Help a mechanical engineer who is playing an electrical engineer

[winnie]

If you short the output right at the source, then you won't have significant voltage to measure at the source. But if you connect your bus bar assemblies to the source, and short at the _far_ end of the bus bar assembly, then at the _source_ side of the bus bar you should be able to measure voltage.

Gar's calculations are an estimate of what the voltage will be, with variations for different connections.

-Jon

 

[Smart $]

150916-0723 EDT
...
With this delta closed the calculated measured resistance between any two terminals are:
Rab = 0.6774 ohms
Rac = 0.6774 ohms.
Rbc = 0.7097 ohms.
....
Instead of closing the delta just short one pair of bus bars at a time, then for:
Between A and B 1.0 ohms.
Between A and C 1.0 ohms.
Between B and C 1.1 ohms.

The results are:
Rab = 2.1 ohms
Rac = 2.1 ohms.
Rbc = 2.0 ohms.
...
Someone ckeck my math.
....

I'd have to check back through all the details, and may still not be able to provide a different calculated result without additional information, but...

Looking through your numbers, I can't quite fathom how say Rab ohms double when only one pair of bus is shorted... i.e. you stated ohms between A and B is 1.0 ohms, yet Rab result is 2.1 ohms???

I think somewhere in the calculations, one would have to account for source impedance because the load impedance is so low.

Anyway, sounds like Girswald has made enough progress to make this possibly moot for his end purpose... perhaps not for future endeavors or other readers in same or similar situation.

 

[gar]

150916-2352 EDT

Smart $:

If the test is on two bus bars only that are shorted together at the far end, then at the input end there are two resistances in series. When only two bus bars are shorted together the load is an open delta.

The Rab resistance of 2.1 ohms results from the series resistance of the 1.0 ohm resistor from point A to C that is added to the 1.1 ohm resistor from point B to C when the 1.0 ohm resistor from A to B of the delta load is removed by not shorting bus A to Bus B at the far end.

.

 

[gar]

150916-2415 EDT

The purpose of this test has not been clearly defined, but I assume it is to look for hot spots. The only likely hot spots are where one section of the bus bar is joined to another segment. I have to also assume that bus bars installed have been designed for whatever the expected currents are.

I would look for bad connections before doing the high current AC test. To do this preliminary test use DC at possibly a 100 A level on individual bars. It is easy to measure DC at microvolt levels over long distances and not have problems from stray magnetic fields. This is not feasible with the high current AC test.

Probing the bus bar under test at various points can farly easily identify bad joint problems. Bad joints can be fixed before doing the high current AC test.

.

 

[Smart $]

150916-2352 EDT

Smart $:

If the test is on two bus bars only that are shorted together at the far end, then at the input end there are two resistances in series. When only two bus bars are shorted together the load is an open delta.

The Rab resistance of 2.1 ohms results from the series resistance of the 1.0 ohm resistor from point A to C that is added to the 1.1 ohm resistor from point B to C when the 1.0 ohm resistor from A to B of the delta load is removed by not shorting bus A to Bus B at the far end.

.

I'm still lost... :happyyes:

I don't think it matters anyway. I don't believe you can test just two buses shorted. The point of thermal rise equilibrium would not be the same as all three buses conducting (i.e. shorted).

 

[Sahib]

To answer some of the questions posed, I will refer to the current generator wiring diagram:

Incoming to our lab is 460VAC, 3P 60Hz, 200A, that runs into a dry-type transformer that is delta to wye with lighting circuit (unused) 150kVA 460/230, the neutral is not used and simply goes to a ground bar at the final current generator. Next step is that the 3 phases go to the variable transformers which are rated at 240 to 0-240VAC. The 3 phases then connect to the final large current transformers, T1 and T2 go to one, T2 and T3 go to the second and T3 and T1 go to the third. No neutral is available. The output of this current generator is a stack of 3 per phase 6 x 1/4 copper bus with shrink tube covers. Again, no neutral bus is available, but we do have a 250MCM lug wired back to the original dry type transformer that is used as a ground for the metering and indicating lights on this system.

If the final current transformers secondaries are connected in star, then the shorted ends of the bus bars could be connected to the star point of the final current transformers secondaries. So, Girswald, is it possible to describe how the terminals of the final current transformers secondaries are connected with the stack of 3 per phase 6 x 1/4 copper bus with shrink tube covers?

Would I need to size the ground/neutral bus and its connections per the amperage that we are developing, i.e. do I need 3 @6 x 1/4 bus bars for it?

Only half that size may be required for the neutral but it has to ascertained if any neutral point on the secondary side of the final current transformers is available.

 

[Girswald]

If the final current transformers secondaries are connected in star, then the shorted ends of the bus bars could be connected to the star point of the final current transformers secondaries. So, Girswald, is it possible to describe how the terminals of the final current transformers secondaries are connected with the stack of 3 per phase 6 x 1/4 copper bus with shrink tube covers?

From the wiring diagram, the X1 of transformer 1 is connected to the A phase bus output, the X1 of transformer 2 is connected to the B phase output and the X1 of transformer 3 is connected to the C phase output. The X2's of all three transformers are connected together with the same bus. I took a couple of pics. showing a single phase and an overall showing the bus connecting the X2's and the output bus below.

 

[gar]

150917-0945 EDT

Smart $:

I am not suggesting the measurement of two bus bars in series as a means of doing the thermal rise measurement. Rather I am suggesting a way to to look for an unbalance in the resistances of the individual bus bars, and correcting the unbalance before doing the thermal rise measurement. This should be a quicker and easire means of finding potential major hot spot problems before the final test. And I suggested a DC measurement of resistance and voltage measurements to find high resistance locations.

.

 

[gar]

150917-0955 EDT

Girswald:

Your voltage sources (current transformers) are connected as a wye (star) source. Your X2s are the neutral point.

To bring the source neutral point to the far end of the horizontal bus bars will require a fourth bus bar or an equivalent cross-section of copper wire and then make your load a wye circuit (shorting bars or cables) instead of your present delta layout. If it is a cable and not another bus bar, then there are magnetic field problems created. If the unbalance is not large, then a smaller neutral could be used. Further if the horizontal bus is 1000 ft long this gets messy, and expensive unless the bus system already includes a neutral.

I believe my DC resistance measurement method would help get you to your final test quicker. This only requires small wires (test leads) for voltage measurements. #22 wire would be more than adequate for a test lead.

.

 

[Smart $]

150917-0945 EDT

Smart $:

I am not suggesting the measurement of two bus bars in series as a means of doing the thermal rise measurement. Rather I am suggesting a way to to look for an unbalance in the resistances of the individual bus bars, and correcting the unbalance before doing the thermal rise measurement. This should be a quicker and easire means of finding potential major hot spot problems before the final test. And I suggested a DC measurement of resistance and voltage measurements to find high resistance locations.

.

With the source being a wye configuration, just a phase/neutral loop would do better yet IMO. A floating neutral at far end is only problematic when there is imbalance in the system. The substantial point would shows itself more obviously if the far end neutral point is connected to the source.... BUT as you mentioned in your next post, getting the neutral to the far end of the buses may be riddled with challenges the output configuration is not set up for.

 

[Sahib]

Girswald:
The X2 in your final current transformers is the neutral. With a AC voltmeter, you measure the voltage between the shorted ends of the bus bars and the X2. By adjusting the steel wraps of the bus bars, you could reduce this voltage: if you bring down this voltage close to zero, at least less than 5V, equality of currents in the R Y B bus bars may be achieved to solve your problem of

The bad part is that each phase also adjusts the other phases, so its a real game to try to keep everything balanced.

 

[Smart $]

Girswald:
... if you bring down this voltage close to zero, at least less than 5V...

IIRC this is a low voltage high current implementation, like a 24V nominal system voltage. 5V difference would be huge.

 

[Sahib]

IIRC this is a low voltage high current implementation, like a 24V nominal system voltage. 5V difference would be huge.

Yes, but let the OP get proficient in adjusting bus bar currents with respect to the bus bars shorted ends to X2 voltage.