Complex Issue..Need Expert Help

[alanElec]

The transformer size is 1600 KVA.With rated amps 2100 Amps...The breaker is 2500A 4P,drawout motorized acb with micrologic trip unit with O/L,S/C & E/F protection-100 KA/1sec,make:schneider elctric,Model:NW25 H2+Micrologic 6.0 A..The locked rotor starting time of pump is 10 sec as per motor data sheet..
I have the following loads connected to Panel:
1.Two chillers of max load current 1003 A.Their starting current is 969 as the 4 compressors in it have inbuilt star/delta starters and are started in sequence.(from data sheet of chiller)
2.Three pumps of FLA 81 and Locked Rotor Amps of 555,out of which only two will be working.
3.One lighting DB
4.One 5.5 KW AHU
5.One 20 KW heater(Intermittent Load)..

Max Current =1003+1003+81+81+12+20+30=2230..Which I hope the transformer & breaker can handle.
However problem arises when one pump fails.When one pump fails and next pump starts Locked rotor amps of 555A will come..
so total current=1003+1003+81+20+30+555=2692..This is a problem ryt even though this occurs only during starting.Can the breaker handlethis current without tripping?What about the transformer?

 

[MikiBg]

The way you calculated the total current will give you the worst case (maximum), not a real total current. You have to calculate total kWs and kvars and then calculate the total current.
Even with 2692A in total, micrologic will hardly trip, if it is adjusted to rated current of 2500A, but you have to check a tripping curve settings. Schneider provides "Curve Direct" software for that purpose. Transformer itself will handle it for sure, but you have to check MV protection settings.

 

[MikiBg]

... having in mind that MV protection of transformer, if properly selected, has to handle transformer inrush current, it can't be possible to trip at such light overcurrent

 

[alanElec]

I already downloaded curve direct..But Im not sure as to the conclusion from that.Y axis it has time in sec..and x-axis they have current..So I need to check the intersection of 2600 A and note how mucg time it can hold right? Can you check out n tell me?

 

[alanElec]

http://hto.power.schneider-electric...en-GB&regulation=EuropeCenelec#Discrimination

I got the curves in the above link for model: Masterpact NW25H2 Micrologic 6.0 A..Can you confirm weather this can hold 2700 A for 10/20 sec?

 

[MikiBg]

Yes, that's the principle, but I can't do it for you, sorry. Even if I want to, I don't know the settings of your micrologic. Pay attention that this tripping curve is adjustable in inverse (overload) area.