ELEC 4302 – Power System Protection
Tutorial submission week 12
Due 22/10/2021 4:00 pm
a) Explain the advantage of neutral overcurrent relay scheme over differential protection scheme for
identifying phase to earth faults of generators. [10 Marks]
b) A three-phase transformer rated for 30 MVA, 33 kV/6.6 kV is connected for wye delta and protected by a
differential relay. The CT ratio on the low voltage side is 2500:5.
i. Identify suitable winding connection type for line-CT secondaries with the aid of circuit diagram. [10
marks]
ii. Determine the CT ratio on high voltage side for the correct operation of the differential protection
scheme. [10 Marks]
iii. Select a suitable CT assuming that transformer never exceeds the full load capacity. Following CTs are
available to select: 500:5, 600:5, 800:5, 900:5, 1000:5 and 1200:5. Provide reasons for your choice
[10 Marks]
iv. Find the operating current setting suitable for differential relays going to be used with the given CT in
(iii) for faults drawing up to 200% of the rated current. [10 Marks]

Marking criteria – To achieve full marks all calculation steps must be clearly presented and any assumptions you
have used should be clearly stated.

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