LA SIZING FOR NON EFFECTIVELY EARTHED SYSTEM

[TPEC]

thread238-46596

Can anyone provide lighting arrester voltage calculation for 66/33 kv substation with 33 kv side of transformer earthed with neutral grounding resistor of 400 A and 50 ohms??

 

[Kiribanda]

Hi TPEC,
From your numbers, since your 33kV system is grounded with a 400A resister, it looks like
that the 33kV side is effectively grounded (low resistance grounded). But you have to perform few system studies
to determine the COG or EFF of the 33kV system.
1) During ground fault simulations, if you find that COG=0.8 (EFF=1.25),
then your system is effectively grounded. That means your lightning arrester MCOV is equal to the
Line-Line voltage x 1.1 divided by 1.732. The arrester rated voltage should be selected 1.25xMCOV
or from a manufacturer's catalog.

2) If not, then the MCOV= Line-Line voltage x 1.10 (some folks use 1.05). The arrester rated voltage
should be selected 1.25xMCOV or from a manufacturer's catalog.

Hope this helps.