Upgrading Utility Distribution Transformer with an NGR

[rockman7892]

A utility customer is considering installing an neutral ground reactor on one of their distribution transformers (12.47kV).

I'm familiar with NGR in several other applications but have not seen many of these installed on distribution transformers. Is it common for utility's to have NGR's on their distribution transformers to limit fault currents or help with coordination?

I know in most industrial facilities the NEC does not permit NGR's to be installed on circuits with L-N loads, and I'm assuming with a utility distribution circuit there would be a similar requirement to prevent a voltage potential between N-G on the system during normal conditions with neutral current flowing through NGR?

Is approach to sizing these NGR's similar to other applications where you evaluate simultaneous objectives of keeping fault current to a minimum on system, while at the same time allowing enough fault current to make sure relays operate in a coordinated fashion? I assume this would be modeling the entire distribution circuit to see what faults on remote end of distribution system appear as for relaying?

If a NGR is being placed on an existing OH distribution line wouldn't the PT's, arrestors, CPT's and other equipment need to be evaluated for adequate ratings for L-G connections during faults when L-G voltage is elevated to L-L (IE proper bushing, winding ratings, etc..)

 

[waross]

As I understand the rules, fault currents are based on line to line faults, not line to ground or line to neutral faults.
If they are impedance grounding distribution circuits I hope that the services do not include a neutral conductor.
The last time I saw impedance grounding it was on the power transformers of a world class mine mill.
There were voltage relays across the impedances.
In the event that a piece of equipment developed a serious ground fault, the impedance ground avoided a general power outage and allowed time to isolate and remove the faulted equipment.
The voltage relays produced an alarm in the event of fault current through the impedance.
These were on a 13,000 Volt secondary system feeding 13,000;480 Volt unit subs..

 

[rockman7892]

Is there an advantage to having a neutral reactor as opposed to a neutral resistor on a distribution circuit?

From what I've seen this is commonly done with generators where the Z0 is much less than Z1 causing zero sequence current to be much higher than positive sequence current and this the generator may not be properly braced for zero sequence. In these instances is appears that the generator can still connect L-N loads? Does a neutral grounding reactor still allow the connection of L-N loads in contrast with a grounding resistor?

It appears that the sweet spot for reactors is limiting the gf current to no less than 25% with 25%-60% being the target range.

 

[waross]

Is there an advantage to having a neutral reactor as opposed to a neutral resisto

Less heat to dissipate.