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#1

# Rating of VCB and Relay panel

can any one tell me, what should be rating of VCB and Relay panel for the protection of 2500kva T/F and what should the value for current % for relay setting (both o/c and earth fault).
08-29-2013 09:09 PM
Top #2
Gopal, What is voltage level? Is trafo oil cooled / air cooled. Is it Power or Distribution trafo ? If it is Power trafo do you want Differential protection also?
08-29-2013 09:10 PM
Top #3
I understand your transformer is 11/0.4 kV delta-star distribution transformer with solidly grounded secondary.

You can use standard 400A or 630A 11kV VCB (depending upon your local standard rating). Most of the 11kV VCB's are now rated 630A.

You can choose any standard relay panel with Overcurrent & Earth Fault protection facility. At our utility we use Micom relays (Alstom) or VIP relays (Schnieder electric).

The common practice for TF over current is 125-150% or above and 20% for earth fault. You need to check what overload limit your transformer is designed for? As for as I know most distribution transformers are designed for 120 or 130% short period overload. You also need to make sure that protection caters for the transformer rush-in current with the time settings.
08-29-2013 09:10 PM
Top #4
thank you Musa, But we use this transformer for WTG,the voltage ratio of the t/f is 33000/690.it is interconnected to grid.
08-29-2013 09:11 PM
Top #5
Hi Gopal, As a quick guide, 2500kVA divided by root 3 (1.732) x Voltage (33000) = 43.74A. 33kV breaker would be minimum 630A anyway. Similarly secondary current 2500kVA divided by root 3 (1.732) x Voltage (690) = 2,092A. Secondary breaker will be 2500A. If you know power factor include in multiplication. If you need rough idea of fault current on secondary side use impedance of transformer i.e. If impedance is 5% 2500kVA divided by 5%, 5 x 1.732 x 690 = 41.8kA
08-29-2013 09:12 PM
Top #6
@gopal i understand that u r using this for wind generation....thats being used in india...as suggested by Mr. John u can go for same rating.....however for relays u can go for various makes of relay ...but as it is going to be mount in outdoor panel...u should be careful of selection of relays.....u can go for any OC & EF economically in India ER,C&S and ASHIDA will be best however u can choose amongst GE, AREVA, ABB that is more commonly used in india....also u need transformer fault auxiliary relay. If you need further info u can send me msg.....
08-29-2013 09:12 PM
Top #7
thanks all of you