The floating star neutral to earth voltage in transformer. 2

[prc]

In a 131/11.5 kV three-phase Dyn Transformer, a balanced 10 kV was applied to the primary, with the secondary neutral floating. The voltages measured on the LV secondary were equal in all phases. The line to neutral voltages were 504 V, the line to earth voltages were 505 V, and the line to line voltages were 875 V. The neutral to earth voltage was 655 V. Why is the N_E voltage higher than the phase voltage? What does it signify?

 

[waross]

655 Volts is close to the average of 504V and 875V.
You are probably looking at capacitively coupled voltages of the average winding voltages.

 

[FPelec]

As said by waross, I also believe that the star point to ground voltage is due to capacitive coupling between the primary and secondary.

 

[prc]

How to explain the capacitive coupling and the reported results? L-E and L-N voltages are almost the same, meaning N and E are at the same potential. Then, how was a more than phase voltage measured between N&E? Since phase voltages are equal, a neutral inversion is ruled out.

 

[waross]

The secondary winding is not grounded.
Capacitive coupling between the primary winding and the secondary winding raises the common mode voltage.
The micro-farad capacity will be very low.
The impedance of the Volt-meter will act as a voltage divider with the capacity and metering error cannot be ruled out.
Also, the cable capacity to ground and the cable resistance to ground will tend to stabilize the line to ground voltages.
A modern high impedance Volt-meter will detect high voltages that the Old "Wigginton Voltage Tester" or "Wiggy" would swamp to zero.
It was a gross exaggeration to say that a Wiggy would dim the lights, but we said it anyway. grin.

 

[prc]

Thank you waross. What is a Wigginton Voltage tester? Moving Iron meter?

 

[waross]

The Wiggy was a solenoid type voltage tester.
It had both an AC and a DC scale.
It vibrated on AC and was steady on DC.
It had a window on the end with a pivoted magnet that would indicate DC polarity.

 

[waross]

The original Wiggy.

 

[stevenal]

Or instead of using an antique voltage tester, you can use a modern device that has a low impedance input. Fluke calls these capacitive voltages "ghost voltages".

 

[prc]

Thank you, Waross and Stevenal. When we say capacitive coupling is the reason for the ghost voltages, can you elaborate a bit more.
As I understand, this is power frequency voltage transfer from primary. Being PF why it is not getting adding up at neutral? How the impedance of voltmeter acting as a voltage divider with H-L capacitance?

 

[waross]

A capacitor is two conductors separated by an insulator.
The primary winding and the secondary winding form a capacitor.
The secondary windings and ground form a capacitor..
The two capacitors in series form a voltage divider between the primary voltage and ground.
The three phases tend to cancel, so for some reason the capacitive effect is stronger iin one phase.

 

[stevenal]

I confess I'm having a hard time imagining phasors that would lead to these values. Just the same I would connect a 2K ohm resistance from N-G and see if the N-G voltage drops to near zero. I understand 2K is what Fluke uses in the Auto-V LoZ setting.

 

[waross]

Imagine the phasors for each phase. They should sum to zero. Then imagine that for some reason one phasor is significantly higher or lower than the other two.

 

[PHovnanian]

At first glance, I'd vote for the capacitive coupling explanation as well.

But if I were building the "perfect" transformer, the capacitances of each primary to secondary winding would be equal. And the resulting coupled voltages would cancel out. Before putting this thing in service, I'd check the primary to secondary and each winding to ground insulation resistances.