Short Time rating of the bus bar 2

[sramesh]

I am preparing the specification for LV switch board with 2 sections connected by a Bus coupler, rating of 415 V , 4000A fed from 2.5 MVA 6% Transformer(2 Nos, one for each section). normal mode of operation is 2 Incomers ON and Buscoupler OFF. There is an option for Synchronising both the section and closing Buscoupler Breaker to have the facility of Isolating one of the Transformer for maintenance with out interruption. I would like to know how much should be the BUS BAR short time Current rating ie Calculating the short time current rating considering only one Transformer source or two Transformer Source ( Since 2 Transformers will be feeding to the BUS during SYNCRO mode operation , ofcourse only for a short duration). Kindly give your valuable opinion that will be helpful for me.

 

[DrDrreeeaaa]

Certainly seems like a waste. You would be needing an approx 100kA switchboard just to facilitate a few seconds of changeover - but if a fault occured during a switching you could get the full fault current.

I think you should rethink the way you implement the requirement for always-on TF maintenance. Perhaps an automatic transfer switch controlling the feed from the two transformers.

Theoretically my interpretation of the bus coupler in this kind of installation is to allow either transformer to power the busbar.

 

[sramesh]

Thanks for your reply, The Transfer operation will take place very fast, once the buscoupler is closed , (with the auxillary contact reference)the selected Transformer breaker will be OFF. Typically less than 300msec all the three breakers will be ON. For this short duration, is it necessary to rate the bus bar for 100kA? Any guidence from IEC or other standards for this ?

 

[alehman]

See the discussion of a similar question at thread238-177926
Bus bracing is relatively inexpensive.

 

[sramesh]

Thank you very much for the reply and the link of previous similar query. Can you please explain further on your suggestion of Bus Bracing...

 

[burnt2x]

Bus bracing - installation of braces(with insulation) on bus bars to improve mechanical strength.

 

[alehman]

The mechanical supports required to provide higher withstand ratings are usually relatively inexpensive. And probably worth the cost if there is any question.

 

[NCTHAI]

Here the decision is based on balance between economic and technical consideration.

What you can do is to find out the fault current with one transformer and select the next higher rating available from OEM.

For MBB (Make before break) operation What control you need to have is once the bus coupler is closed, selected incopmer will trip with time delay..(T1 Sec). This should be part of your synch scheme.

What protection you need to add is if all three breakers are closed, it will trip bus coupler or last breaker which was closed. This must be with time delay (T2 Sec). This time delay should be greater than earlier case time delay. (T2>T1) ..

T1 + CB opening time + Margin = T2
CB Closing Time + Margin = T1

If under voltage timers are installed for incomers (Time Delay T3) , they should be set between T1 and T2.

T1<T3<T2.

If undervoltage timers are installed bus used for trip function (Time Delay T4), that time delay should be more than T2.

T1<T3<T2<T4.

Hope this helps.

NC

 

[sramesh]

Thank you all for the inputs. From the discussion, I understand that there is no International standard reference for this and the risk has to be managed by the Asset owner by taking possible additional precautions.