transformer secondary protection

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I have a new power transformer of 1 MVA (13.8 KV/400V) rating in Dyn11 configuration with neutral solidly grounded.All protections including ground fault protection is provided on the primary side. however presently the complete distribution set up at secondary side does not include ground fault protection at downstream. presently the system is being fed thru generators without the ground fault protection at the generator output (02 gen of 350 KVA each).
Now the secondary of the new transformer is protected thru overload and long time / short time overcurrent protection thru MCCB. introduction of ground fault protection can create tripping problems of the whole area due to any problem at downstream. what is the technical opinion for not using ground fault protection at transformer secondary???
An urgent response is expected please
 

rcwilson

Senior Member
Location
Redmond, WA
Ground fault protection on the 400 V system would be required if this was in the US or Canada. It is strongly recommended.

You are correct that putting GF on the transformer 400V MCCB will cause it to trip for downstream faults. However, the settings could be selected by an engineer to allow the smaller downstream circuit breakers (15-50 Amp) time to trip while still protecting against major ground faults that could burn down the 400 V switchgear and switchboards.

The alternative is to put ground fault on all major feeders downstream.
 

Cold Fusion

Senior Member
Location
way north
---Now the secondary of the new transformer is protected thru overload and long time / short time overcurrent protection thru MCCB. ---

what is the technical opinion for not using ground fault protection at transformer secondary??? ---
There are some regulatory issues. Under the NEC, services (2008 - 230.95) or building disconnects (240.13) over 1000A has to have GFP. There are exceptions. However, you are likely not under the NEC - so first, I recommend you check the codes and standards covering your work.

As for not using GFP:
As you said, the system is solidly grounded, 400V, 1MVA. For a nominal transformer impedance of 5.5%, available short circuit current is around 25kA. In my experience, for solidly grounded systems, most faults start phase to ground. And it takes a while for the current to build up to where the main CB will trip - it may not trip until the fault goes phase to phase. Arcing ground faults really cause a lot of damage. Coordinated GFP will limit/reduce the damage.

If there are safety/process issues requiring an orderly shutdown, then the design criteria changes. You may not want GFP.

For industrial applications, solidly grounded 400V systems are a design headache compared with impedance grounded systems. But I suspect it is way too late to make any changes of that nature.

---. introduction of ground fault protection can create tripping problems of the whole area due to any problem at downstream. ---
That's true, it can. However, one can design the coordination to prevent most of this. All of the GFP systems I have seen have programmable delays. It is a matter of coordinating the trips so that downstream CBs trip first. Coordination is not that difficult. The main CB GF LT could be in the 1000A range, with the delay being 1 sec at 3000A. It should be pretty easy to get down stream feeders to trip sooner. Small faults won't trip the main. Large faults will - and should.

Get out the coordination study and the arc-fault study. Start there.

cf
 
Firstly , thanks to all for responding.

Defining further, i would like to add that the output MCCB is rated 1600 A against the FLA of transformer i.e 1440 A. Area current requirement is around 650 A. Therefore, the main MCCB of 1600 A can be adjust for a setting of 800 A. please note that the trafo manufacturer has confirmed the withstand of neutral for the same fault values as for phases.
With this setting do you think that the system could be protected for ground faults?
 
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