Transformer saturation-driven neutral instability

[Bronzeado]

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

 

[Bronzeado]

David is correct when saying that we need a VERY GOOD information (and very good models) to do a simulation of this phenomenon.

prc, I will waiting for the papers. PLEASE, try to find them. Thank you!

Regards,

H. Bronzeado

 

[slavag]

Hi.
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

 

[Bronzeado]

Hi folks,

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

 

[slavag]

Hi Herivelto.
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

 

[Bronzeado]

Thank you, Slava, for your advice.

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

 

[prc]

Bronzeado, I wish I could locate the old AIEE papers.The neutral inversion or shifting is occurring in an unearthed system due to resonnance of inductance of the transformer winding with the capacitance on the terminals.At certain values of L vs C this happens. By putting a capacitor on the line we are moving away from that resonnance point withe respect to the L &C of the system.It has nothing to do with any of the points mentioned by you.

 

[Bronzeado]

Thank you, prc.

Herivelto