Protection of tapped windings

[elenapredescu]

For power transformer with no load tap changer +/- 10% of HV voltage, liniar; the value of overvoltage which apear on tapped winding is too high. What kind a protection for tapped windings it is necessary to use?

 

[slavag]

Not something special.
Standard protective devices for tap changer tank.
it's base on the oil flow, pressure.

 

[elenapredescu]

No, I didn't talk about the OLTC protection. The tapped windings need, maybe, some arresters to protect against overvoltage but the transformer is already manufactured and it is not enought place inside of the tank.
Thank you for your support.

 

[7anoter4]

Usually the tap changer is connected in star [Y] and the star point is solidly grounded so you don't need a separate overvoltage protection. If the connection is not solidly grounded no overvoltage protection could be useful as the tap changer insulation is reduced with respect to the main winding. But you may check if you may fit an arrester on the star point instead the solidly grounding. Delta connection tap changer with reduced insulation cannot be protected [my opinion].

 

[ScottyUK]

This discussion is quite speculative without any detail of the transformer size or connection. I suspect with an off circuit tap selector - which I assume is what you mean by off-load tapchanger - this is a relatively small transformer.

'OLTC' is universally accepted to mean On-Load Tap Changer; true off load tapchangers which allow a tap to be changed with the circuit energised but carrying no load are a rare device. Off circuit tap selectors require the transformer to be dead and isolated before any attmept is made to change tap. Which type are we talking about here?


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If we learn from our mistakes I'm getting a great education!

 

[elenapredescu]

Thank you!
The transformer is 50 MVA; 132 kV star connection; off load tap changer is provided on the neutral +/-10%. During the impulse test on HV (BIL 550 kV) on the tapped windings apear more than 950 kV. The transformer is already manufactured and I ask for a solution without any changes inside of the windins.

 

[7anoter4]

I don't think you need to test this transformer for BIL while is already mounted on site.
I think also is a kind of resonance. The medium voltage motors are provided with an arrestor beside a capacitor-both grounded- in order to break the resonance.
If it is compulsive to test the transformer, an arrestor provided inside the transformer housing will reduce the BIL to the arrestor residual voltage that means part of the transformer winding will not be tested at the required BIL.
If it is so critic to limit the possible overvoltage from the network a suitable arrestor connected with the transformer terminals may protect entire winding [including tap-changer].

 

[erickench]

Well for one thing the tap is considered a grounded(or neutral)conductor and I don't think your supposed to have an OCPD of any kind in a grounded neutral circuit.

Eric Kench, P.E.

 

[elenapredescu]

Thank you all who have helped me.

 

[prc]

Good transformer designers know how to control the upswing of impulse voltage on floating tapping winding.Since you said transformer is already made ,the solution is to provide zinc-oxide pieces across the regulating tapping leads.Sometimes capacitors (formed by placing paper covered conductors side by side) also tried.All these require careful design and caculations,requiring a transformer expert.

 

[elenapredescu]

Thanks!
I already put ZnO elements on the tapped windings. I tried to get another solution that I didn't know.

 

[odlanor]

If the problem is surge voltage impinged by lightning discharge , the solution is install surge arrester type OZn at HV transformer terminals.
We have some installation with arresters mounting over transformer tank.