Selection Of Electrical Power Cables

Right selection of cables is not only very important for reliability of power supply and safety of devices as well as human beings present around them, it also prevents loss of assets and saves costly business hours. - K Sivakumar

Electrical cables are the nerves of any electrical network. Cables consist of a huge percentage of capital investment in any electrification project. And, they are the most vulnerable to failures too. Most of the cable failures could be attributed to improper selection. This article aims to address the issue of proper selection of electric power cables.

Selection Parameters

i. Voltage Rating: This is the rated voltage of the system, in which the cable is to be installed & used. It is also important to know the method of system earthing. The rated voltage of the cable is generally specified as a dual rating (e.g.) 6.6kV (UE)/11kV (E).

‘UE’ means that the cable can be used for the specified voltage in an unearthed or in a non-effectively earthed system. ‘E’ means that the cable can be used for the specified voltage in a solidly earthed system. Thus, a cable whose rated voltage is specified as 6.6kV (UE)/11kV (E) can be used either in a 6.6kV unearthed or in a 6.6kV non-effectively earthed system or in an 11kV solidly earthed system.

ii. Type of Conductor: The most generally used conductor in a cable is either Copper or Aluminium. As is known, for the same voltage rating, type, insulation, cross sectional area and method of installation, the continuous current rating, the short time current rating and the per unit length cost of a Copper cable is considerably higher than that of an Aluminium cable.

iii. Type of Insulation: Most of today’s cables are insulated either with PVC or with XLPE. Obviously, for the same conductor material, voltage rating, type, insulation, cross sectional area and method of installation, the continuous current rating, the short time current rating and the per unit length cost of an XLPE insulated cable is considerably higher than that of a PVC insulated cable.

iv. Type of Cables: Armoured or Unarmoured Unarmoured cables are used in indoor installations and on above ground installations, such as in cable trays, in pre-built concrete cable trenches, etc. , Armoured cables are mandatory for any underground cable installation.

The armour can be a wire or strip made of Galvanised Iron or Aluminium. In many cases, this armour is connected to the plant earthing system, preferably at one end only, generally, the sending end.

v. Continuous Current Rating: The continuous current rating of cables with Aluminium / Copper conductor are available in different cable manufacturer’s catalogues. But, it should be noted that the continuous current ratings are given in these catalogues for certain standard conditions of laying. In practice, it is not possible to get or to maintain these standard conditions. Thus, certain rating factors are applied to arrive at the practical continuous current rating.

vi. Rating Factors: The following are the general rating factors to be considered:

  • Rating factor for variation in ground temperature or in duct temperature
    • Rating factor for variation in ambient temperature
    • Rating factor for variation in thermal resistivity of soil
    • Group Rating Factor – Vertical Spacing
    • Group Rating Factor – Horizontal Spacing

    All these rating factors for various conditions are also available in the cable manufacturers’ catalogues.

vii. Voltage Drop: Cables consist of resistance & reactance. And, thus the current flowing though such an impedance will cause a voltage drop. This drop should not affect the loads connected by the cable.

Actual voltage drops in cables are given in V/km/A, in the cable manufacturer’s catalogues, for various types of cables. It is also given in Indian Standard IS 1255 (Code of practice for installation and maintenance of power cables up to and including 33 kV rating).

One should not only calculate the steady-state voltage drop, but also the acceleration state voltage drop during the starting of large loads.

And, it must be ensured that the steady state voltage drop at the load terminals is not more than 10% and the acceleration state voltage drop at the load terminals is not more than 15%.

viii) Short Circuit Current Withstand: Any cable’s short time current withstand capacity can be calculated using the following formula:
S = [(I √t) / K]
Where, I = Short Circuit Current, in Amperes, t = Duration of Short Circuit, in seconds, K = Adiabatic Constant (= 115 for PVC /Copper, = 143 for XLPE / Copper, = 76 for PVC / Aluminium and = 92 for XLPE / Aluminium)

Note: It is prudent to select the duration (t) judiciously, based on the fault clearing time of the isolating devices provided in the upstream of the cable. It must be ensured that the co-ordinated fault clearing time of the uppermost isolating device is considered for the purpose.

CASE STUDY: Now, let us select the power cable for the following case:

Motor Full Load Current: (200 x 0.746 x 1000) / (√3 x 415 x 0.8) = 260A

De-rating factors:
i. Ambient Temperature @ 45°C – 0.90
ii. Ground Temperature @ 35°C – 0.94
iii. Soil Resistivity @ 150°C cm/W – 1.00
iv. Depth of Laying @ 75cm – 1.00
v. Grouping factor for horizontal laying, 1 cable laid in isolation – 1.00
vi. Grouping Factor for vertical laying, 1 cable laid in isolation – 1.00
Net De-rating Factor = 0.90 x 0.94 x 1.00 x 1.00 x 1.00 x 1.00 = 0.846

Continuous current rating of the cable to be selected = 260/0.846 = 307A

We can select a 3c x 400 sq.mm AYFY Cable, whose continuous current rating is 335A. Secondly, we have to check the voltage drop of this cable. The estimated voltage drop is: 0.22V/km/A. Consider the cable length is about 120m.

During normal running, the voltage drop in the cable would be: 0.22 x 0.12 x 260 = 6.864V, which is just 1.65% of the nominal system voltage of 415V. As per IS 325, a motor should be capable of producing rated output even with a voltage drop of 6%. So, it is seen that with this cable, the steady state voltage drop is kept within limits.

Assuming that the starting current of the motor is 6 times its rated current, the voltage drop in the cable during motor starting would be: 0.22 x 0.12 x 6 x 260 = 41.18V, which is about 9.92% of 415V. During acceleration state, a voltage drop of up to 15% is permissible. Hence, with the selected cable, even the acceleration state voltage drop is kept within permissible limits.

Lastly, we have to check the short circuit current withstand capacity of the cable. For this, we have to calculate the magnitude of short circuit current at the terminals of the motor.

For this, the transformer impedance has to be converted to Ohmic value from % value.
Z = [5 x 10 x (0.433)2] / 1500 = 0.00625 ohms
The transformer resistance is 1.42 milliohms.
So, the reactance would be: {(0.00625)2 – (0.00142)2} = 0.00609 Ohms.
The cable resistance & reactance will be: 0.0778 Ohm/km & 0.0729 Ohm/km;
For 120m cable it will be 0.0093 ohm & 0.0087 ohm respectively.
The total resistance is: 0.0107 Ohm & the total reactance is: 0.0148 Ohm. The total impedance is: 0.0183 Ohm.
The fault current at the motor terminals would be: 13748A.
The short time withstand rating of cable is given by: t = (K2S2/I2).
K = 76 for PVC Alu.; S = 400 sq.mm; I = 13748A; t = 4.89 seconds.
This cable will withstand this fault current for a duration of 4.89 seconds, whereas the 315A fuse provided in the motor feeder would clear this fault in about 4 milliseconds.
Even if the fuse does not operate, the short circuit release provided in the 2000A ACB, which will be set for 5In (i.e.) 10000A, will trip in about 300 milliseconds.

Hence, the cable will withstand the short-circuit current too till cleared by the fuse or by the ACB release.

Hence, it is established that the 3c x 400 sq.mm AYFY Cable selected is suitable for the application with respect to the continuous current carrying capacity, steady state voltage drop, acceleration state voltage drop & short circuit current withstand.

Conclusion

The selected cable size is OK for the application.


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