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EO310 1 / 6
SCHOOL OF ENGINEERING
MODULAR HONOURS DEGREE COURSE
LEVEL THREE
SEMESTER 2
2005/2006
HIGH VOLTAGE TECHNOLOGY
Examiner: Dr P.A. Howson
Attempt THREE questions only
Time allowed: 2 hours
Total number of questions = 5
All questions carry equal marks.
The figures in brackets indicate the relative weightings
of parts of a question
Special requirements: None
EO310 2 /6
1) Figure 1b shows a three stage Cockcroft Walton Multiplier.
a) Explain why this arrangement is favoured in modern d.c power supplies as a
means of generating high voltage d. c.
(5)
C1
Ck3
C2
Ck2
C3
Ck1
Figure 1b
b) Determine the output voltage of the multiplier when the load current is zero and
if the supply transformer voltage is 5 kV r.m.s
(5)
c) Determine the value of C if the voltage ripple is to be limited to 100 V for a
load current of 10 mA and the supply frequency is 50 Hz.
(5)
d) If the ripple voltage is to be reduced to 10 V for a load current of 10 mA,
determine a suitable new value of frequency to meet this target
(5)
EO310 3 /6
2) Given that the reflection coefficient () for surges traveling from a power line of
surge impedance Z1 into a power line of impedance Z2 is given by :

Z 2  Z1
Z1  Z 2
Estimate the reflection coefficients for the following two power line termination
conditions:
a) Power line termination, open circuit
(3)
b) Power line termination, short circuit.
(3)
c) Consider the following situation. A lightening conductor is attached to a tower
and connected to a lightening earthing rod via a conductor tape with a
characteristic impedance of 350 . The lightening rod has a inherent inductance
of 15 H and capacitance of 6 nF, the ground into which the rod is conducting,
70 . If the lightening conductor is struck by a discharge which causes the
conductor potential to rise to 1 million volts:
Calculate the potential that the ground in contact with the earthing rod, will rise
to.
d) State two principle causes of traveling surges
(10)
(4)
EO310 4 /6
3) a) Describe generally the relative merits and de-merits of the use of a vacuum for
dielectric purposes against its oil and gaseous counterparts?
(5)
b) The formation of voids in a solid dielectric material is inevitable. Describe the
source(s) of formation and possible suppression techniques manufacturers apply
to alleviate damage?
(5)
c) Describe the factors that affect Corona discharge (include typical values) and
any potential influence(s) discharge activity may have.
(5)
d) Explain what you understand by the term “Intrensic Dielectric Strength” of a
electrical insulation material
(5)
EO310 5 /6
4) a) Figure 3 shows a section of insulation of thickness d, across which a field E is
applied.
Insulation
E
Void
d’
d
Figure 3
If a cavity exists in the insulation of thickness d’. Show that the following
expression which will allow you to calculate the r.m.s inception voltage, Vi,
which must be applied between the electrodes for discharge inception to occur
in the void.
(10)
 d  1 
Vi  Fc  d '1    1. 
  d  m 
Ec 
Given
Where
m
&
c
m
.E
c
are the relative permitivities of the insulation and void
respectively and Fc is the field breakdown in the cavity.
b) Obtain the r.m.s at which you would expect partial discharge activity to occur, if
d’ = 0.3 mm..and the breakdown strength of air in the void can be taken as 46
kV(peak)/cm.
The relative value of insulation permittivity = 2.7 and flux
fringing may be neglected.
(3)
c) Assume the same sample is left under test for six months and the homogeneity
of the surroundings insulations remains unchanged. During this period the
diameter of the cavity increases due to erosion. Obtain the new diameter of the
cavity if the inception voltage increases by 50%.
(7)
EO310 6 /6
5) a) What are the advantages of dielectric loss measurement of electrical insulation
compared to partial discharge and simple d.c measuring methods
(3)
b) How can the “Dielectric Loss Angle” be utilised to test the properties of
electrical insulation.
Sketch a suitable circuit to measure the dielectric loss angle and capacitance of a
cable.
(5)
c) The capacitance, Cx and loss angle , of a cable were measured using a
Schering bridge connected as follows :
Arm AB: A high voltage 300 pF, standard air capacitor having negligible loss.
Arm BC: A non inductive resistor of 2180 ohm in parallel with a variable
capacitor, C.
Arm CD: A non inductive resistor, R.
Arm DA: The cable under test.
The bridge was supplied between A and C at 50 Hz and a detector was
connected between B and D. Balance was obtained when R=49.7 ohm and
C=0.0045 F.
Calculate C & tan  .
d) How can this dielectric loss angle measurement method be enhanced.
(12)
(2)