ELEC4611 - Power System Equipment
SAMPLE MID-SESSION EXAM
Duration: 90 minutes, closed book
Q1
(a) Consider a coaxial geometry of dielectric material with inner radius a, outer radius b, and
applied voltage V. The e
...
ELEC4611 - Power System Equipment
SAMPLE MID-SESSION EXAM
Duration: 90 minutes, closed book
Q1
(a) Consider a coaxial geometry of dielectric material with inner radius a, outer radius b, and
applied voltage V. The electric field in the insulation at a radial distance r is:
ln
V
E r
r b a
V/m
A power cable uses a single type of dielectric material where a=15mm, b=25mm. The
material relative permittivity is r 2.3 . The rated voltage across the cable insulation is
19kV (phase-to-ground rms voltage). Due to flaws during the manufacturing process, a
defect in the form of a small air cavity is present in the cable insulation. Considering the
breakdown strength of air is 3kV/mm, show that electrical breakdown will occur in the
cavity.
(b) According to the Overhead Line Design Standard for Transmission & Distribution
Systems TS-107 of South Australia Power Networks, a minimum clearance of 255mm
between 11kV phase-phase conductors shall be maintained under all conditions. This
rated voltage is allowed to vary up to 10%. A range of conductors with diameter of
8.3mm, 14.3mm or 29.3mm are available for use. Calculate the worst-case electric stress
and comment on the result. Note that the breakdown strength of air is 3kV/mm
Hint: Consider two bare conductors (representing two phases) of radius a, separated by
distance d, with a voltage V between them. Figure 1 shows the resultant electric field
intensity in the air space.
Figure 1
The electric field in the air between the lines a r d 2 is given by:
V/m
2 ln
V
E r
d a
r
a
Q2 (a) A fault on a typical low voltage circuit can be represented by a general R-L circuit with a
switch and a sinusoidal voltage source. The instantaneous fault current for a fault
initiating at t = 0, with no prior load current flow, is given by
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