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Home > Archive > Electrical Engineering > June 2006 > neutral loss
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| kennyd 2006-06-08, 4:21 pm |
| On a 120/240 single phase 3 wire system the power company grounds the
neutral at the transformer. The neutral is also grounded at the service
entrance on the customer side. Obviously this makes a parallel path, that is
the neutral conductor and the higher resistance earth are in parallel.
The problem is that if the neutral conductor is opened the transfer switch
for an onsite generator does not sense this. The loads in the building
suddenly are connected in series with bad results unless the load is
ballanced.
Is there a way to sense the loss of the neutral connection and then tell the
generator to start and take the load?
Simple relays wont work because they would also be in series and not drop.
Any suggestions??
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| electrician@electrician2.com 2006-06-08, 4:21 pm |
|
kennyd wrote:
> On a 120/240 single phase 3 wire system the power company grounds the
> neutral at the transformer. The neutral is also grounded at the service
> entrance on the customer side. Obviously this makes a parallel path, that is
> the neutral conductor and the higher resistance earth are in parallel.
> The problem is that if the neutral conductor is opened the transfer switch
> for an onsite generator does not sense this. The loads in the building
> suddenly are connected in series with bad results unless the load is
> ballanced.
> Is there a way to sense the loss of the neutral connection and then tell the
> generator to start and take the load?
> Simple relays wont work because they would also be in series and not drop.
> Any suggestions??
There are two methods recognized by the IEEE Green Book for grounding
onsite generators. Diagrams can be found in one of the latter
chapters. One or the other is generally used.
Method 1: Do not open the neutral with the transfer switch. The
neutral is hardwired through the switch. By this method the onsite
generator is not a separately derived AC system and no additional
grounding electrode is required at the generator. After a transfer is
initiated the neutral connection to the service is rlied upon.
Method 2: The transfer switch does break the neutral connection to the
service and a grounding electrode is required at the generator. The
generator is considered a separately derived AC system. Method 2 would
eliminate the open neutral problem where the open is between the
service and the utility transformer
Which method are you using?
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| Charles Perry 2006-06-08, 5:21 pm |
|
"kennyd" <ken@dieselectric.com> wrote in message
news:%YZhg.41499$YI5.20572@tornado.ohiordc.rr.com...
> On a 120/240 single phase 3 wire system the power company grounds the
> neutral at the transformer. The neutral is also grounded at the service
> entrance on the customer side. Obviously this makes a parallel path, that
> is the neutral conductor and the higher resistance earth are in parallel.
> The problem is that if the neutral conductor is opened the transfer switch
> for an onsite generator does not sense this. The loads in the building
> suddenly are connected in series with bad results unless the load is
> ballanced.
> Is there a way to sense the loss of the neutral connection and then tell
> the generator to start and take the load?
> Simple relays wont work because they would also be in series and not drop.
> Any suggestions??
>
The loss of a neutral can be determined by measuring both "hot" to neutral
voltages. When the neutral is lost, and the load becomes unbalanced, these
two voltages will no longer be equal. Now, you have to allow some "slop" as
they say. Even with a good neutral connection, the normal load unbalance
will cause the voltages to be somewhat different (say 5 volts or so). With
a loose neutral, you can see a difference of as much as 240V. Now, I am not
sure there is a transfer switch this smart.
Another option is to measure both hot legs and transfer any time one of them
goes "low". Again, is there a switch this smart? Don't know. Actually, I
do know, but you don't want a $7k switch. In the "cheap" models, I don't
know if this option exists.
Charles Perry P.E.
| |
| Paul Hovnanian P.E. 2006-06-08, 5:21 pm |
| kennyd wrote:
>
> We are using method 1. But the problem is not with the neutral grounding,
> the problem is that the transfer switch does not sense neutral loss.
Here are two methods for detecting utility neutral loss. Each has their
shortcomings:
1) measure the service neutral voltage imbalance. If the service drop
neutral opens, the ground has a high impedance and the load has some
imbalance, the neutral voltage will drift.
2) sum the service entrance currents (pass all conductors through a
single CT window). If the service neutral opens, the load imbalance
current will return to the utility system through the ground. In this
case, if the ground impedance is very low, some imbalance current may
return through the ground in parallel with a good neutral, so the
sensing threshold must be set to account for this.
If you are concerned about a neutral break between the service equipment
(where the system neutral is bonded to the grounding system), you must
use 'Method 2' (below) and the above detection schemes are no longer
applicable.
[color=darkred]
> <electrician@electrician2.com> wrote in message
> news:1149792361.760087.307740@f6g2000cwb.googlegroups.com...
--
Paul Hovnanian mailto:Paul@Hovnanian.com
------------------------------------------------------------------
Catapultam habeo. Nisi pecuniam omnem mihi dabis, ad caput tuum saxum
immane mittam. (Translation from Latin: "I have a catapult. Give me
all the money, or I will fling an enormous rock at your head.")
| |
| kennyd 2006-06-08, 5:21 pm |
| The plot thickens in that we also have 3 single phase Powerware9170 18 kva
UPS's at this site. On the day we lost the power company neutral connection
(verified) two of the UPS's did not sense the neotral loss as a power
failure, one did. Of coures the batteries wewe quickly depleted because the
generator never got a start signal from the transfer switch.
Powerware says they will have a firmware upgrade to fix the UPS problem, now
I need a fix for the transfer switch which is a 225 amp Kohler.
7k might not be too bad. Is that a Russelectric?
"Charles Perry" <pipesandtobacco@hotmail.com> wrote in message
news:4erc2oF1g3rcsU1@individual.net...
>
> "kennyd" <ken@dieselectric.com> wrote in message
> news:%YZhg.41499$YI5.20572@tornado.ohiordc.rr.com...
> The loss of a neutral can be determined by measuring both "hot" to neutral
> voltages. When the neutral is lost, and the load becomes unbalanced,
> these two voltages will no longer be equal. Now, you have to allow some
> "slop" as they say. Even with a good neutral connection, the normal load
> unbalance will cause the voltages to be somewhat different (say 5 volts or
> so). With a loose neutral, you can see a difference of as much as 240V.
> Now, I am not sure there is a transfer switch this smart.
>
> Another option is to measure both hot legs and transfer any time one of
> them goes "low". Again, is there a switch this smart? Don't know.
> Actually, I do know, but you don't want a $7k switch. In the "cheap"
> models, I don't know if this option exists.
>
> Charles Perry P.E.
>
>
| |
| Charles Perry 2006-06-08, 6:21 pm |
|
"kennyd" <ken@dieselectric.com> wrote in message
news:Be%hg.53109$mh.33688@tornado.ohiordc.rr.com...
> The plot thickens in that we also have 3 single phase Powerware9170 18 kva
> UPS's at this site. On the day we lost the power company neutral
> connection (verified) two of the UPS's did not sense the neotral loss as a
> power failure, one did. Of coures the batteries wewe quickly depleted
> because the generator never got a start signal from the transfer switch.
> Powerware says they will have a firmware upgrade to fix the UPS problem,
> now I need a fix for the transfer switch which is a 225 amp Kohler.
> 7k might not be too bad. Is that a Russelectric?
Is the Powerware capable of 240V input? I would think that a 240V UPS could
care less if there is a neutral. I will have to investigate this when I
return to the office. We have tested 9170s in ways Powerware could never
imagine.
Charles Perry P.E.
| |
| kennyd 2006-06-08, 7:21 pm |
| I agree,but the 9170 needs the neutral according to Powerware, Put a switch
on the neutral going into the UPS. that is what the Powerware people tell me
they did. Worked (that is went to battery) some of the time but not always.
Even if Powerware does solve their sensing of neutral loss problem, that
doesn't solve mine. The batteries will drain without the generator.
The ct's around all three conductors, as in and maybe simply, a ground fault
sensor sounds interesting. what do you think?
"Charles Perry" <pipesandtobacco@hotmail.com> wrote in message
news:4erhnmF1gas12U1@individual.net...
>
> "kennyd" <ken@dieselectric.com> wrote in message
> news:Be%hg.53109$mh.33688@tornado.ohiordc.rr.com...
>
> Is the Powerware capable of 240V input? I would think that a 240V UPS
> could care less if there is a neutral. I will have to investigate this
> when I return to the office. We have tested 9170s in ways Powerware could
> never imagine.
>
> Charles Perry P.E.
>
>
| |
| Paul Hovnanian P.E. 2006-06-09, 10:21 pm |
| kennyd wrote:
>
> I agree,but the 9170 needs the neutral according to Powerware, Put a switch
> on the neutral going into the UPS. that is what the Powerware people tell me
> they did. Worked (that is went to battery) some of the time but not always.
> Even if Powerware does solve their sensing of neutral loss problem, that
> doesn't solve mine. The batteries will drain without the generator.
> The ct's around all three conductors, as in and maybe simply, a ground fault
> sensor sounds interesting. what do you think?
The neutral loss sensing that the transfer switch has may only be
sensing a low voltage on one hot leg. As Charles pointed out, a broken
utility neutral can result in voltage imbalances which cause one hot leg
to go high instead of low. If the UPSs and the transfer switch are
connected to different hot legs, one may think that the utility power is
OK while the other may think it has been lost.
Its more important to get both the transfer switch and the UPS sensing
to agree on loss of power conditions.
--
Paul Hovnanian mailto:Paul@Hovnanian.com
------------------------------------------------------------------
If you can't beat them, arrange to have them beaten.
-- George Carlin
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| John Gilmer 2006-06-12, 10:21 am |
| I don't know whether anyone actually makes such a gadget but an open neutral
could be detected by using two current transformers on the neutral between
your panel bonding point and the utility.
One transformer would be connected to some signal source and would, when the
neutral is "working" cause the test signal current to be injected in the
"loop" formed by the neutral being connected to a "pretty good" ground at
both ends. The other current transformer would detect this injected
current. If the neutral opens, the "test signal" will go away.
Some real world experience would tell you how simple or complex the test
signal and the signal detector would have to be.
This is essentially what your BASIC $8 GFCI does to detect FAULTS between
neutral and ground.
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