neutral ground(ing) resistor
neutral ground(ing) resistor
(OP)
Does anyone out there use neutral grounding resistors on the neutral(s) of the distribution substation transformer WYE secondary windings?
I have a situation involving a distribution substation with high ground fault current, in which the electronic feeder protection module (I'll use the acronym POS to describe it) is tripping and going to lockout after one pickup (shot) of a high-magnitude ground fault (undocumented anomoly). I don't want to change the protection module, but I'm exploring using a neutral resistor to lower the fault current to the point where the module will not immediately go to lockout, but reclose (as intended). We have other situations similar to above, but the protection modules will trip/reclose several times trying to free the dead squirrel off the insulator before going to lockout.
I have read that utilities will install neutral resistors to limit ground fault current, and lower nuisance outages.
What is been your experience?
I have a situation involving a distribution substation with high ground fault current, in which the electronic feeder protection module (I'll use the acronym POS to describe it) is tripping and going to lockout after one pickup (shot) of a high-magnitude ground fault (undocumented anomoly). I don't want to change the protection module, but I'm exploring using a neutral resistor to lower the fault current to the point where the module will not immediately go to lockout, but reclose (as intended). We have other situations similar to above, but the protection modules will trip/reclose several times trying to free the dead squirrel off the insulator before going to lockout.
I have read that utilities will install neutral resistors to limit ground fault current, and lower nuisance outages.
What is been your experience?






RE: neutral ground(ing) resistor
RE: neutral ground(ing) resistor
Also, you won't be able to serve line-to-neutral loads if you add much resistance.
Neutral ground resistance is used for industrial facilities to limit damage from ground faults.
I've never seen it used on a distribution feeder.
RE: neutral ground(ing) resistor
For other relays, this may be programmed through logic with a separate instantaneous element.
RE: neutral ground(ing) resistor
To DavidBeach: We did an extensive bus/fault study years ago, and have gone over the settings of these devices again and again, with no setting to describe the high GF trip/lockout problem. The manufacturer (Cooper) have said this is an internal function which cannot be shutoff, and was designed to go to lockout after one(1) exceptionally high GF. The module is the IM30 series which Cooper introduced some years ago. Very European, but w/many problems. JL Blackburn advised against neutral resistances as well, citing difficulty in detecting GF far away from the substation. I am going to advise installing shield wires over the distribution lines to stop the lighting flashovers.
To DPC: You are correct. As I said above, JL Blackburn also said "NO" to neutral resistances. And under normal loading conditions, such resistances will be "cooking" 24/7 due to single phase loads.
To JGChrist: We have scoured the manual of the IM30 modules, and nothing was mentioned about the undocumented function. We only found out through a support person at Cooper, who admitted this was a problem, and offered no workaround. FYI: We also use the Form 4C controls, which we like, but are wondering as to their accuracy, which Cooper said is lacking in the Form 4C. We like the Form 6, but there again, what about the accuracy (?), which is extremely important.
RE: neutral ground(ing) resistor
I'm not familiar with the IM30, but I've had problems with some other "Cooper" relays that were made for them in Italy.
As for accuracy - for very high current faults, the CTs may be more of the limitation rather than the relays.
RE: neutral ground(ing) resistor
Shield wires on distribution lines are not all that effective. A lot of distribution flashovers are from induced voltage from nearby strokes which wouldn't be affected by the shield wire. With typical distribution line BIL and grounding, backflashes to the conductor from strokes to the shield wire will also cause a lot of trips. Generally, arresters will provide better protection, but then there are problems with arrester failures.
You could also install phase current-limiting reactors. This may cause voltage regulation problems and would surely be more expensive than changing the relays.
RE: neutral ground(ing) resistor
Needless to say, we got burned, and we will do almost anything else to avoid changing these modules. Changing them involves cutting a bigger hole in the plate steel face to accomodate the replacement..........not an easy task considering the number of cubicles.
The IM30 relays were made in Italy, which at the time were the only small, low-cost, integrated relay on the market, and they were very appealing. Oddly enough, Cooper still sells them.
We think we know where the lightning flashovers are occuring, and installing shield protection on an otherwise unshielded line may be worth a try. We have installed shield wires on other feeders, with great success.
RE: neutral ground(ing) resistor
RE: neutral ground(ing) resistor
RE: neutral ground(ing) resistor
I'll see if newer, revamped versions are available. The question is: Will Cooper charge a fee for replacements?
I have d/l'd the latest documentation for the modules, and nothing has changed in six years.
When we first installed these six years ago, we received a defective unit, and Cooper sent out a replacement unit right away. We noticed the output relays had been changed in the replacement unit, which brings up another bad feature of these particular units: Output contact too small (ampere rating).
We had to install interposing relays between the output contacts and the circuit breaker trip mechanisms, since the output relay contact ratings were below the circuit breaker tripping requirement.........big mess. Now you see why I call these P.O.S.
RE: neutral ground(ing) resistor
The NER's also assist in the limitation of substation EPR.
We also commonly encounter NER's on clients 11,000V systems limiting fault current to 800A or 1000A
RE: neutral ground(ing) resistor
RE: neutral ground(ing) resistor
RE: neutral ground(ing) resistor
EPR = Earth Potential Rise
RE: neutral ground(ing) resistor
RE: neutral ground(ing) resistor
RE: neutral ground(ing) resistor
The IM30DRE, being a digital computer, has all the trimmings; CPU, A/D, D/A, relay contact outputs, display, etc. I don't think it's internal architecture is all that whippy, but at the time (2000) it was, to my knowledge, one of a handful of small and integrated feeder protection modules on the market.
Now, Siemens and Areva make some really small, nice, and sophisticated modules.
ABB used to (I haven't checked recently) sell a fully-integrated preotection module which allowed for the connection of a neutral CT. I wonder what affect on performance a separate neutral CT has on GF detection vs. virtual values as in the case of the IM30DRE.
RE: neutral ground(ing) resistor
Now to get your problem defeated. Short the GF CT out at the CT terminals. That will elliminate the circuit all togather.
Unless your required to have the GF at that location. In which case perhaps get a more flexible GF unit external to the Cooper unit and fire it's trip circuit via shunt trip.