SIEMENS LTD

MAD-EFS/ECHT CABLE SIZING CALCULATION Client :Subburaj Papers

Project : 110KV Switchyard

1.0 OBJECTIVE

To size 11kV HT Power cables.

2.0 REFERENCES

3.0 DESIGN CRITERIA

3.1 The cross-section of cable selected shall fulfill the following criteria:

i) To be able to withstand the short-circuit fault current in the circuit for the duration of its circuit interrupting device.

ii)

iii)

4.0 DESIGN BASIS:

a) The maximum symmetrical short circuit fault level at 11kV bus is as follows:

Max. Fault MVA at 11kV Bus = 500 MVA

Max. Fault kA = 26.2 kA

b) Derating factor for continuous current rating:

For current carrying capacity, derating factor due to site conditions is arrived at as follows:

i) For variation in ambient temp. = 0.89

(Site / ambient temp. 50°C)

ii) Derating factor for Multicore cables laid in open = 0.93

Racks in air 3 cables, (max) in a Tray & say in 2 to 3 racks

with spacing equal to cable diameter.

iii) Net derating Factor = 0.89 x 0.93 = 0.8277

5.0 CALCULATIONS:

5.1 11kV switchgear (110kV Switchyard) Incoming cable from 12.5/16MVA Transformer

a) Short-time current withstand capability point of view

i. Duration of short-circuit to be considered = 1.0 sec

ii. Min.cross section required for 11kV(UE) to withstand = Ik √t

26.2 kA for 1.0 sec. k

Where K (constant) for Al. XLPE Cable = 0.094

= 26.2 x √ 1.0

0.094

= 279.19

b) Rated current carrying (continuously) capability:

i) = 839.81 A

ii) Current carrying capacity of 300sq.mm (Al.) cable = 450 A

iii) = 372.47 A

(450 x 0.8277)

iv) No. of runs required = 839.81 = 2.255 run

372

v) Selected Cable Size is = 3 # 3C x 300 sq.mm (Al.) XLPE

Incoming cable to 11kV Switchgear (110kV Switchyard) is sized to withstand 26.2kA for 1.0sec.

HT CABLE SIZING CALCULATION

Current ratings,derating factors are taken from "NICCO" Cable Catalogue for 11kV / 11kV (UE) XLPE insulated, armoured, Power

cables.

To be able to carry the full load current after considering appropriate derating factor for ambient temperature variation, method of

laying (in air and in trays in our case).

Voltage drop limited to 1% while carrying rated current in case of Power feeders /Transformer feeders.

Therefore, minimum cross section of the cable is selected as 3C x 300 sq.mm XLPE (Al.) which can withstand 26.2kA for 1.158

sec.

Therefore, the cross- section of the cable selected as 3#3C x 300 sq.mm (Al.) XLPE is adequate both from continous current

carrying capability point of view.

Full load current of 12.5 / 16 MVA Power transformer

Derated current carrying capacity of 300sq.mm (Al.)

Page 2 of 4 Drg No:(4)G71400-V4013-D015

Date:31.7.07

SIEMENS LTD

MAD-EFS/ECHT CABLE SIZING CALCULATION Client :Subburaj Papers

Project : 110KV Switchyard

HT CABLE SIZING CALCULATION

C)

Typical Calculation :

i) Cable size (selected from above calculation) = 3Cx300 sq.mm (Al.)

ii) Cable length (estimated max.) (L) = 50 m

iii No. of runs (n) = 3

iv) Cable resistance /Km ( R ) per run = 0.13 ohm/km

v) Cable reactance /Km ( X ) per run = 0.094 ohm/km

vi) Cos φ = 0.8

vii) Sin φ = √1−cos2φ = 0.6

viii) Full load current (I) = 839.81 A

ix) Percentage voltage drop (%Vd) = I x L x (Rcosφ + Xsinφ) x 100

= 0.035 %

5.2 11kV Switchgear (110kV Switchyard) outgoing cable to 11kV Switchgear-1(TG)

a) Short-time current withstand capability point of view

i. Duration of short-circuit to be considered = 1.0 sec

ii. Min.cross section required for 11kV(UE) to withstand = Ik √t

26.2 kA for 1.0 sec. k

Where K (constant) for Al. XLPE Cable = 0.094

= 26.2 x √ 1.0

0.094

= 279.19

b) Rated current carrying (continuously) capability:

i) = 839.81 A

ii) Current carrying capacity of 300sq.mm (Al.) cable = 450 A

iii) = 372.47 A

450 x 0.8277

iv) No. of runs required = 839.81 = 2.255 run

372

v) Selected Cable Size is = 3 # 3C x 300 sq.mm (Al.) XLPE

C)

Typical Calculation :

i) Cable size (selected from above calculation) = 3Cx300 sq.mm (Al.)

ii) Cable length (estimated max.) (L) = 600 m

iii No. of runs (n) = 3

iv) Cable resistance /Km ( R ) per run = 0.13 ohm/km

v) Cable reactance /Km ( X ) per run = 0.094 ohm/km

vi) Cos φ = 0.8

vii) Sin φ = √1−cos2φ = 0.6

viii) Full load current (I) = 839.81 A

ix) Percentage voltage drop (%Vd) = I x L x (Rcosφ + Xsinφ) x 100

= 0.424 %

n x 11 / √ 3 x 1000 x 1000

Derated current carrying capacity of 300sq.mm (Al.)

Therefore, the cross- section of the cable selected as 3#3C x 300 sq.mm (Al.) XLPE is adequate from continous current carrying

capability point of view.

Steady state voltage drop:

n x 11 / √ 3 x 1000 x 1000

Thus it is seen, the voltage drop in each phase is within limitation of 1% and hence the selected cable size 3#3C X 300 sq.mm(Al.)

XLPE is adequate.

Outgoing cable from 11kV Switchgear (110kV Switchyard) to 11kV Switchgear-1 (TG) is sized to withstand 26.2kA for 1.0sec.

Therefore, minimum cross section of the cable is selected as 3C x 300 sq.mm XLPE (Al.) which can withstand 26.2kA for 1.158

sec.

Full load current of Transformer 12.5/16MVA (110kV SWYD)

Thus it is seen, the voltage drop in each phase is within limitation of 1% and hence the selected cable size 3#3C X 300 sq.mm(Al.)

XLPE is adequate.

Steady state voltage drop:

Page 3 of 4 Drg No:(4)G71400-V4013-D015

Date:31.7.07

SIEMENS LTD

MAD-EFS/ECHT CABLE SIZING CALCULATION Client :Subburaj Papers

Project : 110KV Switchyard

HT CABLE SIZING CALCULATION

5.3 Cable connecting 12.5/16MVA Power transformer Neutral & Neutral Grouding Cubicle (NGC)

i) Duration of SLG fault current to be considered = 10 sec

ii) Min.cross section required for 11kV(UE) to withstand = Ik √t

0.1 kA for 10 sec. k

Where K (constant) for Al. XLPE Cable = 0.094

= 0.1 x √ 10

0.094

= 3.364

Cable to NGC is sized to withstand 100A for 10sec.

Even though 1C x 25sq.mm will meet the requirement, minimum cable size selected for NGC cable is 1#1C x 185 sq.mm (using 1

of 3 cores of 3Cx185 sqmm being used for other cases).

Page 4 of 4 Drg No:(4)G71400-V4013-D015

Date:31.7.07