IEEE-142-2007 Grounding of Industrial Systems

[genhead]

Could somebody please help me in my understanding of this section from IEEE-142-2007 Green Book, regarding generator earthing.

"If the generator is rated for solidly grounded service, the neutral may be connected directly to the grounding circuit conductor. If a standard generator is used, a reactor should be connected between neutral and the grounded circuit conductor......"

What is meant by a "standard generator" in this context?

"The use of the reactor between the generator neutral and the neutral circuit conductor does not affect the NEC requirement that the neutral conductor be solidly grounded"

How does it not?

 

[rbulsara]

Beats me! Apparently it means ones that are not rated for solid neutral grounding. It may have to do with winding insulation levels for units of high voltage. But I did not know they were called "standard".

Regardless, impedance grounding is not prohibited by NEC, but if you do, you cannot use single phase loads on it. NEC "requires" solid grounding for only systems that can have phase to ground voltage of 150V or less. For other systems solid neutral grounding is "permitted" not mandatory. This does not preclude proper engineering. For ungrounded or impedance grounded system, ground fault detection is still required.



Rafiq Bulsara

http://www.srengineersct.com

 

[dpc]

For most generators, the line-to-ground fault current (at the generator terminals is higher than the three-phase fault current. But a "standard" generator is normally braced only for three-phase fault current. So it is necessary to limit the ground fault current to not more than the three-phase fault current.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[burnt2x]

"For most generators, the line-to-ground fault current at the generator terminals is higher than the three-phase fault current."

True. I have to re-think my theory and knowledge on why this fact won't be the usual case if the generator is solidly grounded!

 

[dpc]

It is the case. That's why generators are often grounded via some type of impedance.



"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[genhead]

Thanks for your replies

I forgot to note that it's in Section 1.7.2, "Single Unparalleled Generator"

Regarding the second question, I've realized that I'm just misinterpreting the wording - I thought this standard was stating that with the reactor between the generator neutral and the neutral conductor, then the neutral conductor was still somehow solidly grounded!

My mistake.

 

[alehman]

Frequently low voltage generators are rated for solid grounding.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[EddyWirbelstrom]

In the IEC world I have not heard of 'non-standard' LV generators where extra generator winding bracing protects against the earthfault ( ground fault ) currents which can be higher than the initial symmetrical rms three phase short-circuit current.
The attached IEC60909 short-circuit current calculation for different types of faults on the terminals of a 600kVA, 415V, 50Hz generator terminals shows the highest fault current to be the earth or ground return current ( I"k2E2 ).
Can this Stamford Frame HCK534/544E, Voltage series star, 415V/240V, 50HZ, Self excited generator withstand the 12.2kA earth (ground) return current ?

 

[xxjohnh]

Most low voltage generators are soldly earthed and small, up to 2500 kVA.Trip on earth fault.
Most medium voltage generators 2 TO 10 Mva are low resistance grounded, 100 to 400amps, 10 secs, to limit damage to expensive machinery.Trip on earth fault.
Larger units 10 to 1000 Mva tend to be unit connected so they are high restistance grounded, 10 amps or so continous through a distribution transfomer or resistor. Sometimes just alarm and don't trip on ground fault.