Short Circuit Capacity with a Trailing Cable

Dear

First you to have to get the shortcircuit level of your substation ( upstream side ) then only is possible.

Why can't you assume a infite bus? Its being supplied by a utility with a very high fault level.

Sir,

These types of plants do pose problems with maintenance and equipment reliability. Fortunately for you, the long distance from the transformer to the OCB and trailing cable limit your available short-circuit fault current to modest levels.

Look at the following on-line training guide and see if you come up with the numbers I have calculated: www.bussmann.com/library/docs/EPR_Booklet.pdf. They are a source which I trust. The best approach is to assume an infinite primary, as you have responded to another person's comment. Although you leave many details out of the question, they are not critical to giving an approximate answer.

Step 1. Transformer secondary current is approx. 650A

Step 2. The short circuit amps multiplier is 10.

Step 3. The available short circuit amps is approx. 6500A

Step 4. The adjustment factor for the 12km overhead line is about 22. This is based on an assumption of a "C" factor of 18000. Are you sure of this distance? It seems quite long to me. If it is 1.2km (more reasonable), then the adjustment factor is 2.2, the multiplier in step 5 is 0.34.

Step 5. The multiplier for short circuit amps at the OCB is about 0.05

Step 6. The available short circuit amps at the OCB is no larger than the full load current. If your line is 1.2km as I suggested in step 4, then the I_SCA at the OCB is approximately 2000.

The 350m trailing cable to the switchgear does not appreciably reduce these numbers.

Therefore, I believe that the bus bar bracing is dependent on the overhead transmission line distance from the transformer to the OCB. If you want to be safe, I would suggest you assume that there will be a future relocation of the plant so this distance is as small as 1km. Therefore, I would recommend that you specify for a minimum of 2,000A, and would be safer with a specification of 5,000A. I doubt that these specifications will be any problem. (I don't have any experience with the standard SCA bracing levels for medium-voltage switchgear, but I suspect that it is at least 5,000A.)

Study the reference I noted above. It will make you much better informed and capable of keeping your plant safe. Please note that in the above calculations I have ignored any contributions from motors in the plant. The reference I have cited gives the corrections for this also.

--John M.

Hi John M -Thank you so much for your resoponse. I'll show you my workings as well and I think the numbers are similar to what you mentioned. You are right,

Step 1: Transformer secondary current = 650 A approx

Step 2: Transformer Impedance approx 10% = 6500 Amps short circuit capacity of transformers

Step 3: Yes I messed up my line distance. Its actually 3 km (minimum). We use Saturn cables in Aus with impedance of 0.135 ohms/km = 0.54 ohms

Step 4: Trailing cable impedance at 350 m = 0.0321 ohms (0.107 ohms /km)

Per uni: Using a base of 11kV and 12.5 MVA: I have the following:

• Base current = 650 Amps
• Base impedance = 16.77 ohms:

Therfore transmission line impedance = 0.54/16.77 = 0.0322 p.u and Trailing cable impedance = 0.0321/16.77 = 0.001914 p.u

Net impedance in p.u at the 11kV bus = p.u of tx + p.u Overhead + p.u trailing = 0.13114:

Therefore, FAult current in p.u at 11kV bus = 1/0.13144 = 7.456 p.u

So the fault current rating of bus bar = 7.4646 * 650 A = 4851.99 Amps = 5kA approx

This is similar to what you have. Do you agree?

Thanks mate again> Much appreciated. NO need to consider Motors connected to this bus as there are five 11kv/415 transformers at the end of it. Therefore during a fault this won't back feed to it so can be ignored. Hope this makes sense.

Sir,

Your response was quick! You might want to reconsider the possible contribution of the secondary transformers. The available fault current is the sum of the contributions from the line side as well as the load side. When you have a fault, the stored energy in the magnetic field of the transformer windings gets released very quickly. Your assumption of minimal contribution from the load side transformers could be valid, but I suggest checking this.

--JMM

Hi,

yes I have the forum up for answers as we are currently working. Very useful website this. Are you able to help here with what you said. Hanging off the 11kV bus is 5 Identical 2MVA 11kV/415V transformers with impedance of 6.8% each. Do I work out the parallel impedance in p.u and use a thevin equivalent to get the fault? If I do this, doesn't this raise the fault level signficiantly?

Are you able to work out some numbers? Sorry to be a pain? I want to make sure I'm doing the right thing here.

Yes, I believe you do. You have a better engineering background than I do, so I would trust your approach. Don't forget to reduce the contribution from each transformer by any reductions you can get from the cables between the swichgear and the individual transformers. Also, remember that these calculations use assumptions designed to give the highest likely numbers. Certainly the results would lead to a safe installation.

--JMM