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.
You've reached the end of your free preview.
Want to read all 6 pages?
- One '14
- Zone Substation