200% Neutrals

[don_resqcapt19]

That's where the neutral was connected.

I still don't understand how a 3 phase load puts current on the neutral.

 

[Besoeker3]

I still don't understand how a 3 phase load puts current on the neutral.

The harmonics are fed into the neutral.

 

[winnie]

1) If you had a pure third harmonic load, no net power would be delivered to the load.

Say you have a perfect sine voltage source connected to a bizarre load that draws perfect 3rd harmonic current. Because of the orthogonality of sine functions the integral of voltage * current would be zero, no power delivered.

2) If a load has no connection to the neutral it can't inject harmonics on the neutral.

A large three phase rectifier with no neutral connection could not draw any third harmonic current.

I could imagine some topologies where a drive had a neutral connection.

Beo: could you explain how these particular drives were connected to the neutral?

Jon

 

[Besoeker3]

1) If you had a pure third harmonic load, no net power would be delivered to the load.

Say you have a perfect sine voltage source connected to a bizarre load that draws perfect 3rd harmonic current. Because of the orthogonality of sine functions the integral of voltage * current would be zero, no power delivered.

2) If a load has no connection to the neutral it can't inject harmonics on the neutral.

A large three phase rectifier with no neutral connection could not draw any third harmonic current.

I could imagine some topologies where a drive had a neutral connection.

Beo: could you explain how these particular drives were connected to the neutral?

Jon

Just to the neutral. The first harmonic is the 5th, not third.

 

[GoldDigger]

Agreed that if there is no L-N load there is no need for a neutral at all, let alone a 200% neutral.

 

[winnie]

Just to the neutral. The first harmonic is the 5th, not third.

But was the rectifier connected to the neutral, or a capacitor center tap, or some other part of the drive? What was the circuit topology that connected the drive to the neutral?

Or are we talking indirect coupling, where the drive has no neutral, but drive harmonics distort the supply voltage which then causes harmonic current on connected line-neutral loads?

5th harmonic is not a zero sequence and doesn't add on the neutral.

Thanks
Jon

 

[Besoeker3]

5th harmonic is not a zero sequence and doesn't add on the neutral.

Thanks
Jon

Then maybe I am wrong. We definitely used harmonic filters.

 

[GoldDigger]

Then maybe I am wrong. We definitely used harmonic filters.

Even though they do not cause neutral current, non-triplet harmonics are still consdiered objectionable when they cause current harmonics to POCO or voltage harmonics on other loads. Hence the need for filtering.

 

[Besoeker3]

Even though they do not cause neutral current, non-triplet harmonics are still consdiered objectionable when they cause current harmonics to POCO or voltage harmonics on other loads. Hence the need for filtering.

Exactly.

 

[SceneryDriver]

Back in the days of incandescent dimming for stage lighting, we ran 6-wire to the dimmer racks - we doubled the neutral cam-lock cable - to account for harmonics. Phase-angle dimming can create nasty harmonics, especially when it was scaled up to the size of large arena rock shows. It's much better now that practically all lighting is LED and arc-source and the power supplies are pretty much all PF controlled.

I still don't like dragging 4/O feeder all over hell's half acre, but I have people for that now


SceneryDriver