Why use a seventh A/D with a 3 phase energy meter?

A 3-phase 4-wire system only needs three CTs and three voltage pickoffs to meter energy (Blondel's Theorem.) The CT on neutral can provide extra info on harmonic imbalances, tampering, stuff like that.

John

The current sense on the neutral? That can be used to detect several things:

a. Imbalance in 3-phase loads.

b. A loose neutral or ground faults where the sums don't compute (potentially dangerous situation).

c. As John mentioned, tampering. For example if a pot growing operations try to make the power consumption look sort of normal and not attract "attention".

How do use the neutral current measurement to detect tampering?

Howard

When the sums don't add up. Probably these days the DEA guys would show up quite soon after ;-)

If the three line currents don't equal the neutral current, someone may have fiddled with one of the CTs or something.

John

In a clean 3-phase motor there wouldn't be any neutral current. In fact, it's not even connected to neutral. Same if you connected a 208V load between phase A and B but not C.

an

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Sorry men I keep thinking of the case where phase A is drawing 10 amps at 0 degrees, phase B is drawing 100 amps at 120 degrees and phase C is drawing 100 amps at -120 degrees. The net result is 90 amps in the neutral at a phase of 180 degrees. This may be an extreme case but it is possible if 3 phase power is used for lighting. Hence I conclude a significant neutral current may be suspicious but it is certainly not conclusive tampering has occurred. John I know you used the word may. But if a technique is used to detect tampering then I was expecting something a lot more conclusive.than a suspicion of tampering.

Howard

I said that one could compare the sum of the line currents to the neutral current. It would be a realtime (instantaneous/vector) comparison, not just magnitude. It would detect paths around CTs (including adding a shorting loop) and also catch line-to-ground faults.

Neutrals can get fried by nasty harmonic loads; that can be detected by a neutral CT too.

John I know you used the word

Suspicion is the first step to investigation and proof.

John

Fascinating.

John

Sorry men I keep thinking of the case where phase A is drawing 10 amps at 0 degrees, phase B is drawing 100 amps at 120 degrees and phase C is drawing 100 amps at -120 degrees. The net result is 90 amps in the neutral at a phase of 180 degrees. This may be an extreme case but it is possible if 3 phase power is used for lighting. Hence I conclude a significant neutral current may be suspicious but it is certainly not conclusive tampering has occurred. John I know you used the word may. But if a technique is used to detect tampering then I was expecting something a lot more conclusive.than a suspicion of tampering.

Actually the sums are still correct. 10 + 100 + 100 = 90 in three phase.

More accurately, 10cos(0) + 100cos(120) + 100cos(240) = 10 -50 -50 = -90

And you could have those readings without a neutral.

Paul

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Thanks John for taking the time to provide the extra explanation. I got off on the wrong track thinking it was just the presence of neutral current that cause a suspicion of tampering. Whereas it was really a comparison of the vector sum of the three line currents with the neutral current. That sounds like an interesting measurement and programming problem that must be put off for some other time.

Howard

Just simultaneous-sample the four currents periodically and compute the RMS of A+B+C+N.

John

On a sunny day (Mon, 27 Jul 2009 16:58:30 -0700) it happened Joerg wrote in :n a clean 3-phase motor there wouldn't be any neutral current. In fact,

The old system in Amsterdam was like that. Got me when I was designing a SCR controller in the sixties. On the workbench there were 3 power outlets, all three between different phases :-) "Where does that phase shift come from ????"

The system has now changed I think, probably all of it, to use neutral. From:

De overgang naar 220/380 is nog steeds niet helemaal voltooid. Volgens netwerkbeheerder Noord West Net waren er in 2003 in Amsterdam nog enkele 127/220 V netdelen die gesaneerd moesten worden.[1] De huisaansluitingen zijn hier tussen de fasen geschakeld, zodat deze gebruikers wel 220 (thans 230) volt op het stopcontact hebben.

So seems in 2003 still some between the phases ;-)

But then the sum of the phase currents will also be zero.

IOW, if the readings appear to violate Kirchoff's current law, someone has added an extra wire.

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Thanks for the hint John. Using your hint it appears all you need to do is to simultaneous sample and compute the sum of the currents (A + B + C - N) at each sampling instant. Or if you wanted to get fancy you could also compute a running average or the RMS value of (A + B + C - N) for one second periods.

Howard

clean 3-phase motor there wouldn't be any neutral current. In fact,

Wow, 127V in Europe? In this century? I didn't know that.

Yep, that's one of the things the extra current sense is for.

On another note, I had a technical discussion with a guy (chemist) one day and then told him that Kirchhoff's law wouldn't allow implementation of an idea he'd had. "Ahm, isn't there any way around Kirchhoff's law?" Scratching my head and thinking about whether there'd be any rogue countries where Kirchhoff doesn't apply I had to tell him "no" ;-)

Pffbt! Do what the physicists do: blast a cathode ray into nullspace. ;-)

Nevermind that, as the device continues to spew off electrons, it gains a positive charge, until the field becomes so great that the electrons simply fall back onto it (obviously, the barrier being how many volts you put on the electron gun). Or on Earth, it'll just slink back along the ionized air path, yielding an even lower potential.

Still, it would be fun to perform half cell reactions one at a time.

Tim