HV Cable sizing

Written By:TDM/AMK

DESIGN MANUAL No. DCB08-05

Approved By:AK

ELECTRICAL DEPARTMENTHIGH VOLTAGE CABLE SIZING

Date: MAY 1998

Page: 1 of 11 Rev. : 0

1.0 INTRODUCTION :

High Voltage Cables are sized based on the short-circuit withstand capacity for the specified fault clearing time, continuous current carrying capacity suitably derated depending on the conditions of installation and permissible voltage drop in the cable.

Short circuit withstand capacity for power feeders shall be equal to the fault level at switchgear bus. For motor feeders, short circuit withstand capacity shall be that of the fault level at the terminals of the motor.



Date: MAY 1998


Fault withstand time can be taken as shown below :

Cables protected by circuit breakers shall be rated for short circuit duty. Cables protected by HRC Fuses, may not be sized for short circuit duty. However a minimum time ‘t’ of 0.02 sec. is considered as safe & indicated as a minimum requirement.



Date: MAY 1998


2.0 STANDARD / REFERENCES :

1. Cable corporation of India Ltd. Catalogue for Tropothen-X cables. (March 1994)

2. Beeman’s Handbook.

3.0 SAMPLE CALCULATION :

INPUT

System fault level = 26.2 kA at 6.6 kVFault level at motor terminal = 10 kA at 6.6 kVFault clearing time = 0.5 sec for power feeder and 0.2 sec for motor

& capacitor feeder.

For motor feeders Maximum voltage drop :- 5% running, 15% starting condition.Cable catalog for resistance and reactances.

Multicore lables are laid on cables tray in air at ambient temperature of 40°C. There are 3 tier cable trays with 6 cables having one diameter spacing between each other.

FORMULA USED

S = , where

S is the minimum cross section area of the conductor in mm². is the fault current in Amperes (rms)t is the fault withstand time in secs.K is the constant. Amp/mm2

For XLPE Cables

KCopper = 143KAluminimum = 94

For PVC Cables

KCopper = 112KAluminimum = 75



Date: MAY 1998


Voltage drop = (R Cos + X Sin)

where,

= is the load current in amperes = is the length of cable in kilometersR = is the resistance of cable in /kmX = is the reactance of cable in /kmCos = is the power factor of the running load.

The resistance & reactor values are at the Maximum conductor temperature of XLPE cable 90°C and PVC cables 70°C.

CABLE DERATION FACTOR :

Cables ampacity has to be derated with respect to conditions of installation. Rating factors to be considered are :

a) Variation in ambient air temperature for cables laid in air / ground temperature for cables laid underground.

b) Cable laying arrangement.

c) Depth of laying for cables laid direct in ground.

d) Variation in thermal resistivity of soil.

Derating factor = Product of Rating factors.Refer Appendix-I for Rating factor tables of XLPE cable.

NOTE :

A typical calculation of XLPE cable is made. In case of PVC cables the vendors catalogues for various derating factors shall be used.



Date: MAY 1998


CALCULATION

Minimum Cross section area ofConductor for XLPE Cable withAluminium conductor for t = 0.5 sec

= = 197.08 mm2

Minimum Cross section area ofConductor for XLPE Cable withAluminium conductor for t = 0.2 sec

= = 47.5 mm2

Standard conductor size to meet the minimum cross section area is 240 sq. mm. having an ampacity of 390A in air for t = 0.5 sec and 26.2 kA at 6.6 kV and 50 sq. mm having an ampacity of 145A in air for t = 0.2 sec and 10 kA at 6.6 kV.

Derating factor = Product of rating factor= Rating factor for variation in ambient temperature x

Rating factor for laying conditions= 1 x 0.89 = 0.89

Therefore, derated ampacity of cable is 390A x 0.89 = 347.1 Amperes for 3 x 240 sq. mm and 145A x 0.89 = 129.05 Amperes for 3 x 50 sq. mm.

CASE - 1 :

200 A power feeder with cable run of 200 meters and power factor 0.9. 1 No. 3 x 240 sq. mm. is sufficient considering minimum cross section area and ampacity.

Cable Resistance R = 0.161 / km Cable Reactance X = 0.093 / km.Voltage drop = X 200 x 0.2 (0.161 x 0.9 + 0.093 x 0.435)

= 12.84 Volts

% Voltage drop = 0.19% which is within limits

1 No. 3 X 240 sq. mm. cable considered is safe.



Date: MAY 1998


CASE - 2 :

430 kW motor with a power factor of 0.9 at full load and 0.3 at starting. Cable run is for 200 meters.Full load current = Amperes

1 No. 3 x 50 sq. mm is sufficient considering minimum cross section area and ampacity.

Locked rotor current = 6 times full load current= 6 x 41.7 = 250.2 Amperes

Cable Resistance R = 0.822 / kmCable Reactance X = 0.117 / km

Starting voltage drop = x 250.2 x 0.2 [0.822 x 0.3 + 0.117 x 0.95]= 31.0 V

% Voltage drop starting = x 100 = 0.46% which is within limits

Full load voltage drop = x 41.7 x 0.2 [0.822 x 0.9 + 0.117 x 0.435]= 11.42 V

% Voltage drop full load = x 100 = 0.17% which is within limits

1 No. 3 X 50 sq. mm. Cable considered is safe.

CASE - 3 :

300 kVAR capacitor feeder with cable run of 50 meters and 0.9 power factor

Current = = 26.24 Amperes

Considering the continuous capacity of the capacitors at 135%, the cable shall be sized to carry charging in rush current of 1.35 times rated capacitor current = 1.35 x 26.24A = 35.4 Amperes.



Date: MAY 1998


1 No. 3 x 240 sq. mm is sufficient considering minimum cross section area and ampacity.

Voltage drop = x 35.4 x 0.05 [0.161 x 0.9 + 0.093 x 0.435]= 0.56 V

% Voltage drop starting = x 100 = 0.01% which is within limits

1 No. 3 X 240 sq. mm. Cable considered is adequate.

CONCLUSION :

3 X 240 sq. mm. XLPE Aluminium cable has been selected for 200 A power feeder and 300 KVAR capacitor feeder. 3 x 50 sq. mm XLPE Aluminum cable has been selected for 430 kw motor feeder.



Date: MAY 1998


APPENDIX - I

Rating factors for variation in ambient air temperature

Air Temperautre - °C 20 25 30 35 40 45 50 55 60Rating Factors

Conductor Temp. 90°C

1.18 1.14 1.10 1.05 1.00 0.95 0.89 0.84 0.78

Rating factors for variation in ground temperature :

Ground Temperautre - °C 15 20 25 30 35 40 45 50Rating Factors

Conductor Temp. 90°C

1.12 1.08 1.04 1.00 0.96 0.91 0.87 0.82

Rating factors for multicore cables laid on open racks in air :

No of No of cables per rack No of No of cables per rackracks 1 2 3 6 9 racks 1 2 3 6 9

1 1.00 0.98 0.96 0.93 0.92 1 1.00 0.84 0.80 0.75 0.732 1.00 0.95 0.93 0.90 0.89 2 1.00 0.80 0.76 0.71 0.693 1.00 0.94 0.92 0.89 0.88 3 1.00 0.78 0.74 0.70 0.686 1.00 0.93 0.90 0.87 0.86 6 1.00 0.76 0.72 0.68 0.66



Date: MAY 1998


Rating factors for single core cable in trefoil circuits laid on open racks in air

No of No of circuits per rackracks 1 2 3

1 1.00 0.98 0.962 1.00 0.95 0.933 1.00 0.94 0.926 1.00 0.93 0.90

Rating factors for grouping of multicore cables laid direct in ground, in horizontal formation :

SPACING No. of cables in group2 3 4 6 8 10

Cables touching 0.79 0.69 0.62 0.54 0.50 0.4615 cm 0.82 0.72 0.66 0.59 0.54 0.5130 cm 0.86 0.76 0.72 0.65 0.62 0.59



Date: MAY 1998

Page: 10 of 11 Rev. : 0

Rating factors for grouping of multicore cables laid direct in ground in tier formation :

Formation of cablesSpacing

Cables touching 0.60 0.51 0.4315 cm 0.64 0.55 0.4630 cm 0.69 0.60 0.50

Rating factors for grouping of single core cable laid in trefoil circuits laid direct in ground in horizontal formation :

Spacing No. of circuits in group2 3 4 6 8 10

Cables touching 0.78 0.68 0.61 0.53 0.48 0.4515 cm 0.81 0.71 0.65 0.57 0.53 0.5030 cm 0.85 0.76 0.71 0.64 0.60 0.58

Rating factors for depth of laying for Cables laid direct in the ground :

Depth of laying (cm)

90 105 120 150 180 or more

6.6kV & 11kV 1.00 0.99 0.98 0.96 0.9522kV & 33kV -- 1.00 0.99 0.97 0.96

Rating factors for variation in thermal resistivity of soil :

Rating factors for value of Thermal Resistivity of Soil in °C cm/Watt100 120 150 200 250 3001.20 1.11 1.00 0.89 0.80 0.73



Date: MAY 1998

Page: 11 of 11 Rev. : 0

Arrangement for which reduction in current rating is not required :

For cables sizes upto 70 sq. mm. conductor section, reduction in rating for grouping for cable in air need not be applied, provided, the circuits are run with a clearance of at least 75 mm and installed in a horizontal plane, when number of circuits are more than three. For cables above 70 sq. mm. conductor section, no reduction in rating be applied provided the circuits are run with a clearance of at least 150 mm, and installed in a horizontal plane as and when the number of circuits exceed four.

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