORTION”

Dr J. C. Attachie/I. Aidoo

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