Odd question - unbalanced load

[mityeltu]

I was talking with a colleague in civil eng. and he was asking about balanced vs unbalanced conditions. He understaood the idea of balanced and unbalanced conditions in both siource and load, but he asked a question I couldn't answer:

If the load is unbalanced in onlyy 1 phase (say, phase c), is it possible to rebalance the system by deliberately adding an unbalanced condition to the source?

I know you wouldn't WANT to, but is that possible? I mean, if the idea is to eliminate the undesirable ground currents, is this THAT bad a solution? You'd get circulating currents in your source, right? Shortening the sources life?

 

[Jraef]

Hmmm... I'm having a little trouble grasping this.

Unbalanced loading causes increased heating in the distribution transformer. So if you "add more unbalance" to the distribution transformer, you are balancing the system, no?

 

[mityeltu]

I don't know, what I was asked was, if there is an unbalance in the in the load on phase c (this causes undesireable ground currents that are a threat to personnel), is it *possible* to deliberately unbalance the source to *counter* the unbalanced load and eliminate the ground current?

I know it's nuts, but as an exercise in academics, he posed the question and I thought it would be interesting to find out.

 

[SAC]

I don't know, what I was asked was, if there is an unbalance in the in the load on phase c (this causes undesireable ground currents that are a threat to personnel), is it *possible* to deliberately unbalance the source to *counter* the unbalanced load and eliminate the ground current?

I know it's nuts, but as an exercise in academics, he posed the question and I thought it would be interesting to find out.

Not in any practical sense, no. The "imbalance" is that phase c is drawing more current than the other phases, resulting in increased neutral current. The only way to balance the situation would be to reduce the phase c current, or increase the other phase currents. The only real way for the source to "compensate" for the imbalance would be to reduce the voltage on phase c so that the resulting current on phase c, and into the neutral, is reduced (assuming a linear load). The net effect is reducing the power delivered to phase c loads - probably not what is desired.

 

[dkarst]

I don't know, what I was asked was, if there is an unbalance in the in the load on phase c (this causes undesireable ground currents that are a threat to personnel), is it *possible* to deliberately unbalance the source to *counter* the unbalanced load and eliminate the ground current?

I know it's nuts, but as an exercise in academics, he posed the question and I thought it would be interesting to find out.

I'm struggling to understand your question a bit so let's try an example. For simplicity we have a wye connected source 208Y/120 3P4W feeding a simple 3 phase panel with a 3 phase motor load (assume balanced) and with a few single phase lighting circuits. All the lights aren't on right now, therefore the phases are not balanced. I lose you when you state you have "undesireable ground currents that are a threat to personnel"... it is true you have some current returning on the neutral conductor but you shouldn't have any ground potential problems unless I'm missing something? Now if you turned all the lights on and you had perfect balance, the neutral current may decrease... how does this help? Hope I helped but maybe not...

 

[roger]

I don't know, what I was asked was, if there is an unbalance in the in the load on phase c (this causes undesireable ground currents that are a threat to personnel),

Can you explain your reasoning of that statement?

The reality is, unless all loads supplied (speaking wye source) are A-B-C with no neutral loads, the system will have some unballance which is perfectly normal and does not create any hazzards.

Roger

 

[charlie b]

. . . this causes undesireable ground currents that are a threat to personnel. . . .

No it doesn't. You might have been using the phrase "ground currents" in either of two contexts, and the statement is not true for either context.

The phrase "ground currents" might mean current through planet Earth, along the dirt beneath our feet. An imbalance in loads will not create a path for current to flow in the dirt.

The phrase "ground currents" might also mean current through the equipment grounding conductors, bonding jumpers, conduits, and external metal enclosures that are all bonded together to provide a path for fault current to return to the source. An imbalance in loads is not the same as having a live metal part come into contact with the metal case of any component, and will therefore not create a path for fault current to flow back to the source.

 

[mityeltu]

Then I must be missing something fundamental. In looking strictly at balanced vs unbalanced systems: a balanced system provides zero curretn in the neutral (ground) because the sum of the 3 phase currents is zero, right?

Well, if the system is unbalanced, do I not have neutral (ground) current? While I realize if the NEC is followed and the ground at any particular piece of equipment is an "effective ground" there will be negligable step/touch potentials and hence no real danger to personnel, the fact should remain that there is a retun current *somewhere*. Is this not correct?

If there is a neutral (ground) current, then I was asking if there was a way, preposterous as it might be, to rebalance the system from the source end to compensate for an unbalance in the load. I know it would not happen, that is I would not design the system to do this, but as an exercise in engineering, I was wondering if it was possible after I was asked by a colleague.

 

[charlie b]

Then I must be missing something fundamental.

You are. It is here:

a balanced system provides zero current in the neutral (ground) because the sum of the 3 phase currents is zero, right?

This statement is correct, except that you are somehow equating the neutral to "ground." That is a fundamental error. The two words do not mean the same thing, and there are separate bus bars and conductors associated with each. Most of the time, the phrase "neutral conductor" and the phrase "grounded conductor" carry the same meaning, but pay attention to the "-ed" at the end of the word "grounded." That is not the same word you are using.

Once again, I will say that the word "ground" is used in two separate contexts, and I don't know which you have in mind. But here again, an imbalance will cause current to flow in the neutral, but that will not cause current to flow in the "Grounding Electrode Conductor" (i.e., the wire that connects the center point of the transformer to planet Earth) or in the "Equipment Grounding Conductor" (i.e., the wire that connects the case of equipment back to the ground bar at the main panel, and that carries current only during a fault).

 

[mityeltu]

Aren't the neutral bus and ground bus tied together and then grounded via a grounding electrode?

Then if there is an unbalanced current, it will follow the path of least impedance in its retun to the source. Assuming the neutral is carried as you said in a 3p4w system, that path will be the neutral, right? But even if the path of least impedance is the neutral, the path between the source and load still has a parallel path in the earth. And since the neutral conductor is grounded to earth via a grounding electrode, won't some of the current flow in the earth?

Either way, back to my origianl question, is it *possible* to rebalance this syetm to eliminate this current from whatever path it is taking?

 

[charlie b]

The neutral and the ground are connected to each other in the main panel, but not again anywhere else within the system. Current in the neutral will not be accompanied by current in the earth, because there is not a complete path. In order for two paths to be in parallel, they have to be connected to each other at both ends. We have a connection only at one end: the main panel, or if you prefer, the source.

If for example there is a three phase, four wire load, and if within that load the currents are higher in phases A and B than the current in phase C, then there will be some current in the neutral wire serving that load. But where, within that heater, is there a path to planet Earth? There is a connection at the main panel, but once current gets to that location it will have already reached its source. There is nothing that will drive current from that point, along earth, towards the source, because it is already at the source. Planet Earth is not a player in this discussion; there is no current flowing through the earth, and there is no safety concern, no threat to life or health.

 

[charlie b]

Either way, back to my original question, is it *possible* to rebalance this system to eliminate this current from whatever path it is taking?

Yes.

Example: Consider a 120/208 volt panel with several loads connected and running. Measuring at the panel, I get a load of 40 amps on Phase A, 20 amps on Phase B, and 20 amps on Phase C. I can either calculate or measure the current on the neutral, but it will be 20 amps in either case. I now connect a new load, a single phase, 208 volt load, with a rating of 4.16KVA (equivalent to 20 amps at 208 volts). I feed it from a 2-pole breaker that gets its power from phases B and C. I turn it on and measure again. I see Phases A, B, and C each have 40 amps, and the neutral has 0 amps. Mission accomplished.

Now the sad news. Two minutes later, someone turns off the lights in one room, and that causes the total load at the panel to become imbalanced again. Oh well. I tried!

 

[mityeltu]

Great. Thank you. Between the 2 of you, my guestion(s) have been answered. Sorry for the confusion along the lines of grounding (pun intended).

Now, let me twist a little. If there *wer* a ground rod at the load in addition to the one at the source, wouldn't there be a gound current (albeit it very small because of the large ground impedance compared to that of the neutral conductor between source and load)?

I ask becasue in my reading of the IEEE 142 I seem to recall seeing a non-separately derived system for a UPS that was fed from a 3p4w source and both source transformer and UPS were grounded to earth ground.

I reference page 56 from the 2007 revision. Figure 1-39

 

[LarryFine]

Then if there is an unbalanced current, it will follow the path of least impedance in its retun to the source. Assuming the neutral is carried as you said in a 3p4w system, that path will be the neutral, right? But even if the path of least impedance is the neutral, the path between the source and load still has a parallel path in the earth. And since the neutral conductor is grounded to earth via a grounding electrode, won't some of the current flow in the earth?

On the power company's side of the service main, this is true. On our side is where the grounding system begins.

If the neutral is grounded at the utility transformer (it is), and again at the service, some of the neutral current travels through the earth.

The same is true for the primary systems, too. It's normal, accepted, and unavoidable.

 

[charlie b]

Now, let me twist a little. If there *were* a ground rod at the load in addition to the one at the source, wouldn't there be a ground current (albeit it very small because of the large ground impedance compared to that of the neutral conductor between source and load)?

I suppose that would depend on what the ground rod was connected to. If you connected it to the case of the equipment (i.e., in parallel with the EGC), it would have no effect whatsoever. But I think you are talking about connecting it to the neutral.

Most items of equipment do not have a separate neutral bar, like a panel would have. But if you could find a way to run a wire from a ground rod to the point within a load at which the neutral wire would be connected, then two things would happen. First, you will have created a code violation. Second, there would be current flowing through planet Earth. I agree that it would be a small current, but (perhaps) not for the reason you gave. There is a significant impedance between the ground rod and the dirt within which it is installed. But do keep in mind that the dirt itself has a very small resistance, when you are talking about an entire planet's worth of dirt, all of which is in parallel with itself.

I don't have a copy of the IEEE standard to which you refer, so I cannot answer that part of your question.

 

[BJ Conner]

NO. IF you change primary voltages you could change secondar currents, but then some of the secondary voltage would be unballanced.
The best you can do is swap single phase loads on the secondary till thing are "close enough". When it gets to the 600MW generator out at the power plant it's ballenced enough.

 

[Jack Linville]

The ground electrode is intended to clamp the voltage level of the grounding system and the neutral conductor at the bonding jumper to ground potential. For current to flow in the ground rod, some sort of potential would need to exist across it. That would only happen of there were a defect in the system such as an un-bonded enclosure shorted to a phase conductor and having no return path to the neutral except through the earth...a system defect IMO.

Now back to the imbalance question. An unbalanced load is effectively unbalancing the whole system. Theoretically it would be possible to unbalance the source in a complementary fashion to achieve perfect balance and eliminate the neutral current. It would be impractical to do so as well as very difficult.

Hope this helps.

 

[glene77is]

Theoretically it would be possible to unbalance the source

Jack,
You have my curiosity up.
Hypothetically, just as an exercise in engineering,
how might this be done.

 

[topgone]

Guys, I did this scenario.Please check if my calcs are correct.
Load:
Phase "A" to neutral = 5,000 watts
Phase "B" to neutral = 5,000 watts
Phase "C" to neutral = 4,000 watts

Supply voltages:
Vab = 128.27 /30 deg. volts
Vbc = 128.27 /-90 deg. volts
Vca = 102.62 /150 deg. volts

The resulting line amps = 67.51 amps (balanced)

 

[rattus]

Guys, I did this scenario.Please check if my calcs are correct.
Load:
Phase "A" to neutral = 5,000 watts
Phase "B" to neutral = 5,000 watts
Phase "C" to neutral = 4,000 watts

Supply voltages:
Vab = 128.27 /30 deg. volts
Vbc = 128.27 /-90 deg. volts
Vca = 102.62 /150 deg. volts

The resulting line amps = 67.51 amps (balanced)

Theoretically this could be done at only one point in the system, say a substation. The hundreds of loads served by the substation could not be balanced in this way.

You might have to shift the phase angle as well as adjust the voltage.

Athough impractical, it is a question to make one think.