Length of Transformer Secondary Cables and Associated protection

[Gerry123]

Hi,

I have a situation to decide whether to move 3MVA 11KV/415V Outdoor transformers about 10m away from a switchroom building inorder to prevent having to build fire barriers / blast walls as required by standards if the transformers are within 7.5 m of the switchroom building.

Situation 1: 3MVA 11KV/415V TX located 10m away from switchroom and protected upstream by IDMT Relay and down stream in switchroom Over Current and Earth Fault. No Blast Walls so less capital outlay

Situation 2: 3 MVA 11KV/415V TX located 1.5m away from switchroom and protected upstream by IDMT Relay and down stream in switchroom Over Current and Earth Fault. Blast Walls required

Question is what are the impacts regarding protection of the transformer secondary or transformer secondary cables if the length to the downstream short circuit protection increases. Is there increased risk to situation 1 transformer due to the increased distance. Are there other protection configs such differential protection which may be more suitable?

Any Info appreciated. Standards I'm referring is an australian standard AS 2067 which similar to IEC standards.

Thank

Gerry

 

[vandal06]

Is the "switchroom" connected to the 11 kV bushing or the 415 V bushing?

 

[vandal06]

And of course it's mentioned in the topic that we are discussing secondary cable lengths...I need more coffee.

If you have a short circuit program available this would be easy to check. When you lengthen the secondary conductors coming out of the transformer you add more impedance to the circuit, further reducing the available fault current at the switchboard. More than likely this will have very little effect on the clearing time for faults at the switchboard bus as the main circuit breaker is probably well in to the short time and instantaneous portion of the curves.

So my answer is "it depends". I don't think it will much of an effect, 10m is pretty short and depending on the utility source there is a lot of fault current available on the secondary of the transformer.

 

[OliverJanke]

Hi Gerry,

the only problem I can think of is, that you might get saturation on your CTs in Situataion 1. The longer the secondary cable, the higher the burden, the more saturation. But this also depends on the relay type, the CTs and the diameter of the cables.

You should find guidelines how to dimension CTs and secondary wiring in the relay manual or in other documents from the vendor. Check these formulas and see, if your system is well dimensioned with longer wires. I guess it should not be a problem, as 10 meters is not much, but you should always check it.

Differential protection is also sensitive to saturation, maybe even more.

Regards,
Oliver

 

[ScottyUK]

I wouldn't see the extra length as a particular problem, other than it would be desirable to have some form of unit protection scheme to protect the vulnerable connection between the LV terminals of the transformer and the LV switchboard incomer. Many numerical relays intended for transformer protection can provide a restricted earth fault relay for a relatively small expenditure, and you get the benefit of rapid fault clearance in the area where the fault level is highest. You might want to check how easy it would be to fit a neutral CT either at the switchboard or the transformer neutral bushing depending on where your neutral-earth link is fitted.

 

[healyx]

Hi Gerry,

So are you saying there is no protection from the transformer LV terminals to the main switchboard housed in the switchroom? If so, you might want to look at section 2.5 of AS3000, particularly 2.5.4.4. It can be done.

Going from 1.5m to 10m in cables length should have little effect. Some points to note:
(1) A reduced maximum fault level at the MSB with longer cable length. It won't be that much (A rough calculation suggests about 5%).
(2) A reduced minimum fault level at the MSB. Again this is unlikely to add much to downstream disconnect time requirements.
(3) If you are going to be moving 4000A to the MSB, you are likely to be using multiple large cables in parallel. At the 10 metre length vs 1.5 metre length, the cable influence on fault level/current imbalances will start to have significance. With large cables, reactance dominates resistance so you need make sure cable geometry is correct, this is even more important with parallel cables. I'd consider transposition too, but most would say that is overkill. I'd leave a safety factor.

 

[Gerry123]

Hi All,

Thank you for the information. Yes, there is no protection on the cable between the transformer LV Terminals and the switchroom. So I was concerned the extra cable length required additional protective measures. I have now read AS3000 section 2.5.4.4. and the requirement for protection can be omitted if improved mechanical protection can be provided and not near flammable material.

Yes the MSB is 4000Amps so will make consideration for large cables in parallel.

thanks

Gerry

 

[AusLee]

Hi Gerry,

I think AS 3000 says you must have protection at the origin if the cable length exceeds 3 meters.