A 33 kV / 11 kV zone substation has two transformers that are
connected in parallel. When the transformers are fully loaded, they
exhibit a voltage drop of 10% of the nominal voltage. Each
transformer is rated at 25 MVA.
(a) Using the transformer % voltage drop and their ratings,
calculate the impedance in ohms of each transformer referred to the
11 kV side (assume impedance is purely reactive).
(b) Ignoring the impedance of the system upstream of the 33 kV
bus, calculate the fault current and the fault level on the 11 kV
bus of the zone substation.
There are two 11 kV overhead feeders that leave the substation
and each feeder is 10 km long. Each feeder has an impedance of 0+j
0.30 Ω/km per phase. During a storm, a tree falls onto one of the
feeders causing a three phase to ground fault at a location half
way along the length of the feeder.
(c) Draw a single-phase equivalent circuit that will allow the
calculation of the fault current and the voltages during this
fault.
(d) Using the above circuit, calculate the magnitude of the
voltage seen by the customers on the healthy parallel feeder during
the fault and express this as a percentage of the nominal
voltage.