Neutral Voltage and Unbalanced Voltages

[charz]

1. If the primary neutral of the VT and the ground is tied together, the neutral voltage (Neutral to ground) voltage should always be zero at the VT side. In other words, the neutral point of the VT Primary and the ground is shorted.Then how come neutral shift (there is a neutral to ground voltage) occur?. What is the relation if the connected system to the VT is ungrounded or grounded. How the neutral shift is calculated by the relays?

2. what is the reason for the unbalance in phase to neutral voltages in ungrounded system?
- Is it because of unbalanced line to ground capacitance. If so how it exactly causes the unbalanced phase to neutral voltages?
- Is it because of unbalanced load. How the unbalanced load causes unbalanced voltages

 

[HamburgerHelper]

1, How are your VTs connected? The relay calculates the residual or 3V0 by adding up all the phases and referencing them to ground.

2. Anything that draws zero sequence components will cause line to neutral voltages to drift from ground on not solidly grounded systems. Ground faults, unbalanced load, untransposed lines.

 

[waross]

This most often happens with long distribution lines with unbalanced line to neutral loads.
A single phase load causes a current to flow in the neutral conductor. The current times the impedance of the neutral conductor equals the voltage drop on the neutral.
Throw in voltage regulators and the situation gets worse.
On long rural lines, the voltage regulators are typically single phase and do a good job of stabilizing the line to neutral voltages.
If one phase is more heavily loaded than the others, there will be a neutral current and a neutral voltage drop. The regulator will cause the neutral current to increase slightly making the neutral voltage drop greater.
The downstream phase to neutral voltages will be equal but the line to line voltages and the phase angles will be unequal.
The independent voltage regulators work great for long rural lines with primarily single phase loading.
Not so great for three phase motors. In this situation it is well to oversize three phase motors.

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

 

[charz]

Hi Hamburger,
There are 3 single phase PT's connected wye- wye grounded at both sides.
"The relay calculates the residual or 3V0 by adding up all the phases and referencing them to ground."
-I suppose the measured voltage is phase to ground Voltages and the summed voltages are 3V0. Is my understanding right?

What happens if primary neutral of VT is disconnected?
What happens if secondary neutral of VT is disconnected?

 

[davidbeach]

The VTs will "correctly" measure the phase-to-ground voltage at their point of connection, but it doesn't mean much. With the primary system ungrounded there is no fixed relationship between source phase-neutral and the measured phase-ground voltage. The measured phase-phase voltages will be correct and they're what matters.

I'm not sure of the implications of leaving the VT primary ungrounded in this configuration, but leaving the secondary ungrounded runs the risk of the having the primary voltage capacitively coupling to the secondary and imposing dangerously high voltages on your secondary circuit.

 

[charz]

Hi David,
Thank you for your reply. Actually I want to intuitively understand the "Neutral Shift" in both the ungrounded and
grounded systems. How does the relay knows there is a Neutral shift?
For ungrounded systems, I can understand if the VT's are connected in broken delta but I couldn't understand
for the VT's connected to phase to ground.
Some relays measure the Neutral Voltage (Vn). I suppose this is Neutral - ground Voltage.
How does the relay measures/ calculates this Neutral voltage as the VT measures just Phase to ground?
what is the formula used by the relay?

 

[HamburgerHelper]

If you want an intuitive understanding use this tool I have linked at the bottom of this page. (

http://powerprotection.wikia.com/wiki/Symmetrical_Components)

- "FREE Power Quality Teaching Toy by Alex McEachern of Power Standards Lab". It is a tool that lets you look at the phasors with varying magnitudes and angles of zero, negative, and positive sequence components. If you play with the zero sequence slider, you will see that it affects all the phase phasors equally. (that is the definition of zero sequence). The result is that it basically shifts your voltages off center, or off from ground. The neutral point will be the center of the triangle formed by your line to line values. The relay measures ground to this neutral point. I really recommend the tool though because it is one thing to be able to use the matrices to go from phasor to sequence values or vice versa but to get good feel is even better.

GIF of varying amounts of zero sequence.

GIFs for phasors of common faults on grounded and ungrounded systems.

http://powerprotection.wikia.com/wiki/Analyzing_Faults_with_Symmetrical_Components

 

[charz]

Hi Hamburger,
Thank you for your reply.

On Neutral shift - "The neutral shift is the deviation of the potential of the load’s neutral point in respect to system neutral, which ideally, should be very close to zero volts in respect to true earth ground potential." Is this understanding correct? This could be undestood in grounded systems.

However on ungrounded systems, relay manual says Vn (Neutral Voltage) and V0(Zero Sequence Voltage)are calculated three Phase voltages? Is Vn= V0? Here three phase voltage refers to Phase to neutral (or) Phase to Ground Voltage?

 

[charz]

The VT's measures Phase to Ground Voltages. I need to measure Neutral to ground Voltages (Vn). How is the neutral point derived/calculated by the relays?