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Grounding Question No.18 - Grounding systems advantage and disadvantage
Author : Guest - From: Syria
 
#1
Sat, June 20th, 2009 - 10:25
Grounding system definition is done base on the grounding impedance ( resistance/reactance ) amount and relevant relationship as following :

1- Solidly grounded: No intentional grounding impedance.
Provides for the highest level of fault current to permit maximum ability for overcurrent protection for isolation of faulted circuit.
Fault current may need to be limited if equipment ratings are to be met. Will trip on first fault (it is this factor that occasionally leads to use of ungrounded circuits). Provides greatest ability for protection against arcing faults. Provides maximum protection against system overvoltages because of lightning, switching surges, static, contact with another (high) voltage system, line to- ground faults, resonant conditions, and restricting ground faults. Limits the difference of electric potential between all un-insulated conducting objects in a local area.

2- Effectively grounded: R0 ≤ X1, X0 ≤ 3X1, where R is the system fault resistance and X is the system fault reactance.
Permits the use of lower-rated (80 percent) surge arresters.
Reduces fault current in comparison with solidly grounded circuits. The reactance limitations provide a fault-relaying current of at least 60 percent of the three-phase short-circuit value.

3- Reactance grounded: X0 ≤ 10X1.
In order to limit the transient overvoltage, X0 ≤ 10X1. This usually results in higher fault currents than resistance-grounded systems. Used to reduce zero-sequence fault current to generator fault-current rating (normally line-to-line rating).

4-Resistance grounded: Intentional insertion of resistance into the system grounding connection;  2X0 ≤ R0 .
5- High-resistance grounded: The insertion of nearly the highest permissible resistance into the grounding connection; R0 ≤ X0c/3, where X0c is the capacitive zero-sequence reactance.

At high resistance, extreme transient overvoltages are limited to 250 percent of normal. This system is intended to reduce fault damage, mechanical stresses, stray currents, and flash hazards. It requires sophisticated relaying. At the low-resistance end, this system allows fairly large fault current so it minimizes high-resistance grounding advantages. It is easier to relay.


6- Grounded for serving line-to-neutral loads: Z ≤ Z1, where Z is the system fault impedance.
It is used for single-phase loads.
 
Author : Hamid - From: IRAN
 
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