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DC Breaker Sizing

DC Breaker Sizing

DC Breaker Sizing

(OP)
1.         Standards for calculating Short circiut currents

According to IEC standards, The fault current on a dc system is given by a rising and decaying exponentials, described as:

I(t)=Ip[1-exp(-t/tr)]/[1-exp(-tp/tr)]  for t<=tp
I(t)=Ip[ρ +(1-ρ)exp(-(t-tp)/td         for t>=tp

  
Where

I(t) is the instantaneous dc current
Ip is the prospective or peak dc current available
Iq is the quasi or steady state dc current after some period of time
ρ is the quasi to peak  current ratio =Iq/Ip
tr is the rising time constant
td is the ramping time constant
tp is the time to reach its peak

In North America, the available literature , which are mainly the procedures developed by GE,  covers the rising time section of the waveform and is given as :

 I(t)=Iss[1-exp(-t/tr)]
 
Where Iss is the prospective steady state value of the fault current.

 
2.    Breaker/Fuse sizing requirements for DC Application:

Breakers and fuses must meet the following requirements:

1.    Certified for DC application
2.    Proper dc voltage rating
3.    Momentary capability larger than the Ip (IEC) or Iss (GE)
4.    Interrupt capability larger than Iq (IEC) for slow breakers and Ip (IEC) for fast breakers or Iss (GE)

Q1)

Some colleages argue that the interrupt rating of the breaker should only be compared to the quasi current Iq and not to the peak current Ip. Does anybody have an opinion  either way? Are there any other requirements?


3.    System Characteristics:


I need to install a DC breaker, whose rated interrupt rating is 42 KA in a location with the following characteristics:

System    250 VDC    
Ip  (Kamps)    60
Tp (msec)    6
Tr  (msec)    23
Td (msec)    7
Iq (Kamps)    45
Iss (Kamps)    61.33
        
    
Q2)

Would this breajker be considered properly sized?




 

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