Contribution from Utility Distribution Load for 3-phase and 1-phase-Ground faults

[musaUKPN]

Hi All,

I am involved in accepting connections of new generators on our LV, 11kV and 33kV distribution network. Most of new generators are either CHP (gas engine) that will run Long Term parallel with utility (or) Diesel Generators (DG) that will run parallel only for short duration (1 min typical). Typical generator size range from 0.5MW to 5MW.

We are reviewing our acceptance criteria for new generators and hence want to hear from you guys. The question is regarding 33 or 11kV fault levels at out primary substation (mainly 132/11kV or 132/33kV) bars and to assess break/ make capability of breakers.

At present, we include the load contribution as 1.1 x maximum demand on the bus bars for both the break as well as make conditions. I believe this comes from G-74 (UK standard).

I want to know what load contribution we should add in our break and make analysis? Please note that in our distribution network the load is mixed commercial/ residential loads which do not include large industrial loads.

musa UKPN

 

[abedneirat]

Hi Musa

The contribution from distribution will be mainly by Motors . it equals 4 x Nominal Current .
also you should consider the asymmetric factor for total load .
I sc_asym = K * I sc_sym ; K = 1 to 2 ; practically it can be considered = 1.7 max ;

Fault Current = K * Isc_sym + 4 * I_motor .

thanks