ELEC4611 Power System Equipment SAMPLE MID-SESSION EXAM

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