Neutral as a CCC

[Flapjack]

A service entrance has a 3 phase, 4 wire circuit from a 480/277 V transformer. The service entrance has a 3 phase, 3 wire feeder to a panelboard where 80% of the load is made up of VFD's for 3 phase motors. Per NEC 310.15(B)(5) the 3 phase, 4 wire circuit to the service entrance is primarily loaded with non-linear loads. According to the code, wouldn't the service entrance conductors result in 4 CCC's in the conduit?

From what I understand, VFD's produce very little triplen harmonics. The 5th, 7th, 11th, 13th harmonics do not result in neutral current. Does the code address this anywhere and provide exceptions to not count the neutral as a CCC? For example, using active harmonic filters.

 

[tom baker]

Are you asking if a EGC or neutral is required (4 wires)?

 

[Smart $]

A service entrance has a 3 phase, 4 wire circuit from a 480/277 V transformer. The service entrance has a 3 phase, 3 wire feeder to a panelboard where 80% of the load is made up of VFD's for 3 phase motors. Per NEC 310.15(B)(5) the 3 phase, 4 wire circuit to the service entrance is primarily loaded with non-linear loads. According to the code, wouldn't the service entrance conductors result in 4 CCC's in the conduit?

From what I understand, VFD's produce very little triplen harmonics. The 5th, 7th, 11th, 13th harmonics do not result in neutral current. Does the code address this anywhere and provide exceptions to not count the neutral as a CCC? For example, using active harmonic filters.

Here's the exact wording and note my underlined portion...

(c) On a 4-wire, 3-phase wye circuit where the major
portion of the load consists of nonlinear loads, harmonic
currents are present in the neutral conductor; the neutral
conductor shall therefore be considered a current-carrying
conductor.

If there is no line-to-neutral load, there is absolutely zero harmonic currents present in the service entrance neutral. The neutral does not count as a current-carrying conductor.

 

[Flapjack]

I am first asking if the scenario I described requires the neutral to count as a CCC, and then if there are any exceptions if harmonic mitigation is used or when the non-linear loads do not result in neutral current.

The service conductors are 3 phases and the neutral. The transformer (wye-wye) and service entrance panel will each be connected to their own independent grounding electrode systems, so no supply side bonding jumper is routed between the two to avoid a parallel path.

 

[Flapjack]

Here's the exact wording and note my underlined portion...

If there is no line-to-neutral load, there is absolutely zero harmonic currents present in the service entrance neutral. The neutral does not count as a current-carrying conductor.

Smart$, I agree with your second statement that the neutral shouldn't be counted, but I am not understanding how the code states that. The underlined portion didn't clarify it for me. :slaphead:

I read the code as saying that there will be harmonic current on the neutral for my scenario. I have a 3 phase, 4 wire wye circuit. And the majority of the downstream load consists of nonlinear loads.

 

[Dennis Alwon]

Smart$, I agree with your second statement that the neutral shouldn't be counted, but I am not understanding how the code states that. The underlined portion didn't clarify it for me. :slaphead:

I read the code as saying that there will be harmonic current on the neutral for my scenario. I have a 3 phase, 4 wire wye circuit. And the majority of the downstream load consists of nonlinear loads.

In your case, the neutral should be counted if the major portion of the load is nonlinear.

 

[Smart $]

Smart$, I agree with your second statement that the neutral shouldn't be counted, but I am not understanding how the code states that. The underlined portion didn't clarify it for me. :slaphead:

I read the code as saying that there will be harmonic current on the neutral for my scenario. I have a 3 phase, 4 wire wye circuit. And the majority of the downstream load consists of nonlinear loads.

You have a 4-wire, 3-phase wye system.

You do not have a 4-wire, 3-phase wye circuit for you have no line-to-neutral loads. Once again, the exact wording...

(c) On a 4-wire, 3-phase wye circuit where the majorportion of the load consists of nonlinear loads, harmonic
currents are present in the neutral conductor; the neutral
conductor shall therefore be considered a current-carrying
conductor.

 

[Flapjack]

You have a 4-wire, 3-phase wye system.

You do not have a 4-wire, 3-phase wye circuit for you have no line-to-neutral loads. Once again, the exact wording...

Ahhhhh. It makes way more sense now that you put it in bold, italics, and underline.

This makes more sense now. Thanks.

 

[GoldDigger]

Ahhhhh. It makes way more sense now that you put it in bold, italics, and underline.

This makes more sense now. Thanks.

It is extremely likely that the rectifier bridges in the VFDs are all line to line, not involving the neutral.
A helpful double check that does not involve examining all of the various loads is simply to put a clamp meter on the neutral to the panel.
Or if you do not actually run the neutral from the panel to the loads there is no point in even doing that.

 

[Smart $]

...
Or if you do not actually run the neutral from the panel to the loads there is no point in even doing that.

Yeah, I don't see that happening anytime soon. :slaphead:

 

[Carultch]

From what I understand, VFD's produce very little triplen harmonics. The 5th, 7th, 11th, 13th harmonics do not result in neutral current. Does the code address this anywhere and provide exceptions to not count the neutral as a CCC? For example, using active harmonic filters.

What about 6th, 9th, 12th, and 15th harmonics?

Or do they specifically produce prime number harmonics?


3 is prime
5 is prime
7 is prime
9 is experimental error
11 is prime
13 is prime

Just be sure, try some other randomly chosen odd numbers. 23 is prime, 37 is prime, 61 is prime

 

[jumper]

What about 6th, 9th, 12th, and 15th harmonics?

Or do they specifically produce prime number harmonics?


3 is prime
5 is prime
7 is prime
9 is experimental error
11 is prime
13 is prime

Just be sure, try some other randomly chosen odd numbers. 23 is prime, 37 is prime, 61 is prime

According to a couple of sites I just glanced at it is only the odd numbers of three (triplen) harmonics causing the problem with 3rd being the worst. 3, 9, 15, 21, etc.

http://www.slideshare.net/acmetransformer/transformer-harmonics

http://ewh.ieee.org/conf/tdc/eliminating_Harmonic_Neutral_Current_Problems.pdf

 

[don_resqcapt19]

In your case, the neutral should be counted if the major portion of the load is nonlinear.

How do 3 phase loads fed from a panel without a neutral cause the service neutral to carry harmonic currents and be required to be counted as a current carrying conductor?

 

[jumper]

How do 3 phase loads fed from a panel without a neutral cause the service neutral to carry harmonic currents and be required to be counted as a current carrying conductor?

Well, um... ya see it's complicated. It happens when the flux capacitor is out of phase with a full moon and Venus is rising in the South....

 

[GoldDigger]

According to a couple of sites I just glanced at it is only the odd numbers of three (triplen) harmonics causing the problem with 3rd being the worst. 3, 9, 15, 21, etc.

http://www.slideshare.net/acmetransformer/transformer-harmonics

http://ewh.ieee.org/conf/tdc/eliminating_Harmonic_Neutral_Current_Problems.pdf

From the point of view of simple physics, harmonics which are even multiples of three rather than odd multiples of three would also add in the neutral rather than cancelling. We don't end up worrying about them because the common non-linear loads do not generate even harmonics.
An even harmonic would correspond to a difference in shape between the positive and negative going waveforms, and except for a half wave bridge load on single phase that does not happen.

 

[Dennis Alwon]

How do 3 phase loads fed from a panel without a neutral cause the service neutral to carry harmonic currents and be required to be counted as a current carrying conductor?

Easily if you don't read the question thoroughly. I assumed a neutral since he was asking about it.

 

[don_resqcapt19]

Easily if you don't read the question thoroughly. I assumed a neutral since he was asking about it.

He does have a neutral, but all of the non-linear loads are 3 phase line to line.

 

[Besoeker]

What about 6th, 9th, 12th, and 15th harmonics?

Or do they specifically produce prime number harmonics?


3 is prime
5 is prime
7 is prime
9 is experimental error
11 is prime
13 is prime

Just be sure, try some other randomly chosen odd numbers. 23 is prime, 37 is prime, 61 is prime

VFDs with a three phase input rectify the voltage to 6-pulse DC. The produce 6n+/-1 harmonics. So 5th, 7th, 11th, 13th, 17th, 19th etc.

 

[GoldDigger]

What about 6th, 9th, 12th, and 15th harmonics?

Or do they specifically produce prime number harmonics?


3 is prime
5 is prime
7 is prime
9 is experimental error
11 is prime
13 is prime
[The hypothesis is probably true. ]

Just be sure, try some other randomly chosen odd numbers. 23 is prime, 37 is prime, 61 is prime

That sequence, for those who have not heard the full joke, is how a physicist reacts when asked whether all odd numbers are prime.
The (electrical) engineer's response is:

3 is prime
5 is prime
7 is prime
9 is prime
11 is prime
13 is prime
The hypothesis is true.

 

[ggunn]

That sequence, for those who have not heard the full joke, is how a physicist reacts when asked whether all odd numbers are prime.
The (electrical) engineer's response is:

3 is prime
5 is prime
7 is prime
9 is prime
11 is prime
13 is prime
The hypothesis is true.

9 is not prime.

The way I heard it was thus:

The engineering proof that all odd numbers are prime:
1..., 3..., 5..., 7..., yep.