Chapter 3 Substation Sitting

3 Optimum siting and sizing of substation 49

Chapter 3 Optimum 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

3 Optimum siting and sizing of substation 50

Chapter 3 Optimum siting and sizing of substations (1.0)Introduction: 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,

3 Optimum siting and sizing of substation 50

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 forecast Land availability Cost of land Existing substation locations Feeder limitation Closeness to load centers

Factors affecting feeder routing selection: 1) 2) 3) 4) 5)

Future load growth Physical barriers Voltage drop Total cost feeder

3 Optimum siting and sizing of substation 50

(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.

3 Optimum siting and sizing of substation 50

D

Load density (KVA/km2)

( kVA) s

Substation KVA capacity

S

Distance between substations (km)

a

Fixed charge on feeder equipment LE/year

b

Total cost of feeder including erection LE/Km

c

That part of substation cost not proportional to substation capacity

d

Cost per KVA of the capacity required to carry the load in the area S2

( kVA) f nf Cs

CT

Feeder KVA capacity =

*V*Icc 3

number of feeders required per km2 Cost of substation per km2 Total annual cost of substation & feeders per km2

3 Optimum siting and sizing of substation 51

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 +d×D S2 The total cost of substation and feeders is then the product of equation (1) and (2) plus equation (3), mathematically, the total cost is,

CT = ( a + b × S / 2) × D / KVA f + c / S + d × D 2

(4)

3 Optimum siting and sizing of substation 50

Minimizing equation (4) with respect to S: (5)

 kVA f c  S = 1.59   bD 

0.333

km

and the corresponding optimum substation size would be (6)

kVAS opt = S 2 D

(3.0)Calculation Sequence: •

KVA Sopt = D ⋅ S 2

•

approximated to the higher integer

n S = area ÷ S 2 ⇒ •

• •

KVAS act = Total Load ÷ n S Loading Percent = KVA Sopt ÷ Substationrating includingthe reserve Get # of transformers needed & add reserve transformer. Try to make total # of transformers to be even for easier protection.

3 Optimum siting and sizing of substation 49

•

Feeders / Substation = Substationrating includingthe 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)Data used in design: (4.1)Draw:

All dimensions in Km

(4.2)load density:

3 Optimum siting and sizing of substation 49

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)Substations: Cost of Outdoor substation=23,000,000 LE Cost of Indoor substation= 30,000,000 LE

(4.4)For OHTL:

kind

Aluminum conductor steel reinforced(ACSR)

Cross section

120 mm2 (Stranded in 6 strands)

CCC

300 ampere

Price

38,400 LE/KM

Assume no derating in the OHTL

3 Optimum siting and sizing of substation 51

(4.5)For UGC: Insulation level

12/20 kV

Kind

Multicore cables, with stranded Aluminum conductors XLPE insulated, steel tape armoured and PVC sheathed

Cross-section

240 mm2

CCC

340 ampere

Price

166 LE/M+12% sales tax+25% erection

De-rating factors

Ground temperature derating factor , burials depth derating 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

(5.0)Calculations: (5.1)Agricultural area: (5.1.1)Design aspects: •

Substations used in the agricultural area are generally outdoor substations due to the low cost of land

3 Optimum siting and sizing of substation 49

•

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/mm2. It is recommended that the loading percent of the substation between 60%

⇒

80%

(5.1.2)Calculations: As we use OHTL

⇒

b=38,400 LE/KM

&

( kVA ) f

= 3 * 11 * 300

3 Optimum siting and sizing of substation 51

As we use Outdoor substation

⇒

C=23,000,000*0.3 LE

D=1.1275 MVA/ Km2

 kVA f c  S = 1.59   bD 

0.333

km

Km

S = 15.41306768 KVA

KVASopt = D ⋅ S 2 = 267851.8937 , approximated to the higher integer (

)

nS = 1

n S = area ÷ S = 0.526176979 2

KVA

KVAS act = Total Load ÷ nS = 140936.8 Transformers used in substation: 66/11 KV, 5×35 + 1×35 MVA Loading percent = 67.1128%

Feeders/ Substation= Substationrating includingthe reserve÷ KVA f = 36.74 Approximated to the integer giving even number of

Feeders/ Substation= 36 feeders/ substation & feeders/ transformer. Substation capacity is bigger than feeders capacity with 4.23 MVA Each transformer supplying 6 feeders.

3 Optimum siting and sizing of substation 51

Current density=0.8562A/mm2

(5.1.3)Sitting of the 66/11 KV substation in agricultural area:

All dimensions in Km

3 Optimum siting and sizing of substation 51

(5.2.0)Residential area: (5.2.1)Design aspects: •

•

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.

3 Optimum siting and sizing of substation 49

•

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/mm2

(5.2.2)Calculations: As we use UGC

⇒

b=227420LE/KM

( kVA ) f

As we use Indoor substation

D=3.3474 MVA/ Km2

 kVA f c  S = 1.59   bD 

&

0.333

km Km

S = 6.012358226

⇒

= 3 *11* 340 * 0.706

C=30,000,000*0.3 LE

3 Optimum siting and sizing of substation 49

KVA

KVASopt = D ⋅ S 2 = 121003.3264 , we have two solutions

nS = area ÷ S 2 = 3.1122

nS = KVAS act = Total Load ÷ nS =

solution 1

solution 2

3

4

125527.7 KVA

94145.775 KVA

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

Each transformer supplying

8 feeders

Current density=

0.572 A/mm

⇒

48

30.6

⇒

32

8 feeders 2

0.6434 A/mm

2

3 Optimum siting and sizing of substation 50

(5.2.3)Sitting of the 66/11 KV substation in the Residential area: Solution 1(

):

nS = 3

All dimensions in Km

Solution 2(

):

nS = 4

3 Optimum siting and sizing of substation 51

All dimensions in Km

(5.3.0)City center: (5.3.1)Design aspects: •

• •

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.

3 Optimum siting and sizing of substation 49

•

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/mm2

(5.3.2)Calculations: As we use UGC

⇒

b=227420LE/KM

&

( 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/ Km2

 kVA f c  S = 1.59   bD 

0.333

km Km

S = 4.823596935 KVA

KVASopt = D ⋅ S = 150824.2406 2

, approximated to integer (

)

nS = 8

nS = area ÷ S 2 = 8.058593534 KVA

KVAS act = Total Load ÷ nS = 151928.75 Transformers used in substation: 66/11 KV, 5×35 + 1×35 MVA

3 Optimum siting and sizing of substation 50

Loading percent = 72.35%

Feeders/ Substation= Substationrating includingthe reserve÷ KVA f = 45.92 Approximated to the integer giving even number of

Feeders/ Substation= 48 feeders/ substation & feeders/ transformer. Each transformer supplying 8 feeders. Current density=0.6922A/mm2

(5.3.3)Sitting of the 66/11 KV substation in the City center area:

All dimensions in Km

(5.4.0)Light industrial:

3 Optimum siting and sizing of substation 51

(5.4.1)Design aspects: • •

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/mm2

(5.4.2)Calculations: As we use UGC

b=227420LE/KM

⇒

( kVA ) f

As we use Indoor substation

D=6.872 MVA/ Km2

 kVA f c  S = 1.59   bD 

&

0.333

km

⇒

= 3 *11* 340 * 0.706

C=30,000,000*0.3 LE

3 Optimum siting and sizing of substation 51

Km

S = 4.730637488 KVA

KVASopt = D ⋅ S 2 = 153788.0141 , approximated to the higher integer (

)

nS = 2

nS = area ÷ S 2 = 1.676 KVA

KVAS act = Total Load ÷ nS = 128854.35 Transformers used in substation: 66/11 KV, 5×35 + 1×35 MVA Loading percent = 61.36%

Feeders/ Substation= Substationrating includingthe reserve÷ KVA f = 45.92

Feeders/ Substation= 48 Approximated to the integer giving even number of feeders/ substation & feeders/ transformer. Each transformer supplying 8 feeders. Current density=0.587A/mm2

(5.4.3)Sitting of the 66/11 KV substation in Light Industrial area:

3 Optimum siting and sizing of substation 51

All dimensions in Km

(5.5.0)Heavy industrial: (5.5.1)Design aspects: •

•

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.

3 Optimum siting and sizing of substation 49

•

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/mm2

(5.5.2)Calculations: As we use UGC

⇒

b=227420LE/KM

As we use indoor substations

&

( kVA ) f ⇒

= 3 *11* 340 * 0.706

C=30,000,000*0.3 LE

3 Optimum siting and sizing of substation 49

D=16.2175 MVA/ Km2

 kVA f c  S = 1.59   bD 

0.333

km Km

S = 3.553198066 KVA

KVASopt = D ⋅ S 2 = 204749.4485 , approximated to integer (

)

nS = 3

nS = area ÷ S 2 = 2.97

KVA

KVASact = Total Load ÷ nS = 202718.7333 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

Each transformer supplying

11 feeders

8 feeders

Current density=

0.6716 A/mm2

0.6926 A/mm2

1. Higher cost due to higher

1. Lower cost.

comments

⇒

66

61.224

⇒

64

3 Optimum siting and sizing of substation 49

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.

(5.5.3)Sitting of the 66/11 KV substation in Heavy industrial area:

3 Optimum siting and sizing of substation 49

All dimensions in Km