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.

 

[jlazucena]

Check the voltages they may not be identical in magnitude and angle, because the voltages from utility are not quite balanced and the transformer is not perfect.
Check the loads they may not be identical in magnitude and angle.
Check the currents they may not have the same angle

Beside that, the system has has a capacitance to ground that allows some current to circulate

 

[busbar]

If the load is 3-wire and insulation is good shape, then the three measurements account for all current flow. There would be no zero-sequence component—only positive and negative.

{What are the respective ø-ø voltages? Is it a single load or a composite?}

 

[dpc]

As mentioned above, you have to know the relative phase angles of the three currents.

The current at each point of the delta must add to zero.

 

[Saut]

I agree with busbar, there'll be no zero sequence component as your conenction to earth does not result in zero sequence becasue the transformer secondary is delta connected, so you'd be having a single point to earth grounded situation with the currents circulated in the phases, to resolve the currents, you need to analyse using phasor components.

 

[SMB1]

What is the purpose of grounding the corner of the delta connected load.
Usualy for star/star high voltage Xfmr with tertiary winding, one corner of the delta winding is grounded to limit the surges transmitted from high voltage system.

 

[jbartos]

Suggestion: Please, provide more info regarding the transformer wye (assumed secondary and supplying the load) system grounding.

 

[jghrist]

I assume that the delta primary of the transformer has the grounded corner, not the delta connected load. You should ground the neutral of the wye secondary, either solidly or through an impedance.

The unbalanced delta load, as stated by busbar, has no zero-sequence current. The positive- and negative-sequence currents come from the primary. If there were unbalanced wye-connected loads, the zero-sequence current would circulate in the delta-primary; the primary line currents would be positive- and negative-sequence.

 

[gcaudill]

Let me provide more information to answer some of the questions.

Application - this is a resistance annealer where the delta connected load is copper wire (3 segments between four sheaves (ph.1 & ph.3), two sheaves grounded).

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

Voltages and connections - 0-480VAC primary 0-70VAC secondary, 60Hz supply, wye-conneced primary, delta-wye transformer, delta connected load.

At the given currents the voltages were as follows:
Ph.1 to gnd - 48VAC
Ph.2 to gnd - 4.8V (this is the grounded corner)
Ph.3 to gnd - 46VAC
Currents:
I leg1 = 1115A
I leg2 = 1000A
I leg3 = 1092A

Transofrmer secondary Neutral is not grounded.

I have not checked the phase angles of the currents and voltages. I wish I knew what they were. Except for the transformer and the conductors from the transformer to the sheaves, I think it would be safe the say the system is primarily resistive and not reactive in nature. But I don't know for sure.

 

[jghrist]

Confusion resulted from your calling the transformer delta-wye when the primary is wye and the secondary is delta. This should be called a wye-delta tranformer.

The unbalance in the secondary is positive- and negative-sequence. This translates into the wye primary as positive- negative- and zero-sequence current.

 

[gcaudill]

More confusion, the transformer is DELTA-WYE, I stated wye-delta in my last response which is INCORRECT!!!!!!!

 

[DanDel]

gcaudill, are you saying that you have a wye connected winding, serving a delta load, with one phase grounded?

 

[gcaudill]

Yes. I have 78A in the ground conductor, which is bonded to the corner of the delta load, which is fed by the wye secondary of the transormer.

 

[DanDel]

You state that the recorded secondary voltage for two legs are 48V and 46V, but the winding voltage is 0-78V and the input voltage is 0-480V. Is there a variable input?

 

[gcaudill]

Yes, it is an SCR voltage controller. Secondary voltage is 0-70V.

 

[DanDel]

With 78A in the ground conductor and the system you describe, there has to be a connection with some resistance to ground somewhere else.
Is it possible to measure the other two phase conductors(not the grounded one) together inside one CT? If the amperage is equal to what you measure in the ground conductor, then you have another ground connection. If it isn't the same, you may have a faulty instrument or some other problem.

 

[gcaudill]

DanDel,
Are you implying that only two of the three transformer secondary phases connected to the delta load are current carrying?

From load measurements of:
I leg1 = 1115A
I leg2 = 1000A
I leg3 = 1092A

You get calculated transformer secondary currents of:
Ix1 = 1931A
Ix2 = 1732A
Ix3 = 1891A

And calculated transformer primarmy currents of:
I L1 = 487A
I L2 = 477A
I L3 = 437A
x-former turns ratio = 480/70 = 6.857

 

[DanDel]

I'm simply asking if you can get the other two phase conductors(to the load) together inside the same CT clamp-on meter. This will allow you to see the vector sum of their currents, which must equal the current in the other conductor, which is connected to ground. If they do not equal, then you have another conductor(ground) in the system. Remember, the vector sum of all the currents in a circuit must be equal to zero.

 

[jghrist]

I've never heard of grounding one phase of a wye transformer winding and floating the neutral!

 

[DanDel]

I'd have to agree with you on that one, jghrist. This appears to be a special application.