Unbalanced load on ungrounded wye source

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Pitt123

Senior Member
I a 3-phase wye source such as a transformer secondary had unbalaned loading on each of its phases and did not have the neutral grounded so it was therefore floating, would the value of the neutral point to ground float and shift similar to a wye connected load that was ungrounded? With the neutral point floating and unbalanced current on each phase could there be a difference between the L-N readings on each phase at the source?
 

nollij

Member
Location
Washington
Ungrounded systems are capacitively coupled to ground (and each other).

I think you are asking: "does the positive sequence current flowing through the individual phases affect the L-N source voltage?"

No.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
If the neutral is unbonded, the line-to-line and line-to-neutral voltages will remain the same, even with imbalanced loading, but the voltages to ground could be almost anything.
 

Pitt123

Senior Member
If the neutral is unbonded, the line-to-line and line-to-neutral voltages will remain the same, even with imbalanced loading, but the voltages to ground could be almost anything.

O.k. with a wye source the voltage from L-N is alawys 120V because it is bound by the voltage ratings on the wye connected coils determined by the primary voltage and turns ratio. The voltage to ground will float due to capacitive coupling correct? What about the neutral to ground voltage?

On the other hand, a wye connected three phase load without a neutral will have the neutral point float above ground and therefore can have different voltage develop across the L-G windings with unbalanced current due to the fact there is nothing to hold the neutral point steady and it can float causing these unequal voltages. Is this correct.

By adding a neutral to the wye load the unbalanced current will flow on the neutral therefore causing the L-N voltages to be balanced. Correct?

Do you ever need to ground the wye point of a wye connected load? I would not think so?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
The voltage to ground will float due to capacitive coupling correct? What about the neutral to ground voltage?
Correct, and it can also vary. It could even rise above 120v if there is enough coupling to a primary supply, not to mention an accidental primary-to-secondary fault. That's why bonding is so important.

Is this correct.
Yes.

Correct again.

Do you ever need to ground the wye point of a wye connected load? I would not think so?
Read Section II of Art. 250, starting with 250.20, then come back and tell us what you think.
 
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Pitt123

Senior Member
Correct, and it can also vary. It could even rise above 120v if there is enough coupling to a primary supply, not to mention an accidental primary-to-secondary fault. That's why bonding is so important.
.

I think I have seen this before rising to above 120V. Can you explain why this happens when it goes above 120V? Is this simply a LV line to HV line coupling capacitcance? Or is a combination of the HV capacitance to ground and the LV capacitance to ground?


Read Section II of Art. 250, starting with 250.20, then come back and tell us what you think.

I will look at this at let you know what I think.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Can you explain why this happens when it goes above 120V? Is this simply a LV line to HV line coupling capacitcance? Or is a combination of the HV capacitance to ground and the LV capacitance to ground?
I can try, but I'm mostly self-taught and field-learned, not schooled like the engineers here. My understanding of electrical theory comes from reading, experience, discussions with my EE friend, and this place. :)

It's a bit of everything. When it comes to indirect coupling, meaning high-impedance capacitance or induction, the influence of everything comes into play, proportionate to the relative amount of coupling.

A wire physically placed halfway between a high-voltage line and earth would have half of the line-to-earth voltage if the amount of coupling is equal, but the line is a wire, and the earth is, as you know, huge.

Hopefully, the brain trust will step in and add factoids that are more accurate than what I said above.
 

Pitt123

Senior Member
Read Section II of Art. 250, starting with 250.20, then come back and tell us what you think.

I believe that you do not want to ever connect the neutral of a wye connected load to ground because then any unbalanced current on the load would be flowing on the ground system. Is this correct?
 

ArchieMedes

Member
I believe that is the reason why you should connect the neutral (star point) of wye connected load. You can connect a CT to this neutral and detect the current which would be your unbalance tripping function. This is from my understanding. Anyone care to enlighten us about this? :)
 

rattus

Senior Member
For What it's Worth:

For What it's Worth:

IMO, if the individual loads are expected to vary, the neutral should be connected in order to stabilize the load voltages. Without a neutral, some loads could see undervoltage while others see overvoltage.
 

hurk27

Senior Member
The neutral is only for line to neutral loads.

three phase loads such as transformers and motors are not design to use the neutral as trying to force the neutral to balance the lines on a three phase Y connect load will cause the load to over heat and over current on the neutral.

I was told once this was due to the eddy currents in common core three phase loads causing the voltage on the neutral to vary as the balance of the three phase voltage varies, Utilities will connect Y to Y when they have 3 single tanks, but not when its a common core single transformer.
 
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