Cable effect on Short Circuit Current

Hello, our discussion is to observe the effect of power cable on the short circuit current leaving the distribution transformer secondary side in case of a downstream fault in the power network.

Whenever the fault occurs in the system, electrical components connected to the system will start to contribute towards the fault point. Today we are looking at the effect of cable on the value of short circuit current.

To see the cable’s effect, first we need to identify some rated parameters of the cable which can be found in the cable’s technical datasheet. These parameters are the rated current carrying capacity of the cable and the rated cable impedance.

It is important to mentioned that these parameter changes with the cable size, therefore, first we need to select the size of the cable based on transformer’s secondary current. We are considering the 2000 kVA transformer with 2510.3 A secondary current. For our today’s calculation we are considering low voltage power cable of 185 mm2. We are considering that the fault occurred at the cable end connected to the switchgear and will see the effect of cable in our calculation for the short circuit current.

Now we start the calculation. We have selected the 4-core 185mm2 cable for our calculation. To calculate the short circuit current, first, we need to calculate the total short circuit impedance of the circuit during a short circuit condition.

Parameters to consider in the equation for calculating the total short circuit impedance are the Ztrafo which is the rated short circuit impedance of the transformer and its value is available from the transformer datasheet or nameplate data. Second parameter is the Zutility which is the ratio of the rated transformer kVA and the transformer’s short circuit kVA and its value is dependent on the short circuit current entering the transformer primary side. These two values we have already calculated during our last discussion,

(https://www.dhirubhai.net/pulse/short-circuit-contribution-transformer-farrukh-habib/)

and we will take the values from our last discussion which were 6% and 3.53%.

The third added value is the Zcable which can be calculated using the formula shown here.

In the formula Zc is the cable impedance and its value is from cable data sheet which is 0.127 omhs per km, this is the corresponding value at 50 Hz frequency and 90 degrees cable end temperature.

L is the length of cable, here we assume that the length of the cable between the transformer terminal and the other side of the cable connection at the LV switchboard is 50 meters. Since the cable impedance is in ohms per km therefore we need to convert the cable length into km as well.

S & Vsec are the transformer rated kVA and the secondary voltage and the values are 2000 kVA and 460 V.

N are the number of cables per phase, which is obtained from the ratio between the rated current of the transformer secondary and the rated current carrying capacity of the selected cable. In our case it found to be 7 cables per phase.

It is important to mention that to keep the calculation simple we are considering the cable installation in an open air and cable bunching effect is neglected.

After filling all the parameters in the equation the value of Z,cable is found to be 0.85%.

Once the total short circuit impedance is calculated, then the short circuit current at the cable end is simply the ratio of rated transformer secondary current and the total short circuit impedance.

In this case rated secondary current is 2510.3 A and the total short circuit impedance is 10.38%. The value of short circuit current is found to be 24.18 kA.

It is important to note that in our last discussion the value of short circuit current at transformer secondary terminal without taking the cable effect was 26.34 kA. Therefore, we can see that the cable helped in reducing the value of short circuit current to 24.18 kA.

We are discussing this second scenario to notice the effect of taking single core cable instead of four core cable of the same size 185 mm2 on the short circuit current.

First thing to notice is that the current carrying capacity of single core cable is more than the four core cable therefore the number of cable per phase are reduced to 6 cables. All other parameters of the equation will remain the same.

With single core cable the value of Zcable is increased to 1%, which helps in further reduction of short circuit value to 23.84 kA.

However, cable being an expensive material and due to considerable difference in price among different types of cables its choice is always a techno-economic decision.

For more details or to develop better understanding please visit below video!!!