1. A 380 / 220 V r.m.s. 3-phase 4-wire circuit carries loads of 8 + j8.1 ohm; 4 + j4.1 ohm; and 2 +j2.1 ohmin the three phases respectively. The length of the circuit is 100 m, and the resistivity
of the phase conductors is 0.0005 Ω/m. The neutral conductor is of the same size as the
phase conductors.
a) Select a reference angle 0for the first phase. Then write down the three phase voltages
in both polar format and complex number format.
b) Make appropriate assumptions, and take temperature effects on resistance is
insignificant. Calculate the currents in the three phases. (Note: Determine conductor
resistance, and apply Kirchhoff’s Voltage Law) Based on the result, suggest a main
switch rating for the circuit.
c) Show your preferred direction of neutral current in a diagram, and then determine its
value. (Note: Apply superposition principle, and Kirchhoff’s Current Law) Use its
magnitude to show that in this circuit, the unbalanced load may increase the voltage
drop in some of the phases. (Note: Also observe the common volt drip in neutral) State
your assumptions, if there are any.
d) Suddenly the neutral wire of the circuit is broken. Estimate the new currents in the first
phase, i.e. Phase L1. State your assumptions, if there are any.
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