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View Poll Results: What is the zero sequence current with one phase lost?
Zero 6 60.00%
100 amps 3 30.00%
173 amps 0 0%
200 amps 0 0%
300 amps 0 0%
346 amps 0 0%
Who the hell knows? 1 10.00%
Voters: 10. You may not vote on this poll

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Old 02-07-2015, 02:15 PM   #1
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Default 3 Phase Zero Sequence Current, What happens when....

Ok, a puzzler that occurred to me last night when I couldn't sleep. :

Here's the setup:

Source voltage: 277/480y
Current On EACH Phase: 100 amps (All three phases under NORMAL operation.)

Neutral is included in the zero sequence metering.

Obviously with ALL THREE phases an the 100 amps equal current on each phase, the zero sequence current will (should) be zero.

Now, the puzzler and reason for the post and the poll:

What will the "zero sequence" current be if any ONE PHASE is LOST? (The other two stay at 100 amps each.)

Vote and discuss.

Last edited by guest; 02-07-2015 at 02:21 PM.
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Old 02-07-2015, 02:17 PM   #2
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How are you summing this, ie is the neutral included?

Neutral current will rise to 100 amps.
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Old 02-07-2015, 02:20 PM   #3
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Quote:
Originally Posted by meadow View Post
How are you summing this, ie is the neutral included?

Neutral current will rise to 100 amps.

Neutral is included. Will update OP.
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Old 02-07-2015, 02:23 PM   #4
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Quote:
Originally Posted by meadow View Post
How are you summing this, ie is the neutral included?

Neutral current will rise to 100 amps.


Neutral would be 100 amps and the sum of the currents would equal '0"

I believe it would be the same formula assuming all single phase loads neutral to phase


BUT with everything I post I could be wrong?

Last edited by Bad Electrician; 02-07-2015 at 02:26 PM.
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Old 02-07-2015, 02:28 PM   #5
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Quote:
Originally Posted by mxslick View Post
Neutral is included. Will update OP.
If the neutral is included in the sum, net current would remain zero as nothing is leaking onto the grounding system.
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Old 02-07-2015, 02:28 PM   #6
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Quote:
Originally Posted by Bad Electrician View Post


Neutral would be 100 amps and the sum of the currents would equal '0"

I believe it would be the same formula assuming all single phase loads neutral to phase


BUT with everything I post I could be wrong?

Only if the power factor is unity
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Old 02-07-2015, 06:00 PM   #7
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Iirc, the term applies to 3 phase symmetry , even though i'm moved to think open delta....~CS~
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Old 02-07-2015, 07:24 PM   #8
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Neutral carries unbalanced portion of the load so if you lost a phase then then connected loaded would also be disconnected therefore the load would remain balanced and nutr current would remain 0
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Old 02-07-2015, 07:46 PM   #9
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Quote:
Originally Posted by FF301 View Post
Neutral carries unbalanced portion of the load so if you lost a phase then then connected loaded would also be disconnected therefore the load would remain balanced and nutr current would remain 0
Not really.

If each phase is drawing 100 amps then the neutral current would be zero. If you open one phase then the load is no longer balanced and the neutral current would rise to 100 amps, assuming the power factor is unity.
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Old 02-07-2015, 08:10 PM   #10
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The only 'zero sequence' anything that I know of is a type of ground fault detection system.

In this system, all phase conductors plus the neutral (if any) but not the grounding conductor pass through a CT. If no ground fault is present, the CT will read zero current. Only when current is going out from a phase conductor and returning through the grounding conductor will current be present on the CT.

Therefore, the answer is zero amps, provided no ground fault is present. The fact that only 2 phases are hot has no effect.
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Old 02-07-2015, 08:12 PM   #11
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Originally Posted by Aussielec View Post
Not really.

If each phase is drawing 100 amps then the neutral current would be zero. If you open one phase then the load is no longer balanced and the neutral current would rise to 100 amps, assuming the power factor is unity.
In a 3 Y connected system using 2 phases and the neutral, the current in the neutral will be very close to the current in the highest phase.

The current in the neutral doesn't begin to decrease until all 3 phases are carrying load.
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Old 02-07-2015, 08:24 PM   #12
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Quote:
Originally Posted by micromind View Post
In a 3 Y connected system using 2 phases and the neutral, the current in the neutral will be very close to the current in the highest phase.

The current in the neutral doesn't begin to decrease until all 3 phases are carrying load.
Pretty sure I just said that??
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Old 02-07-2015, 10:04 PM   #13
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Quote:
Originally Posted by Aussielec View Post
Pretty sure I just said that??
My mistake, you did.
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Old 02-08-2015, 08:38 AM   #14
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I surfed up an electrical god, who apparently made left quite the impression back in his day on the topic....

Charles Legeyt Fortescue

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In a paper[1] presented in 1918, Fortescue demonstrated that any set of N unbalanced phasors that is, any such "polyphase" signal could be expressed as the sum of N symmetrical sets of balanced phasors known as symmetrical components. The paper was judged to be the most important power engineering paper in the twentieth century.[2]

He was awarded the Franklin Institute's 1932 Elliott Cresson Medal for his contributions to the field of electrical engineering.

A fellowship awarded every year by the IEEE in his name commemorates his contributions to electrical engineering.
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Old 02-08-2015, 10:35 AM   #15
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Quote:
Originally Posted by micromind View Post
The only 'zero sequence' anything that I know of is a type of ground fault detection system.

.
Zero Sequence testing is done for tracing ground current and it is physics that is prevalent in all AC systems I would believe.
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Old 02-08-2015, 08:44 PM   #16
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Quote:
Originally Posted by micromind View Post
The only 'zero sequence' anything that I know of is a type of ground fault detection system.

In this system, all phase conductors plus the neutral (if any) but not the grounding conductor pass through a CT. If no ground fault is present, the CT will read zero current. Only when current is going out from a phase conductor and returning through the grounding conductor will current be present on the CT.

Therefore, the answer is zero amps, provided no ground fault is present. The fact that only 2 phases are hot has no effect.
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Pretty sure I just said that??

Guys see post #5



Either way your explanation is better than mine.
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Old 02-08-2015, 09:19 PM   #17
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Great discussion guys, keep it going.

I will reveal that, my first thought was that the current seen by the zero sequence setup would be 100 amps.

I was working on the idea that the three phases are 120 degrees apart...so in that, I pictured that the two live phases, being 120 degrees apart would, in the real world timeline of a 360 degree sequence, would never fully cancel each other out (IMHO the only way they would is if they were 180 degrees apart, as in a 120/240v single phase system.) So that left me with the impression that, over a timeline of the full 360 degree sequence, the remaining 120 degrees of "time" in the sequence would mean that each remaining phase, at any given moment, would contribute 50 amps each to the sequential current reading. Thus my 100 amp answer.

I have to add that in that scenario, I was NOT including the neutral as part of the zero sequence metering (which is of course incorrect in the literal sense of the real-world applications of zero sequence metering.)

So, in the real-world, standard practice of zero sequence metering, you all have the correct answer of zero amps.

Now to throw a wrench into the works, and do not answer the poll with this scenario:

Would I be correct in my initial thoughts that the reading would be 100 amps, IF THE NEUTRAL IS NOT INCLUDED IN THE CURRENT TRANSFORMERS, i.e. the metering was using the three line phases ONLY?

I wish I had access to a true 3-phase source (I have a VFD I could probably use, if I can get it to work with resistive loads instead of my $400 3-phase projector motor) to set up a real-world test.
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Old 02-08-2015, 09:26 PM   #18
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Your MX, cant you get Idaho to drag two extra pigs up that pole?
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Old 02-08-2015, 09:37 PM   #19
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Your MX, cant you get Idaho to drag two extra pigs up that pole?
LOL if I had access to it, I could grab the 3ph 480v from the city water well. The pole and pigs are on my property.
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Old 02-08-2015, 10:08 PM   #20
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LOL if I had access to it, I could grab the 3ph 480v from the city water well. The pole and pigs are on my property.
If you feel rebellious enough Id try it

Ask them though, I heard French Electrician (Mark) had his shop zoned out as commercial just for a 3 phase service.

You are an electrician so just tell them you need the 3 phase for lathes, welders and what not.
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