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ELEC 5204 POWER SYSTEMS 2012 Mid-Semester Assignment – Design of an Urban HV Distribution
** This assignment is due Monday 14 May, 2012. **
Introduction The objective of this assignment is to do the basic planning and protection design of the
sub-transmission and high voltage distribution network of a typical suburban “zone”
substation. It has been designed to tie in with system studies to be done on the Power
System Simulator in the power systems laboratory (the lab work will enable you to
confirm a number of the relay settings determined as part of this assignment).
A zone substation steps “sub-transmission” voltages of 33 kV down to “high voltage
distribution” voltages of 11 kV. Distribution substations on the 11 kV feeders in turn step
voltages down to 415 volts for domestic and commercial use. The sub-transmission
system is fed by a grid supply substation, modelled in this exercise by a single
132kV/33kV star-star transformer (but in reality would be several transformers in
parallel). From this a sub-mesh of 33kV lines feed the sub-transmission network,
modelled in this exercise by a single equivalent line. All impedances are shown on the
The zone substation will be constructed in two stages: initially two 25 MVA 33/11kV
transformers supplying 8 outgoing 11kV feeders and ultimately three 25 MVA
transformers and 15 outgoing 11kV feeders. For sake of analysis, the supply area can be
assumed to be roughly circular and of uniform load (refer fig 1). Initial diversified load
density (i.e. the average electrical load on the network) will be 1.0 MVA/km2 and the
ultimate load density 2 MVA/km2.
On the basis of reliability, the substation always runs one of its transformers as a spare,
i.e. the design capacity of a 2 transformer substation is only the rating of one and that of a
3-transformer substation is the capacity of 2 transformers. No spare capacity is proved at
local distribution substations. The 11kV network is run as a “ring-main” system, where
any 2 feeders can carry the load of 3 (i.e. the design capacity of the cables is based on an
average load equal to 2/3 of the cable maximum rating). However, all the feeders in
practice do not hit peak load at the same time (not uniformly loaded) and a “diversity
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, 2012 P 1 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
Supply area 11kV feeders Distribution
substations Zone
substation Fig 1 -Idealised Diagram of Supply Area, for Initial (25 MVA, 8 feeder
development) ELEC 5204 Mid-semester Assignment, 2012 P 2 of 6 Two zone substation transformers will run in parallel most times, but sometimes only one
will operate. This will lower the 11 kV fault level and you need to consider this when
designing the relay settings. The fault level on the 11 kV side of the zone substation must
not exceed 13.1 kA for three-phase and 8 kA for single-phase faults (cable heating limits)
with two transformers in parallel. The fault level on the 415V side of the distribution
transformers must not exceed 15 kA for three-phase faults on the 750 kVA units and 30
kA on the 1500 kVA units. You will need to check the incoming 33 kV side of the zone
substation fault levels for all three-phase and single phase to earth fault calculations (if
the 3-phase fault level is OK, but the single-phase fault level too high, you may wish to
consider inserting a neutral earthing impedance Zn in the zone transformer star points). F
132 kV 33 kV sub-transmission line
Z1 = j0.25 p.u ; Z0 = j0.5 p.u. 33 kV Grid supply transformers in ||
Z = j0.048 p.u equivalent 33 kV
A 33/11kV (Final Stage) Zone transformers
Z = j0.13 p.u each
Zn Zn
11 kV C
11 kV OC & EF relays are
located at:
A, B, C, D1, D2 & F C D1 C C 11kV/415 V (1500 kVA) D2
11kV/415 V (750 kVA) Note: All impedances are shown in per unit on 100 MVA base.
Fig 2 - Schematic Diagram of Zone Substation, Feeders and Distribution Substations
Available 11 kV (underground) 3-phase cable sizes are 185 mm2, 240 mm2 and 300 mm2
aluminium. Cable impedances and ratings (in Amps) are shown in table 1. The
maximum voltage regulation permissible on the zone substation transformers is 10%
(assume purely inductive, on 0.9 lagging PF load), on the 11 kV distribution feeders it is
3% and on the distribution substation transformers 3.5%. ELEC 5204 Mid-semester Assignment, 2012 P 3 of 6 Table 1 - 11kV Cable Impedances and Ratings
185 mm2
240 mm2
300 mm2 R1
0.13 X1
0.0958 ohms per km RO
0.266 XO ohms per km RATING
Amps/ph FAULT
3-ph/1-ph-E in
kA for 1 sec Standard Overcurrent Relay Characteristics
Choose the current pick-up and time dial settings. The current pick-up must be at least
120% of normal maximum feeder load or 150% of zone or distribution transformer
loading to allow for temporary overloads. In addition the setting must allow for
transformer inrush current (12 times full load for 0.01 second and 6 times for 0.1 second).
The distribution transformer HV setting must act as back-up for the low voltage side and
be able to detect LV arcing faults of 4000 Amps phase-neutral. The earth fault pick-up
settings are normally about 20% of the overcurrent, but again relays must grade correctly.
Note carefully the impact of the delta-earthed star winding arrangement on all
transformers, and the impact that will have on single phase to earth faults on the LV side
of transformers when reflected to the HV side.
The standard IEC formula for the operating time of an overcurrent relay on “standard
inverse” characteristic is:Time = 0.14 * TL /(I 0.02 –1),
Where : Time = the operating time, in seconds
TL = the relay time dial setting,
I = the relay current, in multiples (per unit) of the relay nominal current
(in this case, the latter will be 5 amps). A copy of the curve is attached (Fig 3). An Excel spreadsheet to calculate the relay times
and draw curves is also available, if desired.
As part of the analysis, you will have to consider faults on or near the zone substation
11kV bus-bar and also at the extremity of the 11 kV feeders. The overcurrent relays must
grade properly for these two fault conditions. When calculating faults, assume the
transformers are purely inductive and ignore the R component of cable impedances.
In particular, the following grading must occur• Between the 11kV overcurrent and 33kV overcurrent on the zone substation
transformers, for faults on the zone substation 11 kV busbar, for both one and two
transformers in service (i.e. between relays ‘A’ and ‘B’), • Between the 33kV overcurrent on the zone substation transformers, for faults on
the zone substation 33kV or 11 kV busbars and the overcurrent relay at the start of
the 33kV sub-transmission line (i.e. between relays ‘A’ and ‘F’), ELEC 5204 Mid-semester Assignment, 2012 P 4 of 6 • Between the 11kV feeder overcurrent and the 11kV overcurrent on the zone
substation transformers (i.e. between relays ‘B’ and ‘C’), for faults both near the
zone substation, or near the feeder extremities, • Between the 11kV feeder overcurrent and the overcurrent on the HV side of the
11kV/415 V distribution transformers, located at both the sending end and
extremity of each 11 kV distribution feeder (i.e. between relays ‘C’ and ‘D1/ D2’)
for faults both near the zone substation, or near the feeder extremities. Standard grading curves over page: ELEC 5204 Mid-semester Assignment, 2012 P 5 of 6 Fig 3 – Standard Inverse, Very Inverse and Extremely Inverse Overcurrent Time-Current
Characteristics for 1.0 Time Setting and 100 A current pick-up. NB: At a given current multiple, operating time is proportional to the Time setting ELEC 5204 Mid-semester Assignment, 2012 P 6 of 6

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