earth fault level

[far243]

one plant description :
4 gtg 42 MW(11/33 transformer)
sc level 31.5 KA
load 100 MVA

-we have big motor 11 kv 7MW , with transformer 10MVA 33/11 delta /star and earth fault 30A( star point to earth with resistance)
-for 6KV taransformer earth fault is 50A( star point to earth with resistance)
-for 400 v star point is solidly earth.

i want to know how can i calculate erath fault level in transformer 11/33 ., in fact ? current (star point to earth)??

 

[RRaghunath]

Simple calculation for earth fault current on 11kV side of 33/11kV unit transformer is [(11kV/Root3)/ Neutral earthing resistor value in Ohms].

 

[far243]

but i have earthing fualt level in 11 kv side(30a), and i want to calculate this at 33 kv side 11/33kv conected to main busbar????

 

[rbulsara]

A single phase to earth fault current of 1 p.u. on wye side of the transfomer show up as 0.58 p.u in the other two phases (lines) of the delta side and 0 in the one phase corresponding to one that is faulted on the secondary side.

Refer to some literature of S&C MV fuse applications. You may want to search the web or S&C publications. There must be other references too like IEEE buff book.

 

[wrong100]

RETURN SHOULD BE IN NUT, NOT EARTH.
EARTH SHOULD HAVE SAME FAULT CURRENT AS HOUSEHOLD.
IF I REMEMBER THIS IS .002 TO .006 AMPS, PER USER
OR, PER OUTLET OR STRING, AND
IS CONTROLED IN GOV SAFETY MANDATES,(US).
EARTH IS ALWAYS USED FOR SAFETY GROUND
AND NEVER USED FOR POWER.
EUROPE IS BECOMING MORE RESTRICTIVE,
THE OTHER COUNTRIES DO NOT SEEM TO CARE.
ELECTRIC SUPPLIERS MUST HAVE THEIR SPECIFIC
RULES AND REGULATIOMS FOR SOURCING POWER.
FOR USERS IN THE US THE CURRENT SHOULD
BE BELOW THE LEVAL THAT WILL KILL YOU.