Objective of LT Cable Sizing
The objective is to select an XLPE Armoured Aluminium AC Cable that can safely carry inverter output current to the Inverter Duty Transformer (IDT) while satisfying: Thermal limits Voltage drop limits Power loss limits Short circuit capabilityThe sample calculation in the PDF considers
1C × 400 mm² Aluminium XLPE Armoured Cable between the inverter and IDT.
REFERENCES
Current ratings,derating factors are taken for 1.9/3.3kV (E) LT XLPE insulated, armoured, aluminium, Power cables". (Provided)
DESIGN BASIS:
7 Circuit with 3 No of tray formation/9 Circuit with 3 No of tray formation
Derating factor for continuous current rating:
For variation in Air Temperature (Assumed 50 Deg. C.)-0.89 Refer Annexure 1 (IS-3961 Part-7)
Cables laid in Ladder type Cable trays with trefoil formation-0.9 Refer Annexure 1 (IS-3961 Part-7)
Net derating Factor = 0.89 x 0.9=0.80
Selection of cable based on Rated current carrying (continuously) capability:
Capacity of Each Trafo LV/ Each Inverter (Pr)=3300KVA
Full load current-2887A
I=P/root3×V×PF
Where:
P = Inverter Power (VA)
V = AC Voltage (V)
PF = Power Factor
Capacity of Each Trafo LV/ Each Inverter (Pr)=4400KVA
Full load current-3849A
I=P/root3×V×PF
Where:
P = Inverter Power (VA)
V = AC Voltage (V)
PF = Power Factor
Calculate Derated Current Carrying CapacityFormula
Id=Ic×DF
Where
Id = Derated Current
Ic = Cable Current Rating
DF = Net Derating Factor
Example
Cable Current Rating 593 A
Derating Factor0.80
Calculation593×0.80=474.4A
Therefore,Each 400 mm² cable can safely carry approximately 475 A.
Calculate Number of Parallel Cable RunsFormulaN=FLCI/I
Example 1
N=2887/475
N=6.08
Always round upward. Selected 7 Runs
Example 2
N=3849/475
N=8.10
Always round upward. Selected 9 Runs
Voltage Drop Calculation for LT AC Cable in Solar Power Plant
Formula
Vd = (√3 × I × L × (R cosφ + X sinφ)) / (n × 1000)
Where· Vd = Voltage Drop (V)· I = Full Load Current (A)· L = Cable Length (m)· R = AC Resistance (Ω/km)· X = Cable Reactance (Ω/km)· φ = Power Factor Angle· n = Number of Parallel Cable RunsExample from LT Cable SizingGiven:
Current (I) = 2887 A
Cable Length (L) = 25 m
Resistance (R) = 0.0778 Ω/km
Reactance (X) = 0.088 Ω/km
Power Factor = 1.0
cosφ = 1, sinφ = 0
Parallel Runs (n) = 7
Step 1: Simplify FormulaSince cosφ = 1 and sinφ = 0:
Vd = (√3 × I × L × R) / (n × 1000)
Step 2: Substitute Values Vd = (1.732 × 2887 × 25 × 0.0778) / (7 × 1000)
Step 3: ResultVoltage Drop = 1.21 V Voltage Drop (%) = (1.21 / 660) × 100 = 0.176 %
Design RecommendationFor inverter to IDT cables, voltage drop is generally kept below 2%. The calculated value of 0.176% is well within acceptable limits, indicating the selected cable is suitable.
