Fault Level

Minor correction:

but having voltage level to substations at 33kV from 220kV grid instead of 66kV from 440kV grid.

Pls read it as 33kV stepped up voltage from local generators* instead of 66kV from 440kV grid.

To carry the same power at a lower voltage would require an increase in current. If the physical arrangements are satisfactory for increased current then they will be sized accordingly. Under these circumstances it would be fair to state that one might expect them to withstand higher fault currents until a circuit protective device operates to disconnect the fault. Sizing them to withstand the fault lays within the domain of the Electrical Engineer, who will be specialised in this art.

If we trust in IEC 60076-5/2000 Power transformers –Ability to withstand short circuit Table 2 – Short-circuit apparent power of the system says for 245 kV System [Current North American practice] S220kV=15000 MVA and for 765 kV 83000 MVA.

Let’s say the both transformers are 400 MVA 20% short-circuit impedance.

Then for 220/33 kV we have Zsys=33^2/15000=0.0726 ohm and Ztrf=20%*33^2/400=0.5445 ohm

Then S33kV=33^2/(0.5445+0.0726)=1784.7 MVA

For 440/66 kV Zsys=66^2/830000=.05248 ohm

Ztrf=20%*66^2/400=2.178ohm

S66kV=66^2/(0.05248+2.178)=1952.9 MVA

If transformer short-circuit impedance will be 22% at 400 MVA 66 kV then the transformer impedance will be 2.4 and S66kV=66^2/(0.05248+2.4)=1776.2 MVA

Usually will be the same short-circuit apparent power.

Thank you ..My doubt is cleared to some extent.

If possible please provide the calculations as an example

How the fault level remains same at 33kV & 66kV GIS bus.Is it not fault level at higher voltage (66kV ) would be more than fault level at lower voltage.(33kV), even though fault current will be higher at lower voltage .

Nicely written..It means, though there is huge difference of fault level at upstream voltage levels , there is not much difference at downstream side of transformers.So 33kV and 66kV will have nearly same fault level and fault current.

thanks replying...!!!

Nicely written..It means, though there is huge difference of fault level at upstream voltage levels , there is not much difference at downstream side of transformers.So 33kV and 66kV will have nearly same fault level and fault current.

thanks replying...!!!

Nicely written..It means, though there is huge difference of fault level at upstream voltage levels , there is not much difference at downstream side of transformers.So 33kV and 66kV will have nearly same fault level and fault current.

thanks replying...!!!

Nicely written..It means, though there is huge difference of fault level at upstream voltage levels , there is not much difference at downstream side of transformers.So 33kV and 66kV will have nearly same fault level and fault current.

thanks replying...!!!

Nicely written..It means, though there is huge difference of fault level at upstream voltage levels , there is not much difference at downstream side of transformers.So 33kV and 66kV will have nearly same fault level and fault current.

thanks replying...!!!