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Zero sequence voltage from VT
5

Zero sequence voltage from VT

Zero sequence voltage from VT

(OP)
Hi,

If I have star/star voltage transformer connected to a relay to provide directional sensing. However the neutral of the HV side of the VT is not being earthed, (LV side is earthed). How much impact is there on the zero sequence voltage input? (assuming the relay calculates the zero sequence from the phase inputs). Esp for earth faults if the relay uses zero seq voltage as a polarizing quantity for directional sensing in this case.

Any input greatly appreciated.

RE: Zero sequence voltage from VT

There will be no zero sequence component to the secondary voltage.

RE: Zero sequence voltage from VT

It depends on the type of the system configuration that the PT's are connected to. At the bus or feeder breaker end, 3 phase voltage no longer sum to zero during a L-G fault. Positive and negative seuqence voltage sum to zero but zero sequence. V0 equals the I0*Z0

RE: Zero sequence voltage from VT

Quote (pwrtran)

It depends on the type of the system configuration that the PT's are connected to. At the bus or feeder breaker end, 3 phase voltage no longer sum to zero during a L-G fault. Positive and negative seuqence voltage sum to zero but zero sequence. V0 equals the I0*Z0
But if the VT is connected ungrounded wye on the primary, there is no zero-sequence path and no zero-sequence source on the secondary. I0 = 0, V0 = 0 on the secondary.

RE: Zero sequence voltage from VT

I think the OP was talking about how do the PT's detect a L-G fault in terms of the 3V0 on the system not the fault on the PT secondary, isn't it?

RE: Zero sequence voltage from VT

It does not depend on the system connection. If the VT primary neutral isn't connected to ground there won't be any V0 in the secondary. Full Stop. If there's no V0 in the secondary the relay can't deduce what V0 there might be in the primary. That information is lost and gone forever.

RE: Zero sequence voltage from VT

Any unbalanced 3-phase system can be analyzed by 3 symmetrical sequential components, 1,2 and 0. Assume the system is a 3ph 4W, voltage is dropped on the faulted phase during a L-G fault. PT primary connected in Wye but ungrounded. 3 PT primary windings see voltage but the neutral is floating, the imaginary neutral (floating neutral) has Vn to the true earth. V1 and V2 for all three phases sum to zero but the V0, since V0 is not zero it cause the floating neutral drifted. Why do you think it can't see V0?

RE: Zero sequence voltage from VT

(OP)
The VT is connected to the transformer(transformer is impedance earthed to ground) The relay computes the 3V0 from the phase voltages. In this case the floating neutral may be still locked to true earth.
My original concern was the zero seq volts may not correctly transform across the VT secondary because of the floating neutral on the HV side.

RE: Zero sequence voltage from VT

So, if 3V0 is what you want then try ground the neutral of the PT primary and wire the secondaries in the open corner delta configuration to get true 3V0.

RE: Zero sequence voltage from VT

Look at the sequence diagram for a wye-wye transformer with only one eye-point grounded. Please explain how the zero sequence gets past that open circuit.

Once both sides have a neutral connection the relays can calculate pretty much anything that can be derived from three voltages. Ground the high side neutral and there is no need for that archaic broken delta secondary connection. The broken delta was a great tool in the electromechanical days but has no purpose today.

RE: Zero sequence voltage from VT

Didn't the broken delta connection also require a grounded wye point on the primary side David? Otherwise, we are back to the floating neutral. (Subject to the limits of saturation if the PTs are rated for line to neutral voltage rather than line to line voltage.)

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

RE: Zero sequence voltage from VT

Davidbeach - you are correct that Y-YGrd PTs do not have zero sequence current path. However as long as unbalanced 3phase voltages can be read, 3V0 can be calculated by modern relays.

RE: Zero sequence voltage from VT

How can V0 be calculated from the line to line voltages?

V0 = 1/3(Va-b + Vb-c + Vc-a).

The vectors of Va-b, Vb-c + Vc-a will form a closed triangle. If the system is perfectly symmetrical it will be a nice equilateral triangle. If the system has substantial negative sequence components then the triangle will have different length sides and different angles, but it is still a closed loop, and under those conditions V0 = 0.

RE: Zero sequence voltage from VT

That is why use open corner delta can get 3V0!

RE: Zero sequence voltage from VT

But broken delta also requires a grounded neutral on the high side wye.

RE: Zero sequence voltage from VT

Quote (pwrtran)

Davidbeach - you are correct that Y-YGrd PTs do not have zero sequence current path. However as long as unbalanced 3phase voltages can be read, 3V0 can be calculated by modern relays.
On the primary, there is a V0, but the ungrounded primary neutral will shift so that on the secondary, V0=Va+Vb+Vc=0.
For instance, if because of loading, Øa voltage was depressed and the primary voltages to ground bacame Vag=60@0°, Vbg=120@-120°, Vcg=120@120°, then the ungrounded neutral would shift so that Vng=20@180°, Van=80@0°, Vbn=111.355@-111.052°, Vcn=111.355@111.052°.

RE: Zero sequence voltage from VT

Hi folks,

In the case of ungrounded power systems, if the high voltage side of the Y/Y PTs is grounded, are the secondary measurements correct?

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil
http://www.linkedin.com/profile/view?id=46319837&a...

RE: Zero sequence voltage from VT

Yes, but the phase-neutral values may not have any meaning. The phase-phase values will be just fine.

RE: Zero sequence voltage from VT

Say the neutral is drifting, Van+Vbn+Vcn=Vn, and Vn≠0
So in your calculation how come Van+Vbn+Vcn=0?

RE: Zero sequence voltage from VT

If the high side neutral is not tied down, a single phase to ground fault does not change the shape of the voltage triangle but merely moves it, therefore the "neutral" point of the high side is still at the center of a equilateral triangle but no longer at ground. In that case the low side shows a balanced set of three voltages, all equal in magnitude, all spaced 120 degrees apart. No zero sequence in that, no way for the relay to do anything with math to create a zero sequence voltage.

RE: Zero sequence voltage from VT

And if the transformer feeding the transmission line is impedance grounded, the PT wye point must be connected to the ground, not the transformer wye point.

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

RE: Zero sequence voltage from VT

Quote (pwrtran)

Say the neutral is drifting, Van+Vbn+Vcn=Vn, and Vn≠0
So in your calculation how come Van+Vbn+Vcn=0?
The primary neutral is not drifting, it is shifted with respect to ground. What do you mean by Vn? What is this voltage with respect to? Why is Van+Vbn+Vcn=Vn?
If the voltage from neutral to ground is Vng, voltage from a to ground is Vag, and the voltage from a to neutral is Van, then:

Van=Vag-Vng
Vbn=Vbg-Vng
Vcn=Vcg-Vng
Van+Vbn+Vcn=0, necessary because there is no connection from the ungrounded neutral back to the source

These equations can be solved for Van, Vbn, Vcn, and Vng
Van=2/3*Vag-1/3*Vbg-1/3*Vcg
Vbn=2/3*Vbg-1/3*Vag-1/3*Vcg
Vcn=2/3*Vcg-1/3*Vag-1/3*Vbg
Vng=1/3*Vag+1/3*Vbg+1/3*Vcg

RE: Zero sequence voltage from VT

Quote (davidbeach)

If the high side neutral is not tied down, a single phase to ground fault does not change the shape of the voltage triangle but merely moves it, therefore the "neutral" point of the high side is still at the center of a equilateral triangle but no longer at ground.
Actually, I think it does change the shape of the voltage triangle. The voltage from the unfaulted phases to the faulted phase would become phase-to-ground voltage, while the voltage between unfaulted phases would remain unchanged. The triangle would not be equilateral.

Assume before the fault that Vag=120@0°, Vbg=120@-120°, and Vcg=120@120°. Vab=208@30°, Vbc=208@-90°, Vca=208@150°. If Vag went to zero, and neither Vbg or Vcg were changed, Vab would be 120@60°, Vbc would be 208@-90°, and Vca would be 120@120°

RE: Zero sequence voltage from VT

The fundamental thing about voltage is that it is the potential difference between two points, or put another way, the difference between one point and another fixed reference point. Measuring zero sequence voltage is in effect measuring the neutral point displacement from a nominal central location at the star point. It is the act of tying the star point to ground that gives you the fixed reference point to measure displacement voltage from. If you don't ground the primary star point you have no reference point to measure from. Without the ground connection, you are effectively measuring the distance between two points, each of which is moving around randomly within the equilateral triangle bounded by the phase to phase voltages. The points will move due to inbalanced capacitance, inductance, loading etc. Either point could potentially in some circumstances move outside the triangle. Yes you can get a measurement, but it is meaningless.
Regards
Marmite

RE: Zero sequence voltage from VT

Marmite,
I'd agree if the system were ungrounded, but the OP system was a resistance grounded source with an ungrounded neutral on the primary of the VT. There is a fixed point for the VT primary neutral point, it just isn't at ground potential unless there is no unbalance. The VT primary neutral will be at the voltage (Vng with respect to ground) where Van+Vbn+Vcn=0.

RE: Zero sequence voltage from VT

jghrist - the system is an impedance grounded wye connection so why would Vag going to 0 not change Vbg or Vcg or any of the phase to phase voltages?

Want to think about this in an even simpler manner? This isn't a solidly grounded system so a line to ground fault won't change any of the phase to phase voltages or angles. The PT's are only connected phase to phase so the voltages being measured by the PT's do not change during a ground fault.

RE: Zero sequence voltage from VT

jghrist, Va+Vb+Vc=Vn, Vn=3Vo, the neutral point shifts to Vo, it is same value of Vng in your calculation, I am with you on that now.

RE: Zero sequence voltage from VT

In the event of a ground fault the voltage across the grounding impedance will be equal to the line to neutral voltage.
The voltage to ground of the faulted phase will drop to zero and the voltage to ground of the other two phases will rise from line to neutral voltage to line to line voltage. Think "Corner grounded delta".
The wye point of the PTs will still be at the neutral point, set by the assumed equal impedance of each phase of the PTs.
The PTs will give no information in regards to a grounded phase unless the wye point is grounded.

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

RE: Zero sequence voltage from VT

Bill,
You're correct. My mistake was neglecting the voltage drop across the ground impedance. Including this, if phase a is faulted, Vbg and Vcg will not be unchanged.

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