High and Low Gr. System

[Meinkaun]

Hi,

I am sure many have answered this before:
Is this thubm rule or there is a logic behind this assumption:

1. Low resistance grounded system: 0-25 A
2. High resistance grounded system-25-600 A

Will it be correct to say, if I limit my ground fault current to 50 A, it will be a High resistance grounded system.
Thanks

 

[cuky2000]

I believe you have the high/low resistance effect reversed. As high is the resistance lower is the ground fault current. Representative typical limits of ground fault current are as follow:
Low resistance: 5 to 10% common of 3 phase fault current & trip.
High resistance: Less 0.1% of 3 phase fault & alarm no trip.

 

[Meinkaun]

Grateful Cuky2000.

The debate is some what related actually. selection of a L.A. based on high resistance or low resistance grounding system.
My system is 44 Kv/13.8 KV star grounded 159 ohms I(Gf)50A there is a pole line of approx. 1 km from the sec. of transformer 13.8 KV. what would be your suggestion for be the MCOV of lightning arrester for Line to ground application.
Could you please put a word here?
Thanks

 

[davidbeach]

Unless solidly grounded, your arrestors need to be rated for line-line voltage. Even with a low resistance grounded system you will have a significant neutral shift for a ground fault.

 

[Meinkaun]

Thanks david beach. when you said rated line to line, Does that means MCOV should be line to line or rating of L.A should be line to line?

I beleive rating of L.A is different from MCOV.

Thanks

 

[davidbeach]

I'm not familiar with the details of specifying arrestors, but the MCOV would be some % above the intended operating voltage, usually 15% or more above. Unless solidly grounded you need to be your % above line-line; if solidly grounded you can be at your % above line-neutral. With any type of impedance grounding the unfaulted phases will see an overvoltage considerably more than 15% above normal.

 

[RRaghunath]

MCOV should be maximum system voltage i.e. typically about 105% of the rated system voltage.

 

[cuky2000]

The MCOV of arresters connected phase-to-ground the following approximation is usually used:

* For High Impedance/resistance grounded : MCOV [≥] V_LL
* For solidly & low impedance grounded: 0.63.V_LL [≤]MCOV [≤] 0.80V_LL
See an example in the following link:

http://cuky2000.250free.com/SA1.jpg

If the sequence impedance parameters are known, more accurate results could be obtained determining the coefficient of grounding (COG). The COG take in consideration the overvoltage effect of the grounding impedance and the system parameters at the surge arrester location. The surge arrester MCOV, regarless the type of grounding impedance could be calculated as follow:
MCOV=1.05.COG.V_LL