Unbalanced Three Phase Load 6

[gcaudill]

I have an electrical system which consists of a delta-wye connected transformer supplying a delta connected load. One corner of the delta is grounded. I have the following load current measurements:
I leg1 = 1115A
I leg2 = 1000A
I leg3 = 1092A

Can someone tell me where the unbalanced current will go? Circulate in the load or exit to ground at the grounded corner? The ground is well connected, low impedance, and terminated to the main grounding bar at the switchgear.

 

[gcaudill]

Are you really grounding one phase of the wye or holding one corner of the delta to ground potential?

I'm not sure the measurement suggested by DanDel is possible but I am going to look into it.

 

[DanDel]

gcaudill, apparently, you're grounding one phase of the wye(source) AND holding one corner of the delta(load) to ground potential at the same time.

 

[gcaudill]

DanDel,

By saying that one phase of the wye is grounded, which is how it appears, conjurs images of a phase-to-ground fualt condition. Is the grounded phase of the secondary wye (X2) physically the same as one of the primary delta corners (i.e. H2)?

 

[DanDel]

I was talking about your wye(source) and your delta(load), both the same point, which are connected to the secondary of your transformer.
I'm getting increasingly confused about what you're talking about.....

 

[gcaudill]

I understand your confusion. I wish I could paste a copy of the diagram. Do you know if there is a way?

 

[DanDel]

There is a way, but I believe first you must post the drawing to a website, then use the 'img' part of the 'Process TGML' link below the Reply message box.

 

[jghrist]

Please indicate which phases the three load legs are connected to. For instance, is Leg 1 connected between Ph.1 and Ph.2?

Is the transformer winding ratio 480/70 or is this the voltage ratio? When the primary voltage is 480 volts phase-to-phase, what is the secondary voltage phase-to-phase? With a winding ratio of 480/70, the secondary phase to neutral voltage would be 70 volts and the phase-to-phase voltage would be 121 volts.

 

[busbar]

Some utilities will furnish corner grounding of a 480V-wye secondary to get around the NEC 230-95 ground-fault-protection mandate. The problem is, with molded-case breakers used downstream, they must correctly be applied at their single-pole interrupting rating, that is often only 8.6-12KA even through their “nameplate” is stamped 42-65KA.

The 480V single-pole interrupting rating is typically buried in a manufacturers’ “frame book” and not likely any where else. This potential misapplication applies to non-slash-rated MCCBs as well as slash-rated devices.

 

[gcaudill]

jghrist,

On the secondary X1 (ph.1) is connected between leg 1 and leg 2, X2 between leg 1 and leg 3, X3 between leg 2 and leg 3. The voltage ratio of the transformer is 480:70 delta-wye, I do not know the turns ratio.

When the phase-to-phase primary (supply) voltage is 480V (wye connected) the secondary phase-to-phase voltage is 70V. The transformer secondary is wye connected, the load is delta connected. X2 of the transformer secondary is grounded.

 

[jghrist]

Are there two connections to ground, one at the transformer and one at the load? Is there an equipment grounding conductor between the transformer ground connection and the load ground connection? Were the 48v, 4.8v, and 46v measurements made at the transformer or at the load? It isn't clear how you get 4.8v to ground at the grounded point.

I suspect that the Ph. 2 current is split between the phase conductor and the ground path. This is how you are getting current in the ground connection. This is not zero-sequence current. With an isolated neutral, you would not have any zero-sequence current. This would only make sense if there were a connection to ground at both the load and the transformer. But if this were the case, I don't know where the 4.8v would be measured.

 

[gcaudill]

The only ground connection is at X2 of the transformer. The voltage measurements were at the load, the actual secondary voltages would be a few volts higher because a voltage drop occurs between the x-former and load. The 4.8V to ground was measured at the load end. I believe it is due to voltage drop in the conductors due to the return current back to the transformer, or maybe to ground?

 

[jghrist]

As DanDel said earlier, if there is 78A flowing into the ground, there has to be another connection to ground somewhere on the secondary side of the transformer. There will be some capacitive current from phases 1 and 3 to ground, but at 48 volts, it would be no more than a few milliamps.

 

[busbar]

gcaudill -- Apologies if I overlooked it, but can you describe the low-voltage 3ø load? With the wye secondary, it seems unusual that the secondary-winding neutral/wyepoint is floating. Is there a specific reason for that?

 

[GusD]

Hi GCAUDILL.
It is a good thing that you don't really have a problem.
This thread is getting very interesting.
You did say that you have 70 amps measured on the load Delta ground condutor.Wouldn't it be your natural reason for the umbalance on such a system? I don't think you couldn have Balanced currents with this set up.

Thanks . GusD

 

[gcaudill]

busbar,
Here is a description of the load again. This is a resistance annealer where the delta connected load is copper wire (3 segments between four sheaves (ph.1 & ph.3 "hot&quot, two sheaves grounded to ph.2).

Grounded Corner - one corner of the of the load delta is grounded at the transformer X2 tap for safety and equipment protection reasons.

Voltages and connections - 0-480VAC primary 0-70VAC secondary, 60Hz wye connected supply, delta-wye x-former, delta connected load. Ph.2 of the x-former secondary is grounded only.

I do not know why the neutral is left floating, always been done that way.

GusD,
You are correct that I do not have any problems. I am trying to understand the physics of the system. It is safe to assume that the load will always operate unbalanced due to 1.) differences in leg lengths 2.) differences in load resistance as the wire is heated along its length 3.) differences in cable supply lengths from the x-former to the sheaves. 10 250mcm cables feed each sheave assembly.

When the system is unbalanced, where does the unbalanced current circulate (or go); in the transformer, in the load, to ground?

 

[jghrist]

It is possible that the load resistances are identical and that the unbalanced currents are caused by the unbalanced voltage. Try measuring the voltages across the load legs at the same time that you measure the current in the legs. This would help in analyzing the situation.

The unbalanced voltages could be caused by the phantom second connection to ground.

Unbalanced currents circulate in the transformer. The three phase currents in the wye tranformer legs do not have to be equal, but they must add vectorially to zero because there is no neutral connection.

 

[gcaudill]

jghrist,
Why are you so certain there is a phantom connection to ground? The sheaves are all isolated so that the are not electrically connected to ground. The only connection point to ground is at X2 of the transformer secondary.

You had written previously:
"I suspect that the Ph. 2 current is split between the phase conductor and the ground path. This is how you are getting current in the ground connection. This is not zero-sequence current. With an isolated neutral, you would not have any zero-sequence current. This would only make sense if there were a connection to ground at both the load and the transformer."

One of my questions is this, would X2 of the x-former secondary stand at some voltage potential with no ground attached? If so, then by bolting it to ground it is essentially a continuous ground fault? If this is true, why don't I see a large short circuit in the ground conductor?

The rated impedance of the transformer is 5%. A bolted fault current of Is.c.=70V/.05=1400As.c. should exist on the ground conductor connected to X2?

 

[jghrist]

I'm sure that there is some phantom connection to ground because you measured 78A in the connection from X2 to ground. The current has to go (or come from) somewhere.

Without any ground connection on the transformer, the phase voltages to ground are determined by the relative capacitance of each phase to ground. There is some stray capacitive coupling to ground from cables and load equipment. Connecting one phase to ground would result in the stray capacitive current from the other phases flowing through the ground connection, but this will be very small at 48V.

If you were to ground the neutral and then ground the phase, you would have a bolted single phase-to-ground fault, but with the neutral floating, you would have no fault current.

 

[busbar]

gc — Thank you for restating the application. {This thread is getting quite detailed.}

Small aside — Wouldn't secondary short-circuit current be based on the secondary-winding rated current and not secondary-winding voltage?

Given usual process resistive and reactive variables, the measured unbalance may be quite small and anticipated in the annealing-equipment design. There may not be any significant problem to address.

 

[gcaudill]

You are correct that the leg lengths are adjusted to try to balance the loads based upon "estimated" heating. None of the old timers have been able to adequately explain the transformer arrangement in terms of how exactly it works.

I think what I need to learn from this thread is:
1.) Characteristics of the delta-wye connected transformers and effects of connecting/not connecting the neutral.
2.) Effects of capacitive coupling on the secondary side.
3.) Finding positive and negative zero sequence currents.

Anyone have any brief explanations (primers) on these three topics?