Transformer saturation-driven neutral instability
Transformer saturation-driven neutral instability
(OP)
Hi folks,
Has anybody experience on neutral instability during the energizing of a wye-delta transformer?
The transformer is energized throught the high voltage (230 kV) of which windings are wye connected and the neutral grounded.
The low voltage (69 kV) windings are conected in delta and, therefore, have a "ficticious" neutral point.
My questions are:
1. How this "ficticious" neutral point behaves with relation to the grounding during the transformer energizing?
2. If there is a TP on each phase of the delta side with its primary winding connected between the phase and the grounding, what kind of voltage I will get from this TP?
3. Is it possible to get a phase to grounding voltage bigger the phase to phase voltage?
I would appreciate your thoughts!
Best regards,
Herivelto Bronzeado
Has anybody experience on neutral instability during the energizing of a wye-delta transformer?
The transformer is energized throught the high voltage (230 kV) of which windings are wye connected and the neutral grounded.
The low voltage (69 kV) windings are conected in delta and, therefore, have a "ficticious" neutral point.
My questions are:
1. How this "ficticious" neutral point behaves with relation to the grounding during the transformer energizing?
2. If there is a TP on each phase of the delta side with its primary winding connected between the phase and the grounding, what kind of voltage I will get from this TP?
3. Is it possible to get a phase to grounding voltage bigger the phase to phase voltage?
I would appreciate your thoughts!
Best regards,
Herivelto Bronzeado






RE: Transformer saturation-driven neutral instability
This could result in voltages to ground greater than the phase to phase voltage.
Also,a wye-delta transformer experiences switching surges when energized. I understand that these surges can reach 200%, but I may be corrected here. If your wye grounding does not have good continuity or has a high impedance back to the source, you may be getting switching surges. These surges are transient and will dampen out.
Is your 230 kV/69 kV transformer large enough that the potential transformers may not be able to drain the capacitive voltage from the 69 kV winding?
Bill
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RE: Transformer saturation-driven neutral instability
How do you establish a ground connection at the transformer? There's no way to tie it back to the source unless you use a static wire. In any event, it's probably a fairly high impedance path to ground. For this reason, I would be tempted to treat it more like a floating wye delta connection.
On the distribution scene, this connection has been shown to cause overvoltages due to single-pole energization, with phase to ground voltages of 2.65 per unit possible. How are you energizing the transformer? I'd imagine you're using a circuit breaker or gand operated switch. If that's the case, then I wouldn't worry about getting the 2.65 pu value.
Did you have any arrester failures associated with this?
Finally, what's a TP? Terminal pole doesn't seem to fit.
RE: Transformer saturation-driven neutral instability
For your information, I am talking about a 100MVA, 230/69kV transformer wye-delta connected. The neutral point of the wye (230 kV) is solid grounding.
In general, we have more that one transformer with the same connection in a substation. Normally our delta system (69kV) has a grounding reference given by a zig-zag transformer.
When the second transformer (100MVA, 230/69kV, wye-delta) is energized by closing a 230kV circuit breaker, the delta (69 kV) will be open (including a bay of 50-100 m of cable from transformer bushing to circuit breaker) for some time before closing the 69kV circuit breaker that inserts the transformer to the system. My question is related to what happens with the "ficticious" neutral of the delta side during this period of time (transformer energizing).
May this "ficticious" neutral point reach some "positions" against grounding in which the phase-to-ground is bigger than phase-to-phase voltage?
Please, send me any literature or thougth on that. I need to anderstand this phenomenon.
Thanks Bill for you input. I will consider the capacitive coupling between the windings.
Best regards,
Herivelto
RE: Transformer saturation-driven neutral instability
Your fictitious neutral will tend to be somewhere within the voltage triangle. The placement of that neutral will be determined by the capacitance to ground from each phase. To the extent that those capacitances are balanced (they won't be) your neutral will be in the middle of the triangle. The only way to get the neutral outside the triangle is to charge those capacitances, open and then reclose out of phase with the stored charge.
RE: Transformer saturation-driven neutral instability
Please, have in mind that the transformer is being energized, with the poles of the circuit breaker closing in different instants. So the "capacitances" involved may be charged differently of each other.
Also, inrush currents of high magnitude will flow throught the transformer primary windings (230 kV, wye) and may go into the ground.
Best regards,
H. Bronzeado
RE: Transformer saturation-driven neutral instability
RE: Transformer saturation-driven neutral instability
We have a lot of transformers wye-delta (230/69kV) in our system. During transformer energizing, it has been found phase-to-ground overvoltages in the delta side (69 kV) which remain for some time. These overvoltages seems to be caused by:
a) Neutral instability or
b) Ferro-resonance
What I expect to get from this forum of experts is some technical information, some light, to explain this phenomenon.
Bill (see above) has given some tips which I will work on that.
The voltage measurements comes from PTs with their primary windings connected betweem each phase of the delta system and the ground.
Best regards,
H. Bronzeado
RE: Transformer saturation-driven neutral instability
RE: Transformer saturation-driven neutral instability
The VT primary rating is 69kV.
So far, we have not seen any physical evidence of overvoltage on the 69 kV, only on the secondary of the VTs circuit.
Regards,
H. Bronzeado
RE: Transformer saturation-driven neutral instability
RE: Transformer saturation-driven neutral instability
That is one solutionn for ferro-ressonance. Is it the phenomennon that is happening?
I would like to know a physical explaination on the phenomenon that is really happening.
My guess so far from discussions on "instability" of the neutral point of the delta side are:
1. Voltage rise of the substation grounding network during the transformer energizing due to the high inrush currents going into the ground network.
2. Voltage transfer due to the capacitance divider formed by the capacitances between the windings (high and low voltages) and the windings (delta) and ground (Bill suggestion as far I anderstood).
As a consequence, the delta neutral point will be at a "position" at which phase-to-ground overvoltages may impar the equipment, includind saturation of the PTs and, perhaps, the power transformer.
If ferro-ressonance appears, it should be the effect not the cause.
Do you agree with these comments?
Regards,
H. Bronzeado
RE: Transformer saturation-driven neutral instability
RE: Transformer saturation-driven neutral instability
Are you have some news on the issue?
Could you please explain why you start with this Q's.
My collegue sent me today other case ( but on the same issue), he see this is ferroresonance problem on the VT. But in case of two VT's on the BB w/o any load ( 10kV). In case of one VT, no problem, and this case occure only on the one SS.
Are you have this effect in the special place or on the few installation?
Best Regards.
Slava
RE: Transformer saturation-driven neutral instability
Are you have broken delta connection in the your VT?
I haven't documents on the English.
It's problem of small C ( you need add some 150-200m of cable for avoid this effect). In the Russia lot of MV grid is isolated neutral and this effect is very known.
Neutral point driven, but of VT.
It's effect of saturation of VT cores, and isn't same saturation per one phase VT, C and L are changes up to current resonance (C=L). This also move to increas of phase to ground voltage.
Hope that help.
Best Regards.
Slava
RE: Transformer saturation-driven neutral instability
I will try to get some voltage waveforms (69 kV) in order to show a better picture of the phenomenon we talk about.
I still thinking that ferro-ressonance (if it happens) may be an effect of the phase-to-ground overvoltage, NOT the cause.
Regards,
Herivelto
RE: Transformer saturation-driven neutral instability
We wrote togehter
I haven't option now check this effect, but in the begining of 2009, we'll start commissioning of some SS, where we have this option, but it will be 11kV.
Regards.
Slava
RE: Transformer saturation-driven neutral instability
RE: Transformer saturation-driven neutral instability
RE: Transformer saturation-driven neutral instability
RE: Transformer saturation-driven neutral instability
prc, I will waiting for the papers. PLEASE, try to find them. Thank you!
Regards,
H. Bronzeado
RE: Transformer saturation-driven neutral instability
Additional capacitors, this is a point.
Instead this I would recommend use longer cable from delta side of trafo. Longer cable add capacity to circuit.
Second, in lot of application we are installed zig-zag trafo before trafo CB.
Best Regards.
Slava
RE: Transformer saturation-driven neutral instability
Please, find attached the voltage and current measurements during a 100MVA, 230/69 kV, wye/delta transformer energizing, by closing the 230kV circuit-breaker.
From the top to the bottom, it can be seen:
1. 230 kV voltage waveforms (Va, Vb and Vc)
2. Currents on the 230 kV side (inrush currents, Ia, Ib and Ic)
3. 3V0 voltage waveform (Va+Vb+Vc, 69 kV)
4. 69 kV voltage waveforms (Va, Vb and Vc)
My question is: why the 69 kV voltages behaves like that?
Best regards,
H. Bronzeado
RE: Transformer saturation-driven neutral instability
You need additional capacitance in your installation.
It's VT saturation.
Are you have antiresonance resistor across broken delta connection of VT about 22-27Ohm 350-400W.
Reagrads.
Slava
RE: Transformer saturation-driven neutral instability
However, I need to understand what is happens before applying any solutions.
So, I wonder if somebody could explain why this voltage behaves like that.
1. Is it due to the electrical coupling between high and low windings?
2. Is it due to a grounding rising?
3. This happens due to zero sequence current circulating in the delta windings?
4. As the potential transformer is connected between phase-to-ground in the delta side (69 kV), are the voltage measurements correct (assuming the capacitance of the bay is the same for all phases) or meanless?
5. Is this phenomenon caused by ferro-ressonance or it may cause ferro-ressonace as the VTs may saturate (cause or effect)?
Bill, magoo2, davidbeach, slava, ePOWEReng, prc, you have the words.
Best regards,
H. Bronzeado
RE: Transformer saturation-driven neutral instability
RE: Transformer saturation-driven neutral instability
Herivelto