CT sizing for transformer overcurrent application 1

[ballenden]

Good day,

I'm in an IEC world.

I have a 11kV/0.4kV, 200kVA xfr. This means 10.5A high side nominal current, and 282A low side nominal current.

How do I size the 11kV CT for overcurrent protection? The fault levels on 11kV is 25kA.

b.

 

[ballenden]

Hi,

I meant to say, please provide me with technical papers or bulletins to find this information.

b.

 

[EPEngineer]

Are you sure you are going to install CTs for O/C protection on a 200kVA transformer on 11kV side? Genarally transformers of this capacity are protected by H.V (or M.V) Fuses.
However in IEC world IEC-185 and IEC-44-6 are the standards for CTs.
Regards

 

[ballenden]

EPEngineer,

The xfr is indoor type. This was my thought as well, the use of a microprocessor relay to protect a small xfr like this is not justified.

This I cannot change becos it was requested, priced and accepted before I had to have a look at it.

Thanx,
b.

 

[davidbeach]

You can protect an indoor transformer with fuses. (At least you can in the ANSI world.)

 

[EPEngineer]

Does this surprise only me? I would be more surprised if in IEC world someone would ask you to do so ....

 

[dpc]

What is the relay going to trip?

 

[ballenden]

dpc,

The relay will trip an 11kV circuit breaker that is provided for as well.

EPEngineer,
It surprised me too...

b.

 

[KJvR]

It is not that suprising in heavy industry. Your standard installation is MV circuit breakers feeding transformer sizes that varies between 1MVA and 4MVA. It is simpler to add an additional MV panel with C/B feeding the odd small transformer than install an odd Fuse Isolator switch - most standard MV panels just don't suite a fuse isolator with a preferable visual isolator blades. Your lock-out procedure will also have to be different because the isolator is fixed maounted and not rackable as the circuit breakers. I have seen this type of practise for transformers as small as 315kVA.

Using fuses as standard practise on a plant creates discrimination problems for the larger (3-4MVA) transformers especially in high impedance earthed systems.

 

[ballenden]

Thanks for this invaluable input!

I'm new at Distribution type work, more familiar with 220kV and above. I just need clarification on busbar configurations for 11kV indoor networks:

i) What are typical busbar layouts? Or reference document (IEC).
ii) If a double bus layout; would incomers have isolators to switch them between the two buses? Would you then need two bus-sections?


Thanks.
b.

 

[KJvR]

MV incomers and bus ties are these days all C/B. I have only seen one installation (radially fed - 1975 design) using isolators as incomers. This design used double busbar with a isolator as tie between two busbars and was mechanically interlocked with the two incomers to prevent parr the main transformers. The busbar configuration of the system will depends on the required security (type of process etc.) and load configuration. No need for a double bus without (i) double transformers feeding double LV bus with LV tie or (ii) deviding the load between the two buses that if you do loose one bus you will not stop the complete process. It seems as clients prefers ring fed networks these days and therefore single busbars. The main incoming bus is still a two or three busbar scheme depending on the number of incomers from the utility. The use of Is Limiters in the tie panel is also becoming more common to enable you to operate the main incoming buses with a closed tie.

Regarding your orriginal question on CTs - most common problem in protecting a small transformer using CTs is the large physical size of a low ratio CT - you should not be surprised when your design came back from the switchboard manufacturer because the CT can not be fitted in the switchboard. You should accept heavy satration - just make sure the instantanuous setting is well below saturation. With modern low burden relays the saturation is not as bad as in the past.