CHAPTER 3 Substation Sitting

493 Optimum siting and sizing of substation

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


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,

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) Load forecast2) Land availability3) Cost of land4) Existing substation locations5) Feeder limitation6) Closeness to load centers

Factors affecting feeder routing selection:

1) Future load growth2) Physical barriers3) Voltage drop4) Total cost5) feeder


(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.

Using one square km as a unit area, the number of feeders required to serve this area

is,

n f=D /kVA f (1)

And the cost per feeder is given by

a+b×S /2 (2)

The substation cost to supply this unit area is:

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 Feeder KVA capacity = √3 *V*Icc

n f number of feeders required per km2

C s Cost of substation per km2

CT Total annual cost of substation & feeders per km2


c

S2+d×D

(3)

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

2+d×D (4)

Minimizing equation (4) with respect to S:

S=1.59[ kVAf cbD ]0.333

km (5)

and the corresponding optimum substation size would be

kVAS opt=S2D

(6)

(3.0)Calculation Sequence:

KVASopt=D⋅S

2

nS=area÷S2⇒

approximated to the higher integer

KVASact

=Total Load÷nS

Loading Percent=KVASopt÷Substation rating including the reserve

Get # of transformers needed & add reserve transformer. Try to make total # of

transformers to be even for easier protection.

Feeders /Substation=Substation rating including the reserve÷KVA f

And then approximated to the higher integer giving even no of feeders / substation


Current Density = KVASact÷ ( feeder /Substation×√3×11×csa of feeder )

(4.0)Data used in design:

(4.1)Draw:

All dimensions in Km

(4.2)load density:

ZoneArea (Km2)

Maximum Load (MVA)

D

(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


(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

(4.5)For UGC:

Insulation level 12/20 kVKind Multicore cables, with stranded Aluminum conductors XLPE

insulated, steel tape armoured and PVC sheathedCross-section 240 mm2

CCC 340 amperePrice 166 LE/M+12% sales tax+25% erectionDe-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

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

As we use Outdoor substation ⇒ C=23,000,000*0.3 LE

D=1.1275 MVA/ Km2

S=1.59[ kVAf cbD ]0.333

km

S=15.41306768Km

KVASopt=D⋅S2=267851.8937

KVA

nS=area÷S2=0.526176979

, approximated to the higher integer (

nS=1 )

KVASact=Total Load÷nS=140936 .8

KVA

Transformers used in substation: 66/11 KV, 5×35 + 1×35 MVA

Loading percent = 67.1128%

Feeders /Substation=Substation rating including the reserve÷KVA f=36 .74

Feeders /Substation=36

Approximated to the integer giving even number of

feeders/ substation & feeders/ transformer.

Substation capacity is bigger than feeders capacity with 4.23 MVA

Each transformer supplying 6 feeders.

Current density=0.8562A/mm2


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



(5.2.0)Residential area:


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 feeders per transformer in Residential area 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



As we use UGC⇒ b=227420LE/KM & (kVA )f=√3∗11∗340∗0 .706

As we use Indoor substation ⇒ C=30,000,000*0.3 LE

D=3.3474 MVA/ Km2

S=1.59[ kVAf cbD ]0.333

km

S=6 .012358226Km

KVASopt=D⋅S2=121003.3264

KVA

nS=area÷S2=3.1122

, we have two solutions

solution 1 solution 2nS= 3 4

KVASact=Total Load÷nS=

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 ⇒ 48 30.6 ⇒ 32

Each transformer supplying8 feeders 8 feeders

Current density= 0.572 A/mm2 0.6434 A/mm2


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

Solution 1(nS=3 ):


Solution 2(nS=4 ):



(5.3.0)City center:


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).









It is recommended that the number of feeders per transformer in City center area shouldn't exceed 8 feeders per transformer.







As we use GIS (Gas Insulated Substations) ⇒ C=30,000,000*0.3 LE

D=6.4823 MVA/ Km2

S=1.59[ kVAf cbD ]0.333

km

S=4 .823596935Km

KVASopt=D⋅S2=150824 .2406

KVA

nS=area÷S2=8.058593534

, approximated to integer (

nS=8 )


KVA



Feeders /Substation=Substation rating including the reserve÷KVA f=45.92

Feeders /Substation=48





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



(5.4.0)Light industrial:


Substations used in Light Industrial area are generally the indoor substations Under Ground Cables (UGC) are used in Light Industrial area for the primary










It is recommended that the number of feeders per transformer in Light Industrial area shouldn't exceed 8 feeders per transformer.







As we use Indoor substation ⇒ C=30,000,000*0.3 LE

D=6.872 MVA/ Km2

S=1.59[ kVAf cbD ]0.333

km

S=4 .730637488Km

KVASopt=D⋅S2=153788.0141

KVA

nS=area÷S2=1 .676

, approximated to the higher integer (

nS=2 )


KVA



Feeders /Substation=Substation rating including the reserve÷KVA f=45.92Feeders /Substation=48





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



(5.5.0)Heavy industrial:


Substations used in Heavy industrial area are generally the indoor substations. Under Ground Cables (UGC) are used in Heavy industrial area for the primary











It is recommended that the number of feeders per transformer in City center area shouldn't exceed 8 feeders per transformer.






As we use indoor substations⇒ C=30,000,000*0.3 LE

D=16.2175 MVA/ Km2

S=1.59[ kVAf cbD ]0.333

km

S=3.553198066Km


KVASopt=D⋅S2=204749.4485

KVA

nS=area÷S2=2.97

, approximated to integer (

nS=3 )


KVA

We have two solutions:

Solution 1 Solution 2Transformers 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 supplying11 feeders 8 feeders

Current density=0.6716 A/mm2 0.6926 A/mm2

comments1. Higher cost due to higher capacity.2.transformers of 50 MVA do not produced in Egypt.3. Lower loading percent.4. large # of feeders/ trans.

1. Lower cost.2. Transformers from Egypt.3. Higher loading percent.4. # of trans. Larger than 6 transformers.

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

CHAPTER 3 Substation Sitting

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