Nonlinear loads and neutral current

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mbrooke

mbrooke

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If all none linear loads are connected phase-to-phase, will a wye grounded wye grounded transformer still pass neutral current? And to what degree?
 
Three phase non linear loads are almost always balanced and won’t produce triplens unless there is a voltage imbalance.
5,7,11,and 13 may be there, but they won’t pass neutral current because they aren’t connected to neutral.
 
Hv&Lv said:
Three phase non linear loads are almost always balanced and won’t produce triplens unless there is a voltage imbalance.
5,7,11,and 13 may be there, but they won’t pass neutral current because they aren’t connected to neutral.
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Correct, but the LV windings are in theory none linear, so the triplens should add and produce primary HV neutral current.
 
mbrooke said:
Correct, but the LV windings are in theory none linear, so the triplens should add and produce primary HV neutral current.
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Phase to phase loads with voltages at the same magnitude should produce very little triplen currents, if any. Three phase UPS systems are a good example.
 
mbrooke said:
If all none linear loads are connected phase-to-phase, will a wye grounded wye grounded transformer still pass neutral current? And to what degree?
Click to expand...

Zero neutral current in this situation; in my opinion, none on the primary side, either..
 
190929-0222 EDT

I don't think "none linear" is very good term to use. Usually what I believe is being called "none linear" would be called "nonlinear".

I don't believe an accurate statement of the problem has been provided.

I believe what is being asked is:

Everything is finite, except we can have some infinite impedances.

Given three two terminal voltage sources with one terminal of each source connected at a common point.
And there is no need for any correlation of voltage or frequency between any of the sources.
And there is infinite impedance from any hot terminal to common, or anything else, other than the following delta load..

What is the net current at the common point assuming any finite delta load on the three hot lines? The obvious answer is zero.

Now connect that common point to earth. Is there any current to earth from the common point? Again the obvious answer is zero based on the stated assumptions.

.
 
Right.To restate compactly:
In a wye load situation, a single load connected L to N will always produce neutral current, whether it is non-linear or not. The non-linearity comes in with a balanced load on all three L-N connections. With a linear load and balanced voltages, the neutral currents will cancel. With a non-linear load set, any harmonics which are a multiple of 3 will add instead of canceling.
When the loads are L-L, there is no neutral current from any of the loads, so there is nothing to add, let alone cancel or not cancel.
If you had instead a wye primary and delta secondary, the LL loads are reflected as L-N loads on the primary and you have a possibility of neutral current on the primary side. The problem is that triplen harmonics would end up as circulating current on the delta side, and will not necessarily make it back to the primary.
 
GoldDigger said:
Right.To restate compactly:
In a wye load situation, a single load connected L to N will always produce neutral current, whether it is non-linear or not. The non-linearity comes in with a balanced load on all three L-N connections. With a linear load and balanced voltages, the neutral currents will cancel. With a non-linear load set, any harmonics which are a multiple of 3 will add instead of canceling.
When the loads are L-L, there is no neutral current from any of the loads, so there is nothing to add, let alone cancel or not cancel.
If you had instead a wye primary and delta secondary, the LL loads are reflected as L-N loads on the primary and you have a possibility of neutral current on the primary side. The problem is that triplen harmonics would end up as circulating current on the delta side, and will not necessarily make it back to the primary.
Click to expand...

One of the divisions of our corporation did controlled lighting. They had a project for a large hotel in the Middle East, several MW of dimming. Overheated neutral resulted. I got drafted in because of my experience of harmonics on power electronics. Got me a trip to Dubai.
 
Besoeker3 said:
One of the divisions of our corporation did controlled lighting. They had a project for a large hotel in the Middle East, several MW of dimming. Overheated neutral resulted. I got drafted in because of my experience of harmonics on power electronics. Got me a trip to Dubai.
Click to expand...

230/400Y?
 
gar said:
190929-0222 EDT

I don't think "none linear" is very good term to use. Usually what I believe is being called "none linear" would be called "nonlinear".
Click to expand...

Question- what is the difference? :dunce: Linear to me means sinusoidal or sine wave.

I don't believe an accurate statement of the problem has been provided.

I believe what is being asked is:

Everything is finite, except we can have some infinite impedances.

Given three two terminal voltage sources with one terminal of each source connected at a common point.
And there is no need for any correlation of voltage or frequency between any of the sources.
And there is infinite impedance from any hot terminal to common, or anything else, other than the following delta load..

What is the net current at the common point assuming any finite delta load on the three hot lines? The obvious answer is zero.

Now connect that common point to earth. Is there any current to earth from the common point? Again the obvious answer is zero based on the stated assumptions.
Click to expand...


For a linear load.

Look at it like this: "balanced" nonlinear loads on each phase to neutral will produce current, and that current will appear on the primary neutral in proportion to the transformer's voltage ratio.

Now, take those same loads and place them in delta. What happens? Does current appear on the primary neutral?
 
The original post is asking what happens when you have only L-L loads but fed from a wye:wye transformer. I think that this combination leads to some unexpected results.

1) When we talk about nonlinear loads, we are talking about loads where the current flowing through the load does not vary in a linear fashion with the applied voltage. This introduces harmonic currents to a system even when the applied voltage is perfectly sinusoidal.

2) In the normal situation with nonlinear L-N loads on a 3 phase system, we care about the 'triplen' harmonics, because these are the ones that _add_ on the neutral instead of balancing. In a normal 'MWBC' you supply the different legs from different phases so that the neutral currents will tend to balance out. But even supplied from different phases the 'triplen' harmonics add, as if the MWBC were supplied from the same phase.

3) I would need to do more analysis, but my _guess_ is that if you had balanced 3 phase L-L loads, supplied by a wye secondary, that load induced triplen current would circulate through the loads, and _not_ couple to the transformer at all.

4) If you have a wye:wye transformer any triplen current that does flow through the secondary would show up as triplen current on the primary.

-Jon
 
190929-0745 EDT

mbrooke:

A nonlinear load has nothing to do with sinusoidal.

A nonlinear load is one where the load v-i curve is not a straight line.

For a linear load the instantaneous shape of the current curve will match the shape of the applied voltage curve, not so of a nonlinear load.

.
 
winnie said:
The original post is asking what happens when you have only L-L loads but fed from a wye:wye transformer. I think that this combination leads to some unexpected results.

1) When we talk about nonlinear loads, we are talking about loads where the current flowing through the load does not vary in a linear fashion with the applied voltage. This introduces harmonic currents to a system even when the applied voltage is perfectly sinusoidal.

2) In the normal situation with nonlinear L-N loads on a 3 phase system, we care about the 'triplen' harmonics, because these are the ones that _add_ on the neutral instead of balancing. In a normal 'MWBC' you supply the different legs from different phases so that the neutral currents will tend to balance out. But even supplied from different phases the 'triplen' harmonics add, as if the MWBC were supplied from the same phase.

3) I would need to do more analysis, but my _guess_ is that if you had balanced 3 phase L-L loads, supplied by a wye secondary, that load induced triplen current would circulate through the loads, and _not_ couple to the transformer at all.

4) If you have a wye:wye transformer any triplen current that does flow through the secondary would show up as triplen current on the primary.

-Jon
Click to expand...

What if it wasn't balanced? Still connected phase to phase, but just various nonlinear loads drawing current in various magnitudes. Would the still be no current on the primary neutral?
 
gar said:
190929-0745 EDT
A nonlinear load has nothing to do with sinusoidal.

A nonlinear load is one where the load v-i curve is not a straight line.
Click to expand...

I am going to agree with the above statement being technically correct but disagree with it as a practical matter.

I absolutely agree that 'non-linear load' simply means that the V-I curve is not a straight line. There are a huge number of such loads, for example LEDs supplied from DC supplies. No sine waves there!

However in the _context_ of AC power distribution, 'non-linear load' has taken on the meaning of 'a load that injects harmonic currents into the power system'. If you apply a sinusoidal voltage to a non-linear load, the current which flows will not follow that sine curve. Since AC power distribution almost invariably attempts to provide a sinusoidal AC supply it is perfectly reasonable to focus on the (non) sinusoidal aspect of things.

Going back to the original question, gar's point is quite relevant. It might be helpful to think of the load not as injecting harmonics, but rather its non-linear V-I curve, and compare this to the _instantaneous_ voltages between the phases. I'll be pondering this for a bit :)

-Jon
 
190929-1251 EDT

winnie:

Sixty years ago, if I looked at my supply voltage on a scope, then the voltage looked like a sine wave. I believe I still have pictures somewhere. Today the waveform visually appears distorted. The top is rounded off. It is not a huge amount, but visually observable. But clearly not the severe distortion that you see in the current waveform that is the cause.

In some inverters you see a squared voltage waveform. Probably should be called pulsed. The waveform is basicly 0, then jumps to about 180 V, remains for a time, then returns to 0. Next the same occurs but with reversed polarity. This is a symmetrical wave. The on time of the pulses in relation to the cycle time is adjusted such that the RMS to peak ratio is the same as a sine wave, 0.707 . This will produce the same heating effect in a resistance as a sine wave RMS voltage will, but will provide the peak voltage needed by capacitor input rectifier filter circuits. A huge harmonic content.
 
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