Cap Bank Protection

[ariccardo]

For a 13.8kV ungrounded wye capacitor bank with one main breaker with phase Instantaneous Overcurrent and Time Overcurrent relays and 59N on a neutral VT. How would you calculate the relay pick ups, also for the ground relays neutral to ground alarm and trip settings?? and how would you calculate the neutral VT ratio?
Thanks

 

[7anoter4]

We need some further details. Is your cap bank double Y cap bank?
See for information:

http://store.gedigitalenergy.com/faq/documents/general/shunt.pdf

 

[ariccardo]

It is a double wye

 

[7anoter4]

First of all in the case one_ phase_ to ground is still UN=0 but Uground=UR.
then the voltage on ground transformer will be UR=UL-L/sqrt(3).
In the case of phase to ground to phase the Ishcircuit depends on Short-circuit level of the supply system.
Reactance per phase will be: Xn=(Xc*s)/p s=no.of capacitors in series p=no.of capacitors in parallel.
Let's say one capacitor shorted [no fuse in the cap]is gone.
The fault reactance will be[one cap. less in series chain]: Xf=Xc*(s-1)*[Xc*s/(p-1)]/{Xc*(s-1)+[Xc*s/(p-1)]}
Where Xc=1/2/pi()/frq/Cap one capacitor reactance.
In the case of a fault the neutral potential will be UN.
UN=UR*(Xn-Xf)/(2*Xf+Xn) it is minimum UN[one single cap less]
But Max UN=UL-L=SQRT(3)*UR -in the case in one wye Xf=0 of phase R and in the second wye phase Y or B Xf=0 then UN1=UR and UN2=UY.
In case of a fused capacitor one entire chain is gone. Then Xf=(Xc*s)/(p-1).