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.
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
RE: Zero sequence voltage from VT
RE: Zero sequence voltage from VT
RE: Zero sequence voltage from VT
RE: Zero sequence voltage from VT
RE: Zero sequence voltage from VT
RE: Zero sequence voltage from VT
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
RE: Zero sequence voltage from VT
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
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Zero sequence voltage from VT
RE: Zero sequence voltage from VT
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
RE: Zero sequence voltage from VT
RE: Zero sequence voltage from VT
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
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
RE: Zero sequence voltage from VT
So in your calculation how come Van+Vbn+Vcn=0?
RE: Zero sequence voltage from VT
RE: Zero sequence voltage from VT
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Zero sequence voltage from VT
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
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
Regards
Marmite
RE: Zero sequence voltage from VT
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
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
RE: Zero sequence voltage from VT
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
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.