7 should be applied definition is df total substation

factor” (DF) of 0.7 should be applied (definition is: DF = Total Substation Load/Sum of
individual feeder loads, i.e. zone substation load = 0.7 * individual feeder peak loads;
conversely an individual feeder peak load = zone sub load/no of feeders/0.7).
The distribution substations vary in size from “kiosk” types of 750 kVA to commercial
designs of 1500 kVA capacities. All the zone and distribution transformers are 3-phase,
delta-star types; delta on the HV, earthed star on the LV; the zone transformers may be
earthed via an earthing reactor. The substations and network are all three-phase, 50 Hz.
The Design Task
The design task is as follows. Analyse the system and recommend:-

ELEC 5204 Mid-semester Assignment, 2020 P 2 of 6
• Confirm the suitability of the zone transformer impedance, and zone transformer
neutral earthing impedance (if used),
• The distribution transformer impedances (for both 1500 and 750 kVA sizes),
• The 11kV cable size,
• The zone substation transformer 11 kV CT ratio, to suit the transformer’s
differential protection scheme (allow for star-delta transformation),
• The zone substation transformer overcurrent and earth fault relay settings on HV
(33kV) and LV (11kV) sides (relays ‘A’ and ‘B’),
• The sub-transmission feeder overcurrent and earth fault relay settings,
• The 11 kV feeder overcurrent and earth fault relay setting (relay ‘C’),
• The HV overcurrent relay setting on both the 1500 and 750 kVA distribution
transformers (relays ‘D1’ and ‘D2’). Refer to figure 2 for the A, B, C and D
relays locations.
There are both differential and overcurrent relays on each side of zone substation
transformer, and overcurrent relays on each outgoing 11 kV feeder, and on the HV side of
each distribution substation. The zone substation transformers have 500:5 Amp CTs on
the 33kV side, and a choice of 1200:5, 1500:5 or 1800:5 on the 11kV side. The 11 kV
feeders have 400:5 Amp CTs and for the distribution transformers, there is a choice of
60:5, 80:5, 100:5 or 150:5 Amp CTs. The primary rating CTs on the 11kV side of the
zone substation transformers will be chosen to suit the differential protection and the
overcurrent relays will have the same CT ratio. All CT secondaries (and hence also all
overcurrent relays) will be 5 amps. You will have to specify the secondary current rating
on the 11 kV sides of the differential relays on each zone substation transformer.
System Diagrams
Fig 1 -Idealised Diagram of Supply Area, for Initial (25 MVA, 8 feeder
development)
Supply area
11kV feeders
Distribution
substations
Zone
substation

ELEC 5204 Mid-semester Assignment, 2020 P 3 of 6
Two zone substation transformers will run in parallel most times, but sometimes only one
will operate.