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SCHOOL OF ENGINEERING
B Eng Honours Degree in Electrical and Electronic Engineering
Level Three
June 2007
High Voltage Technology
Examiner: Dr P.A. Howson
Answer only Three questions.
Special requirement None
Time allowed Two Hour hours.
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 15 kV r.m.s
(5)
c. Determine the value of C if the voltage ripple is to be limited to 300 V for a load current of 5 mA and
the supply frequency is 50 Hz.
(5)
d. If the ripple voltage is to be reduced to 100 V for a load current of 5 mA and the value of C remains
the same. Determine a suitable new value of frequency to meet this target
(5)
2.
i. Explain briefly the causes of voltage surges on Power transmission lines
(4)
ii. 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 transmission coefficients for the following two power line termination
conditions:
a. Power line termination, open circuit
b. Power line termination, short circuit.
(3)
(3)
iii. 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 charactoristic impedance
of 300 . The lightening rod has a inherent inductance of 20 H and capacitance of 8 nF,
the ground into which the rod is conducting, 80 . 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, would rise to.
(10)
3.
a. Briefly explain the differences between void, surface and corona partial discharges.
(6)
b.
Figure 1 shows an electrode arrangement for testing the dielectric strength of insulation.
HV
Void
6 mm
t
Polythene
mm
Fig 1
Owing to a manufacturing fault, the polythene sample has a disc shaped void parallel to the
electrodes as shown in Fig 1.
Derive a expression which will allow you to calculate the r.m.s voltage which must be applied
between the electrodes for discharge inception to occur in the void when t = 0.1 mm. Flux
fringing may be neglected. The breakdown strength of air in the void can be taken as 40
kV(peak)/cm.
(14)
4.
b. A polyethylene insulated cable has a inner radius of 7 mm and an outer conductor whose inner
radius is 14 mm. A void is present in the insulation. Show that the discharge inception voltage
can be obtained from the following expression:
  b   1  c 
Vi  E  max  a  ln    ln  
  a   2  b 
Where a=inner conductor radius, c=outer conductor radius, b=radius to void, 1 and 2
= insulation permittivity.
(14)
c. If a o.1 mm thick void exists in the insulation, determine the peak discharge inception voltage
when the void is near the inner conductor and half-way between the conductors. Take the
permittivity of polyethylene to be 2.26 and the breakdown of air in the void to be 6 MV/m.
(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 100 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=39.7 ohm and C=0.0051 F.
Calculate C & tan  .
d How can this dielectric loss angle measurement method be enhanced.
(12)
(2)