3 Optimum siting and sizing of substation
Chapter 3 Optimum siting siting and sizing of substations
Chapter content:
(1.0)Introduction
(2.0)Symbols definitions
(3.0)Calculation Sequence
(4.0)Data used in design
(5.0)Calculations
49
3 Optimum siting and sizing of substation
Chapter 3 Optimum siti siting ng and sizi si zing ng of subst substat atio ion ns (1.0 (1.0))Intro Int roduc ducti tion: on: Substations are the second step in power system after the Electrical load forecast & Starting with Substation “66/22 KV or 11 KV “the distribution network will initiate. Substation will distribute its power using feeders on distribution voltage 22, 11 KV. These feeders can be overhead or underground cables depend on the cost of the system, it's nature & it's location in the Rural or in Cities. Our concern here is to find the number of substations and its distribution on each planning area. The selection of the number of the substations is from economical view. We try to find the optimum number of substations to have minimum cost. We want to equalize the cost of substations and the cost of feeders, to have the minimum total cost for both S/S & feeders, If we have large number of substations (large S/S cost) we will have short feeders (low cost) and if we have small number of substations (low S/S cost) we will have long feeders (large cost). This mean increasing the number of substations for a given load density tends to increase total cost. However, increasing the number of substations reduces the cost of feeders and feeder losses. Clearly then, the least total annual cost is a function of substation size, feeders cost, capacity of feeder and load density. But there are another factors rather than the cost of S/S & feeders effect on the selection the optimum sitting & sizing, so due to the growth of demands and scarcity of available S/S sites, the cost of the location and spacing of substations becoming a major economic problem also. Due to local geography,
50
3 Optimum siting and sizing of substation
distribution system, Security & other conditions beyond control, it may be impractical to select the ideal substation size and spacing. So, these factors should be known so that the ideal conditions may be approached as near as possible from all points of view. The best size and spacing of S/S is that which results in the least annual cost of the sum of fixed charges on S/S, feeders, operation, maintenance, and losses.
Factors affecting substation site selection: 1) 2) 3) 4) 5) 6)
Load Load fore foreca cast st Land Land avai availa labi bili lity ty Cost Cost of land land Existing Existing substat substation ion location locations s Feed Feeder er limi limita tati tion on Close Closenes ness s to load load cent centers ers
Factors affecting feeder routing selection: 1) 2) 3) 4) 5)
Futu Future re load load gro growth wth Phys Physic ical al barr barrie iers rs Volt Voltag age e dro drop p Tota otal co cost feeder
50
3 Optimum siting and sizing of substation
(2.0)Symbols definitions: The following symbols will be used to determine the relationship of the above factors in deriving an equation for total annual cost.
50
3 Optimum siting and sizing of substation
D
Load density (KVA/km 2)
( kVA) s
Substation KVA capacity
S
Distance between substations (km)
a
Fixed charge on feeder equipment LE LE/year
b
Total cost of feeder including erection LE L E/Km
c
That That part part of subs substa tati tion on cost cost not not pro propo port rtio iona nall to to sub subst stat atio ion n cap capac acit ity y
d
Cost Cost per per KVA KVA of of the the capa capaci city ty requ requir ired ed to carr carry y the the loa load d in in the the area area S 2
( kVA) f
n f C s
C T
Feeder KVA capacity =
*V*Icc 3
number of feeders required per km 2
Cost of substation per km 2 Total annual cost of substation & feeders per km 2
50
3 Optimum siting and sizing of substation
Using one square km as a unit area, the number of feeders required to serve this area is, (1)
n f = D / kVA f And the cost per feeder is given by (2)
a + b × S / 2 The substation cost to supply this unit area is: (3)
c 2
S
+
d × D
The total cost of substation and feeders is then the product of equation (1) and (2) plus equation (3), mathematically, the total cost is, (4)
C T
=
( a + b × S / 2 ) × D / KVA f + c / S
2
+
d × D
51
3 Optimum siting and sizing of substation
Minimizing equation (4) with respect to S : (5)
kVAf c S = 1.59 bD
0.333
km
and the corresponding optimum substation size would be (6)
kVAS opt = S 2 D
(3.0) (3. 0)Calculat Calculation ion Seque Sequence nce:: •
KVA S opt = D ⋅ S 2 approximated to the higher integer
•
n S •
•
•
=
area ÷ S 2
⇒
KVAS act = Total Load ÷ nS
Loading Percent = KVA S opt ÷ Substationrating including the reserve Get # of transformers needed & add reserve transformer. Try to make total # of transformers to be even for easier protection.
50
3 Optimum siting and sizing of substation
•
Feeders / Substation = Substationrating including the reserve ÷ KVA f And then approximated to the higher integer giving even no of feeders / substation Current Density =
KVAS act ÷ ( feeder / Substation× 3 × 11 × csa of feeder )
(4.0) (4. 0)Data used in desig design: n: (4.1)Draw:
All dimensions in Km
(4.2)load density:
49
3 Optimum siting and sizing of substation
D
Area (Km2)
Maximum Load (MVA)
(MVA/ Km2)
1.Agricultural
25*5
140.9368
1.1275
2.Residential
15*7.5
376.5831
3.3474
3.City Center
15*12.5
1215.43
6.4823
4.Light Industrial
7.5*5
257.7087
6.872
5.Heavy Industrial
7.5*5
608.1562
16.2175
Zone
(4.3) (4. 3)Substat Subs tations ions:: Cost of Outdoor substation=23,000,000 LE Cost of Indoor substation= 30,000,000 LE
(4.4 (4.4))For For OHT OHTL: L:
kind
Aluminum conductor steel reinforced(ACSR)
Cross section
120 mm 2 (Stranded in 6 strands)
CCC
300 ampere
Price
38,400 LE/KM
Assume no derating in the OHTL
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3 Optimum siting and sizing of substation
(4.5 (4.5))For Fo r UGC: UG C: Insulation level
12/20 kV
Kind
Multicore cables, with stranded Aluminum conductors XLPE insulated, steel tape armoured and PVC sheathed
Cross-section
240 mm 2
CCC
340 ampere
Price
166 LE/M+12% sales tax+25% erection
De-r De-rat atin ing g fac facto tors rs
Grou Ground nd temp temper erat atur ure e der derat atin ing g fac facto torr , bur buria ials ls dept depth h der derat atin ing g factor, soil thermal resistivity derating factor
The following de-rating factors should be considered in design: •
Ground temperature de-rating factor
•
Burial depth derating factor (1 m)
•
Soil thermal resistivity derating factor
D.F = 0.93 × 0.93 × 0.91 = 0.706 D.F
(5.0)Calculations: (5.1) (5. 1)Agricult Agri cultura urall area: are a:
(5.1.1) (5.1 .1) Design Des ign aspect as pects: s: •
Substations used in the agricultural area are generally outdoor substations due to the low cost of land
51
3 Optimum siting and sizing of substation
•
•
Over Head Transmission Lines ( OHTL ) are used in the agricultural area are used for the primary distribution networks ( 11 KV ) due to The lower cost of OHTL makes it more suitable for use in the agricultural areas and UGC may be destroyed while digging or due to irrigation in the agricultural areas. It is recommended that the number of transformers in each substation doesn't exceed 6 transformers including the reserve.
•
It is recommended that the bus bars in the substation are sectionalized and doubled to allow maneuver and that the number of transformers and feeders per section is even number.
•
• •
It is recommended that the number of feeders per transformer in agricultural area shouldn't exceed 6 feeders. A recommended current density for the OHTL it is generally around 2 A/mm 2. It is recommended that the loading percent of the substation between 60%
80% ⇒
(5.1.2) (5.1 .2) Calcul Cal culati ations: ons:
As we use OHTL
b=38,400 LE/KM ⇒
&
( kVA kVA ) f
=
3 * 11 * 300
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3 Optimum siting and sizing of substation
As we use Outdoor substation
C=23,000,000*0.3 LE ⇒
D=1.1275 MVA/ Km2
kVAf c S = 1.59 bD
0.333
km
Km
S = 1 5.4130676 KVA 2 KVAS opt = D ⋅ S = 267851.8937
, approximated to the higher integer (
n S = area ÷ S 2 = 0.526176979
)
n S
=1
KVA
KVAS act = Total Load ÷ nS = 140936.8 Transformers used in substation: substation: 66/11 KV, 5×35 + 1×35 MVA Loading percent = 67.1128%
Feeders / Substation= Substationrating including the reserve÷ KVA f = 36.74 Approximated
to the integer giving even number of
Feeders/ Substation= 3 6 feeders/ substation & feeders/ transformer. Substation capacity is bigger than feeders capacity with 4.23 MVA Each transformer supplying 6 feeders.
51
3 Optimum siting and sizing of substation
Current density=0.8562A/mm 2
(5.1.3) (5.1 .3) Sitting Sit ting o f the 66/11 66/ 11 KV substa su bstatio tion n in agricu agr icultu ltural ral area: ar ea:
All dimensions in Km
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3 Optimum siting and sizing of substation
(5.2.0) (5.2.0) Resident Resi dential ial area: ar ea:
(5.2.1) (5.2 .1) Design Des ign aspect as pects: s: •
Substations used in Residential area are generally the indoor substations. Under Ground Cables (UGC) are used in Residential area for the primary distribution networks (11 KV).
•
From the reliability point of view, it is better to replace one large substation with
•
some smaller substations distributed over the planning area, in spite of the increase in the cost; yet, the increase is not that considerable amount. •
It is recommended that the number of transformers in each substation doesn't exceed 6 transformers including the reserve.
•
It is recommended that the bus bars in the substation are sectionalized and doubled to allow maneuver and that the number of transformers and feeders per section is even number.
•
It is recommended that the number of feeders per transformer in Residential area exceed 8 feeders per transformer.
51
3 Optimum siting and sizing of substation
•
The loading percent in each substation shouldn't exceed 80% for safety and continuity of feeding in case of outage of any unit.
•
A recommended current density for the UGC is 1 A/mm 2
(5.2.2) (5.2 .2) Calcul Cal culati ations: ons: As we use UGC
b=227420LE/KM ⇒
( kVA kVA ) f
As we use Indoor substation
D=3.3474 MVA/ Km 2 0.333
km
Km
S = 6.01235822
=
3 *11 * 340 * 0.706
C=30,000,000*0.3 LE ⇒
kVAf c S = 1.59 bD
&
49
3 Optimum siting and sizing of substation
KVA
KVAS opt = D ⋅ S 2 = 121003.3264 , we have two solutions
nS = area ÷ S 2 = 3.1122
nS =
solution 1
solution 2
3
4
125527.7 KVA
94145.775 KVA
KVAS act = Total Load ÷ nS = Transformers used in substation 66/11 KV
5×35 + 1×35 MVA
3×35 + 1×35 MVA
Loading percent =
59.77%
67.247%
Feeders/ Substation=
45.9
48 ⇒
Each transformer supplying
8 feeders
Current density=
0.572 A/mm
30.6
32 ⇒
8 feeders 2
0.6434 A/mm
2
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3 Optimum siting and sizing of substation
(5.2.3) 5.2. 3)Sitting Sit ting of the th e 66/11 66 /11 KV K V substa su bstatio tion n in the Resident Resi dential ial area : Solution 1(
):
nS = 3
All dimensions in Km
Solution 2(
):
nS = 4
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3 Optimum siting and sizing of substation
All dimensions in Km
(5.3 (5. 3.0)City .0)City center: center: (5.3 .1)Desi .1)D esign gn aspects asp ects:: •
•
•
Substations used in City center area are generally the GIS (Gas Insulated Substations) since the land is very expensive. Under Ground Cables (UGC) are used in City center area for the primary distribution networks (11 KV). From the reliability point of view, it is better to replace one large substation with some smaller substations distributed over the planning area, in spite of the increase in the cost; yet, the increase is not that considerable amount.
•
It is recommended that the number of transformers in each substation doesn't exceed 6 transformers including the reserve.
•
It is recommended that the bus bars in the substation are sectionalized and doubled to allow maneuver and that the number of transformers and feeders per section is even number.
51
3 Optimum siting and sizing of substation
•
•
It is recommended that the number of feeders per transformer in City center area shouldn't exceed 8 feeders per transformer. The loading percent in each substation shouldn't exceed 80% for safety and continuity of feeding in case of outage of any unit.
•
A recommended current density for the UGC is 1 A/mm 2
(5.3 .2)Calc .2)C alculat ulations ions:: As we use UGC
b=227420LE/KM
&
( kVA kVA ) f
⇒
As we use GIS (Gas Insulated Substations)
=
3 *11 * 340 * 0.706
C=30,000,000*0.3 LE ⇒
D=6.4823 MVA/ Km 2
kVAf c S = 1.59 bD
0.333
km
Km
S = 4.82359693 KVA
KVAS opt = D ⋅ S 2 = 150824.2406 , approximated to integer (
nS = area ÷ S 2 = 8.058593534
)
nS = 8 KVA
KVAS act = Total Load ÷ nS = 151928.75 Transformers used in substation: substation: 66/11 KV, 5×35 + 1×35 MVA
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3 Optimum siting and sizing of substation
Loading percent = 72.35%
Feeders / Substation= Substationrating including the reserve÷ KVA f = 45.92 Approximated
to the integer giving giving even number of
Feeders/ Substation= 4 8 feeders/ substation & feeders/ transformer. Each transformer supplying 8 feeders. Current density=0.6922A/mm 2
(5.3.3)Sitting .3)Sitti ng of the 66/11 66 /11 KV K V substation subs tation in the th e City Ci ty center c enter area:
All dimensions in Km
(5.4 (5.4 .0) .0)Ligh Lightt ind indus ustr tria iall:
50
3 Optimum siting and sizing of substation
(5.4 (5 .4 .1) .1)Desi De sign gn asp aspec ects ts:: •
Substations used in Light Industrial area are generally the indoor substations Under Ground Cables (UGC) are used in Light Industrial area for the primary distribution networks (11 KV).
•
From the reliability point of view, it is better to replace one large substation with
•
some smaller substations distributed over the planning area, in spite of the increase in the cost; yet, the increase is not that considerable amount. •
It is recommended that the number of transformers in each substation doesn't exceed 6 transformers including the reserve.
•
It is recommended that the bus bars in the substation are sectionalized and doubled to allow maneuver and that the number of transformers and feeders per section is even number.
•
•
It is recommended that the number of feeders per transformer in Light Industrial area shouldn't exceed 8 feeders per transformer. The loading percent in each substation shouldn't exceed 80% for safety and continuity of feeding in case of outage of any unit.
•
A recommended current density for the UGC is 1 A/mm 2
(5.4 (5 .4 .2) .2)Calc Calcul ulat atio ions ns:: As we use UGC
b=227420LE/KM
( kVA kVA ) f
⇒
As we use Indoor substation
D=6.872 MVA/ Km 2 0.333
km
=
3 *11 * 340 * 0.706
C=30,000,000*0.3 LE ⇒
kVAf c S = 1.59 bD
&
51
3 Optimum siting and sizing of substation
Km
S = 4.73063748 KVA
KVAS opt = D ⋅ S 2 = 153788.0141 , approximated to the higher integer (
nS = area ÷ S 2 = 1.676
)
nS = 2 KVA
KVAS act = Total Load ÷ nS = 128854.35 Transformers used in substation: substation: 66/11 KV, 5×35 + 1×35 MVA Loading percent = 61.36%
Feeders / Substation= Substationrating including the reserve÷ KVA f = 45.92
Feeders/ Substation= 4 8
Approximated
to the integer giving giving even number of
feeders/ substation & feeders/ transformer. Each transformer supplying 8 feeders. Current density=0.587A/mm 2
(5.4 (5.4.3) Sitti Sitting ng of the the 66/11 66 /11 KV subs s ubstat tatio ion n in Light Lig ht Indu I ndust stri rial al area a rea :
51
3 Optimum siting and sizing of substation
All dimensions in Km
(5.5 (5.5 .0) .0)Heav Heavy y indu in dust stri rial al::
(5.5 (5 .5 .1) .1)Desi De sign gn asp aspec ects ts:: •
Substations used in Heavy industrial area are generally the indoor substations. Under Ground Cables (UGC) are used in Heavy industrial area for the primary distribution networks (11 KV).
•
From the reliability point of view, it is better to replace one large substation with
•
some smaller substations distributed over the planning area, in spite of the increase in the cost; yet, the increase is not that considerable amount.
51
3 Optimum siting and sizing of substation
•
It is recommended that the number of transformers in each substation doesn't exceed 6 transformers including the reserve.
•
It is recommended that the bus bars in the substation are sectionalized and doubled to allow maneuver and that the number of transformers and feeders per section is even number.
•
•
It is recommended that the number of feeders per transformer in City center area shouldn't exceed 8 feeders per transformer. The loading percent in each substation shouldn't exceed 80% for safety and continuity of feeding in case of outage of any unit.
•
A recommended current density for the UGC is 1 A/mm 2
(5.5 (5 .5 .2) .2)Calc Calcul ulat atio ions ns:: As we use UGC
b=227420LE/KM
&
( kVA kVA ) f
⇒
As we use indoor substations
=
3 *11 * 340 * 0.706
C=30,000,000*0.3 LE ⇒
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3 Optimum siting and sizing of substation
D=16.2175 MVA/ Km 2
kVAf c S = 1.59 bD
0.333
km
Km
S = 3.55319806 KVA
KVAS opt = D ⋅ S 2 = 204749.4485 , approximated to integer (
nS = area ÷ S 2 = 2.97
)
nS = 3 KVA
KVAS act
=
Total Load ÷ nS = 2 0 2 7 1 8.7 3 3 3
We have two solutions: Solution 1
Solution 2
Transformers used in substation: 66/11 KV
5×50 + 1×50 MVA
7×35 + 1×35 MVA
Loading percent =
67.573%
72.399%
Feeders/ Substation=
65.597
66 ⇒
61.224
64 ⇒
Each transformer supplying
11 feeders
8 feeders
Current density=
0.6716 A/mm 2
0.6926 A/mm 2
1. Higher cost due to higher
1. Lower cost.
comments
49
3 Optimum siting and sizing of substation
capacity. 2.transformers of 50 MVA do not produced in Egypt. 3. Lower loading percent.
2. Transformers from Egypt. 3. Higher loading percent. 4. # of trans. Larger than 6 transformers.
4. large # of feeders/ trans.
(5.5 (5.5.3) Sitti Sitting ng of the the 66/11 66 /11 KV subs s ubstat tatio ion n in Heavy Hea vy indu i ndust stri rial al area are a:
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3 Optimum siting and sizing of substation
All dimensions in Km
49
