Ngr
- Thread starter K.Mohan
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brian john
Senior Member
- Location
- Kilmarnock, Va
- Occupation
- Retired after 52 years in the trade.
To limit fault current to a predetermined level, and thus minimize fault damage.
http://www.postglover.com/Literature/NG112-06_Tech_Info.pdf
http://www.postglover.com/Literature/NG112-06_Tech_Info.pdf
Cold Fusion
Senior Member
- Location
- way north
Most of the time I have seen utilities use NGRs its becaues they have parallel generation with a low impedance between the generators. If one generator develops a fault, the other generators can deliver more SCC than the faulted gen can stand. I have not been involve with this, but I hear it really makes a mess. As 72.5 said, adding a low impedance neutral grounding reactor limits the SCC to where the faulted gen does not melt down before the OCP opens.
For industrial 480V, the main issue is continuity of service. The L-G fault current is generally limited to 5A - 10A. The system is still operational with one line to ground. Rarely are there any trips programmed for L-G faults. A second L-G fault on a different L causes severe hate and discontent. However, you are susposed to be smart enough to track down the first fault REAL SOON - before the second happens.
Also, as brian said, the NGR limits the L-G fault current to no damage. Most grounded system L-G faults in switchboards cause a lot of damage because the current is not high enough to quickly trip but high enough the make a mess. As soon as the fault goes line-to-line, then it trips relatively quickly.
The same scenerio in an impedance grounded system causes no damage on an L-G fault. And for a L-L fault, the current is high and will quickly trip - limiting damage.
For MV systems, the reasoning and outcome are somewhat different.
cf
For industrial 480V, the main issue is continuity of service. The L-G fault current is generally limited to 5A - 10A. The system is still operational with one line to ground. Rarely are there any trips programmed for L-G faults. A second L-G fault on a different L causes severe hate and discontent. However, you are susposed to be smart enough to track down the first fault REAL SOON - before the second happens.
Also, as brian said, the NGR limits the L-G fault current to no damage. Most grounded system L-G faults in switchboards cause a lot of damage because the current is not high enough to quickly trip but high enough the make a mess. As soon as the fault goes line-to-line, then it trips relatively quickly.
The same scenerio in an impedance grounded system causes no damage on an L-G fault. And for a L-L fault, the current is high and will quickly trip - limiting damage.
For MV systems, the reasoning and outcome are somewhat different.
cf
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