14.4 distribution line How to correct power factor

[Fkec350]

I'm a trouble man for a utility company. We have a 14.4 phase to ground 26 phase to phase line. The problem is a 277/480 bank. Under normal source conditions there is no problem. When we switch the line out to another substation our phase to ground voltage is good but the phase to phase goes haywire. 515 480 444. ?? It's a bank of 3 250 kva transformers on a rack. The further I get from the station the worse the voltage gets. I have a 3 phase pad mount another mile down the road and the voltage is even worse. I know there's at least 3000 Kvar of capacitance on the distribution circuit. The customers equipment trips off line immediately when we switch to the other substation. There alarm is an current imbalance of 30% or greater for 1 second. Any help would greatly be appreciated. Thanx

 

[waross]

It sounds as if the line may be passing through a bank of voltage regulators. When there is uneven loading on a long distribution line, the voltage regulators correct the phase to neutral voltage at the cost of the phase angles and the line to line voltages. Capacitors will not generally cause your problem.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Fkec350]

There is a regulator bank down stream from the problem area. When we switch to the other sub we zero the regs out. And they still trip off line. we put them back in auto and the voltage goes further off. We tried to manually adjust the phase to neutral voltage but as you said the line to line got worse. I'm checking cap banks now ( that's my orders). Next I'm checking load balance in the affected area. Thanks for your help Bill if you think of anything else let me know. Steve

 

[waross]

The root cause of the problem is neutral voltage drops due to single phase loading on the wye distribution circuit. To visualize this, draw a vector representation of a balanced wye circuit.
Label the sketch nodes as A, B, C and N.
Now draw a short vector starting from N to represent the neutral voltage drop. It doesn't matter which direction the vector is drawn.
Label the end of this vector as N₁.
Now draw new vectors from N₁ to A, B, and C.
This represents the effect of neutral current and neutral voltage drop on phase to neutral voltages and phase angles upstream of a voltage regulator.
The angles between the phases are no longer exactly 120 degrees.
Now the voltage regulators boost the uneven phase to neutral voltages. The phase with the lowest voltage will cause the most neutral current. This unbalanced neutral current will cause a further displacement of the neutral position and greater phase angle errors.
Back to your vector sketch,increase the length of the vectors, A-N₁, B-N₁, and C-N₁ so as to raise and balance the phase to neutral voltages.
Now the phase to phase voltages are equal but the phase to phase voltages and the phase angles are not equal.
Any three phase motors will act as generators and will try to correct the voltages and phase angles by drawing more current from the phases with the higher voltages. That is the cause of the unbalanced motor currents.
There will also be voltage drops on the phases, A, B, and C, which add to the issues, but for the sake of simplicity we will disregard then for now. The real cause of the problem is the neutral voltage drop.
One solution is a better balance of the single phase loads. This is not always possible due to the location of the loads. Also, different loads may peak at different times of the day, making it impossible to maintain a balanced loading over a 24 hour period.
The distribution transformer connection that I love to hate is the four-wire wye delta connection. However a four wire wye delta bank floating on the line will try to correct the phase angles and phase to phase voltages.
Be aware that in the event of a loss of one primary phase the four-wire wye delta bank will backfeed and try to replace the missing phase.
A line to neutral (ground) fault anywhere on the circuit may result in the primary fuses blowing on the wye delta bank.
If the linemen disconnect the circuit by pulling fuses and thn go back online one phase at a time, expect some refrigerator or deep freeze burnouts.


Bill
--------------------
"Why not the best?"
Jimmy Carter